Coupling of clouds and tropospheric relative humidity in the tropical Western Atlantic: insights from multisatellite observations

Abstract : We investigated the interactions between clouds and moisture at the diurnal scale in the Western Atlantic trade winds region. Profiles of tropospheric relative humidity from the SAPHIR/Megha‐Tropiques sounder are combined with cloud categories obtained from geostationary satellites. In Winter, the mid‐troposphere undergoes strong daytime drying due to air masses coming from the colder upper troposphere. The moistening near the surface triggered by solar radiation precludes the development of low‐level clouds. At night rising moist air in the upper troposphere triggers the formation of high‐altitude clouds and favors their presence. In Summer, daytime high‐altitude clouds shield the solar forcing on the atmosphere and reduce drying from large‐scale subsidence. After sunset, the development of upper tropospheric opaque clouds constitutes a local source of moisture. We argue that modulations of the diurnal cycle of clouds and relative humidity by season may be related to diurnal pulses of the ITCZ.