CloudSat instrument requirements as determined from ECMWF forecasts of global cloudiness

In the years preceding the launch of CloudSat in 2003, important questions regarding instrument requirements sufficient to fulfilling the mission's science objectives must be addressed. Qualified and useful answers to these questions require in turn a careful simulation strategy whereupon the observing system and modeled environment are represented as realistically as possible. In this paper, we consider the W band (94 GHz) cloud radar minimum detectable signal (MDS) requirement in the context of specified boundary fluxes and in-cloud heating rates. Realistic cloud distribution and water contents from short-range forecasts produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) are converted to attenuated equivalent radar reflectivity to yield full-orbit virtual CloudSat observations. The radiative implications of variable radar MDS are examined using a two-stream radiative transfer model. Simulations show that a MDS of similar to -28 dBZ will detect a fraction of the true cloud field sufficient to reconstruct the instantaneous top-of-atmosphere and surface fluxes to within Clouds and the Earth's Radiant Energy System (CERES) requirements. The results of this analysis form collectively a statement on instrument engineering requirements that is predicated on and hence mappable directly to the physical parameters that define CloudSat science objectives.