A Dynamic Study On The Structure And Variation Of The Mesoscale Upper-level Jetstream-Front Cloud Systems And Rainstorms

Upper-level jet-front system is one of the most outstanding weather systems inthe upper troposphere. It plays an important role to the development of themesoscale disaster weather systems. By comprehensive inspection using thesounding, satellite and radar observations, theory analysis and modeling simulationsby WRF model, a high resolution model for meso-scale and micro-scale weatherresearch developed by NCAR U.S.A, this paper is aimed at exploring the structuresand dynamic mechanisms of the subtropical upper-level jet-frontal zone cloudsystems related with three heavy rain processes over China occurred on 9-11 June2005, 24-26 May 2006 and 7-10 June 2006 respectively. The major results made inthe course of this study are listed as follows:1. A typical upper-level jet-front cloud system may have four characteristicfeatures: a baroclinic leaf cloud, a dark area, vortex comma clouds andasequential convective cloud clusters or transversal cloud lines. A baroclinic leafis often observed on the cyclonic side vicinity of an upper level jet orientedsouthwest-northeast. Composed of high-or medium-level clouds, a typical cloudleaf is normally opaque in IR and VIS images. With respect to its microphysicalstructure, the upper-level part of the cloud leaf is mainly composed of dense iceand snow crystals, while the lower-level part plentiful liquid water. Those liquidcloud droplets involved in the cloud leaf are seem to be concentrated within thesouth-west part of the cloud corresponding with the rainfall on the anti-cyclonicside of jets. A dark area is observed on the cyclonic side of the jet distributingalong the jet edge in WV (water vapor) images. This dark area is then advecteddownstream with environment flow and wrapped into the cloud head andgradually evolves the downstream cloud into a comma cloud. Some commacloud on the cyclonic side of jetstream sometimes display as a sequence ofconvective cloud clusters or some transversal cloud lines at its nascent stage.These kinds of convective cloud clusters or some transversal cloud lines areoften seen formed within the left-exit zone of upper-level jet and developedwhen the upper-level jet is closed to or the jet core is strengthened. The microphysical structure of the cloud within this area is generally same as that ofthe cloud leaf on the anti-cyclonic side of the jet, except that the ice and liquidwater content of the former are some lower than the latter.2. The cloud leafs in both south and north sides of the jet stream can be identifiedas a frontal cloud, while with different development machanisms. According tothe trajectory analysis, the cloud leaf is affected by three branches of flowduring its development: southwesterlies, southeasterlies and northwesterlies.Both the south-westerlies and south-easterlies are from the mid-lower level andare responsible for the cloud leaf genesis. A part of cold and dry air parcels onthe dry belt within north-westerlies spreads downward from upper-level andintrudes into the stratified rainfall area at the end of the cloud leafs. This airflowencountered with the dowdraft air by rainfall makes the total dowdraftstrengthening in such a manner as to cause the rear of the cloud dissipating;while the other part turn to an updraft at the middle level after a short termdescending. This branch of airflow superposites on the warm moist air from thelower which cause the local atmosphere instability increase rapidly. Thereforethe whole cloud band on the anti-cyclonic side of jet can be manifested as theoutcome of the interplay of the dry and warm conveyor belts. The cloud band onthe cyclonic side is remained as a frontal cloud. Comparing the cloud betweenthe two sides of the jet, the frontal cloud on the cyclonic side is relativelythinner with lower height. With respect to the genesis of the cloud on thecyclonic side, it is speculated to be generated in the course of the upstreamnorthwest flow to the west of the pressure trough being abruptly changed fromdescending into ascending by encountering a surface low. During the clouddevelopment, the updraft is included with a part of continuous warm and moistairflow.3. The dark area on the cyclonic side of the upper-level jet in WV imageriescorresponds to a low moist and high PV region. In the PV perspective, thisphenomenon signifies that the dark area on the cyclonic side of the upper-leveljet is the source of the dry intrusion from the upper-level. Dry intrusion acts as apromoter to the development of the clouds on both side of upper-level jet. Onthe one hand, it decreases the moisture at the middle level of the tropospheremaking the instability increase which is good for the deep convectiondeveloping in the cloud cluster. On the other hand, dry intrusion can cause the high-value PV at the upper level of the troposphere glide down to thelower-level to trigger a surface cyclone. According to the trajectory analysis, dryintrusion may be a key mechanism for the cloud clusters on the cyclonic side ofthe upper-level jet taking on a comma-shape finally. The moist potentialvorticity distribution shows characteristics of moist baroclinicity along the jetaxes. With respect to the stability, it seems that the conditional instability andthe instability-related tilt updraft are the possible dynamic mechanisms fortriggering and maintaining the cloud leaf on the anti-cyclonic side of theupper-level jet.4. From the characteristics of the energy distribution, it is seen that the instableenergy is not symmetrically distributed in a jet-front cloud system. There aretwo CAPE centers. One is located at the rear of the cloud band on theanti-cyclonic side of the jet; the other is within a small area between the head ofthe comma cloud and its maximum inflection area on the cyclonic side of the jet.While the maximum on the DCAPE distribution are located on the left side ofthe enter area and right side of the exit area of the jet respectively, whichcorrespond to the dark areas in satellite images. This fact seems to further verifythat the dark area on the cyclonic side of the jet can be identified as the token ofthe dry intrusion from the upper level.5. With respect to the kinematic structure of an upper-level jet, ageostrophic windson the two ends of a jet streak (enter area and exit area) have evident cyclonicshear, while ageostrophic winds on the south and north sides of the jet streakanti-cyclonic shear. On the vorticity field of ageostrophic wind, the two endsalong the jet streak correspond to positive vorticity areas, while the south andnorth sides of the jet steak correspond to negtive vorticity areas. Such a structurein ageostrophic vorticity field generally coincides with the "four quadrant"model of ageostrophic vorticity for a straight jet streak suggested byCunningham and Keyser in 2000.6. Diagnosis study based on the Lagrangian Rossby number (Ro) and the residualof nonlinear balance equation (ANBE) shows that the cyclonic side of theupper-level jet is occupied by both the high value areas of Ro and ANBE. Thescale of theâ–³NBE around this area is over 10^-8s^-1. According to the concept ofâ–³NBE, it is manifested that there is an evident ageostrophic on the cyclonic sideof the upper-level jet and with the increasing curvature of the jet, the ageostrophic enhances. Besides, according to the diagnosis study of shearinstability around the jet based on Richardson number (Ri), it is noticed that theexit area of the upper-level jet is a shear instability area where mesoscale gravitywave and heavy rainfall may occur.7. In the light of the advantage of wavelet analysis in the multi-resolutionfrequency filtering, a band pass filter is designed based on wavelet transform foranalyzing the characteristics of the gravity wave around the exit area of anupper-level jet. The analysis results show that the horizontal wavelength of thegravity wave around the exit area of the subtropical upper-level jet in thevicinity area of China is about 100-500km, and the amplitude 1-10hPa. Besides,the wave in the vertical velocity field is about 1/4 wavelength behind that in thepotential temperature field. Such facts display the common characteristics of atypical mesoscale gravity wave. Considering the cloud distribution, it seems thatthe sequential convective cloud clusters or transversal cloud lines on the left sideof the jet exit area are close related with this kind of meso-scale gravity wave.8. With respect to the influence of the geostrophic adjustment and the shearinstability on the mesoscle gravity wave on the left side of the jet exit area, itseems that the mesoscale gravity wave is generated through the quickening ofthe geostrophic adjustment and increase of the shear instability. And the locationof the mesoscale gravity wave is just within the superposition of the downstreamunbalanced energy radiation area and the shear instability area. As for the timesequence of the geostrophic adjustment and the shear instability, the formerseems occur ahead of the latter. Superposing the PV andâ–³NBE on the cyclonicside of the upper-level jet shows the maxima of PV correspond closely to thosepositiveâ–³NBE maxima in this region. According to the definition of PV andâ–³NBE, it is speculated that the mesoscale gravity wave within the exit area ofthe upper-level jet is possible induced by a strong imbalance flow generatedfrom the upper-level jet-front system.9. An elementary diagnosis of the dynamic mechanism of the jet-front systemrainfall in the eastem central portion of Heilongjiang province on 10th June 2005(to be called as "05.06" northeastern rainstorm hereafter) is performed. Theresults show that this rainstorm occurred in the process of a forward tiltingupper-level trough with a diverging dispersive structure, moving eastward anddeepening. The rain-producing MCSs (Meso-scale Convective Systems) systems are in the foreside area of this upper trough, and the large-scale kinetic energysuitable for system development is supplied by the low level convergence-upper level divergence mechanism; a SW-NE oriented moist tongue located atthe lower level of troposphere was obviously seen before the rainstorm occurs,which feeds the rainfall area with favorable moisture condition; and thedifferential advection induced by dry and cold air superposing the warm andmoist air results in the increase in local instability. Furthermore, the distributionof incoming solar radiance at underlying surface shows differential heatingwhich is an important trigger to the MCSs of this rainstorm. As for thecharacteristics of the mesoscale convective system (MCS) during this rainstorm,the MCS is an isolated convective system with a multi-cell storm structure; andthe system is characterized with a backward input and forward output structure.As for the movement direction, this MCS belongs to a left-moving storm.10. As far as the influence of the upper-level jet on the "2005.06" rainstorm isconcerned, the space-temporal characteristics of the rain band and convectivecloud clusters displayed in the satellite and radar imageries finely agree with thatof the mesoscale gravity wave triggered by the upper-level jet on the left side ofits exit area. Such facts imply that the mesoscale gravity wave triggered by theupper-level jet on the left side of its exit area may therefore have played animportant role in initiating and maintaining this rainstorm. It is the gravity waveinduced by the jet that transports the energy at the upper-level downward tostrengthen the wave at the lower-level. The rainfall maxima are located at thearea where the waves at upper and lower levels happen to have the samespace-temporal phase. Furthermore, with respect to the relationship betweenmeso-scale shear line and the MCSs, it is found that over the shear line, theconvective cells located around the bend part of the shear line are the mostintense. After a comprehensive analysis, two possible explanations to thisphenomenon are considered. One is related to the orientations between thesurface meso-scale shear line and the ambient wind fields and the other is tied tothe relationship between the radiation direction of the meso-scale gravity waveand the orientation of the moist tongue. As for the formation mechanism of theleft-moving MCS, the shear instability induced by the upper-level jet may haveacted as a key role.