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  • 1. Ansell, T. J.
    et al.
    Jones, P. D.
    Allan, R. J.
    Lister, D.
    Parker, D. E.
    Brunet, M.
    Moberg, A.
    Jacobeit, J.
    Brohan, P.
    Rayner, N. A.
    Aguilar, E.
    Alexandersson, Hans
    SMHI.
    Barriendos, M.
    Brandsma, T.
    Cox, N. J.
    Della-Marta, P. M.
    Drebs, A.
    Founda, D.
    Gerstengarbe, F.
    Hickey, K.
    Jonsson, T.
    Luterbacher, J.
    Nordli, O.
    Oesterle, H.
    Petrakis, M.
    Philipp, A.
    Rodwell, M. J.
    Saladie, O.
    Sigro, J.
    Slonosky, V.
    Srnec, L.
    Swail, V.
    Garcia-Suarez, A. M.
    Tuomenvirta, H.
    Wang, X.
    Wanner, H.
    Werner, P.
    Wheeler, D.
    Xoplaki, E.
    Daily mean sea level pressure reconstructions for the European-North Atlantic region for the period 1850-20032006In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 19, no 12, p. 2717-2742Article in journal (Refereed)
    Abstract [en]

    The development of a daily historical European-North Atlantic mean sea level pressure dataset (EMSLP) for 1850-2003 on a 5 latitude by longitude grid is described. This product was produced using 86 continental and island stations distributed over the region 25 degrees-70 degrees N, 70 degrees W-50 degrees E blended with marine data from the International Comprehensive Ocean-Atmosphere Data Set (ICOADS). The EMSLP fields for 1850-80 are based purely on the land station data and ship observations. From 1881, the blended land and marine fields are combined with already available daily Northern Hemisphere fields. Complete coverage is obtained by employing reduced space optimal interpolation. Squared correlations (r(2)) indicate that EMSLP generally captures 80%-90% of daily variability represented in an existing historical mean sea level pressure product and over 90% in modern 40-yr European Centre for Medium-Range Weather Forecasts Re-Analyses (ERA-40) over most of the region. A lack of sufficient observations over Greenland and the Middle East, however, has resulted in poorer reconstructions there. Error estimates, produced as part of the reconstruction technique, flag these as regions of low confidence. It is shown that the EMSLP daily fields and associated error estimates provide a unique opportunity to examine the circulation patterns associated with extreme events across the European-North Atlantic region, such as the 2003 heat wave, in the context of historical events.

  • 2. Bjork, Goran
    et al.
    Stranne, Christian
    Borenäs, Karin
    SMHI, Core Services.
    The Sensitivity of the Arctic Ocean Sea Ice Thickness and Its Dependence on the Surface Albedo Parameterization2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 4, p. 1355-1370Article in journal (Refereed)
    Abstract [en]

    In this study, the response of sea ice thickness to changes in the external forcing is investigated and particularly how this response depends on the surface albedo formulation by means of a one-dimensional coupled ocean-ice-atmosphere model. The main focus is on the thickness response to the atmospheric heat advection F-wall, solar radiation F-SW, and amount of snow precipitation S-prec. Different albedo parameterization schemes [ECHAM5, CSIRO, and Community Climate System Model, version 3 (CCSM3)] representing albedos commonly used in global climate models are compared together with more simplified schemes. Using different albedo schemes with the same external forcing produces large differences in ice thickness. The ice thickness response is similar for all realistic albedo schemes with a nearly linear decrease with increasing F-wall in the perennial ice regime and with a steplike transition into seasonal ice when F-wall exceeds a certain threshold. This transition occurs at an annual-mean ice thickness of 1.7-2.0 m. Latitudinal differences in solar insolation generally leads to increasing ice thickness toward the North Pole. The snow response varies significantly depending on which albedo scheme is used. The ECHAM5 scheme yields thinner ice with S-prec, the CSIRO scheme gives ice thickness nearly independent of S-prec, and with the CCSM3 scheme the ice thickness decreases with S-prec. A general result is that the modeled ice cover is rather sensitive to positive perturbations of the external heat supply when it is close to the transition such that just a small increase of, for example, F-wall can force the ice cover into the seasonal regime.

  • 3.
    Devasthale, Abhay
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Thomas, Manu Anna
    SMHI, Research Department, Air quality.
    Sensitivity of Cloud Liquid Water Content Estimates to the Temperature-Dependent Thermodynamic Phase: A Global Study Using CloudSat Data2012In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 25, no 20, p. 7297-7307Article in journal (Refereed)
    Abstract [en]

    The main purpose of this study is to underline the sensitivity of cloud liquid water content (LWC) estimates purely to 1) the shape of computationally simplified temperature-dependent thermodynamic phase and 2) the range of subzero temperatures covered to partition total cloud condensate into liquid and ice fractions. Linear, quadratic, or sigmoid-shaped functions for subfreezing temperatures (down to -20 degrees or -40 degrees C) are often used in climate models and reanalysis datasets for partitioning total condensate. The global vertical profiles of clouds obtained from CloudSat for the 4-yr period June 2006-May 2010 are used for sensitivity analysis and the quantitative estimates of sensitivities based on these realistic cloud profiles are provided. It is found that three cloud regimes in particular-convective clouds in the tropics, low-level clouds in the northern high latitudes, and middle-level clouds over the midlatitudes and Southern Ocean-are most sensitive to assumptions on thermodynamic phase. In these clouds, the LWC estimates based purely on quadratic or sigmoid-shaped functions with a temperature range down to -20 degrees C can differ by up to 20%-40% over the tropics (in seasonal means). 10%-30% over the midlatitudes, and up to 50% over high latitudes compared to a linear assumption. When the temperature range is extended down to -40 degrees C. LWC estimates in the sigmoid case can be much higher than the above values over high-latitude regions compared to the commonly used case with quadratic dependency down to -20 C. This sensitivity study emphasizes the need to critically investigate radiative impacts of cloud thermodynamic phase assumptions in simplified climate models and reanalysis datasets.

  • 4. Endris, Hussen Seid
    et al.
    Omondi, Philip
    Jain, Suman
    Lennard, Christopher
    Hewitson, Bruce
    Chang'a, Ladislaus
    Awange, J. L.
    Dosio, Alessandro
    Ketiem, Patrick
    Nikulin, Grigory
    SMHI, Research Department, Climate research - Rossby Centre.
    Panitz, Hans-Juergen
    Buechner, Matthias
    Stordal, Frode
    Tazalika, Lukiya
    Assessment of the Performance of CORDEX Regional Climate Models in Simulating East African Rainfall2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 21, p. 8453-8475Article, review/survey (Refereed)
    Abstract [en]

    This study evaluates the ability of 10 regional climate models (RCMs) from the Coordinated Regional Climate Downscaling Experiment (CORDEX) in simulating the characteristics of rainfall patterns over eastern Africa. The seasonal climatology, annual rainfall cycles, and interannual variability of RCM output have been assessed over three homogeneous subregions against a number of observational datasets. The ability of the RCMs in simulating large-scale global climate forcing signals is further assessed by compositing the El Nino-Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) events. It is found that most RCMs reasonably simulate the main features of the rainfall climatology over the three subregions and also reproduce the majority of the documented regional responses to ENSO and IOD forcings. At the same time the analysis shows significant biases in individual models depending on subregion and season; however, the ensemble mean has better agreement with observation than individual models. In general, the analysis herein demonstrates that the multimodel ensemble mean simulates eastern Africa rainfall adequately and can therefore be used for the assessment of future climate projections for the region.

  • 5.
    Fitch, Anna
    SMHI, Research Department, Climate research - Rossby Centre.
    Climate Impacts of Large-Scale Wind Farms as Parameterized in a Global Climate Model2015In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 28, no 15, p. 6160-6180Article in journal (Refereed)
    Abstract [en]

    The local, regional, and global climate impacts of a large-scale global deployment of wind power in regionally high densities over land are investigated for a 60-yr period. Wind farms are represented as elevated momentum sinks as well as enhanced turbulence to represent turbine blade mixing in the Community Atmosphere Model, version 5 (CAM5), a global climate model. For a total installed capacity of 2.5 TW, to provide 16% of the world's projected electricity demand in 2050, minimal impacts are found both regionally and globally on temperature, sensible and latent heat fluxes, cloud, and precipitation. A mean near-surface warming of 0.12 +/- 0.07 K is seen within the wind farms, with a global-mean temperature change of -0.013 +/- 0.015 K. Impacts on wind speed and turbulence are more pronounced but largely confined within the wind farm areas. Increasing the wind farm areas to provide an installed capacity of 10 TW, or 65% of the 2050 electricity demand, causes further impacts; however, they remain slight overall. Maximum temperature changes are less than 0.5 K in the wind farm areas. To provide 20 TW of installed capacity, or 130% of the 2050 electricity demand, impacts both within the wind farms and beyond become more pronounced, with a doubling in turbine density. However, maximum temperature changes remain less than 0.7 K. Representing wind farms instead as an increase in surface roughness generally produces similar mean results; however, maximum changes increase, and influences on wind and turbulence are exaggerated. Overall, wind farm impacts are much weaker than those expected from greenhouse gas emissions, with very slight global-mean climate impacts.

  • 6.
    Hieronymus, Magnus
    et al.
    SMHI, Research Department, Oceanography.
    Nycander, Jonas
    Nilsson, Johan
    Doos, Kristofer
    Hallberg, Robert
    Oceanic Overturning and Heat Transport: The Role of Background Diffusivity2019In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 32, no 3, p. 701-716Article in journal (Refereed)
  • 7. Hood, Lon
    et al.
    Schimanke, Semjon
    SMHI, Research Department, Oceanography.
    Spangehl, Thomas
    Bal, Sourabh
    Cubasch, Ulrich
    The Surface Climate Response to 11-Yr Solar Forcing during Northern Winter: Observational Analyses and Comparisons with GCM Simulations2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 19, p. 7489-7506Article in journal (Refereed)
    Abstract [en]

    The surface climate response to 11-yr solar forcing during northern winter is first reestimated by applying a multiple linear regression (MLR) statistical model to Hadley Centre sea level pressure (SLP) and sea surface temperature (SST) data over the 1880-2009 period. In addition to a significant positive SLP response in the North Pacific found in previous studies, a positive SST response is obtained across the midlatitude North Pacific. Negative but insignificant SLP responses are obtained in the Arctic. The derived SLP response at zero lag therefore resembles a positive phase of the Arctic Oscillation (AO). Evaluation of the SLP and SST responses as a function of phase lag indicates that the response evolves from a negative AO-like mode a few years before solar maximum to a positive AO-like mode at and following solar maximum. For comparison, a similar MLR analysis is applied to model SLP and SST data from a series of simulations using an atmosphere-ocean general circulation model with a well-resolved stratosphere. The simulations differed only in the assumed solar cycle variation of stratospheric ozone. It is found that the simulation that assumed an ozone variation estimated from satellite data produces solar SLP and SST responses that are most consistent with the observational results, especially during a selected centennial period. In particular, a positive SLP response anomaly is obtained in the northeastern Pacific and a corresponding positive SST response anomaly extends across the midlatitude North Pacific. The model response versus phase lag also evolves from a mainly negative AO-like response before solar maximum to a mainly positive AO response at and following solar maximum.

  • 8. Kalognomou, Evangelia-Anna
    et al.
    Lennard, Christopher
    Shongwe, Mxolisi
    Pinto, Izidine
    Favre, Alice
    Kent, Michael
    Hewitson, Bruce
    Dosio, Alessandro
    Nikulin, Grigory
    SMHI, Research Department, Climate research - Rossby Centre.
    Panitz, Hans-Juergen
    Buechner, Matthias
    A Diagnostic Evaluation of Precipitation in CORDEX Models over Southern Africa2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 23, p. 9477-9506Article, review/survey (Refereed)
    Abstract [en]

    The authors evaluate the ability of 10 regional climate models (RCMs) to simulate precipitation over Southern Africa within the Coordinated Regional Climate Downscaling Experiment (CORDEX) framework. An ensemble of 10 regional climate simulations and the ensemble average is analyzed to evaluate the models' ability to reproduce seasonal and interannual regional climatic features over regions of the subcontinent. All the RCMs use a similar domain, have a spatial resolution of 50 km, and are driven by the Interim ECMWF Re-Analysis (ERA-Interim; 1989-2008). Results are compared against a number of observational datasets.In general, the spatial and temporal nature of rainfall over the region is captured by all RCMs, although individual models exhibit wet or dry biases over particular regions of the domain. Models generally produce lower seasonal variability of precipitation compared to observations and the magnitude of the variability varies in space and time. Model biases are related to model setup, simulated circulation anomalies, and moisture transport. The multimodel ensemble mean generally outperforms individual models, with bias magnitudes similar to differences across the observational datasets. In the northern parts of the domain, some of the RCMs and the ensemble average improve the precipitation climate compared to that of ERA-Interim. The models are generally able to capture the dry (wet) precipitation anomaly associated with El Nino (La Nina) events across the region. Based on this analysis, the authors suggest that the present set of RCMs can be used to provide useful information on climate projections of rainfall over Southern Africa.

  • 9. Kendon, Elizabeth J.
    et al.
    Jones, Richard G.
    Kjellström, Erik
    SMHI, Research Department, Climate research - Rossby Centre.
    Murphy, James M.
    Using and Designing GCM-RCM Ensemble Regional Climate Projections2010In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 23, no 24, p. 6485-6503Article in journal (Refereed)
    Abstract [en]

    Multimodel ensembles, whereby different global climate models (GCMs) and regional climate models (RCMs) are combined, have been widely used to explore uncertainties in regional climate projections. In this study, the extent to which information can be enhanced from sparsely filled GCM RCM ensemble matrices and the way in which simulations should be prioritized to sample uncertainties most effectively are examined. A simple scaling technique, whereby the local climate response in an RCM is predicted from the large-scale change in the GCM, is found to often show skill in estimating local changes for missing GCM RCM combinations. In particular, scaling shows skill for precipitation indices (including mean, variance, and extremes) across Europe in winter and mean and extreme temperature in summer and winter, except for hot extremes over central/northern Europe in summer. However, internal variability significantly impacts the ability to determine scaling skill for precipitation indices, with a three-member ensemble found to be insufficient for identifying robust local scaling relationships in many cases. This study suggests that, given limited computer resources, ensembles should be designed to prioritize the sampling of GCM uncertainty, using a reduced set of RCMs. Exceptions are found over the Alps and northeastern Europe in winter and central Europe in summer, where sampling multiple RCMs may be equally or more important for capturing uncertainty in local temperature or precipitation change. This reflects the significant role of local processes in these regions. Also, to determine the ensemble strategy in some cases, notably precipitation extremes in summer, better sampling of internal variability is needed.

  • 10.
    Lind, Petter
    et al.
    SMHI, Research Department, Climate research - Rossby Centre.
    Lindstedt, David
    SMHI, Research Department, Climate research - Rossby Centre.
    Kjellström, Erik
    SMHI, Research Department, Climate research - Rossby Centre.
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Spatial and Temporal Characteristics of Summer Precipitation over Central Europe in a Suite of High-Resolution Climate Models2016In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, no 10, p. 3501-3518Article in journal (Refereed)
  • 11.
    Nikulin, Grigory
    et al.
    SMHI, Research Department, Climate research - Rossby Centre.
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Giorgi, Filippo
    Asrar, Ghassem
    Buechner, Matthias
    Cerezo-Mota, Ruth
    Christensen, Ole Bossing
    Deque, Michel
    Fernandez, Jesus
    Haensler, Andreas
    van Meijgaard, Erik
    Samuelsson, Patrick
    SMHI, Research Department, Climate research - Rossby Centre.
    Sylla, Mouhamadou Bamba
    Sushama, Laxmi
    Precipitation Climatology in an Ensemble of CORDEX-Africa Regional Climate Simulations2012In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 25, no 18, p. 6057-6078Article in journal (Refereed)
    Abstract [en]

    An ensemble of regional climate simulations is analyzed to evaluate the ability of 10 regional climate models (RCMs) and their ensemble average to simulate precipitation over Africa. All RCMs use a similar domain and spatial resolution of similar to 50 km and are driven by the ECMWF Interim Re-Analysis (ERA-Interim) (1989-2008). They constitute the first set of simulations in the Coordinated Regional Downscaling Experiment in Africa (CORDEX-Africa) project. Simulated precipitation is evaluated at a range of time scales, including seasonal means, and annual and diurnal cycles, against a number of detailed observational datasets. All RCMs simulate the seasonal mean and annual cycle quite accurately, although individual models can exhibit significant biases in some subregions and seasons. The multimodel average generally outperforms any individual simulation, showing biases of similar magnitude to differences across a number of observational datasets. Moreover, many of the RCMs significantly improve the precipitation climate compared to that from their boundary condition dataset, that is, ERA-Interim. A common problem in the majority of the RCMs is that precipitation is triggered too early during the diurnal cycle, although a small subset of models does have a reasonable representation of the phase of the diurnal cycle. The systematic bias in the diurnal cycle is not improved when the ensemble mean is considered. Based on this performance analysis, it is assessed that the present set of RCMs can be used to provide useful information on climate projections over Africa.

  • 12. Pucik, Tomas
    et al.
    Groenemeijer, Pieter
    Raedler, Anja T.
    Tijssen, Lars
    Nikulin, Grigory
    SMHI, Research Department, Climate research - Rossby Centre.
    Prein, Andreas F.
    van Meijgaard, Erik
    Fealy, Rowan
    Jacob, Daniela
    Teichmann, Claas
    Future Changes in European Severe Convection Environments in a Regional Climate Model Ensemble2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 17, p. 6771-6794Article in journal (Refereed)
  • 13.
    Räisänen, Jouni
    SMHI, Research Department, Climate research - Rossby Centre.
    CO2- and aerosol-induced changes in vertically integrated zonal momentum budget in a GCM experiment1998In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 11, no 4, p. 625-639Article in journal (Refereed)
    Abstract [en]

    The German Climate Computing Center recently conducted a model experiment in which separate runs simulate the climatic response to increasing CO2 alone and to increasing CO2 together with direct radiative forcing by sulfate aerosols. One of the variables that shows interesting differences between the different runs is the nearsurface zonal-mean zonal wind. As compared with the control run, the midlatitude surface westerlies intensify and shift poleward in the CO2-only run in both hemispheres in both the northern winter (DJF) and summer (JJA). However, the aerosol forcing moderates these changes in general and, in particular, reverses the pattern of change in the Northern Hemisphere in JJA. Consistent differences between the various runs occur in the meridional distribution of sea level pressure. The origin of these simulated changes is studied by using the vertically integrated zonal-mean zonal momentum budget, utilizing the intimate linkage between the low-level wind and the surface stress and the close time-mean balance between the surface stress and the other terms in the budget. Regardless of the forcing used, momentum convergence in transient eddies is found to be the dominant agent of change in the extratropical Southern Hemisphere and in the Northern Hemisphere midlatitudes in JJA, In particular, the changes in the contribution of high-pass transients are relatively large and they seem to be qualitatively traceable to the changes in the tropospheric meridional temperature gradient. In the northern extratropics in DJE stationary eddies make an even larger contribution than the transients.

  • 14.
    Räisänen, Jouni
    SMHI, Research Department, Climate research - Rossby Centre.
    CO2-induced changes in atmospheric angular momentum in CMIP2 experiments2003In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 16, no 1, p. 132-143Article in journal (Refereed)
    Abstract [en]

    The response of atmospheric angular momentum to a gradual doubling of CO2 is studied using 16 model experiments participating in the second phase of the Coupled Model Intercomparison Project (CMIP2). The relative angular momentum associated with atmospheric zonal winds increases in all but one of the models, although the magnitude of the change varies widely. About 90% of the 16-model mean increase comes from increasing westerly winds in the stratosphere and the uppermost low-latitude troposphere above 200 hPa. This increase in westerly winds reflects a steepening of the meridional temperature gradient near the tropopause and in the upper troposphere. The simulated temperature gradient at this height increases partly as an indirect consequence of the poleward decrease in the tropopause height, and partly because convection induces a maximum in warming in the tropical upper troposphere. The change in the omega angular momentum associated with the surface pressure distribution is in most models smaller than the change in the relative angular momentum, although its exact value is sensitive to the method of calculation.

  • 15.
    Räisänen, Jouni
    SMHI, Research Department, Climate research - Rossby Centre.
    CO2-induced changes in interannual temperature and precipitation variability in 19 CMIP2 experiments2002In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 15, no 17, p. 2395-2411Article in journal (Refereed)
    Abstract [en]

    CO2-induced changes in the interannual variability of monthly surface air temperature and precipitation are studied using 19 model experiments participating in the second phase of the Coupled Model Intercomparison Project (CMIP2). The magnitude of variability in the control runs appears generally reasonable, but it varies a great deal between different models, almost all of which overestimate temperature variability on low-latitude land areas. In most models the gradual doubling of CO2 leads to a decrease in temperature variability in the winter half-year in the extratropical Northern Hemisphere and over the high-latitude Southern Ocean. Over land in low latitudes and in northern midlatitudes in summer, a slight tendency toward increased temperature variability occurs. The standard deviation of monthly precipitation increases, on average, where the mean precipitation increases but also does so in some areas where the mean precipitation decreases slightly. The coefficient of variation of precipitation (i.e., the ratio between the standard deviation and the mean) also tends to increase in most areas, especially where the mean precipitation decreases. However, the changes in variability are less similar between the 19 experiments than the changes in mean temperature and precipitation, at least partly because they have a much lower signal-to-noise ratio. In addition, the changes in the standard deviation of monthly temperature are generally much smaller than the time-mean warming, which suggests that future changes in the extremes of interannual temperature variability will be largely determined by the latter.

  • 16.
    Räisänen, Jouni
    SMHI, Research Department, Climate research - Rossby Centre.
    CO2-induced climate change in CMIP2 experiments: Quantification of agreement and role of internal variability2001In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 14, no 9, p. 2088-2104Article in journal (Refereed)
    Abstract [en]

    CO2-induced changes in surface air temperature, precipitation, and sea level pressure are compared between model experiments participating in the second phase of the Coupled Model Intercomparison Project (CMIP2). A statistical formalism is applied, in which the average squared amplitude of the simulated climate changes is divided into a common signal and variances associated with internal variability and model differences. In the 20-yr period centered at the doubling of CO2 and for a set of 14-15 models, the dimensionless global relative agreement on gridbox-scale annual mean climate changes is 0.89 for surface air temperature but only 0.22 for precipitation and 0.46 for sea level pressure. A majority of the interexperiment differences are attributed to model differences; the contribution of internal variability to the differences in change is estimated as 16% for temperature, 34% for precipitation, and 32% for sea level pressure. For seasonal rather than annual climate changes, the agreement is lower and the contribution of internal variability to the interexperiment variance larger. Likewise, the relative agreement is worse and internal variability in relative terms more important earlier during the transient experiments than around the doubling of CO2. Conversely, when climate changes are averaged over larger areas than individual grid boxes, the relative agreement improves with increasing averaging domain (especially with precipitation and temperature) and the impact of internal variability decreases.

  • 17.
    Räisänen, Jouni
    et al.
    SMHI, Research Department, Climate research - Rossby Centre.
    Palmer, T N
    A probability and decision-model analysis of a multimodel ensemble of climate change simulations2001In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 14, no 15, p. 3212-3226Article in journal (Refereed)
    Abstract [en]

    Because of the inherent uncertainties in the computational representation of climate and because of unforced chaotic climate variability, it is argued that climate change projections should be expressed in probabilistic form. In this paper, 17 Coupled Model Intercomparison Project second-phase experiments sharing the same gradual increase in atmospheric CO2 are treated as a probabilistic multimodel ensemble projection of future climate. Tools commonly used for evaluation of probabilistic weather and seasonal forecasts are applied to this climate change ensemble. The probabilities of some temperature- and precipitation-related events defined for 20-yr seasonal means of climate are first studied. A cross-verification exercise is then used to obtain an upper estimate of the quality of these probability forecasts in terms of Brier skill scores, reliability diagrams, and potential economic value. Skill and value estimates are consistently higher for temperature- related events (e.g., will the 20-yr period around the doubling of CO2 be at least 1 degreesC warmer than the present?) than for precipitation-related events (e.g., will the mean precipitation decrease by 10% or more?). For large enough CO2 forcing, however, probabilistic projections of precipitation-related events also exhibit substantial potential economic value for a range of cost-loss ratios. The treatment of climate change information in a probabilistic rather than deterministic manner (e.g., using the ensemble consensus forecast) can greatly enhance its potential value.

  • 18.
    Schimanke, Semjon
    et al.
    SMHI, Research Department, Oceanography.
    Meier, Markus
    SMHI, Research Department, Oceanography.
    Decadal-to-Centennial Variability of Salinity in the Baltic Sea2016In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, no 20, p. 7173-7188Article in journal (Refereed)
  • 19.
    Sedlar, Joseph
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Shupe, Matthew D.
    Tjernstrom, Michael
    On the Relationship between Thermodynamic Structure and Cloud Top, and Its Climate Significance in the Arctic2012In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 25, no 7, p. 2374-2393Article in journal (Refereed)
    Abstract [en]

    Cloud and thermodynamic characteristics from three Arctic observation sites are investigated to understand the collocation between low-level clouds and temperature inversions. A regime where cloud top was 100-200 m above the inversion base [cloud inside inversion (CII)] was frequently observed at central Arctic Ocean sites, while observations from Barrow, Alaska, indicate that cloud tops were more frequently constrained to inversion base height [cloud capped by inversion (CCI)]. Cloud base and top heights were lower, and temperature inversions were also stronger and deeper, during CII cases. Both cloud regimes were often decoupled from the surface except for CCI over Barrow. In-cloud lapse rates differ and suggest increased cloud-mixing potential for CII cases. Specific humidity inversions were collocated with temperature inversions for more than 60% of the CCI and more than 85% of the CII regimes. Horizontal advection of heat and moisture is hypothesized as an important process controlling thermodynamic structure and efficiency of cloud-generated motions. The portion of CII clouds above the inversion contains cloud radar signatures consistent with cloud droplets. The authors test the longwave radiative impact of cloud liquid above the inversion through hypothetical liquid water distributions. Optically thin CII clouds alter the effective cloud emission temperature and can lead to an increase in surface flux on the order of 1.5 W m(-2) relative to the same cloud but whose top does not extend above the inversion base. The top of atmosphere impact is even larger, increasing outgoing longwave radiation up to 10 W m(-2). These results suggest a potentially significant longwave radiative forcing via simple liquid redistributions for a distinctly dominant cloud regime over sea ice.

  • 20. Sun, Bomin
    et al.
    Free, Melissa
    Yoo, Hye Lim
    Foster, Michael J.
    Heidinger, Andrew
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Variability and Trends in U.S. Cloud Cover: ISCCP, PATMOS-x, and CLARA-A1 Compared to Homogeneity-Adjusted Weather Observations2015In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 28, no 11, p. 4373-4389Article in journal (Refereed)
    Abstract [en]

    Variability and trends in total cloud cover for 1982-2009 across the contiguous United States from the International Satellite Cloud Climatology Project (ISCCP), AVHRR Pathfinder Atmospheres-Extended (PATMOS-x), and EUMETSAT Satellite Application Facility on Climate Monitoring Clouds, Albedo and Radiation from AVHRR Data Edition 1 (CLARA-A1) satellite datasets are assessed using homogeneity-adjusted weather station data. The station data, considered as "ground truth" in the evaluation, are generally well correlated with the ISCCP and PATMOS-x data and with the physically related variables diurnal temperature range, precipitation, and surface solar radiation. Among the satellite products, overall, the PATMOS-x data have the highest interannual correlations with the weather station cloud data and those other physically related variables. The CLARA-A1 daytime dataset generally shows the lowest correlations, even after trends are removed. For the U.S. mean, the station dataset shows a negative but not statistically significant trend of -0.40% decade(-1), and satellite products show larger downward trends ranging from -0.55% to -5.00% decade(-1) for 1984-2007. PATMOS-x 1330 local time trends for U.S. mean cloud cover are closest to those in the station data, followed by the PATMOS-x diurnally corrected dataset and ISCCP, with CLARA-A1 having a large negative trend contrasting strongly with the station data. These results tend to validate the usefulness of weather station cloud data for monitoring changes in cloud cover, and they show that the long-term stability of satellite cloud datasets can be assessed by comparison to homogeneity-adjusted station data and other physically related variables.

  • 21. Tuomenvirta, H
    et al.
    Alexandersson, Hans
    SMHI.
    Drebs, A
    Frich, P
    Nordli, P O
    Trends in Nordic and Arctic temperature extremes and ranges2000In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 13, no 5, p. 977-990Article in journal (Refereed)
    Abstract [en]

    The national meteorological institutes in the Nordic countries have produced a comprehensive dataset of climatic extreme temperatures (monthly mean daily maximum and minimum, and monthly absolute highest and lowest temperatures) comprising stations from Fenno-Scandia, the Nordic Seas, and Greenland. Mean maximum and minimum temperatures show statistically significant negative trends in western coastal Greenland during the period 1950-95, while over the Nordic Seas and Fenno-Scandia the trends are generally positive. The diurnal temperature range (DTR) is decreasing significantly throughout the study area and is unrelated to regional temperature trends, which show both warming and cooling. The opposite temperature trends between western coastal Greenland and Fenno-Scandia since the 1950s are in accordance with a strengthening of the North Atlantic Oscillation (NAO). However, the simple NAO index fails to explain the decrease of DTR. In Fenno-Scandia, the reliable long-term mean maximum and minimum temperatures show cooling in winter and warming in spring and summer during the period 1910-95. Simultaneously, DTR has been decreasing in all seasons except winter. Most of the decrease has occurred since the 1940s. Atmospheric circulation indices defined by zonal and meridional sea level pressure differences, along with sea level pressure and cloud cover anomalies were used to build a multiple linear regression model for the Fenno-Scandian DTR. During the period 1910-95 the model explains from 53% (winter) to 80% (summer) of the variation in DTR and reproduces the statistically significant decreasing trend on annual level. Cloud cover is the dominant predictor, while circulation provides substantial improvement in explanation.

  • 22. 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.

  • 23. 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.

  • 24. Van den Hurk, B
    et al.
    Hirschi, M
    Schar, C
    Lenderink, G
    Van Meijgaard, E
    Van Ulden, A
    Rockel, B
    Hagemann, S
    Graham, Phil
    SMHI, Research Department, Climate research - Rossby Centre.
    Kjellström, Erik
    SMHI, Research Department, Climate research - Rossby Centre.
    Jones, R
    Soil control on runoff response to climate change in regional climate model simulations2005In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 18, no 17, p. 3536-3551Article in journal (Refereed)
    Abstract [en]

    Simulations with seven regional climate models driven by a common control climate simulation of a GCM carried out for Europe in the context of the (European Union) EU-funded Prediction of Regional scenarios and Uncertainties for Defining European Climate change risks and Effects (PRUDENCE) project were analyzed with respect to land surface hydrology in the Rhine basin. In particular, the annual cycle of the terrestrial water storage was compared to analyses based on the 40-yr ECMWF Re-Analysis (ERA-40) atmospheric convergence and observed Rhine discharge data. In addition, an analysis was made of the partitioning of convergence anomalies over anomalies in runoff and storage. This analysis revealed that most models underestimate the size of the water storage and consequently overestimated the response of runoff to anomalies in net convergence. The partitioning of these anomalies over runoff and storage was indicative for the response of the simulated runoff to a projected climate change consistent with the greenhouse gas A2 Synthesis Report on Emission Scenarios (SRES). In particular, the annual cycle of runoff is affected largely by the terrestrial storage reservoir. Larger storage capacity leads to smaller changes in both wintertime and summertime monthly mean runoff. The sustained summertime evaporation resulting from larger storage reservoirs may have a noticeable impact on the summertime surface temperature projections.

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