Onset, extent, and duration of dehydration in the Southern Hemisphere polar vortex

Satellite observations of water vapor and aerosol extinction along with tem perature trajectory calculations are analyzed for the Southern Hemisphere w inter of 1992 in order to determine the onset, extent, and duration of dehy dration within the polar vortex. Our investigation uses measurements of wat er vapor from the Microwave Limb Sounder (MLS) and aerosol extinction from the Cryogenic Limb Array Etalon Spectrometer (CLAES), both on board the Upp er Atmosphere Research Satellite (UARS). Evidence of persistent ice cloud f ormation, supported by temperature statistics obtained from air parcel traj ectories, suggests that the onset of the dehydration process occurs between late June and early July. By late August-early September water vapor deple ted areas within the vortex no longer coincide with high aerosol extinction s, indicating that severe dehydration has occurred with the irreversible re moval of water vapor over vast areas. Areas with depleted levels of water v apor, below the prewinter values, persist well into November. Evidence for dehydration is found on potential temperature surfaces from 420 K (the lowe r limit of the MLS measurements) to 520 K (approximately 16 to 22 km). The horizontal extent of the dehydrated area at 465 K encompasses up to 35% of the total vortex area equatorward of 80 degreesS. A comparison of CLAES aer osol extinction measurements and model calculations of aerosol extinction s uggests an average ice particle number concentration and size of 10(-2)-10( -3) cm(-3) and 10-30 mum, respectively. We show that the difference between the timing of the onset of dehydration found here and that in a recent ana lysis of Polar Ozone and Aerosol Measurement III (POAM) observations can be explained by the latitudinal sampling pattern of the POAM instrument.