Remote Sensing Of Lightning Heating Effect With A Ground-based Microwave Radiometer

The lightning heating effect refers to the phenomenon that the atmospheric temperature rises during the lightning discharge process,and the local air temperature can reach tens of thousands of degrees.At present,the temperature of the lightning discharge path,especially the highest temperature,mainly depends on spectroscopy methods in the visible(including nearinfrared)band.However,due to the short duration in visible band,this method is difficult to represent the lightning heating effect duration.The duration of microwave radiation produced by lightning is longer than that in visible band,and it has stronger penetration than that in infrared band.Therefore,this study proposes to use the microwave thermal radiation generated by lightning to observe the lightning heating effect.The observation scheme of detecting lightning heating effect with a ground-based microwave radiometer is systematically studied,and the observation experiment is carried out at Guangzhou Field Experiment Site for Lightning research and testing.This paper focuses on the processing of brightness temperature observation data,the improvement of observation scheme,and the inversion and verification of lightning heating effect characteristic parameters.In order to explore the application of microwave radiometer in lightning heating effect observation,combined with the radiative transfer model,the beam filling factor,transmittance and brightness temperature increment with different working parameters of ground-based microwave radiometer are given.The simulation results show that a suitable low elevation and frequencies close to the atmospheric window are more conducive to the microwave remote sensing of lightning heating effect.According to the characteristics of lightning heating effect,the "lightning observation mode" of ground-based microwave radiometer was proposed in 2016,and it was continuously improved in the observation experiment from 2017 to 2019.Based on the lightning observation data in 2017,it is found that the observation efficiency and average pulse amplitude show a "unimodal" distribution with frequency,reaching peak values at 30 GHz and 28 GHz,respectively;the radiometer may observe the lightning heating effect when the elevation angle is set to 30 °-75 °.This is helpful to continue the observation experiment in 2018-2019.This paper systematically analyzes the lightning observation data in the summer of 2017-2019,and the results show that the brightness temperature pulse is generally generated about0.2 s after the trigger time;when the elevation angle is 30 °,and the observation distance is relatively short(99 m in this study),the lightning observation data of the radiometer can be better used to reveal the characteristics of lightning heating effect.Among the 42 triggered lightning,30 were observed by the radiometer,and the observation efficiency of radiometer has increased year by year,totaling about 71.4 %.In the same lightning process,the change trend of the lightning current action integral,the electrical charge quantity and the pulse amplitude are basically the same.Combining the 11 triggered lightning videos and local wind observations,it is found that the moving direction and speed of the high-temperature air cylinder is in good agreement with the wind direction and speed.When the high-temperature air cylinder is gradually away from the beam center because of the local wind,the amplitude and duration of the brightness temperature pulse decrease,which may make the estimation result of the lightning heating effect duration smaller.The lightning heating effect duration of 30 triggered lightning is between 0.1-1.1 s,and the average duration is about 0.5 s.The temperature increment caused by the lightning current is basically between thousands to 40,000 K,with an average of about 19,589 K,which is consistent with the results of the spectroscopy method reported in other literatures.By matching the brightness temperature data with the lightning current data,the possible exponential relationship between the maximum pulse amplitude and the total current action integral in 28 triggered events is given,and the correlation coefficient can reach 0.68.However,the positive correlation between the maximum pulse amplitude and the total electrical charge quantity is not obvious.The changes in the duration of the lightning current and lightning heating effect are relatively consistent.The 25 triggered events that can be statistically analyzed show that the correlation coefficient can reach 0.73,indicating that the radiometer working in the lightning observation mode proposed in this paper can observe the lightning heating effect and estimate its duration.There is an obvious linear relationship between the temperature increment caused by lightning current and the total current action integral in 10 triggered events,and the correlation coefficient is 0.72,which is consistent with the correlation between the temperature of the discharge path and the lightning current action integral reported in other literatures.Based on 38 comparable triggered events,it is found that the ground-based microwave radiometer has the advantage of observing the lightning heating effect,especially for the lightning with weak return strokes but strong heating effects,the radiometer is more likely to detect it.