Cloud Electrification Experiment

This document is divided into FOUR sections:


This is a multi-option, single-aircraft experiment designed to seek out the electrically active convection in TCs for in-depth study. The first option uses the recently installed Desert Research Institute (DRI) electric field mills and the DRI induction ring to obtain both the electric field strength and the charge carried on the hydrometeors within the hurricane eyewall and convective rainbands. The information will help to determine why some hurricane convection is electrically active while other, similar, hurricane convection is not. A second option will investigate the relationship between cloud physics, vertical velocity, and the occurrence and location of cloud-to-ground (CG) lightning within ~325 nmi (600 km) range of the NLDN. Together, these data sources and techniques should lead to a better understanding of the characteristics of the convective processes that lead to lightning in hurricanes and, possibly, to intensity changes of the storms.

[Return to table of contents above]

Program Significance

Cloud electrification has been a topic of great scientific interest for many years, but the lack of suitable instruments for measuring electric fields and particle charges in clouds has hindered research. From anecdotal evidence, meteorologists have considered that hurricanes usually have little electrical activity. However, the introduction of wide-area lightning detection systems along the U.S. coast has resulted in several case studies of lightning from tropical storms and hurricanes. These data show that a larger proportion of TCs produce cloud-to-ground (CG) lightning than was previously known.

Neither the microphysical nor electrical structure of TC clouds that exhibit lightning is known. Laboratory experiments have shown that more charge is separated when ice crystals collide with a rimed target in the presence of supercooled water than is separated without supercooled water. They also showed that the sign of the charge transferred reversed at about -20°C. Other laboratory experiments showed that the growing conditions encountered by the ice particles determined the sign of the charge that was transferred between them during collisions. Observations in continental thunderstorms support this hypothesis and suggest that charge separation occurs most rapidly on the boundary between the main updraft and the downdraft near -15°C. More recent observations showed that sublimating graupel acquire negative charge and graupel undergoing deposition acquire positive charge. As these processes depend critically upon the graupel temperature and cloud liquid water content, it is highly desirable to obtain suitable measurements in natural clouds.

In mature hurricanes, updraft velocities are usually low. In addition, graupel and ice particles are plentiful, but supercooled cloud water is rare in hurricanes at temperatures as warm as -5°C. Studies of two mature Atlantic hurricanes have shown that the little supercooled water present in the strongest eyewall updrafts was immediately adjacent to areas that contained high concentrations of small ice particles. When one considers the lack of supercooled water in mature hurricanes, it is not surprising that mature hurricanes are not always electrified. However, the National Lightning Detection Network (NLDN) detected lightning in several hurricanes and tropical storms as they approached land.

A recent investigation noted that there appeared to be a relationship between the occurrence of CG lightning in the eyewall and a subsequent intensification of the hurricane. A similar relationship was proposed by studies of lightning observations in two developing TCs. In each case, lightning was qualitatively associated with exceptionally strong convection, which occurred when the storms were rapidly intensifying. In addition, recent observational studies of CG lightning in TCs using data from the NLDN showed that CG lightning is most prevalent in the outer convective rainbands of hurricanes with little CG lightning near the eyewall. An apparent paradox is thus created as research shows that vertical velocities in rainbands are weaker than those in the eyewall. It is important to note, however, that rainbands >54 nmi (100 km) outside of the eyewall remain virtually unsampled.

Although these observational studies analyzed lightning in TCs, none of them included cloud microphysics or vertical velocity measurements. The inclusion of these data are critical to better understanding the relationship between cloud physics, vertical velocity, and CG lightning. Combining these three data sets will allow further investigation into the possible implications of CG lightning to intensity changes in TCs.

In view of these observations, we believe that supercooled water and charge separation occasionally occur in the strong convection in TCs. Recent additions to the WP-3D instrumentation that make electrification studies possible are four rotating vane field mills that measure E (the vector electric field) and an induction ring that measures the charge on individual particles. The development and testing of these instruments will continue through 1996.

[Return to table of contents above]


The objectives of this experiment are to study the temporal evolution of the electric field and microphysical and kinematic properties in TCs. The specific goals are:

  1. Measure the sign and magnitude of the vector electric field near the eyewall and in an outer convective rainband.
  2. Document the three dimensional wind field in electrified clouds, including the vertical winds estimated from the Doppler radar.
  3. Determine the polarity and magnitude of the charge on ice precipitation at several temperature levels above the melting level.
  4. Estimate the transport of electrical charge in the storm.
  5. Record the types and concentrations of all particle types observed in the electrically active portions of the storm.

[Return to table of contents above]

Mission Description

This experiment documents the microphysical characteristics of electrically active convection using a single aircraft. The new Particle Measuring System (PMS) 2-D greyscale probes, the new PMS FSSP-100, and the University of Nevada, Desert Research Institute (DRI) field mills are essential. The DRI induction ring, the tail Doppler radar, and the cloud liquid water probes (Johnson-Williams [JW] and King) are highly desirable. Horizontal and vertical wind field measurements will be obtained from the Doppler radar. The aircraft should execute a standard true airspeed (TAS) calibration in clear air prior to entering the storm if conditions permit.

This study requires that one aircraft be equipped with the DRI electric field instruments in addition to the standard instrumentation. The PMS probes must be the best available, and the radars must be fully operational. The experiment is composed of three options. In all options, it is desirable to have 4 to 6 GPS-sondes to obtain soundings outside the convection in the inflow near the areas of interest. The aircraft should loiter in the eye or any other suitable area when it is necessary to service equipment.

There are THREE options associated with this experiment:

[Return to table of contents above]

Back to the Hurricane Field Program Experiment page.