The Study Of Low-frequency Oscillation In Tibetan Plateau And The Occurrence And Evolution Of Plateau Low Vortices

Using the history weather map, radio sounding, daily precipitation and ECMWF daily reanalysis data from 1980-2004, with many statistic and diagnostic methods, first, the basic activity features of Plateau Low Vortices (PLV) were analysed statistically, and the causes of the interannual variation of PLV were also investigated on the basis of these. Second, we studied the characteristics of Atmospheric low-frequency oscillation (LFO) over Tibetan Plateau (TP) and its interannual variation. Finally, the causes of PLV cluster-occur which partly arise from LFO over TP were investigated. The results showed as follows:(1) The occurrence frequency of PLVs in summer has evident interdecadal, interannual and intraseasonal variation characteristics, and shows a decreasing tendency in the 1990s compared with that in the 1980s. July is an active period of PLVs. The four sources PLV forming in the Tibetan Plateau are between Shenzha and Gaize, northeastern Naqu, northeastern Dege, and Songpan, and northeastern Naqu was the most generation center which changes monthly. PLVs moving out of the Tibetan Plateau also have four sources: northeastern Naqu, Qumalai, Dege and Maqin. Some PLVs can survive over 36 h in the Plateau and move eastward including three routes (northeast, southeast and east) that PLVs moving toward northeast are in the majority, but the routes out of the Plateau are different from those in the Plateau that most of PLVs are moving toward east first, then toward northeast and southeast. There are two main routes when PLVs are moving out of the Plateau, one is northeast to Hexi, Ningxia and Loess Plateau, the other is southeast to Sichuan Basin, and PLVs moving to Loess Plateau are the most. PLVs move out Plateau also has evident interannual and intraseasonal variation characteristics. Most of PLVs will weaken and vanish during 12 h after moving out of the Plateau, some can persist for 60 h, and only a few for 100 h, even 192 h. Warm PLVs are nearly two times more than baroclinic PLVs in the beginning state, while after moving out of the Plateau, the characteristics of PLVs change a lot during 12 h and most are baroclinic vortices. In contrast to the former study results mainly concluded by the Lhasa meeting group, there are many significant differences on the following aspects: the original sources, the occurrence frequency, the moving direction, and the moving out sources of PLVs.(2)When PLVs occur more or fewer, the rainfall increased or decreased on TP. When more (fewer) PLVs occur, Iran High (IH) is strong (weak) and shifts northward (southward), West Subtropical High Pressure (WPSHP) is strong (weak) and shift southward (northward), Indian Low Pressure (ILP) is weak (strong), the geopotential height at west is high than (equal) east at Middle and high latitude, the south part of TP is trough, but the north part is ridge (there is a little high pressure on the east part of TP) at 500hPa, South Asian High (SAH) is high (weak) on 100Pa. There is abnormal southwest (east) wind at south part of TP, but abnormal east (west) wind at north part of TP. The positive (negative) vorticity and convergence (divergence) is intensive on low layer of TP, but divergence (convergence) at high layer. The humidity is high (low), and the abnormal vertical upward (downward) motion over TP. All above are the advantages (disadvantages) of PLVs occurrence. The general circulation anomaly were token as the mainly factors of interannual variation of PLVs occurrence preliminarily.(3)The LFO over TP is significant. There are two kinds of teleconnection horizontal structures over Tibetan Plateau (TP), one is the Tibetan Plateau-Europe-North Atlantic-North America,the other is the Tibetan Plateau-northeast-Okhotsk-North Alaskan. There are Low-Frequency Systems (LFS) on TP, and they arise on the southeast part of TP and move for northwest of TP. The Low-Frequency Oscillations (LFO) has a barotropic structure on main body of TP and a baroclinic structure on its southern regions, but a barotropic structure on high latitude. The LFO can propagate in both meridional and zonal direction, the low latitude and high latitude LFO can propagate into TP, but the zonal paopagate is not obvious. And sometimes the LFO caused by TP itself which only vibrated on TP. Generally speaking, TP is a sink of LFO. The LFO over TP have intensive interannual variation. The Teleconnection is not existed on some years. The barotropic structure and the propagation are different in different year.(4)The PLVs intraseasonal occurrence has obviously cluster-occur and intermission-occur characteristics. When the PLVs are cluster-occur, the rainfall will increase on TP. The occurrence frequency of cluster-occur is more than intermission-occur and the former is mainly happen on July. The general circulation characteristics of cluster-occur or intermission-occur resemble with the characteristics of PLVs happens more or fewer years. When the general circulation adjusted, there is intensive southwest wind at south part of TP, but weak north wind or even east wind at north part. The convergence happens on the low layer of TP, but divergence at 200hPa. Massive water vapor convergent and move upward, and then coagulates and releases latent heat which heating high layer atmosphere that maintains the intensive divergence. It can strengthen the convergence at low layer and vertical upward motion due to the theory of quality compensation, and positive vorticity can last. PLVs cluster-occur is through this positive feedback. By contraries, when there is no intensive west wind at south part over TP, east wind at north part of TP, divergence at up layer, water vapor convergence at low layer, the PLV is intermission-occur.(5)It is significant that PLVs cluster-occur during the positive phase of west wind low-frequency oscillation in Tibetan Plateau. The main reason probably is that the structure of this low frequency oscillation, that is the characteristic of north-south reverse phase, will make west wind in the south of Plateau strong and weak in the north of Plateau when the low frequency oscillation is at positive phase in the central and south of Plateau, thus abnormal positive vorticity circulation will be stimulated. It is favorable for plateau vortices cluster-occurring.