湄洲湾海雾的发生规律和成因分析

Analysis of the occurrence and origin of heavy fog in Meizhouwan Bay

利用湄洲湾及近海3个气象站1974～2003年和1个气象站1985～2003年的地面观测资料，统计分析了湄洲湾海雾的天气气候特征。湄洲湾海雾具有明显的年际变化，而且季节差异显著，其中2～5月是海雾的盛季。初步分析了湄洲湾海雾的成因，归纳得出2～5月该海湾海雾生成前的3种环流天气形势：锋面型、入海高压后部型、高压底部型。在对海雾生成的水文气象要素分析的基础上，得出了2～5月海雾发生的一般规律，为海洋预报和服务提供一定的参考依据。

Using the observation data of three weather stations from 1974 to 2003 and one weather station from 1985 to 2003 in the Meizhouwan Bay and its adjacent sea area,statistic analysis is made on the climatic regularity and circulation background during the fog prevailing season. Analysis of the hydrometeorological elements to form the sea fogs has resulted in the general laws of their occurrence during February to May. It can serve as the reference basis for marine service and prediction. Advection fog and radiation fog are two most common fogs over the Meizhouwan Bay and its adjacent sea. Their climatic features are （1）The sea fogs have obvious annual variation. Since the end of 1980s the annual fog days showed a decreasing trend year by year. But from mid 1970s to the beginning of 1980s,the aver age annual fog days of both Putian and Congwu stations presented an increasing trend while they trends to decrease in mid and late 1980s. And the decrease trend is more significant at Putian station further away from the sea. （2）The sea fogs have an obvious seasonal variation. From February every year heavy fog increased and reached the peak in April, then decreased quickly after June. February to May is the prevailing season of the sea fog. （3）Most sea fogs took place between 8 pm to 8 am next day,which is related to the remarkable temperature reduction over the sea surface and the stable air near the surface. About 67 percent of the sea fogs disappeared during 8 am to 14 pm,80 percent of them disappeared between 8 am to 11 am. The occurrence of the sea fog needs right environmental conditions such as wind direction, wind speed, humidity and temperature difference between sea and air. （1）Northerly wind, especially the southern wind from the warm sea surface is essential for the formation of the sea fog. And the stable wind direction is also important. Too strong or too weak of wind is unfavorable for fog formation. （2）The temperature difference between sea and air at the Meizhouwan Bay is negative in April and May,and the cold water of the bay has the cooling effect on the air over the water. After entering into spring the circulation begins to change：warm and humid air mass gets strengthened and moves northward and meets the cold water mass, which results in great temperature difference,intensified heat exchange and saturation of the vapor, and thus the fog forms. （3）Most of the sea fog is formed mostly when the vapor is saturated of nearly saturated. Taking relative humidity as the characteristic factor of vapor content in the air, two third of the samples have the relative humidity of over 80% at 14 hour before the fog day. For the prevailing season of sea fog from February to May, three main weather situations before the occurrence of the sea fog were concluded. They are the inverted Jianghuai cyclone,back of high pressure and in verted weak fronts.