Characteristics Of Water Vapor Content In Henan Province And Its Relationship With Summer Rainfall

Dataset of monthly mean surface vapor pressure (SVP) from49stations over Henan Province for the period1979to2008, the corresponding monthly mean air temperature and depression of the dew point from two sounding stations located in Zhengzhou and Nanyang in addition are exploited to formulate the empirical equations of water vapor content (WVC) which could be applied in Henan Province by the method of least squares regression. On this basis, detailed investigation is carried out to explore the annual and seasonal characteristic of temporal and spatial variation in WVC. Referred to the monthly mean precipitation data from17surface stations in Henan, the relationship between WVC and precipitation on the ground is explored in summer. Primary discussion about circulation background and moist condition contributed to the abnormal WVC is also undertaken in this paper. At last, we analyzed the precipitation conversion rate in summer over Zhengzhou. The specific conclusions are as follows:(1) The method of least squares regression is applied to build the empirical formulas in regard to SVP observed in49surface stations and WVC calculated by using data from Zhengzhou and Nanyang sounding station. Then WVC in each station could be figured out depending on these equations built. The formulas put forward in this paper with higher accuracy are practically convinced to be more suitable in Henan with not large area and less complicated terrain after error analysis and comparison with WVC originated from classic empirical equation and NCEP/NCAR reanalysis data respectively.(2) Spatial distribution characteristic of WVC indicates a uniform descending trend from Southwest to Northeast at both annual and seasonal time-scale while distinct temporal discrepancies in WVC do exist in different season. Summer is the season with maximum difference, followed by spring, autumn, and winter with minimum difference.(3) Consistent change of WVC during four seasons appears to be the principal characteristic on its spatial distribution in Henan. When it comes to temporal evolution, however, WVC is inclined to show multi-time-scale variation feature in seasons. WVC in spring exits three characteristic time scale,4～5a, quasi-7a, and12～14a; WVC in summer and autumn presents the oscillation cycle of quasi-5a, which respectively have the scillation period of9～10a and7～8a, WVC in winter has the variation period of3～4a and12～13a. WVC indicates a positive linear trend for various degrees in seasons. Of which the upward trend is the most significant in winter, followed by spring and summer, with autumn the last.(4)WVC is strongly related to surface precipitation in summer. The result from SVD analysis turns out that the first two modes reveal a consistent and a north-south opposite variation in the spatial distribution of WVC and precipitation respectively in summer over the whole province. When the amount of WVC is higher(lower), more(less) precipitation could be generally observed on the ground in most regions.(5) It is suggest that the anomaly of WVC is associated with atmospheric circulation. In the years with more(less) WVC in Henan, remarkable south(north) wind anomaly is maintained and summer monsoon propagates apparently(unapparently) over the whole East-Asia region which are favorable(unfavorable) to the northward transportation of warm moist vapor from Pacific Ocean and eventually lead to the more(less) WVC than mean in summer.(6) In summer, there are similar interannual variations in precipitation conversion rate and WVC over Zhengzhou for nearly45a(1964-2008), both of which have a linear upward trend and an abrupt mutation in the early21st century.