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David S. Nolan

Abstract

A number of studies in recent years have used wind fields derived from portable Doppler radars in combination with the ground-based velocity track display (GBVTD) technique to diagnose the primary (tangential) and secondary (radial and vertical) circulations in tornadoes. These analyses indicate very strong vertical motions in the vortex core, in some cases with updrafts and downdrafts exceeding 100 m s−1. In addition, many of the analyses indicate strong radial outflow at low levels and in the vicinity of the low-level tangential wind maximum. This paper shows that strong outward motion at this location cannot be consistent with a tornado circulation that lasts more than a few minutes. In addition, using data from numerical simulations as truth, it is shown that using observed radial velocities to diagnose vertical velocities greatly overestimates the intensity of downward motion in the core for two reasons: neglect of the mass flux into the core through the swirling boundary layer, and the likely positive bias in low-level radial velocities due to the centrifuging of debris. Possible methods for accounting for these errors are briefly discussed.

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David S. Nolan

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A new approach is presented for the nondimensionalization of the Navier–Stokes equations for tornado-like vortices. This scaling is based on the results of recent numerical simulations and physical reasoning. The method clarifies and unifies the results of numerous earlier studies that used numerical simulations of axisymmetric incompressible flow to study tornadoes. Some examples are presented.

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David S. Nolan

Abstract

A recent study showed observational and numerical evidence for small-scale gravity waves that radiate outward from tropical cyclones. These waves are wrapped into tight spirals by the radial and vertical shears of the tangential wind field. Reexamination of the previously studied tropical cyclone simulations suggests that the dominant source for these waves are convective asymmetries rotating along the eyewall, modulated in intensity by the preferred convection region on the left side of the environmental wind shear vector. A linearized, nonhydrostatic model for perturbations to a balanced vortex is used to study the waves. Forcing the linear model with rotating and pulsing asymmetric heat sources generates radiating gravity waves with multiple vertical and horizontal structures. The pulsation of the rotating heat source generates two types of waves: fast, deep waves with larger radial wavelengths, and slower, secondary waves with shorter radial and vertical wavelengths. The deeper waves produce surface pressure oscillations that have time scales consistent with surface observations, whereas the shorter waves have little surface indication but produce oscillations in vertical velocity with shorter radial wavelengths that are consistent with aircraft observations. Convective forcing that is either not pulsing or not rotating produces gravity waves but they are not as similar to the observed or simulated waves. The effects of varying the intensity of the cyclone, the asymmetry of the forcing, and the static stability of the surrounding atmosphere are explored.

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James Hlywiak and David S. Nolan

Abstract

The sensitivity of the inland wind decay to realistic inland surface roughness lengths and soil moisture contents is evaluated for strong, idealized tropical cyclones (TCs) of category 4 strength making landfall. Results show that the relative sensitivities to roughness and moisture differ throughout the decay process, and are dependent on the strength and size of the vortex. First, within 12 h of landfall, intense winds at the surface decay rapidly in reaction to the sudden change in surface roughness and decreasing enthalpy fluxes. Wind speeds above the boundary layer decay at a slower rate. Differences in soil moisture contents minimally affects intensity during the first 12 h, as the enhancement of latent heat fluxes from high moisture contents is countered by enhanced surface cooling. After TCs decay to tropical storm intensities, weakening slows and the sensitivity of the intensity decay to soil moisture increases. Increased latent heating becomes significant enough to combat surface temperature cooling, resulting in enhanced convection outside of the expanding radius of maximum winds. This supports a slower decay. Additionally, the decay of the radial wind profile by quadrant is highly asymmetric, as the rear and left-of-motion quadrants decay the fastest. Increasing surface roughness accelerates the decay of the strongest winds, while increasing soil moisture slows the decay of the larger TC wind field. Results have implications for inland forecasting of TC winds and understanding the potential for damages.

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Yumin Moon and David S. Nolan

Abstract

This is the second part of a study that examines spiral rainbands in a numerical simulation of Hurricane Bill (2009). This paper evaluates whether the propagation of inner rainbands in the Hurricane Bill simulation is consistent with previously proposed hypotheses. Results indicate that the propagation of inner rainbands is not consistent with gravity waves, vortex Rossby waves, or squall lines. An alternative hypothesis is offered, arguing that inner rainbands are simply convective clouds that are advected by the rapidly rotating tropical cyclone wind field while being deformed into spiral shapes. A summary and a discussion of the results of both Parts I and II are provided.

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James Hlywiak and David S. Nolan

Abstract

The connection relating upper-ocean salinity stratification in the form of oceanic barrier layers to tropical cyclone (TC) intensification is investigated in this study. Previous works disagree on whether ocean salinity is a negligible factor on TC intensification. Relationships derived in many of these studies are based on observations, which can be sparse or incomplete, or uncoupled models, which neglect air–sea feedbacks. Here, idealized ensemble simulations of TCs performed using the Weather Research and Forecasting (WRF) Model coupled to the 3D Price–Weller–Pinkel (PWP) ocean model facilitate examination of the TC–upper-ocean system in a controlled, high-resolution, mesoscale environment. Idealized vertical ocean profiles are modeled after barrier layer profiles of the Amazon–Orinoco river plume region, where barrier layers are defined as vertical salinity gradients between the mixed and isothermal layer depths. Our results reveal that for TCs of category 1 hurricane strength or greater, thick (24–30 m) barrier layers may favor further intensification by 6%–15% when averaging across ensemble members. Conversely, weaker cyclones are hindered by thick barrier layers. Reduced sea surface temperature cooling below the TC inner core is the primary reason for additional intensification. Sensitivity tests of the results to storm translation speed, initial oceanic mixed layer temperature, and atmospheric vertical wind shear provide a more comprehensive analysis. Last, it is shown that the ensemble mean intensity results are similar when using a 3D or 1D version of PWP.

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Daniel Hodyss and David S. Nolan

Abstract

A linear anelastic-vortex model is derived using assumptions appropriate to waves on vortices with scales similar to tropical cyclones. The equation set is derived through application of a multiple-scaling technique, such that the radial variations of the thermodynamic fields are incorporated into the reference state. The primary assumption required for the model is that the horizontal variations in the thermodynamic variables describing the reference state are appreciably longer than the waves on the vortex. This new version of the anelastic system makes no approximation to the requirements for hydrostatic and gradient wind balance, or the buoyancy frequency, in the core of the vortex. A small but measurable improvement in the performance of the new equation set is demonstrated through simulations of gravity waves and vortex–Rossby waves in a baroclinic vortex.

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Yumin Moon and David S. Nolan

Abstract

This study examines spiral rainbands in a numerical simulation of Hurricane Bill (2009). This paper, the first part of the study, evaluates the structures of spiral rainbands and compares them to previous observations. Four types of spiral rainbands are identified: principal, secondary, distant, and inner rainbands. Principal rainbands tend to be stationary relative to the storm center, while secondary rainbands are more transient and move around the storm center. Both principal and secondary rainbands are tilted radially outward with height and have many of the commonly observed kinematic features, such as overturning secondary circulation and enhanced tangential velocity on their radially outward sides. Principal rainbands are bounded by very dry air on their radially outward sides. Distant rainbands are radially inward-tilting convective features that have dense cold pools near the surface. Inner rainbands are made of shallow convection that appears to have originated from near the eyewall. Differences in the structures of spiral rainbands between observations and the Hurricane Bill simulation are noted. The second part of the study investigates how inner rainbands propagate and makes comparison with previously proposed hypotheses such as vortex Rossby waves.

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Yumin Moon and David S. Nolan

Abstract

Previous studies have suggested that gravity waves can transport a significantly large amount of angular momentum away from tropical cyclones, as much as 10% of the core angular momentum per hour. These previous studies used the shallow-water equations to model gravity waves radiating outward from rapidly rotating inner-core asymmetries. This issue is reinvestigated with a three-dimensional, nonhydrostatic, linear model of the vortex-anelastic equations. The response of balanced, axisymmetric vortices modeled after tropical cyclones to rotating asymmetric heat sources is examined to assess angular momentum transport by gravity waves radiating away from the core region of the vortices. Calculations show that gravity waves do transport angular momentum away from the vortex core; however, the amount transported is several orders of magnitude smaller than recent estimates.

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Yoshiaki Miyamoto and David S. Nolan

Abstract

Structural changes that precede rapid intensification (RI) of tropical cyclones (TCs) are examined in a full-physics model by conducting a large ensemble (270) of idealized TC simulations. The processes leading to RI in a representative case with moderate shear are consistent with previous studies for weakly sheared cases. The most distinct changes are that the vortex tilt and the vortex size begin to decrease more rapidly 6 h before the onset of RI. A vorticity budget analysis for the upper layer around the low-level center reveals that the vertical vorticity is increased by vertical advection, stretching, and tilting terms before RI, whereas the horizontal advection is small. Thus, the upright vortex structure is not achieved through a vortex alignment process but rather is built upward by deep convection.

The ensemble simulations are generated by changing the intensity and size of the initial vortex, the magnitude of vertical wind shear, and the translation speed. The ensemble members that show RI are consistent with the control case and many previous studies: before the onset of RI, the intensity gradually increases, the radius of maximum tangential wind (RMW) decreases, the flow structure becomes more symmetric, the vortex tilt decreases, and the radius of maximum convergence approaches the radius of maximum winds. A dimensionless parameter representing a tendency for the formation of the vertically upright structure is considered. The product of this parameter and the local Rossby number is significantly larger for TCs that exhibit RI in the next 24 h.

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