Analysis on basic characteristics of frontal fog in Guizhou and influence of stationary front on its generation and elimination

Using the fog records from the 84 national meteorological observation stations over Guizhou from 2000 to 2013, NCEP/NCAR reanalysis data and the routine upper-air and surface observations, we have performed an analysis of the frontal fogs in winter, spring and autumn in Guizhou focusing on their basic characteristics, synoptic situation, temperature and humidity characteristics and the influence of stationary front on their generation and elimination based on statistical analysis and composited analysis methods. The main results are as follow. (1) Frontal fog is a kind of weather that appears behind the stationary front and is accompanied by low-temperature and cloudy and rainy condition, which mostly occurs in the central and western Guizhou and are followed by weak frontal precipitation below 4 mm, shallow cloud cover below 700 hPa, weak inversion of below 3 ℃ and surface moisture saturation. (2) When the front fog appeared in different seasons, the high altitude circulation is mostly latitudinal, the short wave trough on the Tibetan-Qinghai Plateau is active, and there is an 8-16 m·s^-1 southerly airflow in the low layer. The Yunnan Guizhou stationary front is in the western Guizhou, and the surface temperature in Guizhou is between 4 ℃ and 14 ℃. The difference of frontal fogs in different seasons mainly show that when they appeared in winter the southwest airflow in front of southern branch trough is stronger than that in spring and autumn, and both cold air and stationary front gradient are the strongest among the three seasons. (3) There are some commonalities in the influence of stationary fronts on the formation and elimination of frontal fogs in the three seasons. That is, stationary fronts are located the junction of Yunnan and Guizhou at the beginning of fogs that appears in the high humidity area with relative humidity over 90% in the western Guizhou. During their strong phase, they are prone to appear in the southeast wind area between stationary front and convergence line. When they weaken, the stationary front moves eastward and northward, and the fog area heats up and dehumidifies. Their differences show that at their beginning both intensity and gradient of stationary front in winter are higher than those in spring and autumn. During their strong stage, the uplift effect induced by upper airflow on the stationary front is more obvious in winter, the cloud water content is more abundant, and the frontal fog range is wider. When they weaken, the thermal depression establishes rapidly between Yunnan and Guizhou in spring, and the stationary front rapidly retreats eastward and moves northward to the central Guizhou. While the thermal depression in winter and autumn is weaker than that in spring, the front zone moves northward by westerly, which made frontal fog in winter and autumn weaken more slowly than in spring.