Observational constraints on the formation of Type Ia polar stratospheric clouds

On January 11, 1989 during AASE I, Type la polar stratospheric clouds (PSCs) were observed over a vast area in the Arctic. Synoptic scale temperature histories for this flight are obtained using the Goddard trajectory model. Mesoscale temperature fluctuations (MTF) are divided into two major categories of background and lee waves. The statistics of background MTFs are derived from measurements by the microwave temperature profiler aboard the ER-2 aircraft. A forecast model is used to determine the occurrence of lee waves. The MTFs are then superimposed upon the Goddard synoptic scale temperatures. Statistical correlations between temperature histories and Type la PSC observations indicate that MTFs can not be solely responsible for the formation of Type la PSCs 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 ∼ 1 day or more should be the main criteria for predicting the occurrence of Type la clouds in the stratosphere.