Research On The Northern Hemisphere Potential Vorticity Distribution Characteristics And Its Relationship With Precipitation Processes

Potential vorticity, which is widely used in the diagnosis and analysis of atmospheric systems, is a physical variable that could synthesized reflect the dynamical and thermodynamics features of atmosphere. By using the geopotential height, temperature and wind field of the NCEP daily reanalysis dataset from1948to2011, and the daily temperature and precipitation data at207meteorological stations in the region of100-110°E,32.4-40°N across the east of Northwest China, this paper calculates the potential vorticity in the Northern hemisphere, and analyzes its horizontal and vertical distribution characteristics as well as the relationship between the potential vorticity and weather process. The results are as follows:First, the horizontal characteristics of potential vorticity in the tropopause mainly appear as zonal distribution in winter and spring. In winter, the high value area mainly locate at the middle-latitude of50°N, and the highest value, which is about5.5PVU, appears in the junction that between Xinjiang and Inner Mongolia of China. The low center mainly locate at the region over the Eastern equatorial Pacific and the Atlantic Ocean. In spring, the high value area move northerly, and the center maximum value increased month by month. The high center appear in the region over the North Pacific and the North Atlantic Ocean. From summer to early autumn(June-September), the distribution of potential vorticity is similar with May’s, and the center of potential vorticity over the North Pacific and North Atlantic Ocean become the strongest. The above maximum center is weakened in autumn, and a low center emerges at the equatorial Pacific Ocean. The value of potential vorticity in the middle troposphere(500hPa), which generally in1PVU or less, is significantly smaller than that of100hPa. The potential vorticity basically appears as zonal distribution in winter and spring. The high values are concentrated in high-latitude(70°N) while the small value focus on low-latitude(from equator to30°N). The maximum center emerges at Northeast Russia, and the value in this center could reach0.75PVU. In addition, the value in the low center are smaller than0.2. In summer and autumn, the potential vorticity in high-latitude forms three high value centers, and the value in these areas could reach0.7PVU. In the low troposphere, the high value of potential vorticity maintain in Alaska, Canada, and eastern Greenland area over the whole year. The high value center mainly exist in eastern Russia and China Bohai area in spring and winter, and the maximum value could reach0.4PVU or more. The low center is located at the equator region, and the minimum value could reach0.1PVU. The main high value area are mainly in60-80°N. This appears as wavy distribution, and the value in the land is higher than that in marine.Second, on the vertical section at30°N, the potential vorticity on each isobaric surface more than400hpa is less than that on the upper isobaric surface, while the potential vorticity on each isobaric surface less than300hpa increases exponentially and very stably. The potential vorticities of all months have little difference at the top of troposphere. On the vertical section at45°N, vertical vorticity presents zonal distribution with parallel contour lines on isobaric surface less than400hpa. The values increase as the pressure decreases and this distribution stays very stable in the whole year. The vertical section at60°N is very similar to that at45°N.Third, the relationship between the weather process in West China and the potential vorticity reveal seasonal differences. Specifically, the relationship is obvious in spring, and an obvious correspondences can be observed in the whole troposphere. The highest correlations appears at500hPa, and the coefficients gradually increased from March to May. The correlations between rainfall and potential vorticity in100hPa are negative while the correlations between rainfall and potential vorticity in500hPa are positive, and the correlations of the third area are the best. The correlation between summer rainfall and potential vorticity is better than spring’s, and the correlation coefficients in three months of summer are generally similar. The correlation between autumn rainfall and potential vorticity is higher than spring and summer because autumn is the rainfall season of East of Northwest China. Specifically, the correlation coefficient in the former two months of autumn are higher than that in November. The correlations significantly decline in winter for the reason that winter is the drought season of East of Northwest China, but the correlations have a slight increasing in February.In short, potential vorticity is a comprehensive physical variable that contains dynamical and thermodynamics features, which has good indication to occurrence, development and evolution of weather systems, is much loved by meteorological workers. This paper uses the long series. a wide range of64years of reanalysis dataset, systematic analyzes the northern hemisphere vortex in the horizontal and vertical distribution characteristics, the research results can provide valuable reference for understanding the spatial distribution characteristics of the northern hemisphere planetary scale weather system and its seasonal changes, especially the research on relation between potential vorticity and precipitation in the eastern part of Northwest China, is an exploration and attempt to find a new starting point for the precipitation in Northwest four subregion.