Abstract
AbstractThe Hadley circulation and tropical rain belt are dominant features of African climate. Moist convection provides ascent within the rain belt, but must be parameterized in climate models, limiting predictions. Here, we use a pan-African convection-permitting model (CPM), alongside a parameterized convection model (PCM), to analyze how explicit convection affects the rain belt under climate change. Regarding changes in mean climate, both models project an increase in total column water (TCW), a widespread increase in rainfall, and slowdown of subtropical descent. Regional climate changes are similar for annual mean rainfall but regional changes of ascent typically strengthen less or weaken more in the CPM. Over a land-only meridional transect of the rain belt, the CPM mean rainfall increases less than in the PCM (5% vs 14%) but mean vertical velocity at 500 hPa weakens more (17% vs 10%). These changes mask more fundamental changes in underlying distributions. The decrease in 3-hourly rain frequency and shift from lighter to heavier rainfall are more pronounced in the CPM and accompanied by a shift from weak to strong updrafts with the enhancement of heavy rainfall largely due to these dynamic changes. The CPM has stronger coupling between intense rainfall and higher TCW. This yields a greater increase in rainfall contribution from events with greater TCW, with more rainfall for a given large-scale ascent, and so favors slowing of that ascent. These findings highlight connections between the convective-scale and larger-scale flows and emphasize that limitations of parameterized convection have major implications for planning adaptation to climate change.
Highlights
The tropical rain belt over Africa is a zone of heavy rainfall that migrates seasonally about the equator between the Northern and Southern Hemispheres
Suggest that CMIP5 projections may overestimate the increase in mean rainfall and underestimate the intensification of wet and dry extremes over tropical Africa
Resolving convection gives similar spatial patterns of projected changes in annual mean rainfall, but the representation of convection remains a source of uncertainty, with correlations of projected annual changes in rainfall between explicit and parameterized convection of 0.60 and 0.77
Summary
The tropical rain belt over Africa is a zone of heavy rainfall that migrates seasonally about the equator between the Northern and Southern Hemispheres. The regions of convection within the mesoscale cloud systems include intense convective updrafts of high MSE air that extend from the boundary layer to the upper troposphere (‘‘hot towers’’; Riehl and Malkus 1958). Ascent within these hot towers accounts for the profile of vertical velocity within the cloud systems with its peak velocity in the mid- to upper troposphere (Williams and Gray 1973)
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