Dry Intrusion Analysis of A Heavy Rainstorm in Southern Shaanxi

[Objective] One regional rainstorm weather in the south area of Shaanxi was expounded.[Method] By employing the NCEP/NCAR 1°×1°reanalysis data,routine observation and automatic precipitation station data,the large scale circulation background field of regional precipitation in the south area of Shaanxi province from July 16-18 in 2010 was expounded.By dint of physical quantity such as the meridional wind,relative humidity and false relative temperature,the characteristics of dry intrusion and its role in rainstorm were expounded.[Result] The mild and high latitude at 500 hPa was stable.The continental high pressure and subtropical high pressure were stable.The 700 and 850 hPa shear lines or low vortexes were the main influence system that resulted into large scale of rainstorm in south Shaanxi.The landing typhoon had distinct influences on the importance of storm.The influence of typhoon and subtropical high was mutual.It not only formed and strengthened the torrent in the low air,also transmitted water vapor and energy incessantly to the south area of Shaanxi,playing an important role in augment of rainstorm.There were two distinct dry intrusions in the rainstorm process,showing a dense dry layer in the high layer of convective layer.The existence of dry layer was conductive to the accumulation of convective unstable energy in the low layer.The release of convective unstable energy could transmit warm and wet airstream in the low level to the higher level,being conductive to the occurrence of precipitation.The analysis of θse in the warp direction vertical profile suggested that there was frontal zone in this rainstorm process and the dry air activity in this rainstorm process was very strong.[Conclusion] The study provided theoretical reference to the forecast in future.

Analyses of Dry Intrusion and Instability during a Heavy Rainfall Event that Occurred in Northern China

The 1°×1° National Center for Environmental Prediction/National Center for Atmospheric Research （NCEP/NCAR） data and mesoscale numerical simulation data are analyzed to reveal a mechanism for the formation of heavy rainfall in Northern China; this mechanism is the non-uniformly saturated instability induced by a dry intrusion. The dry intrusion and the accompanying downward transport of air with a high value of potential vorticity （PV） are maintained during the precipitation event. As the dry air intrudes down into the warm and moist sector in the lower troposphere, the cold, dry air and the warm, moist air mix with each other, and, as a result, the atmosphere becomes non-uniformly saturated. On the basis of this non-uniform saturation, a new Brunt-Vaisaila frequency （BVF） formula is derived and applied to the precipitation event. It is shown that, compared to the conditions of either a dry or a saturated atmosphere, the BVF in a non-uniformly saturated, moist atmosphere （BVF） may be more appropriate for depicting the atmospheric instability in rainy regions.

An Atmospheric Dry Intrusion Parameter and Its Application

Dry intrusion plays an important role in the explosive development of cyclones and the evolution of cold fronts. Characteristics of dry intrusion during a rainfall event that occurred in northern China are analyzed in detail in this paper. The IM （ingredients-based methodology） developed by Doswell et al. in 1996 and Wetzel and Martin in 2001 is utilized. All the physical representations of dry intrusion defined in the past studies, such as low relative humidity, cold advection, and high potential vorticity （on either isobaric or isentropic surfaces）, are combined into a simple and convenient physical parameter to characterize dry intrusion. This is a new attempt to extend the IM that was primarily applied to research on heavy rainfall to the study of dry intrusion. The new dry intrusion parameter is used to analyze the isentropic evolution of dry intrusion during the rainfall event. The results show that this parameter can better quantify the intensity of dry intrusion and diagnose its evolution shown in satellite infrared and water vapor imageries. It is found that dry intrusion maintains during the rainfall period. The intensity of precipitation increases with the increasing dry intrusion, which has pushed the rainy region southeastward. From the results on the isentropic surface and the corresponding isobaric surface, it is inferred that the analyses of dry intrusion on both surfaces are consistent with each other. The isentropic analysis of dry intrusion reveals that cold and dry air at the upper level overruns that in the lower troposphere where moist and warm air is located. Thus, potential instability is built up in the vertical direction, which favors the occurrence of precipitation. In practice, we may identify dry intrusion regions by tracking strong signals of the dry intrusion parameter, and further identify the instability near the dry intrusion regions. This will aid in improving the accuracy of precipitation forecast.

Comparative analysis of dry intrusion in the different position of pre-TC squall line on typhoon Lekima(1909) and Matsa(0509)

Using conventional observation data,FY2 satellite data,typhoon data and NCEP reanalysis data,the different position of Pre-TC squall line accompanying the similar tracks with"Lekima"(No.1909)and"Matsa"(No.0509)is studied.The article analyzes the circulation background and trigger mechanism generated by the two pre-TC squall lines,and uses various physical quantities such as low-level vertical wind shear,humidity,wind field,wet potential vorticity,and vertical vorticity.Also,try to study the dynamic mechanism of dry intrusion during the process using the Slantwise Vorticity Development(SVD)theory.The results show that:the high-altitude system and the ground trigger mechanism produced are significantly different.On the high-altitude system,the"Lekima"squall line is generated between the continental high pressure and the typhoon with the northeast jet.The"Matsa"squall line occurred between the typhoon and the subtropical high,and near the typhoon inverted trough,which was generated with the strong easterlies.On the ground trigger mechanism,the"Lekima"squall line is generated at the convergence line under the intersection of northeast and northwest air currents.The"Matsa"squall line is generated in the ground inverted trough moves westward and meridional degree increases.During the two pre-TC squall lines,low-level vertical wind shear values are above 16 m·s-1.The dry air intrusion in the lower layer came from the upper and middle troposphere in the vertical direction,and in the east from the west along the latitude in the horizontal direction.Theoretical analysis base on the development of wet potential vorticity,vertical vorticity,and inclined vorticity indicate that the downward transmission of the high potential vorticity band on the upper troposphere,the level of dry invasion,and the air mixing of equivalent potential temperature at high and low levels play an important role in the generation and development of the preTC squall line.The slantwise vorticity has a good corresponding relationship with the vertical vorticity over the pre-TC squall line.The two development processes of the pre-TC squall line can be better explained by using the SVD theory.

Characteristics of an Explosive Cyclone over Northeast China Revealed by Satellite Water Vapor Imagery

In this paper,an explosive cyclone(EC)that occurred over Northeast China in the spring of 2016 is studied by using 6.7μm FY satellite water vapor(WV)imagery and NCEP(1°×1°)reanalysis data.Moreover,the evolutions of the upper-level jet stream(ULJ),the vertical motions,and the potential vorticity(PV)are analyzed in detail.Results show that different shapes of the WV image dark zones could reflect different stages of the EC.At the pre-explosion stage,a small dark zone and an S-shaped baroclinic leaf cloud can be found on the WV imagery.Then the dark zone expands and the leaf cloud grows into a comma-shaped cloud at the explosively developing stage.At the post-explosion stage,the dark zone brightens,and the spiral cloud forms.The whole process can be well described by the WV imagery.The dynamic dry band associated with the sinking motion and the ULJ can develop into the dry intrusion later,which is an important signal in forecasting the EC and should be paid attention to when analyzing the WV imagery.Furthermore,the mechanism is also analyzed in detail in this article.EC usually occurs in the left-exit region of the 200-h Pa jet and the region ahead of the 500-h Pa trough where there is significant positive vorticity advection(PVA).When the EC moves onto the sea surface,the decreased friction would favour the development of the EC.The upper-level PVA,the strong convergence at low level,and the divergence at high levels can maintain the strong updraft.Meanwhile,the high PV zone from the upper levels extends downward,approaching the cyclone.Together,they keep the cyclone deepening continuously.

Tropical cyclogenesis:Controlling factors and physical mechanisms

In this review,advances in the understanding of the controlling factors and physical mechanisms of tropical cyclogenesis(TCG)are summarized from recent(2018–2022)research on TCG,as presented in the Tenth International Workshop on Tropical Cyclones(IWTC-10).Observational,theoretical,and numerical modeling studies published in recent years have advanced our knowledge on the influence of large-scale environmental factors on TCG.Furthermore,studies have shown clearly that appropriate convective coupling with tropical equatorial waves enhances the development chances of TCG.More recently,illuminating research has been carried out on analyzing the mechanisms by which oscillations and teleconnections(El Ni˜no Southern Oscillation(ENSO)in particular)modulate TCG globally,in association with changes in the sea surface temperature(SST).In addition to this,recent research has diligently addressed different aspects of TCG.Multiple studies have reported the applicability of unified theories and physical mechanisms of TCG in different ocean basins.Recently,research has been carried out on TCG under different flow pattern regimes,dry air intrusion,importance of marsupial pouch,genesis of Medicanes,wind shear,convection and vertical structure.Furthermore,studies have discussed the possibility of near equatorial TCG provided that there is enough supply of background vertical vorticity and relatively low vertical wind shear.Progress has been made to understand the role of climate change on global and regional TCG.However,there are still significant gaps which need to be addressed in order to better understand TCG prediction.