COMPARISON OF CEILOMETER, SATELLITE, AND SYNOPTIC MEASUREMENTS OF BOUNDARY-LAYER CLOUDINESS AND THE ECMWF DIAGNOSTIC CLOUD PARAMETERIZATIONSCHEME DURING ASTEX

Cloud fraction is a widely used parameter for estimating the effects o f boundary-layer cloud on radiative transfer. During the Atlantic Stra tocumulus Transition Experiment (ASTEX) during June 1992, ceilometer a nd satellite-based measurements of boundary-layer cloud fraction were made in the subtropical North Atlantic, a region typified by a 1-2 km deep marine boundary layer with cumulus clouds rising into a broken st ratocumulus layer underneath an inversion. Both the diurnal cycle and day-to-day variations in low-cloud fraction are examined. It is shown that ECMWF low cloudiness analyses do not correlate with the observed variations in cloudiness and substantially underestimate the mean low cloudiness. In these analyses, the parameterization of low cloud fract ion is primarily based on the inversion strength. A comparison of ECMW F analyses and ASTEX soundings (most of which were assimilated into th e analyses) shows that the thermodynamic structure of the boundary lay er and the inversion strength are well represented (with some small bu t significant systematic biases) in the analyses and preserved (again with some biases) in 5-day forecasts. However, even when applied to th e actual sounding the ECMWF low cloud scheme cannot predict the observ ed day-to-day variations or the diurnal cycle in low cloud. Other diag nostic schemes based on lower tropospheric stability, cloud-top entrai nment instability, boundary-layer depth, and vertical motion do equall y poorly. The only successful predictor of low cloud fraction from sou nding information is the relative humidity in the upper part of the bo undary layer.