Using the Micaps regular date,correlation and statistical analyses between sandstorm days and the upper level jet stream between 30～50°N during 2000—2004 are carried out in this study.The results show that the sandstorms accompanied by the upper level jets can be divided into 2 kinds:single jet stream pattern and double jet stream pattern.For single upper level jet patterns,sandstorm is likely to occur beneath the left side and right rear side of the upper level SW or NW jet stream;and for double(a NW plus a SW) jet streams patterns,sandstorm is likely to takes place beneath the left front side of the NW jet and the left rear side of the SW jet,or beneath the right front side of the NW jet and the left rear side of the SW jet.Dynamic analyses on the vertical distributions of wind filed structure and the vertical structure of circulation field for selected typical sandstorm cases are also carried out.The analysis results suggest that the upper level jet associated with sandstorms includes single jet pattern,double jet pattern and even trijet pattern;generally the height of the north branch upper level jet is lower,at 300 hPa,and the south branch jet is higher,at 200 hPa;the maximum wind speed of the south branch is greater than that of the north branch;the north branch jet tilts to the south with height,while the south branch leans northwards with height.Sandstorms for single jet pattern mostly occur beneath the south side of the descending leg of the secondary circulation of the north branch jet,and sandstorms for double jet pattern are likely to occur beneath the merged descending leg of the two jets,sandstorms could occur either in the desceing area of upper level jet or in the ascending area:in the ascending area,the suction resulted from the low level convegence and the upper level divergence induces the low level strong winds and sandstorms,and in the descending area,the downward transfer of upper level momentum is in favor of the genesis of sandstorm.Between the north and south branch jet streams there always is a vertical circulation cell,which facilitates the genesis of sandstorm,therefore both the north branch jet and the south branch jet jointly impact the genesis and development of surface sandstorms.
In this paper, we give the estimations both of spectral and Frobenius norm condition number of a simple matrix. The estimations can be used to measure the sensitivity of the solution of linear systems.
Using the data of ECMWF (European Center for Medium-range Weather Forecasts) to undertake composite diagnoses of 16 explosive cyclones occurring at the Atlantic and the Pacific Oceans,it is found that there are a lot of obvious discrepancies on the basic fields between these strong and weak explosive cyclones.The major reasons why the explosive cyclones over the Atlantic are stronger than those over the Pacific Ocean are that the non-zonal upper jet and the low-level warm moist flow over the Atlantic are stronger.The non-zonal upper jet offers stronger divergence,baroclinicity and baroclinic instability fields for explosive cyclones.Anticyclonic curvature at the high level of strong explosive cyclones is easy to make the inertia-gravitational wave developing at the moment of northward transfer of energy and stimulate the cyclones deepening quickly.Warm advection and diabatic heating can cause the upper isobaric surface lifting,as a result,the anticyclone curvature of cyclones enlarges,and wave energy develops easily as well.The most powerful period of the development of explosive cyclones is just the time when the positive vorticity advection center is located over the low vortex.At the upper level,when the distribution of potential vorticity contours changes suddenly from rareness to denseness,and the large values of the potential vorticity both in the west and north sides of cyclones extend downwards together,then cyclones are easy to explosively develop.The formation of strong explosive cyclones is closely related with the non-zonality of upper jet and the anticyclonic curvature.
In the context of NCEP/NCAR reanalysis data, the potential vorticity (PV) inversion data and numerical simulation are made to examine the genesis of the 0909 strong tropical storm Etau in Chinese convention system. Analysis of PV inversions indicates that the high- and low-level large-value PV disturbances have great impact on Etau's genesis, showing that when the PV disturbances in both the levels approach each other, the middle-lower-layer convergence between them is strengthened, making pressure decrease so as to trigger the fast development of Etau's embryo. Sensitivity experiments show that the appropriate time when high-level large-value PV meet the disturbance of the monsoon trough acts as an important cause of Etau's production, whereupon a conceptual model is constructed for the cyclogenesis of this kind. Besides, the sensitivity runs indicate that Etau has great impact on the strength and track of typhoon Morakot, which is also one of the aspects of practical significance to the study of Etau's genesis.
In order to reveal the climatic variation characteristics of thunderstorm in Fuding under global warming, Using the observational thunderstorm data of meteorological station in Fuding during 1971-2010,and the characteristics of thunderstorm were anlyzed by climate trend rate method and climate trend coefficient method.The results show that;①the annual average number of thunderstorm days is 48 d in Fuding and the total trend of thunderstorm has been decreased in the recent 40 years,the rate of its climate tendency is 2 d/10 a;②the thunderstorm has the obvious seasonal,about 56%thunderstorms occur in summer,Thunderstorm day is the most in August,and then decreases obviously in September,monthly characteristics of thunderstorm day are similar to those of air temperature and precipitation;③the first thunderstorm day is significantly delayed 4 d/10 a,the last thunderstorm day is significantly delayed 4.4 d/10 a,thus the duration of thunderstorm has been slight increased.
In this paper, statistics were analyzed concerning correlation between the storm rainfall far from typhoon and non-zonal upper-level jet stream. The results show that the jet stream at 200 hPa is constantly SW (90.2 %) during the period in which storm rainfall occurs. Rainfall area lies in the right rear regions of the jet axes. While the storm intensifies, the jet tends to be stronger and turn non-zonal. With the MM4 model, nu-merical simulation and diagnosis were carried out for Typhoon No.9711 (Winnie) on August 19 to 20, 1997. The distant storm rainfall is tightly correlative to the jet and low-level typhoon trough. The divergence field of jet is related to the v component. The upper level can cause the allobaric wind convergence at low level. This is the result of the form of low-level typhoon trough and the strength of the storm. By scale analysis, it is found that there is a branch of middle scale transverse inverse circulation in the right entrance regions behind the jet below the 300-hPa level, which is very important to the maintenance and strengthening of storm rainfall. This branch of inverse circulation is relative to the reinforcement of jet's non-zonal characteristics. From the field of mesoscale divergence field and non-zonal wind field, we know that the stronger symmetry caused by transverse circulation in the two sides of the jet, rainfall抯 feedback and reinforcement of jet抯 non-zonal characteristics had lead to positive feedback mechanism that was favorable of storm rainfall抯 strengthening.
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