Thunderstorm Dynamics

Project Director: Dr. S.D. Pawar

• To understand the characteristics of thunderstorms embedded in monsoon circulation and its effect on monsoon.
• To study the effect of electric forces on cloud microphysical characteristics using vertical wind tunnel and cloud chamber.
• To study the interaction of thunderstorms with environment using micrometeorological observations and numerical models.
• To study the pattern of damages caused by lightning.
• To study of effects of atmospheric ions and aerosols and on cloud properties.

About two-third of the total number of thunderstorms occurring all over the globe, occur in tropics. The dynamical, microphysical and electrical characteristics of the tropical thunderstorms are much different from those that occur in mid-latitudes. Understanding the interactions of microphysical-electrical-dynamical processes in clouds is crucial to understand the rain formation processes, the cloud charging mechanisms, occurrence and distribution of lightning discharges in storms and several phenomenon such as cloud burst, tornado etc. associated with storms. Further, the role of aerosols in microphysical, dynamical and electrical characteristics of thunderstorms is not yet fully understood. Recent studies suggest that the increased aerosol concentration can reduce mean droplet size, suppression of warm rain coalescence process, and enhance the cloud water reaching the mixed phase region, which can increase the lightning activity. Our studies are vital to understand the microphysical, dynamical and electrical characteristics of thunderstorms occurring in the Indian region and to understand the effect of aerosols on thunderstorm characteristics. Also, this study can be useful to reduce the damages caused by lightning.

To achieve these objectives, we are conducting observations and performing simulation experiments in laboratories. A atmospheric electricity observatory has been established where, observations of basic atmospheric parameters like electric field, atmospheric conductivity air-current current  and space charge density are being done for last 25 years.  A vertical Wind tunel has been designed and fabricated where cloud simulation experiments  are being conducted to study the breakup characteristics of water drop which would help in understanding the effect of electrical forces on rain formation processes  Measurements of Air  Ion spectra along with sulphur di-oxide and Radon concentration are made at Air-Ion-Spectrometer lab to study new particle formation.

Lightning Location Network with 20 sensors has been established over Maharashtra to study the lightning characteristics and its relationship with lightning related damages.

Developmental Activities:

• Lightning Location Network: Lightning Location Network installed over Maharashtra to understand the characteristics of lightning occurring over this region has been strengthened with installation of three more sensors at Mahabaleshwar, Nanded and Phalghar. The performance of this network in detecting lightning is being assessed and is found to be performing well. The recent occurrence of large scale thunderstorms with hails in October 2015 has been captured with great accuracy by this network. A protocol is being discussed with India Meteorological Department, Mumbai to give the information about impending lightning strikes to the Government of Maharashtra. The precautions needed to minimise the loss of life due to lightning are communicated to the Department of Disaster Management, Govt. of Maharashtra.

• Micrometeorological Tower: A 6 m micrometeorological tower with sensors for wind speed and direction, air temperature, relative humidity, atmospheric pressure and rainfall at 1 m and 5 m is installed at IITM campus. Shortwave incoming and outgoing and net radiation sensors were installed at 1.6 m. These measurements will be useful for the study of surface boundary layer characteristics during winter season. The data are being collected continuously from all these sensors and stored in the data logger.

Important Results:

• Role of orography in inducing high lightning flash rate at the foothills of the Himalayas

  Many studies in the recent past have shown that the local orography can generate intense vertical velocity and effect a change in lightning flash rate by interacting with prevailing wind and/or large scale processes. Bourscheidt et al. [2009] in their study over South Brazil have shown that the terrain slope has more influence than altitude on thunderstorm occurrence and lightning activity. Role of orography in inducing high lightning flash rate at the foothills of the Himalayas is examined using the surface electric measurements obtained beneath three thunderstorms with almost similar characteristics at Guwahati located close to the Himalayan foothills in northeastern India. All these thunderstorms occurred after midnight and lasted for a short duration of less than an hour, and active stage of these thunderstorms lasted for 10 to 25 minutes. These thunderstorms exhibited very high peak flash rates ranging from 40 to 80 flashes per minute during the active stage, however, no severe weather phenomenon such as heavy precipitation and high winds were observed at the ground during these thunderstorms. The observations suggest that many severe thunderstorms occur over this region during late evening/night hours. Moisture conversion at foothills due to radiative cooling at mountain tops during nighttime may be responsible for triggering such deep convections over Guwahati during the pre-monsoon season. [Pawar S.D., Gopalakrishnan V., Murugavel P., Role of orography in inducing high lightning flash rate at the foothills of Himalaya, Earth, Planets and Space, 67:51, April 2015, DOI:10.1186/s40623-015-0221-3, 1-7]

• Role of solar activity on convection process

  Role of solar activity, convective available potential energy, surface temperature and difference of land-ocean surfaces on convection processes is studied. Different processes for initiation of discharge are discussed. It is observed that events like sprites and halos are caused by the upward quasi-electrostatic fields associated with intense cloud-to-ground discharges while blue starter, blue jet and gigantic jet are caused by charge imbalance in thunderstorm during lightning discharges. Elves are generated by the electromagnetic pulse radiated during lightning discharges. Relationship between lightning activity/global electric circuit and climate is discussed. [Siingh D., Singh R.P., Sarvan Kumar, Dharmaraj T., Singh A.K., Singh Ashok K., Patil M.N., Singh S., Lightning and middle atmospheric discharges in the atmosphere, Journal of Atmospheric and Solar Terrestrial Physics, 134, November 2015, DOI:/10.1016/j.jastp.2015.10.001, 78-101]

• Production mechanism and characteristics of new particle formation

  Observations of mobility distribution of ions are being made continuously at IITM using a Neutral Air Ion Spectrometer (NAIS) to study formation of new particles and their characteristics. Observations made during rain events show that charged nanoparticles are produced by splashing of raindrops. Observations show that mechanism responsible for the generation of intermediate ions is more efficient than that for the generation of heavy large ions during periods of high rain intensity. Relative roles of Lenard and Blanchard effects are suggested in generating excess of negative intermediate ions in the initial stages and excess of positive cluster ions in the later stages of a rain shower respectively. Further, it is observed that splashing generates excess of negative ions in each ion category. Concentrations of rain‐generated ions increase/decrease with rain. (Fig. 1) [Kamra A.K., Gautam A.S., Siingh D., Charged nanoparticles produced by splashing of raindrops, Journal of Geophysical Research, 120, July 2015, DOI:10.1002/2015JD023320, 6669-6681]

Fig. 1: Variations of the fraction concentrations of different categories of ions and rain intensity with local time on 26-27 August 2010

On the association of lightning activity and projected change in climate over the Indian sub-continent

Association of lightning with long-term distribution of CAPE, convective rain, vegetation cover  and Aerosol Optical Depth over India has been studied using satellite data. It is seen that lightning is positively correlated to CAPE, convective rain and AOD and negatively to vegetation cover. These correlations help us in understanding the definite entity that is responsible for changing the lightning activity in different parts of India.  It is shown that the  upper tropospheric water vapor possesses a significant linkage with lightning occurrences associated with convective activities and strong updraft over India. Future projections through CMIP5-based CCSM4 and CESM1-CAM5 models have shown that enhancement  of  AOD, convective rain and specific humidity may result in regional changes in lightning activity over the Indian sub-continent. (Saha et al., Atmos. Res., online, September 2016, DOI:10.1016/j.atmosres.2016.09.001, 173-190)

Fig. Trend Maps of lightning flash rate (units in %) during (a) January-February, (b) March-May, (c) June-September and (d) October-December, for the period 2000-2014 over the Indian sub-continent.

• Pawar S.D., Gopalakrishnan V., Murugavel P., Veremey N.E., Sinkevich A.A., Possible role of aerosols in the charge structure of isolated thunderstorms, Atmospheric Research, 183, September 2017
• Pawar S.D., Gopalakrishnan V., Murugavel P., Role of orography in inducing high lightning flash rate at the foothills of Himalaya, Earth, Planets and Space, 67:51, April 2015
• Veremei N.E., Dovgalyuk Yu. A., Gopalakrishnan V., Komarovskrikh K.F., Murugavel P., Pawar S.D., Sinkevich A.A., Studying the effects of severe Aerosol Pollution of the atmosphere on the dynamics of cumulonimbus cloud charge structure by Numerical Modeling, Russian Meteorology and Hydrology, 40, September 2015

Project Director: Dr. S.D. Pawar, Scientist-E

Associates :  1.  Dr. D. M. Chate, Sci-E, 2.  Dr. B. S. Murthy, Sci-E, 3.  Dr. C. G. Deshpande, Sci-E, 4.  Shri. P. Murugvel, Sci-E, 5. Dr. R. Latha, Sci-D