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Chung-Hsiung Sui and Michio Yanai

Abstract

The relationship between the residual of the large-scale vorticity budget, Z, and organized cumulus convectionis studied using the data taken during Phase III of GARP Atlantic Tropical Experiment (GATE). The appearanceof large vorticity budget residuals is clearly associated with intense cumulus convection. The association isclearest in the upper and middle troposphere.

A positive peak appears in the Phase III-mean vedcal profile of 2 near the 200 mb level. Large day-to-dayvariations, however, exist in the time series of the area-mean 2 near this level. The horizontal distributions of2 at the same level for major convective events show distinct dipole patterns over areas of deep convection.These features are attributed to the detrainment of weaker horizontal momentum from deep clouds. Near 400mb, negative values of 2 are dominant. Peaks of n-egative 2 are associated with the activity of deep cumulusconvection and positive vertical vorticity gradient, δλ/δp, suggesting the effects of vertical advection of the e-scale vorticity due to cumulus-induced subsidence in the environment. In the lower troposphere where δλ/δpk generally negative, the horizontal distributions of 2 tend to be localized and positive values of 2 dominatein the convective area. These are attributable to the vertical advection process and detrainment from shallowclouds. Large negative values of 2 near the sea surface are caused by boundary layer turbulence. The cumulus-induced time rate of change of the rotational part of the wind is estimated from the vorticity budget residual.Organized cumulus convection is found to strongly decelerate the large-scale flow particularly in the uppertroposphere and reduce the vertical wind shear in the lower troposphere.

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Chung-Hsiung Sui and Ka-Ming Lau

Abstract

An improved treatment of diabatic heating due to moist convection is introduced into the dynamical model used in Part I of this paper to further investigate the origin of intraseasonal oscillations in the tropics. The convective heating in the model is parameterized by a simple one-dimensional cloud model which takes into account the available moisture supply in the lower troposphere and the mean thermodynamic states for the entire troposphere. Consequently, the spatial distribution of convective heating in the model can be determined internally as a function of the sea surface temperature consistent with observed convection-SST relationship in the tropics. The periods of low-frequency oscillations excited in the numerical simulations range from 20 to 50 days depending primarily on the vertical distribution of heating through condensation-moisture-convergence feedback or “mobile wave-CISK” The “fast” wave (period around 20 days) is excited by deep convection which has heating maximum at or above the 500 mb level. The “slow” wave (period near 50 days) is excited by heating maximized in the lower troposphere between 500 and 700 mb. A crude parameterization of lower boundary forcing due to heat flux from the ocean surface is incorporated in the model. The boundary forcing tends to further destabilize the mobile wave-CISK modes. It is also found that the boundary forcing plays an important role in sustaining the propagation of intraseasonal oscillations around the globe, especially over the eastern part of ocean where SST is cold and deep convection is strongly inhibited.

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Chung-Hsiung Sui, Ming-Dean Cheng, Xiaoqing Wu, and Michio Yanai

Abstract

A parameterization of cumulus ensemble effects on the large-scale vorticity is tested to interpret the vorticity budget residual, Z, observed during Phase III of GARP Atlantic Tropical Experiment (GATE). The parameterization is derived consistently from the parameterization of cumulus ensemble effects on the momentum equation. Values of the parameterized Z are computed using the cumulus properties (mass flux, mass detrainment and cloud momentum) diagnosed by a spectral cumulus ensemble model. The results confirm the inferences made in Part I of this paper that organized cumulus convection produces significant residuals in the large-scale vorticity budget mainly through 1) the detrainment of excess momentum from clouds, and 2) the vertical advection of the large-scale vorticity due to the subsidence of environmental air compensating the convective mass flux. In addition, the twisting of the horizontal component of the large-scale vorticity into the vertical component due to nonuniform spatial distributions of the convective mass flux plays a significant role in producing Z at the levels where the vertical wind shear is large.

Deep cumulus convection decelerates the mean flow over the area of convection by the detrainment of smaller cloud momentum transported from below. This deceleration produces a positive vorticity tendency to the right of the convective area facing downstream and a negative tendency to the left. In addition, the curl of the excess momentum tends to produce a positive vorticity tendency over the area of convection. These effects explain the observed features of Z in the upper troposphere, i.e., the horizontal dipole pattern and the positive mean values. The vertical advection of the large-scale vorticity by the cumulus-induced subsidence is the dominant mechanism producing negative Z in the middle troposphere where the gradient of vorticity, ∂¯ζ/∂p, is positive. In the lower troposphere where ∂¯ζ/∂p is negative, the vertical advection effect produces positive Z. In addition, detrainment of momentum from shallow clouds is found to be significant near 650 mb and responsible for generating localized patterns in the horizontal distribution of Z.

Results of additional experiments show improvements of the parameterized Z in the lower troposphere by including downdrafts in the diagnosis of mass flux and the potential importance of pressure interactions between the clouds and the environment in the cumulus momentum budget.

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Hsiao-Ling Huang, Ming-Jen Yang, and Chung-Hsiung Sui

Abstract

In this study, the Weather Research and Forecasting model, version 3.2, with the finest grid size of 1 km is used to explicitly simulate Typhoon Morakot (2009), which dumped rainfall of more than 2600 mm in 3 days on Taiwan. The model reasonably reproduced the track, the organization, the sizes of the eye and eyewall, and the characteristics of major convective cells in outer rainbands. The horizontal rainfall distribution and local rainfall maximum in the southwestern portion of the Central Mountain Range (CMR) are captured. The simulated rain rate and precipitation efficiency (PE) over the CMR are highly correlated. In the absence of terrain forcing, the simulated TC’s track is farther north and rainfall distribution is mainly determined by rainbands. The calculated rain rate and PE over the CMR during landfall are about 50% and 15%–20% less than those of the full-terrain control run, respectively. By following major convective cells that propagate eastward from the Taiwan Strait to the CMR, it is found that the PE and the processes of vapor condensation and raindrop evaporation are strongly influenced by orographic lifting; the PEs are 60%–75% over ocean and more than 95% over the CMR, respectively. The secondary increase of PE results from the increase of ice-phase deposition ratio when the liquid-phase condensation becomes small as the air on the lee side subsides and moves downstream. This nearly perfect PE over the CMR causes tremendous rainfall in southwestern Taiwan, triggering enormous landslides and severe flooding.

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Chung-Hsiung Sui, Xiaofan Li, and Ming-Jen Yang

Abstract

A modified definition of precipitation efficiency (PE) is proposed based on either cloud microphysics precipitation efficiency (CMPE) or water cycling processes including water vapor and hydrometeor species [large-scale precipitation efficiency (LSPE)]. These PEs are examined based on a two-dimensional cloud-resolving simulation. The model is integrated for 21 days with the imposed large-scale vertical velocity, zonal wind, and horizontal advections obtained from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). It is found that the properly defined PEs include all moisture and hydrometeor sources associated with surface rainfall processes so that they range from 0% to 100%. Furthermore, the modified LSPE and CMPE are highly correlated. Their linear correlation coefficient and root-mean-squared difference are insensitive to the spatial scales of averaged data and are moderately sensitive to the time period of averaged data.

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Chung-Hsiung Sui, Xiaofan Li, Ming-Jen Yang, and Hsiao-Ling Huang

Abstract

Precipitation efficiency is estimated based on vertically integrated budgets of water vapor and clouds using hourly data from both two-dimensional (2D) and three-dimensional (3D) cloud-resolving simulations. The 2D cloud-resolving model is forced by the vertical velocity derived from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The 3D cloud-resolving modeling is based on the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) simulation of Typhoon Nari (in 2001). The analysis of the hourly moisture and cloud budgets of the 2D simulation shows that the total moisture source (surface evaporation and vertically integrated moisture convergence) is converted into hydrometeors through vapor condensation and deposition rates regardless of the area size where the average is taken. This leads to the conclusion that the large-scale and cloud-microphysics precipitation efficiencies are statistically equivalent. Results further show that convergence (divergence) of hydrometeors would make precipitation efficiency larger (smaller). The precipitation efficiency tends to be larger (even >100%) in light rain conditions as a result of hydrometeor convergence from the neighboring atmospheric columns. Analysis of the hourly moisture and cloud budgets of the 3D results from the simulation of a typhoon system with heavy rainfall generally supports that of 2D results from the simulation of the tropical convective system with moderate rainfall intensity.

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