淺對流層積雲動力模擬

Dynamical Simulation of Shallow Stratocumulus Convection

郭鴻基(Hung-Chi Kuo) ； 游志淇(Tsu-Chi Yu)

《大氣科學》 20卷1期 (1992/03) Pp. 1-18

摘要

低層層積雲覆蓋了極大部份的海洋面，它是海洋和大氣交互作用的有效媒介，它對於地球水汽平衡和輻射收支有很大的影響。為了正確看待層積雲所扮角色，先要了解其動力結構。逸入不穩定為層積雲動力的一個重要問題，傳統的逸入不穩定問題只考慮水滴的蒸發冷卻。本實驗利用二維布新尼高解析模式（2-D Boussinesq high resolution model）來模擬逸入不穩定問題，除了考慮水滴蒸發冷卻外，我們考量逸入在輻射冷卻、大尺度下降氣流及海洋蒸發存在時對層積雲的影響。模擬實驗顯示，在逸入不穩定的探空曲線下，高洋溫及較強的大尺度輻散皆有助層積雲的破裂。在逸入穩定的背景場下，雲頂輻射長波冷卻所產生邊界層環流，將洋面水氣傳送至雲內，增加雲內水汽，層積雲不易破裂。在低洋溫及氣候平均大尺度輻散下，不管層積雲是否滿足逸入不穩定，模擬層積雲皆無破裂，這和Kuo and Schubert（1988）之結論相合，即因蒸發冷卻而產生的逸入是微弱的，不足以使層積雲破裂。模式也顯示哈德雷環流積雲區的高洋溫，加上逸入不穩定的雲頂條件，是層積雲過渡為積雲之原因。

關鍵字

逸入不穩定 ； 邊界層 ； 輻射冷卻 ； 大尺度下降氣流 ； 波譜法

並列摘要

The stratocumulus covers a vast region over the ocean. They are important for the air-sea interaction. We have employed Kuo and Schubert's (1988) twodimensional convection model to simulate the cloud-topped marine boundary layer. In addition to the evaporative cooling in Kuo and Schubert (1988), the effect of radiative cooling, sea surface evaporation and large scale subsidence are considered in the model. In particular, we explore the impact of large scale subsidence, seas surface temperature on the cloud-topped marine boundary layer. Our experiments indicate that the stratocumulus is likely to breakup under the conditions of high sea surface temperature and large scale subsidence. Whether the background soundings satisfies the entrainment instability criteria or not, the stratocumulus will not breakup under low sea surface temperature and climate value of subsidence. Our results also indicates that high sea surface temperature plus the entrainment unstable sounding may be the reason that stratocumulus breakup into trade cumulus in the Hadley circulation.

並列關鍵字

entrainment instability ； boundary layer ； radiative cooling ； large scale subsidence ； spectral method