| Sprites are one of large-scale transient luminous events(TLEs)that usually emerge from 40 km to 90 km altitude above the ground,which are caused by cloud-to-ground lightning strokes and subsequent continuous current.In order to analyze the characteristics of meteorological condition,thunderstorm and lightning flashes that produced sprites,we used the synchronous observation data to carry out detailed analysis of three cases of sprites within 200 km of the North Alabama Lightning Mapping Array.The main results are listed as follows:(1)The meteorological conditions for the formation of the parent thunderstorm of sprites were analyzed.The three thunderstorms occurred under unstable stratification conditions.The thunderstorm(T1)process controlled by the southwest flow was a retrograde low-pressure trough activity,and the thunderstorm occurrence area was in the front of the low-pressure trough.Similarly, Thunderstorm T2,similar to thunderstorm T1,occurred at the front of the high trough and was controlled by the southwest flow.The thunderstorm(T3)process which occurred under the circulation situation of the eastward movement of the high trough was the result of the interaction between the low-altitude vortex system and the southerly wind in front of the trough.(2)Through the analysis of the overall characteristics of the parent thunderstorm of sprites,it is concluded that thunderstorms T1 and T3 were not isolated cell thunderstorm;nevertheless,and thunderstorm T2 was isolated cell thunderstorm. The three parent thunderstorms all had a large area of stratiform cloud precipitation, and the duration of the thunderstorms was long(more than 20 hours).The coldedest cloud top brightness temperature of thunderstorm T1 and T3 was-69°C and-63°C respectively,and the maximum radar echo reached 60?65 d BZ,while the coldest cloud top brightness temperature of thunderstorm T2 was-62°C and the maximum radar echo was 55?60 d BZ.The maximum radar echo top height of thunderstorm T1 reached about 10 km,while the maximum radar echo top heights of thunderstorm T2 and thunderstorm T3 were below 10 km.The maximum area of three thunderstorms(radar reflectivity?10 d BZ)is larger(all greater than 8×10~4 km~2).When the sprite occurred in thunderstorm T1,the convection and stratified precipitation region of the thunderstorm were both developed and enhanced.When the sprite occurred in thunderstorm T2,the thunderstorm convection weakened and the development of stratiform precipitation region was enhanced.When the sprite occurred in thunderstorm T3,the thunderstorm convection weakened and the development of stratiform precipitation region was not significantly changed.(3)The structural characteristics of the parent thunderstorm of sprites were analyzed.Results show that a total of four sprites occurred during the dissipation stage of the thunderstorm.Only one of the three sprites(sprite that produced by thunderstorm T1 did not find its parent CG)had a distance of 62 km from the parent cloud-to-ground flash(CG)and the rest were all within 50 km(25 km and 40.6 km).The sprites produced by thunderstorm T1 was located in the region with the cloud top brightness temperature of-55?-45°C,and the corresponding radar reflectivity was 25?35 d BZ.The sprites produced by thunderstorms T2 were located in the region with the cloud top brightness temperature of-40?-30°C,and its parent CGs were located in the region with the cloud top brightness temperature of-55?-45°C.However,the sprite produced by thunderstorms T3 and its parent CG also located in the region with the cloud top brightness temperature of-55?-45°C.The sprites produced by thunderstorm T2 and T3 were also located in the region of the radar reflectivity of 35?45 d BZ,and their parent CGs were produced above the stratiform region of the thunderstorm,where the radar reflectivity was between 25 and 35 d BZ, and the corresponding radar echo top heights were about 2.5 km,5 km and 2 km respectively.(4)The lightning activity characteristics of the parent thunderstorm of sprites were analyzed.Results indicate that lightning activities of three parent thunderstorms were dominated by negative CG flashes,and positive CG flashes was always at a low level(all less than 60 flashes/6 min).The Percentage of Positive CG to total CG(POP)was higher at the time of sprites,and POP was higher at the weakening stage of the thunderstorm than at the developing stage.Before the occurrence of the sprite in thunderstorm T1,the frequency of negative CG flash decreased from 31 flashes/min to 19 flashes/min and the positive CG flash frequency increased from 7 flashes/min to 12 flashes/min.In thunderstorm T2,the frequency of negative CG flash increased from 3 flashes/6 min to 10 flashes/6 min and the positive CG flash frequency deceased from 7 flashes/6 min to 6 flashes/6 min before the sprite occurred.However,both positive and negative CG flash frequencies increased before the sprites occurred in thunderstorm T3,the frequency of positive CG flash increased from 2 flashes/min to 6 flashes/min,and the negative CG flash frequency increased from 1 flashes/min to 17 flashes/min.(5)Characteristics of sprite images and the lightning flash that produced sprites were discussed.Three of four sprites(one of which was dark and could not be identified)were columnar sprites and the polarity of their parent CGs was positive.In thunderstorm T2,the peak current of two sprites parent CGs was+183 k A and+45k A,and the impulse charge moment change(i CMC)of the parent CGs was+394 C km and+117 C km,respectively.The ultra-low frequency magnetic field recorded the discharge current inside the first case of sprite,indicating that the discharge of this sprite was very strong.In thunderstorm T3,the discharge intensity of the parent CG was weak(+41 k A),and the i CMC of the parent CG was+117 C km.In thunderstorm T1 and T2,the initial height of the positive CG flash which produced sprites was above the height of the freezing layer,and the height of the charge source was around 5.6km MSL,6.6km MSL and 5.7km MSL respectively.However,in thunderstorm T3,the initial height of the positive CG flash that produced sprite was below the height of the freezing layer,and the height of the charge source is around 5.2km MSL. |
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