To investigate the possible impact of urban development on lightning activity, an eight-year (2010–2017) cloud-to-ground (CG) lightning dataset provided by the National-Wide Lightning Detection Network in China was analyzed to characterize the CG lightning activity in the metropolitan area of Beijing. There is a high CG flash density area over the downtown of Beijing, but different from previous studies, the downwind area of Beijing is not significantly enhanced. Compared with the upwind area, the CG flash density in the downtown area was enhanced by about 50%. Negative CG flashes mainly occurred in the downtown and industrial area, while positive CG flashes were distributed evenly. The percentage of positive CG flashes with I_peak ≥ 75 kA is more than six times that of the corresponding negative CG flashes in the Beijing area. The enhancement of lightning activity varies with season and time. About 98% of CG flashes occurred from May to September, and the peak of CG diurnal variation is from 1900 to 2100 local time. Based on the analysis of thunderstorm types in Beijing, it is considered that the abnormal lightning activity is mainly responsible for an enhancement of the discharge number in frontal systems rather than the increase of the number of local thunderstorms. In addition, there is a non-linear relationship between pollutant concentrations and CG flash number, which indicates that there are other critical factors affecting the production of lightning.

This paper presents the temporal characteristics of electromagnetic fields produced by cloud-to-ground (CG) lightning flashes associated with Antarctica storms. A total of 51 positive CG (+CG) and 103 negative CG (−CG) flashes have been recorded by a magnetic direction finder (MDF) sensor in King Sejong Station, King George Island. In total, 47 positive and 107 negative return strokes were located, ranges from 32 to 569 km. All CG flashes were detected to occur over the ocean. The return strokes characteristics include the observations of slow front, fast rising, rise time, and pulse duration with geometric mean values of 3.2, 0.53, 3.21, 13.12, and 67.09 µs for +CG flashes, while 3.9, 0.57, 3.72, 20.75, and 69.86 µs for −CG flashes, respectively. Additionally, the average peak currents of +CG and −CG flashes were 4.13 kA and 3.14 kA, respectively. The temporal characteristics of the return strokes of Antarctica storms are comparable to other geographical regions. The smaller peak currents might be due to small magnitude of Antarctica cloud charges when compared to other geographical regions.

Radio atmospherics (sferics for short) are electromagnetic pulses discharged by lightning return strokes and can be used for D region remote sensing. A frequency-domain full-wave (FDFW) numerical model is developed to simulate sferics propagation in Earth ionosphere waveguide (EIWG). For a certain sampling frequency, electromagnetic (EM) fields of sferics are decomposed into a series of plane waves in the spectral domain via two-dimensional (2-D) spatial Fourier transforms. The magnetized cold plasma medium of ionosphere is divided into several thin layers with the arbitrary anisotropic dielectric constant for each layer. The EM fields at the sferics receiver site are evaluated by recursively computing the reflection and transmission matrices in all layer boundaries for all the plane wave components and performing 2-D numerical integration in the spectral domain. The Earth flattening technique is adopted and the horizontal wave vector of each plane wave component is directly modified in the spectral domain to compensate for Earth curvature effects. Interactions between plane waves and the magnetized cold plasma of ionosphere are analyzed and discussed. Comparisons between the FDFW computation and finite-difference time-domain simulation as well as field measured sferics are implemented to verify the accuracy and reliability of the proposed new model.

The thermal structure of the environmental atmosphere associated with Terrestrial Gamma-ray Flashes (TGFs) is investigated with the combined observations from several detectors (FERMI, RHESSI, and Insight-HXMT) and GNSS-RO (SAC-C, COSMIC, GRACE, TerraSAR-X, and MetOp-A). The geographic distributions of TGF-related tropopause altitude and climatology are similar. The regional TGF-related tropopause altitude in Africa and the Caribbean Sea is 0.1–0.4 km lower than the climatology, whereas that in Asia is 0.1–0.2 km higher. Most of the TGF-related tropopause altitudes are slightly higher than the climatology, while some of them have a slightly negative bias. The subtropical TGF-producing thunderstorms are warmer in the troposphere and have a colder and higher tropopause over land than the ocean. There is no significant land–ocean difference in the thermal structure for the tropical TGF-producing thunderstorms. The TGF-producing thunderstorms have a cold anomaly in the middle and upper troposphere and have stronger anomalies than the deep convection found in previous studies.

In this paper, we examined the initiation events of ten negative cloud-to-ground (CG) flashes detected close to measurement stations on 12 November (CG-1 to CG-5) and 24 November (CG-6 to CG-10), 2019. All CG flashes were accompanied by microwave and very-high frequency (VHF) radiation pulses. We recorded all the CG flashes from two measurement stations (ST1 and ST2), which are separated by 13.3 km and consist of fast and slow antenna (FA and SA) sensors, a magnetic field (B-field) sensor, a VHF sensor (60 MHz), and a microwave sensor (~1 GHz). The key finding is that all of the microwave radiation pulses detected before the onset of initial electric field changes (IECs) preceded all VHF radiation pulses with an average lead time of 1.53 µs, ranging between 0.3 and 3.0 µs (for all CG flashes within reversal distance). A total of 50%, 17%, and 33% CG flashes were initiated by a single radiation pulse, two radiation pulses, and three radiation pulses, respectively, and were followed by IECs. Some VHF pulses are classified as short VHF radiation pulses (pulse duration less than 0.5 µs). The average duration from the first microwave and VHF pulses to the onset of IECs were 53.4 µs and 32.78 µs, respectively. The VHF radiation pulses detected before the IECs are suggested to be emitted by short-length positive streamers. Because the microwave radiation pulses were detected earlier (average lead time of 1.53 µs) than VHF radiation pulses, it is most likely that the microwave pulses were emitted by an electron avalanche process.