Abstract
The study explored the ability of four cumulus parameterization schemes (CPSs) from Weather and Research Forecasting model (WRF) to simulate mean rainfall patterns, number of rainy days (NRD) and vertically integrated moisture flux (VIMF) during the composite of wet years for the core rainfall seasons of March-April-May (MAM; 1989, 1998 and 2012) and Octo-ber-November-December (OND; 1997, 2006 and 2015) seasons. The CPSs used were Kain-Fritsch (KF), Kain-Fritsch with a moisture-advection based trigger function (KFT), Grell Dévényi (GRELL) and Betts Miller Janjic (BML). The simulations by the GRELL and KF schemes were clearly separated by the dry and wet rainfall gradient in the simulations. For example, the GRELL scheme rainfall simulations were drier over the eastern parts of the region bet-ter. The KF and KFT schemes generated wetter rainfall conditions mainly con-fined to the western parts of the region. The BML scheme simulations were not consistent with the observations. The western and eastern parts of the region were characterized by more and fewer NRD, in both the KF and GRELL schemes. The root mean square error (RMSE) and spatial correlation by KF scheme was 2 mm/day and 0.6. The GRELL scheme however simulated low correlation of 0.45 and RMSE of about 3.0 mm/day over most of the sub-domains. The moisture convergence biases were found to be larger conti-nentally and parts of the nearby Indian Ocean. The persisting rainfall biases constituting of too wet and dry conditions were associated with the KF and GRELL cumulus schemes. The findings from the current study are very funda-mental for the improvement of numerical weather prediction (NWP) tools and cumulus modification processes over the region. The accurate and higher skill rainfall forecasts would provide early warning information for disaster risk re-duction and the related risks on the livelihoods.
Highlights
The contrasts between land and water bodies, together with the undulating topography, interact with the Inter-Tropical Convergence Zone (ITCZ) as it traverses the East Africa (EA) region, resulting into complex but interesting spatial and temporal variations of rainfall over the region
The GREL and Betts Miller Janjic (BML) clearly tended to be drier over southern parts of Tanzania and Central parts Uganda and Kenya; these areas coincided with the observations (Figures 2(a)-(c)) during MAM season
The OND season was correctly reproduced by all the cumulus parameterization schemes (CPSs) over northern parts of Equatorial region except for Grell Dévényi (GRELL) CPSs that tended to be drier throughout (Figures 2(f)-(j))
Summary
The contrasts between land and water bodies, together with the undulating topography, interact with the Inter-Tropical Convergence Zone (ITCZ) as it traverses the East Africa (EA) region, resulting into complex but interesting spatial and temporal variations of rainfall over the region. The unique characteristics of the region imply that the spatial and temporal distribution of rainfall develops smaller-scale features that are not generally well represented by coarse-resolution general circulation models (GCMs) [1]. The dynamic downscaling is an acceptable approach globally to bridge the gap between the regional climate information and the GCM. It is an accepted practice to use the regional climate models (RCMs) for downscaling global models, because of the more comprehensive representation of the important physical processes at a finer resolution [4] [5]. Over the EA region, there are still enormous challenge in applicability of the RCMs to correctly represent the rainfall characteristics arising from the complexity of representing the physical processes of precipitation, including cumulus convection, planetary boundary-layer turbulence, cloud microphysics and radiative forcing [6]. The WRF model has numerous cumulus schemes that are extensible being used by the scientific community without proper sensitivity test over the region
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