CLOUD LAYER THICKNESSES FROM A COMBINATION OF SURFACE AND UPPER-AIR OBSERVATIONS

Cloud layer thicknesses are derived from base and top altitudes by com bining 14 years (1975-1988) of surface and upper-air observations at 6 3 sites in the Northern Hemisphere. Rawinsonde observations are employ ed to determine the locations of cloud-layer top and base by testing f or dewpoint temperature depressions below some threshold value. Surfac e observations serve as quality checks on the rawinsonde-determined cl oud properties and provide cloud amount and cloud-type information. Th e dataset provides layer-cloud amount, cloud type, high, middle, or lo w height classes, cloud-top heights, base heights and layer thicknesse s, covering a range of latitudes from 0 degrees to 80 degrees N. All d ata comes from land sites: 34 are located in continental interiors, 14 are near coasts, and 15 are on islands. The uncertainties in the deri ved cloud properties are discussed. For clouds classified by low-, mid -, and high-top altitudes, there are strong latitudinal and seasonal v ariations in the layer thickness only for high clouds. High-cloud laye r thickness increases with latitude and exhibits different seasonal va riations in different latitude zones: in summer, high-cloud layer thic kness is a maximum in the Tropics but a minimum at high latitudes. For clouds classified into three types by base altitude or into six stand ard morphological types, latitudinal and seasonal variations in layer thickness are very small. The thickness of the clear surface layer dec reases with latitude and reaches a summer minimum in the Tropics and s ummer maximum at higher latitudes over land, but does not vary much ov er the ocean. Tropical clouds occur in three base-altitude groups and the layer thickness of each group increases linearly with top altitude . Extratropical clouds exhibit two groups, one with layer thickness pr oportional to their cloud-top altitude and one with small (less than o r equal to 1000 m) layer thickness independent of cloud-top altitude.