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  • 1. Bech, J.
    et al.
    Gjertsen, U.
    Haase, Gunther
    SMHI, Research Department, Atmospheric remote sensing.
    Modelling weather radar beam propagation and topographical blockage at northern high latitudes2007In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no 626, p. 1191-1204Article in journal (Refereed)
    Abstract [en]

    This paper describes a study to evaluate the variability of radio-propagation conditions and to assess their effects upon weather-radar beam blockage corrections for precipitation estimates. Radiosonde observations are examined in order to analyse the propagation conditions at several locations covered by the Nordic Weather Radar Network (NORDRAD). A beam-propagation model is used to simulate the interaction between the radar beam and the topography and to derive correction factors. The model is applied to correct yearly accumulations, assuming standard radio-propagation conditions, and is also used to examine case studies in detail under various propagation conditions. The correction reduces the bias between yearly radar precipitation estimates and gauge records by 1 dB for moderate blockages (1% to 50%), and by up to 3 dB for severe blockages (50% to 70%). The case studies indicate that HIRLAM forecasts show potential to predict the radar coverage and the associated ground- and sea-clutter patterns. This research aims at determining a beam-blockage-correction algorithm to be used within the NORDRAD quality-control system. This is particularly useful for obtaining radar precipitation estimates in environments with complex topography. Copyright (C) 2007 Royal Meteorological Society.

  • 2.
    Bengtsson, Lisa
    et al.
    SMHI, Research Department, Meteorology.
    Körnich, Heiner
    SMHI, Research Department, Meteorology.
    Impact of a stochastic parametrization of cumulus convection, using cellular automata, in a mesoscale ensemble prediction system2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 695, p. 1150-1159Article in journal (Refereed)
    Abstract [en]

    A stochastic parametrization for deep convection, based on cellular automata, has been evaluated in the high-resolution (2.5 km) ensemble prediction system Hirlam Aladin Regional Mesoscale Operational NWP Ensemble Prediction System (HarmonEPS). We studied whether such a stochastic physical parametrization, whilst implemented in a deterministic forecast model, can have an impact on the performance of the uncertainty estimates given by an ensemble prediction system. Various feedback mechanisms in the parametrization were studied with respect to ensemble spread and skill, in both subgrid and resolved precipitation fields. It was found that the stochastic parametrization improves the model skill in general, by reducing a positive bias in precipitation. This reduction in bias, however, led to a reduction in ensemble spread of precipitation. Overall, scores that measure the accuracy and reliability of probabilistic predictions indicate that the net impact (improved skill, degraded spread) of the ensemble prediction system is improved for 6 h accumulated precipitation with the stochastic parametrization and is rather neutral for other quantities examined.

  • 3.
    Bengtsson, Lisa
    et al.
    SMHI, Research Department, Meteorology.
    Steinheimer, Martin
    Bechtold, Peter
    Geleyn, Jean-Francois
    A stochastic parametrization for deep convection using cellular automata2013In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 139, no 675, p. 1533-1543Article in journal (Refereed)
    Abstract [en]

    A cellular automaton (CA) is introduced to the deep convection parametrization of the high-resolution limited-area model Aire Limitee Adaptation/Application de la Recherche a l'Operationnel (ALARO). The self-organizational characteristics of the CA allow for lateral communication between adjacent numerical weather prediction (NWP) model grid boxes and add additional memory to the deep convection scheme. The CA acts in two horizontal dimensions, with finer grid spacing than the NWP model. It is randomly seeded in regions where convective available potential energy (CAPE) exceeds a threshold value. Both deterministic and probabilistic rules, coupled to the large-scale wind, are explored to evolve the CA in time. Case studies indicate that the scheme has the potential to organize cells along convective squall lines and enhance advective effects. An ensemble of forecasts using the present CA scheme demonstrated an ensemble spread in the resolved wind field in regions where deep convection is large. Such a spread represents the uncertainty due to subgrid variability of deep convection and could be an interesting addition to an ensemble prediction system.

  • 4. Couvreux, F.
    et al.
    Roehrig, R.
    Rio, C.
    Lefebvre, M. -P
    Caian, Mihaela
    SMHI, Research Department, Climate research - Rossby Centre.
    Komori, T.
    Derbyshire, S.
    Guichard, F.
    Favot, F.
    D'Andrea, F.
    Bechtold, P.
    Gentine, P.
    Representation of daytime moist convection over the semi-arid Tropics by parametrizations used in climate and meteorological models2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 691, p. 2220-2236Article in journal (Refereed)
    Abstract [en]

    A case of daytime development of deep convection over tropical semi-arid land is used to evaluate the representation of convection in global and regional models. The case is based on observations collected during the African Monsoon Multidisciplinary Analysis (AMMA) field campaign and includes two distinct transition phases, from clear sky to shallow cumulus and from cumulus to deep convection. Different types of models, run with identical initial and boundary conditions, are intercompared: a reference large-eddy simulation (LES), single-column model (SCM) version of four different Earth system models that participated in the Coupled Model Intercomparison Project 5 exercise, the SCM version of the European Centre for Medium-range Weather Forecasts operational forecast model, the SCM version of a mesoscale model and a bulk model. Surface fluxes and radiative heating are prescribed preventing any atmosphere-surface and cloud-radiation coupling in order to simplify the analyses so that it focuses only on convective processes. New physics packages are also evaluated within this framework. As the LES correctly reproduces the observed growth of the boundary layer, the gradual development of shallow clouds, the initiation of deep convection and the development of cold pools, it provides a basis to evaluate in detail the representation of the diurnal cycle of convection by the other models and to test the hypotheses underlying convective parametrizations. Most SCMs have difficulty in representing the timing of convective initiation and rain intensity, although substantial modifications to boundary-layer and deep-convection parametrizations lead to improvements. The SCMs also fail to represent the mid-level troposphere moistening during the shallow convection phase, which we analyse further. Nevertheless, beyond differences in timing of deep convection, the SCM models reproduce the sensitivity to initial and boundary conditions simulated in the LES regarding boundary-layer characteristics, and often the timing of convection triggering.

  • 5.
    Dahlgren, Per
    et al.
    SMHI, Research Department, Meteorology.
    Landelius, Tomas
    SMHI, Research Department, Atmospheric remote sensing.
    Kållberg, Per
    SMHI, Research Department, Meteorology.
    Gollvik, Stefan
    SMHI, Research Department, Meteorology.
    A high-resolution regional reanalysis for Europe. Part 1: Three-dimensional reanalysis with the regional HIgh-Resolution Limited-Area Model (HIRLAM)2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 698, p. 2119-2131Article in journal (Refereed)
  • 6. Dee, D. P.
    et al.
    Uppala, S. M.
    Simmons, A. J.
    Berrisford, P.
    Poli, P.
    Kobayashi, S.
    Andrae, U.
    Balmaseda, M. A.
    Balsamo, G.
    Bauer, P.
    Bechtold, P.
    Beljaars, A. C. M.
    van de Berg, L.
    Bidlot, J.
    Bormann, N.
    Delsol, C.
    Dragani, R.
    Fuentes, M.
    Geer, A. J.
    Haimberger, L.
    Healy, S. B.
    Hersbach, H.
    Holm, E. V.
    Isaksen, L.
    Kållberg, Per
    SMHI, Research Department, Meteorology.
    Koehler, M.
    Matricardi, M.
    McNally, A. P.
    Monge-Sanz, B. M.
    Morcrette, J. -J
    Park, B. -K
    Peubey, C.
    de Rosnay, P.
    Tavolato, C.
    Thepaut, J. -N
    Vitart, F.
    The ERA-Interim reanalysis: configuration and performance of the data assimilation system2011In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 137, no 656, p. 553-597Article, review/survey (Refereed)
    Abstract [en]

    ERA-Interim is the latest global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The ERA-Interim project was conducted in part to prepare for a new atmospheric reanalysis to replace ERA-40, which will extend back to the early part of the twentieth century. This article describes the forecast model, data assimilation method, and input datasets used to produce ERA-Interim, and discusses the performance of the system. Special emphasis is placed on various difficulties encountered in the production of ERA-40, including the representation of the hydrological cycle, the quality of the stratospheric circulation, and the consistency in time of the reanalysed fields. We provide evidence for substantial improvements in each of these aspects. We also identify areas where further work is needed and describe opportunities and objectives for future reanalysis projects at ECMWF. Copyright (C) 2011 Royal Meteorological Society

  • 7. Frogner, Inger-Lise
    et al.
    Singleton, Andrew T.
    Koltzow, Morten O.
    Andrae, Ulf
    SMHI, Research Department, Meteorology.
    Convection-permitting ensembles: Challenges related to their design and use2019In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 145, p. 90-106Article in journal (Refereed)
    Abstract [en]

    Challenges related to the design and use of a convection-permitting ensemble (CPEPS) are discussed. In particular the scale-dependent predictability of precipitation and the use of a CPEPS as well as its potential added value over global ensemble prediction systems (EPS) are investigated. Forecasts of precipitation from the operational CPEPS in Finland, Norway and Sweden (MEPS) are used for the investigations. It is found that predictability for scales smaller than similar to 60 km is lost rapidly within the first 6 h of the forecast with the smallest predictable scale growing more slowly to similar to 100 km over the following 18-24 h. However, there is large case-to-case variability and the ensemble perturbations fail to become fully saturated, especially in winter, suggesting a weakness in the design of the ensemble. The added value of CPEPS over deterministic forecasts and coarser resolution EPSs is discussed with summary statistics and case-studies. It is shown that the added value varies between seasons and lead times. For precipitation there is an added value for both severe precipitation events and for precipitation/no precipitation decisions. The added value is higher in summer compared to winter and for shorter lead times compared to longer lead times.

  • 8. Guemas, Virginie
    et al.
    Blanchard-Wrigglesworth, Edward
    Chevallier, Matthieu
    Day, Jonathan J.
    Deque, Michel
    Doblas-Reyes, Francisco J.
    Fuckar, Neven S.
    Germe, Agathe
    Hawkins, Ed
    Keeley, Sarah
    Koenigk, Torben
    SMHI, Research Department, Climate research - Rossby Centre.
    Salas y Melia, David
    Tietsche, Steffen
    A review on Arctic sea-ice predictability and prediction on seasonal to decadal time-scales2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 695, p. 546-561Article in journal (Refereed)
    Abstract [en]

    Sea ice plays a crucial role in the Earth's energy and water budget and has a substantial impact on local and remote atmospheric and oceanic circulations. Predictions of Arctic sea-ice conditions a few months to a few years in advance could be of interest for stakeholders. This article presents a review of the potential sources of Arctic sea-ice predictability on these time-scales. Predictability mainly originates from persistence or advection of sea-ice anomalies, interactions with the ocean and atmosphere and changes in radiative forcing. After estimating the inherent potential predictability limit with state-of-the-art models, current sea-ice forecast systems are described, together with their performance. Finally, some challenges and issues in sea-ice forecasting are presented, along with suggestions for future research priorities.

  • 9. Guemas, Virginie
    et al.
    Garcia-Serrano, Javier
    Mariotti, Annarita
    Doblas-Reyes, Francisco
    Caron, Louis-Philippe
    SMHI, Research Department, Climate research - Rossby Centre.
    Prospects for decadal climate prediction in the Mediterranean region2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 687, p. 580-597Article in journal (Refereed)
    Abstract [en]

    The Mediterranean region stands as one of the most sensitive to climate change, both in terms of warming and drying. On shorter time-scales, internal variability has substantially affected the observed climate and in the next decade might enhance or compensate long-term trends. Here we compare the multi-model climate predictions produced within the framework of the CMIP5 (Coupled Model Intercomparison Project Phase 5) project with historical simulations to assess the level of multi-year climate prediction skill in the Mediterranean region beyond that originating from the model accumulated response to the external radiative forcings. We obtain a high and significant skill in predicting 4-year averaged annual and summer mean temperature over most of the study domain and in predicting precipitation for the same seasons over northern Europe and sub-Saharan Africa. A lower skill is found during the winter season but still positive for temperature. Although most of this high skill originates from the model response to the external radiative forcings, the initialization contributes to the temperature skill over the Mediterranean Sea and surrounding land areas. The high and significant correlations between the observed Mediterranean temperatures and the observed Atlantic multidecadal oscillation (AMO) in the summer and annual means are captured by the CMIP5 ensemble which suggests that the added skill is related to the ability of the CMIP5 ensemble to predict the AMO. Such a link to the AMO seems restricted to western Africa and summer means only for the precipitation case.

  • 10. Guichard, F
    et al.
    Petch, J C
    Redelsperger, J L
    Bechtold, P
    Chaboureau, J P
    Cheinet, S
    Grabowski, W
    Grenier, H
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Kohler, M
    Piriou, J M
    Tailleux, R
    Tomasini, M
    Modelling the diurnal cycle of deep precipitating convection over land with cloud-resolving models and single-column models2004In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 130, no 604, p. 3139-3172Article in journal (Refereed)
    Abstract [en]

    An idealized case-study has been designed to investigate the modelling of the diurnal cycle of deep precipitating convection over land. A simulation of this case was performed by seven single-column models (SCMs) and three cloud-resolving models (CRMs). Within this framework, a quick onset of convective rainfall is found in most SCMs, consistent with the results from general-circulation models. In contrast, CRMs do not predict rainfall before noon. A joint analysis of the results provided by both types of model indicates that convection occurs too early in most SCMs, due to crude triggering criteria and quick onsets of convective precipitation. In the CRMs, the first clouds appear before noon, but surface rainfall is delayed by a few hours to several hours. This intermediate stage, missing in all SCMs except for one, is characterized by a gradual moistening of the free troposphere and an increase of cloud-top height. Later on, convective downdraughts efficiently cool and dry the boundary layer (BL) in the CRMs. This feature is also absent in most SCMs, which tend to adjust towards more unstable states, with moister (and often more cloudy) low levels and a drier free atmosphere. This common behaviour of most SCMs with respect to deep moist convective processes occurs even though each SCM simulates a different diurnal cycle of the BL and atmospheric stability. The scatter among the SCMs results from the wide variety of representations of BL turbulence and moist convection in these models. Greater consistency is found among the CRMs, despite some differences in their representation of the daytime BL growth, which are linked to their parametrizations of BL turbulence and/or resolution.

  • 11.
    Gustafsson, Nils
    et al.
    SMHI, Research Department, Meteorology.
    Thorsteinsson, S.
    Stengel, M.
    Holm, E.
    Use of a nonlinear pseudo-relative humidity variable in a multivariate formulation of moisture analysis2011In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 137, no 657, p. 1004-1018Article in journal (Refereed)
    Abstract [en]

    We present a reformulation of the humidity part of the HIRLAM (HIgh-Resolution Limited-Area Model) variational data assimilation. The purpose is to rectify some of the shortcomings of the present formulation which uses specific humidity, q, as an assimilation control variable with homogeneous and static covariances. One problem is that specific humidity forecast errors tend to have a non-Gaussian probability distribution, in particular near saturation and near zero humidity. In addition, the variance of the distribution tends to change in space and time due to the dependency of the water vapour saturation pressure on temperature. A modified pseudo-relative humidity variable has been adapted to the statistical balance background constraint, including the associated moisture balance formulation. Background-error statistics for the new moisture control variable and the moisture-related balances were derived, taking differences between forecasts valid at the same time as a proxy for background forecast errors. The background-error statistics were compared with the corresponding statistics for specific humidity as the moisture assimilation control variable. In connection with the nonlinearity of the change of the variable, it was noted that specified background-error standard deviations were chosen to be substantially reduced for nearly dry and saturated states, which can raise difficulties. The impact of the new moisture assimilation control variable is illustrated with simulated observation experiments as well as data assimilation experiments using real observations, for one summer month and one winter month in a 4D-Var assimilation cycle using two outer loop iterations in the 4D-Var minimization. The impact of the new formulation on forecast verification scores is small and essentially neutral, while using the second outer loop in the old formulation has a small positive impact. Copyright (C) 2011 Royal Meteorological Society

  • 12.
    Kahnert, Michael
    SMHI, Research Department, Air quality.
    Information constraints in variational data assimilation2018In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 144, no 716, p. 2230-2244Article in journal (Refereed)
  • 13.
    Kahnert, Michael
    et al.
    SMHI, Research Department, Air quality.
    Nousiainen, T.
    Raisanen, P.
    Mie simulations as an error source in mineral aerosol radiative forcing calculations2007In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no 623, p. 299-307Article in journal (Refereed)
    Abstract [en]

    The role of aerosols remains a major uncertainty for climate and climate change. For the direct radiative forcing by mineral aerosols, the uncertainty in the refractive index in has been regarded as the most important error source, while the impact of aerosol non-sphericity has been considered a minor issue and is neglected in climate models. Here, the errors caused by the spherical particle approximation (SPA) are evaluated by comparing radiative fluxes based on (i) Mie simulations and (ii) laboratory measurements of aerosol optical properties. Furthermore, they are contrasted with the errors related to the uncertainty in the refractive index. These two error sources are found to be of comparable magnitude, although they are strongly dependent on optical depth, surface albedo, and particle size. Thus, our results provide evidence that, contrary to common beliefs, the use of spherical model particles in radiative transfer simulations is probably among the major sources of error in quantifying the climate forcing effect of mineral aerosols. This stems from misrepresentation of the scattering phase function and the asymmetry parameter. Aerosol single-scattering computations based on non-spherical model particles are expected to reduce the shape-related errors and thus significantly improve the accuracy of radiative forcing simulations. Copyright (c) 2007 Royal Meteorological Society.

  • 14.
    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)
  • 15. Lenderink, G
    et al.
    Siebesma, A P
    Cheinet, S
    Irons, S
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Marquet, P
    Muller, F
    Olmeda, D
    Calvo, J
    Sanchez, E
    Soares, P M M
    The diurnal cycle of shallow cumulus clouds over land: A single-column model intercomparison study2004In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 130, no 604, p. 3339-3364Article in journal (Refereed)
    Abstract [en]

    An intercomparison study for single-column models (SCMs) of the diurnal cycle of shallow cumulus convection is reported. The case, based on measurements at the Atmospheric Radiation Measurement program Southern Great Plains site on 21 June 1997, has been used in a large-eddy simulation intercomparison study before. Results of the SCMs reveal the following general deficiencies: too large values of cloud cover and Cloud liquid water, unrealistic thermodynamic profiles, and high amounts of numerical noise. Results are also strongly dependent on vertical resolution. These results are analysed in terms of the behaviour of the different parametrization schemes involved: the convection scheme, the turbulence scheme, and the cloud scheme. In general the behaviour of the SCMs can be grouped in two different classes: one class with too strong mixing by the turbulence scheme, the other class with too strong activity by the convection scheme. The coupling between (subcloud) turbulence and the convection scheme plays a crucial role. Finally, (in part) motivated by these results several models have been successfully updated with new parametrization schemes and/or their present schemes have been successfully modified.

  • 16.
    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

  • 17. 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.

  • 18.
    Michelson, Daniel
    et al.
    SMHI, Core Services.
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Landelius, Tomas
    SMHI, Research Department, Atmospheric remote sensing.
    Collier, C G
    Haase, Gunther
    SMHI, Research Department, Atmospheric remote sensing.
    Heen, M
    'Down-to-Earth' modelling of equivalent surface precipitation using multisource data and radar2005In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 131, no 607, p. 1093-1112Article in journal (Refereed)
    Abstract [en]

    The estimation of surface rainfall from reflectivity data derived from weather radar has been much studied over many years. It is now clear that central to this problem is the adjustment of these data for the impacts of vertical variations in the reflectivity. In this paper a new procedure (known as Down-to-Earth, DTE) is proposed and tested for combining radar measurements aloft with information from a numerical weather-prediction (NWP) model and an analysis system. The procedure involves the exploitation of moist cloud physics in an attempt to account for physical processes impacting on precipitation during its descent from the height of radar echo measurements to the surface. The application of DTE leads to increased underestimation in the radar measurements compared to precipitation gauge observations at short and intermediate radar ranges (0-120 km), but is successful at reducing the bias at further ranges. However the application of DTE does not lead to significant decreases in the random error of the surface rain rate estimate. No improvement is made when attempting to account for the precipitation phase measured by radar. It is concluded that further work on radar data quality control, along with improvements to the NWP model, are essential to improve upon results using such a physically based procedure.

  • 19. Orsolini, Yvan J.
    et al.
    Karpechko, Alexey Yu.
    Nikulin, Grigory
    SMHI, Research Department, Climate research - Rossby Centre.
    Variability of the Northern Hemisphere polar stratospheric cloud potential: The role of North Pacific disturbances2009In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 135, no 641, p. 1020-1029Article in journal (Refereed)
    Abstract [en]

    The potential of the Arctic stratosphere to sustain the formation of Polar Stratospheric Clouds (PSCs) is a key factor in determining the amount of ozone destroyed each winter, and is often measured as a 'PSC volume'. The latter quantity has been shown to closely follow a near-linear compact relationship with winter-averaged column ozone loss, and displays a high variability from monthly to decadal time-scales. We examine the connection between meteorological conditions in the troposphere and the variability of lower polar stratospheric temperatures over the last four decades, and specifically, conditions leading to a high PSC volume. In addition to the well-established connection between the North Atlantic Oscillation (NAO) and the polar vortex, we demonstrate the large influence of precursory disturbances over the North Pacific and the Far East, the region of maximum climatological upward wave activity flux. Namely, very high monthly PSC volume (in the top 12%) predominantly follows the development of positive tropospheric height anomalies over the Far East, which lead to a weakening of the background planetary wave trough, and lessened upward wave activity flux into the stratosphere. Precursory anomalies over the Far East are reminiscent of East Asian monsoon amplification episodes. Copyright (C) 2009 Royal Meteorological Society

  • 20. Porson, Aurore N.
    et al.
    Hagelin, Susanna
    SMHI, Research Department.
    Boyd, Douglas F. A.
    Roberts, Nigel M.
    North, Rachel
    Webster, Stuart
    Lo, Jeff Chun-Fung
    Extreme rainfall sensitivity in convective-scale ensemble modelling over Singapore2019In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870XArticle in journal (Refereed)
  • 21. Raisanen, P.
    et al.
    Haapanala, P.
    Chung, C. E.
    Kahnert, Michael
    SMHI, Research Department, Air quality.
    Makkonen, R.
    Tonttila, J.
    Nousiainen, T.
    Impact of dust particle non-sphericity on climate simulations2013In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 139, no 677, p. 2222-2232Article in journal (Refereed)
    Abstract [en]

    Although mineral aerosol (dust) particles are irregular in shape, they are treated as homogeneous spheres in climate model radiative transfer calculations. Here, we test the effect of dust particle non-sphericity in the ECHAM5.5-HAM2 global aerosol-climate model. The short-wave optical properties of the two insoluble dust modes in HAM2 are modelled using an ensemble of spheroids that has been optimized to reproduce the optical properties of dust-like aerosols, thereby providing a significant improvement over spheres. First, the direct radiative effects (DRE) of dust non-sphericity were evaluated diagnostically, by comparing spheroids with both volume-equivalent and volume-to-area (V/A) equivalent spheres. In the volume-equivalent case, the short-wave DRE of insoluble dust at the surface and at the top of the atmosphere (TOA) was slightly smaller (typically by 3-4%) for spheroidal than for spherical dust particles. This rather small difference stems from compensating non-sphericity effects on the dust optical thickness and asymmetry parameter. In the V/A-equivalent case, the difference in optical thickness was virtually eliminated and the DRE at the TOA (surface) was approximate to 20% (approximate to 13%) smaller for spheroids than for spheres, due to a larger asymmetry parameter. Even then, however, the global-mean DRE of non-sphericity was only 0.055 W m(-2) at the TOA and 0.070 W m(-2) at the surface. Subsequently, the effects of dust non-sphericity were tested interactively in simulations in which ECHAM5.5-HAM2 was coupled to a mixed-layer ocean model. Consistent with the rather small radiative effects noted above, the climatic differences from simulations with spherical dust optics were generally negligible.

  • 22. Siebesma, A P
    et al.
    Jakob, C
    Lenderink, G
    Neggers, R A J
    Teixeira, J
    Van Meijgaard, E
    Calvo, J
    Chlond, A
    Grenier, H
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Kohler, M
    Kitagawa, H
    Marquet, P
    Lock, A P
    Muller, F
    Olmeda, D
    Severijns, C
    Cloud representation in general-circulation models over the northern Pacific Ocean: A EUROCS intercomparison study2004In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 130, no 604, p. 3245-3267Article in journal (Refereed)
    Abstract [en]

    The EUROCS (EUROpean Cloud Systems study) project aims to improve the treatment of cloud systems in regional and global climate and weather prediction models. This paper reports an intercomparison study of cloud representation over the Pacific Ocean for nine climate and weather prediction models. The study consists of an analysis of a June/July/August 1998 period along an idealized trajectory over the Pacific Ocean that encompasses both the ascending and descending branch of the Hadley circulation. The three cloud types that are studied by EUROCS, stratocumulus, shallow cumulus and deep-convective cloud systems, do all occur in a persistent and geographically separated way, along this trajectory. The main focus of this study is on processes related to the hydrological cycle within the Hadley circulation. These include the large-scale dynamics (i.e. the strength of the up- and downwelling branches of the Hadley cell), the cloud processes (i.e. cloud cover, cloud amounts and precipitation), and the impact of these processes on the radiation budget both at the top of the atmosphere and at the ocean's surface. In order to make a quantitative assessment, special care has been taken to select reliable observational datasets. The main conclusions are that (1) almost all models strongly underpredicted both cloud cover and cloud amount in the stratocumulus regions while (2) the situation is opposite in the trade-wind region and the tropics where cloud cover and cloud amount are overpredicted by most models. These deficiencies result in an overprediction of the downwelling surface short-wave radiation of typically 60 W m(-2) in the stratocumulus regimes and a similar underprediction of 60 W m(-2) in the trade-wind regions and in the intertropical convergence zone (ITCZ). Similar biases for the short-wave radiation were found at the top of the atmosphere, while discrepancies in the outgoing long-wave radiation are most pronounced in the ITCZ.

  • 23. 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.

  • 24. 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

  • 25. 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

  • 26. 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)
  • 27. Zilitinkevich, S S
    et al.
    Perov, Veniamin
    SMHI, Research Department, Meteorology.
    King, J C
    Near-surface turbulent fluxes in stable stratification: Calculation techniques for use in general-circulation models2002In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 128, no 583, p. 1571-1587Article in journal (Refereed)
    Abstract [en]

    Practically oriented flux-calculation techniques based on correction functions to the neutral drag and heat/mass transfer coefficients are further developed. In the traditional formulation, the correction functions depend only on the bulk Richardson number. However, data from measurements of turbulent fluxes and mean profiles in stable stratification over different sites exhibit too strong variability in this type of dependencies. Indirect evidence from climate and weather prediction modelling also shows that the traditional flux-calculation technique is not sufficiently advanced. It is conceivable that other mechanisms besides the surface-layer stratification and, therefore, other arguments besides the bulk Richardson number must be considered. The proposed technique includes a newly discovered effect of the static stability in the free atmosphere on the surface-layer scaling and accounts for the general essential difference between the roughness lengths for momentum and scalars. Besides bulk Richardson number, recommended correction functions depend oil one more stability parameter. involving the Brunt-Vaisala frequency in the free atmosphere, and on the roughness lengths.

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