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Nicole P. M. van Lipzig
,
Erik van Meijgaard
, and
Johannes Oerlemans

Abstract

A regional atmospheric climate model with a horizontal grid spacing of 55 km has been used to simulate the Antarctic atmosphere during an austral summer period. ECMWF reanalyses were used to force the atmospheric prognostic variables from the lateral boundaries. Sea surface temperatures and the sea ice mask in the model were prescribed from observations. Parameterizations of the physical processes were taken from the ECHAM4 general circulation model. Before applying the model to Antarctic conditions, several adjustments had been made to the original code. In particular, a better correspondence between model output and measurements was accomplished by 1) the use of a fixed value of 0.8 for the surface albedo rather than applying an albedo that linearly rises with surface temperature and 2) the use of the volumetric heat capacity and the thermal diffusivity of snow rather than employing the values for ice.

The model is evaluated for the period 14–19 January 1993 (P1) on the basis of an extensive dataset compiled from measurements made at a site (Svea) in Dronning Maud Land. This dataset contains boundary layer temperature and specific humidity profiles, snow temperatures, and surface heat fluxes. The surface fluxes were obtained from direct measurements combined with an energy balance model. The atmospheric temperature profiles simulated at the grid points corresponding most closely to Svea are in good agreement with the measured profiles, although the model slightly overestimates the vertical temperature gradient. The model probably underestimates the turbulent transport of heat and moisture to atmospheric layers above roughly 200 m. At Svea a cloud cover of less than 0.5 octas was observed during P1. The model overestimates the cloud cover, which results in an underestimation of shortwave and an overestimation of longwave radiative fluxes at the surface. The simulated values for the net radiative fluxes, the heat flux into the snow, and the turbulent heat fluxes correspond within 4 W m−2 to the fluxes that were inferred from measurements.

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Nicole P. M. van Lipzig
,
Erik van Meijgaard
, and
Johannes Oerlemans

Abstract

The sensitivity of the surface mass balance of the Antarctic ice sheet to a change in temperature and a change in the sea ice extent is studied with a regional atmospheric climate model (RACMO) using a horizontal grid spacing of 55 km. The model is driven at its lateral boundaries by the reanalyses from the European Centre for Medium-Range Weather Forecasts. Sea ice extent and sea surface temperature are prescribed from observations. A control integration is performed for the 5-yr period 1980–84. In a 5-yr sensitivity run, the model is forced by a 2-K increase in temperature at the sea surface and at the lateral boundaries of the model domain, and a reduction in the sea ice extent. The relative humidity at the lateral boundaries is kept constant.

The calculated surface mass balance of the grounded Antarctic ice is found to increase by 30% due to the 2-K warming and the retreat of the sea ice. This value is two to three times as large as previous estimates, which were based on simplified atmospheric models and on statistical relations between the surface temperature and the surface mass balance.

Additional sensitivity runs show that applying the forcing throughout the atmosphere in the lateral boundary zone has a more significant effect than applying the forcing at the sea surface, especially for the interior of the ice sheet. If only an increase in the sea surface temperature or a retreat of the sea ice is prescribed, the increase in temperature and specific humidity is restricted to the lowest 2 km of the atmosphere above the ocean. Sensitivity runs with forcings in the range of −5 to +10 K indicate that the commonly used assumption stating that the surface mass balance responds in proportion to the change in continental saturation specific humidity at the inversion height is an oversimplification.

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Bart J. J. M. van den Hurk
and
Erik van Meijgaard

Abstract

Land–atmosphere interaction at climatological time scales in a large area that includes the West African Sahel has been explicitly explored in a regional climate model (RegCM) simulation using a range of diagnostics. First, areas and seasons of strong land–atmosphere interaction were diagnosed from the requirement of a combined significant correlation between soil moisture, evaporation, and the recycling ratio. The northern edge of the West African monsoon area during June–August (JJA) and an area just north of the equator (Central African Republic) during March–May (MAM) were identified. Further analysis in these regions focused on the seasonal cycle of the lifting condensation level (LCL) and the convective triggering potential (CTP), and the sensitivity of CTP and near-surface dewpoint depressions HIlow to anomalous soil moisture. From these analyses, it is apparent that atmospheric mechanisms impose a strong constraint on the effect of soil moisture on the regional hydrological cycle.

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Wouter Greuell
,
Erik van Meijgaard
,
Nicolas Clerbaux
, and
Jan Fokke Meirink

Abstract

This study compared the Regional Atmospheric Climate Model version 2 (RACMO) with satellite data by simultaneously looking at cloud properties and top-of-atmosphere (TOA) fluxes. This study used cloud properties retrieved from Spinning Enhanced Visible and Infrared Imager (SEVIRI) data and TOA shortwave and longwave outgoing radiative fluxes measured by one of the Geostationary Earth Radiation Budget (GERB) sensors. Both SEVIRI and GERB resolve the diurnal cycle extremely well with 96 images per day. To test the physical parameterizations of the model, RACMO was run for a domain-enclosing Africa and part of the surrounding oceans. Simulations for July 2006, forced at the lateral boundaries by ERA-Interim reanalyses, show generally accurate positioning of the various cloud regimes but also some important model–observation differences, which the authors tried to reduce by altering model parameterizations. These differences are as follows: 1) TOA albedo differences in clear-sky regions like the Sahara and southern Africa. These differences were considerably reduced by prescribing the surface albedo from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data. 2) A considerable overestimation of outgoing longwave radiation within the continental ITCZ caused by the fact that modeled cirrus clouds are far too thin. 3) Underestimation by the model of cloud cover, condensed water path and albedo of the stratocumulus fields off the coast of Angola. The authors reduced these underestimations by suppressing the amount of turbulent mixing above the boundary layer, by prescribing droplet radii derived from SEVIRI data, and by assuming in-cloud horizontal homogeneity for the radiation calculations. 4) Overestimation by the model of the albedo of the trade wind cumulus fields over the Atlantic Ocean. This study argues that this overestimation is likely caused by a model overestimation of condensed water path. In general, the analyses demonstrate the power of the simultaneous evaluation of the TOA fluxes and cloud properties.

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Bart J. J. M. van den Hurk
,
Wim G. M. Bastiaanssen
,
Henk Pelgrum
, and
Erik van Meijgaard

Abstract

In this study, a simple method is described and tested for deriving initial soil moisture fields for numerical weather prediction purposes using satellite imagery. Recently, an algorithm was developed to determine surface evaporation maps from high- and low-resolution satellite data, which does not require information on land use and synoptic data. A correction to initial soil moisture was calculated from a comparison between the evaporation fields produced by a numerical weather prediction model and the satellite algorithm. As a case study, the method was applied to the Iberian Peninsula during a 7-day period in the summer of 1994. Two series of short-term forecasts, initialized from a similar initial soil moisture field, were run in parallel: a control run in which soil moisture evolved freely and an experimental run in which soil moisture was updated daily using the simple assimilation procedure. The simple assimilation resulted in a decrease of the bias of temperature and specific humidity at 2-m height during the daytime and a small decrease of the root-mean-square error of these quantities. The results show that the surface evaporation maps, derived from the satellite data, contain a signal that may be used to assimilate soil moisture in numerical weather prediction models.

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Bart van den Hurk
,
Martin Hirschi
,
Christoph Schär
,
Geert Lenderink
,
Erik van Meijgaard
,
Aad van Ulden
,
Burkhardt Rockel
,
Stefan Hagemann
,
Phil Graham
,
Erik Kjellström
, and
Richard Jones

Abstract

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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Jan T. M. Lenaerts
,
Michiel R. van den Broeke
,
Jan M. van Wessem
,
Willem Jan van de Berg
,
Erik van Meijgaard
,
Lambertus H. van Ulft
, and
Marius Schaefer

Abstract

This study uses output of a high-resolution (5.5 km) regional atmospheric climate model to describe the present-day (1979–2012) climate of Patagonia, with a particular focus on the surface mass balance (SMB) of the Patagonian ice fields. Through a comparison with available in situ observations, it is shown that the model is able to simulate the sharp climate gradients in western Patagonia. The southern Andes are an efficient barrier for the prevalent atmospheric flow, generating strong orographic uplift and precipitation throughout the entire year. The model suggests extreme orographic precipitation west of the Andes divide, with annual precipitation rates of >5 to 34 m w.e. (water equivalent), and a clear rain shadow east of the divide. These modeled precipitation rates are supported qualitatively by available precipitation stations and SMB estimates on the ice fields derived from firn cores. For the period 1979–2012, a slight atmospheric cooling at upper ice field elevations is found, leading to a small but insignificant increase in the ice field SMB.

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Jan Melchior van Wessem
,
Carleen H. Reijmer
,
Willem Jan van de Berg
,
Michiel R. van den Broeke
,
Alison J. Cook
,
Lambertus H. van Ulft
, and
Erik van Meijgaard

Abstract

The latest polar version of the Regional Atmospheric Climate Model (RACMO2.3) has been applied to the Antarctic Peninsula (AP). In this study, the authors present results of a climate run at 5.5 km for the period 1979–2013, in which RACMO2.3 is forced by ERA-Interim atmospheric and ocean surface fields, using an updated AP surface topography. The model results are evaluated with near-surface temperature and wind measurements from 12 manned and automatic weather stations and vertical profiles from balloon soundings made at three stations. The seasonal cycle of near-surface temperature and wind is simulated well, with most biases still related to the limited model resolution. High-resolution climate maps of temperature and wind showing that the AP climate exhibits large spatial variability are discussed. Over the steep and high mountains of the northern AP, large west-to-east climate gradients exist, while over the gentle southern AP mountains the near-surface climate is dominated by katabatic winds. Over the flat ice shelves, where katabatic wind forcing is weak, interannual variability in temperature is largest. Finally, decadal trends of temperature and wind are presented, and it is shown that recently there has been distinct warming over the northwestern AP and cooling over the rest of the AP, related to changes in sea ice cover.

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Reinout Boers
,
Fred Bosveld
,
Henk Klein Baltink
,
Wouter Knap
,
Erik van Meijgaard
, and
Wiel Wauben

Abstract

A dataset of 9 years in duration (2009–17) of clouds and radiation was obtained at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. Cloud radiative forcings (CRF) were derived from the dataset and related to cloud cover and temperature. Also, the data were compared with RCM output. Results indicate that there is a seasonal cycle (i.e., winter, spring, summer, and autumn) in longwave (CRF-LW: 48.3, 34.4, 30.8, and 38.7 W m−2) and shortwave (CRF-SW: −23.6, −60.9, −67.8, and −32.9 W m−2) forcings at CESAR. Total CRF is positive in winter and negative in summer. The RCM has a cold bias with respect to the observations, but the model CRF-LW corresponds well to the observed CRF-LW as a result of compensating errors in the difference function that makes up the CRF-LW. The absolute value of model CRF-SW is smaller than the observed CRF-SW in summer, mostly because of albedo differences. The majority of clouds from above 2 km are present at the same time as low clouds, so the higher clouds have only a small impact on CRF whereas low clouds dominate their values. CRF-LW is a function of fractional cloudiness. CRF-SW is also a function of fractional cloudiness, if the values are normalized by the cosine of solar zenith angle. Expressions for CRF-LW and CRF-SW were derived as functions of temperature, fractional cloudiness, and solar zenith angle, indicating that CRF is the largest when fractional cloudiness is the highest but is also large for low temperature and high sun angle.

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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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