Survey of Cirrus and atmospheric properties from TOVS Path-B: Natural variability and impact of air traffic on cirrus coverage

The TOVS Path-B data set provides atmospheric temperature and water vapor profiles as well as cloud properties over the globe, from 1987 to 1995. Their relatively high spectral resolution yields reliable cirrus properties, day and night. First, we present average cirrus properties and their seasonal and diurnal variations. A reanalysis which will extend the present data set back to 1979 and up to now is at present in preparation. Upper tropospheric relative humidity with respect to ice has been computed from specific humidity over an atmospheric layer between 300 and 100 hPa by using the retrieved temperature profiles. Upper tropospheric relative humidity distributions of clear sky are slightly narrower than those of thin cirrus, but few clear sky scenes are also ice saturated in agreement with other observations. An example of an analysis combining information on upper tropospheric relative humidity and cirrus coverage is given in the investigation of the impact of air traffic on cirrus formation. Contrails form when the hot and humid exhaust gases from the combustion of fuels by an aircraft mix with the ambient, cold atmosphere. Their persistence depends on upper tropospheric relative humidity. Trends of seasonal mean effective high cloud amount have been analyzed in regions with high and low air traffic density. In regions with especially high air traffic density, a significantly stronger increase of effective high cloud amount is found for situations with sufficiently cold and humid air masses favorable for contrails than for all situations in general.