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.

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.

Abstract

The evolution processes for propagating Madden–Julian oscillations with strong magnitude over the Indian Ocean (IO) and Maritime Continent (MC) are investigated through a diagnosis of ECMWF reanalysis data for November–April 1982–2011. A scale-separated lower-tropospheric (1000–700 hPa) moisture budget is analyzed for four stages of composite life cycle: suppressed, cloud developing, convective, and decaying. Overall, the budgets in the IO and MC are dominated by wave-induced boundary layer convergence in the anomalous easterlies (WC) and advection. Starting from the suppressed stage in the central IO, moistening by WC and advection by easterly anomalies contributes to an initiation of the MJO convection in the western IO while surface evaporation and/or shallow convection moistens the central IO. In the following cloud developing and convective stage in the central IO, moistening by WC and advection by the downstream Kelvin–Rossby wave east of central IO lead to eastward propagation of deep convection. In the MC, the suppressed stage coincides with the convective stage in the central IO that promotes anomalous easterlies, subsidence, and enhanced rain rate over islands. Unlike WC and advective moistening in the IO that both occur in the equatorial zone, advective moistening in MC tends to be negative (positive) on windward (leeward) side of the major islands in the equatorial zone and more organized over the Arafura Sea, conducive to a southward detour of the eastward-propagating MJO.

Abstract

Most El Niño events decay after a peak in boreal winter, but some persist and strengthen again in the following year. Several mechanisms for regulating its decay pace have been proposed; however, their relative contributions have not been thoroughly examined yet. By analyzing the fast-decaying and persistent types of the events in a 1200-year coupled simulation, we quantify the key dynamic and thermodynamic processes in the decaying spring that are critical to determining the decay pace of El Niño. The zonal advection due to upwelling Kelvin wave accounts for twice as much the cooling difference as evaporation or meridional advection does. The upwelling Kelvin wave is much stronger in the fast-decaying events than the others, and its strength is equally attributed to the reflected equatorial Rossby wave and the equatorial easterly wind forcing over the western Pacific in the preceding 2-3 months. Relative to the fast-decaying events, the evaporative cooling is weaker but the meridional warm advection is stronger in the persistent events. The former is due to more meridionally asymmetric wind and sea surface temperature anomalies (SSTA) signaling positive Pacific Meridional Mode (PMM). The latter results from the advection of equatorial warm SSTA by climatological divergent flow, and the warmer SSTA persists from the mature stage subject to weaker cloud-radiative cooling in response to the Central-Pacific-type SSTA distribution in the persistent events relative to the fast-decaying events. Our result consolidates the existing knowledge and provides a more comprehensive and physical pathway for the causality of El Niño’s diverse duration.

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.

Abstract

Lower-tropospheric (1000–700 hPa) moistening processes of the two Madden–Julian oscillations (MJOs) over the Indian Ocean during Dynamics of the MJO (DYNAMO)/Cooperative Indian Ocean Experiment on Intraseasonal Variability in Year 2011 (CINDY) are investigated by using soundings, operational assimilation, and satellite data. A scale-separated moisture budget is calculated at the sounding site by using time-decomposed wind and moisture fields. Each budget term is projected onto the intraseasonal moisture anomaly and its time tendency change. The projections and the corresponding temporal correlations are analyzed together with the temporal evolution of the budget terms to identify the dominant moistening process responsible for the MJO evolution. Results indicate that broad-scale advection by low-frequency and MJO flow and moisture fields are dominant moisture sources, while the residual of the moisture budget (−Q ₂) is a dominant sink contributing to the tendency term (propagation) and intraseasonal moisture anomaly (growth and decay). Dividing their life cycles into four phases (suppressed, cloud developing, convective, and decaying phases), the two MJOs exhibit different budget balances in the premoistening stage from the suppressed phase to the cloud-developing phase when low-frequency vertical motion is downward in MJO1 but upward in MJO2. The corresponding drying and moistening are balanced by negative Q ₂ (reevaporation in nonraining cloud) in MJO1 and positive Q ₂ in MJO2. The result implies that low-frequency flow (>60 days) can affect the initiation of MJOs. The premoistening in the lower troposphere by boundary layer moisture convergence leading the deep convection is observed but only in the cloud-developing phase to convective phase of the MJOs. Nonlinear moisture advection by synoptic disturbances always acts as a diffusive term. It is the dominant moisture source in the suppress phase of the two MJOs.

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.

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.

Abstract

The WRF Model is used to simulate 52 tropical cyclones (TCs) that formed in the western North Pacific during 2008–09 to study the influence of the low-frequency mode of environmental vorticity on TC formation [V _max ~ 25 kt (~13 m s⁻¹)]. All simulations, using the same model setting, are repeated at four distinct initial times and with two different initial datasets. These TCs are classified into two groups based on the environmental 850-hPa low-frequency vorticity (using a 10-day low-pass filter) during the period 24–48 h prior to TC formation. Results show that the WRF Model is more capable of simulating the TC formation process, but with larger track errors for TCs formed in an environment with higher low-frequency vorticity (HTC). In contrast, the model is less capable of simulating the TC formation process for TCs formed in an environment with lower low-frequency vorticity (LTC), but with smaller track errors. Fourteen selected TCs are further simulated to examine the sensitivity of previous results to different cumulus parameterization schemes. Results show that the capability of the WRF Model to simulate HTC formation is not sensitive to the choice of cumulus scheme. However, for an LTC, the simulated convection pattern is very sensitive to the cumulus scheme used; therefore, model simulation capability for LTC depends on the cumulus scheme used. Results of this study reveal that the convection process is not a dominant factor in HTC formation, but is very important for LTC formation.

Abstract

This study focuses on the migratory tropical cyclones (TCs) that form in the western North Pacific (WNP) and move into the South China Sea (SCS). Their movements are found to be modulated differently by intraseasonal oscillations (ISOs) and climatological circulations through the TC-active months. The modulating processes of climatological circulations vary from a westward intensifying western Pacific subtropical high (WPSH) in July and August to a southeastward extending monsoon trough (MT) in September, and a strengthening equatorial trough (ET) in October and November. In July and August, enhanced tropical ISO convections in the SCS are accompanied by a 30–60-day anomalous anticyclone to the northeast of the SCS. The migratory TCs move along the southern peripheries of this anomalous anticyclone and the WPSH into the SCS. In September, enhanced ISO convections in the SCS coincide with a meridional 30–60-day circulation pair with an anomalous anticyclone to the north of 20°N and an anomalous cyclone to the south. TCs move in between this meridional 30–60-day circulation pair and the northern periphery of the MT toward the SCS. In October and November, enhanced ISO convections in the SCS and WNP coexist with an overlying 30–60-day anomalous cyclone and an intensified ET. The migratory TCs move along the northern sections of this 30–60-day anomalous cyclone and the ET toward the SCS. With a different track, TCs recurving northward from the tropical WNP into the region east of Taiwan are modulated by completely different variability features of the 30–60-day ISO and climatological circulations.