Application of the NCAR regional climate model to eastern Africa: 1. Simulation of the short rains of 1988

Abstract

Abstract. The National Center for Atmospheric Research (NCAR) Regional Climate Model (RegCM2) is employed to investigate the physical mechanisms that govern the October-December rains over eastern Africa. The model employs the Mercator conformal projection, with a domain of 5580 km x 5040 km centered at 31øE, 4øS, and a horizontal grid point spacing of 60 km. The simulation period is October-December 1988, and the model initial and lateral boundary conditions are taken from ECMWF reanalysis. A number of month-long simulations have been conducted to optimize various parameterizations of the model which include the following factors: cumulus convection, moisture parameterization, radiative transfer formulation, surface processes, boundary layer physics, and the lateral boundary conditions. The model was successfully customized over eastern Africa. The model simulates the large-scale circulation characteristics over the region as well as local features such as the dominant precipitation maxima, the Turkana low-level jet, and the diurnal reversal in the lake/land breeze circulation over Lake Victoria. Several model deficiencies are also identified. They include a negative rainfall bias over the western portions of the domain and the Kenya Highlands and a temperature bias over the tropical forest regions. Systematic analysis of surface water budget reveals that evapotranspiration is a major sink in the water budget over the regions where precipitation is moderate or small, while the role of runoff and drainage becomes important over the regions where precipitation is abundant. The model simulations also suggest that during the short-rains season, the large-scale circulation anomalies play the most important role in shaping the precipitation anomalies. IntroductionOver eastern Africa the large-scale tropical climate regimes, which include several major convergence zones, are superimposed upon regional factors asso'ciated with complex terrain, extreme contrasts in vegetation type, the existence of large inland lakes, and the proximity of the Indian Ocean. As a result, the climate patterns over eastern Africa are extraordinarily complex and require relatively high resolution to be adequately resolved.Recently, a collaborative effort among the North Carolina State University, the National Center for Atmospheric Research (NCAR), the University of Nairobi, and the Drought Monitor Center (DMC) in Kenya has been initiated aimed at the development of a Seasonal Climate Prediction System (SCPS) for eastern Africa. The SCPS is based on the use of the NCAR Regional Climate Model (RegCM) driven by initial and lateral boundary conditions obtained from general circulation model (GCM) simulations. The nesting technique has been extensively used for long-term regional simulations and has been shown to capture regional patterns of precipitation, Model DescriptionThe model used in this study is the NCAR secondgeneration regional climate model (hereinafter referred to as NCAR RegCM2), which is described in detail by Giorgi ½t al. Des...

Variability of East African rainfall based on multiyear Regcm3 simulations

“…over land areas only). The year 1988 short rains season is treated here as being representative of near normal (climatological) condition (Ininda 1994(Ininda , 1998Sun et al 1999a. The ability of RegCM3 to reproduce month-to-month variations in the rainfall amounts as well as the meridional migration (associated with ITCZ position) of regions of rainfall maxima as the season progresses are reasonably reproduced.…”

“…bulk Richardson number), etc. (see full list in Sun et al, 1999a) were adjusted ('tuned') until a combination that gave fairly robust simulation of different aspects of the regional climate system was obtained. However, the key modifications included the configuration of the 1D lake submodel to represent Lake Victoria; with the lake bathymetry derived from digitized global large lakes dataset.…”

“…Consequently, there is often need for comprehensive customization/evaluation of these models in their simulation of climates over different regions to realize optimum performance. The issue of RCM customization has been addressed in a number of studies Seth and Giorgi, 1998;Sun et al, 1999a;Denis et al, 2002;Jones et al, 1995;Mearns, 1999, Laprise et al, 2000 among others). Still, many regional climate modeling applications have often involved shorter model integration periods (a few months to a season) that do not provide sufficient scope for evaluating the downscaling ability of RCMs over a wide spectrum of climate regimes.…”

The Tokar Gap Jet: Regional Circulation, Diurnal Variability, and Moisture Transport Based on Numerical Simulations

“…In this study, a small percentage of simulated back trajectories consistently traced back through the Congo River basin and toward the Gulf of Guinea on the West African side. Sun et al (1999) found similar instances of ''Congo moist air'' originating from the SHH and influencing the flow in the Turkana/East African region in regional climate simulations. An examination of both Atlantic and Indian Ocean influences upon the TG and the TGJ variability over decadal time scales with attention to established climate signals would do much more to clarify its role in the global climate system.…”

“…1). This jet is essential to the advection of Indian Ocean moisture inland and to the accumulation of moisture in the EH/EAH region (Viste and Sorteberg 2013b;Sun et al 1999). The jet flows northwestward and emerges from the Turkana valley near point B in Fig.…”