The Land and Lake Breezes of the Dongting Lake and the Effects on Precipitation

[Abstract] By examining the meteorological data observed in the four months of January, April, July and October by observatories in Yueyang, Changde and Yiyang, the three cities locate on the bank of the Dongting Lake, the author presents a study of the features of land and lake breezes of the Dongting Lake and their effects on rainfall, particularly the occurrence of storm. Analysis indicates a remarkable lake and land breeze in the lake area, which measures about 300 meters in thickness. The conversion between the lake and land breezes can cause a variation in their respective divergence and vorticity which thus leads to an increase in precipitation in the morning and evening. The lake and land breezes are prevailing throughout the year but more evident in summer and obscure in winter. The large bodies of water of the Dongting Lake have a strong influence on the lake and land thermodynamic discrepancy, which, in turn, has a significant effect on the climate of the surrounding areas. Li Lianfang used to make an examination of the features of lake and land breezes by only using the data of the Yueyang Observatory. This author nevertheless uses the meteorology data of the three observatories in Yueyang, Changde and Yiyang in the lake region and further examines the overall features of the lake and land breezes in the Dongting Lake area and their effects on precipitation, particularly the storm. Due to the long-term alluviation and land reclamation, the Dongting Lake has been divided into numerous small lakes and four big ones, the East, West and South Dongting Lake plus the Datong Lake. Therefore the lake and land breezes features in this study actually refer to the comprehensive effects of the big and small lakes and the surrounding hills. Adopting the methods in reference [2], this study is carried out with focus laid on the climatic features of the lake and land breezes. We shall first calculate the monthly mean wind (Component u and Component v) and the rainfall amount observed in January, April, July and October of 1980 by the three stations and then get an approximate annual mean value by averaging the combined values of the four months. Next we get the all-day mean value by averaging the added hourly mean values and then the hourly anomalous value by subtracting the all-day mean value from the hourly mean value. By analyzing the daily variation of the hourly anomalous value, we can have a better understanding of the effect of the lake-land thermodynamic divergence (hereafter referred to as the lake-land effect ) on climatic elements. The lake and land breeze generally refers to the conversion between wind currents that blow perpendicularly to the lake bank to and fro the bodies of water and the land. Only when the all-day mean wind or the system wind is weak enough can the lake and land breezes be observed and they might be obscure in case of a strong system wind. However the thermodynamic divergence of the lake and land actually remains. The lake and land effect can influence the strength of the system wind and is reflected on the anomalous variation of the hourly wind. In view of it, we define in this study the anomalous value of the hourly wind blowing perpendicularly to the lake bank as the anomalous lake and land breeze (hereafter referred to as the lake and land breeze) and the added value of the hourly anomalous value and the all-day mean value as the real lake and land breeze, viz. u or v.