Post Date: 24 October 2017

Inclusion of the Urban Momentum and Thermal Drag Effect within the ACM2 PBL Scheme in the WRF Model to Obtain Wind Profile over the Cities

Abstract

A realistic representation of the planetary boundary layer (PBL) structure and its periodic evolution is crucial for better numerical simulation and forecasting of the regional meteorological conditions and air pollutant dispersion. Various PBL schemes (local and non-local) have been integrated in the Weather Research and Forecasting (WRF) model to incorporate the governing turbulence within the PBL. In addition, these schemes are tied to the different land surface models to account the heat and moisture fluxes evolved from the earth’s surface. A few years back, the urban models such as UCM (Urban Canopy Model), BEP (Building Effect Parameterization) and BEM (Building Energy Model) have been developed and included within the WRF model. The motive behind this was to enhance the representation of the urban surface phenomenon such as the heat island effect and turbulence caused due to the complex built urban structures these days e.g. street canyons. This has improved the PBL structure creation over the urban cities worldwide. However, the widely used BEP-BEM model these days, which runs in parallel with the WRF model increases the computational requirements and the simulation time additionally.

In the recent years, the performance of the four PBL schemes, namely MYJ, Boulac, YSU and ACM2 has been analyzed over the Pearl River Delta (PRD) region in the Southern China. From the results, it was concluded that the non-local ACM2 scheme has shown good correlation with the observation of the meteorological field variables and the PBL height over the Hong Kong city. In this research, an attempt has been made to include the related governing urban physics within the ACM2 PBL scheme itself by modifying it. It is expected that performing simulations over the complex urban structure cities like Hong Kong with the newly modified ACM2 scheme will produce the vertical profile of the horizontal wind-speed as well as the urban roughness sublayer turbulence characteristics both of which are critical for the air pollutant dispersion within less computational time.