Analysis on the Genesis and the Omission of a Torrential Rain over Linqu on 9 July 2012

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

1.Weifang Meteorological Bureau,Weifang 261011,China;

2.Benxi Meteorological Bureau,Benxi 117000,China

Responsible editor:Na LI Responsible proofreader:Xiaoyan WU

Torrential rain is a complicated weather phenomenon.Accurate forecast of rainstorm could directly reduce damage of the rainfall to the drop zone,and Chinese meteorological workers have attached great importance to the research and forecast of rainstorm for a long time,who have done a great deal of work,gradually improving the research and forecasting level of rainstorm in China[1].Improving the monitoring and forecasting ability of rainstorm is of great significance to prevent and reduce disasters and serve industrial and agricultural production[2].The genesis and development of rainstorms in the atmosphere is closely related to the stability of the atmosphere.as for the study on atmospheric stability,Helmholtz[3],Kelvin[4]and Rayleigh[5]had studied the necessities of the unstable shear flow as early as 100 years ago.Based on the analysis on the conventional meteorological data used in daily forecast,we found out the causes of the b nomissions,which provide references to summarize the experience of forecasting torrential rain,improve the accuracy of rainstorm forecasting and reduce omissions.

Subjected to the common effects of upper trough and subtropical high,a rainfall happened in Weifang City from 0 am,July 9 to 8 am,July 10,2012.The precipitation amount of this rainfall in Linqu area was 60.6 mm,and the precipitation in 2 h had reached 57.6 mm,while the amount in the surrounding areas was very small with an average of 10 mm.The precipitation greatly relieved the current damage to crops by drought,and was conductive to the seedlings of summer sown crops and growth of spring-sown crops.The analysis on the data of the afternoon of July 8 showed that the weather forecast showed that in 24h,it would be cloudy and have moderate rain in Weifang City,part with torrential rain,which may mainly appeared in east Gaomi,Zhucheng.However,the real situation was as shown in Fig.1,the torrential rain happened in Linqu area,and therefore,there was obvious errors in the forecast on the rain drop area and rain intensity.

Materials and Methods

General situation of the study area

Linqu area is located in the middle of Shandong Peninsula,southwest of Weifang City,at the northern foot of Yishan,and the upstream of Mihe river.Belonging to the temperate continental monsoon climate,Linqu has four distinctive seasons,the same terms of rainfall and warm weather,obviously wet and dry season,and the rainstorm usually occurs in July and August every year in Linqu.The annual average precipitation is 592 mm with an average temperature of 12.9 ℃.

Data sources

The meteorological data used in this study was taken from Weifang Meteorological Bureau,including the conventional data of height field,temperature filed and humidity field.The data had high resolution,which ensured the accuracy in analyzing the real situation.The ground observation dada came from the automatic weather station in Linqu County,and the time was July 7-11,2012.

Research methods

The meteorological information comprehensive analysis and processing system (MICAPS) 3.1 was used to carry out the overall analysis on general atmospheric circulation.First,it made sure what kind of general atmospheric circulation could make the rainstorm happen,finding out the weather system causing the rainstorm.And then,time profiles were conducted to the various physical quantities during the rainstorm based on the temporal and spatial changes of the physical quantities,which were also compared with the previous summarized rainstorm indicators,systematically analyzing the rainstorm happened in Linqu area.In the meantime,a publishing system was made based on the refined short-time nowcasting of Weifang city to analyze the realtime distribution of precipitation in this area.With fully analysis on upperlower area as well as the ground system configuration,the temporal and spatial changes of various physical quantities as well as their contributions to the development of the rainstorm during the rain were analyzed,and finally the numerical weather prediction was analyzed and inspected,drawing a conclusion.

Results and Analysis

Analysis on the causes of rainfall production and omission

Position field evolutionThrough the analysis on the configuration of position upper-lower air position field(figure omitted),it could conclude that at 8 am,July 7,2012,the whole middle-high latitude area was in the position of 2-trough 2-ridges under the common effects of subtropical high and westerlies high,presenting eastwest zonal distribution.At the same time,there existed a low pressure center in Lake Baikal,and the sliding cold air brought by the west trough in Hetao area made good preparation for the invasion of late cold air[6].There was an obvious jet at 850 hPa,which made the water vapor in the Bay of Bengal well transported,providing well water vapor conditions for the rainstorm.Shandong province was located in the right side of the axis of jet stream,but in a little far south,forming a warm shear line in south Shandong.At 20 pm,July 7,2012,the system moved further eastward,and the westerly trough moved to the east Shandong province,making east Shandong suffer the influence and begin to rain.At this time,the axis of the jet at 850 hPa was also moved eastward and stretched northwards,and west Shandong was in the left side of the axis,making lower-air cooperate the weak shear,accompanied by wind speed convergence.By 8 am,July 8,there formed a new westerly trough,and the frequent activity of the short wave trough within the westerlies was one of the most important reasons causing the rainstorm.At 20 pm,July 8,the analysis on the 500 hPa position field showed that 584 line moved northward than before,and the short wave trough activity was still active.A vortex at 925 hPa formed at southwest Shandong and began to move northwestwards.There still existed strong lower-level jet at this time with the maximum wind speed reaching 16 m/s,and the shear line in west Shandong strengthened and distributed in northeast-southwest direction.The overall precipitation distribution of the whole Shandong Province showed that the heavy rain fell in the right quadrant of the vortex,which was closely related to the existence of well lifting conditions.At 20 pm,July 9,there was a new short wave trough formed in northwest Shandong and moved gradually eastwards.The backside cold air of the Eurasian trough entered into the central Shandong area with non-stop,and the low-level jet provided good water vapor supply for the rainfall,and therefore,the southward movement of the cold air in the upperair Baikal lake along with the short wave trough formed a clear confluence with the lower-level warm air at Weifang City.Subjected to the influence,Linqu began to appear part heavy rain from 18 pm to 20 pm,and the precipitation in 2 h reaching 57.6 mm.At 8,July 19,Shandong was controlled by the long wave trough northwest airflow,and the rain stopped.

It could be concluded form the analysis on the ground position field(figure omitted) that at 8 am,July 7,Shandong Province was in the front of the southwest vortex until 14 pm,when the southwest vortex developed into ground inverted trough,and then Shandong Province was in the front of the ground inverted trough.At 2 am July 8,the ground inverted trough moved northeastwards,and the southeast Shandong area began to be affected and rain.At 8 am July 9,the Changjiang-Huaihe cyclone came into being,making the southwest Shandong Province located outside the cyclone,and then at 14 pm,the cyclone moved northwards,beginning affecting southwest Shandong Province,at which time the blocking high was on the sea; at 20 pm,the Changjiang-Huaihe cyclone began to affect Shandong province in large area.The blocking low in Japan made the high-pressure ridge on the see maintain stable and distribute in south-north position,which was coordinated with the formation of ground inverted trough,providing favorable advantages for antecedent precipitation.On July 9-10,the Japanese high pressure ridge moved eastward into the sea,in which the blocking high played an important role in blocking,making the Changjiang-Huaihe cyclone keep stable and motionless,making the cyclone stay in Shandong Province[7].

Analysis of physical quantity

Water vapor condition analysisWater vapor flux and its divergence reflected the transportation and concentration capability of water vapor during the rainstorm period[8].As was shown in Fig.2a,water vapor showed vertical distribution with the upper level dry and cold and lower level wet and warm,which was conductive to the occurrence of convective rainfall.The distribution of water vapor flux (figure omitted) showed that at 8 am July 9,there was 1 water vapor transportation belt at southern Shandong province,and the maximum central intensity of the water vapor flux reached 20 g/(cm·hpa·s),indicating the water vapor was continuously transported from southwest to Shandong province.With a water vapor flux intensity of 10 g/(cm·hpa·s),Linqu was in the off southwest position of the water vapor conveyer belt,which was also corresponding to the distribution of the precipitation.A bigger negative value of water vapor influx divergence represented stronger convergence,which was more conducive to the production of heavy rainfall[9].As was shown from Fig.2b,when the rainfall began,the negative value center with large water vapor divergence was in southwest Shandong,which was quite consistent with the rainfall drop zone and convergence zone of water vapor flux divergence of this rainfall.Liqu area was in the north of the convergence center,and at 18 pm-20 pm July 9,the precipitation of 2 h reached 54.1 mm,and the water vapor influx divergence before 20 pm was -10 g/(cm·hpa·s),indicating that there was initial moisture energy reserves.In rainstorm forecast,it should give fully consideration to indication of significant water vapor convergence in lower air.

Dynamic condition analysisUpper-air divergence and lower-air convergence with strong suction was conductive to maintain and develop the rising airflow,and strong convergence could promote the rapid release of unstable energy,making the upper-lower air mesoscale weather system generate positive feedback,and therefore resulting in heavy rainstorm[10-12].As was shown in Fig.3a,during this process,there was a divergence center with the central value of 20×10-5/S at 200 hPa in south Linqu,and 1 convergence area at 500 hPa in east Shanxi and Henan of Hetao region,while there was no divergence area appeared at 400 -250 hPa.Generally speaking,there should appear a nonconvergence area at 500 hPa,the reason of which needed further analysis.Starting from July 7,lower-level convergence became stronger,and eventually formed into a low-level convergence center at 20 pm July 9.Due to the weak low-level convergence,the lifting ability was normal with unobvious suction.The corresponding vertical velocity profile (Fig.3b) showed that the whole layer was in the ascending motion area,and the ascending motion center was near 400-300 hPa.However,the low-level lifting ability was normal with general dynamic lifting conditions,which was one of the causes for the neglecting the occurrence of strong convective weather.

Thermal conditionsK index,θseand convective available potential energy CAPE were usually selected from physical quantities in the analysis of the causes of rainstorm.the analysis on the K index distribution(figure omitted) showed that the K index throughout the province increased from 8 am-20 pm,and there was a high value center of 40 ℃in south Shandong province,while the value varied from 36 to 40 ℃throughout the country (it usually appeared rainstorm when K ≥35 ℃).In the early forecasting,this weather process was systematic stable precipitation,ignoring the analysis on K index.However,there appeared partial strong convective weather during the process,which was the mesoand micro-scale weather process caused under the large-scale circulation background.In future forecasting analysis,it should pay attention to not ignoring this case,which was also one for the reasons for the omission.

According to the previous study in Shandong provincial meteorological station,there could appear rainstorm at 850 hPa whenθse≥345[13].The analysis on the potential pseudo-equivalent temperature at 850 hPa (figure omitted) obviously showed that at 8 am,the high energy(more than 350 K)remained in southwest Shandong province,which was also stretched northeastwards with the eastward movement of the westerlies short wave trough.At 20 pm,the whole province was under the influence of high energy region except the northwest Shandong province and north of the peninsula,and theθseof Linqu area was also larger than 350 K.This rain process lasted relatively long time with larger amount,which was closely related with the long-lasting maintenance of the high energy area,and this was also one of the causes for the generation of partial rainstorm.

The CAPE threshold of strong storm was 1 500 J/kg.At 8 am July 9,the CAPE value above Xuzhou was 855 J/kg,reaching 850 J/kg at 20 pm,which showed no significant indication to this strong rainfall.

Generally speaking,continuous rainfall and mesoscale rain mass usually occurred under certain weather scale background,in which mesoscale system was the environmental conditions to keep unstable development,which was the most important in the triggering mechanism.Atmospheric stratification stability was also an important point needed attention in the analysis of mesoscale precipitation[14-16].When atmospheric stability index SI ≤0 ℃,the atmosphere was not stable,while when SI ≤-3℃,the atmosphere was extremely unstable.At 20 July 9,SI was -1.89 ℃ in Xuzhou,indicating that the atmosphere was not stable,which was well corresponding to the precipitation intensity.

The above thermodynamic indicators showed that the heavy rain occurred when the atmosphere was unstable,and under the atmospheric state of high energy and high humidity,the occurrence of rainstorm required favorable thermal environment.

Analysis on the effects of terrain on rainstorm genesisUsually,the production of rainstorm cloud mass needed the cooperation of mesoscale convergence system.Central Shandong is typical hilly area,surrounded with plains,valleys or oceans with an average altitude of less than 100 m.This terrain distribution was well configured with the rainstorm area[17].Linqu station faced with Yishan in southeast,in the leeward slope,and Mihe river in northwest.The water vapor evaporated from the river was transported to Linqu area,providing well water vapor conditions for the precipitation.The mountain ranges in central Shandong greatly lifted the low-level jet in the windward slope,and rainstorm usually happened in the windward slope.However,in this rainfall process,a great deal low-level jet distributed westwards,and Linqu station was located in northwest Yishan,which was also one of the causes of the rainstorm in northwest Yinshan.Due to the common effects of mountains and rivers,the probability of rainstorm occurrence in Liqu area increased in the flood season every year.

Numerical prediction test

EC numerical prediction testAs was compared in Fig.4a,the predicted 500 hPa height field was consistent with the real situation.However,the predicted 584 line position at 8 am July 9 (figure omitted) was off to the north compared with the forecast of 24 h and 48 h,and the predicted westerly trough was also lagged behind the real situation.In Fig.4b,there was a vortex predicted at low-level 850 hPa within 24 h in northern sea area of Linqu,and a poor quality system in southwest Shandong province,while the real situation showed that the 2 poor quality systems did not exist separately at 8 am July 9.However,there existed shear line in northeast-southwest direction in northern Shandong,and the position of jet axis was off the north of the predicted one.The inaccurate prediction of the position of low-level shear line was also one of the reasons for the omission.

Japanese numerical forecast testThe analysis of Fig.5 showed that the predicted rainfall center varied greatly with the real situation.The rainfall center of the 12 h forecast was in northwest and southeast Shandong,respectively at 46 and 56 mm; the rainfall center of 24 h forecast was in southeast Shandong of 37 mm.However,the real area with strong rainfall intensity was in southwest central Shandong and southwest Shandong.There was significant error in the precipitation level of Linqu area,and therefore the Japanese forecast of this rainfall was of little significance.In daily forecast,the precipitation level of Japan was relatively accurate,and therefore the weatherman became strongly rely on the Japan precipitation forecast,which was also one of the important reasons for the omission of the rainfall in Liqu area.

Conclusion and Discussion

The major influencing system of the rainfall in Linqu was the upper trough,low vortex and shear line as well as surface low pressure.The falls of subtropical high in the south and the establishment of upper-lower air jet provided good conditions for the transportation of water vapor.The frequent activity of westerly short wave trough and gradually sliding cold air made preparations for the invasions of late cold air.

The continuous formation and development of westerly short wave trough,convergence of wind direction and speed,larger specific humidity at 850 hPa and the overall good upward movement of whole level convergence as well as the existence of the unstable energy played certain role in the generation of the rainstorm in Linqu.

Due to the relative short life cycle of mesoscale system,it was relatively more difficult in the forecasting process,and therefore,it should pay special attention to the analysis of physical quantity field,which is a good indicator in the meso-and micro-scale rainstorm forecast.

It is easy to appear meso-and micro-scale generated rainstorm in some area during the development of large scale system,which usually occurs after there exists unstable energy and adequate water vapor.Ignoring this point is easily leads to omissions.

Weatherman should take comprehensive consideration to the real conditions of weather,but can not rely excessively on numerical precipitation forecast products.EC,Japan fax and other numerical products have good indications in the forecast of large scale weather situation,but they are still not stable in mesoscale heavy rain forecasting,which could result in certain errors in the falling area of strong precipitation[19].The analytic ability to wea-ther map as well as the correction capability to numerical products of weatherman still needs to be further improved in daily shift to avoid omissions.

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