skip to main content
Explore Scholarly Publications and Datasets in the NSF-PAR

  • NSF Public Access
  • Search Results
  • Accepted Manuscript: How Much Does the Upward Advection of the Supergradient Component of Boundary Layer Wind Contribute to Tropical Cyclone Intensification and Maximum Intensity?
Title: How Much Does the Upward Advection of the Supergradient Component of Boundary Layer Wind Contribute to Tropical Cyclone Intensification and Maximum Intensity?
Abstract Although the development of supergradient winds is well understood, the importance of supergradient winds in tropical cyclone (TC) intensification is still under debate. One view is that the spinup of the eyewall occurs by the upward advection of high tangential momentum associated with supergradient winds from the boundary layer. The other view argues that the upward advection of supergradient winds by eyewall updrafts results in an outward agradient force, leading to the formation of a shallow outflow layer immediately above the inflow boundary layer. As a result, the spinup of tangential wind in the eyewall by the upward advection of supergradient wind from the boundary layer is largely offset by the spindown of tangential wind due to the outflow resulting from the agradient force. In this study, the net contribution by the upward advection of the supergradient wind component from the boundary layer to the intensification rate and final intensity of a TC are quantified through ensemble sensitivity numerical experiments using an axisymmetric TC model. Results show that consistent with the second view above, the positive upward advection of the supergradient wind component from the boundary layer by eyewall updrafts is largely offset by the negative radial advection due more » to the outflow resulting from the outward agradient force. As a result, the upward advection of the supergradient wind component contributes little (often less than 4%) to the intensification rate and but it contributes about 10%–15% to the final intensity of the simulated TC due to the enhanced inner-core air–sea thermodynamic disequilibrium. « less
Authors:
; ;
Award ID(s):
1834300
Publication Date:
NSF-PAR ID:
10216228
Journal Name:
Journal of the Atmospheric Sciences
Volume:
77
Issue:
8
Page Range or eLocation-ID:
2649 to 2664
ISSN:
0022-4928
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ( , Journal of the Atmospheric Sciences)
    Abstract The existence of supergradient wind in the interior of the boundary layer is a distinct feature of a tropical cyclone (TC). Although the vertical advection is shown to enhance supergradient wind in TC boundary layer (TCBL), how and to what extent the strength and structure of supergradient wind are modulated by vertical advection are not well understood. In this study, both a TCBL model and an axisymmetric full-physics model are used to quantify the contribution of vertical advection process to the strength and vertical structure of supergradient wind in TCBL. Results from the TCBL model show that the removal of vertical advection of radial wind reduces both the strength and height of supergradient wind by slightly more than 50%. The removal of vertical advection of agradient wind reduces the height of the supergradient wind core by ~30% but increases the strength of supergradient wind by ~10%. Results from the full-physics model show that the removal of vertical advection of radial wind or agradient wind reduces both the strength and height of supergradient wind but the removal of that of radial wind produces a more substantial reduction (52%) than the removal of that of agradient wind (35%). However, both themore »intensification rate and final intensity of the simulated TCs in terms of maximum 10-m wind speed show little differences in experiments with and without the vertical advection of radial or agradient wind, suggesting that supergradient wind contributes little to either the intensification rate or the steady-state intensity of the simulated TC.« less
  2. ; ( , Journal of the Atmospheric Sciences)
    Abstract

    The formation of a plausible secondary eyewall is examined with two principal simulation experiments that differ only in the fixed value of rain fall speed, one with a value of 70 m s−1(approaching the pseudo-adiabatic limit) that simulates a secondary eyewall, and one with a value of 7 m s−1that does not simulate a secondary eyewall. Key differences are sought between these idealized three-dimensional simulations. A notable expansion of the lower-tropospheric tangential wind field to approximately 400-km radius is found associated with the precursor period of the secondary eyewall. The wind field expansion is traced to an enhanced vertical mass flux across the 5.25-km height level, which leads, in turn, to enhanced radial inflow in the lower troposphere and above the boundary layer. The inflow spins up the tangential wind outside the primary eyewall via the conventional spinup mechanism. This amplified tangential wind field is linked to a broad region of outwardly directed agradient force in the upper boundary layer. Whereas scattered convection is found outside the primary eyewall in both simulations, the agradient force is shown to promote a ring-like organization of this convection when boundary layer convergence occurs in a persistent, localized region of supergradient winds. Themore »results support prior work highlighting a new model of secondary eyewall formation emphasizing a boundary layer control pathway for initiating the outer eyewall as part of the rotating convection paradigm of tropical cyclone evolution.

    « less
  3. ; ; ; ( , Journal of the Atmospheric Sciences)
    Abstract The dynamics of an asymmetric rainband complex leading into secondary eyewall formation (SEF) are examined in a simulation of Hurricane Matthew (2016), with particular focus on the tangential wind field evolution. Prior to SEF, the storm experiences an axisymmetric broadening of the tangential wind field as a stationary rainband complex in the downshear quadrants intensifies. The axisymmetric acceleration pattern that causes this broadening is an inward-descending structure of positive acceleration nearly 100 km wide in radial extent and maximizes in the low levels near 50 km radius. Vertical advection from convective updrafts in the downshear-right quadrant largely contributes to the low-level acceleration maximum, while the broader inward-descending pattern is due to horizontal advection within stratiform precipitation in the downshear-left quadrant. This broad slantwise pattern of positive acceleration is due to a mesoscale descending inflow (MDI) that is driven by midlevel cooling within the stratiform regions and draws absolute angular momentum inward. The MDI is further revealed by examining the irrotational component of the radial velocity, which shows the MDI extending downwind into the upshear-left quadrant. Here, the MDI connects with the boundary layer, where new convective updrafts are triggered along its inner edge; these new upshear-left updrafts are foundmore »to be important to the subsequent axisymmetrization of the low-level tangential wind maximum within the incipient secondary eyewall.« less
  4. ; ; ; ( , Journal of the Atmospheric Sciences)
    Abstract The thermodynamic effect of downdrafts on the boundary layer and nearby updrafts are explored in idealized simulations of category-3 and category-5 tropical cyclones (Ideal3 and Ideal5). In Ideal5, downdrafts underneath the eyewall pose no negative thermodynamic influence because of eye-eyewall mixing below 2-km altitude. Additionally, a layer of higher θ e between 1 and 2 km altitude associated with low-level outflow that extends 40 km outward from the eyewall region creates a “thermodynamic shield” that prevents negative effects from downdrafts. In Ideal3, parcel trajectories from downdrafts directly underneath the eyewall reveal that low-θ e air initially moves radially inward allowing for some recovery in the eye, but still enters eyewall updrafts with a mean θ e deficit of 5.2 K. Parcels originating in low-level downdrafts often stay below 400 m for over an hour and increase their θ e by 10-14 K, showing that air-sea enthalpy fluxes cause sufficient energetic recovery. The most thermodynamically unfavorable downdrafts occur ~5 km radially outward from an updraft and transport low-θ e mid-tropospheric air towards the inflow layer. Here, the low-θ e air entrains into the updraft in less than five minutes with a mean θ e deficit of 8.2 K. In general,more »θ e recovery is a function of minimum parcel altitude such that downdrafts with the most negative influence are those entrained into the top of the inflow layer. With both simulated TCs exposed to environmental vertical wind shear, this study underscores that storm structure and individual downdraft characteristics must be considered when discussing paradigms for TC intensity evolution.« less
  5. ; ( , Monthly Weather Review)
    Abstract

    The evolution of the tropical cyclone boundary layer (TCBL) wind field before landfall is examined in this study. As noted in previous studies, a typical TCBL wind structure over the ocean features a supergradient boundary layer jet to the left of motion and Earth-relative maximum winds to the right. However, the detailed response of the wind field to frictional convergence at the coastline is less well known. Here, idealized numerical simulations reveal an increase in the offshore radial and vertical velocities beginning once the TC is roughly 200 km offshore. This increase in the radial velocity is attributed to the sudden decrease in frictional stress once the highly agradient flow crosses the offshore coastline. Enhanced advection of angular momentum by the secondary circulation forces a strengthening of the supergradient jet near the top of the TCBL. Sensitivity experiments reveal that the coastal roughness discontinuity dominates the friction asymmetry due to motion. Additionally, increasing the inland roughness through increasing the aerodynamic roughness length enhances the observed asymmetries. Last, a brief analysis of in situ surface wind data collected during the landfall of three Gulf of Mexico hurricanes is provided and compared to the idealized simulations. Despite the limited in situmore »data, the observations generally support the simulations. The results here imply that assumptions about the TCBL wind field based on observations from over horizontally homogeneous surface types—which have been well documented by previous studies—are inappropriate for use near strong frictional heterogeneity.

    « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email