A numerical modeling study of local circulations in and around a high-altitude, mountainous city, Ulaanbaatar, Mongolia

Thermally induced local circulations appear in and around mountainous cities around the world. Understanding the local circulations and their interactions is important in a view of mesoscale dynamics and air pollution. This study examines local circulations and their interactions in Ulaanbaatar, capital of Mongolia, and its surrounding area using the Weather Research and Forecasting (WRF) model coupled with the Seoul National University Urban Canopy Model (SNUUCM). The city has a population of ~1.1 million and is located at an altitude of ~1350 m and in a valley between the northern foot of the Bogd Khan mountain and the southern foot of branches in the Khentiin Nuruu mountain range. Idealized summertime fair-weather conditions under no synoptic winds are considered. The simulated urban heat island is stronger in the nighttime than in the daytime. In the daytime, local circulations are characterized by upslope and up-valley winds and urban breeze circulation. The upslope winds precede the up-valley winds. The strong urban breeze circulation forms in the late afternoon. The up-valley winds intensify over the city. Near-surface wind convergence zone and updraft cell associated with the urban breeze circulation move downwind by intensified up-valley winds. In the nighttime, down-valley winds, which are generally weaker than the daytime up-valley winds, and weak downslope winds are developed. The daytime winds are stronger than the nighttime winds, but winds can be intensified due to channeling effect in the nighttime. The sensitivities of the local circulations to the urban fraction, atmospheric stability, and soil moisture content are examined. The urban fraction greatly affects slope and valley winds and their interactions with urban breeze circulation. The near-surface temperature and convective boundary layer growth are strongly dependent on the atmospheric stability. Change in soil moisture content has a less important influence on the local circulations in comparison with the influences due to changes in urban fraction and atmospheric stability.