Post Date: 12 November 2024

Improving the Parameterization of Vertical Turbulent Mixing Processes in the Atmospheric Boundary Layer

Abstract:

The planetary boundary layer (PBL) modeling is a primary contributor to uncertainties in any numerical weather prediction model due to difficulties reproducing surface layer fluxes' turbulent transport. The widely used Weather Research and Forecasting (WRF) model has provided many PBL schemes that may feature a non-local transport component driven by large eddies or a one-and-half order turbulence closure model, but few of them possess the two features at once. In the first chapter of this QE report, a turbulent kinetic energy (TKE)-based eddy diffusivity/viscosity method is integrated into the non-local Asymmetric Convective Model version 2 (ACM2) PBL scheme and implemented in WRF. The original first-order eddy-diffusivity term in ACM2 is discarded and an extra prognostic equation for TKE, which considers the tendency of TKE due to buoyancy, wind shear, vertical transport, and dissipative processes, is supplied to calculate the diffusivity/viscosity. Non-local transport is modeled the same as ACM2 using the transilient matrix method. Idealized tests using prescribed surface heat flux and roughness length are performed. The new PBL scheme, TKE-ACM2, displays advantages over the PBL scheme developed by Bougeault and Lacarrère (hereinafter referred to as Boulac) and ACM2 in the wind speeds profile because it better matches LES results in the surface momentum flux. Real case simulations show that TKE-ACM2 generally outperforms in the diurnal vertical profiles of wind speeds under stable conditions. TKE-ACM2 also produces a better alignment under moderately unstable conditions in the early nighttime at the urban LiDAR station. However, the model exhibits discrepancies more apparently under a more unstable condition during the winter daytime.

Subsequently, the second chapter of this QE report further demonstrates the enhanced applicability of the TKE-ACM2 PBL scheme over built-up land by coupling it with the Building Effect Parameterization (BEP) in WRF. This work is motivated by the fact that the domain of interest, i.e., the Pearl River Delta (PRD) region consists of several tremendously developed giant cities in a relatively small area. The presence of heterogeneous surface obstacles (buildings) that have comparable dimensions with the vertical resolution requires further complexity and design in the PBL scheme. Thus, a numerical method is developed in this chapter to couple one of the recently validated PBL schemes, TKE-ACM2, with the BEP model in WRF. The performance of TKE-ACM2+BEP has been examined under idealized convective atmospheric conditions with a simplified building layout. Furthermore, its reproducibility is benchmarked with one of the more advanced and state-of-the-art large-eddy simulation models, PALM, which can explicitly resolve the building aerodynamics. The result indicates that TKE-ACM2+BEP outperforms the other operational PBL scheme (Boulac) coupled with BEP by reducing the bias in both the potential temperature ($\theta$) and wind speed ($u$). Following this, real case simulations are conducted for a highly urbanized domain, i.e., the PRD region in China. The high-resolution wind speed LiDAR observations suggest that TKE-ACM2+BEP can mitigate the overestimation in the lower part of the boundary layer compared to the bulk method at a densely built LiDAR site. In addition, the surface temperature and relative humidity can be considerably improved in TKE-ACM2+BEP at surface urban stations. However, it is revealed that BEP may not always imply a better reproduction of surface wind speed.