Enhancements to Cloud Overlap Radiative Effects for Weather Forecasting and Tropical Cyclone Prediction

With support from the Developmental Testbed Center (DTC), NCAR and NOAA, this research is evaluating enhancements to the treatment of cloudy radiative transfer within the RRTMG radiation code in the contexts of tropical cyclone prediction using the Hurricane Weather Research and Forecasting (HWRF) model and short-term weather forecasts using the NOAA FV3GFS global model. The default maximum-random (MR) method in RRTMG for representing the sub-grid radiative coupling, or vertical cloud overlap, of fractional clouds has been replaced with more advanced methods known as exponential (EXP) and exponential-random (ER) to examine their impact and potential benefit to weather prediction. In the presence of partial cloudiness, the new methods relax the strict MR assumption of maximum overlap through adjacent cloudy layers by allowing the vertical correlation of clouds to transition exponentially from maximum to random with increasing layer thickness and with distance through cloudy layers. The ER method adds a further level of randomization between non-adjacent blocks of clouds with clear layers in between. In general, the net effects of EXP and ER are to increase the total cloud cover as seen by the radiation code, to reduce the amount of shortwave radiation reaching the surface, and to alter the profile of radiative heating rates, which can subsequently induce a response in the predicted atmospheric state. The EXP and ER methods include a decorrelation length that affects the rate of exponential transition of cloud overlap through cloudy layers, and the optimal rate may depend on atmospheric parameters such as cloud properties, wind shear, and vertical velocity. Different approaches have been evaluated for specifying the decorrelation length either as a constant or with variations in latitude and day of the year. Testing within HWRF has demonstrated that the EXP and ER overlap methods significantly alter radiative heating rates in and around tropical cyclones and in some cases sufficiently modify the atmospheric state to change the track and intensity of tropical cyclones. Testing with EXP overlap for a large sample of cases showed sufficient improvement for NOAA to adopt that method within the operational HWRF during 2018, and the result of subsequent similar experiments with the ER method will be presented. Results of testing both EXP and ER cloud overlap within the global forecast model for an eastern United States severe winter storm case and for the Hurricane Harvey extreme precipitation case will also be presented.