環境濕度的垂直結構對颱風大小之影響

Abstract

颱風大小是用來描述颱風外圍結構及其潛在破壞力的重要指標。過去的研究已指出乾空氣在水平上的分布會影響颱風大小，然而乾空氣在垂直上的位置是否也對颱風大小有影響則仍未被探明。為了探討西北太平洋環境濕度的垂直結構對颱風大小之影響，本研究進行了一系列準理想化模擬。在控制組實驗，背景場為10年平均的ERA5熱帶西北太平洋熱力場資料，並植入2018年山竹颱風的作為其初始渦旋。敏感性實驗的部分，我們將原先背景場低層(850百帕以下；L)以及中層(850及500百帕間；M)的水氣混和比放置各自調整為原先的百分之80或60，如此便會產生以下四組實驗：L08、L06、M08以及M06。所有實驗皆持續對環境濕度做修改，以維持環境乾空氣的存在並持續影響颱風。 模擬結果顯示，L08及M08皆與CTL有幾乎一樣颱風大小。當乾空氣位於低層時，只要環境濕度並非如同L06顯著較乾，海氣交互作用便足以迅速重新加濕大氣，使外圍雨帶僅微幅減弱並外擴，因此外核風場並不會顯著減弱，颱風大小也得以維持。而中層乾空氣則會提升外核的潛在不穩定度有利於對流發展，此時外核的潛熱釋放雖然會增加產生局地次環流，使得低層絕對角動量平流增加，但由於影響的範圍較小，颱風的大小亦未顯著增加。M06雖然相比M08有更高的潛在不穩定度，但逸入作用亦較強可能抵銷潛在不穩定度的正貢獻。未來工作的部分，需要進行更多的深度分析來定量分析乾空氣的影響。此外，亦可進行包含更複雜的乾空氣分布以及更多不同環境參數之實驗，使模擬更接近真實大氣配置。

Size is one of the critical features measuring the outer structure and potential damage of tropical cyclones (TCs). Although previous studies have depicted that the horizontal distribution of dry air can affect TC size, the role of vertical variation of dry air on TC size remains unknown. In order to investigate the impact of the different vertical profile of environmental humidity on TC size in western North Pacific (WNP), a series of quasi-idealized simulations with different vertical structures of humidity are conducted. In the control run, initial background field is derived from the 10-year-averaged thermodynamic conditions in the summertime tropical WNP interpolated from the ERA5 reanalysis product, with the bogused initial vortex obtained from the momentum and thermodynamic fields of Typhoon Mangkhut (2018). For the sensitivity experiments, the background water vapor mixing ratio in the low-level (below 850 hPa, L) and mid-level (850-500 hPa, M) atmosphere is reduced to 80 or 60 percent of its original value, which would set up 4 sensitivity runs (L08, M08, L06, and M06). The humidity profiles are nudged throughout the simulations to maintain the dry-air influences. Results show that the TC in M08 has the strongest outer-core convection and is nearly the same size as TC in CTL, which mainly results from the enhanced potential instability. The enhanced outer-core potential instability could strengthen outer-core diabatic heating and local overturning circulation, thus the low-level absolute angular momentum advection increase. Although drier mid-level atmosphere in M06 leads to a larger potential instability than that in M08, the negative impact of dry air entrainment could reduce the positive contribution of the increased potential instability, resulting in a smaller TC size. Meanwhile, a drier low-level environment may suppress the convection and diabatic heating in TC inner-core. Despite the increase due to the larger thermodynamical disequilibrium, the surface fluxes in L06 may not be able to adequately moisten the low-level air. Thus, the weaker outer-core diabatic heating and local secondary circulation is present, resulting in a smaller TC size. Other in-depth analyses are needed to quantify the exact impact of the dry air. Moreover, sensitivity experiments with different dry area, and the inclusion of other parameters that could create a more realistic background field remain to be investigated.