Thermodynamic Aspects of Tropical Cyclone Formation

2012 ◽  
Vol 69 (8) ◽  
pp. 2433-2451 ◽  
Author(s):  
Zhuo Wang
Keyword(s):  
Tropical Cyclone ◽  
Deep Convection ◽  
Potential Temperature ◽  
Air Entrainment ◽  
Spatial Average ◽  
Low Level ◽  
Equivalent Potential ◽  
Thermodynamic Conditions ◽  
Short Incubation Time ◽  
Convective Organization

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 θe 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 θe 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.

The Global Atmospheric Circulation in Moist Isentropic Coordinates

2010 ◽  
Vol 23 (11) ◽  
pp. 3077-3093 ◽  
Author(s):  
Olivier Pauluis ◽  
Arnaud Czaja ◽  
Robert Korty
Keyword(s):  
Mass Transport ◽  
Total Mass ◽  
Potential Temperature ◽  
Equivalent Potential Temperature ◽  
Moist Air ◽  
Low Level ◽  
Equivalent Potential ◽  
Entropy Transport ◽  
Mean Circulation ◽  
The Tropics

Abstract Differential heating of the earth’s atmosphere drives a global circulation that transports energy from the tropical regions to higher latitudes. Because of the turbulent nature of the flow, any description of a “mean circulation” or “mean parcel trajectories” is tied to the specific averaging method and coordinate system. In this paper, the NCEP–NCAR reanalysis data spanning 1970–2004 are used to compare the mean circulation obtained by averaging the flow on surfaces of constant liquid water potential temperature, or dry isentropes, and on surfaces of constant equivalent potential temperature, or moist isentropes. While the two circulations are qualitatively similar, they differ in intensity. In the tropics, the total mass transport on dry isentropes is larger than the circulation on moist isentropes. In contrast, in midlatitudes, the total mass transport on moist isentropes is between 1.5 and 3 times larger than the mass transport on dry isentropes. It is shown here that the differences between the two circulations can be explained by the atmospheric transport of water vapor. In particular, the enhanced mass transport on moist isentropes corresponds to a poleward flow of warm moist air near the earth’s surface in midlatitudes. This low-level poleward flow does not appear in the zonally averaged circulation on dry isentropes, as it is hidden by the presence of a larger equatorward flow of drier air at same potential temperature. However, as the equivalent potential temperature in this low-level poleward flow is close to the potential temperature of the air near the tropopause, it is included in the total circulation on moist isentropes. In the tropics, the situation is reversed: the Hadley circulation transports warm moist air toward the equator, and in the opposite direction to the flow at upper levels, and the circulation on dry isentropes is larger than that on moist isentropes. The relationship between circulation and entropy transport is also analyzed. A gross stratification is defined as the ratio of the entropy transport to the net transport on isentropic surfaces. It is found that in midlatitudes the gross stability for moist entropy is approximately the same as that for dry entropy. The gross stratification in the midlatitude circulation differs from what one would expect for either an overturning circulation or horizontal mixing; rather, it confirms that warm moist subtropical air ascends into the upper troposphere within the storm tracks.

scholarly journals A High-Resolution Simulation of Asymmetries in Severe Southern Hemisphere Tropical Cyclone Larry (2006)

2009 ◽  
Vol 137 (12) ◽  
pp. 4171-4187 ◽  
Author(s):  
Hamish A. Ramsay ◽  
Lance M. Leslie ◽  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Southern Hemisphere ◽  
Inner Core ◽  
Mesoscale Model ◽  
Deep Convection ◽  
Vertical Motion ◽  
Vertical Shear ◽  
Low Level ◽  
Frictional Convergence

Abstract Advances in observations, theory, and modeling have revealed that inner-core asymmetries are a common feature of tropical cyclones (TCs). In this study, the inner-core asymmetries of a severe Southern Hemisphere tropical cyclone, TC Larry (2006), are investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) and the Kepert–Wang boundary layer model. The MM5-simulated TC exhibited significant asymmetries in the inner-core region, including rainfall distribution, surface convergence, and low-level vertical motion. The near-core environment was characterized by very low environmental vertical shear and consequently the TC vortex had almost no vertical tilt. It was found that, prior to landfall, the rainfall asymmetry was very pronounced with precipitation maxima consistently to the right of the westward direction of motion. Persistent maxima in low-level convergence and vertical motion formed ahead of the translating TC, resulting in deep convection and associated hydrometeor maxima at about 500 hPa. The asymmetry in frictional convergence was mainly due to the storm motion at the eyewall, but was dominated by the proximity to land at larger radii. The displacement of about 30°–120° of azimuth between the surface and midlevel hydrometeor maxima is explained by the rapid cyclonic advection of hydrometeors by the tangential winds in the TC core. These results for TC Larry support earlier studies that show that frictional convergence in the boundary layer can play a significant role in determining the asymmetrical structures, particularly when the environmental vertical shear is weak or absent.

scholarly journals Tropical Cyclone–Relative Environmental Helicity and the Pathways to Intensification in Shear

2016 ◽  
Vol 73 (2) ◽  
pp. 869-890 ◽  
Author(s):  
Matthew J. Onderlinde ◽  
David S. Nolan
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Latent Heat ◽  
Wind Shear ◽  
Potential Temperature ◽  
Reanalysis Data ◽  
Heat Fluxes ◽  
Equivalent Potential ◽  
Vertical Variations ◽  
The Difference

Abstract Tropical cyclone–relative environmental helicity (TCREH) is a measure of how the wind vector changes direction with height, and it has been shown to modulate the rate at which tropical cyclones (TCs) develop both in idealized simulations and in reanalysis data. The channels through which this modulation occurs remain less clear. This study aims to identify the mechanisms that lead to the observed variations in intensification rate. Results suggest that the difference in intensification rate between TCs embedded in positive versus negative TCREH primarily results from the position of convection and associated latent heat fluxes relative to the wind shear vector. When TCREH is positive, convection is more readily advected upshear and air parcels that experience larger fluxes are more frequently ingested into the TC core. Trajectories computed from high-resolution simulations demonstrate the recovery of equivalent potential temperature downwind of convection, latent heat flux near the TC core, and parcel routes through updrafts in convection. Differences in trajectory characteristics between TCs embedded in positive versus negative TCREH are presented. Contoured frequency-by-altitude diagrams (CFADs) show that convection is distributed differently around TCs embedded in environments characterized by positive versus negative TCREH. They also show that the nature of the most intense convection differs only slightly between cases of positive and negative TCREH. The results of this study emphasize the fact that significant variability in TC-intensification rate results from vertical variations in the environmental wind direction, even when the 850–200-hPa wind shear vector remains unchanged.

Life of a Six-Hour Hurricane

2009 ◽  
Vol 137 (1) ◽  
pp. 51-67 ◽  
Author(s):  
Kay L. Shelton ◽  
John Molinari
Keyword(s):  
Deep Convection ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Vertical Shear ◽  
Vortex Center ◽  
Dry Air ◽  
Sheared Flow ◽  
The Core ◽  
Equivalent Potential

Abstract Hurricane Claudette developed from a weak vortex in 6 h as deep convection shifted from downshear into the vortex center, despite ambient vertical wind shear exceeding 10 m s−1. Six hours later it weakened to a tropical storm, and 12 h after the hurricane stage a circulation center could not be found at 850 hPa by aircraft reconnaissance. At hurricane strength the vortex contained classic structure seen in intensifying hurricanes, with the exception of 7°–12°C dewpoint depressions in the lower troposphere upshear of the center. These extended from the 100-km radius to immediately adjacent to the eyewall, where equivalent potential temperature gradients reached 6 K km−1. The dry air was not present prior to intensification, suggesting that it was associated with vertical shear–induced subsidence upshear of the developing storm. It is argued that weakening of the vortex was driven by cooling associated with the mixing of dry air into the core, and subsequent evaporation and cold downdrafts. Evidence suggests that this mixing might have been enhanced by eyewall instabilities after the period of rapid deepening. The existence of a fragile, small, but genuinely hurricane-strength vortex at the surface for 6 h presents difficult problems for forecasters. Such a “temporary hurricane” in strongly sheared flow might require a different warning protocol than longer-lasting hurricane vortices in weaker shear.

scholarly journals Evolution of Pre- and Postconvective Environmental Profiles from Mesoscale Convective Systems during PECAN

2019 ◽  
Vol 147 (7) ◽  
pp. 2329-2354 ◽  
Author(s):  
Stacey M. Hitchcock ◽  
Russ S. Schumacher ◽  
Gregory R. Herman ◽  
Michael C. Coniglio ◽  
Matthew D. Parker ◽  
...  
Keyword(s):  
Potential Temperature ◽  
Cold Pool ◽  
Temperature Perturbation ◽  
Convective System ◽  
Equivalent Potential Temperature ◽  
Wind Maximum ◽  
Low Level ◽  
Equivalent Potential ◽  
Mesoscale Convective ◽  
Low Levels

Abstract During the Plains Elevated Convection at Night (PECAN) field campaign, 15 mesoscale convective system (MCS) environments were sampled by an array of instruments including radiosondes launched by three mobile sounding teams. Additional soundings were collected by fixed and mobile PECAN integrated sounding array (PISA) groups for a number of cases. Cluster analysis of observed vertical profiles established three primary preconvective categories: 1) those with an elevated maximum in equivalent potential temperature below a layer of potential instability; 2) those that maintain a daytime-like planetary boundary layer (PBL) and nearly potentially neutral low levels, sometimes even well after sunset despite the existence of a southerly low-level wind maximum; and 3) those that are potentially neutral at low levels, but have very weak or no southerly low-level winds. Profiles of equivalent potential temperature in elevated instability cases tend to evolve rapidly in time, while cases in the potentially neutral categories do not. Analysis of composite Rapid Refresh (RAP) environments indicate greater moisture content and moisture advection in an elevated layer in the elevated instability cases than in their potentially neutral counterparts. Postconvective soundings demonstrate significantly more variability, but cold pools were observed in nearly every PECAN MCS case. Following convection, perturbations range between −1.9 and −9.1 K over depths between 150 m and 4.35 km, but stronger, deeper stable layers lead to structures where the largest cold pool temperature perturbation is observed above the surface.

scholarly journals Impact of Stratiform Rainband Heating on the Tropical Cyclone Wind Field in Idealized Simulations

2019 ◽  
Vol 76 (8) ◽  
pp. 2443-2462 ◽  
Author(s):  
Chau-Lam Yu ◽  
Anthony C. Didlake
Keyword(s):  
Tropical Cyclone ◽  
Wind Field ◽  
Deep Convection ◽  
Cold Pool ◽  
Low Level ◽  
Secondary Eyewall Formation ◽  
Heating And Cooling ◽  
Storm Evolution ◽  
Spiral Rainbands ◽  
Diabatic Forcing

Abstract Using idealized simulations, we examine the storm-scale wind field response of a dry, hurricane-like vortex to prescribed stratiform heating profiles that mimic tropical cyclone (TC) spiral rainbands. These profiles were stationary with respect to the storm center to represent the diabatic forcing imposed by a quasi-stationary rainband complex. The first profile was typical of stratiform precipitation with heating above and cooling below the melting level. The vortex response included a mesoscale descending inflow and a midlevel tangential jet, consistent with previous studies. An additional response was an inward-spiraling low-level updraft radially inside the rainband heating. The second profile was a modified stratiform heating structure derived from observations and consisted of a diagonal dipole of heating and cooling. The same features were found with stronger magnitudes and larger vertical extents. The dynamics and implications of the forced low-level updraft were examined. This updraft was driven by buoyancy advection because of the stratiform-induced low-level cold pool. The stationary nature of the rainband diabatic forcing played an important role in modulating the required temperature and pressure anomalies to sustain this updraft. Simulations with moisture and full microphysics confirmed that this low-level updraft response was robust and capable of triggering sustained deep convection that could further impact the storm evolution, including having a potential role in secondary eyewall formation.

A Comparison between Moist and Dry Tropical Cyclones: The Low Effectiveness of Surface Sensible Heat Flux in Storm Intensification

2021 ◽  
Author(s):  
Zhanhong Ma ◽  
Jianfang Fei
Keyword(s):  
Heat Flux ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Potential Temperature ◽  
Sensible Heat Flux ◽  
Equivalent Potential Temperature ◽  
Sensible Heat ◽  
Budget Analysis ◽  
Equivalent Potential ◽  
Surface Sensible Heat Flux

AbstractRecent numerical modeling studies demonstrate that dry tropical cyclones can be stably sustained via supply of surface sensible heat flux. This raises questions of whether surface sensible heat flux (SHX) and latent heat flux (LHX) have the same effect on the intensity evolution of tropical cyclones. An estimation of equivalent potential temperature budget in the boundary layer shows that LHX leads to larger increase in equivalent potential temperature than SHX even when they possess the same magnitude. By formulating these two kinds of surface heat fluxes with the same mathematical framework, the simulated intensifications of moist and dry tropical cyclones are compared, with the former driven exclusively by LHX and the latter by SHX. Results show significantly larger intensification rates for the tropical cyclone driven by LHX than that by SHX, revealing low effectiveness of SHX in the intensification of tropical cyclones. The diabatic heating in the moist tropical cyclone occurs accompanying the convection, while it is merely pronounced near the surface in the dry tropical cyclone and is decoupled from the dry convection. A new surface pressure tendency equation is proposed, without incorporating implicit pressure tendency term on the right-hand side. The budget analysis indicates that the SHX is less effective than LHX in lowering surface central pressure and therefore in tropical cyclone intensification. A series of sensitivity experiments suggest that the threshold of energy input required for spinning up a tropical cyclone is lower in the form of LHX than that of SHX.

scholarly journals A Modeling Study of Hurricane Landfall in a Dry Environment

2006 ◽  
Vol 134 (7) ◽  
pp. 1901-1918 ◽  
Author(s):  
Sytske K. Kimball
Keyword(s):  
Numerical Simulations ◽  
Potential Temperature ◽  
Equivalent Potential Temperature ◽  
Initial Amount ◽  
Dry Air ◽  
Low Level ◽  
The Core ◽  
Equivalent Potential ◽  
Radial Extent ◽  
Heat And Moisture

Abstract The effects of dry air intrusion on landfalling hurricanes are investigated using eight numerical simulations. The simulations differ in the initial amount of moisture in the storm core and its horizontal extent from the storm center. The storms evolve very differently during the 36-h simulation. Storms with a small radial extent of moisture develop minimal rainbands, intensify rapidly in the first 3 h, and weaken as dry air from the 800–850-hPa layer wraps cyclonically and inward around the storm core. As the air approaches the core, it sinks (possibly by eyewall downdrafts or as a result of evaporative cooling), reaches the storm’s inflow layer, and entrains into the eyewall updrafts. Storms with large radial extent of moisture develop into larger storms with large rainbands, having smaller intensification rates initially, but continue to intensify for a longer period of time. Rainband downdrafts release low equivalent potential temperature air into the moat region. Low-level convergence into the rainbands reduces the magnitude of eyewall inflow. Both factors reduce storm intensification initially. Simultaneously, the rainbands act as a barrier between the moist core and the dry environment, preventing dry air from penetrating the storm core. As land is approached, inflowing air is no longer replenished with heat and moisture. Eventually, rainband convection erodes and dry air approaches the storm core from the landward side causing the storms to weaken. Without the presence of land, a hurricane can sustain itself in a dry environment, provided its moist envelope is large enough.

scholarly journals Role of Cumulus Congestus in Tropical Cyclone Formation in a High-Resolution Numerical Model Simulation

2014 ◽  
Vol 71 (5) ◽  
pp. 1681-1700 ◽  
Author(s):  
Zhuo Wang
Keyword(s):  
High Resolution ◽  
Tropical Cyclone ◽  
Deep Layer ◽  
Dominant Role ◽  
Rapid Development ◽  
Deep Convection ◽  
Cyclonic Circulation ◽  
Low Level ◽  
Cumulus Congestus

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.

Deep Eye Clouds Observed in Tropical Cyclone Trami (2018) during T-PARCII Dropsonde Observations

2021 ◽  
Keyword(s):  
Tropical Cyclone ◽  
Potential Temperature ◽  
Coupled Model ◽  
Ocean Model ◽  
Convective Heating ◽  
Convective Clouds ◽  
Warm Core ◽  
Thermodynamic Conditions ◽  
Favorable Conditions ◽  
Eye Region

Abstract The sporadic formation of short-lived convective clouds in the eye of Tropical Cyclone (TC) Trami (2018) is investigated using dropsonde data and simulation results from a coupled atmosphere–ocean model. According to the satellite data, top height of the convective clouds exceeds 9 km above mean sea level, considerably taller than that of typical hub clouds (2–3 km). These clouds are located 10–30 km away from the TC center. Hence, these convective clouds are called deep eye clouds (DECs) in this study. The dropsonde data reveal increase in relative humidity in the eye region during the formation of DECs. Short-lived convective clouds are simulated up to the middle troposphere in the eye region in the coupled model. Investigation of thermodynamic conditions shows a weakened low-level warm core and associated favorable conditions for convection in the eye region during the formation of DECs. DECs are formed after the weakening and outward displacement of convective heating within the eyewall. To elucidate the influence of the changes in convective heating within the eyewall on the formation of DECs, we calculate secondary circulation and associated adiabatic warming induced by convective heating within the eyewall using the Sawyer–Eliassen equation. In the eye region, weakenings of subsidence and associated vertical potential temperature advection are observed as DECs are formed. This suggests that the weakening and outward displacement of convective heating within the eyewall create favorable conditions for the sporadic formation of DECs.

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