Reconsideration of the Cause of Dry Air in the

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Aug 1, 1995 - Jim Russell contributed extensive- ly to this paper. Bob Harwood, Alan O'Neill, Anne Par- daens and Peter Stott made many helpful comments.
GEOPHYSICAL RESEARCH LETTERS, VOL. 22, NO. 15, PAGES 2025-2028, AUGUST 1, 1995

Reconsideration of the cause of dry air in the southern middle latitude stratosphere Philip W. Mote Department of Meteorology,University of Edinburgh

Abstract.

A previous comparison of data from the

Halogen Occultation Experiment (HALOE) and of output from the NCAR Community Climate Model

HALOE

Data:

Update

HALOE [Russellet al. 1993]is a solaroccultationin-

(CCM2) in australspringoverstatedthe effectof de- strument which, becauseof the inclination of the UARS hydration in the southern polar vortex on lower strato- orbit, samplesthe atmosphereat two differentlatitudes sphericwater vapor in part due to shortcomingsin the (oneat sunrise,one at sunset)approximately15 times changes HALOE retrieval algorithm and in part to remnants eachper day. The meanlatitudeof observation with time as the UARS orbit precesses, so that zonal of the initially dry stratosphere in the CCM2 run. A averages as shown below represent a sweep from 60øN new HALOE retrieval and a longer CCM2 run both on Sept. 21, 1992 to 80øS on Oct. 15, 1992. HALOE show more water vapor than previously, but HALOE still showslessin the southern hemispherethan in the is self-calibratingsince the signal being processedto northern in September-October 1992. A "dried-air" retrieve constituentmixing ratio is a ratio of solar ratracer in the CCM2 suggeststhat polar dehydration diance transmitted through the atmosphereto the soinfluencesmiddle latitudes only below about 100 hPa, lar radiancemeasuredabovethe atmosphere.Any long in instrumentcharacteristics (e.g. by optiand seasonalvariationsof HALOE H20 imply no influ- termchanges

caldegradation or changein gain)areof noconsequence (J.M. RussellIII, personalcommunication, 1994). MHRB usedversion8 (VS) HALOE data, Pierceet al. [1994]usedV9 and V16 data, and Bithell et al. [1994]usedV16 data, but V17 is now availableand

ence north of 50øS.

Introduction

In the southernpolar stratospherefrom May to Sep- I use it in this paper. V17 incorporatesseveral imtember, very low temperaturespermit the formationof provementsto the retrieval algorithm which will be dispolar stratosphericclouds,which promote photochem- cussed fully elsewhere ("A Summaryof HALOE Water ical destruction of ozone. When these cloud particles Vapor Observations",J. M. RussellIII et al., destined fall out, they leave behind dehydrated and denitrified for JGR). The largestchanges in H20 are about 20% air whichmay be identifiedat lowerlatitudes[Kelly et at the 100 hPa level and diminish to