Local winds in a valley city

Local winds were studied around a valley city, by using a high resolution two-dimensional mesoscale model forced by surface temperatures from a measurement campaign around Lanzhou City, China, during stagnant conditions. In the 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 circulation (UHIC). Hence winds remain weak and variable around a city in a narrow valley during daytime. These conditions may lead to severe air quality problems day and night. The local circulations are sensitive to the widths of the valley and/or city, and also latitude, as is demonstrated by model experiments. Interestingly, in a flat and calm environment an extratropical daytime UHIC cell may turn into a weak 'anti-UHIC' by the morning, due to frictional decoupling after sunset and subsequent inertial oscillation during the night, analogously to the land breeze and nocturnal low-level jet formation.