Studies On Characteristics Of Thunderstorm And Lightning Activity Over The Asian Monsoon Region

Thunderstorm,a severe convective weather process accompanied by lightning and precipitation,plays an important role in global atmospheric electric cycle and hydrosphere cycle.Lightning,as an important catastrophic weather phenomenon during thunderstorms,has great impact on electricity,transportation,broadcasting,communication,petroleum,chemical industry,aviation and aerospace,etc.Affected by the interaction between the Aisan summer monsoon and the Tibetan Plateau(TP),the lightning and thunderstorms are very active over the plateau and the Asian Monsoon Region(AMR),and present unique regional characteristics.Based on the 16-year(1998?2013)observations of the Tropical Rainfall Measuring Mission(TRMM)satellite,the climatology of lightning and thunderstorm activities over the AMR were investigated.Combined with atmosoheric reanalysis datasets,the lightning and thunderstorm activities and their relationship with surface thermodynamics over the Tibetan Plateau were systematically studied.The main conclusions were as follows:1)There is significant difference between the land and the ocean in the lightning activity over the ARM.The lightning density is high over the land and low over the ocean,while the lightning properties(i.e.optical radiance,duration,and footprint area)are low over the land and high over the ocean.The flashes over the far sea are characterized by larger optical radiance and spatial scale,while the flashes with longer temporal scale are concentrated over the offshore region.In spring,the flash density over the low-latitude region was high,while the maximum of lightning optical radiance appears over the East China Sea.In summer,the flash density increases(decreases)over the mid-latitude(low-latitude)region,but the lightning properties weaken(strengthen).In autumn,the flash density over the AMR decreases significantly,but the flashes are characterized by the largest discharge intensity and spatial-temporal scale.It is found that the spatial distribution of total rain rate and convective rain rate matched well with that of lightning in the mid-latitude region,but the centers of lightning and precipitation presents significant differences in the low-latitude region.2)The thunderstorms are mainly concentrated in continental and coastal areas,the land/ocean ratio of thunderstorm is about 4.4:1,and the ratio of intense thunderstorm(top 10%thunderstorm with flash rate greater than 11.7 fl/min)is nearly 7.4:1.It is found that the thunderstorms in the 0?10°N latitude zone account for the largest proportion of the total thunderstorms(31.7%of the total),while intense thunderstorms are most active in the subtropical zone 20?30°N(34.5%of the total).The distribution of thunderstorm is similar with that of lightning density in low-latitude region,however,different distribution patterns are seen in mid-latitude region between 25?36°N,i.e.the thunderstorm(lightning density)presents an increasing(decreasing)trend from west(the TP)to east(Eastern China).However,the distribution of intense thunderstorms is similar with that of lightning density in all latitude.3)Affected by the ASM,the intense thunderstorms in low-latitude region are more likely to occur in spring,and the strong and sub-center center of intense thunderstorms are located at the eastern tip of the South Himalaya Front(SHF)and Indochina Peninsula.The intense thunderstorms are most active in mid-latitude region in summer,when the strong and sub-center center transfer to the western tip of SHF and the Yangtze-Huaihe river basin,respectively.The continental thunderstorms mainly occur from afternoon to evening,and there are frequent nocturnal thunderstorms in some regions because of the affection of local circulation and meteorological conditions,while the maritime thunderstorms are more likely to occur between midnight and early morning.The intensity of thunderstorms over the Eastern China is the strongest,reflected as more ice-particle content,larger lightning flash rate,and higher 40 d BZ echo-top height.The horizontal scale and vertical development height of thunderstorms are the highest over the Marine Continents,while the intensity and scale of thunderstorm over TP are the lowest.4)The geographical distribution of lightning flash density over the TP is decreasing from east to west.The average flash density is 4.3 fl/km~2/yr over the Eastern Tibetan Plateau(ETP),1.7 fl/km~2/yr over the Western Tibetan Plateau(WTP),and 2.6fl/km~2/yr over the Central Tibetan Plateau(CTP).The lightning over the TP mainly occurred in boreal summer,and 96.8%of the lightning occurred from May to September.There are two obvious flash centers over the main highland of TP in the climatological distribution,i.e.,the Nagqu area in the Central Plateau and the Yushu area in the Northern Plateau,and the locations of the two centers swing toward the west with the season.The complex terrain of TP affects the lightning activity,which is manifested in that the flash density gradually decreases with the increase of altitude.The lightning actives weak(strong)when the plateau is controlled by the prevailing westerly wind(summer monsoon).5)Compared with the thunderstorms over the main highland of TP(WTP and CTP),thunderstorms in ETP have greater thunderstorm intensity,which are manifested in higher lightning flash rate,more ice particle content,higher cloud top height and greater convective form volumetric rain.The convective intensity of thunderstorm in the CTP is close to but slightly higher than that in the WTP.The thermal instability conditions of ETP and CTP are similar,but there is a huge difference in thunderstorms flash rate of them.The difference of surface specific humidity between them may explain the difference.Both the variations of surface latent heat flux and specific humidity indicate that thunderstorms over WTP can be triggered under drier conditions.The structure of thunderstorm over TP is significantly affected by the topography of higher west-lower east.The scale and intensity of thunderstorms gradually increase from the WTP to the ETP in terms of vertical top height,development depth and horizontal scale.6)Compared the seasonal variation of lightning with that of precipitation rate over the TP,a phenomenon of‘lower rainfall but more lightning'is found.This phenomenon seems not only occurred in spring over the CTP,but also in summer over the WTP.This phenomenon is not obvious over the moist ETP.The total heat flux seems to be a better parameter for describing the seasonal variation of lightning activity.However,the magnitude of the total heat flux in different regions is not proportional to that of the flash density.The method of modifying the rainfall with the Bowen ratio seems only to work in the CTP for our results.In the drier WTP,the modified rainfall peaks deviate from the lightning.A new parameter,product of rainfall,Bowen ratio(defined as the ratio of the sensible heat fluxes to latent heat fluxes)and surface specific humidity,is proposed and shows a better correlation with lightning activity,generally displaying a larger correlation coefficient than rainfall alone or other parameters over the study sub-regions.