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Sylvie Lorsolo and Altuğ Aksoy

Abstract

Performing wavenumber decomposition on azimuthally distributed data such as those in tropical cyclones can be challenging when data gaps exist in the signal. In the literature, ad hoc approaches are found to determine maximum gap size beyond which not to perform Fourier decomposition. The goal of the present study is to provide a more objective and systematic method to choose the maximum gap size allowed to perform a Fourier analysis on observational data. A Monte Carlo–type experiment is conducted where signals of various wavenumber configurations are generated with gaps of varying size, then a simple interpolation scheme is applied and Fourier decomposition is performed. The wavenumber decomposition is evaluated in a way that requires retrieval of at least 80% of the original amplitude with less than 20° phase shift. Maximum allowable gap size is then retrieved for wavenumbers 0–2. When prior assessment of signal configuration is available, the authors believe that the present study can provide valuable guidance for gap size beyond which Fourier decomposition is not advisable.

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Paul D. Reasor, Robert Rogers, and Sylvie Lorsolo

Abstract

Following a recent demonstration of multicase compositing of axisymmetric tropical cyclone (TC) structure derived from airborne Doppler radar measurements, the authors extend the analysis to the asymmetric structure using an unprecedented database from 75 TC flights. In particular, they examine the precipitation and kinematic asymmetry forced by the TC's motion and interaction with vertical wind shear. For the first time they quantify the average magnitude and phase of the three-dimensional shear-relative kinematic asymmetry of observed TCs through a composite approach. The composite analysis confirms principal features of the shear-relative TC asymmetry documented in prior numerical and observational studies (e.g., downshear tilt, downshear-right convective initiation, and a downshear-left precipitation maximum). The statistical significance of the composite shear-relative structure is demonstrated through a stratification of cases by shear magnitude. The impact of storm motion on eyewall convective asymmetry appears to be secondary to the much greater constraint placed by vertical wind shear on the organization of convection, in agreement with prior studies using lightning and precipitation data.

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Sylvie Lorsolo, John Gamache, and Altug Aksoy

Abstract

The Hurricane Research Division Doppler radar analysis software provides three-dimensional analyses of the three wind components in tropical cyclones. Although this software has been used for over a decade, there has never been a complete and in-depth evaluation of the resulting analyses. The goal here is to provide an evaluation that will permit the best use of the analyses, but also to improve the software. To evaluate the software, analyses are produced from simulated radar data acquired from an output of a Hurricane Weather Research and Forecasting (HWRF) model nature run and are compared against the model “truth” wind fields. Comparisons of the three components of the wind show that the software provides analyses of good quality. The tangential wind is best retrieved, exhibiting an overall small mean error of 0.5 m s−1 at most levels and a root-mean-square error less than 2 m s−1. The retrieval of the radial wind is also quite accurate, exhibiting comparable errors, although the accuracy of the tangential wind is generally better. Some degradation of the retrieval quality is observed at higher altitude, mainly due to sparser distribution of data in the model. The vertical component of the wind appears to be the most challenging to retrieve, but the software still provides acceptable results. The tropical cyclone mean azimuthal structure and wavenumber structure are found to be very well captured. Sources of errors inherent to airborne Doppler measurements and the effects of some of the simplifications used in the simulation methodology are also discussed.

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Robert Rogers, Paul Reasor, and Sylvie Lorsolo

Abstract

Differences in the inner-core structure of intensifying [IN; intensity increase of at least 20 kt (24 h)−1, where 1 kt = 0.51 m s−1] and steady-state [SS; intensity remaining between ±10 kt (24 h)−1] tropical cyclones (TCs) are examined using composites of airborne Doppler observations collected from NOAA P-3 aircraft missions. The IN dataset contains 40 eyewall passes from 14 separate missions, while the SS dataset contains 53 eyewall passes from 14 separate missions. Intensifying TCs have a ringlike vorticity structure inside the radius of maximum wind (RMW); lower vorticity in the outer core; a deeper, stronger inflow layer; and stronger axisymmetric eyewall upward motion compared with steady-state TCs. There is little difference in the vortex tilt between 2 and 7 km, and both IN and SS TCs show an eyewall precipitation and updraft asymmetry whose maxima are located in the downshear and downshear-left region. The azimuthal coverage of eyewall and outer-core precipitation is greater for IN TCs. There is little difference in the distribution of downdrafts and weak to moderate updrafts in the eyewall. The primary difference is seen at the high end of the vertical velocity spectrum, where IN TCs have a larger number of convective bursts. These bursts accomplish more vertical mass flux, but they compose such a small portion of the total vertical velocity distribution that there is little difference in the shape of the net mass flux profile. The radial location of convective bursts for IN TCs is preferentially located inside the RMW, where the axisymmetric vorticity is generally higher, whereas for SS TCs the bursts are located outside the RMW.

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Sylvie Lorsolo, John L. Schroeder, Peter Dodge, and Frank Marks Jr.

Abstract

Data with high temporal and spatial resolution from Hurricanes Isabel (2003) and Frances (2004) were analyzed to provide a detailed study of near-surface linear structures with subkilometer wavelengths of the hurricane boundary layer (HBL). The analysis showed that the features were omnipresent throughout the data collection, displayed a horizontal and vertical coherency, and maintained an average orientation of 7° left of the low-level wind. A unique objective wavelength analysis was conducted, where wavelength was defined as the distance between two wind maxima or minima perpendicular to the features’ long axis, and revealed that although wavelengths as large as 1400 m were observed, the majority of the features had wavelengths between 200 and 650 m. The assessed wavelengths differ from those documented in a recent observational study. To evaluate the correlation between the features and the underlying near-surface wind field, time and spectral analyses were completed and ground-relative frequency distributions of the features were retrieved. High-energy regions of the power spectral density (PSD) determined from near-surface data were collocated with the features’ ground-relative frequency, illustrating that the features have an influence on the near-surface wind field. The additional energy found in the low-frequency range of the PSDs was previously identified as characteristic of the hurricane surface flow, suggesting that the features are an integral component of the HBL flow.

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Sylvie Lorsolo, Jun A. Zhang, Frank Marks Jr., and John Gamache

Abstract

Hurricane turbulent kinetic energy (TKE) was computed using airborne Doppler measurements from the NOAA WP-3D tail radars, and TKE data were retrieved for a variety of storms at different stages of their life cycle. The geometry of the radar analysis coupled with the relatively small beam resolution at ranges <8 km allowed for the estimation of subkilometer turbulent processes. Two-dimensional profiles of TKE were constructed and revealed that the strongest turbulence was generally located in convective regions, such as the eyewall, with magnitudes often exceeding 15 m2 s−2 and in the boundary layer with values of 5–10 m2 s−2 in the lowest kilometer. A correlation analysis showed that the strong turbulence was generally associated with strong horizontal shear of vertical and radial wind components in the eyewall and strong vertical shear of horizontal wind in the boundary layer. Mean vertical profiles of TKE decrease sharply above the hurricane boundary layer and level off at low magnitude for all regions outside the radius of maximum wind.

The quality of the retrieval method was evaluated and showed very good agreement with TKE values directly calculated from the three-dimensional wind components of in situ measurements. The method presented here provides a unique opportunity to assess hurricane turbulence throughout the storm, especially in high-wind regions, and can be applied on extensive datasets of past and future airborne hurricane penetrations.

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Robert Rogers, Sylvie Lorsolo, Paul Reasor, John Gamache, and Frank Marks

Abstract

The multiscale inner-core structure of mature tropical cyclones is presented via the use of composites of airborne Doppler radar analyses. The structure of the axisymmetric vortex and the convective and turbulent-scale properties within this axisymmetric framework are shown to be consistent with many previous studies focusing on individual cases or using different airborne data sources. On the vortex scale, these structures include the primary and secondary circulations, eyewall slope, decay of the tangential wind with height, low-level inflow layer and region of enhanced outflow, radial variation of convective and stratiform reflectivity, eyewall vorticity and divergence fields, and rainband signatures in the radial wind, vertical velocity, vorticity, and divergence composite mean and variance fields. Statistics of convective-scale fields and how they vary as a function of proximity to the radius of maximum wind show that the inner eyewall edge is associated with stronger updrafts and higher reflectivity and vorticity in the mean and have broader distributions for these fields compared with the outer radii. In addition, the reflectivity shows a clear characteristic of stratiform precipitation in the outer radii and the vorticity distribution is much more positively skewed along the inner eyewall than it is in the outer radii. Composites of turbulent kinetic energy (TKE) show large values along the inner eyewall, in the hurricane boundary layer, and in a secondary region located at about 2–3 times the radius of maximum wind. This secondary peak in TKE is also consistent with a peak in divergence and in the variability of vorticity, and they suggest the presence of rainbands at this radial band.

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Jun A. Zhang, Frank D. Marks, Michael T. Montgomery, and Sylvie Lorsolo

Abstract

This study analyzes the flight-level data collected by research aircraft that penetrated the eyewalls of category 5 Hurricane Hugo (1989) and category 4 Hurricane Allen (1980) between 1 km and the sea surface. Estimates of turbulent momentum flux, turbulent kinetic energy (TKE), and vertical eddy diffusivity are obtained before and during the eyewall penetrations. Spatial scales of turbulent eddies are determined through a spectral analysis. The turbulence parameters estimated for the eyewall penetration leg are found to be nearly an order of magnitude larger than those for the leg outside the eyewall at similar altitudes. In the low-level intense eyewall region, the horizontal length scale of the dominant turbulent eddies is found to be between 500 and 3000 m, and the corresponding vertical length scale is approximately 100 m. The results suggest also that it is unwise to include eyewall vorticity maxima (EVM) in the turbulence parameter estimation because the EVMs are likely to be quasi-two-dimensional vortex structures that are embedded within the three-dimensional turbulence on the inside edge of the eyewall.

This study is a first attempt at estimating the characteristics of turbulent flow in the low-level troposphere of an intense eyewall using in situ aircraft observations. The authors believe that the results can offer useful guidance in numerical weather prediction efforts aimed at improving the forecast of hurricane intensity. Because of the small sample size analyzed in this study, further analyses of the turbulent characteristics in the high-wind region of hurricanes are imperative.

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Altuğ Aksoy, Sim D. Aberson, Tomislava Vukicevic, Kathryn J. Sellwood, Sylvie Lorsolo, and Xuejin Zhang

Abstract

The Hurricane Weather Research and Forecasting (HWRF) Ensemble Data Assimilation System (HEDAS) is developed to assimilate tropical cyclone inner-core observations for high-resolution vortex initialization. It is based on a serial implementation of the square root ensemble Kalman filter (EnKF). In this study, HWRF is used in an experimental configuration with horizontal grid spacing of 9 (3) km on the outer (inner) domain. HEDAS is applied to 83 cases from years 2008 to 2011. With the exception of two Hurricane Hilary (2011) cases in the eastern North Pacific basin, all cases are observed in the Atlantic basin. Observed storm intensity for these cases ranges from tropical depression to category-4 hurricane.

Overall, it is found that high-resolution tropical cyclone observations, when assimilated with an advanced data assimilation technique such as the EnKF, result in analyses of the primary circulation that are realistic in terms of intensity, wavenumber-0 radial structure, as well as wavenumber-1 azimuthal structure. Representing the secondary circulation in the analyses is found to be more challenging with systematic errors in the magnitude and depth of the low-level radial inflow. This is believed to result from a model bias in the experimental HWRF caused by the overdiffusive nature of the planetary boundary layer parameterization utilized. Thermodynamic deviations from the observed structure are believed to be caused by both an imbalance between the number of the kinematic and thermodynamic observations in general and the suboptimal ensemble covariances between kinematic and thermodynamic fields. Future plans are discussed to address these challenges.

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Altuğ Aksoy, Sylvie Lorsolo, Tomislava Vukicevic, Kathryn J. Sellwood, Sim D. Aberson, and Fuqing Zhang

Abstract

Within the National Oceanic and Atmospheric Administration, the Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory has developed the Hurricane Weather Research and Forecasting (HWRF) Ensemble Data Assimilation System (HEDAS) to assimilate hurricane inner-core observations for high-resolution vortex initialization. HEDAS is based on a serial implementation of the square root ensemble Kalman filter. HWRF is configured with a horizontal grid spacing of km on the outer/inner domains. In this preliminary study, airborne Doppler radar radial wind observations are simulated from a higher-resolution km version of the same model with other modifications that resulted in appreciable model error.

A 24-h nature run simulation of Hurricane Paloma was initialized at 1200 UTC 7 November 2008 and produced a realistic, category-2-strength hurricane vortex. The impact of assimilating Doppler wind observations is assessed in observation space as well as in model space. It is observed that while the assimilation of Doppler wind observations results in significant improvements in the overall vortex structure, a general bias in the average error statistics persists because of the underestimation of overall intensity. A general deficiency in ensemble spread is also evident. While covariance inflation/relaxation and observation thinning result in improved ensemble spread, these do not translate into improvements in overall error statistics. These results strongly suggest a need to include in the ensemble a representation of forecast error growth from other sources such as model error.

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