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Philip J. Smith and Chih-Hua Tsou

Abstract

The role of static stability (σ) is diagnosed for an intense extratropical cyclone that developed over the central United States during 9–11 January 1975. Results indicate that minimum σ, values occurred in the lower troposphere at 0000 UTC 10 January 1975, during the period of slow cyclone development, and then increased as rapid development proceeded. Further, the upward advection of smaller static stabilities in the cyclone area, a forcing process in the height tendency equation, resulted in a significant reduction of height falls attributed to vorticity advection, thermal advection, and latent heat release.

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Chih-Hua Tsou, Huang-Hsiung Hsu, and Pang-Chi Hsu

Abstract

This study formulates a synoptic-scale eddy (SSE) kinetic energy equation by partitioning the original field into seasonal mean circulation, intraseasonal oscillation (ISO), and SSEs to examine the multiscale interactions over the western North Pacific (WNP) in autumn. In addition, the relative contribution of synoptic-mean and synoptic-ISO interactions to SSE kinetic energy was quantitatively estimated by further separating barotropic energy conversion (CK) into synoptic-mean barotropic energy conversion (CKSM) and synoptic-ISO barotropic energy conversion (CKS−ISO) components.

The development of tropical SSE in the lower troposphere is mainly attributed to CK associated with multiscale interactions. Mean cyclonic circulation in the lower troposphere consistently provides kinetic energy to SSEs (CKSM > 0) during the ISO westerly and easterly phases. However, CKS−ISO during the ISO westerly and easterly phases differs considerably. During the ISO westerly phase, the enhanced ISO cyclonic flow converts energy to SSEs (CKS−ISO > 0). The magnitude of the downscale energy conversion from mean and ISO to SSEs is related to the strength of the SSEs. During the ISO westerly phase, a stronger SSE extracts more kinetic energy from mean and ISO circulation. This positive feedback between SSE-mean and SSE–ISO interactions causes further strengthening of SSEs during the ISO westerly phase.

By contrast, upscale energy conversion from SSEs to ISO anticyclonic flow (CKS−ISO < 0) was observed during the ISO easterly phase. The weaker SSE activity during the ISO easterly phase occurred because the mean circulation provides less energy to SSEs and, at the same time, SSEs lose energy to ISO during the ISO easterly phase. The two-way interaction between the ISO and SSEs has considerable effects on the development of tropical SSEs over the WNP in autumn.

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Chih-hua Tsou, Phillip J. Smith, and Patricia M. Pauley

Abstract

The primary goal of this study is to compare the principal adiabatic and diabatic mechanisms responsible for the behavior of an extratropical cyclone system. To accomplish this goal, the height tendency is solved from two forms of the height tendency equation. One is the classical quasi-geostrophic (QG) form; the other is a modified form identified as the ‘extended’ height tendency equation. The latter retains the essential components of the QG form but replaces the geostrophic wind where it appears in the equation by the observed wind, adds the effects of diabatic heating (latent heat release), and allows three dimensional varying static stability. In this study, the behavior of an intense cyclone is poorly described by the QG form but is realistically represented by the extended form.

Using this latter diagnostic tool, the evolution of a 9–11 January 1975 cyclone event is analyzed with the following major results: 1) a comparison of the terms included in the extended height tendency equation indicates that the vorticity advection has a primary influence on the movement and propagation of the wave system, particularly in the upper troposphere; 2) the differential thermal advection plays a secondary, but important, role in the wave development and is comparable to the vorticity advection in the lower troposphere; 3) in general, the influence of latent heating is less than the other mechanisms, although it forces significant height falls over a limited region at lower levels at the time of maximum precipitation; and 4) the vertical advection of static stability makes a significant contribution and, in general, opposes the other forcings, thus acting to slow the wave propagation and development.

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Pang-chi Hsu, Tim Li, and Chih-Hua Tsou

Abstract

The role of scale interactions in the maintenance of eddy kinetic energy (EKE) during the extreme phases of the intraseasonal oscillation (ISO) is examined through the construction of a new eddy energetics diagnostic tool that separates the effects of ISO and a low-frequency background state (LFBS; with periods longer than 90 days). The LFBS always contributes positively toward the EKE in the boreal summer, regardless of the ISO phases. The synoptic eddies extract energy from the ISO during the ISO active phase. This positive barotropic energy conversion occurs when the synoptic eddies interact with low-level cyclonic and convergent–confluent ISO flows. This contrasts with the ISO suppressed phase during which the synoptic eddies lose kinetic energy to the ISO flow. The anticyclonic and divergent–diffluent ISO flows during the suppressed phase are responsible for the negative barotropic energy conversion.

A positive (negative) EKE tendency occurs during the ISO suppressed-to-active (active-to-suppressed) transitional phase. The cause of this asymmetric EKE tendency is attributed to the spatial phase relation among the ISO vorticity, eddy structure, and EKE. The southwest–northeast-tilted synoptic disturbances interacting with cyclonic (anticyclonic) vorticity of ISO lead to a positive (negative) EKE tendency in the northwest region of the maximum EKE center.

The genesis number and location and intensification rate of tropical cyclones in the western North Pacific are closely related to the barotropic energy conversion. The enhanced barotropic energy conversion favors the generation and development of synoptic seed disturbances, some of which eventually grow into tropical cyclones.

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Pang-Chi Hsu, June-Yi Lee, Kyung-Ja Ha, and Chih-Hua Tsou

Abstract

By analyzing observation-based high-resolution surface air temperature (SAT) data over the Asian monsoon region (here called “monsoon Asia”) for 1981–2007, the modulation by boreal summer intraseasonal oscillation (BSISO) of heat wave (HW) occurrence is examined. Strong SAT variability and a high probability of HW occurrence on intraseasonal time scales are found consistently in the densely populated regions over central India (CI), the Yangtze River valley in China (YR), Japan (JP), and the Korean Peninsula (KP). The two distinct BSISO modes (30–60-day BSISO1 and 10–30-day BSISO2) show different contributions to HW occurrence in monsoon Asia. A significant increase in HW occurrence over CI (YR) is observed during phases 2–3 (8–1) of BSISO2 when the 10–30-day anticyclonic and descending anomaly induce enhanced upward thermal heating and sensible heat flux (warm advection) around the areas. On the other hand, the northeastward propagating BSISO1 is closely connected to the increased HW probability over JP and KP. During phases 7–8 of BSISO1, the 30–60-day subsidence along with the low-level anticyclonic anomaly moves into northeastern Asia, leading to enhanced diabatic (adiabatic) warming near surface in JP (KP). Analysis of a three-dimensional streamfunction tendency equation indicates that diabatic cooling induced by the BSISO-related suppressed convections is the main forcing term of anticyclonic anomaly although it is largely offset by the decreased static stability associated with adiabatic warming. The BSISO-related vorticity advection leads to an anticyclonic (cyclonic) tendency to the northwestern (southeastern) part of the center of anticyclonic anomaly, favoring northwestward development of the BSISO anomalous circulations and thus providing a favorable condition for HW occurrence over the western Pacific–East Asia sector.

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Chi-Cherng Hong, Chih-Hua Tsou, Pang-Chi Hsu, Kuan-Chieh Chen, Hsin-Chien Liang, Huang-Hsiung Hsu, Chia-Ying Tu, and Akio Kitoh

Abstract

The future changes in tropical cyclone (TC) intensity and frequency over the western North Pacific (WNP) under global warming remain uncertain. In this study, we investigated such changes using 20-km resolution HiRAM and Meteorological Research Institute (MRI) models, which can realistically simulate the TC activity in the present climate. We found that the mean intensity of TCs in the future (2075–99) would increase by approximately 15%, along with an eastward shift of TC genesis location in response to the El Niño–like warming. However, the lifetime of future TCs would be shortened because the TCs tend to have more poleward genesis locations and move faster due to a stronger steering flow related to the strengthened WNP subtropical high in a warmer climate. In other words, the enhancement of TC intensity in the future is not attributable to the duration of TC lifetime. To understand the processes responsible for the change in TC intensity in a warmer climate, we applied the budget equation of synoptic-scale eddy kinetic energy along the TC tracks in model simulations. The diagnostic results suggested that both the upper-level baroclinic energy conversion (CE) and lower-level barotropic energy conversion (CK) contribute to the intensified TCs under global warming. The increased CE results from the enhancement of TC-related perturbations of temperature and vertical velocity over the subtropical WNP, whereas the increased CK mainly comes from synoptic-scale eddies interacting with enhanced zonal-wind convergence associated with seasonal-mean and intraseasonal flows over Southeast China and the northwestern sector of WNP.

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