Estimating Precipitation Recycling in Sudan using a BulkModel

Abstract

The objective of this study was to estimate precipitation recycling ratio (PRR) in Sudan. The numerical
bulk model of Eltahir and Brass was used. It was adopted to fit the moisture flux in the domain. The data was
retrieved from the European Re-Analysis (ERA-40) of the European Centre for Medium and long-range Weather
Forecast (ECMWF) archive. Surface evaporation, horizontal wind speed and specific humidity for 11 levels
between the ground surface and the tropopause were used. The grid resolution was 2.5° 2.5° latitude by
longitude. The period was the rainy season for the years 1984, 1988 and 2001. These years represent a dry, a wet
and a normal year respectively. By calculating the vertical integrals for moisture fluxes the troposphere was
reduced to a 2-dimansional domain. PRR values indicated a strong coupling between the ground surface
characteristics and the rain patterns. Precipitation recycling ratio was found to be 19% in July, 21% in August and
27% in September. The contribution of the evaporation in South Sudan to the local rains exceeded 25%. It
approached zero north of latitude 15°N. While evaporation from South Sudan was recycled, evaporation from
North Sudan was not entrained in rain-producing systems due to air subsidence. This raised the question about
the benefit of storing water in the desert.