Stratospheric Water-Vapor Upper Limits Inferred From Upper-Air Observations: Part I. Northern Hemisphere

Abstract

Observational data from the lower stratosphere are examined to determine to what extent an upper limit can be placed on the water-vapor content of that region of the atmosphere, a quantity which is presently in controversy by half an order of magnitude. Presented herein are the details of a systematic search of 37 winter months (November 1964?December 1971) of Northern Hemispheric 30-mb and 50-mb synoptic maps, as well as a search of over 170,000 individual radiosonde 30-mb temperature soundings for the same period. In addition, the results of a search of all available high-latitude Northern Hemispheric meteorological rocket data and a study of the 30-mb climatology for Jan Mayen Island are presented. The results of these studies are compared with the results of a careful literature search, as well as an inquiry to an airline pilot's volunteer organization, for reports of stratospheric cloud observations. The extreme sparsity of such observations, even with allowance for possible obscuring effects of tropospheric cloud cover, leads to the conclusion that water-vapor saturation is seldom reached in the lower stratosphere of the Northern Hemisphere. This conclusion, coupled with the results of the fairly extensive stratospheric observational data mentioned above, appears to be inconsistent with the concept of an average ?moist? (10 ppm or greater) lower stratosphere. From the results of the present investigation, it may be inferred that the average 30-mb and 50-mb water-vapor mixing ratios, for the dates investigated, is almost certainly less than 11 ppm, and probably less than 6?8 ppm. The data appear to constitute reasonable evidence for setting an upper limit on the average 30-mb water-vapor mixing ratio at approximately the 6?8 ppm level for high latitudes in the Northern Hemispheric winter. These 30-mb results fall in between the values quoted by investigators reporting a ?dry? (a few ppm mixing ratio) and ?moist? (≥10 ppm mixing ratio) lower stratosphere, and thus appear to provide a reasonably definitive choice on the question, independent of instrument contamination and calibration problems that have led to discrepancies in stratospheric water-vapor measurements in the past.