冷空气对夏季高原涡移出高原后长久与短期活动影响的对比分析

A Comparative Analysis of Cold Air Influences on Short-and Long-time Maintenance of the Tibetan Plateau Vortex after It Moves Out of the Plateau

本文利用NCEP/NCAR-FNL再分析资料、历史天气图和青藏高原低涡切变线年鉴,通过分析1998～2016年高原涡活动情况,对在高原以东活动时间大于96h的高原涡(长持续涡)和在高原以东活动时间不大于30h的高原涡(短持续涡),进行了环流与冷空气活动特征与位涡诊断的对比分析,得出了长、短持续涡的环境场、冷空气活动特征,揭示了冷空气活动、高空锋区对长、短持续涡的影响。结果表明:(1)长持续涡移出高原后是在受较明显冷空气影响情况下加强并持续的,短持续涡则没有明显受冷空气影响。长持续涡所处的低槽较深,槽后的冷温度槽较明显,副热带高压(简称副高)偏南;短持续涡处在分裂槽中,有冷舌,副高偏北;说明影响低涡活动的天气系统强,槽后的冷温度槽明显,副高偏南是低涡能较长时间持续的重要环流条件。(2)长持续涡不仅受到较强冷平流的影响,还处在有狭长的干冷与暖湿空气相遇的地带,使涡区极易产生对流不稳定和低涡扰动,利于低涡加强并持续,短持续涡则远不及长持续涡。(3)长持续涡移出高原后受两个不同方向冷空气影响,涡区内一般伴有两个高位涡中心区,而短持续涡的只有一个高位涡中心区,且位涡值比长持续涡小,长持续涡活动过程中的斜压性也比短持续涡强。另外,长持续涡活动过程中相应的高空急流较强,在增强、东伸、南压作用下,造成200 hPa高空有高位涡下传到低涡,而短持续涡所伴的西风急流平直、弱,造成了短持续涡只受到400 hPa高位涡下传的影响。(4)冷空气影响高原涡维持的作用有:使影响高原涡活动的天气系统加强;使高原涡斜压性增强、对流不稳定增强;使高空有高位涡下传至低涡附近层次,造成低涡区域正位涡异常,垂直涡度发展,低涡加强。

By using the NCEP/NCAR-FNL reanalysis data, historical weather graphs and the Tibetan Plateau vortex and shear line year books, activities of the Tibetan Plateau vortex（TPV） during the period of 1998 to 2016 are analyzed. The persistence（longer than 96 hours） and short-time maintenance（shorter than 30 hours） of the Tibetan Plateau Vortex examples after they moved out of the plateau, the corresponding atmospheric circulations, cold air activity characteristics and potential vorticity are investigated. The background fields and cold air activities associated with the long-time persistence of TPV（LTPV） and short time maintenance of TPV（STPV） are analyzed. The influences of cold air activity and high-level fronts to the two types of TPV are also revealed. Results are as follows.（1） The LTPV is strengthened and persisted due to the influences of obvious cold air, and the STPV is under no influences of obvious cold air. The LTPV is often located in a deep trough with obvious cold trough behind the trough and the subtropical high is leaning to the south. The STPV is usually located in a split trough with cold tongue and the subtropical high is leaning to the north. This means that the weather system that influences the TPV is strong. The distinct cold trough behind the height trough and the northward-leaning subtropical high are important conditions for the LTPV.（2） The LTPV is not only influenced by relatively strong cold advection, but also located in the long and narrow area where the dry cold air and warm wet air converge. This condition easily leads to convective instability and low vortex turbulence, which is favorable for the reinforcement and persistence of the LTPV. The STPV＇s condition is far less than that of the LTPV.（3） The LTPV is influenced by cold air from two different directions and usually accompanied with two high potential vorticity centers; the STPV only has one high potential vorticity center, and the potential vorticity value is lower than that of the LTPV. The LTPV has stronger baroclinity than that of the STPV. Furthermore, the LTPV is often accompanied with consistent and strong high-level jet stream, which leads to the transfer of the 200 h Pa high potential vorticity to the TPV when strengthening and expanding to the east and pushing to the south. But the STPV corresponds to a weak and straight westerly jet stream, which can only transfer high potential vorticity from 400 hPa to the TPV.（4） The mechanism for cold air to influence the maintenance of TPV is that the cold air strengthens the corresponding weather systems, increases the baroclinity and convective instability of the TPV. And the cold air enables the high level potential vorticity to spread to the low level area close to the TPV and increases the positive vorticity in the TPV region, which increases vertical vorticity and strengthens the TPV.