The statistical distribution law of relative humidity in the global tropopause region

Cloud cleared Microwave Limb Sounder (MLS) data of upper tropospheric humidity are evaluated in order to determine the global statistics of relative humidity with respect to ice, RHi. The evaluation is performed for the 215 hPa level, in order to compare the results with earlier results from the Measurement of ozone by Airbus in-service aircraft (MOZAIC) project. In agreement with the earlier study we find that in the lower-most stratosphere the probability to get a certain value of relative humidity decreases exponentially with the relative humidity. In the Antarctic data class (data south of 55degreesS, mainly winter data) we also find an exponential distribution for RHi but with less steep slope. There is no change in the slope of the exponential distribution function at ice saturation. In the upper troposphere there are corresponding exponential distributions for RHi in ice-supersaturated regions and in subsaturated regions (for 20% < RHi < 100%), however with different slopes, viz. a steeper one in ice-supersaturated regions. In the cases of the tropospheric and antarctic data there is no indication of homogeneous ice nucleation at high humidities of RHi > 150%, and the exponential distributions extend without change of slope up to 180-200%. Such extreme humidity events occur mostly in the tropics and at the edge of Antarctica, and could result from incidental lack of aerosol.