Relationship between Rainfall anomalies in eastern and Southern Africa and El-Nino Southern Oscillation. Ibid

Abstract

Meteorological elements are observed to display diurnal and/ or semi diurnal variability. Therefore the ability of the UK 11-level GCM to correctly simulate these diurnal variations of weather elements may be used as a measure of how realistic the model is compared to the real atmosphere. The model is forced by the diurnal variation of insolation. In order to study the diurnal cycle of the GCM, the simulated April 1986 sub-daily fields were kept, sampled at every 6 hours. All the data were instantaneous values, apart from rainfall, snowfall, evaporation and sensible heat fluxes which were each accumulated over each time step (10 minutes) for the 6 hours period. The variables were then averaged over the model month (30 days). The mean daily fields were computed separately for 0600 GMT, 1200 GMT, 1800GMT and 0000 GMT. The diurnal variation of three variables, namely; surface temperature, rainfall and wind were examined. The simulated surface temperature indicated similar diurnal characteristic as the observed. Thus there was an increase in temperature from morning to afternoon, followed by a general decrease at night. The simulated diurnal variation of the precipitation, particularly the afternoon maximum, was consistent with the general observed diurnal variation of precipitation over many areas in East Africa. The simulated wind anomaly vector was observed to rotate clockwise in the northern hemisphere and anticlockwise in the southern hemisphere. Such a diurnal behaviour of the wind has been observed in the real atmosphere. This characteristic is associated with the zonal movement of the region of maximum insolation (low pressure) from east to west.