Response of marine stratocumulus to African savannah burning

Episodic plumes of smoke from African savannah burning are transported offshore of the western coast of subtropical Africa. Beneath this elevated layer of smoke is a persistent deck of bright marine stratocumulus clouds. The semi-direct effect of the absorbing smoke aerosols on the stratocumulus clouds is studied using AIRS, OMI, MODIS, and other NASA A-train datasets. The liquid water path of clouds beneath high levels of smoke is systematically greater than that of clouds occurring during low smoke conditions regardless of the SST underlying the boundary layer cloud. The thickening of clouds during periods of smoke is coincident with radiative warming at the 700 hPa level immediately above the boundary layer revealed in AIRS tropospheric temperature retrievals. These results lend support to large-eddy simulations showing a thickening of clouds in response to the semi-direct effect for smoke overlaying clouds. An empirical estimate of the radiative forcing of smoke and clouds is also presented.