Gantuya Ganbat

Characteristics of the urban heat island in a high-altitude metropolitan city, Ulaanbaatar, Mongolia

Ulaanbaatar, the capital city of Mongolia, with a population of 1.1 million is located at an altitude of about 1350 m and in a valley. This study is the first to document the characteristics of the urban heat island (UHI) in Ulaanbaatar. Data from two meteorological stations, an urban site and a rural site, for the 31-year period 1980–2010 are used for UHI analysis. The average UHI intensity is 1.6°C. The UHI intensity exhibits a large seasonal dependence, being strongest in winter (3.3°C) and weakest in summer (0.3°C). The average daily maximum UHI intensity is 4.3°C. The strongest daily maximum UHI intensity occurs in winter with an average intensity of 6.4°C, and the weakest one occurs in summer with an average intensity of 2.5°C. The occurrence frequency of the daily maximum UHI intensity in the nighttime is 5.6 times that in the daytime. A multiple linear regression analysis is undertaken to examine the relative importance of meteorological parameters (previous-day maximum UHI intensity, wind speed, cloudiness, and relative humidity) that affect the daily maximum UHI intensity. The half of the variance (49.8%) is explained by the multiple linear regression model. The previous-day maximum UHI intensity is the most important parameter and is positively correlated with the daily maximum UHI intensity. Cloudiness is the second most important parameter and is negatively correlated with the daily maximum UHI intensity. When the data are classified into daytime/nighttime and season, the relative importance of the meteorological parameters changes. The most important parameter in spring and summer is cloudiness, while in autumn and winter it is the previous-day maximum UHI intensity.

Theoretical calculations of interactions between urban breezes and mountain slope winds in the presence of basic-state wind

Many big cities around the world are located near mountains. In city-mountain regions, thermally and topographically forced local winds are produced and they affect the transport of pollutants emitted into the urban atmosphere. A better understanding of the dynamics of thermally and topographically forced local winds is necessary to improve the prediction of local winds and to cope with environmental problems. In this study, we theoretically examine the interactions of urban breezes with mountain slope winds in the presence of basic-state wind within the context of the response of a stably stratified atmosphere to prescribed thermal and mechanical forcing. The interactions between urban breezes and mountain slope winds are viewed through the linear superposition of individual analytical solutions for urban thermal forcing, mountain thermal forcing, and mountain mechanical forcing. A setting is considered in which a city is located downwind of a mountain. In the nighttime, in the mountain-side urban area, surface/near-surface horizontal flows induced by mountain cooling and mountain mechanical forcing cooperatively interact with urban breezes, resulting in strengthened winds. In the daytime, in the urban area, surface/near-surface horizontal flows induced by mountain heating are opposed to urban breezes, giving rise to weakened winds. It is shown that the degree of interactions between urban breezes and mountain slope winds is sensitive to mountain height and basic-state wind speed. Particularly, a change in basic-state wind speed affects not only the strength of thermally and mechanically induced flows (internal gravity waves) but also their vertical wavelength and decaying rate. The examination of a case in a setting in which a city is located upwind of a mountain reveals that basic-state wind direction is an important factor that significantly affects the interactions of urban breezes with mountain slope winds.

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

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.

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

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.

Dynamics of Reversed Urban Breeze Circulation

AbstractThe urban breeze circulation (UBC) is a thermally forced mesoscale circulation that is characterized by low-level inward flow toward the urban center, updrafts near the urban center, upper-level outward flows, and weak downdrafts outside the urban area. Previous numerical modeling studies indicate that in the early morning the direction of the UBC can be reversed. Here, the dynamics of a reversed UBC is studied in the context of the response of the atmosphere to specified thermal forcing, which represents diurnally varying urban heating. For this, a linearized, two-dimensional, hydrostatic, Boussinesq airflow system in a rotating frame with specified thermal forcing is solved using the Fourier transform method. The occurrence of a reversed UBC in the early morning is confirmed. The Coriolis parameter affects the strength and vertical structure of the UBC, whose role is similar to that of the coefficient of Rayleigh friction and Newtonian cooling. The occurrence condition, strength, and vertical st...