Control Mechanisms of Water Vapour in the Upper Troposphere: Large Scale Subsidence in Regions of Tropical Cb-Convection

Abstract

Water vapour measurements from the MOZAIC (Measurement of Ozone and Water Vapour by Airbus In-Service Aircraft) aircraft platform in combination with satellite observations of sea surface temperature (SST) and cloud top temperature (CTT) have been used to investigate the control mechanisms of upper tropospheric humidity (UTH) content in regions of tropical cumulonimbus (Cb) convection over the equatorial Atlantic Ocean. We developed a concept, whereby MOZAIC air samples of convective origin are linked with the location of convection by back tracing with 3-D trajectories in combination with satellite observations of the cloud top temperature along the trajectories. At this location the SST can be related to the water vapour measured by MOZAIC. The concept has been successfully employed for the tropical Atlantic region using a five year MOZAIC record of UTH observations (from 1996 to 2000). The relative humidity in the outflow of Cb convection decreases in time much more slowly than subsidence under cloud-free conditions would predict. Under the assumption that there is no mixing along the trajectory and the specific humidity stays constant, the derived mean cooling rates in the outflow region subject to subsidence are 0.55 K per day, 1.24 K per day and 0.22 K per day over the periods 0–15 h, 15 h-24 h and 24–48 h, respectively, between convection and sampling at the aircraft. The results of the first and third period are more than a factor two lower compared with clear sky radiative transfer model results. The most plausible explanation is either a humidity or a energy source originated from the possible presence of local sub-visible cirrus clouds or by entrainment of moist air through sub-scale mixing processes.