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  • 51.
    Kållberg, Per
    et al.
    SMHI, Research Department, Meteorology.
    Montani, A.
    A case study carried out with two different NWP systems2006In: Natural hazards and earth system sciences, ISSN 1561-8633, E-ISSN 1684-9981, Vol. 6, no 5, p. 755-760Article in journal (Refereed)
    Abstract [en]

    A model intercomparison between two atmospheric models, the non-hydrostatic Lokal Modell (LM) and the hydrostatic HIgh Resolution Limited Area Model (HIRLAM) is carried out for a one-week period, including a case of cyclogeneis leading to heavy precipitation over Northern Italy. The two models, very different in terms of data-assimilation and numerics, provide different results in terms of forecasts of surface fields. Opposite diurnal biases for the two models are found in terms of screen level temperatures. HIRLAM wind speed forecasts are too strong, while LM precipitation forecasts have larger extremes. The intercomparison exercise identifies some systematic differences in the weather products generated by the two systems and sheds some light on the biases of the two numerical weather prediction systems.

  • 52.
    Kållberg, Per
    et al.
    SMHI, Research Department, Meteorology.
    Uppala, Sakari
    Simmons, Adrian
    The real first weather satellite picture2010In: Weather, ISSN 0043-1656, E-ISSN 1477-8696, Vol. 65, no 8, p. 211-213Article in journal (Refereed)
  • 53.
    Landelius, Tomas
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Dahlgren, Per
    SMHI, Research Department, Meteorology.
    Gollvik, Stefan
    SMHI, Research Department, Meteorology.
    Jansson, A.
    Olsson, Esbjörn
    SMHI, Research Department, Meteorology.
    A high-resolution regional reanalysis for Europe. Part 2: 2D analysis of surface temperature, precipitation and wind2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 698, p. 2132-2142Article in journal (Refereed)
  • 54. Lewinschal, Anna
    et al.
    Ekman, Annica M. L.
    Körnich, Heiner
    SMHI, Research Department, Meteorology.
    The role of precipitation in aerosol-induced changes in northern hemisphere wintertime stationary waves2013In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, no 3-4, p. 647-661Article in journal (Refereed)
    Abstract [en]

    The coupled climate model EC-Earth2 is used to investigate the impact of direct radiative effects of aerosols on stationary waves in the northern hemisphere wintertime circulation. The direct effect of aerosols is simulated by introducing prescribed mixing ratios of different aerosol compounds representing pre-industrial and present-day conditions, no indirect effects are included. In the EC-Earth2 results, the surface temperature response is uncorrelated with the highly asymmetric aerosol radiative forcing pattern. Instead, the anomalous extratropical temperature field bears a strong resemblance to the aerosol-induced changes in the stationary-wave pattern. It is demonstrated that the main features of the wave pattern of EC-Earth2 can be replicated by a linear, baroclinic model forced with latent heat changes corresponding to the anomalous convective precipitation generated by EC-Earth2. The tropical latent heat release is an effective means of generating stationary wave trains that propagate into the extratropics. Hence, the results of the present study indicate that aerosol-induced convective precipitation anomalies govern the extratropical wave-field changes, and that the far-field temperature response dominates over local effects of aerosol radiative forcing.

  • 55.
    Lindskog, Magnus
    et al.
    SMHI, Research Department, Meteorology.
    Dee, Dick
    Tremolet, Yannick
    Andersson, Erik
    Radnoti, Gabor
    Fisher, Mike
    A weak-constraint four-dimensional variational analysis system in the stratosphere2009In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 135, no 640, p. 695-706Article in journal (Refereed)
    Abstract [en]

    A weak-constraint four-dimensional variational (4D-Var) analysis system designed to correct stratospheric model errors has been evaluated. Verifications against upper-level radiosonde temperature observations and Stratospheric Sounding Unit (SSU) radiance data show that the addition of a weak constraint in the stratosphere call greatly reduce analysis bias. Both single-observation analysis experiments and extended assimilations have been performed to help us understand the impact of the model error covariance specifications required for the weak-constraint formulation. It is found that the use of multivariate balance constraints similar to those implemented in background-error covariances can be problematic. Copyright (C) 2009 Royal Meteorological Society

  • 56.
    Lindskog, Magnus
    et al.
    SMHI, Research Department, Meteorology.
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    Mogensen, Kristian S.
    Representation of background error standard deviations in a limited area model data assimilation system2006In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 58, no 4, p. 430-444Article in journal (Refereed)
    Abstract [en]

    Two different approaches for improving the representation of background error standard deviations have been developed and introduced into the HIRLAM high-resolution limited area model 3-D variational data assimilation scheme. One of the methods utilizes a horizontally varying climatological background error standard deviation field, estimated from a time-series of innovations. The second approach attempts to take temporal and spatial variations of the background error standard deviations into account by applying an Eady instability measure to the background field. The two approaches are described in detail and their functionality is demonstrated. Parallel data assimilation and forecasts experiments indicate a slightly positive impact on average verification scores, and in addition a positive impact is demonstrated for an individual synoptically active case.

  • 57.
    Lindskog, Magnus
    et al.
    SMHI, Research Department, Meteorology.
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    Navascues, B
    Mogensen, K S
    Huang, X Y
    Yang, X
    Andrae, Ulf
    SMHI, Research Department, Meteorology.
    Berre, Loik
    SMHI, Research Department, Atmospheric remote sensing.
    Thorsteinsson, S
    Rantakokko, J
    Three-dimensional variational data assimilation for a limited area model Part II: Observation handling and assimilation experiments2001In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 53, no 4, p. 447-468Article in journal (Refereed)
    Abstract [en]

    A 3-dimensional variational data assimilation (3D-Var) scheme for the HIgh Resolution Limited Area Model (HIRLAM) forecasting system is described. The HIRLAM 3D-Var is based on the minimisation of a cost function that consists of one term, J(b), which measures the distance between the resulting analysis and a background field, in general a short-range forecast, and another term. J(o), which measures the distance between the analysis and the observations. This paper is concerned with J(o) and the handling of observations, while the companion Paper by Gustafsson et al. (2001) is concerned with the general 3D-Var formulation and with the J(b) term. Individual system components. such as the screening of observations and the observation operators, and other issues, such as the parallelisation strategy for the computer code, are described. The functionality of the observation quality control is investigated and the 3D-Var system is validated through data assimilation and forecast experiments. Results from assimilation and forecast experiments indicate that the 3D-Var assimilation system performs significantly better than two currently used HIRLAM systems. which are based on statistical interpolation. The use of all significant level data from multilevel observation reports is shown to be one factor contributing to the superiority of the 3D-Var system. Other contributing factors are most probably the formulation of the analysis as a single global problem, the use of non-separable structure functions and the variational quality control, which accounts for non-Gaussian observation errors.

  • 58.
    Lindskog, Magnus
    et al.
    SMHI, Research Department, Meteorology.
    Jarvinen, H
    Michelson, Daniel
    SMHI, Core Services.
    Assimilation of radar radial winds in the HIRLAM 3D-Var2000In: Physics and chemistry of the earth. Part B: Hydrology, oceans and atmosphere, ISSN 1464-1909, E-ISSN 1873-4677, Vol. 25, no 10-12, p. 1243-1249Article in journal (Refereed)
    Abstract [en]

    During the last decade several attempts of assimilating radar wind data into atmospheric models have been reported by various research groups. Some of these are briefly reviewed here. A three-dimensional variational data assimilation (3D-Var) scheme for the High Resolution Limited Area Model (HIRLAM) forecasting system has been developed and prepared for assimilation of low elevation angle radar radial wind superobservations. The HIRLAM 3D-Var is based on a minimization of a cost function that consists of one term measuring the distance between the resulting analysis and a background field, which is a short-range forecast, and another term measuring the distance between the analysis and the observations. The development required for assimilating the radial wind data includes software for generating and managing the superobservations from polar volume data, a quality control algorithm and an observation operator for providing the model counterpart of the observation. The functionality of the components have been evaluated through assimilation experiments using data from Finnish and Swedish radars and further studies are underway. (C) 2000 Elsevier Science Ltd. All rights reserved.

  • 59.
    Lindskog, Magnus
    et al.
    SMHI, Research Department, Meteorology.
    Landelius, Tomas
    SMHI, Research Department, Atmospheric remote sensing.
    Prognoser av Solstrålning2018In: Polarfront, no 168, p. 41-44Article in journal (Other academic)
  • 60.
    Lindskog, Magnus
    et al.
    SMHI, Research Department, Meteorology.
    Ridal, Martin
    SMHI, Research Department, Meteorology.
    Thorsteinsson, Sigurdur
    Icelandic Meteorological Office, Reykjavík, Iceland.
    Ning, Tong
    Lantmäteriet.
    Data assimilation of GNSS zenith total delays from a Nordic processing centre2017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 22, p. 13983-13998Article in journal (Refereed)
  • 61.
    Lindskog, Magnus
    et al.
    SMHI, Research Department, Meteorology.
    Salonen, K
    Jarvinen, H
    Michelson, Daniel
    SMHI, Core Services.
    Doppler radar wind data assimilation with HIRLAM 3DVAR2004In: Monthly Weather Review, ISSN 0027-0644, E-ISSN 1520-0493, Vol. 132, no 5, p. 1081-1092Article in journal (Refereed)
    Abstract [en]

    A Doppler radar wind data assimilation system has been developed for the three-dimensional variational data assimilation (3DVAR) scheme of the High Resolution Limited Area Model (HIRLAM). Radar wind observations can be input for the multivariate HIRLAM 3DVAR either as radial wind superobservations (SOs) or as vertical profiles of horizontal wind obtained with the velocity-azimuth display (VAD) technique. The radar wind data handling system, including data processing, quality control, and observation operators for the 3DVAR, are described and evaluated. Background error standard deviation (sigma(b)) in observation space for wind and radial wind have been estimated by the so-called randomization method. The derived values of sigma(b) are used in the quality control of observations and also in the assignment of radar wind observation error standard deviations (sigma(o)). Parallel data assimilation and forecast experiments confirm reasonably tuned error statistics and indicate a small positive impact of radar wind data on the verification scores, for both inputs.

  • 62.
    Ljungemyr, Patrik
    et al.
    SMHI, Core Services.
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    Omstedt, Anders
    SMHI, Research Department, Oceanography.
    Parameterization of lake thermodynamics in a high-resolution weather forecasting model1996In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 48, no 5, p. 608-621Article in journal (Refereed)
    Abstract [en]

    A model for the parameterization of lake temperatures and lake ice thicknesses in atmospheric models is presented. The model is verified independently, and it is also tested within the framework of the High Resolution Limited Area Model(HIRLAM), applied operationally for short range weather forecasting at the Swedish Meteorological and Hydrological Institute (SMHI). The lake model is a slab model based upon energy conservation and treats the lakes as well mixed boxes with depths represented by the mean depths. The model is forced by near surface fluxes calculated from total cloudiness, air temperature, air humidity and low-level winds. A data base, describing 92000 Swedish lakes. provides the model with lake mean depths, areal sizes and locations. When the model is used for parameterization of lake effects in the atmospheric model, all the smaller lakes and the fractions of larger lakes within each horizontal grid square of the atmospheric model are parameterized by four model lakes, representing the lake size distribution. The verification of the lake model is done by comparing it with a more advanced, vertically resolved model, including parameterization of turbulent mixing processes, as well as by comparison with observations. A sensitivity test shows great interannual variations of the ice-covered season, which implies that lake models should be used instead of climate data. The results from an experiment with two-way coupling of the lake model to the atmospheric model are verified by comparing forecasted weather parameters with routine meteorological observations. These results show that the impact of lake effects can reach several degrees C in air temperatures close to the surface.

  • 63. Megner, L.
    et al.
    Tan, D. G. H.
    Körnich, Heiner
    SMHI, Research Department, Meteorology.
    Isaksen, L.
    Horanyi, A.
    Stoffelen, A.
    Marseille, G. -J
    Linearity aspects of the ensemble of data assimilations technique2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 687, p. 426-432Article in journal (Refereed)
    Abstract [en]

    We examine the linearity of the Ensemble of Data Assimilations (EDA) technique with respect to the amplitude of the applied observation perturbations. We provide explicit examples to assess the linear relationship between such modifications of the observing system and the resulting changes in the EDA ensemble spread. The results demonstrate that, for a state-of-the-art numerical weather prediction (NWP) system, such linearity between the applied observation perturbations and the EDA ensemble spread holds well for temporal and spatial regimes relevant to global medium-range weather prediction: specifically, for forecast lead-times of up to approximately 5 days, in the vertical throughout the troposphere up to the lower and middle stratosphere and for broad horizontal scales.

  • 64. Molinder, Jennie
    et al.
    Körnich, Heiner
    SMHI, Research Department, Meteorology.
    Olsson, Esbjörn
    SMHI, Research Department, Meteorology.
    Bergstrom, Hans
    Sjoblom, Anna
    Probabilistic forecasting of wind power production losses in cold climates: a case study2018In: Wind Energy Science, ISSN 2213-3968, E-ISSN 2366-7443, Vol. 3, no 2, p. 667-680Article in journal (Refereed)
  • 65. Mota, Fernando de Brito
    et al.
    Rivelino, Roberto
    Medeiros, Paulo V. C.
    Medeiros, Paulo
    SMHI, Research Department, Meteorology.
    A critical assessment on the electron transport through dehydrogenated intrinsically conducting channels in graphane-graphene hybrids2019In: MATERIALS RESEARCH EXPRESS, ISSN 2053-1591, Vol. 6, no 8, article id 085618Article in journal (Refereed)
  • 66. Mueller, Malte
    et al.
    Homleid, Mariken
    Ivarsson, Karl-Ivar
    SMHI, Core Services.
    Koltzow, Morten A. O.
    Lindskog, Magnus
    SMHI, Research Department, Meteorology.
    Midtbo, Knut Helge
    Andrae, Ulf
    SMHI, Research Department, Meteorology.
    Aspelien, Trygve
    Berggren, Lars
    SMHI, Core Services.
    Bjorge, Dag
    Dahlgren, Per
    SMHI, Research Department, Meteorology.
    Kristiansen, Jorn
    Randriamampianina, Roger
    Ridal, Martin
    SMHI, Research Department, Meteorology.
    Vignes, Ole
    AROME-MetCoOp: A Nordic Convective-Scale Operational Weather Prediction Model2017In: Weather and forecasting, ISSN 0882-8156, E-ISSN 1520-0434, Vol. 32, no 2, p. 609-627Article in journal (Refereed)
  • 67. Olofsson, B
    et al.
    Olsson, Esbjörn
    SMHI, Research Department, Meteorology.
    Andersson, S
    Martensson, T
    Martensson, E
    A new algorithm to estimate aircraft icing in the HIRLAM model2003In: Meteorological Applications, ISSN 1350-4827, E-ISSN 1469-8080, Vol. 10, no 2, p. 111-114Article in journal (Refereed)
    Abstract [en]

    A new index to estimate aircraft icing in clouds from operational meteorological models has been developed by Swedish meteorologists. Although rather simple it takes into account, directly or indirectly, all the principal meteorological variables for icing. The index has been evaluated during three winter seasons and is now operational in the Swedish HIRLAM model. A graphical representation of the index is presented.

  • 68.
    Olsson, Jonas
    et al.
    SMHI, Research Department, Hydrology.
    Pers, Charlotta
    SMHI, Research Department, Hydrology.
    Bengtsson, Lisa
    SMHI, Research Department, Meteorology.
    Pechlivanidis, Ilias
    SMHI, Research Department, Hydrology.
    Berg, Peter
    Körnich, Heiner
    SMHI, Research Department, Meteorology.
    Distance-dependent depth-duration analysis in high-resolution hydro-meteorological ensemble forecasting: A case study in Malmo City, Sweden2017In: Environmental Modelling & Software, ISSN 1364-8152, E-ISSN 1873-6726, Vol. 93, p. 381-397Article in journal (Refereed)
  • 69.
    Olsson, Jonas
    et al.
    SMHI, Research Department, Hydrology.
    Simonsson, Lennart
    SMHI, Research Department, Hydrology.
    Ridal, Martin
    SMHI, Research Department, Meteorology.
    Rainfall nowcasting: predictability of short-term extremes in Sweden2015In: Urban Water Journal, ISSN 1573-062X, Vol. 12, no 1, p. 3-13Article in journal (Refereed)
    Abstract [en]

    Our current knowledge of the character of rainfall events in Sweden associated with extreme short-term accumulations and their predictability by forecasting, is very limited. In this study, observations from automatic stations and weather radars in Sweden were analysed to identify and characterise extreme short-term events. Often shorter-duration (1-6 h) extreme events were associated with small-scale structures, dominated by single cells, and longer-duration (12-24 h) events with less variable, larger-scale fields. For lead time 3 h, similar to 20% of the events were forecasted at the correct place with an error of <25% by the operational Swedish nowcasting system. If allowing for a 25 km displacement of the forecasted events, the hit rate increased by 10-15 percentage points. Some predictability was found for lead time 8 h but not for 24 h. The results suggest a potential added gain of increasing the temporal resolution of the Swedish flood forecasting system to sub-daily steps.

  • 70.
    Olsson, Jonas
    et al.
    SMHI, Research Department, Hydrology.
    Simonsson, Lennart
    SMHI, Research Department, Hydrology.
    Ridal, Martin
    SMHI, Research Department, Meteorology.
    Rainfall nowcasting: predictability of short-term extremes in Sweden2014In: Urban Water Journal, ISSN 1573-062X, Vol. 11, no 7, p. 605-615Article in journal (Refereed)
    Abstract [en]

    Our current knowledge of the character of rainfall events in Sweden associated with extreme short-term accumulations and their predictability by forecasting, is very limited. In this study, observations from automatic stations and weather radars in Sweden were analysed to identify and characterise extreme short-term events. Often shorter-duration (1-6 h) extreme events were associated with small-scale structures, dominated by single cells, and longer-duration (12-24 h) events with less variable, larger-scale fields. For lead time 3 h,,20% of the events were forecasted at the correct place with an error of <25% by the operational Swedish nowcasting system. If allowing for a 25 km displacement of the forecasted events, the hit rate increased by 10-15 percentage points. Some predictability was found for lead time 8 h but not for 24 h. The results suggest a potential added gain of increasing the temporal resolution of the Swedish flood forecasting system to sub-daily steps.

  • 71. Pappenberger, F.
    et al.
    Cloke, H. L.
    Persson, Anders
    SMHI, Research Department, Meteorology.
    Demeritt, D.
    HESS Opinions "On forecast (in)consistency in a hydro-meteorological chain: curse or blessing?"2011In: Hydrology and Earth System Sciences, ISSN 1027-5606, E-ISSN 1607-7938, Vol. 15, no 7, p. 2391-2400Article in journal (Refereed)
    Abstract [en]

    Flood forecasting increasingly relies on numerical weather prediction forecasts to achieve longer lead times. One of the key difficulties that is emerging in constructing a decision framework for these flood forecasts is what to dowhen consecutive forecasts are so different that they lead to different conclusions regarding the issuing of warnings or triggering other action. In this opinion paper we explore some of the issues surrounding such forecast inconsistency (also known as "Jumpiness", "Turning points", "Continuity" or number of "Swings"). In thsi opinion paper we define forecast inconsistency; discuss the reasons why forecasts might be inconsistent; how we should analyse inconsistency; and what we should do about it; how we should communicate it and whether it is a totally undesirable property. The property of consistency is increasingly emerging as a hot topic in many forecasting environments.

  • 72.
    Persson, Anders
    SMHI, Research Department, Meteorology.
    Early operational Numerical Weather Prediction outside the USA: an historical introduction - Part III: Endurance and mathematics - British NWP, 1948-19652005In: Meteorological Applications, ISSN 1350-4827, E-ISSN 1469-8080, Vol. 12, no 4, p. 381-413Article, review/survey (Refereed)
  • 73.
    Persson, Anders
    SMHI, Research Department, Meteorology.
    Early operational numerical weather prediction outside the USA: an historical introduction. Part I: Internationalism and engineering NWP in sweden, 1952-692005In: Meteorological Applications, ISSN 1350-4827, E-ISSN 1469-8080, Vol. 12, no 2, p. 135-159Article, review/survey (Refereed)
  • 74.
    Persson, Anders
    SMHI, Research Department, Meteorology.
    Early operational Numerical Weather Prediction outside the USA: an historical introduction: Part II: Twenty countries around the world2005In: Meteorological Applications, ISSN 1350-4827, E-ISSN 1469-8080, Vol. 12, no 3, p. 269-289Article in journal (Other academic)
  • 75. Raschke, E
    et al.
    Meywerk, J
    Warrach, K
    Andrae, Ulf
    SMHI, Research Department, Meteorology.
    Bergström, Sten
    SMHI, Research Department, Hydrology.
    Beyrich, F
    Bosveld, F
    Bumke, K
    Fortelius, C
    Graham, Phil
    SMHI, Research Department, Climate research - Rossby Centre.
    Gryning, S E
    Halldin, S
    Hasse, L
    Heikinheimo, M
    Isemer, H J
    Jacob, D
    SMHI.
    Jauja, I
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Keevallik, S
    Koistinen, J
    van Lammeren, A
    Lass, U
    Launianen, J
    Lehmann, A
    Liljebladh, B
    Lobmeyr, M
    Matthaus, W
    Mengelkamp, T
    Michelson, Daniel
    SMHI, Core Services.
    Napiorkowski, J
    Omstedt, Anders
    SMHI, Research Department, Oceanography.
    Piechura, J
    Rockel, B
    Rubel, F
    Ruprecht, E
    Smedman, A S
    Stigebrandt, A
    The Baltic Sea Experiment (BALTEX): A European contribution to the investigation of the energy and water cycle over a large drainage basin2001In: Bulletin of The American Meteorological Society - (BAMS), ISSN 0003-0007, E-ISSN 1520-0477, Vol. 82, no 11, p. 2389-2413Article, review/survey (Refereed)
    Abstract [en]

    The Baltic Sea Experiment (BALTEX) is one of the five continental-scale experiments of the Global Energy and Water Cycle Experiment (GEWEX). More than 50 research groups from 14 European countries are participating in this project to measure and model the energy and water cycle over the large drainage basin of the Baltic Sea in northern Europe. BALTEX aims to provide a better understanding of the processes of the climate system and to improve and to validate the water cycle in regional numerical models for weather forecasting and climate studies. A major effort is undertaken to couple interactively the atmosphere with the vegetated continental surfaces and the Baltic Sea including its sea ice. The intensive observational and modeling phase BRIDGE, which is a contribution to the Coordinated Enhanced Observing Period of GEWEX, will provide enhanced datasets for the period October 1999-February 2002 to validate numerical models and satellite products. Major achievements have been obtained in an improved understanding of related exchange processes. For the first time an interactive atmosphere-ocean-land surface model for the Baltic Sea was tested. This paper reports on major activities and some results.

  • 76.
    Ridal, Martin
    SMHI, Research Department, Meteorology.
    Isotopic ratios of water vapor and methane in the stratosphere: Comparison between ATMOS measurements and a one-dimensional model2002In: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, ISSN 0747-7309, Vol. 107, no D16, article id 4285Article in journal (Refereed)
    Abstract [en]

    [1] A one-dimensional model simulating the transport and chemistry of methane and water vapor including their isotopic ratios in the tropical stratosphere is compared to measurements by the Atmospheric Trace Molecule Spectroscopy experiment (ATMOS) instrument. The model and measurements show good agreement in the isotopic ratio profiles. The deltaD depletion for water vapor is -600parts per thousand to -500parts per thousand at the tropopause with a small increase up to similar to10 hPa. Above this altitude the modeled isotopic ratio shows a strong increase due to methane oxidation. The measured profiles are rather noisy above 10 hPa but give an indication of a stronger increase in the isotopic ratio than modeled. If the isotopic ratio of water vapor is allowed to vary at the tropopause simulating an annual cycle in the input values, a wave pattern that is transported upwards arises on the vertical profile. This is a similar effect as the "tape recorder'' for water vapor. A wave pattern can also be detected in the tropical deltaD profiles from ATMOS. The methane isotopic ratio shows behavior similar to that of water vapor but without the wave pattern. The increase in methane deltaD above 10 hPa is very strong. The measured profiles are again rather noisy above this altitude, but measurements from inside the polar vortex show that the methane isotopic ratio in the upper stratosphere is very high. The deltaD values are in the range of +300parts per thousand to +500parts per thousand at altitudes as low as 40 hPa (similar to25 km) in the polar vortex.

  • 77.
    Ridal, Martin
    et al.
    SMHI, Research Department, Meteorology.
    Dahlbom, Mats
    Assimilation of Multinational Radar Reflectivity Data in a Mesoscale Model: A Proof of Concept2017In: Journal of Applied Meteorology and Climatology, ISSN 1558-8424, E-ISSN 1558-8432, Vol. 56, no 6, p. 1739-1751Article in journal (Refereed)
  • 78.
    Ridal, Martin
    et al.
    SMHI, Research Department, Meteorology.
    Lindskog, Magnus
    SMHI, Research Department, Meteorology.
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    Haase, Günther
    SMHI, Research Department, Atmospheric remote sensing.
    Optimized advection of radar reflectivities2011In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 100, no 2-3, p. 213-225Article in journal (Refereed)
    Abstract [en]

    A nowcasting system for generation of short-range precipitation forecasts has been developed at the Swedish Meteorological and Hydrological Institute (SMHI). The methodology consists of utilising a time-series of radar reflectivity composites for deriving an advection field, which will give a better representation of the motion of the precipitation pattern compared to a model wind field. The advection field is derived applying a 4-dimensional variational data assimilation technique. The resulting field is then used for a semi-Lagrangian advection of the latest available reflectivity field forward in time. During the forecast, the advected field is gradually replaced by a numerical weather prediction forecast in order to include the onset of convection and advection into the radar coverage area. In an idealised example with simulated observations the functionality of the method is demonstrated. For a case study of a full scale example the resulting precipitation forecast shows large improvements compared to the operational numerical weather prediction model used at SMHI, especially for forecasts up to three hours, where the largest influence from the radar advection occurs. In an objective validation of the structure, amplitude and location of modelled precipitation, where the forecasts are compared to radar observations, these findings are confirmed. The same validation of model runs over a longer time period also clearly indicates that the amplitude, structure and location of the precipitation patterns are significantly improved as compared to a short-range forecast from the operational forecast model used at SMHI. (C) 2010 Elsevier B.V. All rights reserved.

  • 79.
    Ridal, Martin
    et al.
    SMHI, Research Department, Meteorology.
    Murtagh, D P
    Merino, F
    Pardo, J R
    Pagani, L
    Microwave temperature and pressure measurements with the Odin satellite: II. Retrieval method2002In: Canadian journal of physics (Print), ISSN 0008-4204, E-ISSN 1208-6045, Vol. 80, no 4, p. 455-467Article in journal (Refereed)
    Abstract [en]

    The millimetre receiver on the Swedish satellite Odin, will be used for detection of the 118.750 GHz oxygen line. The temperature and pressure will be determined from the output of a three-channel filter bank measurement. One frequency bin is centred over the emission-line frequency while the other two cover parts of the line wing, where the opacity is less, providing a useful signal at lower altitudes. The bandwidth of each channel is 40 MHz. The signal in the frequency bin covering the line centre is modeled by a high-resolution model including the Zeeman effect, developed by the Observatoire de Paris-Meudon. The other two 40 MHz bins are modeled using the much faster standard Odin forward model, developed at the Department of Meteorology at Stockholm University together with Chalmers University of Technology. The operational retrievals employ an iterative method that uses simulated signals from a reference atmosphere as a lookup table for the pressure. The temperature is then calculated from the equation of hydrostatic equilibrium, and a new lookup table computed. This process is repeated until a convergence criterion is reached. Simulations, including known error sources, show that the temperature can be retrieved with a root mean square (rms) around 3 K, in the altitude range similar to25-90 km using the operational temperature retrieval method (the filter bank method). A sub-millimetre receiver on board Odin will also be used to observe the oxygen line at 487.249 GHz. Both this line and the 118.750 GHz line can be observed in high resolution (150 kHz) for detailed studies of the Zeeman splitting. Retrievals from the high-resolution measurements are expected to give a precision of +/-2 K rms at that resolution. However, this kind of observation will occupy an entire spectrometer and will not be made on a regular basis.

  • 80. Rontu, L.
    et al.
    Obleitner, F.
    Gollvik, Stefan
    SMHI, Research Department, Meteorology.
    Zingerle, C.
    Tijm, S.
    HIRLAM experiments on surface energy balance across Vatnajokull, Iceland2009In: Meteorology and atmospheric physics (Print), ISSN 0177-7971, E-ISSN 1436-5065, Vol. 103, no 1-4, p. 67-77Article in journal (Refereed)
    Abstract [en]

    The aim of this study is to investigate the skill of the High Resolution Limited Area Model (HIRLAM) to reproduce the near-surface atmospheric conditions across the Vatnajokull Ice Cap in Iceland. This model-observation comparison study is based on a mesoscale glaciometeorological observation campaign, which has been performed during summer 1996 and provided a wealth of meteorological and glaciological data. Fine-scale hydrostatic HIRLAM experiments are based on downscaling ERA-40 analyses and the application of upper-air and surface data assimilation. The simulation results are compared to a subset of observations following a height transect across Breidamerkurjokull, a southern outlet glacier of Vatnajokull. After introduction of improvements, suggested by comparison of a reference run with observations, HIRLAM successfully simulates the surface energy balance and the driving meteorological parameters. For a correct simulation, a proper description of the constant and temporary varying physical properties of the underlying surface turned out to be crucial. The results are valuable for further improvement of operational mesoscale NWP in mountainous and high latitude environments.

  • 81. Rydblom, Staffan
    et al.
    Thornberg, Benny
    Olsson, Esbjörn
    SMHI, Research Department, Meteorology.
    Field Study of LWC and MVD Using the Droplet Imaging Instrument2019In: IEEE Transactions on Instrumentation and Measurement, ISSN 0018-9456, E-ISSN 1557-9662, Vol. 68, no 2, p. 614-622Article in journal (Refereed)
  • 82.
    Samuelsson, Patrick
    et al.
    SMHI, Research Department, Climate research - Rossby Centre.
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Willén, Ulrika
    SMHI, Research Department, Climate research - Rossby Centre.
    Ullerstig, Anders
    SMHI, Research Department, Climate research - Rossby Centre.
    Gollvik, Stefan
    SMHI, Research Department, Meteorology.
    Hansson, Ulf
    SMHI, Research Department, Climate research - Rossby Centre.
    Jansson, Christer
    SMHI, Research Department, Climate research - Rossby Centre.
    Kjellström, Erik
    SMHI, Research Department, Climate research - Rossby Centre.
    Nikulin, Grigory
    SMHI, Research Department, Climate research - Rossby Centre.
    Wyser, Klaus
    SMHI, Research Department, Climate research - Rossby Centre.
    The Rossby Centre Regional Climate model RCA3: model description and performance2011In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, no 1, p. 4-23Article in journal (Refereed)
  • 83.
    Samuelsson, Patrick
    et al.
    SMHI, Research Department, Climate research - Rossby Centre.
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Willén, Ulrika
    SMHI, Research Department, Climate research - Rossby Centre.
    Ullerstig, Anders
    SMHI, Research Department, Climate research - Rossby Centre.
    Gollvik, Stefan
    SMHI, Research Department, Meteorology.
    Hansson, Ulf
    SMHI, Research Department, Climate research - Rossby Centre.
    Kjellström, Erik
    SMHI, Research Department, Climate research - Rossby Centre.
    Nikulin, Grigory
    SMHI, Research Department, Climate research - Rossby Centre.
    Wyser, Klaus
    SMHI, Research Department, Climate research - Rossby Centre.
    The Rossby Centre Regional Climate Model RCA3: Model description and performance2011In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 63A, no 1, p. 4-23Article in journal (Refereed)
  • 84. Sanchez Arriola, Jana
    et al.
    Lindskog, Magnus
    SMHI, Research Department, Meteorology.
    Thorsteinsson, Sigurdur
    SMHI, Research Department, Meteorology.
    Bojarova, Jelena
    Variational Bias Correction of GNSS ZTD in the HARMONIE Modeling System2016In: Journal of Applied Meteorology and Climatology, ISSN 1558-8424, E-ISSN 1558-8432, Vol. 55, no 5, article id UNSP 1259Article in journal (Refereed)
  • 85. Schlutow, M.
    et al.
    Becker, E.
    Körnich, Heiner
    SMHI, Research Department, Meteorology.
    Positive definite and mass conserving tracer transport in spectral GCMs2014In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 119, no 20, p. 11562-11577Article in journal (Refereed)
    Abstract [en]

    A new scheme that solves the advection-diffusion equation for tracers in a spectral General Circulation Model (GCM) is presented. The main ideas are (1) using a monotonic and smooth functional of the tracer as prognostic variable to ensure positive definite concentrations and continuity of all derivatives and (2) defining an adjustable tracer-mass correction as a multiplication of the tracer in grid space, giving rise to an efficient correction in spectral space. Common standard benchmark tests for two-dimensional horizontal advection using deformational wind fields show that the new scheme is accurate and essentially not diffusive. A three-dimensional test is proposed in order to validate vertical transport. Additionally to standard error norms and global tracer mass, the entropy of mixing is introduced as another conservation constraint and utilized to determine the strength of the mass correction which is a free parameter. The transport scheme is applied in a mechanistic spectral GCM from the surface to the lower thermosphere. It is extended such that the mass correction takes the diffusion and other nonconservative effects explicitly into account. By this method we estimate the mean age of air along with its dependence on the turbulent horizontal Schmidt number.

  • 86.
    Schöld, Sofie
    et al.
    SMHI, Core Services.
    Hellström, Sverker
    SMHI, Core Services.
    Ivarsson, Cajsa-Lisa
    SMHI, Professional Services.
    Kållberg, Per
    SMHI, Research Department, Meteorology.
    Lindow, Helma
    SMHI, Core Services.
    Nerheim, Signild
    SMHI, Professional Services.
    Schimanke, Semjon
    SMHI, Research Department, Oceanography.
    Södling, Johan
    SMHI, Professional Services.
    Wern, Lennart
    SMHI, Core Services.
    Vattenståndsdynamik längs Sveriges kust2018Report (Other academic)
    Abstract [sv]

    För att skapa ett samhälle väl anpassat till dagens och framtidens havsnivåer behövs besluts- och planeringsunderlag. Skyddsåtgärder och designnivåer för kustskydd är högaktuella frågor och många aktörer är intresserade av information kring potentiella maxnivåer för vattenstånd på olika tidshorisonter. SMHI har därför analyserat de mätdataserier för havsvattenstånd som idag finns tillgängliga från stationer längs Sveriges kust. Det primära syftet var att ta fram en metod för att beräkna det högsta möjliga havsvattenståndet vid mätstationer längs Sveriges kust. Metoden beskrivs i Schöld m.fl.(2017).

    I föreliggande rapport beskrivs allmänt havsnivåer, mätdata, modeller och de resultat som erhölls från olika analyser av mätdata. Mätstationerna indelades i åtta olika kustområden inom vilka vattenståndet samvarierar. Det väder och de specifika stormbanor, som under de senaste 40 åren orsakat de högsta stormfloderna på olika platser längs den svenska kusten kartlades, och vattenståndsdynamiken vid olika mätstationer studerades.

    Kortvariga höjningar av vattenståndet undersöktes, både med avseende på kraftiga vattenståndshöjningar orsakade av passerande väderssystem och med avseende på förhöjda utgångslägen, som i sin tur kan bidra till att stormfloder blir extra höga.

    Det högsta beräknade havsvattenstånd som presenteras är de högsta möjliga stormfloder som skulle kunna inträffa baserat på empiriska analyser av mätdata vid de olika stationerna. Kända extrema händelser, som ägt rum före det att vattenståndet började registreras, ingår inte eftersom de inte har kunnat kvantifieras. Framtida förändringar av medelvattenståndet orsakade av den globala klimatförändringen behandlas inte i denna rapport.

    Resultaten från studien visar att vattennivåerna i Östersjön generellt blir som högst i Bottenviken och i de södra delarna. De höga vattenstånden i större delen av Östersjön är inte lika höga som på västkusten och i Öresund. I Östersjön förefaller också utgångsläget, havsnivån före stormen, utgöra en större del av den resulterande vattenståndshöjningen. Vid flera stationer i de centrala delarna av Östersjön är havsnivån före storm i stort sett hälften av det högsta beräknade havsvattenståndet. Längs västkusten är istället de nettohöjningar som orsakas av rena stormeffekter den viktigaste stormflodskomponenten. Lokala förhållanden, till exempel om stationen är belägen vid en öppen, rak kust eller inne i en vik, påverkar hur högt vattenståndet kan förväntas bli på en viss plats.

    Analyserna visar att stormfloder skulle kunna bli omkring 20-40 cm högre än hittills observerade maximala nivåer i olika kustområden. En osäkerhetsmarginal på runt +15 cm är lämplig att addera, särskilt i de områden där tidvatten förekommer.

  • 87. Stengel, M.
    et al.
    Lindskog, Magnus
    SMHI, Research Department, Meteorology.
    Unden, Per
    SMHI, Research Department, Meteorology.
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    The impact of cloud-affected IR radiances on forecast accuracy of a limited-area NWP model2013In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 139, no 677, p. 2081-2096Article in journal (Refereed)
    Abstract [en]

    The impact of cloud-affected satellite radiances on numerical weather prediction (NWP) accuracy is investigated. The NWP model used is the HIgh Resolution Limited Area Model (HIRLAM). Its four-dimensional variational data assimilation (4D-Var) system was used to assimilate cloud-affected infrared (IR) radiances from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI). Cloud parameters are modelled internally in the observation operator and used in the radiative transfer calculations. The interaction between the cloud parameters and the model control vector variables is incorporated in the adjoint version of the observation operator, which is used to derive cloud-affected Jacobians prior to the inner-loop minimization of the cost function. The developed framework supports an extensive usage of satellite observations with spatial coverage extended into cloudy regions, which therefore provides additional analysis increments and supports a more accurate description of the atmospheric state. In extended assimilation and forecast experiments the total number of assimilated satellite observations could be increased by approximately 10%. This was associated with a clear indication of a positive impact of cloud-affected radiances on the moisture and geopotential height fields of the NWP model analysis and forecast accuracy when used on top of clear-sky radiance observations. This is revealed by reduced analysis errors of the total integrated water vapour and by reduced forecast errors in the mid and upper troposphere.

  • 88. Stengel, M.
    et al.
    Lindskog, Magnus
    SMHI, Research Department, Meteorology.
    Unden, Per
    SMHI, Research Department, Meteorology.
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    Bennartz, R.
    An extended observation operator in HIRLAM 4D-VAR for the assimilation of cloud-affected satellite radiances2010In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 136, no 649, p. 1064-1074Article in journal (Refereed)
    Abstract [en]

    An extended observation operator for the direct assimilation of cloud-affected infrared satellite radiances in the High Resolution Limited Area Model (HIRLAM) is examined. The operator includes a simplified moist-physics scheme, which enables the diagnosis of cloudiness in itself using background values of temperature, moisture and surface pressure. Subsequently, a radiative transfer model provides simulated cloud-affected radiances to be used as background equivalents to the satellite observations. The observation operator was evaluated by using infrared observations measured by the Spinning Enhanced Visible and Infrared Imager (SEVIRI). An observation-screening procedure, which incorporates SEVIRI cloud-retrieval products, supports an improved selection of usable cloudy scenes, leading to good agreement between the observations and background equivalents. The tangent-linear observation operator was verified against finite differences from its nonlinear formulation. The increments revealed a near-linear behaviour for the selected channels for a large number of cases. The adjoint observation operator was used to derive brightness-temperature sensitivities with respect to temperature and moisture changes in the presence of radiance-affecting clouds. Differences from the clear-sky sensitivities were found in and below clouds. In a four-dimensional variational data assimilation experiment, cloud-affected SEVIRI observations were assimilated, resulting in additional increments in both moisture and wind fields. The corresponding analysis fields revealed a reduced deviation from the observations for the majority of all cloudy scenes and a reduced bias for wind and temperature in the upper troposphere against independent radiosonde observations. Overall, our results highlight the capability of this observation operator in the HIRLAM assimilation system and encourage its application for the extended usage of cloudy satellite observations in numerical weather prediction. Copyright (C) 2010 Royal Meteorological Society

  • 89. Stengel, M.
    et al.
    Undén, Per
    SMHI, Research Department, Meteorology.
    Lindskog, Magnus
    SMHI, Research Department, Meteorology.
    Dahlgren, Per
    SMHI, Research Department, Meteorology.
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    Bennartz, R.
    Assimilation of SEVIRI infrared radiances with HIRLAM 4D-Var2009In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 135, no 645, p. 2100-2109Article in journal (Refereed)
    Abstract [en]

    Four-dimensional variational data assimilation (4D-Var) systems are ideally suited to obtain the best possible initial model state by utilizing information about the dynamical evolution of the. atmospheric state from observations, such as satellite measurements, distributed over a certain period of time. In recent years, 4D-Var systems have been developed for several global and limited-area models. At the same time, spatially and temporally highly resolved satellite observations, as for example performed by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) on board the Meteosat Second Generation satellites, have become available. Here we demonstrate the benefit of a regional NWP model's analyses and forecasts gained by the assimilation of those radiances. The 4D-Var system of the High Resolution Limited Area Model (HIRLAM) has been adjusted to utilize three of SEVIRI's infrared channels (located around 6.2 mu m, 7.3 mu m, and 13.4 mu m, respectively) under clear-sky and low-level cloud conditions. Extended assimilation and forecast experiments show that the main direct impact of assimilated SEVIRI radiances on the atmospheric analysis were additional tropospheric humidity and wind increments. Forecast verification reveals a positive impact for almost all upper-air variables throughout the troposphere. Largest improvements are found for humidity and geopotential height in the middle troposphere. The observations in regions of low-level clouds provide especially beneficial information to the NWP system, which highlights the importance of satellite observations in cloudy areas for further improvements in the accuracy of weather forecasts. Copyright (C) 2009 Royal Meteorological Society

  • 90. Sukoriansky, S
    et al.
    Galperin, B
    Perov, Veniamin
    SMHI, Research Department, Meteorology.
    A quasi-normal scale elimination model of turbulence and its application to stably stratified flows2006In: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 13, no 1, p. 9-22Article in journal (Refereed)
    Abstract [en]

    Models of planetary, atmospheric and oceanic circulation involve eddy viscosity and eddy diffusivity, Km and K-H, that account for unresolved turbulent mixing and diffusion. The most sophisticated turbulent closure models used today for geophysical applications belong in the family of the Reynolds stress models. These models are formulated for the physical space variables; they consider a hierarchy of turbulent correlations and employ a rational way of its truncation. In the process, unknown correlations are related to the known ones via "closure assumptions" that are based upon physical plausibility, preservation of tensorial properties, and the principle of the invariant modeling according to which the constants in the closure relationships are universal. Although a great deal of progress has been achieved with Reynolds stress closure models over the years, there are still situations in which these models fail. The most difficult flows for the Reynolds stress modeling are those with anisotropy and waves because these processes are scale-dependent and cannot be included in the closure assumptions that pertain to ensemble-averaged quantities. Here, we develop an alternative approach of deriving expressions for KM and KH using the spectral space representation and employing a self-consistent, quasi-normal scale elimination QNSE) algorithm. More specifically, the QNSE procedure is based upon the quasi-Gaussian mapping of the velocity and temperature fields using the Langevin equations. Turbulence and waves are treated as one entity and the effect of the internal waves is easily identifiable. This model implies partial averaging and, thus, is scale-dependent; it allows one to easily introduce into consideration such parameters as the grid resolution, the degree of the anisotropy, and spectral characteristics, among others. Applied to turbulent flows affected by anisotropy and waves, the method traces turbulence anisotropization and shows how the dispersion relationships for linear waves are modified by turbulence. In addition, one can derive the internal wave frequency shift and the threshold criterion of internal wave generation in the presence of turbulence. The spectral method enables one to derive analytically various one-dimensional and three-dimensional spectra that reflect the effects of waves and anisotropy. When averaging is extended to all scales, the method yields a Reynolds-averaged, Navier-Stokes equations based model (RANS). This RANS model shows that there exists a range of Ri, approximately between 0.1 and 1, in which turbulence undergoes remarkable anisotropization; the vertical mixing becomes suppressed while the horizontal mixing is enhanced. Although KH decreases at large Ri and tends to its molecular value, KM remains finite and larger than its molecular value. This behavior is attributable to the effect of internal waves that mix the momentum but do not mix a scalar. In the Reynolds stress models, this feature is not replicated; instead, all Reynolds stress models predict K-M -> 0 at some value of Ri <= 1 which varies from one model to another. The presented spectral model indicates that there is no a single-valued critical Richardson number Ri at which turbulence is fully suppressed by stable stratification. This result is in agreement with large volume of atmospheric, oceanic and laboratory data. The new spectral model has been implemented in the K-epsilon format and tested in simulations of the stably stratified atmospheric boundary layers. The results of these simulations are in good greement with the data collected in BASE, SHEBA and CASES99 campaigns. Implementation of the QNSE-derived K-M and K-H in the high-resolution weather prediction system HIRLAM results in significant improvement of its predictive skills.

  • 91. Sukoriansky, S
    et al.
    Galperin, B
    Perov, Veniamin
    SMHI, Research Department, Meteorology.
    Application of a new spectral theory of stably stratified turbulence to the atmospheric boundary layer over sea ice2005In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 117, no 2, p. 231-257Article in journal (Refereed)
    Abstract [en]

    A new spectral closure model of stably stratified turbulence is used to develop a K - epsilon model suitable for applications to the atmospheric boundary layer. This K - epsilon model utilizes vertical viscosity and diffusivity obtained from the spectral theory. In the epsilon equation, the Coriolis parameter-dependent formulation of the coefficient C-1 suggested by Detering and Etling is generalized to include the dependence on the Brunt-Vaisala frequency, N. The new K - epsilon model is tested in simulations of the ABL over sea ice and compared with observations from BASE as simulated in large-eddy simulations by Kosovic and Curry, and observations from SHEBA.

  • 92. Sundqvist, H. S.
    et al.
    Holmgren, K.
    Fohlmeister, J.
    Zhang, Q.
    Bar Matthews, M.
    Spoetl, C.
    Körnich, Heiner
    SMHI, Research Department, Meteorology.
    Evidence of a large cooling between 1690 and 1740 AD in southern Africa2013In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, article id 1767Article in journal (Refereed)
    Abstract [en]

    A 350-year-long, well-dated delta O-18 stalagmite record from the summer rainfall region in South Africa is positively correlated with regional air surface temperatures at interannual time scales. The coldest period documented in this record occurred between 1690 and 1740, slightly lagging the Maunder Minimum (1645-1710). A temperature reconstruction, based on the correlation between regional surface temperatures and the stalagmite delta O-18 variations, indicates that parts of this period could have been as much as 1.4 degrees C colder than today. Significant cycles of 22, 11 and 4.8 years demonstrate that the solar magnetic and the El Nino-Southern Oscillation cycle could be important drivers of multidecadal to interannual climate variability in this region. The observation that the most important driver of stalagmite delta O-18 on interannual time scales from this subtropical region is regional surface temperature cautions against deterministic interpretations of delta O-18 variations in low-latitude stalagmites as mainly driven by the amount of precipitation.

  • 93. Sundstrom, Nils
    et al.
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    Kruglyak, Andrey
    Lundberg, Angela
    Field evaluation of a new method for estimation of liquid water content and snow water equivalent of wet snowpacks with GPR2013In: HYDROLOGY RESEARCH, ISSN 1998-9563, Vol. 44, no 4, p. 600-613Article in journal (Refereed)
    Abstract [en]

    Estimates of snow water equivalent (SWE) with ground-penetrating radar can be used to calibrate and validate measurements of SWE over large areas conducted from satellites and aircrafts. However, such radar estimates typically suffer from low accuracy in wet snowpacks due to a built-in assumption of dry snow. To remedy the problem, we suggest determining liquid water content from path-dependent attenuation. We present the results of a field evaluation of this method which demonstrate that, in a wet snowpack between 0.9 and 3 m deep and with about 5 vol% of liquid water, liquid water content is underestimated by about 50% (on average). Nevertheless, the method decreases the mean error in SWE estimates to 16% compared to 34% when the presence of liquid water in snow is ignored and 31% when SWE is determined directly from two-way travel time and calibrated for manually measured snow density.

  • 94. Swap, R
    et al.
    Garstang, M
    Macko, S A
    Tyson, P D
    Maenhaut, W
    Artaxo, P
    Kållberg, Per
    SMHI, Research Department, Meteorology.
    Talbot, R
    The long-range transport of southern African aerosols the tropical South Atlantic1996In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 101, no D19, p. 23777-23791Article in journal (Refereed)
    Abstract [en]

    Two episodes of long-range aerosol transport (4000 km) from southern Africa into the central tropical South Atlantic are documented. Stable nitrogen isotope analysis, multielemental analysis, and meteorological observations on local and regional scales are used to describe the observed surface aerosol chemistry during these transport episodes. The chemical, kinematic, and thermodynamic analyses suggest that for the central tropical South Atlantic, west Africa between 0 degrees and 10 degrees S is the primary air mass source region (over 50%) during austral spring. Over 70% of all air arriving in the lower and middle troposphere in the central tropical South Atlantic comes from a broad latitudinal band extending from 20 degrees S to 10 degrees N. Air coming from the east subsides and is trapped below the midlevel and trade wind inversion layers. Air from the west originates at higher levels (500 hPa) and contributes less than 30% of the air masses arriving in the central tropical South Atlantic. The source types of aerosols and precursor trace gases extend over a broad range of biomes from desert and savanna to the rain forest. During austral spring, over this broad region, processes include production from vegetation, soils, and biomass burning. The aerosol composition of air masses over and the atmospheric chemistry of the central South Atlantic is a function of the supply of biogenic, biomass burning, and aeolian emissions from tropical Africa. Rainfall is a common controlling factor for all three sources. Rain, in turn, is governed by the large-scale circulations which show pronounced interannual variability. The field measurements were taken in an extremely dry year and reflect the circulation and transport fields typical of these conditions.

  • 95. Tyson, P D
    et al.
    Garstang, M
    Swap, R
    Kållberg, Per
    SMHI, Research Department, Meteorology.
    Edwards, M
    An air transport climatology for subtropical southern Africa1996In: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 16, no 3, p. 265-291Article in journal (Refereed)
    Abstract [en]

    An air transport climatology is derived for subtropical southern Africa (Africa south of 15 degrees S) by classifying daily synoptic situations into predominant circulation types. The annual variation of these provides the basis for determining month-by-month transport. Percentage zonal transport in easterly and westerly directions, levels of transport, and times of transit are derived from forward trajectory analyses using European Centre for Medium-range Weather Forecasts (ECMWF) data for a 7-year period. It is shown that semi-permanent subtropical continental anticyclones, transient mid-latitude ridging anticyclones and midlatitude westerly disturbances produce major transport into the south-western Indian Ocean in the Natal plume. Only quasistationary tropical easterly waves result in appreciable transport into the tropical South Atlantic Ocean in the Angolan plume. Total transport is a function of circulation type and frequency, as well as plume dimensions. Transport in continental highs follows an annual cycle reaching peak values in excess of 70 per cent in winter. That in easterly waves also exhibits an annual cycle, but one peaking in summer, when up to 55 per cent transport may occur in north-western regions. Transport in ridging highs and westerly perturbations is much less and occurs throughout the year, with a slight tendency to peak in spring. Recirculation of air is shown to be considerable when anticyclonic conditions prevail. Monthly, seasonal, and annual mass fluxes over and out of southern Africa are determined from transport fields, frequency of occurrence of circulation types and from measurements of aerosol concentrations. An annual mass flux of aerosols some 134 Mtons is generated over the subcontinent. About 60 Mtons year(-1) are deposited, and approximately 29 Mtons year(-1) are exported westward over the Atlantic Ocean and 45 Mtons year(-1) eastward over the Indian Ocean. Twenty-six million tons of the 74 Mtons of aerosols exported annually to the adjacent oceans on each coast are a product of recirculation. Deposition within 10 degrees latitude of the coast is nearly 10 times greater on the east than on the west coast.

  • 96. Van den Dool, H. M.
    et al.
    Peng, Peitao
    Johansson, Åke
    SMHI, Research Department, Meteorology.
    Chelliah, Muthuvel
    Shabbar, Amir
    Saha, Suranjana
    Seasonal-to-decadal predictability and prediction of North American climate - The Atlantic influence2006In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 19, no 23, p. 6005-6024Article in journal (Refereed)
    Abstract [en]

    The question of the impact of the Atlantic on North American (NA) seasonal prediction skill and predictability is examined. Basic material is collected from the literature, a review of seasonal forecast procedures in Canada and the United States, and some fresh calculations using the NCEP-NCAR reanalysis data. The general impression is one of low predictability (due to the Atlantic) for seasonal mean surface temperature and precipitation over NA. Predictability may be slightly better in the Caribbean and the (sub) tropical Americas, even for precipitation. The NAO is widely seen as an agent making the Atlantic influence felt in NA. While the NAO is well established in most months, its prediction skill is limited. Year-round evidence for an equatorially displaced version of the NAO (named ED_NAO) carrying a good fraction of the variance is also found. In general the predictability from the Pacific is thought to dominate over that from the Atlantic sector, which explains the minimal number of reported Atmospheric Model Intercomparison Project (AMIP) runs that explore Atlantic-only impacts. Caveats are noted as to the question of the influence of a single predictor in a nonlinear environment with many predictors. Skill of a new one-tier global coupled atmosphere-ocean model system at NCEP is reviewed; limited skill is found in midlatitudes and there is modest predictability to look forward to. There are several signs of enthusiasm in the community about using "trends" (low-frequency variations): (a) seasonal forecast tools include persistence of last 10 years' averaged anomaly (relative to the official 30-yr climatology), (b) hurricane forecasts are based largely on recognizing a global multidecadal mode (which is similar to an Atlantic trend mode in SST), and (c) two recent papers, one empirical and one modeling, giving equal roles to the (North) Pacific and Atlantic in "explaining" variations in drought frequency over NA on a 20 yr or longer time scale during the twentieth century.

  • 97. van den Dool, H M
    et al.
    Saha, S
    Johansson, Åke
    SMHI, Research Department, Meteorology.
    Empirical orthogonal teleconnections2000In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 13, no 8, p. 1421-1435Article in journal (Refereed)
    Abstract [en]

    A new variant is proposed for calculating functions empirically and orthogonally from a given space-time dataset. The method is rooted in multiple linear regression and yields solutions that are orthogonal in one direction, either space or time. In normal setup, one searches for that point in space, the base point (predictor). which, by linear regression, explains the most of the variance at all other points (predictands) combined. The first spatial pattern is the regression coefficient between the base point and all other points, and the first time series is taken to be the time series of the raw data at the base point. The original dataset is next reduced; that is, what has been accounted for by the first mode is subtracted out. The procedure is repeated exactly as before for the second, third, etc., modes. These new functions are named empirical orthogonal teleconnections (EOTs). This is to emphasize the similarity of EOT to both teleconnections and (biorthogonal) empirical orthogonal functions (EOFs). One has to choose the orthogonal direction for EOT. In the above description of the normal space-time setup, picking successive base points in space, the time series are orthogonal. One can reverse the role of time and space-in this case one picks base points in time, and the spatial maps will be orthogonal. If the dataset contains biorthogonal modes, the EOTs are the same for both setups and are equal to the EOFs. When applied to four commonly used datasets, the procedure was found to work well in terms of explained variance (EV) and in terms of extracting familiar patterns. In all examples the EV for EOTs was only slightly less than the optimum obtained by EOF. A numerical recipe was given to calculate EOF, starting from EOT as an initial guess. When subjected to cross validation the EOTs seem to fare well in terms of explained variance on independent data las good as EOF). The EOT procedure can be implemented very easily and has, for some (but not all) applications, advantages over EOFs. These novelties, advantages, and applications include the following. 1) One can pick certain modes (or base point) first-the order of the EOTs is free, and there is a near-infinite set of EOTs. 2) EOTs are linked to specific points in space or moments in time. 3) When linked to Row at specific moments in time, the EOT modes have undeniable physical reality. 4) When linked to flow at specific moments in time, EOTs appear to be building blocks for empirical forecast methods because one can naturally access the time derivative. 5) When linked to specific points in space, one has a rational basis to define strategically chosen points such that an analysis of the whole domain would benefit maximally from observations at these locations.

  • 98. van Meijgaard, E
    et al.
    Andrae, Ulf
    SMHI, Research Department, Meteorology.
    Rockel, B
    Comparison of model predicted cloud parameters and surface radiative fluxes with observations on the 100 km scale2001In: Meteorology and atmospheric physics (Print), ISSN 0177-7971, E-ISSN 1436-5065, Vol. 77, no 1-4, p. 109-130Article in journal (Refereed)
    Abstract [en]

    Cloud parameters and surface radiative fluxes predicted by regional atmospheric models are directly compared with observations for a 10-day period in late summer 1995 characterized by predominantly large-scale synoptic conditions. Observations of total cloud cover and Vertical cloud structure are inferred from measurements with a groundbased network of Lidar ceilometers and IR-radiometers and from satellite observations on a 100 kilometer scale. Groundbased observations show that at altitudes below 3 km, implying liquid water clouds, there is a considerable portion of optically non-opaque clouds. Vertical distributions of cloud temperatures simultaneously inferred from the groundbased infrared radiometer network and from satellite can only be reconciled if the occurrence of optically thin cloud structures at mid- and high tropospheric levels is assumed to be frequent. Results of three regional atmospheric models, i.e. the GKSS-REMO, SMHI-HIRLAM. and KNMI-RACMO, are quantitatively compared with the observations. The main finding is that all models predict too much cloud amount at low altitude below 900 hPa, which is then compensated by an underestimation of cloud amount around 800 hPa. This is likely to be related with the finding that all models tend to underestimate the planetary boundary layer height. All models overpredict the high-level cloud amount albeit it is difficult to quantify to what extent due to the frequent presence of optically thin clouds. Whereas reasonably alike in cloud parameters, the models differ considerably in radiative fluxes. One model links a well matching incoming solar radiation to a radiatively transparent atmosphere over a too cool surface, another model underpredicts incoming solar radiation at the surface due to a too strong cloud feedback to radiation, the last model represents all surface radiative fluxes quite well on average: but underestimates the sensitivity of atmospheric transmissivity to cloud amount.

  • 99. Wolters, L
    et al.
    Cats, G
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    Wilhelmsson, T
    Data-parallel numerical methods in a weather forecast model1995In: Applied Numerical Mathematics, ISSN 0168-9274, E-ISSN 1873-5460, Vol. 19, no 1-2, p. 159-171Article in journal (Refereed)
    Abstract [en]

    The results presented in this paper are part of a research project to investigate the possibilities to apply massively parallel architectures for numerical weather forecasting. Within numerical weather forecasting several numerical techniques are used to solve the model equations. This paper compares the performance of implementations on a MasPar system of two techniques, finite difference and spectral, that are adopted in the numerical weather forecasting model HIRLAM. The operational HIRLAM model is based on finite difference methods, while the spectral model is still in a research phase. Also the differences in relative performance of these methods on the MasPar and vector architectures will be discussed.

  • 100. Zaplotnik, Ziga
    et al.
    Zagar, Nedjeljka
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    An intermediate-complexity model for four-dimensional variational data assimilation including moist processes2018In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 144, no 715, p. 1772-1787Article in journal (Refereed)
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