MEASUREMENTS OF RADIATIVE COOLING THROUGH TWO INTENSE BAROCLINIC ZONES IN THE MIDDLE TROPOSPHERE

Abstract

Balloon-borne radiometers were used to measure infrared radiative cooling through two intense, dry baroclinic zones in the middle troposphere. In both cases, a warming peak was associated with the baroclinic zone, with cooling of 2 or 3 deg C per day below and above. In one case, the warming peak was near the base of the zone; in the other, it was centered on the zone, although exact positioning of the peaks is uncertain because of the limited resolution of the radiometer in passing through the thin zones. In one of the two cases, upward radiative flux gradually increased upward above the baroclinic zone, although air temperature decreased upward. A probable explanation is lateral drift of the instrument past orographic clouds. The result calls attention to the limitation of the balloon-borne radiometer to use in situations of horizontally homogeneous surface and atmospheric radiative fluxes, and it indicates that upward radiative flux as measured from earth satellites may not agree closely with measurements from balloon-borne radiometers. The region of radiative destabilization within the baroclinic zone in one of the measurements coincides with the region of potential vorticity decrease as measured in a similar baroclinic zone. A decrease of cooling is noted as the tropopause is approached from below, in agreement with other measurements and with theory.