OBSERVATIONAL CONSTRAINTS ON THE FORMATION OF TYPE IA POLAR STRATOSPHERIC CLOUDS

On January 11, 1989 during AASE I, Type Ia polar stratospheric clouds (PSCs) were observed over a vast area in the Arctic. Synoptic scale te mperature histories for this flight are obtained using the Goddard tra jectory model. Mesoscale temperature fluctuations (MTF) are divided in to two major categories of background and lee waves. The statistics of background MTFs are derived from measurements by the microwave temper ature profiler aboard the ER-2 aircraft. A forecast model is used to d etermine the occurrence of lee waves. The MTFs are then superimposed u pon the Goddard synoptic scale temperatures. Statistical correlations between temperature histories and Type Ia PSC observations indicate th at MTFs can not be solely responsible for the formation of Type Ia PSC s in the stratosphere. Instead, on a synoptic scale, the exposure time of air masses to temperatures below the nitric acid trihydrate (NAT) frost point for similar to 1 day or more should be the main criteria f or predicting the occurrence of Type Ia clouds in the stratosphere.