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Tsutomu Takahashi

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Tsutomu Takahashi

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An axisymmetric warm cloud model (Takahashi, 1975b), modified in both the calculation scheme and diffusion term, is used to investigate the development of electricity in a warm cloud. The study focuses on four electric charge separation mechanisms: 1) the ion-drop interaction under an electric field; 2) the competitional attachment of small ions to drops by the mobility difference between positive and negative small ions; 3) the polarization effect when drops collide and rebound under an electric field., and 4) the ion-drop interaction during drop condensation and evaporation. Comparison with observed data supports the conclusion that ion-drop interaction during drop condensation and evaporation is the major electric charge separation mechanism in warm clouds.

The investigation stresses the importance of drizzle and raindrop formation near the cloud top for both the development of rain and electric charge separation.

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Tsutomu Takahashi

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Hail development was studied in a two-dimensional, time-dependent, axisymmetric cloud model with detailed microphysics. A strong relationship between the dynamics and the microphysics of the model appears to govern hail formation through three different stages.

When the cloud is developing, ice crystals which are nucleated on ice nuclei near the cloud top are carried down along the cloud boundary by downdrafts and are reintroduced into the major updraft region by inflow near the cloud base.

When riming of relatively large recycled ice crystals occurs, graupel is formed. The graupel falls along the cloud boundary and the downdraft is intensified. Below the melting level, graupel pellets melt and large drops break up. The resulting small drops are carried into major updraft areas and grow larger by collection processes.

When the updraft in the cloud becomes weaker, because of the accumulation of precipitation elements and the propagation of downward momentum from cloud boundary to cloud center, graupel and small hail near the cloud top fall through the major updraft column. During their fall they capture large drops and relatively large hail is thus formed. Drag forces due to the hail accelerate the downdraft, the cloud then dissipates, and small hail follows. The lifetime of the cloud is approximately 40 min.

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Tsutomu Takahashi

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The surface electric potential of a growing ice crystal was studied for cases of growth by condensation of water vapor. Positive potential was found during the first stage of condensation. It was found that this positive potential was related to the thin liquid water layer which was produced on the ice surface by the condensation process. Also, it was found that this condensation process was necessary for the production of this positive potential. It was shown that H2O molecules arranged themselves so that protons were directed outward from the liquid surface at condensation. It was also seen that negative potential observed at the later condensation stage corresponded to the growth of ice crystals, this negative potential being produced by the negatively electrified dislocations on the ice crystal surface.

It is suggested that these observations are very relevant to electrification in natural clouds.

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Tsutomu Takahashi

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The physical effects of a liquid surface on the electrification of water drops was studied during condensation and evaporation by controled variation of temperature and small-ion concentration. Experiments show that water drops acquire a negative charge during condensation and a positive charge during evaporation. Two processes were studied for their effect in producing negative electrification: the first was the positive surface potential effect of liquid water, and the second the water vapor pressure effect. These two effects, along with Gunn's diffusion process, may account for the principal electrification phenomena in the liquid phase of clouds, especially in warm clouds.

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Tsutomu Takahashi

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Ground-based observations of the electrical properties of oceanic tropical clouds were made in Ponape (7°N, 158°E), Micronesia, in both October 1976 and October 1977. In addition, specially designed radiosonde systems were used for in-cloud measurements during the latter observational period.

Warm clouds in Ponape, although higher than those in Hawaii, produce the same variations in electric field at the ground, and raindrops exhibit the same electrical charge as those from warm clouds in Hawaii.

Radiosonde observations in thunderstorms indicate that negatively charged particles, probably graupel, form in the temperature region between 0 and −40°C, and fall to the ground as negatively electrified large raindrops. Radiosonde observations of the electrical charge on particles in thunderstorms indicate that the in-cloud distribution of charge in Ponapean thunderstorms is the same as the well-recognized electrical structure in midlatitude thunderstorms. In-cloud electrical charge separation may be explained by the graupel electrification process, which has been studied recently in laboratory experiments (Takahashi, 1978). The relatively weak thunderstorm electrification in the Ponape area is probably due to the high liquid water content in tropical maritime clouds.

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Tsutomu Takahashi

Abstract

The electrffication of growing ice crystals during condensation and evaporation was studied in ionized air of various small-ion concentrations. It was found that ice crystals were electrified positively during condensation and negatively during evaporation. Because of the difficulty of controlling humidity during evaporation, we worked quantitatively only on electrification during condensation. Forty percent more positive small ions than negative ones were caught preferentially on the growing surfaces of the ice crystals. This phenomenon is explained by the interaction of small ions and ice crystals through the negative surface potential of the ice, where the negative surface potential is caused by the negatively electrified dislocations at the ice surface. This positive electrification of ice crystals suggests that clouds consisting of the solid phase have excess positive space charge, in contrast with the author's recent finding of excess negative space charge of warm clouds.

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Tsutomu Takahashi

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Tsutomu Takahashi

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Tsutomu Takahashi

Abstract

Precipitation mechanisms in shallow convective clouds are studied using an axisymmetric cloud model. Clouds are classified into continental and maritime clouds, and further subdivided into warm and cool clouds. Different microphysical factors were selected in the model to represent these different cloud systems. In maritime clouds condensation and collection processes are sufficient to develop precipitation, while in continental clouds graupel formation appears to be a necessary step. In the latter case recirculation of ice crystals is required to initiate riming so that it takes a longer time for rainfall initiation than in the maritime case. In maritime clouds inclusion of the ice phase does not change the rainfall pattern, although both raindrops and graupel contribute equally to precipitation.

The possibility of cloud modification is studied by. increasing the ice nuclei concentration. In continental clouds an increase in ice nuclei concentration of 100 times more than the natural ice nuclei concentration causes activation of a higher number of ice nuclei at warmer temperatures, so that ice crystals can grow large enough to initiate riming during a single upward journey in the cloud without requiring recirculation. In this case it rains heavier and earlier than in the case of normal ice nuclei concentration.

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