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Alexei Karpetchko and Grigory Nikulin

Abstract

Using NCEP–NCAR reanalysis data the authors show that the November–December averaged stratospheric eddy heat flux is strongly anticorrelated with the January–February averaged eddy heat flux in the midlatitude stratosphere and troposphere. This finding further emphasizes differences between early and midwinter stratospheric wave flux behavior, which has recently been found in long-term variations. Analysis suggests that the intraseasonal anticorrelation of stratospheric heat fluxes results from changes in the upward wave propagation in the troposphere. Stronger (weaker) upward wave fluxes in early winter lead to weaker (stronger) upward wave fluxes from the troposphere during midwinter. Also, enhanced equatorward wave refraction during midwinter (due to the stronger polar night jet) is associated with weak heat flux in the early winter. It is suggested that the effect of enhanced midwinter upward wave flux from the troposphere in the years with weak early winter heat flux overcompensates the effect of increased equatorward wave refraction in midwinter, leading to a net increase of midwinter upward wave fluxes into the stratosphere.

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Ivan Güttler, Igor Stepanov, Čedo Branković, Grigory Nikulin, and Colin Jones

Abstract

The hydrostatic regional climate model RCA, version 3 (RCA3), of the Swedish Meteorological and Hydrological Institute was used to dynamically downscale ERA-40 and the ECMWF operational analysis over a 22-yr period. Downscaling was performed at four horizontal resolutions—50, 25, 12.5, and 6.25 km—over an identical European domain. The model-simulated precipitation is evaluated against high-resolution gridded observational precipitation datasets over Switzerland and southern Norway, regions that are characterized by complex orography and distinct climate regimes.

RCA3 generally overestimates precipitation over high mountains: during winter and summer over Switzerland and during summer over central-southern Norway. In the summer, this is linked with a substantial contribution of convective precipitation to the total precipitation errors, especially at the coarser resolutions (50 and 25 km). A general improvement in spatial correlation coefficients between simulated and observed precipitation is observed when the horizontal resolution is increased from 50 to 6 km. The 95th percentile spatial correlation coefficients during winter are much higher for southern Norway than for Switzerland, indicating that RCA3 is more successful at reproducing a relatively simple west-to-east precipitation gradient over southern Norway than a much more complex and variable precipitation distribution over Switzerland. The 6-km simulation is not always superior to the other simulations, possibly indicating that the model dynamical and physical configuration at this resolution may not have been optimal. However, a general improvement in simulated precipitation with increasing resolution supports further use and application of high spatial resolutions in RCA3.

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Evangelia-Anna Kalognomou, Christopher Lennard, Mxolisi Shongwe, Izidine Pinto, Alice Favre, Michael Kent, Bruce Hewitson, Alessandro Dosio, Grigory Nikulin, Hans-Jürgen Panitz, and Matthias Büchner

Abstract

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 Niño (La Niña) 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.

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Tomáš Púčik, Pieter Groenemeijer, Anja T. Rädler, Lars Tijssen, Grigory Nikulin, Andreas F. Prein, Erik van Meijgaard, Rowan Fealy, Daniela Jacob, and Claas Teichmann

Abstract

The occurrence of environmental conditions favorable for severe convective storms was assessed in an ensemble of 14 regional climate models covering Europe and the Mediterranean with a horizontal grid spacing of 0.44°. These conditions included the collocated presence of latent instability and strong deep-layer (surface to 500 hPa) wind shear, which is conducive to the severe and well-organized convective storms. The occurrence of precipitation in the models was used as a proxy for convective initiation. Two climate scenarios (RCP4.5 and RCP8.5) were investigated by comparing two future periods (2021–50 and 2071–2100) to a historical period (1971–2000) for each of these scenarios. The ensemble simulates a robust increase (change larger than twice the ensemble sample standard deviation) in the frequency of occurrence of unstable environments (lifted index ≤ −2) across central and south-central Europe in the RCP8.5 scenario in the late twenty-first century. This increase coincides with the increase in lower-tropospheric moisture. Smaller, less robust changes were found until midcentury in the RCP8.5 scenario and in the RCP4.5 scenario. Changes in the frequency of situations with strong (≥15 m s−1) deep-layer shear were found to be small and not robust, except across far northern Europe, where a decrease in shear is projected. By the end of the century, the simultaneous occurrence of latent instability, strong deep-layer shear, and model precipitation is simulated to increase by up to 100% across central and eastern Europe in the RCP8.5 and by 30%–50% in the RCP4.5 scenario. Until midcentury, increases in the 10%–25% range are forecast for most regions. A large intermodel variability is present in the ensemble and is primarily due to the uncertainties in the frequency of the occurrence of unstable environments.

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Grigory Nikulin, Colin Jones, Filippo Giorgi, Ghassem Asrar, Matthias Büchner, Ruth Cerezo-Mota, Ole Bøssing Christensen, Michel Déqué, Jesus Fernandez, Andreas Hänsler, Erik van Meijgaard, Patrick Samuelsson, Mouhamadou Bamba Sylla, and Laxmi Sushama

Abstract

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 ~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.

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Hussen Seid Endris, Philip Omondi, Suman Jain, Christopher Lennard, Bruce Hewitson, Ladislaus Chang'a, J. L. Awange, Alessandro Dosio, Patrick Ketiem, Grigory Nikulin, Hans-Jürgen Panitz, Matthias Büchner, Frode Stordal, and Lukiya Tazalika

Abstract

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 Niño–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.

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Jonathan Spinoni, Paulo Barbosa, Edoardo Bucchignani, John Cassano, Tereza Cavazos, Jens H. Christensen, Ole B. Christensen, Erika Coppola, Jason Evans, Beate Geyer, Filippo Giorgi, Panos Hadjinicolaou, Daniela Jacob, Jack Katzfey, Torben Koenigk, René Laprise, Christopher J. Lennard, M. Levent Kurnaz, Delei Li, Marta Llopart, Niall McCormick, Gustavo Naumann, Grigory Nikulin, Tugba Ozturk, Hans-Juergen Panitz, Rosmeri Porfirio da Rocha, Burkhardt Rockel, Silvina A. Solman, Jozef Syktus, Fredolin Tangang, Claas Teichmann, Robert Vautard, Jürgen V. Vogt, Katja Winger, George Zittis, and Alessandro Dosio

Abstract

Two questions motivated this study: 1) Will meteorological droughts become more frequent and severe during the twenty-first century? 2) Given the projected global temperature rise, to what extent does the inclusion of temperature (in addition to precipitation) in drought indicators play a role in future meteorological droughts? To answer, we analyzed the changes in drought frequency, severity, and historically undocumented extreme droughts over 1981–2100, using the standardized precipitation index (SPI; including precipitation only) and standardized precipitation-evapotranspiration index (SPEI; indirectly including temperature), and under two representative concentration pathways (RCP4.5 and RCP8.5). As input data, we employed 103 high-resolution (0.44°) simulations from the Coordinated Regional Climate Downscaling Experiment (CORDEX), based on a combination of 16 global circulation models (GCMs) and 20 regional circulation models (RCMs). This is the first study on global drought projections including RCMs based on such a large ensemble of RCMs. Based on precipitation only, ~15% of the global land is likely to experience more frequent and severe droughts during 2071–2100 versus 1981–2010 for both scenarios. This increase is larger (~47% under RCP4.5, ~49% under RCP8.5) when precipitation and temperature are used. Both SPI and SPEI project more frequent and severe droughts, especially under RCP8.5, over southern South America, the Mediterranean region, southern Africa, southeastern China, Japan, and southern Australia. A decrease in drought is projected for high latitudes in Northern Hemisphere and Southeast Asia. If temperature is included, drought characteristics are projected to increase over North America, Amazonia, central Europe and Asia, the Horn of Africa, India, and central Australia; if only precipitation is considered, they are found to decrease over those areas.

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