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.

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.

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.

Combined Effects of Midlevel Dry Air and Vertical Wind Shear on Tropical Cyclone Development. Part II: Radial Ventilation

2020 ◽  
Author(s):  
Joshua J. Alland ◽  
Brian H. Tang ◽  
Kristen L. Corbosiero ◽  
George H. Bryan
Keyword(s):  
Tropical Cyclone ◽  
Wind Shear ◽  
Inner Core ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Equivalent Potential Temperature ◽  
Dry Air ◽  
Equivalent Potential ◽  
Radial Outflow

AbstractThis study demonstrates how midlevel dry air and vertical wind shear (VWS) can modulate tropical cyclone (TC) development via radial ventilation. A suite of experiments was conducted with different combinations of initial midlevel moisture and VWS environments. Two radial ventilation structures are documented. The first structure is positioned in a similar region as rainband activity and downdraft ventilation (documented in Part I) between heights of 0 and 3 km. Parcels associated with this first structure transport low-equivalent potential temperature air inward and downward left-of-shear and upshear to suppress convection. The second structure is associated with the vertical tilt of the vortex and storm-relative flow between heights of 5 and 9 km. Parcels associated with this second structure transport low-relative humidity air inward upshear and right-of-shear to suppress convection. Altogether, the modulating effects of radial ventilation on TC development are the inward transport of low-equivalent potential temperature air, as well as low-level radial outflow upshear, which aid in reducing the areal extent of strong upward motions, thereby reducing the vertical mass flux in the inner core, and stunting TC development.

Combined Effects of Midlevel Dry Air and Vertical Wind Shear on Tropical Cyclone Development. Part I: Downdraft Ventilation

2020 ◽  
Author(s):  
Joshua J. Alland ◽  
Brian H. Tang ◽  
Kristen L. Corbosiero ◽  
George H. Bryan
Keyword(s):  
Tropical Cyclone ◽  
Mass Flux ◽  
Wind Shear ◽  
Inner Core ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Equivalent Potential Temperature ◽  
Areal Extent ◽  
Dry Air ◽  
Equivalent Potential

AbstractThis study examines how midlevel dry air and vertical wind shear (VWS) can modulate tropical cyclone (TC) development via downdraft ventilation. A suite of experiments was conducted with different combinations of initial midlevel moisture and VWS. A strong, positive, linear relationship exists between the low-level vertical mass flux in the inner core and TC intensity. The linear increase in vertical mass flux with intensity is not due to an increased strength of upward motions but, instead, is due to an increased areal extent of strong upward motions (w > 0:5 m s−1). This relationship suggests physical processes that could influence the vertical mass flux, such as downdraft ventilation, influence the intensity of a TC.The azimuthal asymmetry and strength of downdraft ventilation is associated with the vertical tilt of the vortex: downdraft ventilation is located cyclonically downstream from the vertical tilt direction and its strength is associated with the magnitude of the vertical tilt. Importantly, equivalent potential temperature of parcels associated with downdraft ventilation trajectories quickly recovers via surface fluxes in the subcloud layer, but the areal extent of strong upward motions is reduced. Altogether, the modulating effects of downdraft ventilation on TC development are the downward transport of low-equivalent potential temperature, negative-buoyancy air left-of-shear and into the upshear semicircle, as well as low-level radial outflow upshear, which aid in reducing the areal extent of strong upward motions, thereby reducing the vertical mass flux in the inner core, and stunting TC development.

Tropical Cyclone Formation in a Sheared Environment: A Case Study

2004 ◽  
Vol 61 (21) ◽  
pp. 2493-2509 ◽  
Author(s):  
John Molinari ◽  
David Vollaro ◽  
Kristen L. Corbosiero
Keyword(s):  
Tropical Cyclone ◽  
Wind Shear ◽  
Potential Temperature ◽  
Radial Profile ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Temperature Fields ◽  
Equivalent Potential Temperature ◽  
The Core ◽  
Equivalent Potential

Abstract The development of Hurricane Danny (1997) from depression to hurricane was examined using cloud-to-ground lightning data, reconnaissance aircraft data, and satellite imagery. Vertical wind shear between 850 and 200 hPa of 5–11 m s−1 produced persistent downshear convective outbreaks that became progressively more intense and closer to the center during the development. Early in the period the storm intensified steadily in the presence of this downshear convection. During the last and most intense outbreak, a second vortex appeared to develop within the convection. Evidence is presented that the new downshear vortex became the dominant vortex and absorbed the original. Based on these events, it is hypothesized that the presence of moderate vertical wind shear accelerated the early development process. Equivalent potential temperature fields within 500 m of the surface were examined. Only well after the period of vortex interaction did the characteristic mature tropical cyclone radial profile of equivalent potential temperature appear. This came about by the virtual elimination of both low θe values in the core and high θe values outside the core that had been present at previous hours. The growth of Hurricane Danny is viewed in terms of the wind-induced surface heat exchange (WISHE) theory. During the tropical depression and early tropical storm (“pre-WISHE”) periods, few if any of the assumptions of WISHE were met: vertical wind shear exceeded 5 m s−1, considerable azimuthal asymmetry was present, transient highly buoyant convection occurred, and low values of θe in the storm core suggested the presence of convective downdrafts. It is proposed that 1) vortex interactions and subsequent axisymmetrization produced a single dominant vortex at the surface, and 2) vertical mixing of moist entropy by strong convection moved the sounding toward moist neutrality. By this reasoning, the disturbance then met the key tenets of the known finite-amplitude WISHE instability, and the storm intensified to hurricane strength.

What are the Best Thermodynamic Quantity and Function to Define a Front in Gridded Model Output?

2019 ◽  
Vol 100 (5) ◽  
pp. 873-895 ◽  
Author(s):  
Carl M. Thomas ◽  
David M. Schultz
Keyword(s):  
Real Time ◽  
Potential Temperature ◽  
Thermodynamic Quantity ◽  
Kinematics And Dynamics ◽  
Horizontal Gradient ◽  
Model Output ◽  
Equivalent Potential Temperature ◽  
Thermal Front ◽  
Equivalent Potential ◽  
Definition Of

AbstractFronts can be computed from gridded datasets such as numerical model output and reanalyses, resulting in automated surface frontal charts and climatologies. Defining automated fronts requires quantities (e.g., potential temperature, equivalent potential temperature, wind shifts) and kinematic functions (e.g., gradient, thermal front parameter, and frontogenesis). Which are the most appropriate to use in different applications remains an open question. This question is investigated using two quantities (potential temperature and equivalent potential temperature) and three functions (magnitude of the horizontal gradient, thermal front parameter, and frontogenesis) from both the context of real-time surface analysis and climatologies from 38 years of reanalyses. The strengths of potential temperature to identify fronts are that it represents the thermal gradients and its direct association with the kinematics and dynamics of fronts. Although climatologies using potential temperature show features associated with extratropical cyclones in the storm tracks, climatologies using equivalent potential temperature include moisture gradients within air masses, most notably at low latitudes that are unrelated to the traditional definition of a front, but may be representative of a broader definition of an airmass boundary. These results help to explain previously published frontal climatologies featuring maxima of fronts in the subtropics and tropics. The best function depends upon the purpose of the analysis, but Petterssen frontogenesis is attractive, both for real-time analysis and long-term climatologies, in part because of its link to the kinematics and dynamics of fronts. Finally, this study challenges the conventional definition of a front as an airmass boundary and suggests that a new, dynamically based definition would be useful for some applications.

scholarly journals MSE Minus CAPE is the True Conserved Variable for an Adiabatically Lifted Parcel

2015 ◽  
Vol 72 (9) ◽  
pp. 3639-3646 ◽  
Author(s):  
David M. Romps
Keyword(s):  
Potential Energy ◽  
Thermodynamic Equilibrium ◽  
Convective Available Potential Energy ◽  
Potential Temperature ◽  
Equivalent Potential Temperature ◽  
Moist Static Energy ◽  
Neutral Buoyancy ◽  
Equivalent Potential ◽  
Nonhydrostatic Pressure ◽  
Static Energy

Abstract For an adiabatic parcel convecting up or down through the atmosphere, it is often assumed that its moist static energy (MSE) is conserved. Here, it is shown that the true conserved variable for this process is MSE minus convective available potential energy (CAPE) calculated as the integral of buoyancy from the parcel’s height to its level of neutral buoyancy and that this variable is conserved even when accounting for full moist thermodynamics and nonhydrostatic pressure forces. In the calculation of a dry convecting parcel, conservation of MSE minus CAPE gives the same answer as conservation of entropy and potential temperature, while the use of MSE alone can generate large errors. For a moist parcel, entropy and equivalent potential temperature give the same answer as MSE minus CAPE only if the parcel ascends in thermodynamic equilibrium. If the parcel ascends with a nonisothermal mixed-phase stage, these methods can give significantly different answers for the parcel buoyancy because MSE minus CAPE is conserved, while entropy and equivalent potential temperature are not.

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