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A Preliminary Numerical Simulation of a Shower

John Y. C. WangChinese Naval Academy, Tsoying, Taiwan, China

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Abstract

A quasi-one-dimensional, time-dependent and precipitating cumulus cloud model incorporated with a time-dependent PBL model has been used to simulate the precipitation record of a local summer afternoon shower induced by sea breeze. The system is so designed that it is subjected only to the variations of the parameters pertaining to the PBL. Through the use of this system, we have obtained relatively good agreement between the observed and model produced rainfall pattern, peak rainfall intensity and total rainfall; we are also confident of reproducing the time of onset of the shower. It has also been found that the PBL influences the precipitation characteristic of a shower in a complicated way. For a given water vapor content, greater thickness of the PBL, which implies a greater heat supply to the cloud activity above, will delay the onset of a shower, reduce its total rainfall and produce multi-peaked intensities. When the thickness is increased to a critical value, no shower can be produced. If the thickness is increased further, a shower can be produced again. For a given thickness, increasing water vapor will greatly expedite the onset of a shower which also has greater total rainfall and multi-peaked rainfall intensities. Only when the heat content is accompanied by a proper water vapor content can a shower of single-peaked rainfall intensity be produced.

In this model, we have introduced an immediate environmental (IE) region which allows the cloud to generate its own inner or immediate environment, enriched by water mass in both liquid and vapor form. Our work has shown that the IE region plays an important role in enhancing cloud development, delaying the showers to afternoon hours and in providing a recycling process of water mass so that heavy rainfall can be produced by this simple model.

Abstract

A quasi-one-dimensional, time-dependent and precipitating cumulus cloud model incorporated with a time-dependent PBL model has been used to simulate the precipitation record of a local summer afternoon shower induced by sea breeze. The system is so designed that it is subjected only to the variations of the parameters pertaining to the PBL. Through the use of this system, we have obtained relatively good agreement between the observed and model produced rainfall pattern, peak rainfall intensity and total rainfall; we are also confident of reproducing the time of onset of the shower. It has also been found that the PBL influences the precipitation characteristic of a shower in a complicated way. For a given water vapor content, greater thickness of the PBL, which implies a greater heat supply to the cloud activity above, will delay the onset of a shower, reduce its total rainfall and produce multi-peaked intensities. When the thickness is increased to a critical value, no shower can be produced. If the thickness is increased further, a shower can be produced again. For a given thickness, increasing water vapor will greatly expedite the onset of a shower which also has greater total rainfall and multi-peaked rainfall intensities. Only when the heat content is accompanied by a proper water vapor content can a shower of single-peaked rainfall intensity be produced.

In this model, we have introduced an immediate environmental (IE) region which allows the cloud to generate its own inner or immediate environment, enriched by water mass in both liquid and vapor form. Our work has shown that the IE region plays an important role in enhancing cloud development, delaying the showers to afternoon hours and in providing a recycling process of water mass so that heavy rainfall can be produced by this simple model.

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