Coupling of Clouds and Tropospheric Relative Humidity in the Tropical Western Atlantic: Insights From Multisatellite Observations

다국어 초록 (Multilingual 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 midtroposphere 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 Intertropical Convergence Zone.
The heat engine of the climate is the atmospheric water cycle. Understanding the relationships between clouds and the atmospheric relative humidity is crucial to improve our understanding of climate variability. Here we focus on the diurnal fluctuations of vertical profiles of tropospheric relative humidity and collocated clouds in the tropical Western Atlantic trade winds region. Profiles of water vapor and distribution of clouds, classified by altitudes, are obtained from spaceborne instruments, and the other environmental characteristics such as sea surface and air temperatures are extracted from a weather model. Their joint diurnal variations are examined. Winter and summer seasons are compared. The diurnal water vapor cycle of water vapor evolves from the bottom to the top of the atmosphere and is strongly associated with cloud types, with a seasonal modulation. During daytime a strong drying is associated with the downward dry air masses moving downward. The diurnal cycle of upper‐level cloudiness plays a critical role in inhibiting daytime drying by shielding the midtroposphere from this downward transport. The upper‐level moisture feeds upper‐level clouds and modulates their diurnal lifetime.


The daytime drying of the midtroposphere is associated with the large‐scale subsidence and presumably with the diurnal pulses of ITCZ
The diurnal cycle of upper‐level cloudiness is essential for tropospheric moistening by inducing local lifting
The upper‐level moisture feeds upper‐level clouds and modulates their diurnal cycle