A. S. Ramachandra Murty

Freezing characteristics of rain water drops with different solutes and their implication on anomalous ice crystal concentrations in clouds

In an attempt to examine the probable cause for the reported anomaly between the ice crystal-and ice nucleus concentrations in clouds, the authors have carried out laboratory experiments on the freezing of rain water drops, under various conditions of temperature, with various solutes. Sulphates, when present in adequate concentration, have been found to accelerate remarkably the process of ice-formation in the drops. The contrast noticed has been greater at warmer temperatures. The ice-forming ability of silver iodide has also improved remarkably in the presence of sulphates. Chlorides, on the contrary, not only decelerated natural ice-formation in the drops but also deactivated silver iodide. Based on certain considerations, the features observed have been interpreted to account for at least part of the anomalous ice crystal concentrations reported in clouds.

Some thermodynamical and microphysical aspects of monsoon clouds

The thermodynamical and microphysical characteristics of monsoon clouds in the Poona, Bombay and Rihand regions were investigated using extensive aircraft in-cloud observations. The number of clouds sampled at Poona, Bombay and Rihand is 2199, 169 and 104 respectively.The temperatures inside the cloud are colder than its environment at Poona and Rihand. The maximum difference is about 3°C at the cloud base level and the difference decreased with height. At Bombay the difference is less than 1°C and at some levels the temperatures inside the cloud are warmer than its environment.The lapse rates of temperatures inside the cloud are slightly less than those in the immediate environment of the cloud. The environmental lapse rates are nearly equal to the saturated adiabatic value.The positive increments in liquid water content (LWC) are associated with the increments in temperature inside the cloud. Similarly positive increments in temperatures inside the cloud are associated with the increments in temperature of its immediate environment at the same level or the layer immediately above.The maximum cloud lengths observed at Poona and Bombay respectively are 14 and 3 km. The horizontal cross-section of LWC showed a maximum number of 13 peaks in clouds at Poona while only 7 peaks were observed at Bombay. The location of maximum LWC in the horizontal cross-section is more or less at the centre of the cloud. The LWC profile showed an increase with height from the base of the cloud at Poona and Bombay. There is no marked variation of LWC with height at Rihand.The total droplet concentration at different altitudes at Poona and Bombay is in the range 28–82 cm−3. The size distribution of cloud droplets experienced a broadening effect with increase in height from the cloud base at Poona. The broadening effect at Bombay is not as marked as that at Poona.

Electrical and microphysical measurements in warm cumulus clouds before and after seeding

Abstract Electrical and microphysical measurements were made in warm maritime and inland cumulus clouds, before and after seeding, by repeated aircraft penetrations at a single level, a few hundred meters above cloud base. Such measurements were also made in non-seeded clouds by single penetrations during transient flights. The electric field initially was negative in the maritime clouds which developed rain. In the cloud case which dissipated without rain it was initially positive. The field showed sign reversal with time, occasionally preceded by intensification, in all maritime clouds. The field initially was positive in inland clouds. It showed no time variation except in one cloud case where both positive and negative fields were recorded during the period of heavy rain. The droplet charge, droplet median volume diameter and liquid water content showed no marked time variation in either maritime or inland clouds. However, in the cloud case which developed heavy rain marked increases in droplet median...

Diurnal and seasonal variations of space charge, electric field and cloud condensation nuclei in the lowest layer of the atmosphere

Diurnal and seasonal variations of space charge, vertical electric field and cloud condensation nuclei in the lowest layers of the atmosphere during winter, pre-monsoon, monsoon and post-monsoon seasons were studied. The curve showing the diurnal variation of space charge exhibited a double oscillation corresponding to that of the electric field during winter and pre-monsoon seasons. Similar features are absent during monsoon and post-monsoon seasons. During pre-monsoon the semi-diurnal components of space charge and electric field are in phase while in winter the semi-diurnal component of space charge occurs 1 h ahead of the electric field component. The mean values and diurnal ranges of both space charge and electric field are high during winter/pre-monsoon and are low during monsoon/post-monsoon seasons. The diurnal range of temperature and concentration of cloud condensation nuclei are positively correlated with space charge and the electric field. The pre-sunrise minimum in the electric field is associated with the characteristics of the F-region of the ionosphere rather than with the ground sunrise time. The variations in the F, layer critical frequency (foFJ are reflected in the diurnal and ter-diurnal components of the surface electric field. The results of the present study support the hypothesis that the increase in the electrosphere potential could be the source of the atmospheric electric sunrise effect (Muir, 1975).

Summary of Observations Indicating Dynamic Effect of Salt Seeding in Warm Cumulus Clouds

Abstract Measurements of cloud liquid water content and temperature were made along with visual observations in 32 traverses carried out in six warm cumulus clouds subjected to salt seeding. The results showed (i) a rise, of 1–2°C, in temperature, (ii) an increase, sometimes exceeding 200%, in liquid water content, and (iii) vertical growth, up to 60%, in seeded clouds which developed rain. The features noticed could be due to the possible dynamic effect of salt seeding in warm clouds.

Airborne electrical and microphysical measurements in clouds in maritime and urban environments

Abstract A study by Khemani and Ramana Murty (1973) has indicated significant increases of rainfall downwind of the urban industrial complex at Bombay during the period of increased industrialisation. In order to understand the physical processes responsible for the observed increases in rainfall, aircraft measurements of cloud electrical and microphysical parameters and of free-air temperatures were made in the maritime (upwind) and urban (downwind) regions at Bombay. The study has pointed out differences in the electrical and microphysical state of clouds in maritime and urban environments. In the maritime clouds the electric field was both positive and negative and the cloud droplet charges were positive. In urban clouds the electric field was negative and the droplet charge was also negative. The cloud condensation nuclei and the total measured droplet concentrations were higher in the urban environment. In urban clouds the droplet spectra showed a multimodal distribution and in maritime clouds it was unimodal. Also, the concentration of large droplets of diameter 50 μm and above, and the integrated cloud droplet liquid water content were more in the urban environment. The tail of the droplet spectra extended to higher sizes in urban clouds. The free air temperature in the urban environment was higher by about 1°C.

Measurements of cloud droplet size distributions in seeded warm cumulus clouds

SummaryAircraft observations were made on cloud droplet size distribution in 19 cases of seeded warm cumulus clouds in three meteorologically different regions. An examination of the data suggested the following: (i) In all the 5 cases at Bombay (maritime), and in 6 out of the 7 cases at Poona (modified maritime) the values of median volume diameter, maximum cloud droplet size and liquid water content showed increase after seeding as compared to what they were before. The increases noticed in the median volume diameter ranged up to 48% at Bombay and 478% at Poona. The liquid water content and the maximum droplet size respectively increased upward of 56% and 25% at Bombay and 60% and 31% at Poona. (ii) In 4 out of the 7 cloud cases at Rihand (continental), the values of the parameters referred to above showed increase. The median volume diameter increased up to 372%. The liquid water content and the maximum droplet size increased upward of 160% and 66% respectively. (iii) In the cloud cases wherein the values of the parameters decreased after seeding, the decreases noticed ranged up to 94%. (iv) More unstable conditions prevailed on the days when the seeded clouds showed visible rain than on the days when they did not.

Role of dust on rainfall in northwest India

The inter-dependences between local dust, thermal stability of the air and precipitation have been examined from the available data of two selected pairs of years for the station, Delhi, in northwest India. The indications are that the dust enters into the precipitation picture by virtue of its role in radiation balance during the relatively dry period, January to June, and by virtue of its role in cloud microstructure during the wet period, July to September.SummaryThe inter-dependences between local dust, thermal stability of the air and precipitation have been examined from the available data of two selected pairs of years for the station, Delhi, in northwest India. The indications are that the dust enters into the precipitation picture by virtue of its role in radiation balance during the relatively dry period, January to June, and by virtue of its role in cloud microstructure during the wet period, July to September.

Conditions Governing Drop Freezing at Warm Temperatures

Abstract In an attempt to investigate conditions under which ice crystals can form at comparatively warm temperatures in supercooled regions of a cloud, the scope of experiments on drop freezing, previously undertaken by the authors, has been enlarged. The results of the present extensive series of experiments have confirmed that supercooling drops, when subjected to evaporation, freeze more readily due to what has been called the “dynamic effect” of evaporation. When sodium sulphate was added to the drops in arbitrary concentration, their freezing probability also showed a significant inctease. These findings indicate that, in supercooled regions of a cloud where prevailing conditions are similar to those of the reported experiments, the ice crystal concentration should be two to three orders of magnitude higher than the existing ice nucleus concentration.

A numerical technique for simulation of cloud seeding experiments

Two numerical cloud seeding experiments, using historic rainfall for the Deccan plateau region in Maharashtra state, were performed adopting different simulation techniques. The data used consisted of 1-day total rainfall for the 5-year period 1951–55. A double-area cross-over design with area randomisation was adopted.The first experiment, EXP-TR, was based on the simulation technique of Twomey and Robertson which involves about 100 hr of Robotron EC-1040 computer time. The second experiment, EXP-MMM was based on a different simulation technique proposed in the present study.The results of EXP-TR and EXP-MMM have shown close agreement. The numerical simulation technique of EXP-MMM is more promising for the following two reasons: (i) the computational time is reduced by about an order of magnitude without compromising the scientific value of the results, and (ii) a direct estimate of the lower limit of the double ratio value which can be detected at 5% level of significance is defined.The results of the two numerical experiments suggested that, for the Deccan plateau region, 15 and 20% increases in rainfall due to seeding could be detected with 80% or more probability in 5 years.