土壤溼度與降水耦合現象的先備條件

Abstract

由於地表條件作為提供大氣能量的主要來源，且與人類生活密切相關，自1990年代以降便受到越來越多關注。藉由衛星與模式資料，許多過去的研究指出下午降水傾向發生在早晨土壤比較濕的時候，但在土壤比較乾的地方。這很可能肇因於在乾土壤上空，地表溫度在空間上的異質性所激發的輻合、隨後發生的上升氣流、以及在這段較氣候值為潮濕的時期，較多水氣可供蒸發。近期研究進一步發現，當某些土壤條件滿足時，所謂的「空間上土壤溼度與降水之負回饋機制」會變成正回饋。然而，由於某些重要變數很難獲得全球尺度的觀測資料，諸如地表通量、溫濕度在高度上垂直的變化等，這兩種正負回饋機制在現實中是否僅在降水發生當天就被建立起，至今很少被討論。為了連結觀測與模式、並釐清前幾天大氣與土壤濕度的狀態， ERA5(ECMWF Reanalysis 5th Generation)再分析資料將被使用在本研究的大部分內容中。統計結果顯示無論是哪一種回饋機制，都在背景環境的發展中存在溫度及絕對溼度的正距平，這不是直接來自當地就是來自白天透過對流的傳送。兩正負回饋機制的差異在於：正回饋因為有較豐富的水氣，大氣會先被加濕並藉此累積能量；而負回饋的加濕與積能速度更快，卻僅發生在能透過強烈對流從周遭將水氣帶入的降水事件當天。這導致在事件當天早上的大氣加濕現象，正回饋從地表往上到大氣中；而負回饋則先從大氣開始加濕，再延伸至地表。雖然只使用一種再分析資料，且仍舊無法下因果的結論，本研究突顯在前幾天先備條件的重要性，並顯示很可能與在事件發生當天空間上的回饋機制有關。

As playing a critical feedback role to the atmosphere and closely relating to human life, land surface condition draws more and more attention since the 1990s. Based on satellite data and idealized model results, many previous studies stated that precipitation in the afternoon tends to occur when soil is wetter and where it is dryer. This is probably because of the convergence induced by spatial surface temperature heterogeneity and the subsequent updraft over dryer soil, as well as more available water vapor evaporated during wetter period. A recent study further shows that this negative “spatial soil moisture – precipitation feedback (spatial SM-P feedback)” may shift to positive when some soil conditions fulfill. Nevertheless, whether the development of both positive and negative spatial SM-P feedback emerges merely on the morning of the afternoon precipitation event or not is rarely discussed so far. It is because the lack of variables in observation that hard to be obtained globally, and because of the necessity of sufficient computational resources if using model. To act as a bridge between observation and model results, and figure out what had happened before the event day, ERA5, a state-of-the-art reanalysis data released only in recent years, is used in this study. Statistical results show that no matter the coupling is positive or negative, there is development of positive temperature and specific humidity anomaly, which can help destabilize the low atmosphere in the first place. The difference is that positive coupling moistens the air and accumulates energy which starts earlier due to the presence of richer soil moisture while the negative does it faster only on the event day with the help of fiercer convection that brings moisture in from the surroundings. This causes the moistening on the event day morning from the wet surface up to the atmosphere under positive cases while causing the moistening first happened in the atmosphere then down to the dry surface under negative ones. Though applying only one reanalysis data and not including firm causality, this study highlights the importance of the preconditions in the previous days. This may help improve the knowledge of spatial SM-P feedback, which may apply to forecasts on short to medium range, and climate change.