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Jun A. Zhang

Abstract

Data collected in the low-level atmospheric boundary layer in five hurricanes by NOAA research aircraft are analyzed to measure turbulence with scales small enough to retrieve the rate of dissipation. A total of 49 flux runs suitable for analysis are identified in the atmospheric boundary layer within 200 m above the sea surface. Momentum fluxes are directly determined using the eddy correlation method, and drag coefficients are also calculated. The dissipative heating is estimated using two different methods: 1) integrating the rate of dissipation in the surface layer and 2) multiplying the drag coefficient by the cube of surface wind speed. While the latter method has been widely used in theoretical models as well as several numerical models simulating hurricanes, these analyses show that using this method would significantly overestimate the magnitude of dissipative heating. Although the dataset used in this study is limited by the surface wind speed range <30 m s−1, this work highlights that it is crucial to understand the physical processes related to dissipative heating in the hurricane boundary layer for implementing it into hurricane models.

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Jun A. Zhang and Michael T. Montgomery

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This study examines further the characteristics of turbulent flow in the low-level region of intense hurricanes using in situ aircraft observations. The data analyzed here are the flight-level data collected by research aircraft that penetrated the eyewalls of category-5 Hurricane Hugo (1989), category-4 Hurricane Allen (1980), and category-5 Hurricane David (1979) between 1 km and the sea surface. Estimates of horizontal eddy momentum flux, horizontal eddy diffusivity, and horizontal mixing length are obtained. It is found that the horizontal momentum flux and horizontal diffusivity increase with increasing wind speed. The horizontal mixing length increases slightly with wind speed also, but the mixing length is not significantly dependent on the wind speed. The magnitude of the horizontal momentum flux is found to be comparable to that of the vertical momentum flux, indicating that horizontal mixing by turbulence becomes nonnegligible in the hurricane boundary layer, especially in the eyewall region.

Within the context of simple K theory, the results suggest that the average horizontal eddy diffusivity and mixing length are approximately 1500 m2 s−1 and 750 m, respectively, at about 500 m in the eyewall region corresponding to the mean wind speed of approximately 52 m s−1. It is recalled also that the mixing length is a virtual scale in numerical models and is quantitatively smaller than the energy-containing scale of turbulent eddies. The distinction between these two scales is a useful reminder for the modeling community on the representation of small-scale turbulence in hurricanes.

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Jun A. Zhang and Frank D. Marks

Abstract

This study examines the effects of horizontal diffusion on tropical cyclone (TC) intensity change and structure using idealized simulations of the Hurricane Weather Research and Forecasting Model (HWRF). A series of sensitivity experiments were conducted with varying horizontal mixing lengths (L h), but kept the vertical diffusion coefficient and other physical parameterizations unchanged. The results show that both simulated maximum intensity and intensity change are sensitive to the L h used in the parameterization of the horizontal turbulent flux, in particular, for L h less than the model’s horizontal resolution. The results also show that simulated storm structures such as storm size, kinematic boundary layer height, and eyewall slope are sensitive to L h as well. However, L h has little impact on the magnitude of the surface inflow angle and thermodynamic mixed layer height. Angular momentum budget analyses indicate that the effect of L h is to mainly spin down a TC vortex. Both mean and eddy advection terms in the angular momentum budget are affected by the magnitude of L h. For smaller L h, the convergence of angular momentum is larger in the boundary layer, which leads to a faster spinup of the vortex. The resolved eddy advection of angular momentum plays an important role in the spinup of the low-level vortex inward from the radius of the maximum wind speed when L h is small.

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Guang Jun Zhang and Norman A. McFarlane

Abstract

The upper-air sounding data from PRE-STORM are used to investigate the convective stabilization effect on the large-scale atmosphere. To facilitate comparison between different stages of cumulus convection, the data are divided into four categories: environment, presystem, insystem, and postsystem. It is found that the convective available potential energy of the atmosphere is reduced substantially after cumulus convection, most of which is consumed during the transition from presystem to insystem. Examination of the temperature and moisture changes during cumulus convection suggests that cooling and drying in the subcloud layer are the most important factors in stabilizing the atmosphere. In general, virtual potential temperature profiles in all categories are close to reversible moist adiabats below the 600-mb level and nearly parallel to moist pseudoadiabats above it.

The effect of entrainment on parcel buoyancy is also studied. It is found that a small amount of entrainment of ambient air can lead to a pronounced decrease of parcel buoyancy. Furthermore, for diluted parcel ascent, the convective available potential energy is greater for the insystem category than for the postsystem one, whereas the opposite is true for undiluted parcel ascent.

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Jun A. Zhang and Robert F. Rogers

Abstract

This study investigates the role of the parameterized boundary layer structure in hurricane intensity change using two retrospective HWRF forecasts of Hurricane Earl (2010) in which the vertical eddy diffusivity K m was modified during physics upgrades. Earl undergoes rapid intensification (RI) in the low-Km forecast as observed in nature, while it weakens briefly before resuming a slow intensification at the RI onset in the high-Km forecast. Angular momentum budget analysis suggests that K m modulates the convergence of angular momentum in the boundary layer, which is a key component of the hurricane spinup dynamics. Reducing K m in the boundary layer causes enhancement of both the inflow and convergence, which in turn leads to stronger and more symmetric deep convection in the low-Km forecast than in the high-Km forecast. The deeper and stronger hurricane vortex with lower static stability in the low-Km forecast is more resilient to shear than that in the high-Km forecast. With a smaller vortex tilt in the low-Km forecast, downdrafts associated with the vortex tilt are reduced, bringing less low-entropy air from the midlevels to the boundary layer, resulting in a less stable boundary layer. Future physics upgrades in operational hurricane models should consider this chain of multiscale interactions to assess their impact on model RI forecasts.

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Jun A. Zhang and William M. Drennan

Abstract

Although vertical eddy diffusivity or viscosity has been extensively used in theoretical and numerical models simulating tropical cyclones, little observational study has documented the magnitude of the eddy diffusivity in high-wind conditions (>20 m s−1) until now. Through analyzing in situ aircraft data that were collected in the atmospheric boundary layer of four intense hurricanes, this study provides the first estimates of vertical distributions of the vertical eddy diffusivities for momentum, sensible heat, and latent heat fluxes in the surface wind speed range between 18 and 30 m s−1. In this work, eddy diffusivity is determined from directly measured turbulent fluxes and vertical gradients of the mean variable, such as wind speed, temperature, and humidity. The analyses show that the magnitudes of vertical eddy diffusivities for momentum and latent heat fluxes are comparable to each other, but the eddy diffusivity for sensible heat flux is much smaller than that for the latent heat flux. The vertical distributions of the eddy diffusivities are generally alike, increasing from the surface to a maximum value within the thermodynamic mixed layer and then deceasing with height. The results indicate also that momentum and latent heat are mainly transferred downgradient of the mean flow and that countergradient transport of the sensible heat may exist. The observational estimates are compared with the eddy diffusivities derived from different methods as used in planetary boundary layer (PBL) parameterization schemes in numerical models as well as ones used in previous observational studies.

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Jun A. Zhang and Eric W. Uhlhorn

Abstract

This study presents an analysis of near-surface (10 m) inflow angles using wind vector data from over 1600 quality-controlled global positioning system dropwindsondes deployed by aircraft on 187 flights into 18 hurricanes. The mean inflow angle in hurricanes is found to be −22.6° ± 2.2° (95% confidence). Composite analysis results indicate little dependence of storm-relative axisymmetric inflow angle on local surface wind speed, and a weak but statistically significant dependence on the radial distance from the storm center. A small, but statistically significant dependence of the axisymmetric inflow angle on storm intensity is also found, especially well outside the eyewall. By compositing observations according to radial and azimuthal location relative to storm motion direction, significant inflow angle asymmetries are found to depend on storm motion speed, although a large amount of unexplained variability remains. Generally, the largest storm-relative inflow angles (<−50°) are found in the fastest-moving storms (>8 m s−1) at large radii (>8 times the radius of maximum wind) in the right-front storm quadrant, while the smallest inflow angles (>−10°) are found in the fastest-moving storms in the left-rear quadrant. Based on these observations, a parametric model of low-wavenumber inflow angle variability as a function of radius, azimuth, storm intensity, and motion speed is developed. This model can be applied for purposes of ocean surface remote sensing studies when wind direction is either unknown or ambiguous, for forcing storm surge, surface wave, and ocean circulation models that require a parametric surface wind vector field, and evaluating surface wind field structure in numerical models of tropical cyclones.

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Sim D. Aberson, Jun A. Zhang, and Kelly Nuñez Ocasio

Abstract

During a routine penetration into Hurricane Felix late on 2 September 2007, NOAA42 encountered extreme turbulence and graupel, flight-level horizontal wind gusts of over 83 m s−1, and vertical wind speeds varying from 10 m s−1 downward to 31 m s−1 upward and back to nearly 7 m s−1 downward within 1 min. This led the plane to rise nearly 300 m and then return to its original level within that time. Though a dropwindsonde was released during this event, the radars and data systems on board the aircraft were rendered inoperable, limiting the amount of data obtained.

The feature observed during the flight is shown to be similar to that encountered during flights into Hurricanes Hugo (1989) and Patricia (2015), and by a dropwindsonde released into a misovortex in Hurricane Isabel (2003). This paper describes a unique dataset of a small-scale feature that appears to be prevalent in very intense tropical cyclones, providing new evidence for eye–eyewall mixing processes that may be related to intensity change.

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John Molinari, Jun A. Zhang, Robert F. Rogers, and David Vollaro

Abstract

Hurricane Frances (2004) represented an unusual event that produced three consecutive overlapping eyewall replacement cycles (ERCs). Their evolution followed some aspects of the typical ERC. The strong primary eyewalls contracted and outward-sloping secondary eyewalls formed near 3 times the radius of maximum winds. Over time these secondary eyewalls shifted inward, became more upright, and replaced the primary eyewalls. In other aspects, however, the ERCs in Hurricane Frances differed from previously described composites. The outer eyewall wind maxima became stronger than the inner in only 12 h, versus 25 h for average ERCs. More than 15 m s−1 outflow peaked in the upper troposphere during each ERC. Relative vorticity maxima peaked at the surface but extended to mid- and upper levels. Mean 200-hPa zonal velocity was often from the east, whereas ERC environments typically have zonal flow from the west. These easterlies were produced by an intense upper anticyclone slightly displaced from the center and present throughout the period of multiple ERCs. Inertial stability was low at almost all azimuths at 175 hPa near the 500-km radius during the period of interest. It is hypothesized that the reduced resistance to outflow associated with low inertial stability aloft induced deep upward motion and rapid intensification of the secondary eyewalls. The annular hurricane index of Knaff et al. showed that Hurricane Frances met all the criteria for annular hurricanes, which make up only 4% of all storms. It is argued that the annular hurricane directly resulted from the repeated ERCs following Wang’s reasoning.

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Ping Zhu, Jun A. Zhang, and Forrest J. Masters

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Using wavelet transform (WT), this study analyzes the surface wind data collected by the portable wind towers during the landfalls of six hurricanes and one tropical storm in the 2002–04 seasons. The WT, which decomposes a time series onto the scale-time domain, provides a means to investigate the role of turbulent eddies in the vertical transport in the unsteady, inhomogeneous hurricane surface layer. The normalized WT power spectra (NWPS) show that the hurricane boundary layer roll vortices tend to suppress the eddy circulations immediately adjacent to rolls, but they do not appear to have a substantial effect on eddies smaller than 100 m. For low-wind conditions with surface wind speeds less than 10 m s−1, the contributions of small eddies (<236 m) to the surface wind stress and turbulent kinetic energy (TKE) decrease with the increase of wind speed. The opposite variation trend is found for eddies greater than 236 m. However, for wind speeds greater than 10 m s−1, contributions of both small and large eddies tend to level off as wind speeds keep increasing. It is also found that the scale of the peak NWPS of the surface wind stress is nearly constant with a mean value of approximately 86 m, whereas the scale of the peak NWPS of TKE generally increases with the increase of wind speed, suggesting the different roles of eddies in generating fluxes and TKE. This study illustrates the unique characteristics of the surface layer turbulent structures during hurricane landfalls. It is hoped that the findings of this study could enlighten the development and improvement of turbulent mixing schemes so that the vertical transport processes in the hurricane surface layer can be appropriately parameterized in forecasting models.

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