High lapse rates in AIRS retrieved temperatures in cold air outbreaks

The Atmospheric Infrared Sounder (AIRS) experiment , orbiting on NASA’s EOS Aqua spacecraft since May 2002, provides 300,00 daily retrievals of temperature profiles, along with profiles of humidity and minor gases, and, surface and cloud properties. These retrievals have a nominal vertical resolution of 1 km, with 50 km spacing between adjacent retrievals in a scan swath 1500 km wide. A preliminary analysis of AIRS temperature retrievals taken during January 2003 reveals extensive areas of very high lapse rates in the lowest kilometer of the atmosphere. These areas are found predominantly east of North America over the Gulf Stream, and, of East Asia over the Kuroshio Current. Accompanying the high lapse rates are low air temperatures, large sea-air temperature differences, and low relative humidities. Imagery from a Visible / Near Infrared instrument on the AIRS experiment shows accompanying clouds. These lines of evidence all point to shallow convection in the bottom layer of a cold air mass overlying warm water, with overturning driven by heat flow from ocean to atmosphere. Crucial to this interpretation are the high lapse rates retrieved by AIRS; many profiles are superadiabatic between the surface and 925 hPa. We determine the validity of the AIRS lapse rates in these putative shallow convective regions by comparing our retrieved temperatures with radiosondes from oceanic and coastal stations in Japan and adjacent islands.