Mesoscale Convective Systems As a Source of Water Vapor in the Tropical Tropopause Transition Layer

Abstract:

Clouds and moisture in the tropical tropopause transition layer (TTL) are composited as a function of distance from mesoscale convective systems (MCSs) identified in observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E). Anvil clouds spread horizontally from the MCS in the upper troposphere and TTL. Cloud-Aersol and Infrared Pathfinder Satellite Observations (CALIPSO) profiles indicate enhanced cloudiness above 150 hPa out to 15 degrees for MCSs over the Maritime Continent and the equatorial eastern Pacific, while MCSs over land areas such as South America and South Africa have less extensive anvils.

Ice water content (IWC) from the Microwave Limb Sounder (MLS) is maximum near the MCS center and decreases rapidly with distance, particularly in oceanic MCSs. Enhanced IWC above the climatological-mean is observed as high as 120 hPa. MLS water vapor is also enhanced near the MCS center but decreases more gradually with distance, with values above the climatological mean observed up to 120 hPa and outward beyond 15 degrees in most regions. These results suggest that MCSs are a major source of water vapor in the TTL.

Water vapor concentrations are suppressed at 100 hPa above the MCSs, consistent with weak subsidence above cloud top.

Composites of European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) wind and vertical velocity are consistent with the satellite-based observations, indicating strong upward motion near the MCS centers and outflow from the MCSs in the TTL.