Analysis on a Gale Hail Strong Convective Weather in Weifang Region of Shandong

Meng YANG,Huanyi WANG,Wenbo WANG,Kedong YANG,Xin SONG,Shijie WANG

1.Weifang Meteorological Bureau,Weifang 261011,China;

2.Benxi Meteorological Bureau,Benxi 117000,China

Analysis on a Gale Hail Strong Convective Weather in Weifang Region of Shandong

Meng YANG1*,Huanyi WANG2,Wenbo WANG1,Kedong YANG1,Xin SONG1,Shijie WANG1

1.Weifang Meteorological Bureau,Weifang 261011,China;

2.Benxi Meteorological Bureau,Benxi 117000,China

To better study occurrence and development mechanism and characteristics of strong convective weather in Weifang,reduce the harm brought by disastrous weather,such as gale and hail,extract available forecast index for future strong convective weather forecast work in spring and summer,and improve forecast accuracy,diagnostic analysis of the strong convective weather process was conducted. By using conventional weather data for profile and sounding analysis,and combining short-term numerical forecast products for cause analysis,such as radar and satellite cloud chart,occurrence characteristics and type of such strong convective weather were obtained.Results showed that the important reason of the strong convective weather occurrence was high-altitude trough split out in eastward movement process of northeast cold vortex.In the afternoon of 2ndwhen hail occurred, temperature rise was obvious at low layer,and energy was sufficient.High-altitude trough with forward tilting structure made that dry cold air at high layer overlapped on warm wet air at lower layer,thereby causing stable stratification formation and triggering the occurrence of strong convective weather.

Strong convection;Hail;Gale

S trong convective weather is a kind of disastrous weather, and often occurs in summer, which is often accompanied by thunderstorm,gale and hail.It is ferocious, and destructive force is strong,thereby bringing serious loss to people’s life and property safety.As development of detection means,the application of advanced instrument,such as radar and lightning positioning,makes people continuously and deeply understand signs and features showed when strong convective weather approaches.Many mature research findings have been applied in short-term nowcasting[1-4].It generally occurs under the atmospheric circulation situation that high-altitude configuration is forward-tilting trough,and the configuration of such circulation situation is up-cold-down-warm.It is often affected by northeast cold vortex in spring and summer over Weifang,thereby generating strong convective weather. Northeast cold vortex is an important component of East Asian middle-highlatitude atmospheric circulation,and is also the unique important weather system of northeast China[5].The existence of convergence belt in southerly airflow of side warm zone and its relationship with thunderstorm and rainstorm have induced attentions of science research workers and the majority of forecasters[6].In the process,during eastward movement process of northeast cold vortex,there was continuous cold air supplement in southeast Lake Baikal,which went south. High-altitude trough split out from its bottom was the true influence system. Old saying indicates that“thunderstorm does not exceed three days”, that is to say,strong convective weather(thunderstorm)generally occurs for continuous three days in Weifang.It is because that maintaining time of northeast cold vortex is generally 4-5 days,and cold air and high-al-

Data and Methods

General situation of research zone and actual weather condition

Weifang belongs to warm temperate continental monsoon climate. Its north is Bohai Sea,and season is clear.In spring,it is windy and rainless,and spring drought often occurs. In summer,it is hot and rainy,and temperature is high,and humidity is large. In autumn,the sky is high and the weather is fine,and drought often occurs in late autumn.It is dry and cold in winter,and temperature is lower.Annual average temperature is 12.3℃, and annual average rainfall is about 650 mm.It is often controlled by northeast cold vortex in summer,and strong convective weather is easy to occur, such as thunderstorm and gale.The strong convective weather occurred from afternoon to night on June 2 (Fig.1).During 16:30-18:00 on 2nd, some streets in Hanting District and Changyi City were struck by hail,and the maximum wind velocity was 17.8 m/s.The maximum rainfall was 18.5 mm,and the biggest hail diameter was 20 mm.It caused that some farmland crops were affected,and direct economic loss was about 0.139 billion yuan.

Data source

The used meteorological data were from automatic receiving system of Cmacast meteorological satellite data and 152 regional meteorological stations of Weifang,including height field,temperature field,humidity field, etc.Higher data resolution can guarantee accuracy of live analysis.Usage time of data was during 1-2 June, 2012.

Research method

By using comprehensive analysis processing system MICAPS 3.1 of meteorological information,overall analysis on atmospheric circulation situation was conducted.First,we should determine that the gale and hail occurred under circulation background of northeast cold vortex control,and find weather system inducing strong convective weather.Then,time profile of each physical quantity generating hail in occurring process of strong convective weather was conducted.By studying temporal-spatial evolution situation of each physical quantity, available forecast indicators could be found.Meanwhile,using new generation of Doppler radar and satellite cloud chart product for reanalysis,the indicators for short-term nowcasting could be found,which could provide support for future short-term nowcasting and pre-warning work.

Results and Analysis

Cause of strong convective weather Evolution of situation fieldDuring 16:00-18:00 on June 2,strong convective weather occurred in Weifang region of Shandong,and thunderstorm and gale appeared,which was accompanied by very dense hail.

In the day before strong convection occurred,at 500 hPa of situation field at 20:00 on 1st,there was a cold vortex in northwest Lake Baikal,and cold air continuously split out and went south,which made cold vortex in east Inner Mongolia strengthen.To 08:00 on 2nd(Fig.2),cold vortex center slowly moved toward southeast direction,and center was at 118°E,45°N.Cold vortex was deep and thick,and developed to 250 hPa.At 500 hPa,high-altitude trough was split out from bottom of cold vortex,and 700 hPa of trough line was at Weifang region.At 850 hPa,trough line was at central Shandong and west Weifang,and there was warm tongue extending eastward from west.By contrasting high-altitude situations at the three layers,it was found that trough line had obvious forward tilting,which was conducive to the development of convection.Meanwhile,temperature difference between 500 and 850 hPa was 26℃.At 20:00on 2nd,cold vortex slightly moved northward,and high-altitude trough moved eastward to North Korea. Contour was straight,and shear line at 700 and 850 hPa moved toward northeast direction and moved out of Weifang region,which was by north. Wind field converted into anti-cyclonic circulation,and precipitation finished. Analyzed from ground chart,Weifang was affected by circulation at the rear of high pressure at 14:00 on 2nd,and air pressure field was weak.There existed a cyclonic circulation in west Hetao region,and center air pressure was 1 000 hPa,which gradually moved eastward.Convergence at ground wind field in Weifang strengthened at 17:00.The closed convergence center appeared,and gradually moved eastward,which was consistent with time and pathway of hail shooting.It needs to further study if the triggering effect of ground mesosmall-scale convergence line can be as triggering mechanism of local convective wind storm system quickly generating,developing and strengthening.

Analysis on physical quantities

(1)Water vapor condition.In fact, because cold stratification was deep and thick,strong cold vortex was difficult to induce strong convective weather,but its split positive vorticity or weak cold low vortex coordinating with warm wet condition at low layer often forms sudden strong convective weather[7].Seen from high-altitude humidity configuration at the three layers, relative humidity at 500 hPa of Weifang was less than 10%at 08:00 on 2nd,and relative humidity at 700, 850 and 925 hPa was during 70%-80%.It only illustrated that it was relatively dry at high altitude of Weifang and was wet at low layer.The system was similar with general precipitation. But if further analyzing,relative humidity below 80%at low layer was not conducive to occurrence of precipitation.It showed that occurrence of strong convective weather did not have very high requirement on water vapor.Especially when there was hail, too much water vapor analysis wouldhinder research thinking,which did not have too much significance at the referable forecast indicator aspect.

(2)Dynamic condition.Vertical velocity was a basic parameter of weather analysis forecast,but it could not be directly observed like horizontal velocity.Especially atmospheric vertical motion induced by cumulus convection had smaller horizontal scale, but its strength and vertical scale were incomparable by motion[8].Profile analysis of vertical velocity field at 08:00 on 2ndwas conducted.It was found that there was strong vertical ascending motion at bottom layer of troposphere over Shandong,and ascending motion maintained to 100 hPa,which provided forceful uplifting condition for strong convection.On the one hand, strong sinking motion could convert potential energy into heat energy to heat air.On the other hand,it could roll warm air outside density flow in to form sinking temperature inversion.

(3)Thermal condition.Major generation zone of hail was the layer from 0 to-20℃,which was called as supercooled water accumulative zone. Hail generally occurred under the situation that heights of 0 and-20℃layers were suitable.Height of 0℃layer suitable for hail shooting was generally 4 km,and height of-20℃layer was near or below 400 hPa of isobaric surface height.Seen from T-ln P chart in Jinan at 08:00,it was shallow and thin wet layer during 950-1 000 hPa.Height of 0℃layer was 3 444.8 m,while height of-20℃layer was 6 585.5 m.It illustrated that there was cold air entering at high altitude,and“dry warm cover”was formed.“Dry warm cover”could inhibit small-scale thermal convection development at boundary layer,and make instable energy in the boundary layer not release too early until there was one mechanism enough to make blocking layer destroy and disappear and small-scale thermal convective bull strongly develop[9].Stratification tended to be instable,and height difference between 0 and-20℃layers was small,and supercooled water concentration was large,which was conducive to forming hail.

(4)Atmospheric stratification stability.Atmospheric stratification stability is decided by distribution situation of atmospheric temperature and humidity at vertical direction,and it is an important factor judging if convection could occur and develop.There is a potentialinstable energy accumulation process at bottom layer of hail zone.Under the situation with external dynamic condition,atmospheric potential instable energy will release,thereby triggering strong convective weather.Many researches[10]pointed out that when it was warm and wet at low layer of troposphere and especially dry and cold at high layer,such atmospheric stratification was extremely unstable,that is,it was conducive to forming strong convective weather[11].We selected SI index and K index to explore.SI index was the difference between temperature(T’)of air parcel uplifting from 850 to 500 hPa and 500 hPa environmental temperature(T500),namely,SI= T500-T’.When SI＞0,it showed atmospheric layer was stable.When SI＜0, atmospheric layer was instable.K index was a diagnostic quantity representing thunderstorm or rainstorm potential constructed based on vertical lapse rate of atmospheric temperature and water vapor condition at low layer. Generally considering,K＞28 was one of thermal indicators of strong convective weather[12].

Seen from sounding chart at Zhangqiu station at 20:00 on 1st(Fig.3), K index was 25.9℃;SI index was 2.52℃;CAPE value was 102.4 J/kg;CIN value was 190.3 J/kg.At this time, ground temperature was 25℃and was lower,and stratification was stable(lower K value and SI＞0),and potential instable energy was weak (lower CAPE value).Higher CIN value showed that instable energy was inhibited.In addition,it was conducive to accumulation of unstable energy. Seen from vertical wind field,it was warm advection at low layer and cold advection at high layer,which was conducive to strengthening instability. At this time,lifting condensation level (LCL)was higher than 850 hPa,and it was obvious dry layer during 850-650 and 600-350 hPa.

Seen from Fig.4,K index was 27.7℃;SI index was-0.27℃;CAPE value was 56.2 J/kg;CIN value was 115.2 J/kg.At this time,stratification gradually changed into unstable(K value slightly increasing and SI changing into negative value).Although CAPE value was lower,instability inhibition weakened(CIN value decreasing when compared with that at 20:00 of prior day).Vertical distribution was still warm advection at low layer and cold advection at high layer,which was conducive to inducing instability to continuously increase in afternoon.At this time,LCL declined to 900 hPa, and there existed relatively dry layer at middle and high layers,which was more obvious than that at 20:00 of prior day.Before convection occurred, there was no temperature inversion in the environment.

According to that temperature was 29℃,dew point temperature was 16℃in Weifang at 14:00,sounding chart of Zhangqiu station at 08:00 on 2ndwas corrected.Fig.5 and Fig.6 respectively used Micaps and Shanghai software,and analysis result was consistent.CAPE value:2 146.1 J/kg (Micaps)▏2 134.0 J/kg(Shanghai software),CIN value:0 J/kg(Micaps). CAPE value obviously increased,and heating after the noon made low-layer temperature profile line become super heat insulation.At this time,although vertical wind shear of 0-6 km became weak and was 11.67 m/s,CAPE value was higher.Moreover,0℃layer height reached 3.5 km,and-20℃layer height was about 6.8 km,which was still conducive to occurrence of hail weather.

(5)SI index and K index.Seen from change tendencies of SI index and K index at Zhangqiu from 08:00 on 1stto 08:00 on 2nd,SI index gradually declined,while K index gradually increased.It illustrated that atmospheric layer from 08:00 on 1stto 08:00 on 2ndtended to be instable,and there was instable energy accumulation at nightfall of 1st,which provided favorable condition for occurrence of strong convection.At 17:00 on 2nd,strong convection occurred under favorable triggering condition.It illustrated that there was a potential unstable energy accumulation process at bottom layer of hail zone.Under the situation with external dynamic condition,atmospheric potential instable energy will release,thereby triggering strong convective weather.

(6)Pseudo equivalent potential temperature.Seen from vertical distribution ofθseat 08:00 on 2nd(Fig.7),as height rose from ground to 600 hPa,θsealso rose.But over 600 hPa,θseobviously declined as height,and there was convective instability.Moreover,θseat 300 hPa declined by 8℃in 24 h, and Δθse(300-700)changed from 15 to 10℃.Although it was stable at middle and high layers,stability decreased when compared with that in prior day.

(7)Vertical wind shear.Seen from the corrected T-ln P chart at Zhangqiu at 08:00 on 2nd,it was southwester below 850 hPa and northwester over 850 hPa.Wind direction converted clockwise as height,and there was warm advection.Wind velocity was about 12 m/s at 500 hPa,and 6 m/s at 700 hPa. Vertical wind shear at 0-6 km was 11.67 m/s.Although vertical wind shear was not very large,seen from whole layer,it was cold advection at high layer and warm advection at low layer,which was conducive to formation and strengthening of convective unstable stratification.

Radar and satellite cloud chart

Radar productSeen from basic reflectivity chart of radar 713 in Weifang (Fig.8),there was small piece of echo generating in northwest Weifang after the noon on 2nd,and range gradually enlarged.At 15:46,there was linear multi cell storm in west Weifang.Echo center intensity in south Changle reached 45 dBz and stably maintained,which corresponded to 55.3 mm of short-term strong precipitation appearing during 16:00-17:00 at Tangwu of Changle.At 16:03,there was＞45 dBz echo generating in south Shouguang.There were three strong echo centers at Weifang,and they were respectively in central Qingzhou, south Changle and south Shouguang, and strong echo center stably maintained.At 16:35,strong echo over Qingzhou maintained,and strong echo in south Changle moved southward and affected west Linqu.Strong echo in south Shouguang moved eastward and affected Hanting,and new strong echo was generated at the junction of Shouguang and Qingzhou.At 16:46, echo over Hanting enhanced,and range enlarged.At 16:30,hail shooting appeared in Hanting.At 17:21,echo over Hanting started to weaken,move eastward and affect Changyi.After reaching Changyi,it started to gradually strengthen again.At 18:07,strongecho center in north Changyi reached over 50 dBz,and hail fell in Changyi at 18:00.Hail shooting occurred at strong development stage of echo,but not the stage that echo was the strongest. It could be as a referable index of future short-term nowcasting and prewarning work.

Seen from cloud top height chart of radar(Fig.9),at 16:42,cloud top height of strong echo center was close to 14 km,and strong echo cloud top height exceeded 8 km,and echo center intensity exceeded 60 dBz.It illustrated that thunderstorm cell development was strong and reached the generating condition of hail,which corresponded to hail shooting in Hanting. At 17:28,cloud top height of strong echo reached 12 km again,and echo center intensity reached 50 dBz.At 18:00,hail shooting occurred in Changyi.ETPPI,VIL,RZ and HARL had consistent change,which reflected short-term abrupt change of physical characteristic quantity in hail cloud and the feature gradually forming hail. By comprehensively analyzing the above products,we could detailedly grasp development trend of strong convective weather and issue strong convective weather pre-warning[13].

Characteristic analysis on satellite cloud chartSunny cumulonimbus clouds showed the condition of atmospheric layer.In one day,the earlier the occurrence time,the larger the cumulus cluster,the more unstable the stratification,which was conducive to occurrence of strong convective weather[13].FY2E satellite brightness temperature chart could detailedly reveal development evolution of convective cloud cluster(Fig.10).At 14:00 on 2nd,there was convective cloud cluster developing in east Weifang.At this time,weak convective cloud cluster was gradually generated in west Weifang and developed.It illustrated that convective cloud cluster was developing in flow in region of low layer. At 15:00,two cloud clusters merged together,and quickly strengthened, developed and moved toward east direction.Seen from evolution chart,at 17:00,most area in east of central Shandong was in cold cloud zone of TBB＜-40℃,and-41℃cold center was in south Weifang.To 17:00-18:00, convective cloud cluster development was the strongest,and the area of cold cloud cover that cloud top brightness temperature was lower than-40℃reached the maximum,and center zone was mainly in south Weifang.At this time,isotherm in east and northeast cloud cluster was dense,and whole Changyi City occupied three isotherms with 18℃interval.It showed that temperature gradient in front of cold cloud cover was very large,and convection in cloud cluster was active.At 20:00,cold cloud cover of-40℃basically moved out of east Weifang and reached Shandong Peninsula and southeast Shandong, and intensity also weakened.According to actual situation,starting-ending time of hail shooting in Hanting and south Changyi was 16:30-18:00.It showed that hail shooting occurred when convective cloud cluster development was the strongest,and was in front of cold cloud cover moving direction and big-value zone of temperature gradient.As cold cloud cover moved out,the hail process also finished.

Density of TBB isotherm surrounding cold cloud cover reflected cloud top temperature gradient,which had certain indication significance for the development of hail cloud[14-16].The hail occurred when convective cloud cluster development was the strongest,and position was from the front of 232 K cold cloud cover moving direction to big-value zone of isotherm gradient.

Conclusions and Discussion

(1)The strong convective weather occurred in movement process of northeast cold vortex toward northeast direction,but its true influence system should be high-altitude trough split out from its bottom.By analyzing three high-altitude layers,it was found that it was forward tilting trough,and warm air at low layer lift.Although cold front obviously weakened,there was steady cold air at the rear of low vortex,which overlapped on warm wet airflow at low layer and caused unstable stratification.

(2)The strong convective weather had strong local feature,and ground meso-small-scale convergence line was triggering mechanism of the strong convection.On the other hand, the strong convective weather was dominated by hail,which was obviously different from short-term strong precipitation at sounding aspect.It was helpful for forecasting strong convective weather type according to sounding feature,thereby better conducting pre-warning.

(3)The hail process occurred when convective cloud cluster development was the strongest,and position was from the front of 232 K cold cloud cover moving direction to bigvalue zone of isotherm gradient.It was because that the cloud cluster generally moved toward the region with sufficient energy.

(4)Convective cloud top development height especially strong center height was a better indicator judging hail.In short-term nowcasting,we only used radar to monitor and forecast. Seen from basic product of radar-reflectivity factor,it could form strong hail when strong echo center intensity was more than or equal to 50 dBz and expanded over-20℃layer,and suitable height of 0℃layer was 600 hPa.

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Responsible editor:Ping SONG

Responsible proofreader:Xiaoyan WU

山東濰坊地區(qū)的一次大風(fēng)冰雹強(qiáng)對流天氣分析

楊萌1*，王煥毅2，王文波1，楊可棟1，宋欣1，王世杰1
（1.濰坊市氣象局，山東濰坊261011；2.本溪市氣象局，遼寧本溪117000）

為了更好地研究濰坊地區(qū)強(qiáng)對流天氣的發(fā)生發(fā)展機(jī)制及特點(diǎn)，減少大風(fēng)冰雹等災(zāi)害性天氣帶來的危害，為日后的春夏季節(jié)強(qiáng)對流天氣預(yù)報(bào)工作提取可利用的預(yù)報(bào)指標(biāo)，提高預(yù)報(bào)準(zhǔn)確率，特對此次強(qiáng)對流天氣過程進(jìn)行診斷分析。本文利用常規(guī)氣象資料進(jìn)行剖面和探空分析，同時(shí)結(jié)合雷達(dá)和衛(wèi)星云圖等短時(shí)臨近數(shù)值預(yù)報(bào)產(chǎn)品進(jìn)行成因分析，得出此種強(qiáng)對流天氣的發(fā)生特點(diǎn)和類型。結(jié)果表明，此次強(qiáng)對流天氣產(chǎn)生的重要原因是東北冷渦東移過程中分列出的高空槽。冰雹發(fā)生在2日的下午，此時(shí)低層升溫明顯，能量充足。前傾結(jié)構(gòu)的高空槽使高層干冷空氣疊加在低層暖濕空氣上，導(dǎo)致不穩(wěn)定層結(jié)出現(xiàn)，從而觸發(fā)了強(qiáng)對流天氣的發(fā)生。

強(qiáng)對流；冰雹；大風(fēng)titude trough continuously split out from its bottom,move eastward and go south.

山東省氣象局氣象科學(xué)技術(shù)研究項(xiàng)目“影響山東的臺(tái)風(fēng)特征分析與典型疑難個(gè)例研究”（2014sdqxm14）。

楊萌（1981-），男，山東濰坊人，工程師，碩士，研究方向：中短期天氣預(yù)報(bào)，E-mail：13863636361@163.com。*通訊作者。

2015-04-02

修回日期 2015-05-20

Supported by Meteorological Science and Technology Research Project of Shandong Provincial Meteorological Bureau(2014sdqxm14).

*Corresponding author.E-mail:442091917@qq.com

Received:April 12,2015 Accepted:May 20,2015