A distribution law for relative humidity in the upper troposphere and lower stratosphere derived from three years of MOZAIC measurements

摘要

Data from three years of MOZAIC measurementsmade it possible to determine a distribution law for the relative humidity inthe upper troposphere and lower stratosphere. Data amounting to 13.5% of thetotal were obtained in regions with ice supersaturation. Troposphere andstratosphere are distinguished by an ozone concentration of 130 ppbv asthreshold. The probability of measuring a certain amount of ice supersaturationin the troposphere decreases exponentially with the degree of icesupersaturation. The probability of measuring a certain relative humidity in thestratosphere (both with respect to water and ice) decreases exponentially withthe relative humidity. A stochastic model that naturally leads to theexponential distribution is provided. Mean supersaturation in the troposphere isabout 15%, whereas ice nucleation requires 30% supersaturation on the average.This explains the frequency of regions in which aircraft induce persistentcontrails but which are otherwise free of clouds. Ice supersaturated regions are3-4 K colder and contain more than 50% more vapour than other regions in theupper troposphere. The stratospheric air masses sampled are dry, as expected,having mean relative humidity over water of 12% and over ice of 23%,respectively. However, 2% of the stratospheric data indicate icesupersaturation. As the MOZAIC measurements have been obtained on commercialflights mainly between Europe and North America, the data do not provide acomplete global picture, but the exponential character of the distribution lawsfound is probably valid globally. Since water vapour is the most importantgreenhouse gas and since it might enhance the anthropogenic greenhouse effectsvia positive feedback mechanisms, it is important to represent its distributioncorrectly in climate models. The discovery of the distribution law of therelative humidity makes possible simple tests to show whether the hydrologicalcycle in climate models is represented in an adequate way or not.Key words. Atmospheric composition and structure(troposphere · composition and chemistry)