Model simulations of rainfall over southern Africa and its eastern escarpment

Zane Dedekind,Francois A Engelbrecht,Jacobus Van Der Merwe

doi:10.4314/wsa.v42i1.13

Abstract

Rainfall simulations over southern and tropical Africa in the form of low-resolution Atmospheric Model Intercomparison Project (AMIP) simulations and higher resolution National Centre for Environmental Prediction (NCEP) reanalysis downscalings are presented and evaluated in this paper. The model used is the conformal-cubic atmospheric model (CCAM), a variable-resolution global atmospheric model. The simulations are evaluated with regards to rainfall totals, spatial distribution, seasonality and inter-annual variability. Since both Global Circulation Models (GCMs) and Regional Climate Models (RCMs) are known to have relatively large biases and shortcomings in simulating rainfall over the steep eastern escarpment of southern Africa and in particular Lesotho, the paper has a focus on evaluating model performance over these regions. It is shown that in the reanalysis simulations the model realistically represents the seasonal cycle in rainfall. However, the AMIP simulations are prone to the model overestimating rainfall totals in spring. The spatial distribution of rainfall is simulated realistically; however rainfall totals are significantly overestimated over the escarpment areas of both southern Africa and East Africa. When nudged within the observed circulation patterns of the reanalysis data, the model is capable of realistically simulating inter-annual rainfall variability over the eastern parts of southern Africa.

Highlights

Rainfall over southern and tropical AfricaSouthern and tropical Africa (STA) are prone to the occurrence of droughts and floods (e.g. Mason and Joubert, 1997; Rouault and Richard, 2003; Lyon and DeWitt, 2012), which constitutes a highly variable climate
A pronounced rainfall maximum is evident over the Great Rift Valley in both the cubic atmospheric model (CCAM)-Atmospheric Model Intercomparison Project (AMIP) and CCAM-National Centre for Environmental Prediction (NCEP) simulations, in relation to the complex topography found in this region (e.g. Zhang et al, 2012)
The model simulations generally represent the seasonality of rainfall over southern Africa well, spring rainfall is significantly overestimated in the CCAM-AMIP simulations, over the eastern parts of southern Africa

Summary

Introduction

Rainfall over southern and tropical AfricaSouthern and tropical Africa (STA) are prone to the occurrence of droughts and floods (e.g. Mason and Joubert, 1997; Rouault and Richard, 2003; Lyon and DeWitt, 2012), which constitutes a highly variable climate. During the austral summer when the ITCZ is displaced to the south of the equator, north-easterly flow of low-level moisture takes place around the Indian Ocean High (IOH) into southern Africa (here defined as Africa south of 15°S), forming a convergence zone in combination with the Angola Low (Reason et al, 2006). This region is referred to as the South Indian Convergence Zone, and is associated with the formation of tropical-temperate cloud bands (Taljaard 1986; Walker and Lindesay, 1989; D’Abreton and Tyson, 1995; Todd et al, 2004, Hart et al, 2010). This results in southern Africa being largely a summer rainfall region, with the exception

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