Research On Spatial-temporal Distribution And Forecasting Of Cloud Cover For Bohai Sea Region

Cloud is a common weather phenomenon in the atmosphere.It plays an important role in weather and climate change,and has a direct impact on flight and military activities.Bohai Sea region is one of main economic development area in China.Therefore,studying the variation characteristics and refined forecast of cloud cover in this area is of great significance for understanding the mechanism of regional climate change,serving the daily life of residents,and ensuring the normal development of flight and military activities.In this paper,based on the cloud observation data,ERA-Interim reanalysis datasets and prediction products from T639 model,the spatial-temporal variation characteristics and causes of cloud cover,including total cloud cover,low cloud cover,convective cloud cover and non-convective cloud cover,in Bohai Sea region are studied,the correlation between cloud cover and the main meteorological factors affecting cloud formation is analyzed,and time refined quantitative forecast models of cloud cover are established by using multiple stepwise regression.The main results are as follows:(1)There is more annual average,autumn and winter total cloud cover in the south of Bohai Sea region than in the north,and more in the west than in the east.The total cloud cover is relatively evenly distributed in spring,and decreases from southeast to northwest in summer.The annual average and the seasonal low cloud cover distribution is roughly the same.There is more low cloud cover in the south than in the north,and more in the east than in the west.The convective cloud cover in the northern mountainous area is more than that in the southern plain area.The non-convective cloud cover decreases from southeastern coast to inland northwest.The low clouds are dominated by convective clouds in the north and dominated by non-convective clouds in the south.The spatial distribution of total cloud cover is positively correlated with the spatial distribution of ascending movement and water vapor.The spatial distribution of convective cloud cover is positively correlated with large-scale sinking and negatively correlated with water vapor.The spatial distribution of the non-convective cloud cover is only positively correlated with the water vapor in the autumn and winter,and only with the large-scale ascending movement in the spring and summer.The total cloud cover at the two representative stations,Weichang and Juxian,convective cloud cover in Weichang and non-convective cloud cover in Juxian are all significantly changed during a year.The cloud cover is the most in summer,the least in winter and it is more in spring than in autumn.This is mainly caused by the annual variation of vertical movement and water vapor.The diurnal variations of total cloud cover at the two stations are not obvious at all seasons.The convective cloud cover and non-convective cloud cover showed obvious diurnal variation in summer.The convective cloud cover is the least at 08 o'clock.Then the convective activity develops and the cloud increases in the afternoon.The maximum value of cloud cover is reached by 14 o'clock.The maximum value of non-convective cloud cover in summer appears at 05 o'clock,and reaches the minimum value in the afternoon.(2)The effects of various forecast factors on total cloud cover,low cloud cover and non-convective cloud cover in Bohai Sea region are similar.Water vapor,atmospheric instability and atmospheric vertical velocity are significantly correlated with total cloud cover,low cloud cover and non-convective cloud cover almost all year round.Humidity of the whole layer,atmospheric precipitable water and relative humidity of the whole layer,which are related to water vapor,are significantly correlated with cloud cover.Average vertical velocity from 850 hPa to 400 hPa mandatory level is significant,too.K-index and geopotential instability index,which are related to atmospheric instability are also significant.The effects of weather system intensity factors on total cloud cover,low cloud cover and non-convective cloud cover are not obvious.The correlation between forecast factors and convective cloud cover is not as significant as total cloud cover and non-convective cloud cover.Water vapor and atmospheric instability are mainly in the spring,summer and autumn,vertical upward movement of the atmosphere is mainly in summer and autumn,and the weather system intensity factors is mainly in the afternoon of spring and winter,related to the convective cloud cover.The low cloud cover products directly exported by the T639 numerical model is significantly related to total cloud cover,low cloud cover,convective cloud cover and non-convective cloud cover,especially in winter half year.(3)By means of multiple stepwise regression,time refined forecast models of total cloud cover,low cloud cover,convective cloud cover and non-convective cloud cover in Bohai Sea region is established.The complex correlation coefficients of the total cloud cover,low cloud cover and non-convective cloud cover forecast equations are mostly above 0.5,and the complex correlation coefficients of the convective cloud cover forecast equations are mostly above 0.2.The results of back generation test show that the average absolute error of total cloud cover,low cloud cover and non-convective cloud cover are 22,18 and 21 percent,and the average relative errors are 54.6%,62.6% and 67.6%.The forecast verification of total cloud cover shows that the average absolute error of the model is 23 percent and the average relative error is 48.1%.Case studies show that the forecast effect at the two representative stations,Xinmin and Yiyuan is ideal.The forecast values and trends of total cloud cover are close to the actual observations in most periods.The model has good forecast performance and some practical application value.The results in this paper can provide some reference for climate research in Bohai Sea region and the local operational weather forecast.