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Tsutomu Takahashi

Abstract

Drop charge in warm clouds was simulated numerically in a cloud model which includes four basic electric charge generation processes. These basic physical mechanisms are Wilson's induction ion capture effect, Gunn's diffusion effect, the surface potential effect, and the vapor pressure effect.

The diffusion and surface potential effects contribute to the negative electrification in–cloud at the developing stage. Evaporation causes drops to become positively charged at the cloud top in the mature stage. It is only during the dissipating stage that the Wilson effect becomes primary; at this stage, electric charge separation in the air is at the maximum so that high potential gradient develops.

The numerical calculations compare favorably with recent observations in Hawaii.

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Tsutomu Takahashi

Knowledge about precipitation particles in clouds is a prerequisite for the study of precipitation and cloud electrification mechanisms. A videosonde that can inform in-cloud precipitation particle shape and charge has been designed and more than 200 videosondes have been launched into monsoon clouds from 13 different locations in east Asia during the past 12 years.

Rain is divided into three different regions with respect to the precipitation mechanisms: “cool” in inland China, “mixed” over the Maritime Continent, and “frozen” over the west Pacific. Low concentrations of both ice crystal and graupel were observed over the west Pacific where lightning activities are weak. Ice crystals grew primarily on frozen drops and varied highly with different drop-size distributions near the melting level. Large cloud drops over the open ocean due to low CCN are slow to form ice crystals and suggest inactive ice crystal production. Electric charge measurements helped to study the particle evolution in rain systems. In squall lines, extensive recirculation of precipitation particles and rapid growth of frozen drops through capturing of supercooled drops from forward cells correlate with intense rainfall.

Information about precipitation particles in heavy rain areas is essential for the study of precipitation and cloud electrification mechanisms. The purpose of this article is to demonstrate that a videosonde that can fly into any area of a rain system is a powerful tool to study rain and cloud electrification. The videosonde provides images of in-cloud precipitation particles ranging from cloud drop and ice crystal to raindrop, graupel, and hail as well as charges of precipitation particles. This article shows how important results were derived from each function of videosondes launched in East Asia monsoon clouds. More than 200 videosondes ascended from 13 different locations during the past 12 years. Highlights of this project are presented. It is hoped that this videosonde system will be used by workers in many different research fields related to the study of rain and electricity; enabling the sharing of knowledge obtained on East Asian monsoon rain.

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Tsutomu Takahashi

Abstract

The existence of the electric potential of liquid water on an ice surface is investigated both theoretically and experimentally.

The electric potential produced at the boundary surface between the liquid water and bulk ice is set up by the difference of the proton activation energy between liquid water and ice. Negative charge is developed on the side of liquid water and positive charge on the side of ice. The expected value of the potential of liquid water on ice is confirmed by the experiments on specimens of highly purified single crystals of ice. The electric properties of bulk ice have been considered to explain the electric charge generation of graupel pellets and hailstones. In this paper, it is shown that the electric properties at the ice surface were important in explaining the charge generation in thunderstorms, especially the explanation of positively electrified graupel pellets and hailstones. This idea is extended to explain the positive charge of snow crystals in the process of melting.

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Tsutomu Takahashi

Abstract

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

Abstract

Warm cloud electricity was extensively studied using a system comprised of raindrop charge radiosondes, raindrop charge-size radiosondes, cloud droplet charge radiosondes, space charge radiosondes, and electric potential.gradient radiosondes. More than 90 radiosondes were launched by balloon, mainly over the land, and also dropped from a helicopter. Special efforts were made to identify visually the cloud life stage at the sounding. The data were analyzed and arranged to present a composite picture of the electric charge profile with respect to the cloud life cycle. It was found that warm clouds produce excess negative charge in-cloud throughout their life cycle.

Two charge separation processes appear to occur: one results in the negative charging of cloud droplets and raindrops in-cloud, which occurs mainly in the developing stage. The other produces the positive charging of raindrops near the cloud top, occurring mainly in the mature stage.

The raindrop charge distribution in clouds with cloud top height lower than 3 km differs strongly from the distribution in clouds higher than 3 km. This difference parallels the difference in rainfall mechanisms in shallow and deep warm clouds. In the shallow cloud, large raindrops are formed in-cloud and fall against the updraft. In the deep cloud, most drops are carried to the cloud top where large raindrops grow because of the higher updraft.

A model of warm cloud electricity has been completed, and the charge separation processes are discussed in this paper. When the cloud is shallow, negatively charged raindrops and cloud droplets generated in-cloud fall, while small droplets evaporate and form a negatively charged layer near the ground. Positively charged raindrops originating from the upper part of the cloud fall through this negative charge layer, and their positive charge is almost cancelled out by that of the negatively charged group. However, when the cloud in deep, large positive raindrops are formed near the cloud top and fall through the regions dominated by negative raindrops and cloud droplets. The heavy rainshower produces a strong downdraft, and the negatively charged group at the cloud top is carried down toward the ground and produces a large negative potential gradient at the ground. The positive raindrops are dispersed over a wide region in space and time, during which negatively charged particles are carded by downdraft for a brief period.

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Tsutomu Takahashi

Abstract

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

Abstract

The development of electricity in a shallow wintertime cumulus was studied using an axisymmetric cloud model containing both microphysical and electrical charge separation processes during graupel formation. The charge separation mechanisms considered included ion induction, ion diffusion, polarization and riming electrification.

An unexpected result was that polarization did not intensify cloud electrification. Instead, riming electrification appears to be the principal charge separation process acting to intensify cloud electricity.

The cloud is electrified during graupel formation, and a relatively large positive potential gradient forms initially near the cloud top along the cloud boundary between negative graupel and positive ions. Later, graupel particles, electrified positively through riming electrification, produce a relatively strong negative potential gradient between the positive space charge and the upper-level negative space charge produced by snow crystals. As these positively charged graupel particles fall, the positive potential gradient at the ground increases. Due to ion induction, negatively charged snow crystals change sign as they fall through the negative potential gradient field in later stages of the cloud life cycle; thew positively charged snow crystals maintain the positive potential gradient at the ground. When the positive potential gradient is a maximum at the surface, snow crystals become negatively charged near the surface due to ion induction, producing a “mirror image” between the snow crystal charges and the surface electric potential gradient. Numerical results compare favorably with field observations.

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Tsutomu Takahashi

Abstract

A three-dimensional warm rain model that included microphysics was used to study the reasons for ease of rainfall in Hawaiian clouds and the long-lasting rainfall from certain rainbands. It was found that drop recirculation occurs within these clouds to enhance the raindrop growth rate, and that the locations of recirculation differ with cloud types, which are determined by wind shear profiles. As has been noted in observations, rain lasts longest when the wind is parabolic with height and a strong wind blows at the middle of the trade-wind layer. In the model, a very long-lasting rainfall is calculated when the wind veers greatly below the cloud base.

In Hawaiian warm clouds, dry air acts to dissipate clouds rather than to enhance them as occurs in other types of squall lines. In Hawaii, the cloud system that produces long-lasting rainfall is one where dry air does not intrude, and the drag force by raindrops creates a low-level convergence without blocking the low-level inflow of moist air.

Kessler's parameterizations on cloud development and rainfall have also been examined as a part of this study.

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Tsutomu Takahashi

Abstract

Abstract not available.

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Tsutomu Takahashi

Abstract

The heavy, short-lived, and localized rainfall pattern usually observed in tropical areas is simulated in a two-dimensional slab model incorporating detailed cloud physical processes. The rainfall pattern is determined by the re-entry of drops from the cloud boundary into the cloud, by the strong downdraft, and by the inflow of warm, dry air from outside the cloud, which suppresses the updraft and triggers the sudden downdraft and fall of accumulated raindrops. The airflow pattern is characterized by the development of a relatively strong inflow near the cloud base at the developing stage of the cloud life cycle, and by the formation of a new airflow cycle near the ground at the dissipating stage. Furthermore, in the model, rainfall intensity increases with increasing cloud height, the relationship usually observed in the field. The contrast in rainfall patterns characteristic of deep vs shallow clouds are reflected in the size distributions of drops. Next, the calculated raindrop size distribution is compared with the raindrop distribution in warm rain obtained by Blanchard, and, finally, the present model is compared with other models.

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