An Evaluation of the Grell-Freitas Cumulus Scheme in the Hurricane Weather Research and Forecasting System

The cumulus parameterization in numerical weather prediction models plays an important role in modulating tropical cyclone (TC) structure, intensity and evolution. The Hurricane Weather Research and Forecasting system (HWRF), the National Oceanic and Atmospheric Administration’s operational model for tropical cyclone prediction, currently uses the scale-aware Simplified Arakawa-Schubert (SASAS) cumulus scheme. In past years, the Developmental Testbed Center (DTC) has run experiments of HWRF with a variety of cumulus schemes, but none outperformed the operational configuration. However, recent tests indicate promising improvements, particularly for TC intensity and capturing rapid intensification events using the Grell-Freitas (GF) cumulus scheme.

The GF scheme is a state-of-the-art scheme that employs an ensemble approach to the representation of convection, using a collection of parameters and algorithms to represent convective triggers, vertical mass flux, and closures. It is also scale aware, which makes it suitable for use across a wide range of model horizontal grid spacings. The scheme is already used in the Rapid Refresh (RAP) model that is run operationally by the National Centers for Environmental Prediction (NCEP).

To assess the suitability of GF for HWRF simulations of TCs, retrospective HWRF forecasts of select TCs occurring in the Atlantic basin during the 2015-2017 hurricane seasons were simulated using the GF scheme and compared to those produced by the operational HWRF. Assessments were done through a combination of physics process diagnostics (e.g. temperature tendencies from the cumulus schemes), as well as traditional model verification metrics of track and intensity. Evaluation of large– and storm-scale fields including comparisons between the modeled and observed air temperature, moisture, winds, and precipitation will be presented. Additionally, observations such as Coyote Uncrewed Aircraft System (UAS) measurements and dropsondes in the vicinity of the storm will be used to further demonstrate the model performance for select storms.