西北太平洋上熱帶氣旋降水的近期趨勢

Abstract

熱帶氣旋伴隨的強降雨可引發洪水、土石流等嚴重災害，對人類社會構成重大威脅。在全球暖化所導致的氣候變遷背景下，了解近年熱帶氣旋降水的變化趨勢已成為重要的科學議題。儘管少數研究指出熱帶氣旋降水有逐年增加的趨勢，但其背後所隱含的物理及動力機制仍未被釐清。本研究利用多衛星降水產品－the Integrated Multi-satellitE Retrievals for GPM (IMERG) 的降水資料與聯合颱風警報中心（Joint Typhoon Warning Center, JTWC）所提供的熱帶氣旋最佳路徑數據，針對2000～2022年間508個生成於西北太平洋上的熱帶氣旋伴隨的降水進行分析。同時，配合歐洲中期天氣預報中心（European Centre for Medium-Range Weather Forecasts）的再分析資料ERA5進行環境分析，探討影響熱帶氣旋降雨趨勢的潛在環境因素。 結果顯示，熱帶氣旋平均降雨率呈些微上升趨勢，其中內核區域平均降雨率呈顯著的下降趨勢，而外圍區域則為上升趨勢。內（外）圍區域降水呈現逐年減少（增加）的趨勢，主要歸因於該區域對流性降水的減弱（增強）。環境因子分析顯示外圍區域的大氣靜力穩定度逐年上升，而對流可用位能與對流不穩定度呈現下降趨勢。這些熱力參數的變化一致顯示環境逐漸朝向不利於降水發展的方向演變，難以解釋外圍區域降水強度仍呈現逐年增加的現象。此外，熱帶氣旋環境風切（storm-scale vertical wind shear）雖有逐年增強趨勢，但降水增強的地方集中於風切左右兩側的外圍區域，而非先前研究所指出的下風切處，顯示外圍區域降水的增強可能並非由環境風切所引發的渦度平流或相關動力效應所致。然而，在外圍對流性降水區域，局部低層垂直風切與冷池強度之間的差異呈現逐漸縮小的趨勢。根據RKW理論（Rotunno–Klemp–Weisman theory），此情形代表該區環境正逐漸接近理論上的最佳化階段（optimal state），使上升運動得以加強，進而造成熱帶氣旋外圍降水的逐年增強。進一步分析顯示，局部低層垂直風切的增強可能與全球暖化下熱帶氣旋強度上升所伴隨的近地層入流加強有關；另一方面，冷池強度的逐年減弱則可能與環境對流不穩定度的降低有關。此外，分析結果顯示，儘管外圍區域之海氣熱力背景隨時間呈現增暖趨勢，海表面溫度的上升並非主導外圍區域降水增強的主要因素，亦未對降水之年際振盪產生直接影響。

Heavy rainfall associated with tropical cyclones (TCs) can lead to severe disasters such as flooding and landslides, posing substantial threats to human societies. Under climate change driven by global warming, understanding recent trends in TC precipitation is thus an important scientific issue. In this study, we analyze precipitation associated with 508 tropical cyclones that formed over the northwestern Pacific Ocean between 2000 and 2022, using the multi-satellite precipitation product, the Integrated Multi-satellitE Retrievals for GPM (IMERG). Best track data from Joint Typhoon Warning Center (JTWC) and the reanalysis data from the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) (ERA5) are also used to investigate environmental characteristics potentially influencing long-term TC rainfall trends. The results indicate that the average rainfall rate of TCs shows a modest increasing trend. The rainfall rate within the inner-core is characterized by a prominent decreasing trend, whereas the outer rainfall shows a slightly increasing trend. The year-to-year decrease (increase) of rainfall in the inner (outer) region is mainly attributed to the weakening (strengthening) of convective precipitation in those respective regions. Environmental analyses indicate that the atmospheric dry static stability in the outer region has been gradually increasing, while convective available potential energy (CAPE) and equivalent potential temperature deficit have shown decreasing trends. In theory, these changes should be unfavorable for precipitation development; however, the statistical results reveal that rainfall in the outer region continues to increase, implying that other dynamical or physical processes may be playing a key role. Moreover, although storm-scale vertical wind shear has gradually strengthened, the enhanced precipitation is concentrated in the outer region on both sides of the shear vector, rather than on the downshear side as suggested by previous studies. This indicates that the increasing outer rainfall is unlikely to be caused by shear-induced vorticity advection or related dynamical effects. Nevertheless, within the convective outer region, the difference between low-level vertical wind shear (LVWS) and cold-pool strength has been progressively decreasing. According to the Rotunno–Klemp–Weisman (RKW) theory, this evolving relationship suggests that the environment in this region is approaching the theoretical “optimal state”, where enhanced vertical motion can be more easily sustained, thereby contributing to the observed long-term increase in outer region precipitation. Further analysis suggests that the increase in LVWS may be associated with enhanced near-surface inflow resulting from the strengthening of TCs under global warming. In contrast, the gradual weakening of the cold-pool appears to be related to a long-term reduction in environmental convective instability. In addition, the analysis indicates that although the air-sea thermodynamic background in the outer region has warmed over time, the increase in sea surface temperature is not the primary driver of the enhancement of outer region precipitation, nor does it have a direct impact on the interannual oscillation of precipitation.