九华山雾日时间变化特征及其形成的气象条件分析

Analysis on the temporal change features and meteorological conditions of fog day in Jiuhua Mountain

利用1991—2010年九华山不同海拔高度上的国家气象观测站及区域气象观测站资料,分析九华山雾日时间变化特征及其形成的气象条件。结果表明:近10年来,九华山山坡雾日年际变化较大,并逐年减少,山脚雾日逐年增多;山脚季平均雾日,秋(8 d)、冬(7 d)季多于春(3 d)、夏(2 d)季,而山坡表现为冬(50 d)、春(45 d)季雾日多于夏(27 d)、秋(26 d)季;平地雾在05—07时最易形成,山坡雾在04—08时最易形成,山脚雾主要在08—10时消散,山坡雾主要在09—11时消散,两者持续时间也有较大差异;夜间降温≥6℃时出现的雾日占总雾日的74.4%,气温日较差≥7.0℃时出现的雾日占总雾日的80.9%,65.2%的雾日前一日20时至当日08时平均相对湿度在90%以上,雾日多出现在风速<3 m.s-1的条件下,83.9%的雾日近地面有逆温层存在。未饱和湿空气随气流进入喇叭口后,湿度条件发生改变,有利于在喇叭口底部区域形成雾,山区风场辐合作用有利于雾的形成与维持。

The conventional observation data and the intensive observation of automatic weather station from 1991 to 2010 in different alti- tude of Jiuhua Mountain have been employed to analyze the temporal change features and meteorological conditions of fog day in the mountain area. The results show that the interannual change of fog day on the mountainside is bigger and decreases yearly but fog day on the foot of moun- tain increases yearly in the latest 10 years. The mean fog day （8 d） in autumn and mean fog day （7 d） in winter is more than the one （3 d） in spring and one （2 d） in summer on the foot of mountain, but the mean fog day （50 d） in winter and mean fog day （45 d） in spring is more than the one （27 d） in summer and one （26 d） in autumn on the mountainside. The fog occurs easily from 5 a.m.to 7 a.m. and disappears mainly from 8 a.m.to 10 a. m. on the foot of mountain, but the fog occurs easily from 4 a.m.to 8 a.m. and disappears mainly from 9 a.m.to 11 a.m. on the mountainside, and there is a difference in the fog duration between mountainside and the foot of mountain. The fog day accounts for 74.4% of the total when tem- perature in the nighttime fall more than 6 ~C ; the fog day accounts for 80.9% of the total when diurnal range of air temperature is more than 7.0 ℃. 65.2% of the total fog days have the feature that mean relative humidity is over 90% from 20 p.m. in a day to 08 a.m. on the following day. Most of fog days occur when wind speed is less than 3 m· s^-1, and 83.9% of the total fog days occur when an inversion layer exists near the ground. The change of humidity condition is favorable to originate fog in the trumpet-shaped topography areas after unsaturated wet air gets in- to the terrain; meanwhile, the convergence of wind field in the mountain area is favorable to the formation and maintenance of fog.