Local circulations in and around the Ulaanbaatar, Mongolia, metropolitan area

Abstract

Many cities around the world are located in mountainous areas. Understanding local circulations in mountainous urban areas is important for improving local weather and air quality prediction as well as understanding thermally forced mesoscale flow dynamics. In this study, we examine local circulations in and around the Ulaanbaatar, Mongolia, metropolitan area using the Weather Research and Forecasting model coupled with the Seoul National University Urban Canopy Model. Ulaanbaatar lies in an east–west-oriented valley between the northern base of Mt. Bogd Khan and the southern base of branches of the Khentiin Nuruu mountain range. Idealized summertime fair-weather conditions with no synoptic winds are considered. In the daytime, mountain upslope winds, up-valley winds, and urban breeze circulation form and interact with each other. Mountain upslope winds precede up-valley winds. It is found that the transition of upslope winds to downslope winds on the urban-side slope of Mt. Bogd Khan occurs and the downslope winds in the afternoon strengthen due to urban breezes. In the nighttime, mountain downslope winds and down-valley winds are prominent and strong channeling flows form over the city. The sensitivities of local circulations to urban fraction, atmospheric stability, and soil water content are examined. As urban fraction increases, daytime up-valley winds over the city and daytime downslope winds on the urban-side slope of Mt. Bogd Khan strengthen. Daytime near-surface up-valley winds in the city strengthen with increasing atmospheric stability. As soil water content decreases, daytime near-surface up-valley winds in the city weaken. The daytime urban atmospheric boundary-layer height is found to be sensitive to atmospheric stability and soil water content. This study is a first attempt to examine local circulations in and around the Ulaanbaatar metropolitan area and demonstrates that the city alters mountain slope winds and up-/down-valley winds.