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A Study of Hawaiian Warm Rain Showers Based on Aircraft Observation

Tsutomu TakahashiCloud Physics Observatory, Department of Meteorology, University of Hawaii, Hilo 96720

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Abstract

Warm rain off the island of Hawaii was followed by aircraft beneath the cloud base from the beginning of the shower to the end, and newly designed instruments were used to observe time and spatial distributions of rainwater content and raindrop size distribution as a function of cloud-top height.

Based on 110 h of observation, rain showers were classified into two main groups: isolated showers and line showers. Isolated showers were subdivided into base showers (showers developing under the cloud base) and side showers (showers developing outside of the cloud area). Line showers were subdivided into showers moving at right angles to the cloud line, showers moving parallel to the cloud line, standing showers and showers which formed successively upwind of the present shower line. These cloud classes are intimately related to the profile of upper wind.

Two shower peaks, with weak showers at the cloud center are observed along the flight line when the upper wind is weak, irrespective of cloud type, while only a single shower peak is seen when a strong wind shear exists near the cloud top. Initial weak rainfall with small raindrops is followed by heavy rainfall with large raindrops (1.5–2.5 mm in diameter). Then the rain gradually weakens with small raindrops. On the other hand, in many cases raindrops from side rain are small and there are no peak raindrops.

Differences were observed in different cloud types with respect to shower lifetime and magnitude of rainwater content. For isolated side showers, lifetime is about 10 min, while for isolated base showers it is 20 min; for line showers, it is 30 min. In isolated clouds, rain starts when the cloud top height reaches 2 km (cloud base is around 600 m); in line clouds, rain starts when the cloud top reaches 1.5 km. Higher clouds produce heavier rain. An isolated cloud with cloud-top height of 2.5 km produces maximum rainwater of 1 g m−3, while a line shower from clouds of similar height produces rain two to three times heavier. Line showers also have higher mean time and space rainwater content than isolated showers.

Observational results from base showers are compared with an axisymmetric cloud model. The reason for longer shower life and heavier rainfall from line clouds may be the existence of strong inflow near the surface and updraft over a broad area which maintains the line cloud.

Abstract

Warm rain off the island of Hawaii was followed by aircraft beneath the cloud base from the beginning of the shower to the end, and newly designed instruments were used to observe time and spatial distributions of rainwater content and raindrop size distribution as a function of cloud-top height.

Based on 110 h of observation, rain showers were classified into two main groups: isolated showers and line showers. Isolated showers were subdivided into base showers (showers developing under the cloud base) and side showers (showers developing outside of the cloud area). Line showers were subdivided into showers moving at right angles to the cloud line, showers moving parallel to the cloud line, standing showers and showers which formed successively upwind of the present shower line. These cloud classes are intimately related to the profile of upper wind.

Two shower peaks, with weak showers at the cloud center are observed along the flight line when the upper wind is weak, irrespective of cloud type, while only a single shower peak is seen when a strong wind shear exists near the cloud top. Initial weak rainfall with small raindrops is followed by heavy rainfall with large raindrops (1.5–2.5 mm in diameter). Then the rain gradually weakens with small raindrops. On the other hand, in many cases raindrops from side rain are small and there are no peak raindrops.

Differences were observed in different cloud types with respect to shower lifetime and magnitude of rainwater content. For isolated side showers, lifetime is about 10 min, while for isolated base showers it is 20 min; for line showers, it is 30 min. In isolated clouds, rain starts when the cloud top height reaches 2 km (cloud base is around 600 m); in line clouds, rain starts when the cloud top reaches 1.5 km. Higher clouds produce heavier rain. An isolated cloud with cloud-top height of 2.5 km produces maximum rainwater of 1 g m−3, while a line shower from clouds of similar height produces rain two to three times heavier. Line showers also have higher mean time and space rainwater content than isolated showers.

Observational results from base showers are compared with an axisymmetric cloud model. The reason for longer shower life and heavier rainfall from line clouds may be the existence of strong inflow near the surface and updraft over a broad area which maintains the line cloud.

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