GEOPHYSICAL RESEARCH LETTERS, VOL. 17, NO. 4, PAGES 537-540, MARCH SUPPLEMENT 1990 THE POLAR STRATCSPHERIC CLOUD EVENT OF JANUARY 24, 1989, PART !. MICROPHYSICS L. R. Poole I , S. Solomon 2, B. W. Gandrud 3, K. A. Powell •, J. E. Dye 3, R. L. Jones 5, and D. S. McKenna 5 •. Rapid adiabatic cooling induced by meteorological events. In this paper, the synoptic forcing led to polar stratospheric cloud microphysical/photochemical trajectory model IPSC) formation on January 24, 1989, at altitudes described by Jones et al. (this issue, a) is used sampled by the ER-2 aircraft. Particle charac- to study the evolution of PSCs during the January teristics measuredby the Forward Scattering 24 event, and modelcalculations are compared to Spectrometer Probe (FSSP) on the ER-2 were observations from the FSSP (Forward Scattering compared to those calculated using a theoretical Spectrometer Probe) instrument (Dye et al., this PSC microphysicsmodel. Although calculations issue) mounted aboard the ER-2. A companion weresensitive to local changes in cooling rate, paper (Jones et al., this issue, b) discusses the generally favorable agreementwas found, that in influence of the January 24 PSC on gas-phase particle surface area being especially important photochemistry. In particular, the temporal since this parameter dictates heterogeneous chemi- development of the cloud apparently led to a •al rates. The overall model performance HUg- diurnal asymmetry in reactive chlorine manifested gests that the current approach for simulating in the ClO observations, providing direct obser- Type 1 (nitric acid trihydrate) PSC formation vational evidence for photochemical perturbations under rapid cooling conditions is well founded induced by heterogeneous processes. and can be used to study the effects of hetero- geneous chemistryon stratospheric composition. Air Parcel Origin and Model Initialization Introduction The January 24 ER-2 flight proceededfrom the AASE base at Sola Airfield up the Norwegian Airborne lidar sightings of polar strato- coast and then due north toward Spitsbergen, spheric clouds (PSCs) were common (McCormick et where the aircraft turned before flying back ai., this issue) during the 1989 Airborne Arctic along the sametrack. Potential temperatures (8) Stratospheric Expedition (AASE), due largely to at flight level ranged from =420K-480K except very cold local stratospheric temperatures during a dive near 75øN on the return leg. The (Naujokat et al., 1989). Although many of these origin of air parcels moving into the area is PSCs were at altitudes above the operational illustrated in Figure ! by trajectories on the range of the ER-2 aircraft, an important excep- •=470K surface. Flow had been nearly zonal for %ion occurred on January 24, when clouds extended the previous 5 days except for a modest nortkerly as low as =17 km (and poleward to at least 84øN). component during the final half day. Parcels In situ measurements of PSCswere made from the were ascending as they approached the Norwegian ER-2 on this day at latitudes from =67øN to 79øN. coast and had cooled adiabatically by =12-15K These PSCs were formed by rapid adiabatic during the previous 24 hours, resulting in PSC cooling inducedby polewardpenetration of a mid- formation. Model calculations suggest that the troposphericridge. Diagnosis of a similar event leading edge of the PSC at 470K (and on lower 8 in the Antarctic by McKenna et at. (1989) showed surfaces) expanded to the south and west during that the sudden appearance of local "mini2holes" the day due to this rapid cooling. Hence, in total ozone is caused largely by reversible parcels intercepting the afternoon ER-2 flight lifting of the isentropic surfaces rather than leg from =67-71øN were thus subjected to more actual ozone depletion. However, rapid flow of extensive PSCexposure than their morning counter- air through quasi-stationary PSCs formed during parts, from which might be expected marked %hese events may indeed produce significant per- differences in parcel chemical composition. •urbations in stratospheric composition due to For detailed simulations of the January 24 he'•erogeneous chemical processing. In particu- PSC encounter, we used trajectories for which 8 lar• the associated enhancements in chlorine was adjusted by latitude to match ER-2 levels. monoxide (C10) maylead to subsequent ozone deple- Final temperatures were also adjusted where •ion. Therefore, an essential elementin under- necessary (by 1-2K) to match ER-2 observations. s%anding the depletion process is an accurate Cooling rates were in general agreement with moael of PSC formation during such transient local rates of temperature change indicated by radiosonde data and were not adjusted. The '--¾NAsA At•spheric Sciences Division, Langley variation with latitude of initial H20mixing Research Center ratio (4.5-5.0 ppmv) was based on ER-2 meas- 2NOAA Aeronomy Laboratory urements by Kelly et al. (this issue), while that 3Nationa! Center for Atmospheric Research of initial HNO3 mixing ratio was based on •STSystems Corporation reported values of NOy* (as inferred from ER-2 5United Kingdom Meteorological Office measurements of N20; Kawa eta!., this issue) and a nominal HNO3/NOy fraction of 0.75 (with excep- This paperis not subject to U.S. copyright. Pub- tions noted by Jones et a!., this issue, b). lis,•hed in !990 by the American Geophysical Union. Saturation HNO3 vaporpressures werebased on the nitric acid trihydrate (NAT) relationship of •per n•r 90•330. Hanson and Mauersberger(!988) . 537