Local winds in a valley city

Local winds were studied around a valley city, by using a high resolution t wo-dimensional mesoscale model forced by surface temperatures from a measur ement campaign around Lanzhou City, China, during stagnant conditions. In t he simulations nighttime winds are purely katabatic downslope winds without urban effects, despite the fact that the city is 6-7 degreesC warmer than its surroundings all night. In contrast, daytime near-surface winds result from upslope flow resisted by an opposing simultaneous urban heat-island ci rculation (UHIC). Hence winds remain weak and variable around a city in a n arrow valley during daytime. These conditions may lead to severe air qualit y problems day and night. The local circulations are sensitive to the widths of the valley and/or cit y, and also latitude, as is demonstrated by model experiments. Interestingl y, in a flat and calm environment an extratropical daytime UHIC cell may tu rn into a weak 'anti-UHIC' by the morning, due to frictional decoupling aft er sunset and subsequent inertial oscillation during the night, analogously to the land breeze and nocturnal low-level jet formation.