Abstract

The role of cumulus congestus (shallow and congestus convection) in tropical cyclone (TC) formation is examined in a high-resolution simulation of Tropical Cyclone Fay (2008). It is found that cumulus congestus plays a dominant role in moistening the lower to middle troposphere and spinning up the near-surface circulation prior to genesis, while deep convection plays a key role in moistening the upper troposphere and intensifying the cyclonic circulation over a deep layer. The transition from the tropical wave stage to the TC stage is marked by a substantial increase in net condensation and potential vorticity generation by deep convection in the inner wave pouch region.

This study suggests that TC formation can be regarded as a two-stage process. The first stage is a gradual process of moisture preconditioning and low-level spinup, in which cumulus congestus plays a dominant role. The second stage commences with the rapid development of deep convection in the inner pouch region after the air column is moistened sufficiently, whereupon the concentrated convective heating near the pouch center strengthens the transverse circulation and leads to the amplification of the cyclonic circulation over a deep layer. The rapid development of deep convection can be explained by the power-law increase of precipitation rate with column water vapor (CWV) above a critical value. The high CWV near the pouch center thus plays an important role in convective organization. It is also shown that cumulus congestus can effectively drive the low-level convergence and provides a direct and simple pathway for the development of the TC protovortex near the surface.

Abstract

The statistics of convective processes and vertical vorticity from the tropical wave to tropical cyclone stage are examined in a high-resolution simulation of Tropical Cyclone Fay (2008). The intensity of vertical velocity follows approximately the truncated lognormal distribution in the model simulation, which is consistent with previous observational studies. The upward motion at the pregenesis stage is weaker compared to mature hurricanes or midlatitude thunderstorms. The relatively strong upward velocities occupying a small areal fraction make a substantial contribution to the upward mass and moisture fluxes and condensation.

It is also found that upward motion and downward motion both intensify with time, but the former is stronger than the latter, and the mean vertical motion and the mean vertical mass flux thus increase with time. By contrast, the maximum anticyclonic vorticity is comparable to the maximum cyclonic vorticity in magnitude. Both cyclonic vorticity and anticyclonic vorticity intensify with time, but the former covers a larger areal fraction in the lower and middle troposphere and becomes dominant throughout the troposphere after genesis.

Sensitivity tests with different model resolutions were carried out to test the robustness of the results. When the horizontal grid spacing is reduced, the size of updrafts decreases and the number of updrafts increases, but the areal fraction of updrafts, the mean vertical velocity, and the mean vertical mass flux are rather insensitive to the model resolution, especially in the lower troposphere and when the model resolution is 1 km or higher. This may explain why models with relatively coarse resolution can simulate tropical cyclogenesis reasonably well.

Abstract

The thermodynamic aspects of tropical cyclone (TC) formation near the center of the wave pouch, a region of approximately closed Lagrangian circulation within the wave critical layer, are examined through diagnoses of a high-resolution numerical simulation and dropsonde data from a recent field campaign. It is found that the meso-β area near the pouch center is characterized by high saturation fraction, small difference in equivalent potential temperature θ_e between the surface and the middle troposphere, and a short incubation time scale. Updrafts tend to be more vigorous in this region, presumably because of reduced dry air entrainment, while downdrafts are not suppressed. The thermodynamic conditions near the pouch center are thus critically important for TC formation.

The balanced responses to convective and stratiform heating at the pregenesis stage are examined using the Sawyer–Eliassen equation. Deep convection is concentrated near the pouch center. The strong radial and vertical gradients of latent heat release effectively force the transverse circulation and spin up a surface protovortex near the pouch center. Stratiform heating induces modest midlevel inflow and very weak low-level outflow, which contributes to the midlevel spinup without substantially spinning down the low-level circulation.

The analysis of dropsonde data shows that the midlevel θ_e increases significantly near the pouch center one to two days prior to genesis but changes little away from the pouch center. This may indicate convective organization and the impending TC genesis. It also suggests that the critical information of TC genesis near the pouch center may be masked out if a spatial average is taken over the pouch scale.

Abstract

Infrared brightness temperature data are used to investigate convective evolution during tropical cyclone (TC) formation in a quasi-Lagrangian framework. More than 150 named Atlantic storms during 1989–2010 were examined. It is found that both convective intensity and convective frequency increase with time in the inner pouch region but change little, or even weaken slightly, in the outer pouch region. Convection thus appears to concentrate toward the circulation center as genesis is approached. However, large variability is found from storm to storm in convective intensity, area, and duration, and the convective evolution of individual storms does not resemble the composite mean. Further analysis suggests that the composite mean or the median represents the probability of occurrence of convection instead of a recurrent pattern. Three distinct spatial patterns of convection are identified using cluster analysis. Substantial differences in convection intensity and area are found among the clusters and can be attributed to the impacts of environmental conditions. These differences suggest that convection intensity or area is not a key feature of convection for tropical cyclogenesis. In particular, a small and weak convective system is not necessarily associated with a weak vortex. A simple proxy of the radial gradient of convection is found to be similar among the clusters. Furthermore, convection is most effective in strengthening the TC protovortex when its maximum occurs near the pouch center. These findings suggest that organized convection near the pouch center is a key feature of convection for tropical cyclogenesis and that emphasizing convective intensity or frequency without considering the spatial pattern may be misleading.

Abstract

The impacts of dry air on tropical cyclone formation are examined in the numerical model simulations of ex-Gaston (2010) and pre-Fay (2008). The former, a remnant low downgraded from a short-lived tropical cyclone, can be regarded as a nondeveloping system because it failed to redevelop, and the latter developed into a tropical cyclone despite lateral dry air entrainment and a transient upper-level dry air intrusion. Water vapor budget analysis suggests that the mean vertical moisture transport plays the dominant role in moistening the free atmosphere. Backward trajectory analysis and water budget analysis show that vertical transport of dry air from the middle and upper troposphere, where a well-defined wave pouch is absent, contributes to the midlevel drying near the pouch center in ex-Gaston. The midlevel drying suppresses deep convection, reduces moisture supply from the boundary layer, and contributes to the nondevelopment of ex-Gaston. Three-dimensional trajectory analysis based on the numerical model simulation of Fay suggests that dry air entrained at the pouch periphery tends to stay off the pouch center because of the weak midlevel inflow or gets moistened along its path even if it is being wrapped into the wave pouch. Lateral entrainment in the middle troposphere thus does not suppress convection near the pouch center or prevent the development of Tropical Storm Fay. This study suggests that the upper troposphere is a weak spot of the wave pouch at the early formation stage and that the vertical transport is likely a more direct pathway for dry air to influence moist convection near the pouch center.

Abstract

Evolution of the water vapor budget from the tropical wave stage to the tropical cyclone stage is examined using a high-resolution numerical model simulation. The focus is on a time window from 27 h prior to genesis to 9 h after genesis, and the diagnoses are carried out in the framework of the marsupial paradigm. Analysis shows that the vertically integrated inward moisture flux accounts for a majority of the total condensation and that its fractional contribution increases from the tropical wave stage to the tropical cyclone stage. The fractional contribution of the local evaporation is much smaller and decreases from the tropical wave stage to the tropical cyclone stage. It is also shown that the radial moisture flux above 850 hPa is rather weak prior to genesis but increases significantly after genesis because of the deepening of the inflow layer. The decrease in the fractional contribution of the local evaporation, or the increase in the fractional contribution of the vertically integrated inward moisture flux, is due to the strengthening of the low-level convergence associated with the secondary circulation. The intensification of the secondary circulation can be attributed to the organized convection and concentrated diabatic heating near the circulation center. The results suggest that the local evaporation and its positive interaction with the primary circulation may not be as important as generally appreciated for tropical cyclone development. By contrast, the increase in the fractional contribution by the inward moisture flux with the storm intensification implies the importance of the positive feedback among the primary circulation, the secondary circulation, and convection for tropical cyclone development.

Abstract

The simultaneous precipitation and column water vapor retrievals from the SSM/I and SSMIS passive microwave instruments were used to examine the convective and moisture evolution during tropical cyclone formation. Using a wave-pouch-track dataset, composites of precipitation and column water vapor were constructed with more than 2000 satellite overpasses for a 3-day time period prior to genesis. It was found that high column water vapor occurs near the pouch center and starts to increase about 42 h prior to genesis while a substantial increase in precipitation occurs within 24 h prior to genesis. These features are consistent with a recently proposed two-stage conceptual model for tropical cyclone formation, in which gradual moisture preconditioning precedes an abrupt transition to sustained deep convection leading up to genesis.

The relationship between precipitation and saturation fraction (SF) is examined for the developing waves and compared with the general tropical North Atlantic. Precipitation rate is found to increase at the same exponential rate above the same critical point of SF in the two groups, but convection in the developing waves has a higher probability of occurrence near and above criticality. This can be attributed to the positive feedback between convection and the low-level moisture convergence, which counteracts the negative feedback of convection on water vapor and makes convection in a developing tropical cyclone more sustainable.

Abstract

Atmospheric blocking is a prolific producer of extreme weather with significant socioeconomic impacts. Different physical mechanisms for blocking onset have been proposed and are generally focused on two sectors: the Eurasian and the North Pacific. Here, we objectively separate blocking into four regions and investigate how the blocking onset mechanisms vary from one region to another, focusing on three factors: scale interactions between three frequency bands, Rossby wave breaking (RWB), and diabatic heating. Atlantic blocks are dominated by the low-frequency flow evolution that resembles the negative phase of the North Atlantic Oscillation and are influenced by cyclonic RWB toward the western edge of the anticyclone. Europe blocks are influenced by high-frequency, traveling waves across the Atlantic Ocean and develop rapidly, mainly attributed to strong anticyclonic RWB and interaction between high- and intermediate-frequency flow components. Asian blocks are fixated within a stationary wave train that spans upstream to the western Atlantic Ocean and do not have strong potential vorticity or RWB features. The Pacific blocks are mainly influenced by an intermediate-frequency retrograding wave train, while a low-frequency component resembling the Pacific–North American pattern is evident. The Pacific blocks also contain precursor signals in the stratosphere. Backward trajectory analysis revealed that 35%–45% of parcels initialized within the Atlantic, Europe, and Pacific blocking anticyclones experience heating and ascent, while adiabatic processes dominate Asian blocking. Overall, our analysis demonstrates the importance of decomposing the flow into three frequency bands and illustrates different blocking onset mechanisms over four sectors.

Abstract

Rossby wave propagation theory predicts that Rossby waves in a tropical easterly flow cannot escape from the Tropics to the extratropics. Here the authors show that a southerly flow component in the basic state (a southerly conveyor) may transfer a Rossby wave source northward; thus, a forcing embedded in the deep tropical easterlies may excite a Rossby wave response in the extratropical westerlies. It is shown that the southerly conveyor determines the location of the effective Rossby wave source and that the extratropical response is relatively insensitive to the location of the tropical forcing, provided that the tropical response can reach the southerly conveyor. A stronger southerly flow favors a stronger extratropical response, and the spatial structure of the extratropical response is determined by the extratropical westerly basic flows.

Abstract

The formation of pre–Hurricane Felix (2007) in a tropical easterly wave is examined in a two-part study using the Weather Research and Forecasting (WRF) model with a high-resolution nested grid configuration that permits the representation of cloud system processes. The simulation commences during the wave stage of the precursor African easterly-wave disturbance. Here the simulated and observed developments are compared, while in of the study various large-scale analyses, physical parameterizations, and initialization times are explored to document model sensitivities.

In this first part the authors focus on the wave/vortex morphology, its interaction with the adjacent intertropical convergence zone complex, and the vorticity balance in the neighborhood of the developing storm. Analysis of the model simulation points to a bottom-up development process within the wave critical layer and supports the three new hypotheses of tropical cyclone formation proposed recently by Dunkerton, Montgomery, and Wang. It is shown also that low-level convergence associated with the ITCZ helps to enhance the wave signal and extend the “wave pouch” from the jet level to the top of the atmospheric boundary layer. The region of a quasi-closed Lagrangian circulation within the wave pouch provides a focal point for diabatic merger of convective vortices and their vortical remnants. The wave pouch serves also to protect the moist air inside from dry air intrusion, providing a favorable environment for sustained deep convection. Consistent with the authors’ earlier findings, the tropical storm forms near the center of the wave pouch via system-scale convergence in the lower troposphere and vorticity aggregation. Components of the vorticity balance are shown to be scale dependent, with the immediate effects of cloud processes confined more closely to the storm center than the overturning Eliassen circulation induced by diabatic heating, the influence of which extends to larger radii.