Reconciling ground-based and space-based estimates of the frequency of occurrence and radiative effect of clouds around Darwin, Australia

Description

The main purpose of this work is to investigate whether estimates of the frequency of occurrence of hydrometeors and associated cloud radiative forcing as derived from ground-based and satellite active remote sensing and radiative transfer calculations can be reconciled over a well-instrumented active remote sensing site located in Darwin, Australia. It is found that the ground-based millimeter wavelength cloud radar (MMCR)-micropulse lidar (MPL) combination at the Darwin site does not detect most of the thin cirrus clouds above 10 km height, and that the CloudSat-CALIPSO combination strongly underreports the hydrometeor frequency of occurrence below 1.5 m height. The radiative impact of these differences in cloud frequency of occurrence and the resulting cloud radiative forcing is found to be quite large, especially on the shortwave radiative fluxes. Also, although the general shape of the mean vertical profile of radiative heating rate as derived from ground and satellite radar-lidar instruments and RT calculations agree pretty well, large differences (up to 0.35 Kday-1 for the shortwave and 0.8 Kday-1 for the longwave, above the melting layer) are found at some heights in the troposphere. The general conclusion of this study is that current state-of-the-art ground-based and satellite estimates of the cloud frequency of occurrence, surface downwelling fluxes, top-of-atmosphere upwelling fluxes, and mean profile of radiative heating rate cannot be fully reconciled over Darwin, although there are heights in the troposphere where good agreement is found. Caution should therefore be exercised when evaluating the representation of clouds and cloud-radiation interactions in large-scale models, and limitations of each set of instrumentation should be considered when interpreting model-observations differences.