Electrical, microphysical and dynamical observations in summer monsoon clouds

Atmospheric Research, 26 ( 1991 ) 19-32

19

Elsevier Science Publishers B.V., Amsterdam

Electrical, microphysical and dynamical observations in summer monsoon clouds A. Mary Selvam, R. Vijayakumar, G.K. Manohar and A.S.R. Murty Indian Institute of Tropical Meteorology, Pune 411 005, India (Received August 26, 1987; accepted after revision February 20, 1990)

ABSTRACT Mary Selvam, A., Vijayakumar, R., Manokar, G.K. and Murty, A.S.R., 1991. Electrical, microscopical and dynamical observations in summer monsoon clouds. Atmos. Res., 26:19-32. Electrical, microphysical and dynamical conditions in ground-based clouds were studied using the observations carded out at Mahabaleshwar ( 17 ° 56' N, 73 ° 40' E; 1382 m ASL), a hill station, during the summer monsoon season of 1977. There is a significant correlation between the rain intensity and the corresponding atmospheric electric potential gradient. The diurnal curves of rainfall and negative electric potential gradient exhibited two peaks. The peak in rainfall during the morning hours and the afternoon peak in electric potential gradient were statistically significant. The early morning peak in rainfall was attributed to the enhanced convergence caused by the radiational imbalance in the cloud and cloud-free regions during active monsoon conditions. The afternoon peak in the electric potential gradient was attributed to the cloud formation due to convection. The atmospheric electric potential gradient showed sign reversal from its normal fair weather positive to negative at the time of the onset of rain. The reversal of the electric potential gradient and the incidence of positively charged raindrops at the surface were almost simultaneous. Positive raindrop charges were recorded 1-2 minutes prior to the occurrence of negative electric potential gradient showing a steep increase during heavy rain spells. The raindrop charges were predominantly positive. The possible physical mechanisms for electrification of monsoon clouds have been discussed. The cloud condensation nuclei and surface temperature also showed peaks during the morning hours. The microphysical observations suggest that the rain formation takes place in monsoon clouds both by the collision-coalescence and ice crystal processes. A simple 1-D model was used to compute different cloud physical parameters. For a cloud with its top at 9 km the precipitation efficiency is in the range 60-70% of the adiabatic liquid water content. It has an average rainfall rate of 15 mm h r - i, vertical velocity of 3-5 m s- ~ and average life time of 30 minutes. RI~SUMI~ On a 6tudi6 les caract&istiques ~lectriques, microphysiques et dynamiques de nuages au sol ~ l'aide d'observations faites h la station de montagne de Mahabaleshwar ( 17 ° 56'N, 73 °40' E; 1382 m ASL), pendant la mousson d'rt6 de 1977. On trouve une corrrlation significative entre l'intensit6 de la pluie et le gradient potentie161ectrique atmosphrrique, les courbes j ournalirres de pluie et de gradient nrgatif prrsentent deux pics. Le pic de pluie du matin et celui de gradient potentie161ectrique de l'aprrs-midi sont statistiquement significatifs. ie pic matinal de pluie est attribu6 ~ I'augmentation de convergence par drsrquilibre radiatif entre

0169-8095/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

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A. MARYSELVAMET AL.

le nuage et les r6gions sans nuage dans des conditions de mousson active. Le pic de gradient de l'apresmidi est attribu6 h la formation de nuage par convection. Le gradient potentiel 61ectrique, normalement positif par beau temps, deviem n6gatif en d6but de pluie. Ce changement de signe et l'arriv6e de gouttes de pluie charg6es positivement sont pratiquement simultan6s. On observe des charges positives sur des gouttes une h deux minutes avant une augmentation prononc6e du gradient n6gatif pendant de fortes averses, les charges des gouttes sont essentiellement positives. On discute des m6canismes physiques possibles d'61ectrification des nuages de mousson. Les noyaux de condensation nuageuse et la temp6rature au sol presentent egalement des pics dans la matin6e, les observations microphysiques sugg~rent que la pluie se forme dans les nuages de mousson ~ la fois par des processus de collision-coalescence et de glaciation. On a calcul6 ~ t'aide d'un module unidimensionnel simple les diff6rents parambtres physiques des nuages. Pour un nuage culminant h 9 kin, le rendement en pr6cipitation est de l'ordre de 60~t 70% du contenu en eau liquide adiabatique, le nuage a un taux moyen de pr6cipitation de 15 mm par ehure, une vitesse verticale de 3h 5 m s - 1, et une dur6e de vie de 30 minutes.

INTRODUCTION

Observations on the electrical, microphysical and dynamical parameters in monsoon clouds in the Indian region are sparse (Murty et al., 1975, 1976; Mary Selvam et al., 1978, 1980). Such observations would be of value for the understanding of the influence of electrical forces on cloud microphysical processes and interactions between the cloud microphysical and dynamical parameters. The observations would also help to identify the electrification and rain formation mechanisms in monsoon clouds. Observations at hill stations with sufficient altitude, where cloud formation can take place, would be useful for the type of investigations listed above. A field observational programme was, therefore, undertaken during the summer monsoon of 1977 at a hill station, mahabaleshwar (17 ° 56'N, 73 °40' E; 1382 m above ASL), which is located directly in the path of the summer monsoon westerly flow. The results of the study are presented below. OBSERVATIONS AND MEASUREMENT TECHNIQUES

Simultaneous observations on ( 1 ) atmospheric electric potential gradient, (2) raindrop charge, ( 3 ) point discharge current, (4) rain intensity, ( 5 ) giant (r> 1 #m ) condensation nuclei, (6) giant non-hygroscopic nuclei, ( 7 ) cloud condensation nuclei active at 0.1% supersaturation, (8) ice nuclei active at - 15°C, (9) surface temperature, (10) station level pressure and ( 11 ) relative humidity were made at Malaabaleshwar during August 23-30, 1977. The values of (i) entrainment, (ii) adiabatic liquid water content, (iii) vertical velocity at the cloud-base level and (iv) maximum cloud-top heights were computed using the 1-D cloud model (Simpson and Wiggert, 1969; Rogers, 1976 ) with the temperature and the humidity profiles obtained from the radiosonde ascents at Bombay ( 18°65'N, 72°49'E; 11 m ASL).

OBSERVATIONSIN SUMMER MONSOON CLOUDS

21

The details of measurements of the parameters listed at ( 1 ) to (6) were described elsewhere (Murty et al., 1976; Mary Selvam et al., 1977 ). Ice nuclei active at - 15 °C were measured using a thermal diffusion chamber (Gagin and Aroyo, 1969). Standard meteorological self-recording instruments viz., (a) bimetallic thermograph, (b) sylphon bellows precision barograph, (c) hair hygrograph were used for the measurement of the parameters listed at (9), (10) and ( 11 ), respectively. The accuracies of the measurement of rainfall, temperature, relative humidity and station pressure were 0.1 ram, 0.3 ° C, 0.5% and 0.2 hPa, respectively. The areal cloud cover over the Indian peninsula and the surrounding region as obtained from the NOAA-5 satellite pictures at 0800 and 1900 LST were also considered in the present study. Also, inferences on the type of cloud, rain process viz., warm/cold were derived from the radar observations of precipitation echoes obtained from the clouds forming in the area of the field observational programme. These observations were obtained using the 10-cm weather radar of the India Meteorological Department located at Bombay which is about 70 km in the upwind region of the observational site. The beam width of the radar was 2 ° and the accuracy of the height of precipitation echoes (cloud top) was estimated to be +__1.0 kin. The characteristics of the precipitation echoes/presence of melting band as evaluated from the radar cloud observations were utilised to infer the type of rain process (warm/cold) associated with the monsoon clouds forming in the area of the field observational programme. METEOROLOGICAL CONDITIONS

Summer monsoon season (June-September) Mahabaleshwar is located on the crest of the Western Ghats. The normal rainfall at the station for the summer monsoon season is 5936 m m out of which 1764 m m is received during the month of August. Large influx of moist air is brought inland from the Arabian sea by the westerly flow in the lower troposphere. During the active monsoon conditions, the station receives continuous heavy to moderate rain. Occasionally tall cumulus clouds form over the Western Ghats with their roots embedded in the low-level stratiform cloud deck (Miller and Keshava Murty, 1967 ).

Period of observations (August 24-29, 1977) During the period August 24-26, under the influence of a low-pressure system there was thick extensive cloud cover over the region, which gave rise to heavy rainfall. During the period August 27-29 the areal extent and thickness of the cloud cover were less. The satellite cloud pictures indicate that the total

22

A MARY SELVAMETAL.

areal cloud cover and areal extent of thick clouds were higher during the period August 24-26 as compared to the period August 27-29. RESULTS A N D D I S C U S S I O N

Daily variations The daily variations of different parameters measured are shown in Fig. 1. The concentrations of giant hygroscopic nuclei, non-hygroscopic nuclei, cloud condensation nuclei and ice nuclei, the atmospheric electric potential gradient, the total rainfall, radar-estimated cloud-top heights were more during August 25-26 when the monsoon activity was strong. The higher concentrations of different nuclei could be attributed to the transport of pollutants from the urban industrial complexes located in the upwind region of the observational site. Under active monsoon conditions the transport of the pollutants into the region takes place due to enhanced meso-scale convergence. The cloud tops during August 25-26, 1977 exceeded 8 km and the rainfall was heavy. The electric potential gradient was negative during the period of rain. The raindrop charges were predominantly positive when the rainfall was heavy.

Diurnal variations The diurnal variation of electric potential gradient, hourly total rainfall, radar-estimated cloud-top heights, pressure and temperature were studied and z hi

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23

OBSERVATIONS IN SUMMER MONSOON CLOUDS

the results for the days with strong and weak monsoon conditions are shown in Fig. 2 and 3, respectively. The diurnal variations in different types of nuclei for August 23-29 are also shown in Fig. 4.

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On days with strong monsoon conditions (25-26 August) the rainfall showed two peaks, one during the early morning hours and the other during the afternoon hours (Fig. 2 ). The early morning peak in rainfall was more pronounced than the afternoon peak during the strong monsoon conditions, when extensive thick cloud cover prevailed over the region. Under such conditions the diurnal range of temperature was small (0.4-1.3°C). The morning maximum in rainfall could be attributed to enhanced convergence caused by nocturnal radiational imbalance between the region of extensive thick could cover and surrounding cloud-free region. The results of the present study are in agreement with those of the others who had investigated the diurnal variations Of rainfall in the region (Gray and Jacobson, 1977 ). The rainfall maximum observed during the afternoon hours may be attributed to the cloud formation caused by surface heating. The early morning rainfall maximum is associated with the maximum in negative electric potential gradient, positively charged raindrops and maximum cloud-top heights (Fig. 2 and 3 ) and these observations are consistent. The concentration of cloud condensation nuclei in the cloud-free air is more (27%) during the afternoon hours (Fig. 4 ). The concentration of cloud condensation nuclei in cloud air is more ( 11% during early morning hours).

Regression and harmonic analyses of electricpotential gradient and rainfi~ll The data of electric potential gradient and rainfall were examined using harmonic and regression analyses. The results are shown in Tables 1 and 2.

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TABLE 2 Results of the harmonic analysis of different parameters Date August 1977 (time LST )

Parameter (three hour means)

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*The numbers within brackets give the time of occurrence of peak in LST.

For harmonic analysis, mean data obtained at three successive hourly intervals were utilised. For regression analysis, data obtained at 1-minute intervals were utilised.

Regression analysis There is a close agreement between the variations in the electric potential gradient and rainfall (Fig. 1 ). The peak in rainfall coincided with that in negative electric potential gradient. The correlation coefficient computed using one-minute resolution data of rainfall intensity and electric potential gradient is found to be highly significant on all days of observations (Table 1 ). The regression line for a heavy rainfall day is found to be different from that for a moderate rainfall day. For the given rainfall rates ( 10 m m h - ~) the electric potential gradient was less on a day (August 26) with maximum number of charged raindrops as compared to a day (August 25 ) with minimum number of charged raindrops. Both August 25 and 26 were strong monsoon days with August 26 having the highest total rainfall. On August 24 the maximum number of charged raindrops were observed.

Harmonic analysis On a day (August 26 ) with strong monsoon activity the major contribution to the rainfall is due to the fundamental component with a peak at 1400 LST. The major contribution to the associated electric field is due to the fundamental component which had a peak at 1330 LST and corresponded with the time of peak of semi-diurnal component of the rainfall during afternoon hours. Analogous to the above, the peak at 0936 LST in the semi-diurnal component of the electric potential gradient showed correspondence with major peak at 0942 LST of the fundamental component of the rainfall. The peak in the

OBSERVATIONS IN SUMMER MONSOON CLOUDS

27

morning hours of the fundamental component of rainfall may be explained on the physical hypothesis of Gray and Jacobson (1977) mentioned in Section "Diurnal variations". The peak in the afternoon hours of the fundamental component of the electric field may be attributed to convection in the!ower layers of the atmosphere caused by surface heating.

Electrical characteristics of clouds The maximum value of the electric potential gradient recorded was - 840 V m - 1. This value is smaller as compared to that generally observed when tall cumulonimbus clouds are present. The electrical activity associated with clouds whose top did not exceed - 15 °C level (about 7 km) was less (Fig. 2 ) and the rainfall received during such occasions was low. The heavy rain was received from cumulus clouds with cloud tops exceeding 8 km. However, during such occasions no measurable corona discharge current, lightning and thunder were recorded. Hence, it may be inferred that these clouds may not be of cumulonimbus type. Aircraft measurements made in the region indicated that monsoon clouds with appreciable vertical extent contain high liquid water content (Mary Selvam et al., 1980). The weak electrification noticed in the tall cumulus clouds in the region may be due to the high liquid water content (Takahashi, 1978). Measurable raindrop charges ( > 10-13 C ) were present only during heavy rainfall occasions with cloud-top heights exceeding 8 km. The maximum raindrop charge recorded was 9.5 X 10-12 C. On most of the occasions ( > 90%) the raindrops were positively charged. A critical examination of the simultaneous recordings of the electric potential gradient, raindrop charges and rain intensity at Mahabaleshwar indicated that positive raindrop charges were recorded 1-2 minutes prior to the electric potential gradient showing a steep negative increase during the onset of heavy rain. Release of positively charged raindrops from the cloud top gives rise to the net increase in the negative electric potential gradient (Fig. 5 ). Surface observations on atmospheric electrical parameters made at Poona about 40 km in the upwind region of the present observational site indicated that the reversal of the electric potential gradient at the surface from the usual fair weather positive to negative takes place at about 10-12 minutes before the onset of rain from the clouds with their bases of about 1.8 km above MSL (Sivaramakrishnan, 1960). In the case of Mahabaleshwar the reversal in the electric potential gradient and the incidence of positively charged raindrops at the surface were almost simultaneous (Fig. 5 ). This is because the cloud base almost rests on the hill top at Mahabaleshwar. The positive raindrop charges noticed may be explained by the charge separation mechanism proposed by Takahashi ( 1978 ). According to the above

28

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mechanism the sign and magnitude of the electrical charges depend upon the temperature and cloud liquid water content. The adiabatic liquid water content in clouds at Mahabaleshwar is estimated to be 5 g m - 3 (Mary Selvam et al., 1980). For the cloud temperatures less than - 15 ° C and liquid water content greater than 6 g m - 3, the presence of positive raindrop charges are expected according to the mechanism proposed by Takahashi (1978). Also, the ice crystals on melting may acquire positive charges (Dinger and Gunn, 1946). This mechanism may also be, partially, contributing to the observed positive raindrops at Mahabaleshwar.

Microphysical conditions The average concentrations of giant condensation nuclei and giant nonhygroscopic nuclei were 82 1- ' and 11 1- ', respectively. The concentrations of cloud condensation nuclei at 0.1% supersaturation in the cloud and cloudfree air were 263 cm -3 and 220 cm -3, respectively. The variations in the concentrations of cloud condensation nuclei, giant condensation nuclei, giant non-hygroscopic nuclei and ice nuclei, in general, followed those of rainfall (Fig. 1 ). The concentrations of giant condensation nuclei and ice nuclei were 100% more on days of higher rainfall. The higher concentrations may be attributed to the influx of the nuclei into the region due to enhanced convergence. The average cloud condensation nuclei concentration was more in cloud air than in cloud-free air and this may be attributed to the influx of nuclei into the cloud due to enhanced convergence in the sub-cloud layers and the subsequent scavenging of cloud condensation nuclei inside the clouds by the cloud droplets. The radar observations indicated that cloud-top height on some occasions extended up to 9 km where the temperatures will be about - 20°C (Table 3 ). From the m a x i m u m measured ice nuclei concentrations (0.071- ' at - 15 °C ),

OBSERVATIONS IN SUMMER

MONSOON

CLOUDS

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A, M A R Y SELVAM ET AL.

it is expected that the ice crystal concentration at - 20 ° C level could be about 11- 7. Since the cloud top on heavy rain occasions extended beyond the - 15 oC level (about 7 k m ) the dominant mechanism of rain-formation on such occasions could be the ice crystal process. These results are in agreement with those reported by others (Sivaramakrishnan, 1960; Srivastava et al., 1966 ). As already stated the electrical activity associated with the clouds, whose tops did not exceed the - 15 ° C level (about 7 km ), was less. Also, under such cloud conditions the rainfall received was less. In such clouds, ice crystals may be absent and mainly consist of water droplets. The dominant mechanisms of rain formation in such clouds could be the collision-coalescence process.

Dynamical characteristics of clouds It is seen from Fig. 1 that days with heavy rainfall (August 24-26) are associated with positive temperature departures. The high temperatures recorded during heavy rainfall occasions below the cloud base may be the result of the transfer of latent heat of condensation by downdrafts in the fall-streak zone of raining clouds. It is to be recognised that decrease of temperature in the downdraft region due to evaporation of raindrops may not be a significant process since the atmosphere is saturated during he s u m m e r monsoon. Also, the present observations were carried out at the cloud-base levels where the relative humidities were nearly 100%. The above hypothesis is corroborated from the aircraft observations of the microphysical and thermodynamical parameters of monsoon clouds which indicated that higher temperatures observed inside the cloud could be due to the latent heat of condensation (Mary Selvam et al., 1980). The m a x i m u m cloud-top heights were computed with a simple I-D cloud model of Simpson and Wiggert ( 1969 ) using the radiosonde temperature and humidity data. An average value of 1000 m was used for the cloud-base radius in the computations. The results are given in Table 3. The computed cloudtop heights were compared with the radar-estimated cloud-top heights. There is a good agreement in the computed and measured values of cloud-top heights in the region. The results of the above cloud model may be applicable only when tall cumulus clouds are present. For a cloud with its top at 9 km the precipitation efficiency (ratio of the computed precipitation from the cloud when there is no fall out, to the adiabatic liquid water content) of cloud is in the range of 60-70%. It has an average rainfall rate of 15 m m h r - ~ and an average life time of 30 minutes. The computed vertical velocity is in the range of 3-5 m s- t. In the above cloud model computations a value of 0.651R was used for the entrainment parameter where R is the radius of the cloud. The observations made in the region suggest that formation of tall cumulus clouds takes place on days with

OBSERVATIONSIN SUMMERMONSOONCLOUDS

31

favourable meteorological conditions during the summer monsoon (Miller and Keshava Murty, 1967) and the results of the Simpson-Wiggert cloud model will be of some value. CONCLUSIONS

Observations of the electrical, microphysical and dynamical conditions in monsoon clouds were made at a hill station during the summer monsoon season of 1977. The important results are given below. (1) There is a significant correlation between the rain intensity and the corresponding atmospheric electric potential gradient. (2) The electric potential gradient showed sign reversal from its normal fair weather positive to negative during the onset of rain. The field reversal and the incidence of positively charged raindrops at the surface were almost simultaneous. Positive raindrop charges were recorded 1-2 minutes prior to the negative electric potential gradient showing a steep increase during the heavy rain spells. The raindrop charges were predominantly positive. (3) The variations in the concentration of cloud condensation nuclei, giant condensation nuclei, giant non-hygroscopic nuclei and ice nuclei, i'n general, followed of those in rainfall. The concentrations of giant condensation nuclei and ice nuclei are 100% more on days of heavy rainfall. From the radar-evaluated cloud-top heights and the microphysical observations it is hypothesised that rain formation takes place in monsoon clouds by both the collision-coalescence and ice crystal processes. (4) The diurnal curve of rainfall showed two maxima, one during early morning hours and the other during the afternoon hours. The early morning maximum in rainfall was more pronounced than the afternoon maximum during strong monsoon conditions when extensive thick cloud cover prevailed over the region. The diurnal curves of negative electric potential gradient, cloud condensation nuclei and surface temperatures also showed significant peaks during the morning hours. ( 5 ) The computations of a simple cloud model suggest that for a cloud with its top at 9 km the precipitation efficiency is in the range of 60-70% of the adiabatic liquid water content. It has an average rainfall rate of 15 m m h - 1, vertical velocity of 3-5 m s- 1 and average life time of 30 minutes.

REFERENCES Dinger, J.E. and Gunn, R., 1946. Electrical effects associated with a change of state of water. Terr. Magn. Atmos. Electr., 51: 477-494. Gagin, A. and Aroyo, M., 1969. A thermal diffusion chamber for the measurement of ice nuclei concentrations. J. Rech. Atmos., 4:115-122.

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A. MARY SELVAM ET AL.

Gray, W.M. and Jacobson Jr., R.W., 1977. Diurnal variation of deep cumulus convection. Mon. Weather Rev., 105:1171-1188. Mary Selvam, A., Manohar, G.K., Khemani, L.T. and Murty, Bh.V.R., 1977. Characteristics of raindrop charge and associated electric field in different types of rain. J. Atmos. Sci., 34: 1791-1796. Mary Selvam, A., Murty, A.S.R., Vijayakumar, R., Paul, S.K., Manohar, G.K., Reddy, R.S., Mukherjee, B.K. and Murty, Bh.V.R., 1980. Some thermodynamical and microphysical aspects of monsoon clouds. Proc. Indian Acad. Sci., Sect. A, 89:215-230. Miller, F.R. and Keshava Murty, R.N., 1967. Structure of an Arabian Sea Summer Monsoon System. East-West Centre Press, Univ. of Hawaii, Honolulu, USA, 94 pp. Murty, A.S.R., Mary Selvam, A. and Murty, Bh.V.R., 1975. Summary of the observations indicating dynamic effect of salt seeding in warm cumulus clouds. J. Appl. Meteorol., 14: 629637. Murty, A.S.R., Mary Selvam, A., Vijayakumar, R., Paul, S.K. and Murty, Bh.V.R., 1976. Electrical and microphysical measurements in warm cumulus clouds before and after seeding. J. Appl. Meteorol., 15: 1295-1301. Rogers, R.R., 1976. A Short Course in Cloud Physics. Pergamon Press, Oxford, 227 pp. Simpson, J. and Wiggert, V., 1969. Models of precipitating cumulus towers. Mon. Weather, Rev., 97:471-489. Sivaramakrishnan, M.V., 1960. The relation between raindrop size distribution, rate of rainfall and the electrical charge carried down by rain in the tropics. Indian J. Meteorol. Geophys., 11: 258-268. Srivastava, G.P., Huddar, B.B. and Srinivasan, V., 1966. Radar observations of monsoon precipitation. Indian J. Meteorol. Geophys., 17: 249-252. Takahashi, T., 1978. Electrical properties of oceanic tropical clouds at Ponape, Micronasia. Mon. Weather Rev., 106:1598-1612.