生物氣膠影響降水過程之數值模擬

Abstract

冰晶的形成在冷雲降水中是重要關鍵，而異質核化可在較高溫度下進行，因此一般認為是產生冰晶的主要機制。異質核化過程必須靠冰核來發生，而冰核的來源包括礦物沙塵以及生物氣膠。後者包括細菌、花粉和孢子等，觀測和實驗證實它們具有高數量濃度、高核化溫度的良好冰核特性，故在植披豐富的地區很可能是降水形成的重要因素。 本研究利用中尺度氣象模式MM5進行模擬生物氣膠對降水的影響。雲微物理過程主要選用CLR參數法，此參數法是由Cheng et al.,（2007a）將Chen and Liu（2004）針對暖雲所提出的two-moment bulk water parameterization植入MM5雲微物理過程中，並與Reisner 2參數法相結合，除了可以反映凝結核與的影響，也適用於分析生物氣膠的異質核化對於降水的作用與影響。 一系列模擬測試顯示細菌數量濃度的多寡會影響地面降雨量，增加數量濃度會使雨量增多但有其極限，當數量濃度超過極限反而會使與量減少。實驗指出在相同數量濃度下，細菌所造成的地面雨量超過沙塵，顯示生物氣膠對降水過程有顯著與重要的貢獻。

A large proportion of precipitation formation is governed by the cold cloud processes associated with cloud ice, snow, graupel and hail particles. Heterogeneous ice nucleation occurs through a subset of the atmospheric aerosols called ice nuclei (IN), which are generally insoluble particles, such as certain mineral dusts, soot, as well as some biological materials that include bacteria, pollen, sport etc. Many laboratory and observational studies demonstrate that bio-aerosols have the unique capability of catalyzing ice formation at relatively warm environment by means of heterogeneous nucleation processes. In this endeavor, sensitivity of cold cloud microphysical processes to bio-aerosols has been tested for one rainfall event. A non-hydrostatic mesoscale cloud model (MM5) has been used for this simulation. This model is modified by including the two-moment warm-cloud parameterization of Chen and Liu (2004; hereafter as the “CL-scheme”) which considers the effect of condensation nuclei (CN) and explicitly predicts the masses and numbers of cloud drops and raindrops (Cheng et al., 2007a). The warm cloud scheme is further coupled with the ice-phase processes of Reisner et al. (1998) by Cheng et al. (2007b), here denoted as the “CLR-scheme,” to examine the importance of ice processes on precipitation formation. Furthermore, the diagnostic formula of heterogeneous ice nucleation in the Reisner scheme is replaced with new prognostic formulas to allow the inclusion of different types of ice nuclei, such as biological aerosols and mineral dust, and their conversion into ice particles. The simulation results indicate that ice nucleation intensity is limited by available water vapor, and precipitation enhancement varies nonlinearly with IN concentration. As they can be nucleated at higher temperature, bio-aerosols increase the total ice and rain water mixing ratio and contribute to precipitation more efficiently than dust or other natural IN. Thus, bio-aerosols play a key role in precipitation process especially over densely vegetated area.