华北地区平流雾过程湍流输送及演变特征

Characteristics of Turbulent Transfer and Its Temporal Evolution during an Advection Fog Period in North China

利用2006年2月在天津市南部地区开展的大气湍流观测资料,分析了平流雾生消过程中湍流输送及演变特征。结果表明:平流雾生消过程中,大气稳定度参数数值主要集中在弱不稳定到弱稳定范围内,且多呈现弱不稳定状态。雾前,大气呈弱不稳定状态,热量的垂直和水平输送短时加强,且水平输送较垂直输送更显著;雾生成初期,大气呈弱稳定层结,湍流动量和热量的垂直及水平输送均很弱;雾中,大气多呈弱不稳定状态,稳定度参数、动量、热量及水平与垂直热通量之比值随时间均呈多峰形振荡,随着雾的消散,大气由弱不稳定逐渐演变为稳定状态;雾后,低层大气再次呈现弱不稳定层结特征,动量和热量输送呈单峰形日变化特征。平流雾生消过程中,热量的水平输送随不稳定度增强而迅速减小;大气呈稳定层结时,热量的水平输送显著大于垂直输送,其比值约为2。同时,摩擦速度、水平与垂直热通量之比值呈多峰形振荡,振幅分别可达1.2m/s及50。

Based on the observational atmospheric turbulent data obtained in the south of Tianjin City in February 2006,the characteristics of turbulent transfer and its temporal evolution have been analyzed during an advection fog process.The results show that the absolute value of stability parameter is mainly in the range less than one,and the unstable condition is dominant during the advection fog.Before the fog formation,the atmosphere is slightly unstable;heat transfer in horizontal and vertical directions intensifies in a short period;the horizontal heat transfer is much larger than the vertical transfer.In the stage of fog formation,it turns to be stable,with turbulent momentum and heat fluxes being weak in every direction.During the fog,the atmosphere is mainly unstable with the multi-peak undulation characteristics for the temporal variations of stability parameter,turbulence momentum,heat fluxes,and the ratio of heat fluxes in horizontal and vertical directions.The slight instability gradually changes to slight stability as the fog dissipate.The lower atmosphere is unstable after fog with the momentum and heat fluxes showing a diurnal variation feature（large at noon and weak at night）.During the fog process,horizontal heat fluxes decrease quickly with the stability parameter increasing.The variations of the standard deviations of wind speed with the stability parameter satisfy the 1/3 power law.Under the stable condition,the ratio of heat fluxes in horizontal and vertical directions is approximately equal to 2,and the standard deviations of wind speed increase with stability while the standard deviation of potential temperature decreases with stability.Both the friction velocity and the ratio of heat fluxes in horizontal and vertical directions show multi-peak distributions during the fog,with their peaks reaching 1.2 m/s and 50,respectively.