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University of Colorado at Boulder (2014)

Investigating daily summertime circulation and precipitation over West Africa with the WRF model : a regional climate model adaptation study

Noble, Erik Ulysses

Titre : Investigating daily summertime circulation and precipitation over West Africa with the WRF model : a regional climate model adaptation study

Auteur : Noble, Erik Ulysses

Université de soutenance : University of Colorado at Boulder

Grade : Doctor of Philosophy (PhD) 2014

Résumé
This dissertation a) evaluates the performance of the NCAR Weather and Research Forecasting (WRF) model as a West African Sahel regional-atmospheric model and b) investigates the utility of regional modeling to meeting user-needs. This work represents the beginning of an effort to adapt the model as a regional climate model (RCM) for the Sahel. Two independent studies test WRF sensitivity to 64 configurations of alternative parameterizations in a series of September simulations. In all, 104 12-day simulations during 11 consecutive years are examined. Simulated daily and mean circulation results are validated against NASA’s Modern-Era Retrospective Analysis for Research and Applications (MERRA) and NCEP Reanalysis-2. Modeled daily and total precipitation results are validated against NASA’s Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) data. Particular attention is given to westward-propagating precipitation maxima associated with transient African Easterly Waves (AEWs). A wide range of 700-hPa vorticity and daily precipitation validation scores demonstrates the influence of alternative parameterizations. The best WRF performers achieve circulation correlations against reanalysis of 0.40-0.60 and realistic amplitudes of spatiotemporal variability for the 2006 focus year, but they get time-longitude precipitation correlations (against GPCP) of between 0.35-0.42. A parallel-benchmark-simulation by the NASA Regional Model-3 (RM3) achieves higher correlations but less realistic spatiotemporal variability. The largest favorable impact on WRF vorticity and precipitation validation is achieved by selecting the Grell-Devenyi cumulus convection scheme, resulting in higher correlations against reanalysis and GPCP than simulations using the Kain-Fritch convection. Other parameterizations have less-obvious impact. A comparison of reanalysis circulation against two NASA-radiosonde stations confirms that both reanalyses represent observations well enough to validate WRF results. A rain-gauge comparison does the same for GPCP and TRMM.

Mots Clés : Health and environmental sciences, Earth sciences, African Easterly Waves, Numerical weather prediction, Precipitation, Regional climate model, West Africa, Atmospheric circulation

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Page publiée le 9 décembre 2014, mise à jour le 19 novembre 2018