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Robert G. Fovell and Pei-Hua Tan

Abstract

The authors study herein the convective cell life cycle and the cell generation process in mature, multicellular squall-line storms possessing well-developed subcloud cold pools using two- and three-dimensional models. The multicellular storm establishes new cells on its forward side, in the vicinity of the forced updraft formed at the pool boundary, that first intensify and then decay as they travel rearward within the storm’s upward sloping front-to-rear airflow. The principal effort is expended on the two-dimensional case owing to the strong similarity in basic behavior seen in the two geometries.

The cell life cycle is examined in several complementary fashions. The cells are shown to be convectively active entities that induce local circulations that alternately enhance and suppress the forced updraft, modulating the influx of the potentially warm inflow. This transient circulation also drives the episodic mixing of stable air into the inflow that results in the cell’s ultimate dissipation. The timing of cell regeneration is also examined; an explanation involving two separate and successive phases, each with their own timescales, is proposed. The second of these phases can be shortened if a “convective trigger,” another by-product of the cell’s circulation, is present in the storm’s inflow environment. Sensitivity of the results to strictly numerical model details is also discussed.

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Pei-Hua Tan, Chia Chou, and Jien-Yi Tu

Abstract

Hemispherically and temporally asymmetric tropical precipitation responses to global warming are evaluated in 13 different coupled atmosphere–ocean climate model simulations. In the late boreal summer, hemispherical averages of the tropical precipitation anomalies from the multimodel ensemble show a strong positive trend in the Northern Hemisphere and a weak negative trend in the Southern Hemisphere. In the late austral summer, on the other hand, the trends are reversed. This implies that the summer hemisphere becomes wetter and the winter hemisphere becomes a little drier in the tropics. Thus, the seasonal range of tropical precipitation, differences between wet and dry seasons, is increased. Zonal averages of the precipitation anomalies from the multimodel ensemble also reveal a meridional movement, which basically follows the seasonal migration of the main convection zone. Similar asymmetric features can be found in all 13 climate model simulations used in this study. Based on the moisture budget analysis, the vertical moisture advection associated with mean circulation is the main contribution for the robustness of the asymmetric distribution of the tropical precipitation anomalies. Under global warming, tropospheric water vapor increases as the temperature rises and most enhanced water vapor is in the lower troposphere. The ascending motion of the Hadley circulation then transports more water vapor upward, that is, anomalous moisture convergence, and enhances precipitation over the main convection zones. On the other hand, the thermodynamic effect associated with the descending motion of the Hadley circulation, that is, anomalous moisture divergence, reduces the precipitation over the descending regions.

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Jau-Ming Chen, Pei-Hua Tan, and Ching-Feng Shih

Abstract

Tropical cyclones (TCs) moving northwestward/westward across northern Taiwan are defined to have a type-2 track. This study aims to analyze heavy rainfall associated with type-2 TCs in Taiwan, focusing on the modulation processes of the intraseasonal oscillation (ISO). During 1958–2011, 21 summer type-2 TCs are separated into three rainfall types: strong, moderate, and weak. For the strong rainfall type, both 30–60-day and 10–24-day ISOs propagate northwestward across Taiwan. During landfall and the ensuing two days, both ISOs exhibit a cyclonic anomaly centering northwest of Taiwan that causes anomalous westerly flows (or enhance seasonal southwesterly flows) from the South China Sea (SCS) onto Taiwan. Persistent moisture supply and strong moisture convergence result in prolonged heavy rainfall on the windward side over western Taiwan. TCs with weak rainfall are accompanied by a northward-propagating 30–60-day ISO from the tropical western Pacific toward Japan and a westward-propagating 10–24-day ISO along 20°N latitude. During the landfall stage both ISOs have a cyclonic anomaly with its center south of Taiwan. Major anomalous westerly flows are displaced southward across the central SCS, leading to a weak moisture supply and rainfall in Taiwan. The moderate rainfall type features a 30–60-day (10–24 day) ISO resembling that of the weak (strong) rainfall type. The amount of rainfall thus ranges between the strong and weak rainfall types. Major processes regulating the rainfall of type-2 TCs relate to the intensity of the moisture supply associated with anomalous westerly flows from the SCS onto Taiwan, which is jointly modulated by 30–60-day and 10–24-day ISOs.

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Chia Chou, Li-Fan Huang, Lishan Tseng, Jien-Yi Tu, and Pei-Hua Tan

Abstract

The annual cycle of precipitation over the western North Pacific and East Asian (WNP–EA) sector has five major periods: spring, the first and second wet periods, fall, and winter. In this study, processes that induce precipitation in each period are examined from a large-scale point of view. The wet phase over this sector has two distinct periods, which are dominated by the Asian summer monsoon circulation induced by the land–ocean contrast of net energy into the atmospheric column (F net). In the first wet period, the pre-mei-yu/mei-yu rainband is directly associated with a moisture flux convergence caused by the southwesterly Asian summer monsoon flow and the southeasterly trade winds, and indirectly associated with a dynamic feedback induced by this horizontal moisture convergence. The tropical convection, in the meantime, is associated with a rising motion that is induced by positive F net. In the second wet period, the WNP summer monsoon gyre dominates the rainfall of this region, which is partially associated with warmer local sea surface temperature (SST) via positive F net. The land–sea contrast of F net and the atmosphere–ocean interaction also play an important role in establishing the monsoon gyre. The dry phase over the WNP–EA region is the winter period in which precipitation is associated with winter storm activities and large-scale lifting associated with a pressure surge. In the two transition phases, due to a difference in heat capacity, the atmosphere and ocean have distinct impacts on precipitation, albeit similar solar insolations during the two periods. In the spring period, the atmospheric condition is favorable for convection, while the ocean surface is relatively colder, so the horizontal moisture advection associated with the westward extent of the Pacific subtropical high, which is different from a typical winter frontal system, is a major source for the spring rain. In the fall period, however, the atmospheric conditions dominated by the Asian winter monsoon circulation suppress convection, while relatively warmer SST still maintains tropical convection over the southern part of the WNP–EA region. Over the northern part of the WNP–EA region, the fall precipitation is associated with frontal systems, similar to those in winter.

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Chia Chou, Chao-An Chen, Pei-Hua Tan, and Kuan Ting Chen

Abstract

Global warming mechanisms that cause changes in frequency and intensity of precipitation in the tropics are examined in climate model simulations. Under global warming, tropical precipitation tends to be more frequent and intense for heavy precipitation but becomes less frequent and weaker for light precipitation. Changes in precipitation frequency and intensity are both controlled by thermodynamic and dynamic components. The thermodynamic component is induced by changes in atmospheric water vapor, while the dynamic component is associated with changes in vertical motion. A set of equations is derived to estimate both thermodynamic and dynamic contributions to changes in frequency and intensity of precipitation, especially for heavy precipitation. In the thermodynamic contribution, increased water vapor reduces the magnitude of the required vertical motion to generate the same strength of precipitation, so precipitation frequency increases. Increased water vapor also intensifies precipitation due to the enhancement of water vapor availability in the atmosphere. In the dynamic contribution, the more stable atmosphere tends to reduce the frequency and intensity of precipitation, except for the heaviest precipitation. The dynamic component strengthens the heaviest precipitation in most climate model simulations, possibly due to a positive convective feedback.

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Jau-Ming Chen, Pei-Hua Tan, Liang Wu, Hui-Shan Chen, Jin-Shuen Liu, and Ching-Feng Shih

Abstract

This study examines the interannual variability of summer tropical cyclone (TC) rainfall (TCR) in the western North Pacific (WNP) depicted by the Climate Forecast System Reanalysis (CFSR). This interannual variability exhibits a maximum region near Taiwan (19°–28°N, 120°–128°E). Significantly increased TCR in this region is modulated by El Niño–Southern Oscillation (ENSO)-related large-scale processes. They feature elongated sea surface temperature warming in the tropical eastern Pacific and a southeastward-intensified monsoon trough. Increased TC movements are facilitated by interannual southerly/southeasterly flows in the northeastern periphery of the intensified monsoon trough to move from the tropical WNP toward the region near Taiwan, resulting in increased TCR. The coherent dynamic relations between interannual variability of summer TCR and large-scale environmental processes justify CFSR as being able to reasonably depict interannual characteristics of summer TCR in the WNP. For intraseasonal oscillation (ISO) modulations, TCs tend to cluster around the center of a 10–24-day cyclonic anomaly and follow its northwestward propagation from the tropical WNP toward the region near Taiwan. The above TC movements are subject to favorable background conditions provided by a northwest–southeasterly extending 30–60-day cyclonic anomaly. Summer TCR tends to increase (decrease) during El Niño (La Niña) years and strong (weak) ISO years. By comparing composite TCR anomalies and correlations with TCR variability, it is found that ENSO is more influential than ISO in modulating the interannual variability of summer TCR in the WNP.

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