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Formation of Electric Charges in Melting Layer

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Formation of Electric Charges in Melting Layer 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 clouds 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 precipitation 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 cloud type. Hence, the model should be also breaking of large droplets emerging from melting valid for explanation of the processes in ice 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 thunderstorm, 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.
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