The montane circulation on Kilimanjaro, Tanzania, and its influence on the summit ice-fields: comparison of surface mountain climate with equivalent reanalysis parameters

We compare surface climate (temperature and moisture) measured on an hourly basis at ten elevations on Kilimanjaro with equivalent observations in the free atmosphere from NCEP/NCAR reanalysis data, for September 2004–July 2008. On the lower forested slopes the mountain surface is consistently cooler and moister than the atmospheric boundary layer. In contrast, temperatures and moisture on the higher slopes above treeline (~ 3000 m) are decoupled from the free atmosphere, showing substantial heating/cooling by day/night and import of moisture up from lower elevations during daylight hours. The mountain is universally warmer than the background atmosphere at 1500 EAT, the sparsely vegetated upper slopes acting as the focus for the most intense heating. The persistent vapour pressure excesses (>5 mb) in the forest zone move upslope during daylight and subside downslope at night. Strong seasonal contrasts are shown in the vigour of this process, the resultant mountain thermal circulation and its consequences. The synoptic forcing of this process (as represented by flow indices developed from reanalysis wind components), although evident, is relatively weak. This means that upslope flow from the forest zone is an important supplementary source of moisture for the upper slopes of the mountain and that free-air variability, although important, alone cannot account for all the variability in the summit moisture regime. Long-term ice retreat at the summit of Kilimanjaro therefore is most likely to be influenced by changes in local land-use as well as more regional free-air changes.