Diurnal Evolution of the Urban Boundary Layer Structure During a Snow Process in Urumqi

For the purpose of studying the boundary layer characteristics in snowy day and non-snowy day of Urumqi, 20 times of continuous observations from 11 to 13 in January 2008 and 12 times non-snowy day's observations from 21 to 23 in January 2008 are conducted based upon the Vaisala tethered balloon in the middle of the city. The detecting elements include temperature, dew point, wind speed and wind direction etc. The potential temperature and depression of the dew point isoline and wind profile are studied for investigating the characteristics of the boundary layer and the influence of warm advection and snowing on the boundary layer.The convective boundary layer (CBL) is very typical with clear ultra-adiabatic lapse layer, mixed layer (ML) and capping inversion layer (CIL) in the daytime, and there tends to be a surface inversion during the nighttime. In fact, the temperature stratification isn't affected much by snowing. When the warm advection invades, an intense surface inversion occurs in the daytime, but it's still weaker than that in the nighttime. Furthermore, compared with the nighttime, the temperature inversion is even fiercer when warm advection approaches.In the CBL, vapor is blocked at the bottom of the CIL, forming a wet center, as height increases, vapor decreases to form a dry center. The height of the wet center in the snowing day is lower than usual. Under the influence of warm advection, large area of wet center appears near the ground surface. In the evening, weak humidity inversion occurs in the surface layer, besides, the upper layer has an obvious dry center.Although wind direction is dispersed in the surface layer, the main direction still can be estimated as "N". It changes by clockwise or anti-clockwise from the surface layer to the higher layer, the main direction in the upper air is "SE". The wind speed follows the "high-low-high-low" rule at certain heights, and the extreme speed appears at the height where wind direction shifts. In conclusion, wind speed tends to rise by fluctuates with wind direction shifts.