Sensitivities of Polarimetric Radar Forward Operators

Recent advances in microphysics schemes have resulted in a more complete prediction of microphysical processes that allows for more diverse hydrometeor distributions to be modeled.   Computational resources that allow for finer-scale numerical simulations of convective storms have been expanding at the same time as our understanding and the availability of polarimetric weather radar data has increased. A polarimetric radar forward operator lets us view numerical simulations using quantities commonly available from polarimetric weather radars; the direct comparison of output from numerical models and polarimetric radar data has applications in, amongst others, data assimilation and model validation.  However, there are often many factors that influence observed radar quantities that may or may not be explicitly predicted by a numerical model.  For example, most microphysics schemes do not prognose mass water fraction for icy species and, thus, cannot predict mixed-phase hydrometeors.  The polarimetric fields, however, can be very sensitive to the mass water fraction of mixed-phase hydrometeors (and the distribution of water on the hydrometeors); errors associated with estimating (or ignoring) the mass water fraction can be substantial.  Additionally, recent applications of polarimetric forward operators have relied upon precomputed scattering amplitudes loaded as lookup tables to speed the calculation of the radar variables. Using a various combinations of assumptions that have often been made in past modeling studies, we will discuss some of the practical impacts of different “shortcuts” in hopes of providing useful guidance for numerical modeling users that are interested in polarimetric radar forward operators.