| Atmospheric visibility refers to the greatest level of target distance that one person with normal vision can see under certain weather conditions, it is closely related with the cleanliness of the atmosphere. Atmospheric visibility in the city to some extent is influenced by the urban construction, industry and the development of transportation. Poor visibility is harm to aviation, marine and urban traffic. Therefore, the study of urban low-visibility will be beneficial to the traffic safety around the city especially on the thruway, as well as the qualities of people's lives. Bohai Coastal Region is located in the east of China, with the longitude from 115.5°E to 124°E and the latitude from 36°N to 41.5°N. The range of Bohai Coastal Region covers five provinces or cities, they are Liaoning, Hebei, Beijing, Tianjin, and Shandong. This range is with lower elevation, and based on the plains intertwined with hills to form a more complex terrain mainly. At the same time, Bohai Coastal Region is the important economic developing area with developed industry, convenient transportation and developed thruways. However, with the effect of the Bohai Sea, the climate change in this area shows some specificity, especially the low-visibility caused by the heavy fog. The low-visibility makes important effect in the regional economic development, human living and transportation. Therefore the study on spatial-temporal distribution and forecasting of visibility for BoHai Coastal Region are important to the economic and social development in Bohai Coastal Region.The visibility data of the 25 representative stations from 1971 to 2005 in Bohai Coastal Region are used to analyze the regional climate characteristics including the characteristics of the annual and monthly changes of visibility. The results showed that:1. Visibility distribution in Bohai Coastal Region is significantly different. Visibility near the Bohai Sea is relatively high, away from the coast, visibility is relatively low; in industrialized areas, visibility is relatively low, that indicate that the visibility are inextricably linked with urban development and human activities;2. The trend of visibility during 1961-2005 was mainly decline in Bohai Coastal Region. Before 1980's, the level of visibility is higher than that after 1980's. But since 2003 the visibility has gradually increased.3. Monthly by monthly changes of visibility show two high-value pattern, and high values appear in mid-May and late September to early October, the low values appear in late July.4. Level of visibility in Bohai Coastal Region show obviously diurnal variation, the lowest is at 08:00, second lowest is at 02:00, highest is at 14:00, and second highest is at 20:00.5. In Bohai Coastal Region fog is the main reason for the low visibility. weather situation at 500hPa that heavy fog occurs can be divided mainly into three types:anterior ridge and behind trough type, anterior trough type, and ridge controlled type. The three type accounted for 87%of the total heavy fog weather, weather situation at surface that heavy fog occurs can also be divided mainly into three types:bottom of mobility high pressure, anterior frontal cyclone, and equal pressure or saddle field. The three type accounted for 91%of the total heavy fog weather.6. Low-visibility prediction equation have been established for the 5 representative meteorological station (Jinzhou, Qinhuangdao, Huimin, Chengshantou, Dalian), and tested the prediction equation by return prediction. The accuracy was 77%,74%,81%,82%,77% respectively for Jinzhou, Qinhuangdao, Chengshantou, and Dalian.
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