Analysis on the TBB Bright Temperature and Vertical Helicity in a Heavy Rain at Anqiu in 2010

Wenbo WANG,Yaqiong FENG,Qian LIANG,Peng WANG,Kedong YANG,Xu WANG

1.Weifang Meteorological Bureau,Weifang 261011,China;

2.Dongying Meteorological Bureau,Dongying 257091,China

Responsible editor:Na LI Responsible proofreader:Xiaoyan WU

Belonging to the disastrous weather,heavy rain often cause floods,landslides,debris flow and so on,bringing great harm to people’s production and life.The key element in heavy rain forecast is to do a good job in convective forecasting.Black Body Temperature,short for TBB[1-3],which is usually referred to as the brightness temperature,is used to represent the radiation sent from the cloud top and earth surface of cloud sector or partly cloudy sector to space.A lower TBB temperature is usually corresponding to higher cloud top height,and also stronger convection current.Previous study has shown that when TBB ＜-20 ℃,there will be strong convection weather activity.Therefore,based on these characteristics of TBB data,it can conclude the changes of weather system as well as the occurrence of weather phenomena.Vertical helicity[4-6]is determined by the vertical vorticity and vertical velocity,and upper-lower air configuration with upper divergence and lower convergence can further contribute to the building of unstable layer,promoting the development of vertical convection; on the contrary,it would suppress the convection energy accumulation,and the coupled structure of upper-lower air vertical helicity can promote the development and maintain local heavy rainfalls[7-8].

Due to the special geographical locations of Anqui City with hills in the southwest and plains in the northwest,there are relatively fewer heavy rain,especially heavy rainstorms in recent years.At early July,2010,a heavy rainfall suddenly struck Anqiu,causing great impact to local.Therefore,based on the large-scale circulation when the local heavy rainfall happened in Anqiu,we analyzed the TBB images of the heavy rain process,revealing the TBB image characteristics before and after the occurrence of heavy rain in Anqiu area,and we also diagnose the vertical helicity features of this heavy rain with the aim to provide further references for the forecasting of similar heavy rains.

Materials and Methods

TBB data[9]was the 9210 download from the products of Fenyun No.2 E satellite,and FORTRAN and other programming languages were used to interpret binary file into the required grd files,which could be converted into images by using GrADS and other drawing software.The brightness temperature data used in this study had been converted into the needed Celsius temperature.

The 6 h of NCEP/NCAR 1°×1°reanalysis data were downloaded from the UCAR&NCAR web site of USA[10-13],which were the binary files in the format of FNL,and the DOS system was used to process the files into readable CTL and IDX files to GrADS,which was used to convert them into image files.

Description of the Heavy Rain

Subjected to the subtropical high edge warm air,upper trough and ground cyclone,Weifang was stricken by rainfall weather throughout the city.The average precipitation in Weifang was 33.2 mm.However,the rainfall range was unevenly distributed with heavier rainfall in eastern Weifang,in which Anqiu suffered the heaviest rainfall within Weifang with the average precipitation of 104.8 nn,

At 20 pm,July 17,2010,the 500 hPa subtropical high(Fig.1a)stretched out to the west and north with 588 line stretching out to E110°N 29°.Shandong was located around the 584 line.The westerlies trough stretched to the east Bay Lake into the region of Hetao Plain,and there was also a south trough in Sichuan Basin.The lower 700 hPa toned with the north trough generated 1 lower trough,while the south trough had 1 vortex in south Sichuan.The 850 hPa (Fig.1b) had 1 shear line respectively from east Hetao to Shaanxi and east Sichuan.In terms of the lower level wind field,the lower-air southwest jet had already built up[14],and Weifang City was just in the the convergence region of upperair north trough front wind field.On the surface chart,at 20 pm July 17(Fig.1b),there was an area with 850 hPa shear line corresponding to central noth Shanxi,generating a cyclone,and at 23 pm,the cyclone moved eastward to central Shandong.Weifang City was located near the cyclone,and thus at the night of July 17,the precipitation in Weifang City reached the largest until 5 am on July 18,when the cyclone was reduced into a shear line and moved westward to southeast Shandong Province,making the rainfall become weaker.At 8 am,July 18,the 500 mPa subtropical high became stable,and the north trough moved eastward and northward,making the convergence area with large lower-air wind filed reach Shandong Peninsula.The wind field shear on the earth map also happened around the peninsula,thus the heavy rain moved eastwards to the peninsula,and the rainfall in Weifang City almost stopped.

Results and Analysis

Analysis on the TBB cloud image of mesoscle convective system

The distribution of brightness temperature isoline and gradient features of TBB[15]could be used to infer the distribution of vertical upper wind above the troposphere,and thereby diagnosed the possible weather phenomena and rainfall area.In the cloud sector,a lower TBB temperature represented higher cloud top height and stronger convection activity.

As was shown in the FY-2C infrared TBB cloud image (Fig.2),as for this rainfall process happened in Anqiu area,weak rainfall air mass began to form in south Wfiang at 20 pm,July 17,2010 (Fig.2a),corresponding to the 500 hPa upper trough and ground cyclone.Guided by the upper-air airflow,the upper trough moved southwards,making the rainfall air mass expanded and moved eastwards and northwards until 0 am,July 18(Fig.2c),when convective cloud clusters with low cloud top temperature generated continuously in south Weifang,and continuously developed.

The convective cloud clusters in south Weifang continuously moved eastward and became strengthening,expanding into large area of convective cloud clusters with brightness temperature ＜-60 ℃ throughout Weifang city and western Shandong Peninsula until 2 am,July 18 (Fig.2d),when the minimum brightness temperature of upper air convective cloud clusters reached -70 ℃ in south Weifang.The convective cloud clusters in Anqiu area were the strongest,and the lower-air positive vorticity[16]went corresponding to the ascending motion center in this area.The continuous strengthening of lower-air positive vorticity and the development of vertical ascending motion promoted the further strengthening development of the convective cloud clusters.The precipitation intensity of Anqiu tended to the maximum,and in the meantime,the north area corresponding with lower TBB brightness temperature was related with the hilly terrain in Anqiu area.In the plains and similar topographic conditions,rain falls tended to go corresponding to low TBB brightness temperature.At 6 am,July 18(Fig.2f),the strong cloud clusters weakened and moved eastward,making the rainfall in Anqiu region basically stop.

The above analysis showed that mesoscale convective cloud clusters maintained stable,developed and moved slowly northeastwards,which meant the lower-air shear line and ground cyclone also maintained stable.During the northeastwards movement of the strong convective cloud clusters,the upper-air of Anqiu maintained a strong convective cloud clusters with TBB≤-60 ℃,which developed quickly and vigorously,causing a heavy rainfall happened in Anqiu in the following 6 h.therefore,TBB field could directly reflected the distribution and intensity of the heavy rain fall,and rainfall intensity increased with the decrease of the TBB value of cloud clusters.The heavy rainfall center in Anqiu area located in the north of low TBB value center.

Vertical Helicity Analysis

The study on vertical helicity[17-18]shoed that it could significantly instruct the forecasting of heavy rain falls.When there was downward movement,the value of w was negative,and therefore vertical helicity was positive,while if there was negative vorticity and upward movement,the vertical helicity was negative.As was seen from the distribution and evolution of vertical helicity on each isobaric surface when a rain fall happened,the correspondence of vertical helicity distribution and weather system at 850 hPa was superior to the other layers.Therefore,the vertical helicity distribution at 850 hPa(Fig.3a)showed that at 2 am on July 18 m,the helicity center was located around Anqui,which also appeared at the upper-air layer of 500 hPa(Fig.3b),but the vertical helicty remained positive at around 10-4hPa/s,indicating high convective cloud top and strong convective lifting movement.However,the convective cloud top was also close to 400 hPa,which was showed as 0 in the map because it had specified that if there was upward movement and negative vortex,the vertical helicity was equal to 0,but in reality,there existed strong divergence.

At 2 am,July 18,2010,the positive helicity center at 850 hPa (Fig.4a)was above the Anqiu area,and the vertical helicity remained the same for about 6 h above Anqiu,when the position of ground cyclone and lower-air shear line were basically identical with the maximum vertical helicity.Moreover,the shear line moved northward,which was consistent with the stretching direction of the maximum positive vertical helicity.At this time,the wind speed around the cyclone increased,making the cyclone become stronger,and the shearing convergence in Anqiu became more obvious.At the same time,upper-air southwester jet appeared in front of the west trough at upper-air 200 hPa(figure omitted),and the negative vorticity and divergence developed quickly on the right rear side of the upper-air southwester jet,forming a strong negative helicity center,which was cooperated with the lower-air positive helicity center,making warm air rising from the lower-air cyclonic convergence outflow from the upper-air anticyclonic convergence.Therefore,the lower-air convergence rose,providing sufficient dynamic conditions for the development and maintenance of mesoscale convective cloud clusters.

In the process of the rainfall,the middle-low layer of upper-air helicity was the positive center of the rainfall area,while the upper-air was the negative center.The positive vertical helicity center[19-21]well corresponded with the movement and variation of ground cyclone and lower-air shear line,and therefore,the variation of vertical helicity was basically consistent with the changes of rainfall intensity.The coupled structure of upper-lower air vertical helicity,namely,the lower-air convergence and upper-air divergence,played an important role in promoting the development and maintenance of rainfalls.Moreover,the positive vertical helicity center well corresponded with the movement and variation of ground cyclone and lower-air shear line.

Conclusion

Mesoscale convective system was an important influencing system of this rainfall.Due to the strong convection,the convective cloud clusters with lower TBB cloud top temperature directly resulted in the occurrence and development of the rainfall.And due to the terrains and other factors,the area with heavy rainfall in Anqiu area lied in the north of the low TBB value center during the rainfall.

The water vapor supply of the rain fall mainly came from the middle-low layer of the troposphere.the major circulation background of this rainfall was the continuous water vapor supply of the strong southwest jet on the edge of the upper trough and subtropical high,while the trigger mechanism to the heavy rain fall was the lower-air cyclonic shear (or vortex) and lower-air jet.

The coupled structure of upperlower air vertical helicity,namely,the lower-air convergence and upper-air divergence,played an important role in promoting the development and maintenance of rainfalls.Moreover,the positive vertical helicity center well corresponded with the movement and variation of ground cyclone and lowerair shear line.

Discussion

Based on the TBB data,we analyzed the features of brightness temperature,finding that the reason why the heavy rain occurred in the north of the low TBB brightness temperature area might be that as the upper ground cyclone was the low brightness temperature area,with hilly area in east Anqiu,the water vapor brought by the southeast flow of the front cyclone was on the windward slope,that is,gathered above Anqiu area,and therefore,the rain fall happened in the north of the area with low brightness temperature value,namely,the first quadrant of the cyclone.Regardless of the influence of terrains,the previous study on TBB brightness temperature showed that the rain fall should appear at the central region of the area with low TBB value.However,the variation of vertical helicity was consistent with the changes of the rainfall intensity,and there was a maximum center of vertical helicity at the middle layer of 700 and decreased toward the ground and upper layer,which was consistent with the analysis results of JU et al.[4].Thus,vertical helicity reached the maximum at the middle layer during the heavy rain.

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