skip to main content

Title: Development of a Misaligned Tropical Cyclone

A cloud-resolving model is used to examine the virtually shear-free evolution of incipient tropical cyclones initialized with different degrees of misalignment between the lower- and middle-tropospheric centers of rotation. Increasing the initial displacement of rotational centers (the tilt) from a negligible value to several hundred kilometers extends the time scale of hurricane formation from 1 to 10 days. Hindered amplification of the maximum tangential velocity υm at the surface of a strongly perturbed system is related to an extended duration of misalignment resulting from incomplete early decay and subsequent transient growth of the tilt magnitude. The prolonged misalignment coincides with a prolonged period of asymmetric convection peaked far from the surface center of the vortex. A Sawyer–Eliassen model is used to analyze the disparity between azimuthal velocity tendencies of selected pre–tropical storm vortices with low and high degrees of misalignment. Although no single factor completely explains the difference of intensification rates, greater misalignment is linked to weaker positive azimuthal velocity forcing near υm by the component of the mean secondary circulation attributed to heating by microphysical cloud processes. Of note regarding the dynamics, enhanced tilt only modestly affects the growth rate of kinetic energy outside the core of the more » surface vortex while severely hindering intensification of υm within the core for at least several days. The processes controlling the evolution of the misalignment associated with inefficient development are examined in detail for a selected simulation. It is found that adiabatic mechanisms are capable of driving the transient amplification of tilt, whereas diabatic processes are essential to ultimate alignment of the tropical cyclone.

« less
Award ID(s):
Publication Date:
Journal Name:
Journal of the Atmospheric Sciences
Page Range or eLocation-ID:
p. 79-111
American Meteorological Society
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    A cloud-resolving model is used to examine the intensification of tilted tropical cyclones from depression to hurricane strength over relatively cool and warm oceans under idealized conditions where environmental vertical wind shear has become minimal. Variation of the SST does not substantially change the time-averaged relationship between tilt and the radial length scale of the inner core, or between tilt and the azimuthal distribution of precipitation during the hurricane formation period (HFP). By contrast, for systems having similar structural parameters, the HFP lengthens superlinearly in association with a decline of the precipitation rate as the SST decreases from 30° to 26°C. In many simulations, hurricane formation progresses from a phase of slow or neutral intensification to fast spinup. The transition to fast spinup occurs after the magnitudes of tilt and convective asymmetry drop below certain SST-dependent levels following an alignment process explained in an earlier paper. For reasons examined herein, the alignment coincides with enhancements of lower–middle-tropospheric relative humidity and lower-tropospheric CAPE inward of the radius of maximum surface wind speedrm. Such moist-thermodynamic modifications appear to facilitate initiation of the faster mode of intensification, which involves contraction ofrmand the characteristic radius of deep convection. The mean transitional values ofmore »the tilt magnitude and lower–middle-tropospheric relative humidity for SSTs of 28°–30°C are respectively higher and lower than their counterparts at 26°C. Greater magnitudes of the surface enthalpy flux and core deep-layer CAPE found at the higher SSTs plausibly compensate for less complete alignment and core humidification at the transition time.

    « less
  2. The FV3GFS is the current operational Global Forecast System (GFS) at the National Centers for Environmental Prediction (NCEP), which combines a finite-volume cubed sphere dynamical core (FV3) and GFS physics. In this study, FV3GFS is used to gain understanding of rapid intensification (RI) of tropical cyclones (TCs) in shear. The analysis demonstrates the importance of TC structure in a complex system like Hurricane Michael, which intensified to a category 5 hurricane over the Gulf of Mexico despite over 20 kt (10 m s−1) of vertical wind shear. Michael’s RI is examined using a global-nest FV3GFS ensemble with the nest at 3-km resolution. The ensemble shows a range of peak intensities from 77 to 159 kt (40–82 m s−1). Precipitation symmetry, vortex tilt, moisture, and other aspects of Michael’s evolution are compared through composites of stronger and weaker members. The 850–200-hPa vertical shear is 22 kt (11 m s−1) in the mean of both strong and weak members during the early stage. Tilt and moisture are two distinguishing factors between strong and weak members. The relationship between vortex tilt and humidification is complex, and other studies have shown both are important for sheared intensification. Here, it is shown that tilt reductionmore »leads to upshear humidification and is thus a driving factor for intensification. A stronger initial vortex and early evolution of the vortex also appear to be the key to members that are able to resist the sheared environment.

    « less
  3. Abstract

    Idealized numerical simulations of weak tropical cyclones (e.g., tropical depressions and tropical storms) in sheared environments indicate that vortex tilt reduction and convective symmetrization are key structural changes that can precede intensification. Through a series of ensembles of idealized numerical simulations, this study demonstrates that including radiation in the simulations affects the timing and variability of those structural changes. The underlying reason for those effects is a background thermodynamic profile with reduced energy available to fuel strong downdrafts; such a profile leads to weaker lower-tropospheric ventilation, greater azimuthal coverage of clouds and precipitation, and smaller vortex tilt with radiation. Consequently, the simulations with radiation allow for earlier intensification at stronger shear magnitudes than without radiation. An unexpected finding from this work is a reduction of both vortex tilt and intensity variability with radiation in environments with 5 m s−1 deep-layer shear. This reduction stems from reduced variability in nonlinear feedbacks between lower-tropospheric ventilation, cold pools, convection, and vortex tilt. Sensitivity experiments confirm the relationship between those processes and suggest that microphysical processes (e.g., rain evaporation) are major sources of uncertainty in the representation of weak, sheared tropical cyclones in numerical weather prediction models.

  4. Abstract. The fundamental mechanism underlying tropical cyclone (TC) intensification may be understood from the conservation of absolute angular momentum, where the primary circulation of a TC is driven by the torque acting on air parcels resulting from asymmetric eddy processes, including turbulence. While turbulence is commonly regarded as a flow feature pertaining to the planetary boundary layer (PBL), intense turbulent mixing generated by cloud processes also exists above the PBL in the eyewall and rainbands. Unlike the eddy forcing within the PBL that is negative definite, the sign of eyewall/rainband eddy forcing above the PBL is indefinite and thus provides a possible mechanism to spin up a TC vortex. In this study, we show that the Hurricane Weather Research & forecasting (HWRF) model, one of the operational models used for TC prediction, is unable to generate appropriate sub-grid-scale (SGS) eddy forcing above the PBL due to lack of consideration of intense turbulent mixing generated by the eyewall and rainband clouds. Incorporating an in-cloud turbulent mixing parameterization in the PBL scheme notably improves HWRF's skills on predicting rapid changes in intensity for several past major hurricanes. While the analyses show that the SGS eddy forcing above the PBL is only aboutmore »one-fifth of the model-resolved eddy forcing, the simulated TC vortex inner-core structure and the associated model-resolved eddy forcing exhibit a substantial dependence on the parameterized SGS eddy processes. The results highlight the importance of eyewall/rainband SGS eddy forcing to numerical prediction of TC intensification, including rapid intensification at the current resolution of operational models.

    « less
  5. Numerous field observations of tsunami-induced eddies in ports and harbours have been reported for recent tsunami events. We examine the evolution of a turbulent shallow-water monopolar vortex generated by a long wave through a series of large-scale experiments in a rectangular wave basin. A leading-elevation asymmetric wave is guided through a narrow channel to form a flow separation region on the lee side of a straight vertical breakwater, which coupled with the transient flow leads to the formation of a monopolar turbulent coherent structure (TCS). The vortex flow after detachment from the trailing jet is fully turbulent ( $Re_h \sim O(10^{4}\text {--}10^{5}$ )) for the remainder of the experimental duration. The free surface velocity field was extracted through particle tracking velocimetry over several experimental trials. The first-order model proposed by Seol & Jirka ( J. Fluid Mech. , vol. 665, 2010, pp. 274–299) to predict the decay and spatial growth of shallow-water vortices fits the experimental data well. Bottom friction is predicted to induce a $t^{-1}$ azimuthal velocity decay and turbulent viscous diffusion results in a $\sqrt {t}$ bulk vortex radial growth, where $t$ represents time. The azimuthal velocity, vorticity and free surface elevation profiles are well described through anmore »idealised geophysical vortex. Kinematic free surface boundary conditions predict weak upwelling in the TCS-centre, followed by a zone of downwelling in a recirculation pattern along the water column. The vertical confinement of the flow is quantified through the ratio of kinetic energy contained in the secondary and primary surface velocity fields and a transition point towards a quasi-two-dimensional flow is identified.« less