ESRL/PSD Seminar Series

Despite the fact that microphysics parameterization schemes used in numerical weather prediction (NWP) models can be as complex as being capable of resolving the evolution of hydrometeor size spectra, operational centers still cannot computationally afford to run any NWP models with spectrum-resolving schemes for daily weather prediction. To strike an optimal balance between computational cost and physical effect, there is a need to understand what minimal complexity of microphysics parameterizations is required in operational NWP models that are run at affordable resolutions. To this end, we have been working with our NCEP colleagues to identify whether or not the microphysics schemes used currently in NOAA’s operational NWP models is complex enough to enable us to use these models for high-resolution prediction of severe weather events in the near future.

We started the work with a study in which the Weather Research and Forecasting (WRF) model was used to investigate the impact of parameterized warm-rain processes in four widely-used bulk microphysics parameterization schemes on the model-simulated tropical cyclone (TC) development. The schemes investigated, ranging from a single-moment simple 3-category scheme to a complex double-moment 6-category scheme, produce different TC intensification rates and average vertical hydrometeor distributions, as well as different accumulated precipitation. By diagnosing the source and sink terms of the hydrometeor budget equations, we found that the differences in the warm-rain production rate, particularly by conversion of cloud water to rain water, contribute significantly to the variations in the frozen hydrometeor production and in the overall latent heat release above the freezing level. These differences in parameterized warm-rain production reflect the differences of the four schemes in the definition of rain droplet size distribution and consequently in spectrum-dependent microphysical processes such as accretion growth of frozen hydrometeors and their sedimentation. Hydrometeor budget analysis of the four schemes indicates that the assumed pathways to the production of frozen hydrometeors are quite sensitive to the amount of available super-cooled rain water and, thus, the uncertainties in the parameterized warm-rain processes can affect the intensification and structure of the model-simulated tropical cyclone. Results from this study strongly suggest that the advantage of double-moment formulations can be overshadowed by the uncertainties in the spectral definition of individual hydrometeor categories and spectrum-dependent microphysical processes.