陸地大氣交互作用對於熱帶島嶼日降水強度之影響

The impact of land - atmosphere interactions on the diurnal intensity of precipitation over tropical islands

陳柏言(Po-Yen Chen) ； 吳健銘(Chien-Ming Wu)

《大氣科學》 47卷2期 (2019/06) Pp. 120-138

https://doi.org/10.3966/025400022019064702001

摘要

本研究透過三維渦度向量方程式的雲解析模式（Vector-Vorticity cloud-resolving Model, VVM）耦合陸地模式（Noah Land-Surface Model, LSM），分別探討在都市、牧場和草地三種的理想熱帶島嶼上，有無直接陸地大氣交互作用（Land -Atmosphere Interactions, LAI）時對於降水日變化強度的影響。我們設計了兩組實驗，第一組為有直接的陸地大氣交互作用（VVM耦合LSM）；第二組為沒有直接陸地大氣交互作用的結果（只有VVM），利用第一組實驗中包含高時空頻率的地表輻射通量和溫度等資訊做為大氣地表邊界變化。最後去看在不同蒸發比的陸面下，兩組實驗中日降水變化和對流系統在時空上的差距，做為有無直接陸地大氣交互作用的影響。實驗結果顯示，在三種陸面環境下，有直接陸地大氣交互作用的降水日變化幅度（最大陸面平均降水值）與極端降水事件強度都高於沒有直接陸地大氣交互作用。另一部份則是在都市的實驗下，降水日變化幅度與極端降水強度差異明顯高於在牧場與草地的實驗，差異最高可達71%。而造成降水日變化幅度差異的機制，主要是在都市的陸面實驗中，有較大的對流核心雲產生較強的冷池，而在較強冷池的交互作用下有利於激發更強的對流核心雲，因此在有直接陸地大氣交互作用的實驗下冷池對於對流發展為一個正貢獻過程。而在沒有直接陸地大氣交互作用的實驗中，地面冷區會對上方大氣產生微弱的下沉運動並讓對流偏好在冷區邊緣發展。在牧場與草地的實驗則因為地面冷區強度逐漸偏弱，使抑制對流發展效果降低。從此結果初步了解到在經歷都市化或砍伐森林過程的熱帶島嶼上，降水的日變化強度和極端降水事件對於陸地大氣交互作用的過程更加敏感。

關鍵字

午後熱對流 ； 冷池 ； 陸地大氣交互作用 ； 熱帶島嶼降雨日變化

並列摘要

This study investigates the impact of land-atmosphere interactions (LAI) on the diurnal intensity of precipitation over a tropical island. Idealized simulations are performed with three different land surface conditions urban, pasture and grass using a 3-D Vector Vorticity equation cloud-resolving Model (VVM) coupled with the Noah Land Surface Model (LSM). Two sets of experiments are performed in this study. The first set considers direct LAI in which VVM is fully coupled with LSM. The second set of experiment eliminates direct LAI by prescribing surface fluxes in VVM, in which the high spatiotemporal variabilities are preserved from the fully coupled VVM/LSM. With this approach, the difference in temporal and spatial evolution of precipitation and convective systems can be interpreted as the impact of LAI. The results show that the diurnal amplitude (the maximum averaged precipitation over land) and extreme precipitation is stronger with direct LAI than without under all land surface conditions. The impact of LAI is profound on the diurnal amplitude with urban experiment, which is 71% larger compared with grass experiment. This is due to strong cold pool intensity by convective systems with large updraft core clouds in urban experiment. There is a positive contribution to convective updraft core cloud development by strong cold pool intensity. The surface cooling in the experiments without direct LAI, on the other hand, produces a weak divergence to suppress the convection development. The convective cells prefer to develop at the boundary of surface cold patch. In experiments over pasture and grass, the strength of surface cooling becomes weaker and weaker impact on the convection development. These findings imply that the diurnal intensity of precipitation as well as the extreme events are sensitive to LAI over tropical islands through processes such as urbanization or deforestation.

並列關鍵字

afternoon thunderstorm ； cold pool ； land atmosphere interaction ； tropical island ； diurnal cycle of precipitation