LAGRANGIAN TRANSPORT CALCULATIONS USING UARS DATA .1. PASSIVE TRACERS

The transport of passive tracers observed by the Upper Atmosphere Rese arch Satellite is simulated using computed three-dimensional trajector ies of approximate to 100 000 air parcels initialized on a stratospher ic grid, with horizontal winds provided by the United Kingdom Meteorol ogical Office data assimilation system, and vertical (cross isentropic ) velocities computed using a fast radiation code. The conservative ev olution of trace constituent fields is estimated over 20-30-day period s by assigning to each parcel the observed mixing ratio of the long-li ved trace gases N2O and CH4 observed by the Cryogenic Limb Array Etalo n Spectrometer(CLAES) and H2O observed by the Microwave Limb Sounder ( MLS) on the initialization date. Agreement between calculated and obse rved fields is best inside the polar vortex and is better in the Arcti c than in the Antarctic. Although there is not always detailed agreeme nt outside the vortex, the trajectory calculations still reproduce the average large-scale characteristics of passive tracer evolution in mi dlatitudes. In late winter, synoptic maps from trajectory calculations reproduce all major features of the observations, including large ton gues or blobs of material drawn from low latitudes into the region of the anticyclone during February-March 1993. Comparison of lower-strato spheric observations of the CLAES tracers with the calculations sugges ts that discontinuities seen in CLAES data in the Antarctic late winte r lower stratosphere are inconsistent with passive tracer behavior. In the Arctic, and in the Antarctic late winter, MLS H2O observations sh ow behavior that is inconsistent with calculations and with that expec ted for passive tracers inside the polar vortex in the middle-to-upper stratosphere. Diabatic descent rates in the Arctic lower stratosphere deduced from data are consistent with those from the calculations. In the Antarctic lower stratosphere, the calculations appear to underest imate the diabatic descent. The agreement between large-scale features of calculated and observed tracer fields supports the utility of thes e calculations in diagnosing trace species transport in the winter pol ar vortex.