FORMATION OF ELECTRIC CHARGES IN MELTING LAYER

A.V. Kochin

Central Aerological Observatory, Dolgoprudny, Moscow Region, 141700 RUSSIA

1. INTRODUCTION cover, which could be electrically charged. In response, the initially neutral droplet acquires an The mechanisms for generation of electric equal charge of the opposite sign. Similarly, the charges in have been intensively studied air current overflowing a melting graupel pellet, in many countries for a long time, which is an blows off small charged droplets thus charging indication of both scientific and applied the graupel pellet with a charge of the opposite importance of the subject. The majority of sign. A large number of electrification theoretical models describe processes in mechanisms have been examined (Mason, cumulonimbus clouds. Models dealing with 1971; Muchnik, 1974). nimbostratus clouds are few and more of a However, no significant electric fields are qualitative nature. They do not offer any reliable induced because the carriers of charges of quantitative estimates of the observed effects. different signs are close to each other and However, the electric charges are being mutually compensate for the induced electric generated not only in Cb but also in Ns. fields. Moreover, about 80% of the electric discharges After the microscale partitioning is completed, to the aircraft occur in Ns clouds (Brylev et al., the charges of different signs should gather in 1989). This is not an indication of a large electric relatively small and spatially separated areas, activity of Ns clouds but, rather is a result of the with the concentration of charges of a given sign fact that any flying in vicinity of the exceeding that of the opposite sign (i.e., a cumulonimbus clouds is prohibited because of macroscale charge partitioning). strong vertical drafts inside them. Thus, development of a model to explain 3. A PHENOMENOLOGICAL DESCRIPTION electrification of the Ns clouds is viewed as an OF THE PROPOSED MODEL urgent task. Besides, one can reasonably assume that, similar to development This paper suggests a mechanism for in Ns and Cb, the generation of the electric generation of electric charges within clouds. charges in different types of clouds is based on According to this mechanism some common principles, but also has specific -the microscale partitioning of electric charges features resulting from peculiarities of the given takes place within the low melting layer due to type. Hence, the model should be also breaking of large droplets emerging from melting valid for explanation of the processes in particles, cumulonimbus clouds. -the macroscale partitioning is induced by different rates of gravitational sedimentation of 2. ORIGIN OF ELECTRIC FIELD IN CLOUDS the different charge carriers, -the emerging electric field increases the Any model intended for description of electric charge generation rate in the course of the charge formation and origin of the electric field in microscale partitioning. the clouds should explain two phenomena, which could be called micro- and macroscale 3.1 Large droplet formation and breaking partitioning of the charges. At the stage of the charge microscale Precipitation particles form from the water partitioning, the electrically neutral hydrometeors vapour mainly in the cold part of the cloud. Then, (droplets, snow flakes, hailstones) are charged the ice particles (snow flakes, snow aggregates, by elementary electrification mechanisms. For graupel pellet) fall down into warm area and turn example, a rapid freezing of a droplet is into droplets thus forming a so called melting accompanied by breaking small pieces off the ice ______Corresponding author’s address: A.V. Kochin , CAO, Pervomaiskaya 3, Dolgoprudny, Moskow region, 141700 Russia, E-mail: [email protected] layer. Its vertical extension is several hundred positively charged region at some distance below metres. the melting layer. A distance covered by a falling ice particle before its total melting is approximately 3.4 Enhancement of charge generation proportional to a cubic root of the particle mass (Kochin, 1994). In the low melting layer the Initial large droplets break in the Earth's largest ice particles will be transformed into electric field (~150 V/m) and then the drop largest droplets. Drops which diameter exceeds fragments (small and large droplets) leave the 4 mm are unstable and tend to break into a large layer where the drops are breaking. After that, number of smaller droplets. The process is as the electric field in this layer is controlled by follows. A droplet with an initial diameter of 4-5 interaction of negatively charged small droplets mm grows to 40-50 mm, transforming its shape and positively charged large droplets. The small to that of parachute. Then the parachute top droplets induce a field which sign coincides with breaks into a large number of small droplets, that of the Earth's field; a contribution of small while its bottom yields a few large ones (Mason, droplets into the resulting field will be greater 1971). If the ice particle spectrum contains since they are closer to the droplet breaking crystals, which produce drops exceeding 4 mm layer. Hence, the electric field strength in that in diameter, these drops will break. The radar layer grows up. The value of the breaking charge observations have proved droplet breaking in the is proportional to the field strength. Thus, a low melting layer in Ns and Cb (Kochin, 1994). positive feedback arises which will lead to a continuos growth of the field strength because 3.2 Electric charge generation each subsequent portion of droplets breaks in a stronger electric field and produces a larger A rain droplet in the electric field behaves as charge than the previous one. a conducting sphere and polarises in response to electric field strength. This is why breaking of a 4. QUANTITATIVE ESTIMATES OF ELECTRIC large droplet in the electric field produces FIELD STRENGTH GROWTH electrically charged fragments (Mason, 1971). The upper part of the break droplet transforms The above description of a charge generation into small droplets with electric charges, which mechanism presents only the basics of the sign corresponds to that of the vertical proposed model. In order to give quantitative component of electric field. The lower part of the estimates one needs to derive an equation droplet transforms into large droplets with describing the charge generation process with an charges of the opposite sign. account of the affecting factors, i.e. vertical and Usually the vertical component of the Earth's horizontal extension of the droplet breaking electric field is negative, so the small and large layer, ice particle size distribution, vertical draft drops will be charged negatively and positively, velocity within the melting layer, etc. respectively. The values of the forming charges This equation was derived and numerical are proportional to the field strength (Mason, simulations of electric field strength growth were 1971). Thus, a microscale partitioning of charges made by using of the vertical draft velocity and takes place. precipitation rate as variables. Ice particle distribution was taking in accordance with 3.3 Macroscale partitioning Rogers (1976). The length of the path of a particle before Macroscale partitioning of the charges occurs melting is expressed as a linear function in due to different falling velocities of the carriers of accordance with Kochin (1994) charges of different signs. Since the velocities of L(d ) = 100d (1) small droplets are smaller, their concentration in where d and L(d) are given in mm and m, the vicinity of the melting layer will be much respectively. The horizontal dimention of the greater than concentration of the large droplets. droplet breaking layer was taking 300 m in Thus, a negatively charged region forms there. As the distance from the melting layer accordance with radar data. increases, the concentration of small droplets 5. RESULTS OF NUMERICAL SIMULATIONS declines. This results in a relative growth of large droplet concentration and inception of a The numerical simulations show that in the V/m (Mason, 1971) which requires precipitation course of time the vertical profile of the field rate at least 10 mm/h for more than 15 min. strength assumes a complicated shape of varying sign, caused by the vertical extension of 7. ON FEASIBILITY OF PROPOSED the droplet breaking layer. In general, the MECHANISM IN CB CLOUDS maximum field strength grows up as precipitation enhances. Depending on the vertical draft Apart from others, the proposed mechanism velocity the maximum field strength in the is likely to contribute to electrification of Cb melting layer can either be a monotonous clouds as well. This belief is supported by the function of time or have an oscillating nature, fact that the model also agrees with the with the charge generation rate reaching experimental data on the electrical activity of the maximum in the downdrafts with velocities of 0.5 Cb clouds. - 1 m/s. According to the model results, under a near Temporal variation of the maximum electric zero updraft velocity the electric field strength field strength E max in a downdraft with a velocity attains a value of 300 - 3000 V/m with no further of 0.5 - 1 m/s is perfectly described by (for build-up of the electric field strength. In strong I>5mm/h, t>60s) updrafts the electric field strength varies within a range -300 - +300 V/m. This corresponds to the  2 −   I 20  observed values in the non-lightening convective E = 250exp 0.01 t (2 ) clouds. max    I 2  In order that the field strength, typical of the   , is achieved, it is essential that the where Emax is expressed in V/m, I is precipitation precipitation area coincides with the downdrafts. rate, mm/h, t is time, sec. Usually, an updraft concentrates in the centre The results of numerical simulations have of the thunderstorm while downdrafts occupy the also shown that cloud periphery. The lower part of the cloud i) within updrafts or weak (less than 0.3 m/s) converges the horizontal air currents as the downdrafts the electric field strength is about 300 upper part is a region of divergence. In - 3000 V/m, with the maximum values only accordance with the proposed model, the slightly depending on precipitation rate and charges are to generate on the cloud's periphery. having a spatial and temporal variability of The area of charge generation, comprising charge sign; individual cells, forms a narrow ring or a sickle. ii) maximum electric field strength is attained This phenomenon has been already discovered in a layer with temperature +2°C; from the experiments (Williams, 1989) but failed iii) when the downdraft velocity is 0.5 - 1 m/s to be explained theoretically. the charge generation rate is the largest and Besides that, the following phenomena should electric field strength growth rate increases as be observed near the melting layer: precipitation enhances. In this case the electric - a fast change of value and sign of the field strength reaches values of 105- 106 V/m hydrometeor's charge; after 15 min of raining at a rate of 10-15 mm/h. - maximum values of charges on hydrometeors . 6. COMPARISON WITH EXPERIMENTAL Both effects have been found during aircraft DATA spiral descents on the periphery of thunderstorm (McCready and Proudfit, 1965). The experimental data on electrical activity of Two-charge clouds are most frequent to the Ns clouds are in good agreement with the occur, with positively charged upper parts and theoretical results: negatively charged middles (Mason, 1971). So, - typical electrical field strength in Ns clouds one may believe that positive charges are coincides with the model results according to descending, captured by updraft and carried to item i) (Brylev et al., 1989); the cloud top. A similar process is observed in - maximum frequency of lightening strokes to Cb clouds during the hailstone growth, namely, aircraft is observed at a level with temperature the large rain droplets are carried to the cloud +1°C (Brylev et al., 1989) ; top. Studies of the clouds have shown - according to estimates by different (Muchnik, 1974) that the hailstone growth is investigators the electric discharging begins accompanied by a strong radio emission, typical when electric field strength reaches 105 - 106 of the thunderstorm inception and development As the electric field strength exceeds 105 V/m, stages, a fact that supports this belief. another factor comes into play, i.e. the In 10-15 min from start of precipitation, the electrostatic interaction becomes comparable positive charges, captured by updraft, will reach with the aerodynamical drag effect. This can alter the cloud top. By that time the field strength will the falling velocity of charged droplets. build-up to 105 - 106 V/m, resulting in lightening A detail analysis of these processes is the activity development. next stage in the proposed model improvement. In case of a sharp enhancement of downdrafts in a precipitating cloud the change of 9. CONCLUSION electric field sign within the melting layer is possible. The reason is that at the early stage of In conclusion, one has to mention that despite development the field sign is oscillating, so an a satisfactory agreement between obtained increase in charge generation rate can "fix" the theoretical results and the experimental data, sign of field, which existed at the moment of the special experiments are needed for model downdraft enhancement. As a result, an opposite verification. The experiment should include (as compared to described above) distribution of registration of starting moment and rate of the cloud charges emerges, something which has droplet breaking (e.g., with radar methods) and been proved by observation (Mason, 1971). measurements of electric field strength. Such an experiment will allow to conclude whether 8. OUTLOOK FOR FURTHER STUDIES physical bases for the proposed model are correct. The described model, however, fails to account for several factors, which strongly affect 10. ACKNOWLEDGEMENT the charge generation rate. There is a process, similar to droplet The author is grateful to I. L. Bushbinder and V. breaking, taking place in the melting layer and L. Kuznetsov for useful discussions and also to capable of generating charges. The thing is that V.R. Megalinsky and colleagues from the Central the peripheral branches of melting snow crystals Aerological Observatory for their assistance in subject to breaking off. If this process is taking conducting the experiments. place in the electric field, the charged fragments emerge, similar to charge partitioning during REFERENCES droplet breaking. However, this process has not been studied in detail and there are only 1. Brylev G.B., S.B. Gashina, B.F. Evteev and preliminary theoretical estimates of its I.I. Kamaldina, 1989. Kharacteristiki electricheski effectiveness. Based on these results, a aktivnyh zon v sloistoobraznyh oblakah.- quantitative estimation of electric field growth Leningrad, Hydrometizdat, 158 p.p. rate has been made from the model equation (5). 2. Kochin A.V., 1994. Radiolokatsionnie It has been found that under exponential ice issledovania microphysicheskih protsessov v particle size distribution the crystal destruction polose tayania .- Meteorologia i hydrologia, No provides a fast growth of the electric field up to 10, p.p 34 - 40 103 V/m and its further oscillations. 3. Mason B. J., 1971. The physics of clouds.- If the ice particle size spectrum is transformed Clarendon Press, Oxford, 671 p.p. to a monodispersal one, the described process 4. Muchnik V.M., 1974. Fizika grozi.- can lead to field strength values of 105 - 106 V/m Leningrad, Hydrometeoizdat, 352 p.p. even in a light precipitation (about 1-3 mm/h). 5. Rogers R.R., 1976. A short course in Another possibility is charge generation in .- Pergamon Press, 232 p.p. precipitating winter-time warm frontal clouds. 6. McCready J.B. and A. Proudfit, 1965. This situation can provoke formation of a thin Observations of hydrometeor charge evolution in layer with positive temperatures, resulting in the thunderstorm.- Quart. J. Roy. Met. Soc., V.91, same effect as produced by a transformation of No 387, p.p. 44 - 53 the particle size spectrum to a monodispersal 7. Williams E.R., 1989. The tripole structure one. of .- J. Geoph. Res., v. 94, No 11, Besides, as soon as the electric field strength p.p. 13151 - 13179 reaches 104 - 105 V/m, the smaller droplets become unstable and begin breaking, thus giving birth to additional charge carriers.