Tropical Cyclones Lightning Activity Features And Its Relationship With Cyclone Characteristic Evolution Research

Due to the lack of supercooled water and ice particles, electrification is inadequate andlightning activity is weak in tropical ocean convections. However, in recent years, more andmore research found that lightning occur in tropical cyclones (TCs), especially in some stages ofTC change, the eyewall lightning had a phenomenon of sudden increase (i.e., eyewall lightningoutbreak). Lightning activity is closely related to the dynamic and microphysical processes incloud. Increased lightning activity indicates the enhanced internal convection in TCs, thuslightning activity and structure changes and characteristics evolution (intensity change andtrajectory turn) of TCs are intrinsically related. Cloud-to-ground (CG) lightning in tropicalcyclones over the Northwest Pacific were investigated in this study. With data from the WorldWide Lightning Location Network (WWLLN), the GuangDong Lightning Locating System(GDLLS), TC intensity and trajectory, Doppler Radar and Tropical Rainfall Measuring Mission(TRMM) satallite, the study firstly analyzsed the spatial and temporal characteristics of lightningactivity in the TC’s life, particularly during the landfall periods, and secondly investigated thestatistical relationship between lightning activity and TC intensity change, and then discussed thepredictive values of eyewall lightning outbreak to TC changes of intensity and trajectory.Moreover, the landfalling Typhoon Molave (0906) was studied with observational data andnumurical model to reveal the relationship between temporal and spatial variation of lightningand TC convection evolution. Based on the results of dynamic and microphysical processesfrom WRF model, the reasons for the formation of the temporal and spatial distribution oflightning during the evolution of TC structure were explored.The main conclusions and results from the research are as follows:(1) There was no significant correlation between lightning frequency and TC intensity level.Lightning was more likely to occur in intensity level of tropical depression (TD) and tropicalstorm (TS). Lightning activity in the inner core and outer rainband demonstrated consistent trendwith TC intensity levels, with firstly increased and then decreased. When over the water, mostTCs were in a relatively stable state, and less experienced rapid intensity change (i.e., rapidintensification and rapid weaken). There were differences in geographic distribution betweenrapid intensification (RI) and rapid weaken (RW). The former occurred more frequently than thelatter and lightning occurred during both RI and RW periods. Lightning density in TCs duringthe intensify process was greater in radius than that during the weakening process.(2) Lightning density was the highest in the inner core when the storm experienced RI.Lightning in the inner core had predictive values to TC intensity changes. Lightning activity in the inner core can provide instructions to RI and RW changes. Lightning frequency increased24h before the rapid strengthen happened. When RW occurred, the inner core lightning densitysharply reduced and no lightning occurred12h after RW happening. The TBB values reachedthe lowest when the storm rapidly strengthened, and the inner core lightning showed periodicoutbreaks. The largest cloud top and lowest TBB appeared in the inner core during the RI stages,which gave the reason why lightning frequency increased during the RI stages.(3) Lightning activity varied among TCs and the spatial distributions were different amongdifferent TC intensity levels. With the enhancement of TC intensity, lightning distribution hadthe trend of gradually transferring outward from the eyewall to outer rainbands. The highestlightning density ratio of eyewall to outer rainband occurred in TSs and the lowest in strongtyphoon (STY). The positive CG ratio in the eyewall and inner rainband were greater than that inthe outer rainbands. Spatial distributions of lightning differentiated before and after landfall indifferent TC intensity levels. Lightning in the outer rainbands weakened but in the eyewallstrengthened after TSs landed. While lightning in the eyewall, inner raiband and outer rainbandall weakened after strong tropical storms (STSs) landed. Lightning occurred mainly in the outerrainbands during pre-and post-landfall and there were three distinct lightning density regions inradial distribution after the storm landed. Lightning frequency in STY reduced after the stormlanded.(4) Lightning outbreaks in the eyewall region had indication information to TC changes ofintensity and path. When storms were during weak or gradually deepened process, lightningoutbreak indicated the coming rapidly intensity increase and the outbreak was ahead of themaximum intensity about7.1hours. When storms were in greater strength and relatively stable,lightning outbreak represent that the storm would reach the maximum intensity. When theeyewall lightning outbreak with high positive CG ratio, it indicated that storm intensity wouldweaken or come to an end. The sudden increase of eyewall lightning in storm of steady statemay indicate the coming change of its trajectory.(5) There were three distinct lightning density regions in radial distribution in TyphoonMolave (0906) during its landfall period, with a strong maximum in the outer rainbands, asignificant maximum in the eyewall regions and a minimum in the inner rainbands with nearlyzero. During the period of landfall, lightning activities showed significant spatial asymmetry andoccurred mainly on the left side of typhoon moving direction. When Molave was in the sea, ithad a complete structure but lightning activity was weak. Strong convective core apparent in theouter rainband after it landfall, thus resulted strong surface rainfall and intense lightning activity.When typhoon landed, friction between spiral rainbands and land, and the effects ofenvironmental flow were the main causes of stronger lightning activity and the enhancedconvection in the outer rainbands. (6) When Molave was approaching landing, storm intensity gradually weakened, but stronglightning activity still occurred. After the typhoon landing, lightning density rapidly weakened.The maximum lightning frequency in the eyewall region was ahead of that in the whole TCregion. The eyewall lightning outbreak occurred3times during the TC intensification and theoutbreaks of eyewall lightning may indicate deepening of the cyclone and strength enhancement.The ratio of positive to CG lightning reached its maximum one hour before the maximumtyphoon intensity. CG lightning weakened rapidly after the typhoon landed, but the ratio ofpositive lightning increased, especially during the dissipating stage with the average ratio ofabove20%.(7) Different characteristics of lightning activity were determined by convective structuresand precipitation features in different regions. Strong lightning activities in the outer rainbandswere caused by the strong updraft, large concentration of precipitable particles, large ice particledensity in mixed region and the large vertical and horizontal distribution of convective cloud.The outer rainbands have stronger convective precipitation characteristics than the innerrainbands, which determine the more active lightning activity in the outer rainbands than in theinner ones.(8) The vertical distributions of ice and water particles in cloud were ice, snow, graupel andcloud water. Among the concentrations of the four particles, the mixing ratio of ice was thelowest. The eye wall lightning outbreaks did not occur during the maximum wind spend stage,but in advance of the occurrence of the strongest intensity, and had predictive value to typhoon’sintensify. The increased content density and distribution height of graupel and cloud water, andthe enlarged area and speed of updraft were the main reasons for the eyewall lightning outbreak.The peak of TC lightning frequency reached its maximum during the strongest strength oftyphoon. The outer rainbands played an important role in TC lightning frequency changes. Theincreased density of ice-phase particles and the emergence of strong convective core were themain reasons for the increase of TC lightning frequency.