scholarly journals Effects of Vertical Shears and Midlevel Dry Air on Tropical Cyclone Developments*

2013 ◽  
Vol 70 (12) ◽  
pp. 3859-3875 ◽  
Author(s):  
Xuyang Ge ◽  
Tim Li ◽  
Melinda Peng
Keyword(s):  
Tropical Cyclone ◽  
Vertical Shear ◽  
Weather Research And Forecasting ◽  
Moist Convection ◽  
Mean Flow ◽  
Vertical Alignment ◽  
Shear Forces ◽  
Dry Air ◽  
Vertically Aligned ◽  
The Impact

Abstract A set of idealized experiments using the Weather Research and Forecasting model (WRF) were designed to investigate the impacts of a midlevel dry air layer, vertical shear, and their combined effects on tropical cyclone (TC) development. Compared with previous studies that focused on the relative radial position of dry air with no mean flow, it is found that the combined effect of dry air and environmental vertical shear can greatly affect TC development. Moreover, this study indicates the importance of dry air and vertical shear orientations in determining the impact. The background vertical shear causes the tilting of an initially vertically aligned vortex. The shear forces a secondary circulation (FSC) with ascent (descent) in the downshear (upshear) flank. Hence, convection tends to be favored on the downshear side. The FSC reinforced by the convection may overcome the shear-induced drifting and “restore” the vertical alignment. When dry air is located in the downshear-right quadrant of the initial vortex, the dry advection by cyclonic circulation brings the dry air to the downshear side and suppresses moist convection therein. Such a process disrupts the “restoring” mechanism associated with the FSC and thus inhibits TC development. The sensitivity experiments show that, for a fixed dry air condition, a marked difference occurs in TC development between an easterly and a westerly shear background.

scholarly journals The Impact of Dry Midlevel Air on Hurricane Intensity in Idealized Simulations with No Mean Flow

2012 ◽  
Vol 69 (1) ◽  
pp. 236-257 ◽  
Author(s):  
Scott A. Braun ◽  
Jason A. Sippel ◽  
David S. Nolan
Keyword(s):  
Vortex Core ◽  
Inner Core ◽  
Weather Research And Forecasting ◽  
Forecasting Model ◽  
Vortex Center ◽  
Latent Heating ◽  
Initial Radius ◽  
Mean Flow ◽  
Dry Air ◽  
The Impact

Abstract This study examines the potential negative influences of dry midlevel air on the development of tropical cyclones (specifically, its role in enhancing cold downdraft activity and suppressing storm development). The Weather Research and Forecasting model is used to construct two sets of idealized simulations of hurricane development in environments with different configurations of dry air. The first set of simulations begins with dry air located north of the vortex center by distances ranging from 0 to 270 km, whereas the second set of simulations begins with dry air completely surrounding the vortex, but with moist envelopes in the vortex core ranging in size from 0 to 150 km in radius. No impact of the dry air is seen for dry layers located more than 270 km north of the initial vortex center (~3 times the initial radius of maximum wind). When the dry air is initially closer to the vortex center, it suppresses convective development where it entrains into the storm circulation, leading to increasingly asymmetric convection and slower storm development. The presence of dry air throughout the domain, including the vortex center, substantially slows storm development. However, the presence of a moist envelope around the vortex center eliminates the deleterious impact on storm intensity. Instead, storm size is significantly reduced. The simulations suggest that dry air slows intensification only when it is located very close to the vortex core at early times. When it does slow storm development, it does so primarily by inducing outward-moving convective asymmetries that temporarily shift latent heating radially outward away from the high-vorticity inner core.

Impact of Dry Midlevel Air on the Tropical Cyclone Outer Circulation

2019 ◽  
Vol 76 (6) ◽  
pp. 1809-1826 ◽  
Author(s):  
Shuai Wang ◽  
Ralf Toumi
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Horizontal Gradient ◽  
Latent Heating ◽  
Moist Convection ◽  
Overturning Circulation ◽  
Dry Air ◽  
High Enthalpy ◽  
Air Ventilation ◽  
The Impact

Abstract The impact of dry midlevel air on the outer circulation of tropical cyclones is investigated in idealized simulations with and without a moist envelope protecting the inner core. It is found that a dry midlevel layer away from the cyclone center can broaden the outer primary circulation and thus the overall destructive potential at both developing and mature stages. The midlevel outer drying enhances the horizontal gradient of latent heating in the rainbands and drives the expansion of the outer circulation. The moist convection at large radii is suppressed rapidly after the midlevel air is dried in the outer rainbands. An enhanced horizontal gradient of latent heating initiates a radial–vertical overturning circulation anomaly in the rainbands. This anomalous overturning circulation accelerates the radial inflow of the main secondary circulation, increases the angular momentum import, and thus increases the cyclone size. The dry air, mixed into the boundary layer from the midtroposphere, is “recharged” by high enthalpy fluxes because of the increased thermodynamical disequilibrium above the sea surface. This recharge process protects the eyewall convection from the environmental dry-air ventilation. The proposed mechanism may explain the continuous expansion in the tropical cyclone outer circulation after maturity, as found in observations.

Mesoscale Structural Evolution of Three Tropical Weather Systems Observed during PREDICT

2012 ◽  
Vol 69 (4) ◽  
pp. 1284-1305 ◽  
Author(s):  
Christopher A. Davis ◽  
David A. Ahijevych
Keyword(s):  
Tropical Cyclone ◽  
Deep Convection ◽  
Structural Evolution ◽  
Vertical Shear ◽  
Moist Convection ◽  
Dry Air ◽  
Mesoscale Dynamics ◽  
Tropospheric Circulation ◽  
Deep Moist Convection ◽  
Tropical Weather

Abstract Three well-observed Atlantic tropical weather systems that occurred during the 2010 hurricane season are analyzed. One case was former Tropical Storm Gaston that failed to redevelop into a tropical cyclone; the other two cases were developing storms Karl and Matthew. Geostationary satellite, multisensor-derived precipitation, and dropsondes from the National Science Foundation (NSF)–NCAR Gulfstream V (GV), NASA DC-8, and the NOAA Gulfstream IV (G-IV) and WP-3D Orion (P-3) aircraft are analyzed in a system-following frame to quantify the mesoscale dynamics of these systems. Gaston featured extensive dry air surrounding an initially moist core. Vertical shear forced a misalignment of midtropospheric and lower-tropospheric circulation centers. This misalignment allowed dry air to intrude above the lower-tropospheric center and severely limited the area influenced by deep moist convection, thus providing little chance of maintaining or rebuilding the vortex in sheared flow. By contrast, Karl and Matthew developed in a moister environment overall, with moisture increasing with time in the middle and upper troposphere. Deep moist convection was quasi-diurnal prior to genesis. For Karl, deep convection was initially organized away from the lower-tropospheric circulation center, creating a misalignment of the vortex. The vortex gradually realigned over several days and genesis followed this realignment within roughly one day. Matthew experienced weaker shear, was vertically aligned through most of its early evolution, and developed more rapidly than Karl. The evolutions of the three cases are interpreted in the context of recent theories of tropical cyclone formation.

scholarly journals Impact of environmental moisture on tropical cyclone intensification

2015 ◽  
Vol 15 (11) ◽  
pp. 16111-16139 ◽  
Author(s):  
L. Wu ◽  
H. Su ◽  
R. G. Fovell ◽  
T. J. Dunkerton ◽  
Z. Wang ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Inner Core ◽  
Wrf Model ◽  
Weather Research And Forecasting ◽  
Convective Activity ◽  
Dry Air ◽  
Moving Storm ◽  
Lagrangian Boundary ◽  
Moisture Increase

Abstract. The impacts of environmental moisture on the intensification of a tropical cyclone (TC) are investigated in the Weather Research and Forecasting (WRF) model, with a focus on the azimuthal asymmetry of the moisture impacts. A series of sensitivity experiments with varying moisture perturbations in the environment are conducted and the Marsupial Paradigm framework is employed to understand the different moisture impacts. We find that modification of environmental moisture has insignificant impacts on the storm in this case unless it leads to convective activity in the environment, which deforms the quasi-Lagrangian boundary of the storm. By facilitating convection and precipitation outside the storm, enhanced environmental moisture ahead of the northwestward-moving storm induces a dry air intrusion to the inner core and limits TC intensification. However, increased moisture in the rear quadrants favors intensification by providing more moisture to the inner core and promoting storm symmetry, with primary contributions coming from moisture increase in the boundary layer. The different impacts of environmental moisture on TC intensification are governed by the relative locations of moisture perturbations and their interactions with the storm Lagrangian structure.

scholarly journals Toward an Understanding of Tropical Cyclone Formation with a Nonhydrostatic, Mesoscale-Convection-Resolving Model§

2013 ◽  
Vol 7 (1) ◽  
pp. 37-50
Author(s):  
Masanori Yamasaki
Keyword(s):  
Tropical Cyclone ◽  
Numerical Experiments ◽  
Vertical Shear ◽  
Initial Time ◽  
Convective System ◽  
Vortex Center ◽  
Moist Convection ◽  
Mesoscale Convection ◽  
Tropical Cyclone Formation ◽  
Convective Systems

This paper describes results from numerical experiments which have been made toward a better understanding of tropical cyclone formation. This study uses a nonhydrostatic version of the author’s mesoscale-convection-resolving model that was developed in the 1980s to improve paramerization schemes of moist convection. In this study the horizontal grid size is taken to be 20 km in an area of 6,000 km x 3,000 km, and a non-uniform coarse grid is used in two areas to its north and south. Results from two numerical experiments are presented; one (case 1) without any environmental flow, and the other (case 2) with an easterly flow without low-level vertical shear. Three circular buoyancy perturbations are placed in the west-east direction at the initial time. Convection is initiated in the imposed latently unstable (positive CAPE) area. In both cases, a vortex with a pressure low is formed, and two band-shaped convective systems are formed to the north and the south of the vortex center. The vortex and two convective systems are oriented in the westsouthwest – eastnortheast direction, and their horizontal scales are nearly 2,000 km. In case 1, the band-shaped convective system on the southern side is stronger, and winds are stronger just to its south. In contrast, in case 2, the northern convective system is stronger, and winds are stronger just to its north. Therefore, the distributions of the equivalent potential temperature in the boundary layer and latent instability (positive buoyancy of the rising air) are also quite different between cases 1 and 2. The TC formation processes in these different cases are discussed, with an emphasis on the importance of examining the time change of latent instability field.

Examination of the Combined Effect of Deep-layer Vertical Shear Direction and Lower-Tropospheric Mean Flow on Tropical Cyclone Intensity and Size Based on the ERA5 Reanalysis

2021 ◽  
Author(s):  
Buo-Fu Chen ◽  
Christopher A. Davis ◽  
Ying-Hwa Kuo
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Vertical Wind Shear ◽  
Reanalysis Data ◽  
Vertical Shear ◽  
Shear Direction ◽  
Mean Flow ◽  
Representative Subset ◽  
Favorable Conditions ◽  
The Relationship

AbstractIdealized numerical studies have suggested that in addition to vertical wind shear (VWS) magnitude, the VWS profile also affects tropical cyclone (TC) development. A way to further understand the VWS profile’s effect is to examine the interaction between a TC and various shear-relative low-level mean flow (LMF) orientations. This study mainly uses the ERA5 reanalysis to verify that, consistent with idealized simulations, boundary-layer processes associated with different shear-relative LMF orientations affect real-world TC’s intensity and size. Based on analyses of 720 TCs from multiple basins during 2004–2016, a TC affected by an LMF directed toward downshear-left in the Northern Hemisphere favors intensification, whereas an LMF directed toward upshear-right is favorable for expansion. Furthermore, physical processes associated with shear-relative LMF orientation may also partly explain the relationship between the VWS direction and TC development, as there is a correlation between the two variables.The analysis of reanalysis data provides other new insights. The relationship between shear-relative LMF and intensification is not significantly modified by other factors [inner-core sea surface temperature (SST), VWS magnitude, and relative humidity (RH)]. However, the relationship regarding expansion is partly attributed to environmental SST and RH variations for various LMF orientations. Moreover, SST is critical to the basin-dependent variability of the relationship between the shear-relative LMF and intensification. For Atlantic TCs, the relationship between LMF orientation and intensification is inconsistent with all-basin statistics unless the analysis is restricted to a representative subset of samples associated with generally favorable conditions.

scholarly journals Kelvin–Helmholtz Billows in the Eyewall of Hurricane Erin

2006 ◽  
Vol 134 (3) ◽  
pp. 1036-1038 ◽  
Author(s):  
Sim D. Aberson ◽  
Jeffrey B. Halverson
Keyword(s):  
Tropical Cyclone ◽  
High Altitude ◽  
Shear Instability ◽  
Vertical Shear ◽  
Vertically Aligned ◽  
Aircraft Pilots

Abstract A photograph of vertically aligned Kelvin–Helmholtz billows in the eastern eyewall of Hurricane Erin on 10 September 2001 is presented. The vertical shear instability in the horizontal winds necessary to produce the billows is confirmed with a high-altitude dropwindsonde observation. This shear instability is not known to be common in tropical cyclone eyewalls and is likely only in cases with a very large eyewall tilt. However, research and reconnaissance aircraft pilots need to be aware of the possibility of their existence, along with other types of hazardous conditions, in such rare circumstances.

scholarly journals Impact of environmental moisture on tropical cyclone intensification

2015 ◽  
Vol 15 (24) ◽  
pp. 14041-14053 ◽  
Author(s):  
L. Wu ◽  
H. Su ◽  
R. G. Fovell ◽  
T. J. Dunkerton ◽  
Z. Wang ◽  
...  
Keyword(s):  
Tropical Cyclone ◽  
Moisture Transport ◽  
Inner Core ◽  
Wrf Model ◽  
Weather Research And Forecasting ◽  
Convective Activity ◽  
Dry Air ◽  
Moving Storm ◽  
Lagrangian Boundary ◽  
Moisture Increase

Abstract. The impacts of environmental moisture on the intensification of a tropical cyclone (TC) are investigated in the Weather Research and Forecasting (WRF) model, with a focus on the azimuthal asymmetry of the moisture impacts relative to the storm path. A series of sensitivity experiments with varying moisture perturbations in the environment are conducted and the Marsupial Paradigm framework is employed to understand the different moisture impacts. We find that modification of environmental moisture has insignificant impacts on the storm in this case unless it leads to convective activity that deforms the quasi-Lagrangian boundary of the storm and changes the moisture transport into the storm. By facilitating convection and precipitation outside the storm, enhanced environmental moisture ahead of the northwestward-moving storm induces a dry air intrusion to the inner core and limits TC intensification. In contrast, increased moisture in the rear quadrants favors intensification by providing more moisture to the inner core and promoting storm symmetry, with primary contributions coming from moisture increase in the boundary layer. The different impacts of environmental moisture on TC intensification are governed by the relative locations of moisture perturbations and their interactions with the storm Lagrangian structure.

scholarly journals A Numerical Study of the Impacts of Dry Air on Tropical Cyclone Formation: A Development Case and a Nondevelopment Case

2013 ◽  
Vol 70 (1) ◽  
pp. 91-111 ◽  
Author(s):  
Cody Fritz ◽  
Zhuo Wang
Keyword(s):  
Numerical Model ◽  
Tropical Cyclone ◽  
Trajectory Analysis ◽  
Air Entrainment ◽  
Vertical Transport ◽  
Moist Convection ◽  
Upper Troposphere ◽  
Dry Air ◽  
Tropical Cyclone Formation ◽  
Budget Analysis

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.

scholarly journals Possible Linkage between the Monsoon Trough Variability and the Tropical Cyclone Activity over the Western North Pacific

2012 ◽  
Vol 140 (1) ◽  
pp. 140-150 ◽  
Author(s):  
Liang Wu ◽  
Zhiping Wen ◽  
Ronghui Huang ◽  
Renguang Wu
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Monsoon Trough ◽  
Vertical Shear ◽  
Mean Flow ◽  
Tropical Eastern Pacific ◽  
Barotropic Energy Conversion ◽  
Upper Level

Abstract The present study investigates the influence of the monsoon trough (MT) on the interannual variability of tropical cyclone (TC) activity over the western North Pacific during July–November for the period 1979–2007. It is shown that the TC activity is closely related to the MT location. During the years when the MT extends eastward (retreats westward), more (less) TCs form within the southeastern quadrant of the western North Pacific. Such a relationship can be explained by the changes in large-scale environmental factors associated with the movement of the MT. An eastward extension of the MT coincides with warmed ocean surface, enhanced convection, increased relative humidity in the lower and midtroposphere, reduced vertical shear of zonal wind, intensified upper-level divergence, and low-level anomalous cyclonic vorticity over the southeast quadrant of the western North Pacific. These conditions associated with the eastern extension of the MT are favorable for TC genesis, while those associated with the westward retreat of the MT are not. Diagnosis of the barotropic energy conversion indicates that synoptic-scale disturbances moving westward from tropical eastern Pacific will gain the energy from the mean flow when they meet with the eastward-extending MT. This is an important reason for the linkage between MT variability and TC genesis over the western North Pacific.

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