Wintertime winds in and around the Ulaanbaatar metropolitan area in the presence of a temperature inversion

Abstract

Temperature inversions are frequently observed in mountainous urban areas and can cause severe air pollution problems especially in wintertime. This study investigates wintertime winds in and around the Ulaanbaatar, the capital of Mongolia, metropolitan area in the presence of a temperature inversion using the Weather Research and Forecasting (WRF) model coupled with the Seoul National University Urban Canopy Model (SNUUCM). Ulaanbaatar is located in complex terrain and in a nearly east-west-oriented valley. A wintertime scenario with clear skies, weak synoptic winds, and a temperature inversion under the influence of a Siberian high-pressure system is selected. Local winds are weak in the presence of the temperature inversion. In the daytime, weak mountain upslope winds develop, up-valley winds appear to be stronger in the urban area than in the surrounding areas, and channeling winds are produced in the main valley. The bottom of the temperature inversion layer rises up in the urban area, and winds below the bottom of the temperature inversion layer strengthen. In the nighttime, mountain downslope winds and down-valley winds develop. Urban effects in the presence of the temperature inversion are examined by comparing the results of simulations with and without the city. It is shown that in the daytime the urban area acts to elevate the bottom of the temperature inversion layer and weaken the strength of the temperature inversion layer. Winds east of the city weaken in the afternoon and down-valley winds develop later in the simulation with the city.