Impact of Revisions to RRTMG Cloudy Radiative Transfer on Tropical Cyclone Evolution in HWRF

With support from the Developmental Testbed Center (DTC) Visitor Program, this project has implemented and tested revisions to HWRF related to the treatment of cloudy radiative transfer in the RRTMG radiation code developed at AER. The default assumption in RRTMG related to the radiative coupling of fractional clouds and the treatment of vertical cloud overlap, known as maximum-random (MR), is compared to two other cloud overlap approaches, known as exponential (EXP) and exponential-random (ER). In areas with partial cloudiness, these methods relax the strict MR assumption of maximum overlap through adjacent cloud layers by allowing the vertical correlation of clouds to transition exponentially from maximum to random with distance through the cloud, with the ER method adding a further level of randomization between non-adjacent blocks of clouds. Preliminary HWRF forecasts designed to test the effect of using EXP and ER on tropical cyclones have been completed in conjunction with DTC for multiple forecast cycles of Atlantic Hurricanes Gonzalo (2014) and Joaquin (2015), and Pacific Hurricane Dolores (2015).

Diagnostic HWRF radiative heating rate profiles illustrate details of the changes to the inner structure of tropical cyclones related to the use of the new cloud overlap techniques. These radiative heating changes in turn modify the atmospheric state both within the tropical cyclone and in the surrounding environment over time.

A short-term focus of ongoing work with the DTC is to transition the EXP and ER techniques into the 2018 operational HWRF, pending testing by EMC of track and intensity performance on an expanded set of tropical cyclones, including 2016 Atlantic Hurricane Matthew and 2017 Atlantic Hurricanes Harvey, Irma and Maria. We will show the effect on track and intensity forecasts of applying the EXP and EP cloud overlap to recent noteworthy Atlantic hurricanes.