IMPACT OF LARGE-SCALE CIRCULATION ON THE INTERDECADAL VARIATIONS OF THE WESTERN NORTH PACIFIC TROPICAL CYCLONE

Based on the annual frequency data of tropical cyclones from 1960 to 2005 and by the polynomial fit and statistical analysis, this work has discovered that TC activity in the 46a exhibits significant decadal-scale variability. It has two high frequency periods (HFP) and two low frequency periods (LFP). Significant differences in the number of TCs between HFP and LFP are found in active TC seasons from July to October. Differences of large-scale circulation during HFP and LFP have been investigated with NCEP/NOAA data for the season. In HFP, the condition includes not only higher sea surface temperature, lower sea level pressure, larger divergence of upper air, larger relative vorticity at low levels and smaller vertical shear, but also 500-hPa wind vector being more available for TC activity and moving to western North Pacific, the position of the subtropical anticyclone over the western Pacific shifting more northward, and South Asian Anticyclone at 100-hPa being much smaller than that in LFP. The precipitation of western North Pacific has no clear influence on TC activity.

The Large Scale Circulation of the Snow Disaster in Southern China in the Beginning of 2008

This study analyzed the heaviest snowfalls or icy-rainfalls occurring in southern China from January to the beginning of February 2008.The results are summarized as follows：the disaster was induced by the persistent front of warm/cold air masses in southern China,which displayed an interaction between the weather systems in higher and lower latitudes.There was an adjustment for circulation at hemisphere scale during mid January by a variation of sign of the Arctic Oscillation（AO）index from negative to positive.The long lasting precipitation well coincided with a blocking situation centered near 80°E from mid January to the beginning of February.A diagnostic analysis indicates that stationary waves with an energy dispersion accompanying the blocking high propagated from upstream region in high latitudes to the south of the Yangtze River,which formed a maintaining energy source for the cyclonic circulation in situ.This resulted in that the large mass of cold air in high latitudes could not easily invade into the south but slowly shifted southward.On the other hand,the sea surface temperature（SST）over the warm pool of the western Pacific increased with a new history record due to the effect of the strong La Nina episode,which also blocked the cold air mass from the north.The blocking high collapsed around 30 January and the energy source for the local cyclonic circulation was cut off.Thus,the precipitation in southern China ceased after 1 February.

A Fourth Order Numerical Method for the Primitive Equations Formulated in Mean Vorticity

A fourth-order finite difference method is proposed and studied for the primitive equations(PEs)of large-scale atmospheric and oceanic flow based on mean vorticity formulation.Since the vertical average of the horizontal velocity field is divergence-free,we can introduce mean vorticity and mean stream function which are connected by a 2-D Poisson equation.As a result,the PEs can be reformulated such that the prognostic equation for the horizontal velocity is replaced by evolutionary equations for the mean vorticity field and the vertical derivative of the horizontal velocity.The mean vorticity equation is approximated by a compact difference scheme due to the difficulty of the mean vorticity boundary condition,while fourth-order long-stencil approximations are utilized to deal with transport type equations for computational convenience.The numerical values for the total velocity field(both horizontal and vertical)are statically determined by a discrete realization of a differential equation at each fixed horizontal point.The method is highly efficient and is capable of producing highly resolved solutions at a reasonable computational cost.The full fourth-order accuracy is checked by an example of the reformulated PEs with force terms.Additionally,numerical results of a large-scale oceanic circulation are presented.