Statistical analysis of the physical quantities of regional advection fogs in south of the Yangtze River

Routine upper-air and surface observations, and NCEP 6-hourly reanalysis data with 1°×1° resolution from 2000 to 2012 were used to select 54 advection fog events occurred in south of the Yangtze River (23°-32°N, 110°-122°E) in China, from which we have obtained the statistics of regional advection fogs by analyzing physical quantities such as inversion, temperature difference, low-level humidity and vertical velocity when regional advection fogs occurred. The results show that (1) warm advection is evident in the low-level when regional advection fogs occurred in south of the Yangtze River, and it is stronger at 925 hPa than at 850 hPa. (2) There is generally one-layer inversion, isothermal layer or multilayer inversion structure in advection fog events. A great positive value center of temperature difference at 925 hPa extends eastward from Guangxi to Zhejiang and Fujian, which is propitious to the establishment of the inversion structure. The top height of advection fog is lower than that of radiation fog. (3) The relative humidity between 925 hPa and 850 hPa tends to increase from nocturnal to 08:00 BT of the following day with the occurrence of advection fog. Moreover, it tends to be low at 500 hPa while mostly exceeding 90% in ground level. (4) The vertical velocity at 925 hPa changes from -0.1 to 0.2 Pa·s^-1 when regional advection fogs occurred.