Regional analysis of 3D (three-dimensional) Nephanalysis total cloud amounts for July 1983

Abstract

There is a need for cloud information with which to conduct validation of the International Satellite Cloud Climatology Project (ISCCP) cloud retrievals. . The U.S. Air Force (USAF) nephanalysis is at present the only operational realtime global cloud analysis and the available archive from this offers a potential database for comparison with ISCCP retrievals. The performance of the USAF 3D (three dimensional) Nephanalysis during the month of July 1983, one of the months designated for validation/intercomparison, is examined here on a regional basis. The cloud field retrieved by the USAF Nephanalysis is examined in relation to features unique to the archive such as the inclusion, where appropriate, of conventional observations. A direct comparison with ground-based observations illustrates the effect of conventional data on the archive over a range of time scales. Examination of the Nephanalysis over difficult areas such as polar and desert regions reveals some of the problems pertinent to most current cloud algorithms. Comparisons between 3D Nephanalysis archives separated by 4 years (1979 and 1983) show interannual and synoptic effects and illustrate the evolution of this retrieval algorithm. The most obvious change is due to modifications to the technique employed for spreading conventional observations (surface and aircraft) into the satellite-based analysis. The more recent method results in input from isolated stations being more obvious in the final archive although the values reported are generally close to those in the surrounding satellite retrieval. Bogussing (the imposition of cloud amounts derived, at the time, by an analyst usually from geostationary satellite data) is another feature unique to the USAF Nephanalyses. These two characteristics mean that unsupervised (i.e. purely computer-based) comparison between real-time (RT) Nephanalysis and, for example, ISCCP would be unwise. On the other hand, the year-to-year consistency of the USAF retrieval algorithm, the clear lack of reference to previously determined climatological fields and the time and space resolution and continuity of the Nephanalysis product suggest that the USAF Nephanalysis offer a valuable data set for comparison with ISCCP provided that comparisons are undertaken with care.