An approach for retrieving the leader channel charge density, leader current, ambient electric field, and space charge along a leader channel path from ground observations with a complicated ground condition was presented. With this approach, properties of two downward positive leaders (DPL1 and DPL2) struck at high‐rise buildings were studied based on electric and optical measurements made on the roof of a 100 m high building in Guangzhou, China. It shows that the leader‐produced electric field on the roof of the 100 m high building was about 4 times of that on flat ground. The 2‐D speed for both leaders showed a general increasing trend as the leader going down, in the range of 1.8 to 32.3 × 10⁵ and 1.7 to 46.9 × 10⁵ m/s, respectively. The channel line charge density for both leaders showed firstly a sharp increasing trend and then a decreasing trend as the leader going down, in the range of 0.4 to 8.6 and 0.4 to 15.2 mC/m, respectively. The current of the two leaders varied in the range of 0.7 to 5.4 and 0.7 to 4.6 kA, respectively. The ambient electric field (downward is positive) for both leaders showed an alternating polarity along the leader path, in the range of −120 to +176 and −250 to +180 kV/m, respectively. The space charge for both leaders showed also an alternating polarity along the leader path, in the range of −12.3 to +4.2 and −13.9 to +7.8 nC/m³, respectively, which may reflect the in‐cloud electric structure and the corona charge distribution between the cloud and ground.

Hybrid data assimilation (DA) methods have received extensive attention in the field of numerical weather prediction. In this study, a hybrid gain data assimilation (HGDA) method that combined the gain matrices of ensemble and variational methods was first applied in the mesoscale version of the Global/Regional Assimilation and Prediction System (GRAPES_Meso). To evaluate the performance of the HGDA method in the GRAPES_Meso model, different DA schemes, including the three-dimensional variational (3DVAR), local ensemble transform Kalman filter (LETKF), and HGDA schemes, were compared across eight tropical cyclone (TC) cases, and FY-4A atmospheric motion vectors were assimilated. The results indicated that the HYBRID scheme outperformed the 3DVAR and LETKF schemes in TC position forecasting, and with ensemble forecasting techniques, the HYBRID scheme promoted the accuracy of the prediction TC intensity. The threat score (TS) values for the light and medium precipitation forecasts obtained in the HYBRID experiment were higher than those for the forecasts obtained in the 3DVAR and LETKF experiments, which may be attributed to the forecasting accuracy for the TC position. Regarding heavy and extreme rainfall, the HYBRID scheme achieved a more stable effect than those of the 3DVAR and LETKF schemes. The results demonstrated the superiority of the HGDA scheme in TC prediction with the GRAPES_Meso model.

Studies show that intense lightning discharges can perturb nighttime lower ionosphere through induced electromagnetic fields. We report here new evidence that every lightning stroke, regardless of intensity, has an impact on both daytime and nighttime lower ionosphere. Through analysis of thousands of negative cloud‐to‐ground lightning strokes in southern China, we find that skywave features of a lightning sferic are closely related to its source intensity. Variations with increasing lightning intensity show a higher reflectivity from nighttime ionosphere but a larger time delay from daytime ionosphere. It also shows that the ionosphere recovers from the lightning impact in a time shorter than tens of milliseconds. This suggests that there exists a direct and fast nonlinear coupling between lightning‐generated electromagnetic fields and lower ionosphere, making the ionospheric response to a lightning stroke dependent of the lightning intensity. A candidate mechanism for this is the Joule heating effect, but it needs future studies.

This article introduces a time delay calibration technique designed for processing broadband lightning interferometer data with the aim of solving the problem of increased noises in the location results when reducing the length of the data analysis window. The locating performances using three analysis window lengths, 1024 ns, 256 ns, and 128 ns, were compared and analyzed using a cloud-to-ground lightning record as an example. Without using the time delay calibration, the locating noises significantly increased as the length of the analysis window decreased. After the calibration, the problem was solved. Using statistical analysis of the least squares residuals and the signal correlation coefficients within the analysis windows, it was found that overall, there was no significant change in the distribution of residuals after using the time delay calibration method, but the correlation coefficients were significantly improved. The results indicate that the time delay calibration technique can improve the correlation of signals within the analysis window, thereby greatly reducing the ineffective locating results generated after narrowing down the analysis window. The article also analyzed the locating results, as well as the correlation coefficients and signal strength characteristics at the analysis window of 32 ns, the smallest ever reported before. Even at such a small window, the time delay calibration method can still ensure computational stability. The relevant analysis suggests that according to data usage, the correlation coefficient can be flexibly used as a quality control condition of the located results.

Measurement of lightning current plays a critical role in the field of atmospheric electricity. Traditionally, Rogowski coils or low-resistance shunts were employed for measuring lightning currents in the range from several amperes up to several hundreds of kilo-amperes, and high-value resistors were utilized for measuring corona discharge currents at sub-ampere levels. However, these approaches were not suitable for continuously recording the vast range of lightning currents. For this sake, we have developed a lightning current measurement system equipped with a shock-tolerant low-noise amplifier module. With the system installed on a tall tower, sub-ampere level currents just before the lightning initiation were observed for the first time. To confirm the authenticity of the recorded currents, the background noise of the measurement system and surrounding environment were identified, and a digital multi-frequency notch filter was proposed for de-noising. Results show that the system can achieve a current identification level of 50 mA even in complex electromagnetic environments, while having a measurement capability of 220 kA.