COMPUTATIONS OF DIABATIC DESCENT IN THE STRATOSPHERIC POLAR VORTEX

A radiation model, together with National Meteorological Center temper ature observations, was used to compute daily net heating rates in the northern hemisphere (NH) for the Arctic late fall and winter periods of both 1988-1989 and 1991-1992 and in the southern hemisphere (SH) fo r the Antarctic fall and winters of 1987 and 1992. The heating rates w ere interpolated to potential temperature (theta) surfaces between 400 K and 2000 K and averaged within the polar vortex, the boundary of wh ich was determined by the maximum gradient in potential vorticity. The averaged heating rates were used in a one-dimensional vortex interior descent model to compute the change in potential temperature with tim e of air parcels initialized at various theta values, as well as to co mpute the descent in log pressure coordinates. In the NH vortex, air p arcels which were initialized at 18 km on November 1, descended about 6 km by March 21, while air initially at 25 km descended 9 km in the s ame time period. This represents an average descent rate in the lower stratosphere of 1.3 to 2 km per month. Air initialized at 50 km descen ded 27 km between November 1 and March 21. In the SH vortex, parcels i nitialized at 18 km on March 1, descended 3 km, while air at 25 km des cended 5-7 km by the end of October. This is equivalent to an average descent in the lower stratosphere of 0.4 to 0.9 km per month during th is 8-month period. Air initialized at 52 km descended 26-29 km between March 1 and October 31. In both the NH and the SH, computed descent r ates increased markedly with height. The descent for the NH winter of 1992-1993 and the SH winter of 1992 computed with a three-dimensional trajectory model using the same radiation code was within 1 to 2 km of that calculated by the one-dimensional model, thus validating the vor tex averaging procedure. The computed descent rates generally agree we ll with observations of long-lived tracers, thus validating the radiat ive transfer model.