Evolution of Instability before and during a Torrential Rainstorm in North China

NCEP-NCAR reanalysis data were used to analyze the characteristics and evolution mechanism of convective and symmetric instability before and during a heavy rainfall event that occurred in Beijing on 21 July 2012.Approximately twelve hours before the rainstorm,the atmosphere was mainly dominated by convective instability in the lower level of 900-800 hPa.The strong southwesterly low-level jet conveyed the moist and warm airflow continuously to the area of torrential rain,maintaining and enhancing the unstable energy.When the precipitation occurred,unstable energy was released and the convective instability weakened.Meanwhile,due to the baroclinicity enhancement in the atmosphere,the symmetric instability strengthened,maintaining and promoting the subsequent torrential rain.Deriving the convective instability tendency equation demonstrated that the barotropic component of potential divergence and the advection term played a major role in enhancing the convective instability before the rainstorm.Analysis of the tendency equation of moist potential vorticity showed that the coupled term of vertical vorticity and the baroclinic component of potential divergence was the primary factor influencing the development of symmetric instability during the precipitation.Comparing the effects of these factors on convective instability and symmetric instability showed some correlation.

Spatiotemporal Evolution of Stimulated Raman Scattering in the Absolute and Convective Regimes

A three-wave interaction （3WI） code is developed to study the stimulated Raman scattering （SRS） in both absolute and convective regimes. In the simulations, the time and spatial evolutions of a plasma wave are described by temporal growth rate and spatial factor, respectively. The spatial factors in different phases and different instability regimes are investigated. It is found that the spatial factor is caused by the finite velocity of the pump wave in the first phase and by damping in the last phase. With inclusion of the spatial factor, the temporal growth rate decreases and the threshold for SRS for a finite frequency mismatch increases. Meanwhile, the effects of wave frequency mismatch on the temporal growth rate are also discussed.

Absolute and Convective Instabilities of Two-Plasmon Decay in an Inhomogeneous Magnetized Plasma

Three-wave resonant parametric decay instability of extraordinary wave decay into two upper hybrid waves in an inhomogeneous plasma is studied theoretically. Analytical expressions of the local absolute growth rate, convective amplification factor and threshold intensity are obtained. The calculated results show that the effects of magnetic field and ky （ICy is the component of the wavenumber of upper hybrid wave perpendicular to pump wave k0） on the growth rate, amplification factor and threshold intensity are extremely dependent on their strength. The absolute growth rate and convective amplification factor increase with the plasma density while the threshold decreases. Moreover, the expression indicates that the inhomogeneity scale length of density and linear damping will reduce the convective amplification factor.

Influence of South China Sea SST and the ENSO on Winter Rainfall over South China

The present study investigates the influence of South China Sea （SCS） SST and ENSO on winter （January-February-March; JFM） rainfall over South China and its dynamic processes by using station observations for the period 1951-2003, Met Office Hadley Center SST data for the period 1900-2008, and ERA-40 reanalysis data for the period 1958-2002. It is found that JFM rainfall over South China has a sig- nificant correlation with Nio-3 and SCS SST. Analyses show that in El Nio or positive SCS SST anomaly years, southwesterly anomalies at 700 hPa dominate over the South China Sea, which in turn transports more moisture into South China and favors increased rainfall. A partial regression analysis indicates that the independent ENSO influence on winter rainfall occurs mainly over South China, whereas SCS SST has a larger independent influence on winter rainfall in northern part of South China. The temperature over South China shows an obvious decrease at 300 hPa and an increase near the surface, with the former induced by Nio-3 and the latter SCS SST anomalies. This enhances the convective instability and weakens the potential vorticity （PV）, which explains the strengthening of ascending motion and the increase of JFM rainfall over South China.

Control of Karman Vortex Street behind a Thin Airfoil at Low Reynolds Number

To control the Karman vortex street formed behind a thin airfoil at a certain incidence, a control cylinder was placed at the suction side apart from the airfoil in the downstream region. Both smoke visualization and hot-wire measurements confirmed that the Karman vortex street was completely suppressed. The mechanism for suppression of the vortex street was examined with a hot-wire survey.

A Comparative Study of the Atmospheric Layers below First Lifting Condensation Level for Instantaneous Pre-Monsoon Thunderstorm Ocurence at Agartala(23°30′N,91°15′E) and Ranchi(23°14′N,85°14′E) of India

An attempt has been made to investigate the role of vertical wind shear, convective instability and the thermodynamic parameter ( θ es -θ e ) below the first lifting condensation level (FLCL) in the occurrence of instantaneous premonsoon thunderstorm over Agartala (AGT) and Ranchi (RNC) at 12 GMT. Radiosonde data of 1988 have been utilized here. The study has however been confined to 1000 hPa-500 hPa range at most. Here the convectively unstable layers with positive vertical wind shear upto 500 hPa have been termed as ‘Favourable Layers’ (FL) and the level at which an initially stable layer turns out to be convectively unstable for the first time has been termed as ‘Transition Level’ (TL). It is observed that the changes in vertical wind shear are positive at TL at the time of occurrence of thunderstorm (TS) and the corresponding change is negative on fair-weather situation. Moreover, the 90% confidence interval for ( θ es -θ e ) reveals that for AGT the upper layer thermodynamic characteristic is important at the time of occurrence of TS whereas for RNC, the value of ( θ es -θ e ) at the surface is much more effective.

Mesoscale Modeling Study of Severe Convection over Complex Terrain

Short squall lines that occurred over Lishui, southwestern Zhejiang Province, China, on 5 July 2012, were investigated using the WRF model based on 1°× 1° gridded NCEP Final Operational Global Analysis data. The results from the numerical simulations were particularly satisfactory in the simulated radar echo, which realistically reproduced the generation and development of the convective cells during the period of severe convection. The initiation of this severe convective case was mainly associated with the uplift effect of mesoscale mountains, topographic convergence, sufficient water vapor, and enhanced low-level southeasterly wind from the East China Sea. An obvious wind velocity gradient occurred between the Donggong Mountains and the southeast coastline, which easily enabled wind convergence on the windward slope of the Donggong Mountains; both strong mid–low-level southwesterly wind and low-level southeasterly wind enhanced vertical shear over the mountains to form instability; and a vertical coupling relation between the divergence on the upper-left side of the Donggong Mountains and the convergence on the lower-left side caused the convection to develop rapidly. The convergence centers of surface streams occurred over the mountain terrain and updrafts easily broke through the lifting condensation level（LCL） because of the strong wind convergence and topographic lift, which led to water vapor condensation above the LCL and the generation of the initial convective cloud. The centers of surface convergence continually created new convective cells that moved with the southwest wind and combined along the Donggong Mountains, eventually forming a short squall line that caused severe convective weather.

Instabilities of thermocapillary-buoyancy convection in open rectangular liquid layers

This article presents the experimental investigation on instabilities of thermocapillary-buoyancy convection in the transition process in an open rectangular liquid layer subject to a horizontal temperature gradient. In the experimental run,an infrared thermal imaging system was constructed to observe and record the surface wave of the rectangular liquid layer. It was found that there are distinct convection longitudinal rolls in the flow field in the thermocapillary-buoyancy convection transition process. There are different wave characterizations for liquid layers with different thicknesses. For sufficiently thin layers, oblique hydrothermal waves are observed, which was predicted by the linear-stability analysis of Smith ＆ Davis in 1983. For thicker layers, the surface flow is distinct and intensified, which is because the buoyancy convection plays a dominant role and bulk fluid flow from hot wall to cold wall in the free surface of liquid layers. In addition, the spatiotemporal evolution analysis has been carried out to conclude the rule of the temperature field destabilization in the transition process.

Three dimensional simulations of penetrative convection in a porous medium with internal heat sources

The problem of penetrative convection in a fluid saturated porous medium heated internally is analysed. The linear instability theory and nonlinear energy theory are derived and then tested using three dimensions simulation.Critical Rayleigh numbers are obtained numerically for the case of a uniform heat source in a layer with two fixed surfaces. The validity of both the linear instability and global nonlinear energy stability thresholds are tested using a three dimensional simulation. Our results show that the linear threshold accurately predicts the onset of instability in the basic steady state. However, the required time to arrive at the basic steady state increases significantly as the Rayleigh number tends to the linear threshold.

Analysis on Climatic and Synoptic Characteristics of Thunderstorm Gale Weather in Jiangsu

By using observation and sounding data at 68 artificial observatories of Jiangsu Province during 2009- 2013,thunderstorm gale weather and its climatic characteristics in Jiangsu were conducted statistics. The characteristics of some instability indexes and strong convection parameters were analyzed,and environmental parameters of dry and wet thunderstorm gales were contrasted. Results showed that thunderstorm gale in Jiangsu had the characteristics of high occurrence frequency,local feature and stronger intensity. It was mostly accompanied by precipitation,and had obvious seasonal and daily change characteristics. Synoptics analysis showed that temperature-humidity profile characteristics before thunderstorm gale appeared in Jiangsu mainly had four types： bell mouth type,inverted V type,dry unstable type and wet unstable type. Before thunderstorm gale occurred,atmospheric instability was stronger,and some strong convection parameters all had certain instructions. But the forecasts of some thunderstorm gale processes were easy to be missed by only considering CAPE. Environmental condition difference of generating dry and wet thunderstorm gale was that instability of dry thunderstorm gale was stronger than that of wet thunderstorm gale. Before dry thunderstorm gale occurred,environmental temperature at middle-low layer was lower; lapse rate was larger; humidity was small. Before wet thunderstorm gale occurred,environmental temperature was higher; lapse rate was small; humidity was large. At dynamic structure,vertical wind shear at 0- 6 km of dry thunderstorm gale was significantly stronger than that of wet thunderstorm gale.

Analysis of a Convective Storm Crossing Poyang Lake in China

A convective storm crossing Poyang Lake(PL)in China during 1200-1600 UTC on 13 May 2015 is examined.The results show that this storm occurs ahead of a 500-hPa trough with weak low-level temperature advection and a convectively stable layer between 925 and 850 hPa,and the tail of the storm is enhanced when its spearhead sweeps over PL after the sunset.Due to the heating and moistening of PL,the convectively stable layer over PL is destabilized;and instead,a deep(below 700 hPa)convectively unstable layer is organized.Moreover,both the radiative cooling and the storm-induced cooling result in a rapid air(near-surface)and land temperature decrease in the surrounding areas.Thus,a large lake-land temperature difference(about 6℃)occurs,which is conducive to generating land-lake breeze and enhancing the convergence of the low-level wind.Finally,the PL-induced deep convectively unstable layer and the enhanced low-level convergence jointly strengthen the crossing storm.To further confirm this,two simulations(with or without PL)are conducted with the Weather Research and Forecast(WRF)model.The simulation with PL successfully reproduces the evolution of the storm crossing PL,while the simulation without PL fails.In the simulation with PL,a highμse tongue at 850 hPa associated with the storm moves eastward and downward,and merges with the PL-induced lake boundary layer,forming a deep convectively unstable layer under 700 hPa.However,in the simulation without PL,the stable layer constantly maintains under 900 hPa.In addition,the 900-hPa wind difference between the simulations with and without PL shows a land-lake breeze circulation that strengths the convergence of the low-level wind.

A Diagnostic Case Study on the Comparison Between the Frontal and Non-Frontal Convective Systems

Two major mesoscale convective clusters of different characters occurred during the heavy rainfall event in Guangxi Region and Guangdong Province on 20 June 2005,and they are preliminarily identified as a frontal mesoscale convective system（MCS1;a frontal cloud cluster） and a non-frontal MCS（MCS2;a warm sector cloud cluster）.Comparative analyses on their convective intensity,maintenance mechanism, and moist potential vorticity（MPV） structure were further performed.The convective intensity analysis suggests that the ascending motion in both the frontal MCS1 and the warm sector MCS2 was strong,so it is hard to conclude whether the intensity of the frontal convective cluster was stronger than that of the nonfrontal convective cluster,and their difference in precipitation might result from differences in their moisture conditions.The comparative analysis of the maintenance mechanisms of matured MCS1 and MCS2 show that in MCS1 there were strong northerly inflows at middle and upper levels,and the convection was mainly maintained through convective-symmetric instability;while in MCS2,the water vapor was abundant,and the convection was maintained by moist convective instability.The structural analysis of MPV indicates that（1） the two clusters were both potentially symmetric unstable at middle and low levels;（2） there were interactions between the cold/dry air and the warm/wet air in the frontal MCS1,and the interactions between the upper- and low-level jets in the warm sector MCS2;（3） the high- and low-level jets and moisture condition nearby the convective clusters exerted different impacts on the two types of convective systems, respectively.

HE SPATIO-TEMPORAL STRUCTURE OF BINARY FLUID CONVECTION WITH HORIZONTAL FLOW

The convection structure in a rectangular channel with a horizontal flow forΓ = 12 was studied. The simulations were preformed by solving the hydrodynamic equations using theSIMPLE method. The convective behavior in an absolutely and convectively unstable regime wasstudied. The results show that the two types of convection patterns in this system appear dependingon the convection intensity and horizontal flow. A periodically localized traveling wave state wasfound in this system.

Comparison between the Roles of Low-Level Jets in Two Heavy Rainfall Events over South China

Two heavy rainfall events occurred over the Pearl River Delta during 20-22 May 2020:the first was a warm-sector event and the second a frontal event.Based on ERA5 reanalysis data and observations from wind profilers and Doppler weather radars,the structures and roles of low-level jets(LLJs)during these two heavy rainfall events were analyzed.The results show that:(1)South China was affected by a low-level vortex and a low-level shear line during the two processes.The two heavy rainfall events were both associated with a synoptic-system-related low-level jet(SLLJ)and a boundary layer jet(BLJ).The coupling of the convergence at the exit of the BLJ and the divergence at the entrance of the SLLJ produced strong lifting for the warm-sector heavy rainfall,and the strong convergence between the LLJs and northerly winds as the cold front moved southwards was the main lifting reason for the frontal heavy rainfall.(2)The BLJ was the main transport of water vapor during the two processes.The coupling of the BLJ and SLLJ caused the water vapor convergence to be concentrated in the boundary layer during the first process,whereas the strong convergence between the LLJs and northerly winds led to the lower and middle troposphere having strong water vapor convergence during the second process.(3)During the period of these two heavy rainfall events,the lower and middle troposphere remained unstable.Further analysis show that the differences in the intensity,location,and direction between the BLJ and SLLJ resulted in the pseudo-equivalent potential temperature advection in the boundary layer being significantly larger than in the lower and middle troposphere,which compensated for the energy loss caused by heavy rainfall and maintained the convective instability.These findings add to our knowledge on the roles of LLJs in the pre-summer rainfall over South China.

Study on Formation and Development of a Mesoscale Convergence Line in Typhoon Rananim

This study investigated the formation and development of a mesoscale convergence line （MCL） within the circulation of Typhoon Rananim （0414）, which eventually led to torrential rainfall over inland China. The study is based on satellite, surface and sounding data, and 20 km×20 km regional spectral model data released by the Japan Meteorological Agency. It is found that midlatitude cold air intruded into the typhoon circulation, which resulted in the formation of the MCL in the northwestern quadrant of the typhoon. The MCL occurred in the lower troposphere below 700 hPa, with an ascending airflow inclined to cold air, and a secondary vertical circulation across the MCL. Meso-/~ scale convective cloud clusters emerged and developed near the MCL before their merging into the typhoon remnant clouds. Convective instability and conditional symmetric instability appeared simultaneously near Diagnosis of the interaction between the MCL and kinetic energy and positive vorticity for its further the MCL, favorable for the development of convection. the typhoon remnant implies that the MCL obtained development from the typhoon remnant in the lower troposphere. In turn, the development of the MCL provided kinetic energy and positive vorticity at upper levels for the typhoon remnant, which may have slowed clown the decaying of the typhoon.

Three-dimensional simulation for problem of penetrative convection near the maximum density

The problem of penetrative convection with the cubic and fifth-order equations of state proposed by Merker, Waas and Grigull is studied. Both linear instability and nonlinear stability analyses are performed to assess the suitability of linear theory to predict destabilisation of the convection. The validity of both the linear instability and global nonlinear energy stability thresholds are tested using three-dimensional simulation. The results show that the linear threshold accurately predicts the onset of instability in the basic steady state solution.