The Role of the Turbulent Heat Flux Parameterization on Simulating the Low Level Jet Intensity Using WRF

A new turbulence closure scheme has recently been introduced and incorporated into the Weather Research and Forecasting (WRF) model, specifically for the stable boundary layer (SBL). The scheme solves prognostic equations for kinematic heat flux, temperature variance, and turbulent kinetic energy. In this new scheme, the prognostic equations for these turbulent quantities are calculated at the mass points, considering the Arakawa C-grid. This approach allows for the direct computation of turbulent flux divergence through centered finite differences, effectively eliminating the need for spatial averaging. This departure deviates from the conventional approach employed in other WRF schemes. In the present work, the WRF single-column mode is employed to evaluate various planetary boundary layer (PBL) parameterizations, including the new one, in capturing the nocturnal maximum near-surface wind speed (low-level jet nose). Twenty-three simulations for each PBL parameterization were conducted, keeping the geostrophic zonal velocity component constant in both time and height for each simulation with values ranging from 0.5 m/s to 12.0 m/s. In all simulations surface temperature is estimated using the Unified Noah land-surface model, while radiation is computed using the rapid radiative transfer model for longwave parameterization and the Dudhia shortwave parameterization. The results indicate that the BouLac scheme simulates the deepest SBL due to the lack of a diabatic function, resulting in heightened turbulence compared to other parametrizations. The other parameterizations show similar heights for maximum wind speed. The new parameterization, solving heat flux prognostically, yields a sharper and more intense maximum wind speed compared to Mellor–Yamada– Nakanishi–Niino parameterization, as it naturally incorporates stratification dependence on various characteristics of mean and turbulent flows, without the need of prescribing diabatic functions.