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Downscaling GISS ModelE boreal summer climate over Africa

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Abstract

The study examines the perceived added value of downscaling atmosphere–ocean global climate model simulations over Africa and adjacent oceans by a nested regional climate model. NASA/Goddard Institute for Space Studies (GISS) coupled ModelE simulations for June–September 1998–2002 are used to form lateral boundary conditions for synchronous simulations by the GISS RM3 regional climate model. The ModelE computational grid spacing is 2° latitude by 2.5° longitude and the RM3 grid spacing is 0.44°. ModelE precipitation climatology for June–September 1998–2002 is shown to be a good proxy for 30-year means so results based on the 5-year sample are presumed to be generally representative. Comparison with observational evidence shows several discrepancies in ModelE configuration of the boreal summer inter-tropical convergence zone (ITCZ). One glaring shortcoming is that ModelE simulations do not advance the West African rain band northward during the summer to represent monsoon precipitation onset over the Sahel. Results for 1998–2002 show that onset simulation is an important added value produced by downscaling with RM3. ModelE Eastern South Atlantic Ocean computed sea-surface temperatures (SST) are some 4 K warmer than reanalysis, contributing to large positive biases in overlying surface air temperatures (Tsfc). ModelE Tsfc are also too warm over most of Africa. RM3 downscaling somewhat mitigates the magnitude of Tsfc biases over the African continent, it eliminates the ModelE double ITCZ over the Atlantic and it produces more realistic orographic precipitation maxima. Parallel ModelE and RM3 simulations with observed SST forcing (in place of the predicted ocean) lower Tsfc errors but have mixed impacts on circulation and precipitation biases. Downscaling improvements of the meridional movement of the rain band over West Africa and the configuration of orographic precipitation maxima are realized irrespective of the SST biases.

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Acknowledgments

The research was performed with the support of NASA Cooperative Agreement NNX11AR63A. TRMM data were acquired using the GES-DISC Interactive Online Visualization and Analysis Infrastructure (Giovanni), part of NASA’s Goddard Earth Sciences (GES) Data and Information Services Center (DISC). GPCP data were obtained from NASA/GSFC/MAPL. ERA-I data were provided by the European Center for Medium Range Weather Forecasts, facilitated by the CORDEX project. NCEP/DOE reanalysis 2 data used in the study were obtained from the NOAA-ESRL Physical Sciences Division, Boulder CO online from www.esrl.noaa.gov/psd. The authors gratefully acknowledge the cooperation of GISS colleagues, Dr. K. Lo for accessing ModelE data and Dr. I. Aleinov for discussions about ModelE. Many constructive suggestions from two anonymous reviewers are also gratefully acknowledged.

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Authors and Affiliations

  1. NASA/Goddard Institute for Space Studies, 2880 Broadway, New York, NY, 10025, USA

    Leonard M. Druyan & Matthew Fulakeza

  2. Department of Applied Physics and Math, Columbia University, 2880 Broadway, New York, NY, 10025, USA

    Leonard M. Druyan

  3. Center for Climate Systems Research, Columbia University, 2880 Broadway, New York, NY, 10025, USA

    Matthew Fulakeza

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  1. Leonard M. Druyan
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  2. Matthew Fulakeza
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Correspondence to Leonard M. Druyan.

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This paper is a contribution to the special issue on West African climate decadal variability and its modeling, consisting of papers from the West African Monsoon Modeling and Evaluation (WAMME) and the African Multidisciplinary Monsoon Analyses (AMMA) projects, and coordinated by Yongkang Xue, Serge Janicot, and William Lau.

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Druyan, L.M., Fulakeza, M. Downscaling GISS ModelE boreal summer climate over Africa. Clim Dyn 47, 3499–3515 (2016). https://doi.org/10.1007/s00382-015-2880-y

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