A five-day back trajectory climatology for Rukomechi research station (northern Zimbabwe) and the impact of large-scale atmospheric flows on concentrations of airborne coarse and fine particulate mass

Abstract

The climatology of slow and fast air mass transport towards Rukomechi research station in northern Zimbabwe (16.1°S, 29.4°E) is examined through an analysis of 5-day kinematic back trajectories arriving at 1180 m above ground level (~850 hPa) for the period 1994-99. The trajectories are computed daily by the HYSPLIT 4 model using the National Oceanic and Atmospheric Administration's re-analysed wind fields as input. Objective classification of trajectories into different flow regimes is done using a non-hierarchical cluster algorithm that is applied to all the trajectories at once, to examine the temporal and spatial characteristics. The synoptic-scale atmospheric circulation associated with each cluster was also studied. Four major transport corridors and seven large-scale flows were found to provide an indication of source regions of the air masses and transport pathways influencing Rukomechi. The dominant transport features include (1) the late dry season to wet season eastern air flows that contribute 35% of the total flow and which are driven by an anticyclone that wraps around the subcontinent and stretches into the Mozambique Channel, (2) the late wet season to dry season southeastern pathway that accounts for 44% of the total flow and is associated with the South Atlantic anticyclone and the tropical depression in the Indian Ocean, (3) the fast (11 m s-1) dry season southern flow (contributes 8%) that is driven by a continental anticyclone over South Africa coupled to a South Atlantic anticyclone, and (4) the north-northwesterly flow that contributes 6% to the total flow and is associated with the Intertropical Convergence Zone. A further, very slow (1.8 m s-1) flow regime, consisting of regionally re-circulated air results from regional differential heating at the surface. Concentrations of particulate mass, measured during 1994-2000 at the Rukomechi site, are shown to be significantly correlated with the occurrence of those seven large-scale flows, which have been identified by objective trajectory classification. High particulate mass concentrations are associated only with air masses carried along the fast easterly flow, the slow southeasterly flow and the southerly flow (dry season flows); low particulate mass concentrations are exclusively found with wet season flows.