Comment on "A reexamination of the 'stratospheric fountain' hypothesis" byA. E. Dessler

Dessler (1998) analyzed 60,000 radiosonde profiles to re-examine an analysi s done nearly 20 years earlier by Newell and Gould-Stewart (1981) (hereafte r: NGS). Contrary to NGS, Dessler finds that the mean tropical tropopause s aturation mixing ratio (SMR) is sufficient to explain stratospheric dryness and that there is no need to assume a seasonal or regional preference for water vapor entering the stratosphere. However, in using the SMR to compute the water vapor amount, he implicitly assumes that the air is saturated, s ince only under this condition are temperature and water vapor physically c onnected. If the air is not saturated, the average computed by Dessler will overestimate the amount of water crossing the tropical tropopause ([H2O],) , while supersaturation and the presence of ice particles will underestimat e [H2O],. These processes, which are likely to have different regional and temporal distributions, may have fortuitously canceled each other in Dessle r's analysis. NGS studied the distribution of tropopause temperatures and f ocused on the regions and seasons, in which dehydration is more likely to t ake place. Recent studies indicate that the tropical tropopause has been co oling over the last 25 years and show that the years used by Dessler have t he coldest tropical tropopause temperatures. Thus his conclusion may have b een different if other years had been studied. These differing viewpoints e mphasize the need for a detailed understanding of the stratospheric dehydra tion mechanism.