Study of electric fields and turbulence in thunderstorm clouds

In recent years, our knowledge of turbulence statistics inside cumulus, stratiform, and stratocumulus clouds is more complete thanks to a number of measurement campaigns and subsequent detailed analyses of collected data. However, similar cannot be stated for cumulonimbus clouds. This is primarily because very few measurements have been performed, majorly owing to safety issues amidst harsh atmospheric conditions. At the same time, even though the extent of electrification is present in all kinds of clouds, cumulonimbus clouds are particularly significant because of the final result of electrification, in the form of lightning. Thus the necessity to understand the evolution of electric field in such conditions is highly crucial. In this study, we use data from the campaign, Severe Thunderstorm Electrification and Precipitation Study (STEPS), performed in the May of 2000 in Kansas, USA. The campaign consisted of aircraft penetrations into the mature thunderstorm cloud and several balloon soundings. This involved in-situ measurements of electric field, vertical velocity, liquid water content, etc. 

Charges inside the clouds are under constant motion, owing to convective motions such as updraft and downdraft. This causes them to be scattered around in various regions of the clouds and form clusters depending on how turbulent these regions are. In our study, we  compared the evolution of turbulence and electric field inside the clouds. Our results show negative correlation between the turbulent kinetic energy dissipation rates and modules of the electric field vector, which suggests the growth of electric field in regions of weak turbulence and vice versa. This could mean that larger charges exists in those regions where turbulence is on the verge of decay or it is in the process of development. Vice versa, the presence of strong turbulence destroys the charges clusters. We also investigate the intermittency, which is a notable indicator for turbulent fields.  Specifically, we calculated the probability density functions of electric field differences at two points. For small differences those functions are clearly non-Gaussian, with long stretched tails and conical tip, which is a very typical picture for intermittency. For larger lags, the distributions are closer to gaussian, thereby signifying a homogenous arrangement of charges.