A student in our department, Peter Ehrengruber, recently finished his diploma thesis in which he studied back trajectories of air masses reaching Mt Kenya. Back trajectories are used quite heavily in studies of air pollution to determine the sources of pollutants measured at a point in space and time. An atmospheric model is used to compute the 3D trajectory path of a parcel of air reaching your site of interest over a specified interval back in time. The goal of this study was to investigate the sources of moisture in the atmosphere during snowfall events at the summit of Mt Kenya. The Indian Ocean is by far the main source of moisture to east Africa, but we wondered if particular zones were more important for creating conditions that resulted in snowfall at the summit.
Back trajectories were computed using the METEX tool, which you can use yourself online here: http://db.cger.nies.go.jp/metex/. We used data from the 0.5° resolution Japanese Meteorological Association Global Spectral Model, so the trajectories we compute are trajectories in this model, not necessarily in the real world and as the model does not represent the terrain of east Africa in a very realistic way so it cannot capture all the interaction with the land surface. We computed trajectories 5 days back in time at hourly intervals from 12:00 UTC (15:00 LT) at an elevation of 250m above the model surface at the latitude and longitude of the summit of Mt Kenya.
As Mt Kenya is on the equator and the earth is spinning it is no surprise that the dominant airflow is from the east, but the cluster analysis separates out 5 clear groupings. The switch from SE to NE airflow is related to the seasonal migration of the intertropical convergence zone (ITCZ) which dictates the pattern of the surface trade winds.
This can be seen more clearly when we look at the trajectories on days when snowfall was recorded on Mt Kenya. In the figure below trajectories for snowfall events are shown, color coded by season: red = Oct-Dec (which is the regional “short rains” season) and yellow is Mar-May (which is the regional “long rains” season). Both of the rainy seasons occur when the ITCZ crosses over the region. The sampled months of Jan/Feb (orange) and Jun/Jul (black) are generally dry months in the region.
Precipitation can occur at all times of year, but during the wet seasons the Indian Ocean south of the equator in the region north of Madagascar appears to have been the most common origin of air masses during precipitation events during the studied period.