Numerical modeling of natural and anthropogenic impacts on the regional climate of Kilimanjaro (Tanzania)

Glacier recession on Kilimanjaro has been linked to reduction in precipitation and cloudiness largely due to large scale changes in tropical climate. However, local terrain interaction with large scale flow field is an important modulator of precipitation and cloudiness at the top of Kilimanjaro and land use change has the potential to impact conditions at the peak. This study examines a typical annual cycle of orographic cloud and precipitation formation over Kilimanjaro utilizing Regional Atmospheric Modeling System (RAMS) at 1km grid spacing to simulate evolution of atmospheric conditions over this region for the period of July 2007 through June 2008. The RAMS simulation is analyzed to determine processes that modulate climate at the peak, and additional RAMS experiments are conducted to determine how these processes are impacted if current land cover is replaced by pristine and deforested landscapes.;Four flow regimes are found in RAMS experiment: southerly (JJA), east-southeasterly (SON), northeasterly (DJF) and southeasterly (MAM). Cloud and precipitation at the top of Kilimanjaro is most impacted during the northeasterly and southeasterly regimes. Flow oriented parallel to topographical gradient along the rim of a concave terrain feature is the major contributor to high precipitation events at the peak. Terrain interactions with large scale flow contributes to ∼60% and ∼20% of the total precipitation at lower (2000-4000m) and higher elevations (>5000m), respectively, with the influence of terrain generated thermal circulations extending to the peaks of Kilimanjaro. Land cover change indeed impact cloud and precipitation formation at peak, with reforestation and total deforestation both increasing precipitation at the peak. The processes though which mountain top conditions are affected differs, with reforestation increasing the strength of the terrain generated circulation. Deforestation leads to an increase in surface wind speeds, low-level convergence which in turn enhances precipitation at higher elevations. However, deforestation reduces precipitation at lower elevations, while reforestation produces a general increase in precipitation at all elevations. This study finds that regional climate forcing due to land cover change impacts cloud formation and precipitation at Kilimanjaro and is a factor to be considered in the interpretation of observed changes, namely glacier recession.