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Abstract. The joint effects of aerosol, thermodynamic, and cloud-related factors on cloud-to-ground lightning in Sichuan were investigated by a comprehensive analysis of ground-based measurements made from 2005 to 2017 in combination with reanalysis data. Data include aerosol optical depth, cloud-to-ground (CG) lightning density, convective available potential energy (CAPE), mid-level relative humidity, lower- to mid-tropospheric vertical wind shear, cloud-base height, total column liquid water (TCLW), and total column ice water (TCIW). Results show that CG lightning density and aerosols are positively correlated in the plateau region and negatively correlated in the basin region. Sulfate aerosols are found to be more strongly associated with lightning than total aerosols, so this study focuses on the role of sulfate aerosols in lightning activity. In the plateau region, the lower aerosol concentration stimulates lightning activity through microphysical effects. Increasing the aerosol loading decreases the cloud droplet size, reducing the cloud droplet collision–coalescence efficiency and inhibiting the warm-rain process. More small cloud droplets are transported above the freezing level to participate in the freezing process, forming more ice particles and releasing more latent heat during the freezing process. Thus, an increase in the aerosol loading increases CAPE, TCLW, and TCIW, stimulating CG lightning in the plateau region. In the basin region, by contrast, the higher concentration of aerosols inhibits lightning activity through the radiative effect. An increase in the aerosol loading reduces the amount of solar radiation reaching the ground, thereby lowering the CAPE. The intensity of convection decreases, resulting in less supercooled water being transported to the freezing level and fewer ice particles forming, thereby increasing the total liquid water content. Thus, an increase in the aerosol loading suppresses the intensity of convective activity and CG lightning in the basin region.

The basic characteristics of cloud water, precipitation, and the dependence of precipitation efficiency (PE) on the influencing factors over the Tibetan Plateau (TP) are investigated. Results found that the liquid water path shows a significant downward trend in winter over the TP, and the ice water path shows a significant upward trend in the pre‐monsoon and winter seasons and a significant downward trend in the monsoon season in the western TP from 1998 to 2015. In the eastern TP, the precipitation in the monsoon season also shows a significant downward trend, which may be related to the weakening of the South Asian monsoon. Results have determined that precipitation depends more on the ice water cloud than on the liquid water cloud over the TP. Moreover, the convective available potential energy (CAPE) and the low‐tropospheric relative humidity (RH) are two environmental factors that have a prominent influence on the PE. During the monsoon season, higher CAPE and RH were conducive to a larger PE over the TP. The results suggest that the CAPE has a positive effect on the PE, which means that the PE is directly dependent on the convective precipitation, mainly due to the frequent convective activity and dominant convective precipitation over the TP.

The lightning-induced-damages in the mid-latitude regions are usually caused during severe thunderstorms. But the discharge parameters of natural lightning are difficult to be measured. Five lightning flashes have been artificially triggered with the rocket-wire technique during the passage of two severe thunderstorms. The discharge current and close electric field of return stroke in artificially triggered lightning have been obtained in microsecond time resolution by using current measuring systems and electric field change sensors. The results show that the five triggered lightning flashes include 1 to 10 return strokes, and the average return stroke current is 11.9 kA with a maximum of 21.0 kA and a minimum of 6.6 kA, similar to the subsequent return strokes in natural lightning. The half peak width of the current waveform is 39 µs, which is much larger than the usual result. The peak current of stroke I p (kA) and the neutralized charge Q(C) has a relationship of I p = 18.5Q 0.65 . The radiation field of return stroke is 5.9 kV·m −1 and 0.39 kV·m −1 at 60 m and 550 m, respectively. The radiation field decreases as r −1.119 with increase of horizontal distance r from the discharge channel. Based on the well-accepted transmission line model, the speed of return stroke is estimated to be about 1.4×10 8 m·s −1 , with a variation range of (1.1-1.6)×10 8 m·s −1 . Because of the similarities of the triggered lightning and natural lightning, the results in this article can be used in the protection design of natural lightning.artificially triggered lightning, characteristic discharge parameters, current waveform, electric field change in close distance, severe thunderstormThe serious lightning-induced damages are usually caused by the severe thunderstorm because of the very frequent lightning activities in the mid-latitude region. However, the discharge current and close electric and magnetic fields for natural lightning flashes are very difficult to be measured because of the low probability of striking at a certain object, and the understanding of lightning physical mechanism has been lagged, consequently. As a matter of fact, the lightning discharge can be triggered artificially by means of rocket-wire-trailing technique. The first triggered lightning over terrene was accomplished in France in 1973 [1] . A common technique for artificially triggering lightning involves launching a small rocket trailing a thin, grounded copper wire toward the charged cloud overhead. This technique for triggered lightning is called "classical" triggering. The lightning will be triggered successfully when the rocket reaches a height of several hundreds of meters in a favorable condition of thunderstorm electricity. Most frequently, triggered lightning flashes are negative, that is, they occur when the background electric field is oriented

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Global flash activity shows great contrast in different regions. Temperature and humidity are the essential meteorological factors that result in this difference. The regional response of lightning activities to the relative humidity (RH) is examined at different longitudes and latitudes using lightning data based on satellite and NCEP metrological observations (from 1995 to 2002). The result indicates that lightning activities respond positively to RH within longitudinal belts where RH is more than 74% and negatively for the region where RH is less than 72%. A similar response is found in latitudinal belts. It can be concluded that higher relative humidity results in more lighting activities at dry regions and less lighting activities at wet regions. The watershed of relative humidity for lightning production is about 72%~74%. Six special regions have been chosen to validate above results and an agreement has been found.

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