Examining polarimetric forward operators for use with numerical models

As microphysics schemes become more complex, the increasingly-diverse hydrometeor size distributions that can be predicted by such schemes allow for more accurate capturing of microphysical processes that occur in convective storms. Sophisticated microphysics schemes, including both multimoment bulk and spectral bin schemes and the widespread deployment of polarimetric weather radars presents an opportunity for the bridging of polarimetric radar observations and numerical model simulations. Two forward operators designed for use with bulk and bin microphysics have recently been developed. These forward operators allow users to compare numerical model results with observations, in turn letting one assess the performance of the model's microphysics; the forward operators can also be used to simulate and study commonly-observed polarimetric signatures such as ZDR columns. Despite the advancements in these schemes, nearly every scheme does not predict all of the necessary characteristics of the hydrometeors required to calculate the radar variables with high accuracy -- for example, most schemes do not allow the density of hail and graupel to vary, and most do not explicitly track the liquid water fraction of hail, graupel, and snow (particularly as a function of size). A general overview of the forward operators is provided, and some of the challenges and complexities involved in calculating commonly-used polarimetric radar variables using output from numerical models are examined. In addition, some of the sensitivities that arise from required assumptions and estimations (such as having to diagnose liquid water fraction) are noted.