Interactions between Microphysical Parametrizations and All-Sky Infrared Data Assimilation on Hurricane Harvey

Current geostationary satellites provide the majority of observations when Tropical Cyclones (TC) are over the open ocean. These observations are generally under-utilized in operational global and regional forecasting models as most of the cloudy and precipitation-affected regions are discarded. Recent work done using the PSU WRF-EnKF has shown that the assimilation of all-sky IR radiances from these geostationary satellites improves track, intensity and rainfall forecasts for Hurricane Harvey. However, we have learned that a different choice of a microphysics scheme will lead to a different forecast when all-sky IR radiances are assimilated. This is because model hydrometeors fields that are used to simulate IR radiances are strongly constrained by the model’s microphysics scheme and, model-simulated IR radiances are an important aspect when assimilating observed all-sky IR radiances. This study examines how two different microphysical parametrization schemes impact all-sky IR radiance assimilation and subsequent forecasts. It also examines the sensitivities that exist between these schemes with regard to Harvey’s practical predictability. Results show that the forecast errors are primarily controlled by different dynamical fields in the EnKF analyses and to a lesser extent, at certain stages of Harvey’s development, the hydrometeor and moisture fields. This was demonstrated by using these fields as initial conditions for multiple sensitivity forecasts. These initial fields produced by the EnKF analyses are influenced by the choice of the microphysics scheme during EnKF cycles. Since different schemes produce different cloud top brightness temperatures (IR radiances), different schemes lead to different updates to these fields. On the other hand, the microphysics scheme used during the free forecasts generally has a very small impact on Harvey’s intensity forecasts.