A key element of GPM GV is its physically based approach to satellite algorithm validation. This approach requires greater insight into the properties and behavior of both ice microphysics and land surface processes. The validation effort for GPM thus devotes significant effort and resources to improve the basic understanding needed for physically based algorithms

One objective of the physically based approach to GPM GV is the development of a Satellite Simulator Model (SSM). The SSM begins with cloud system-resolving model (CRM) output. Based on these data, atmospheric properties are defined consistently for wavelengths applicable the GPM radar and radiometer, as well as ground-based instruments. Radiative transfer calculations are performed to simulate the atmospheric radiation (for the radiometer) and reflectivity (for the radar) arriving at the space-based and ground-based instruments. These data are then acted on by retrieval algorithms to produce inferred atmospheric properties (e.g., rain rate and drop size distribution), which can be compared directly to the input fields. Simulations thus allow for comparison of ground- and space-based observations of the same, idealized atmosphere. Also, radiation calculations at climatologically significant wavelengths can be done on both input and inferred fields. This produces radiative flux errors that stem from the measurement/inversion process.

It is anticipated that SSM work will be conducted in parallel with ongoing European, Japanese, and Canadian EarthCARE simulator efforts. Both communities will benefit from exchange of scientific ideas during individual development of their respective simulators.

An additional objective of GPM GV modeling and simulation is to develop a coupled model system (land surface, CRM, radiative) to isolate parameters that produce the largest sensitivity in simulated results. The results of this analysis will provide a framework to assess weaknesses in algorithm performance and to provide direction on specific measurements that can isolate the most sensitive algorithm parameters.

SSM concept: observations or cloud-resolving model output to serve as input
to forward radiance / radar reflectivity simulations