THE EVOLUTION OF THE DEHYDRATION IN THE ANTARCTIC STRATOSPHERIC VORTEX

In 1994 an intensive program of balloon-borne frost point measurements was performed at McMurdo, Antarctica, During this program a total of 19 frost point soundings was obtained between February 7 and October 5, which cover a wide range of undisturbed through strongly dehydrated situations, Together with several soundings from South Pole station between 1990 and 1994, they give a comprehensive picture of the general development of the dehydration in the Antarctic stratospheric vortex, The period of dehydration typically starts around the middle of June, and a rapid formation of large particles leads to a fast dehydration of the vortex. The evaporation of falling particles leads to rehydration layers, which have significantly higher water vapor concentrations than the undisturbed stratosphere, Through the formation of these rehydration layers in the early stages of the dehydration we can estimate a particle fall speed of 1/3 km/d and thus a mean particle size of 4 mu m. Ice saturation was observed over McMurdo in only two cases and only well after the onset of the dehydration. From the inspection of synoptic maps it then follows that a small cold region inside the vortex seems to be sufficient to dehydrate the entire vortex. Above 20 km the dehydration is completed by the end of July, From the descent of the upper dehydration edge we can estimate a mean descent rate inside the vortex of 1.5 km/month. In McMurdo we observed occasional penetration of the vortex edge in cases where the vortex edge was close to McMurdo, however, these cases seem to have little effect on the bulk of the vortex, A sounding from November 3, 1990, at South Pole shows that the dehydration may persist into November and indicates that there is no significant transport into the vortex throughout winter and early spring.