Cloud liquid water path derived from AVHRR data using APOLLO

Cloud liquid water path (LWP) is an important parameter to validate forecasts obtained from circulation models of high spatial resolution. At present, it is not measured on a routine basis. Being part of the Advanced Very High Resolution Radiometer (AVHRR) Processing scheme Over cLoud, Land and Ocean (APOLLO) a parametrization scheme to derive optical thickness from cloud reflectance and, further, the LWP has been adopted and adjusted to AVHRR channel-1 counts. From these counts, top-of-atmosphere bidirectional reflectance data are obtained. Using APOLLO-derived fully cloudy pixels only, the directional hemispherical cloud reflectance is derived to which the parametrization scheme refers. The LWP of each fully cloudy pixel is determined. As a first application, the mean LWP of 63-5 × 63-5 km2 boxes is computed and compared to a 14-hour forecast of the LWP made with the Europa-Modell of the Deutsche Wetterdienst. The location of the clouds seems to be forecasted rather well by the model. However, the LWP computed by the model is higher (by a factor of 4 or 5) than that derived from AVHRR data using APOLLO. A first validation by means of aircraft measurements shows that the APOLLO-derived LWP is too low by about 50 per cent. This reduces the discrepancy to the computed data but the model predicted LWP still seems to be too high by a factor of 3.