Comparison of observed data and high-resolution regional climate simulations on process based modelling

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Tropical and subtropical cloud transitions in weather and climate prediction models: The GCSS/WGNE pacific cross-section intercomparison (GPCI)

International audienceA model evaluation approach is proposed in which weather and climate prediction models are analyzed along a Pacific Ocean cross section, from the stratocumulus regions off the coast of California, across the shallow convection dominated trade winds, to the deep convection regions of the ITCZ-the Global Energy and Water Cycle Experiment Cloud System Study/Working Group on Numerical Experimentation (GCSS/WGNE) Pacific Cross-Section Intercomparison (GPCI). The main goal of GPCI is to evaluate and help understand and improve the representation of tropical and subtropical cloud processes in weather and climate prediction models. In this paper, a detailed analysis of cloud regime transitions along the cross section from the subtropics to the tropics for the season June-July-August of 1998 is presented. This GPCI study confirms many of the typical weather and climate prediction model problems in the representation of clouds: underestimation of clouds in the stratocumulus regime by most models with the corresponding consequences in terms of shortwave radiation biases; overestimation of clouds by the 40-yrECMWFRe-Analysis (ERA-40) in the deep tropics (in particular) with the corresponding impact in the outgoing longwave radiation; large spread between the different models in terms of cloud cover, liquid water path and shortwave radiation; significant differences between the models in terms of vertical cross sections of cloud properties (in particular), vertical velocity, and relative humidity. An alternative analysis of cloud cover mean statistics is proposed where sharp gradients in cloud cover along the GPCI transect are taken into account. This analysis shows that the negative cloud bias of some models and ERA-40 in the stratocumulus regions [as compared to the first International Satellite Cloud Climatology Project (ISCCP)] is associated not only with lower values of cloud cover in these regimes, but also with a stratocumulus-to-cumulus transition that occurs too early along the trade wind Lagrangian trajectory. Histograms of cloud cover along the cross section differ significantly between models. Some models exhibit a quasi-bimodal structure with cloud cover being either very large (close to 100%) or very small, while other models show a more continuous transition. The ISCCP observations suggest that reality is in-between these two extreme examples. These different patterns reflect the diverse nature of the cloud, boundary layer, and convection parameterizations in the participating weather and climate prediction models. © 2011 American Meteorological Society

Distinct influences of large-scale circulation and regional feedbacks in two exceptional 2019 European heatwaves

Two separate heatwaves affected western Europe in June and July 2019, in particular France, Belgium, the Netherlands, western Germany and northeastern Spain. Here we compare the European 2019 summer temperatures to multi-proxy reconstructions of temperatures since 1500, and analyze the relative influence of synoptic conditions and soil-atmosphere feedbacks on both heatwave events. We find that a subtropical ridge was a common synoptic set-up to both heatwaves. However, whereas the June heatwave was mostly associated with warm advection of a Saharan air mass intrusion, land surface processes were relevant for the magnitude of the July heatwave. Enhanced radiative fluxes and precipitation reduction during early July added to the soil moisture deficit that had been initiated by the June heatwave. We show this deficit was larger than it would have been in the past decades, pointing to climate change imprint. We conclude that land-atmosphere feedbacks as well as remote influences through northward propagation of dryness contributed to the exceptional intensity of the July heatwaveWe acknowledge the E-OBS dataset from http://www.uerra.eu (EU-FP6), the Copernicus Climate Change Service (https://cds.climate.copernicus.eu), the data providers in ECA&D (https://www.ecad.eu), the NCEP Reanalysis and GISTEPM data provided by the NOAA/OAR/ESRL PSL (Boulder, Colorado, USA, from their web site at https://psl.noaa.gov/), and the climate reconstructions provided by the World Data Center for Paleoclimatology (https://www.ncdc.noaa.gov). This work was partially supported by national funds through FCT (Fundação para a Ciência e a Tecnologia, Portugal): P.M.S. thanks project HOLMODRIVE - North Atlantic Atmospheric Patterns influence on Western Iberia Climate: From the Lateglacial to the Present (PTDC/CTA-GEO/29029/2017) and R.M.T. acknowledges project FireCast (PCIF/GRF/0204/2017). This work was also partially funded by project INDECIS, which is part of ERA4CS, an ERA-NET initiated by JPI Climate, with co-funding by the European Union (Grant 690462). C.O. acknowledges funding from the Ramón y Cajal Programme of the Spanish Ministerio de Economía y Competitividad under grant RYC- 2014-15036. We also acknowledge support from STEADY (CGL2017- 21 83198-R), project funded by the Spanish Ministerio de Economía, Industria y Competitividad, and JEDIS (RTI2018-096402-B-I00), project funded by the Spanish Ministerio de Ciencia, Innovación y Universidades. P.M.M.S. wishes to acknowledge the LEADING project (PTDC/CTA-MET/28914/2017). The IDL authors (P.M.S, R.M.T and P.M.M.S) would like to acknowledge the financial support FCT through project UIDB/50019/2020 – Instituto Dom Luiz. Reference

The added value of km-scale simulations to describe temperature over complex orography: the CORDEX FPS-Convection multi-model ensemble runs over the Alps

International audienceThe increase in computational resources has enabled the emergence of multi-model ensembles of convection-permitting regional climate model (CPRCM) simulations at very high horizontal resolutions. An example is the CORDEX Flagship Pilot Study on "Convective phenomena at high resolution over Europe and the Mediterranean", a set of kilometre-scale simulations over an extended Alpine domain. This first-of-its-kind multi-model ensemble, forced by the ERA-Interim reanalysis, can be considered a benchmark dataset. This study uses a recently proposed metric to determine the added value of all the available Flagship Pilot Study hindcast kilometre-scale simulations for maximum and minimum temperature. The analysis is performed using state-of-the-art gridded and station observations as ground truth. This approach directly assesses the added value between the high-resolution CPRCMs against their driving global simulations and coarser resolution RCM counterparts. Overall, models display some modest gains, but also considerable shortcomings are exhibited. In part, these deficiencies can be attributed to the assimilation of temperature observations into ERA-Interim. Although the gains for the use of kilometrescale resolution for temperature are limited, the improvement of the spatial representation of local atmospheric circulations and land-atmosphere interactions can ultimately lead to gains, particularly in coastal areas

The added value of simulated near-surface wind speed over the Alps from a km-scale multimodel ensemble

The advancement of computational resources has allowed researchers to run convection-permitting regional climate model (CPRCM) simulations. A pioneering effort promoting a multimodel ensemble of such simulations is the CORDEX Flagship Pilot Studies (FPS) on “Convective Phenomena over Europe and the Mediterranean” over an extended Alps region. In this study, the Distribution Added Value metric is used to determine the improvement of the representation of all available FPS hindcast simulations for the daily mean near-surface wind speed. The analysis is performed on normalized empirical probability distributions and considers station observation data as the reference. The use of a normalized metric allows for spatial comparison among the different regions (coast and inland), altitudes and seasons. This approach permits a direct assessment of the added value between the CPRCM simulations against their global driving reanalysis (ERA-Interim) and respective coarser resolution regional model counterparts. In general, the results show that CPRCMs add value to their global driving reanalysis or forcing regional model, due to better-resolved topography or through better representation of ocean-land contrasts. However, the nature and magnitude of the improvement in the wind speed representation vary depending on the model, the season, the altitude, or the region. Among seasons, the improvement is usually larger in summer than winter. CPRCMs generally display gains at low and medium-range altitudes. In addition, despite some shortcomings in comparison to ERA-Interim, which can be attributed to the assimilation of wind observations on the coast, the CPRCMs outperform the coarser regional climate models, both along the coast and inland.publishedVersio