Estimating precipitable water over West Africa from surface equivalent potential temperature and related potential instability indices

Abstract

Monthly mean aerological data from 1960–1970 for West Africa have been used to develop two models for estimating precipitable water ( W) from surface equivalent potential temperature ( θ e) and related potential instability indices ( I) in the northern and southern zones of West Africa. The models are of the form 1n W = aθ e + b, and W = cI + d for the northern and southern zones respectively. W may be replaced by total precipitable water ( W T), precipitable water between 1000-700 hPa levels ( W 7), and precipitable water between 1000-500 hPa levels ( W 5). I may be replaced by I 7 and I 5 defined as the difference between the equivalent potential temperature values at 1000 and 700 hPa levels and 1000 and 500 hPa levels respectively. The constants a, b, c and d assume different values for W = W T, W 7, W 5 and I = I 7, I 5. Calculated W T and actual W T correlated more strongly in the northern zone ( r = 0.9936) than in the southern zone ( r = 0.8384). The model for the southern zone may be of little predictive use because it needs the vertical structure of moisture to compute I. The model for the northern zone, however, may be useful on the fringes of the desert regions of West Africa since it predicts a column variable from a surface variable.