Radiative impact of mineral dust on monsoon precipitation variability over West Africa

The radiative forcing of dust and its impact on precipitation over the West Africa monsoon (WAM) region is simulated using a coupled meteorology and aerosol/chemistry model (WRF-Chem). During the mon- soon season, dust is a dominant contributor to aerosol optical depth (AOD) over West Africa. In the control simulation, on 24-h domain average, dust has a cool- ing effect ( 6.11 W m 2 ) at the surface, a warming effect (6.94 W m 2 ) in the atmosphere, and a relatively small TOA forcing (0.83 W m 2 ). Dust modifies the surface energy bud- get and atmospheric diabatic heating. As a result, atmo- spheric stability is increased in the daytime and reduced in the nighttime, leading to a reduction of late afternoon pre- cipitation by up to 0.14 mm/h (25%) and an increase of noc- turnal and early morning precipitation by up to 0.04 mm/h (45%) over the WAM region. Dust-induced reduction of diurnal precipitation variation improves the simulated diur- nal cycle of precipitation when compared to measurements. However, daily precipitation is only changed by a relatively small amount ( 0.17 mm/day or 4%). The dust-induced change of WAM precipitation is not sensitive to interannual monsoon variability. On the other hand, sensitivity simula- tions with weaker to stronger absorbing dust (in order to rep- resent the uncertainty in dust solar absorptivity) show that, at the lower atmosphere, dust longwave warming effect in the nighttime surpasses its shortwave cooling effect in the daytime; this leads to a less stable atmosphere associated with more convective precipitation in the nighttime. As a result, the dust-induced change of daily WAM precipitation varies from a significant reduction of 0.52 mm/day ( 12%, weaker absorbing dust) to a small increase of 0.03 mm/day (1%, stronger absorbing dust). This variation originates from the competition between dust impact on daytime and night-