Cirrus properties from TOVS Path-B

The TOVS (TIROS-N Operational Vertical Sounder) Path-B data set provides atmospheric temperature and water vapor profiles as well as cloud properties over the globe, from 1987 to 1995. Monthly averages of cloud properties are now available at the website http://www.ara.lmd.polytechnique.fr. The relatively high spectral resolution of TOVS yields reliable cirrus properties, day and night. First we present average cirrus properties and their seasonal and diurnal variations, in comparison to ISCCP. A reanalysis which will extend the present data set back to 1979 and up to now is at present in preparation. Diurnal cycles of high cloud amount over South America and Africa during boreal winter show strong similarities, whereas the Indonesian islands show a seasonal and diurnal behavior strongly influenced by the surrounding ocean. Deeper precipitation systems over Africa than over South America do not seem to be directly reflected in the horizontal coverage and large-scale effective emissivity of high clouds. For semi-transparent cirrus (0.7 < vis. opt. thickness < 3.8) mean effective ice crystal diameter (De) and ice water path (IWP) have been retrieved within the framework of the European project CIRAMOSA. These clouds constitute about half of all high clouds. Their global averages (from 60°N to 60°S) of De and IWP are 55 micron and 30 gm-2, respectively. Recently, upper tropospheric relative humidity with respect to ice has been determined for situations of clear sky and thin cirrus. Distributions for clear sky are slightly narrower than those of thin cirrus, but few clear sky scenes are also ice saturated in agreement with other observations.