土壤水分與降雨在時空耦合上之交互作用

Abstract

水氣再循環與土壤水分-降雨之時空回饋皆是氣候系統中重要的議題。近年來的研究指出，土壤較濕且土壤水分異質性較高時較易出現降雨，而降雨位置則傾向發生在該地區相對較乾的土壤上。本研究藉由遙測資料，探討土壤水分之時空分布對潛在降雨位置的影響；以及降雨後，新的土壤水分分布在隔天早上是否有利對流發生。結果顯示，當土壤狀態較乾時，土壤水分-降雨之空間負回饋較強，且午後降雨傾向維持或提升土壤水分異質性，新的土壤水分空間分布則在隔天有利激發對流。另外，前人研究指出高異質性的土壤水分空間分佈與濕土壤皆對對流發展有利，本研究結果則顯示土壤水分異質性與土壤濕度，在影響降雨的位置所扮演的角色為競爭關係，這與過去對於土壤-降雨之時空回饋的觀念並不衝突，反而更完整的呈現土壤濕度、土壤空間異質性及降雨偏好發生的位置三者的關係。此外，我們提出了土壤水分分布影響降雨位置之可能的機制，該機制可用數值模式做進一步的探討，而若能使模式呈現如現實世界中土壤-降雨之關係亦有助於了解水氣再循環以及增進降雨預報。

The notions of moisture recycling and negative spatial soil moisture-precipitation coupling have recently been reconciled: rain tends to occur when conditions are wet and heterogeneous, but over locally relatively drier patches. This study focuses on the impacts of soil moisture spatiotemporal patterns on the preferential precipitation location and whether the subsequent soil moisture distribution on the next day's morning is advantageous to trigger another convection by using remote-sensing based datasets. Our results show that when soil conditions are dry, negative spatial coupling is stronger, and afternoon precipitation events tend to maintain or increase the soil moisture heterogeneity. This implies that the new soil moisture spatial distribution after the precipitation event might continue favoring convective triggering. In addition, both soil moisture heterogeneity and soil wetness condition jointly determine the position of precipitation. Strong soil moisture heterogeneity and wetter soil moisture both favor developing of convection, but they compete for the location of precipitation. Such responses in fact do not conflict with positive temporal coupling as indicated in previous studies and the mechanisms we proposed can be further investigated using numerical models. The observed relationship between soil moisture pattern and preferred precipitation location establishes new horizon of land-atmosphere interaction and have crucial implications for moisture recycling and precipitation predictions.