Temporal precipitation variability versus altitude on a tropical high mountain: Observations and mesoscale atmospheric modelling

The Occurrence of maximum precipitation amounts (P-max) on tropical high mountains at mid elevations is well documented (spatial variability), but little is known about the temporal variability of precipitation in different elevation zones. The present study targets this issue by combining various types of observations on and around Kilimanjaro (East Africa, 5895 in a.s.l.) with mesoscale atmospheric modelling. In observations, the vertical moisture profiles of air masses approaching the mountain clearly differ during 'normal' and 'significant' daily precipitation events on Kilimanjaro's summit, while model experiments illustrate that the elevation zone of P-max is shifted upslope substantially during the latter events. Large (small) differences in precipitation therefore appear in the dry (moist) high-elevation (mid- and low-elevation) zones of the mountain. This space-time pattern is evident in observations of seasonal precipitation variability on Kilimanjaro and on nearby Mount Kenya for anomalous wet seasons, which are controlled by the frequency of 'significant' daily events. The upslope shift of P-max in the model is favoured by dynamical factors (stronger air ascent and higher static instability of clouds), microphysical processes (greater efficiency in converting condensate to precipitation over high-elevation terrain), and related feedback mechanisms (stronger latent heating). Beyond the seasonal scale, tropical climate change also entails changed frequencies of dry- and moist-air advection, which is demonstrated for the Kilimanjaro region over 1979-2006. Thus, long-term changes of precipitation are probably most drastic in the dry summit zones of tropical high mountains. Copyright (C) 2009 Royal Meteorological Society