Understanding How GCM Momentum Flux Parameterization Impacts Simulated Boundary Layers in Tropical Cyclones

Abstract:

General circulation models (GCMs) are typically restricted to grid spacings of 1 degree or coarser for seasonal prediction and beyond. However, recent studies have demonstrated that novel modeling techniques and improved computational capacities now allow GCMs to produce such simulations at higher resolutions more typical of those applied in numerical weather prediction (NWP). These resolutions open the door to improvements in the prediction of extreme weather phenomena such as tropical cyclones (TCs). Before these advances can be fully realized, however, improvements in how GCMs simulate processes such as turbulence need to be improved in these cutting-edge configurations.

Here, we simulate TCs using an idealized configuration of the Community Atmosphere Model version 6 (CAM6), the atmospheric GCM within the Community Earth System Model (CESM). In spite of advances, CAM6 exhibits noted biases in the modeled structure and climatology of TCs. To improve TC representation, we focus on improving model depictions of the TC planetary boundary layer (PBL) using the Cloud Layers Unified by Binormals (CLUBB) parameterization scheme. Specifically, we use a new version of CLUBB containing a prognostic momentum flux treatment within the TC PBL. We further apply a sensitivity analysis to assess the degree to which modifying certain momentum flux parameters within CLUBB affects a range of TC PBL metrics such as inflow angle and height of maximum wind. Using this method, we demonstrate the relative importance of certain namelist parameters within the TC PBL and provide guidance regarding the anticipated effects of perturbing these parameters. Finally, we introduce several combinations of CLUBB input parameters that most realistically depict momentum flux in the TC PBL when compared to observations and simulations using Cloud Model 1 (CM1) and, as a result, produce a more realistic pressure-wind relationship.