逸入 (氣象學)

上圖顯示出不同積雨雲中逸入率 (${\displaystyle \lambda }$)對雲內垂直溫度的影響。逸入率越高，則雲內溫度越傾向環境溫度(T)的分布。

逸入是環境中的空氣與氣柱裡或雲裡的空氣混和的作用，部分環境中的空氣性質會進入到雲內。由於逸入作用會改變雲的性質，進而影響到降水、輻射...等，因此雲內的逸入係數設定對氣候模式的模擬影響相當大。[1]

逸入作用可分成同質及異質混和過程。同質混和過程是一個假設雲內混和作用的時間尺度小於蒸發的時間尺度的模式。這意味著當環境裡的未飽和乾空氣在使雲水蒸發之前，已有足夠的時間逸入到雲內，並與雲內空氣混和。透過同質混和過程的逸入作用，並不影響雲內的雲滴數量，只會使雲滴的部分液態水蒸發。[2][3] 與同質混和相對應的是異質混和過程，其雲滴蒸發的時間尺度短於混和的時間尺度，因此雲內的飽和氣塊會在逸入作用的區域被完全蒸發，使總雲滴數量減少。

這兩種模式的差距主要在於雲滴粒徑譜上的表現。在同質混和中，由於大、小雲滴的過飽和水氣壓並不相同，改變了粒徑分布曲線的形狀，因此同質混和後的雲滴粒徑分布將會變窄，而異質混和則不影響雲滴粒徑分布。

積雲在大氣中扮演著傳輸能量和水氣的重要角色，也影響著地球的氣候，但是在大尺度的模式當中，積雲無法直接被模擬而需要參數化。逸入率的決定又牽動著積雲內的性質，因此這個參數對積雲參數化來說相當重要。相關的研究中以Henry Stommel為積雲內逸入係數研究的先鋒科學家。[4]

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