| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 102 === Our study focuses on how convective organizations and the associated precipitation extremes respond to various environments. Thus, we use a 3D cloud resolving model VVM with explicit physical process to examine the im-pacts. The model is imposed with sounding data of large-scale forcing observed in GATE (GARP Atlantic Tropical Experiment) to change the environmental moisture. Combining the vertical wind shear, we set a series of idealized sensitivity tests and do statistical analyses with model outputs.
Results reveal that a dramatic increasing of cloud size diagnosed by the four-connected segmentation method is shown from 1.5~5km height under each environment, which indicates a strong correlation between wind shear and the cloud size. Environmental moisture is crucial to the cloud size and organization. The difference of size between w/wo wind shear is much larger in 10% and 1% analysis, so the first 10% clouds can be considered as ones with larger and more organized structures. In 1% analysis, the cloud size can be even 8 times larger under the moistest environment. Cloud size distribution is broadened and the ratio of small cloud size increases when vertical wind shear exits.
The effect of wind shear on enhancing precipitation becomes significant in the 1% extreme. Besides, relative enhancement at the 99.9 percentile is especially greater in the driest environment (~60%) than the moistest one (~20%). To explore the relationship between cloud size and precipitation strength, we combine core cloud size with surface precipitation and show a proportional relation. Moreover, the precipitation strength of first 10% size-averaged shows a significant enhancement, especially in moistening cases, which implies the first 10% clouds can be considered as organized convections leading to the enhancement of 1% extreme precipitation.
Key words: convection, organization, wind shear , moisture, extreme precipitation
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