Wind Radii Research Articles

Using Routinely Available Information to Estimate Tropical Cyclone Wind Structure

AbstractA relatively simple method to estimate tropical cyclone (TC) wind radii from routinely available information including storm data (location, motion, and intensity) and TC size is introduced. The method is based on a combination of techniques presented in previous works and makes an assumption that TCs are largely symmetric and that asymmetries are based solely on storm motion and location. The method was applied to TC size estimates from two sources: infrared satellite imagery and global model analyses. The validation shows that the methodology is comparable with other objective methods based on the error statistics. The technique has a variety of practical research and operational applications, some of which are also discussed.

Maximum wind radius estimated by the 50 kt radius: improvement of storm surge forecasting over the western North Pacific

Abstract. Even though the maximum wind radius (Rmax) is an important parameter in determining the intensity and size of tropical cyclones, it has been overlooked in previous storm surge studies. This study reviews the existing estimation methods for Rmax based on central pressure or maximum wind speed. These over- or underestimate Rmax because of substantial variations in the data, although an average radius can be estimated with moderate accuracy. As an alternative, we propose an Rmax estimation method based on the radius of the 50 kt wind (R50). Data obtained by a meteorological station network in the Japanese archipelago during the passage of strong typhoons, together with the JMA typhoon best track data for 1990–2013, enabled us to derive the following simple equation, Rmax = 0.23 R50. Application to a recent strong typhoon, the 2015 Typhoon Goni, confirms that the equation provides a good estimation of Rmax, particularly when the central pressure became considerably low. Although this new method substantially improves the estimation of Rmax compared to the existing models, estimation errors are unavoidable because of fundamental uncertainties regarding the typhoon's structure or insufficient number of available typhoon data. In fact, a numerical simulation for the 2013 Typhoon Haiyan as well as 2015 Typhoon Goni demonstrates a substantial difference in the storm surge height for different Rmax. Therefore, the variability of Rmax should be taken into account in storm surge simulations (e.g., Rmax = 0.15 R50–0.35 R50), independently of the model used, to minimize the risk of over- or underestimating storm surges. The proposed method is expected to increase the predictability of major storm surges and to contribute to disaster risk management, particularly in the western North Pacific, including countries such as Japan, China, Taiwan, the Philippines, and Vietnam.

POWERFUL, ROTATING DISK WINDS FROM STELLAR-MASS BLACK HOLES

We present an analysis of ionized X-ray disk winds observed in the Fe K band of four stellar-mass black holes observed with Chandra, including 4U 1630-47, GRO J1655-40, H 1743-322, and GRS 1915+105. High-resolution photoionization grids were generated in order to model the data. Third-order gratings spectra were used to resolve complex absorption profiles into atomic effects and multiple velocity components. The Fe XXV line is found to be shaped by contributions from the intercombination line (in absorption), and the Fe XXVI line is detected as a spin-orbit doublet. The data require 2-3 absorption zones, depending on the source. The fastest components have velocities approaching or exceeding 0.01c, increasing mass outflow rates and wind kinetic power by orders of magnitude over prior single-zone models. The first-order spectra require re-emission from the wind, broadened by a degree that is loosely consistent with Keplerian orbital velocities at the photoionization radius. This suggests that disk winds are rotating with the orbital velocity of the underlying disk, and provides a new means of estimating launching radii -- crucial to understanding wind driving mechanisms. Some aspects of the wind velocities and radii correspond well to the broad-line region (BLR) in active galactic nuclei, suggesting a physical connection. We discuss these results in terms of prevalent models for disk wind production and disk accretion itself, and implications for massive black holes in active galactic nuclei.

Assimilation of Tropical Cyclone Track and Wind Radius Data with an Ensemble Kalman Filter

Abstract Improving tropical cyclone (TC) forecasts is one of the most important issues in meteorology, but TC intensity forecasting is a challenging task. Because the lack of observations near TCs usually results in degraded accuracy of the initial fields, utilizing TC advisory data in data assimilation typically has started with an ensemble Kalman filter (EnKF). In this study, TC minimum sea level pressure (MSLP) and position information were directly assimilated using the EnKF, and the impacts of these observations were investigated by comparing different assimilation strategies. Another experiment with TC wind radius data was carried out to examine the influence of TC shape parameters. Sensitivity experiments indicated that the direct assimilation of TC MSLP and position data yielded results that were superior to those based on conventional assimilation of TC MSLP as a standard surface pressure observation. Assimilation of TC radius data modified the outer circulation of TCs closer to observations. The impacts of these TC parameters were also evaluated by using the case of Typhoon Talas in 2011. The TC MSLP, position, and wind radius data led to improved TC track forecasts and therefore to improved precipitation forecasts. These results imply that initialization with these TC-related observations benefits TC forecasting, offering promise for the prevention and mitigation of natural disasters caused by TCs.

Multifractal intensity in dynamical behaviors of multifractals

We study the dynamical behavior of multifractals in typhoons. A significant and fascinating feature of this behavior is that it provides a proper interpretation for the pattern of a typhoon in terms of the numerical values of its generalized dimension and scaling exponent. For our case, these statistical quantities can be estimated numerically from four meteorological factors, i.e., the moving speed, the central pressure, the strong wind radius, and the maximum wind speed, before and after the landing of a typhoon. We perform a computer simulation by using a 5-year typhoon data covering 2008 to 2012 in the East Asian region. The result from the multifractal structure allows us to calculate a definite and reliable fractal dimension. In particular, we discover the strength of the multifractal structures from the four meteorological factors obtained from five years of typhoon data. The multifractal strength in the moving speed is a relatively larger value compared to the other meteorological factors in three time intervals.

An “extended fetch” model for the spatial distribution of tropical cyclone wind–waves as observed by altimeter

An extensive database of altimeter measurements of wind speed and significant wave height is analysed to investigate the spatial distribution of significant wave height within tropical cyclones. The database includes transects of 440 tropical cyclones. As such, the data covers the full range of expected values of the velocity of forward movement, maximum wind velocity and radius to maximum winds. The data confirms previous measurements that JONSWAP scaling can be used to represent such waves. In addition, the data supports the concept of an extended fetch in such systems. The maximum waves occur when the wind direction and direction of propagation of the storm are aligned. In such cases, the waves move forward with the storm and experience an extended fetch. A parametric model based on JONSWAP scaling and representing the extended fetch is developed and optimized using the data. Combined with previous in situ measurements, this model can reproduce the spatial distribution of significant wave height, as well as estimate the full directional spectrum.

A Numerical Simulation Of Severe Cyclone Sidr-2007

The most severe tropical cyclones are formed over the Bay of Bengal because of its complex geometric structure and shallow coastal bathymetry. These natural disasters significantly impact our lives as well as the environment. To quantify the impact of such disasters, we analyzed data pertaining to the severe cyclone Sidr-2007, considering the cyclone’s duration and angle of land fall, as well as the bathymetry of the near-coastal zones. We propose a reliable hydrodynamic model to simulate the severity of such cyclones. The model’s reliability is verified by comparison of the numerical results with actual storm-surge data. Our model requires meteorological and hydrological inputs, including the time-dependent positions of the storm centre, the maximum wind radius, as well as pressure-drop and bathymetry data. The model results are in good agreement with the reported data. DOI: http://dx.doi.org/10.3329/ganit.v32i0.13645 GANIT J. Bangladesh Math. Soc. (ISSN 1606-3694) 32 (2012) 43-54

Uncertainty and Intercalibration Analysis of H*Wind

Abstract The Hurricane Research Division (HRD) Real-time Hurricane Wind Analysis System (H*Wind) is a software application used by NOAA’s HRD to create a gridded tropical cyclone wind analysis based on a wide range of observations. These analyses are used in both forecasting and research applications. Although mean bias and RMS errors are listed, H*Wind lacks robust uncertainty information that considers the contributions of random observation errors, relative biases between observation types, temporal drift resulting from combining nonsimultaneous measurements into a single analysis, and smoothing and interpolation errors introduced by the H*Wind analysis. This investigation seeks to estimate the total contributions of these sources, and thereby provide an overall uncertainty estimate for the H*Wind product. A series of statistical analyses show that in general, the total uncertainty in the H*Wind product in hurricanes is approximately 6% near the storm center, increasing to nearly 13% near the tropical storm force wind radius. The H*Wind analysis algorithm is found to introduce a positive bias to the wind speeds near the storm center, where the analyzed wind speeds are enhanced to match the highest observations. In addition, spectral analyses are performed to ensure that the filter wavelength of the final analysis product matches user specifications. With increased knowledge of bias and uncertainty sources and their effects, researchers will have a better understanding of the uncertainty in the H*Wind product and can then judge the suitability of H*Wind for various research applications.

Determining Selected Tropical Cyclone Characteristics Using QuikSCAT's Ultra-High Resolution Images

Operational SeaWinds on QuikSCAT data can be enhanced to yield a 2.5 km ultra-high resolution (UHR) wind product, which can be used to help estimate tropical cyclone (TC) characteristics such as TC center and wind radii. This paper provides the results of two studies in which the QuikSCAT UHR wind product's effectiveness in estimating these TC characteristics is evaluated. First, a comparison is made between an analyst's choice of center location based on UHR images and interpolated best track position. In this analysis, the UHR images are divided into two categories based on the analyst's confidence level of finding the center location. In each category, statistical error quantities between the analyst's choice of center location and interpolated best track location are computed. UHR images within the high-confidence category can provide, for a given year and basin, mean error distance as small as 19 km with a 10 km standard deviation. Second, a comparison of QuikSCAT's performance in estimating wind radii is made. QuikSCAT's performance is gauged against the H*wind dataset and the extended best track (EBT) dataset. Results show that QuikSCAT UHR data yields the correct 34 kt wind radius most of the time regardless of the TC category when compared to both H*wind and EBT, whereas the 50 kt and 64 kt wind radii estimates do not always agree with H*wind and EBT. A more sophisticated method is implemented to automatically estimate wind radii based on a model fit to QuikSCAT data. Results from this method are compared with EBT wind radii. The 50 kt and 64 kt wind radii obtained from QuikSCAT model fit are generally highly correlated with EBT estimated wind radii.

Geostationary Doppler Radar and Tropical Cyclone Surveillance

Abstract The potential usefulness of spaceborne Doppler radar as a tropical cyclone observing tool is assessed by conducting a high-resolution simulation of an intense hurricane and generating synthetic observations of reflectivity and radial velocity. The ground-based velocity track display (GBVTD) technique is used to process the radial velocity observations and generate retrievals of meteorologically relevant metrics such as the maximum wind (MW), radius of maximum wind (RMW), and radius of 64-kt wind (R64). Results indicate that the performance of the retrieved metrics compares favorably with the current state-of-the-art satellite methods for intensity estimation and somewhat better than current methods for structure (i.e., wind radii).

An Analytic Vortex Initialization Technique for Idealized Tropical Cyclone Studies in AGCMs

Abstract The paper discusses the design of idealized tropical cyclone experiments in atmospheric general circulation models (AGCMs). The evolution of an initially weak, warm-core vortex is investigated over a 10-day period with varying initial conditions that include variations of the maximum wind speed and radius of maximum wind. The initialization of the vortex is built upon prescribed 3D moisture, pressure, temperature, and velocity fields that are embedded into tropical environmental conditions. The initial fields are in exact hydrostatic and gradient-wind balance in an axisymmetric form. The formulation is then generalized to provide analytic initial conditions for an approximately balanced vortex in AGCMs with height-based vertical coordinates. An extension for global models with pressure-based vertical coordinates is presented. The analytic initialization technique can easily be implemented on any AGCM computational grid. The characteristics of the idealized tropical cyclone experiments are illustrated in high-resolution model simulations with the Community Atmosphere Model version 3.1 (CAM 3.1) developed at the National Center for Atmospheric Research. The finite-volume dynamical core in CAM 3.1 with 26 vertical levels is used, and utilizes an aquaplanet configuration with constant sea surface temperatures of 29°C. The impact of varying initial conditions and horizontal resolutions on the evolution of the tropical cyclone–like vortex is investigated. Identical physical parameterizations with a constant parameter set are used at all horizontal resolutions. The sensitivity studies reveal that the initial wind speed and radius of maximum wind need to lie above a threshold to support the intensification of the analytic initial vortex at horizontal grid spacings of 0.5° and 0.25° (or 55 and 28 km in the equatorial regions). The thresholds lie between 15 and 20 m s−1 with a radius of maximum wind of about 200–250 km. In addition, a convergence study with the grid spacings 1.0°, 0.5°, 0.25°, and 0.125° (or 111, 55, 28, and 14 km) shows that the cyclone gets more intense and compact with increasing horizontal resolution. The 0.5°, 0.25°, and 0.125° simulations exhibit many tropical cyclone–like characteristics such as a warm-core, low-level wind maxima, a slanted eyewall-like vertical structure and a relatively calm eye. The 0.125° simulation even starts to resolve spiral rainbands and reaches maximum wind speeds of about 72–83 m s−1 at low levels. These wind speeds are equivalent to a category-5 tropical cyclone on the Saffir–Simpson hurricane scale. It is suggested that the vortex initialization technique can be used as an idealized tool to study the impact of varying resolutions, physical parameterizations, and numerical schemes on the simulation and representation of tropical cyclone–like vortices in global atmospheric models.

Validation of QuikSCAT wind vectors by dropwindsonde data from Dropwindsonde Observations for Typhoon Surveillance Near the Taiwan Region (DOTSTAR)

The accuracy of 10 m wind vectors derived from the QuikSCAT satellite near tropical cyclones is validated against soundings from 457 GPS dropwindsondes deployed by Dropwindsonde Observations for Typhoon Surveillance Near the Taiwan Region (DOTSTAR) during 2003–2007. To maximize the database, the surface to 40 m wind speed in the dropwindsondes is averaged and interpolated to the 10 m wind speed by linear regression. After removing rain‐flagged data, the root‐mean‐square differences between QuikSCAT and dropwindsonde data were 2.6 m s−1 or 18% (wind speed) and 17° (wind direction) on the basis of 896 matching samples. Further analyses also indicate that the QuikSCAT data slightly underestimates the wind speed of medium‐wind regime (between 10 and 17.2 m s−1) and possesses some clockwise directional bias in the high‐wind regime (above 17.2 m s−1). In summary, this study suggests that the QuikSCAT wind vectors below tropical storm wind strength (17.2 m s−1) are accurate enough for forecasters to determine the critical wind radius of 34 knot wind, while a new error bound of the QuikSCAT wind estimate in high‐wind regimes near tropical cyclones is suggested to be set at about 4 m s−1.

Influence of mesoscale topography on vortex intensity

The effect of mesoscale topography on multi-vortex self-organization is investigated numerically in this paper using a barotropic primitive equation model with topographic term. In the initial field there are one DeMaria major vortex with the maximum wind radius r m of 80 km at the center of the computational domain, and four meso-β vortices in the vicinity of r m to the east of the major vortex center. When there is no topography present, the initial vortices self-organize into a quasi-final state flow pattern, i.e. a quasi-axisymmetric vortex whose intensity is close to that of the initial major vortex. However, when a mesoscale topography is incorporated, the spatial scale of the quasi-final state vortex reduces, and the relative vorticity at the center of the vortex and the local maximum wind speed remarkably increase. The possible mechanism for the enhancement of the quasi-final state vortex might be that the negative relative vorticity lump, generated above the mesoscale topography because of the constraint of absolute vorticity conservation, squeezes the center of positive vorticity towards the mountain slope area, and thus reduces the spatial range of the major vortex. Meanwhile, because the total kinetic energy is basically conservative, the squeezing directly leads to the concentration of the energy in a smaller area, i.e. the strengthening of the vortex.

Comparison of observed gale radius statistics

Forecasts of tropical cyclone track and intensity have long been used to characterize the evolution and expected threat from a tropical storm. However, in recent years, recognition of the contributions of subtropical cyclogenesis to tropical storm formation and the process of extratropical transition to latter stages of the once-tropical storm’s lifecycle have raised awareness about the importance of storm structure. Indeed, the structure of a cyclone determines the distribution and intensity of the significant weather associated with that storm. In this study, storm structure is characterized in terms of significant wind radii. The radii of tropical storm, damaging, and hurricane-force winds, as well as the radius of maximum winds are all analyzed. These wind radii are objectively derived from the H*Wind surface wind analysis system. Initially, six years of these data are examined for consistency with previous studies. Having ascertained that the H*Wind radii are realistic, detailed comparisons are performed between the H*Wind and NHC Best Track wind radii for two years (2004 and 2005) of North Atlantic tropical storm and hurricane cases. This intercomparison reveals an unexpected bias: the H*Wind radii are consistently larger than the NHC Best Track for all but the smallest and least intense storms. Further examination of the objectively-determined H*Wind tropical storm force wind radius data compared to subjectively-determined radii for the same storm times demonstrates that the objective wind radii are underestimating the extent of the tropical storm force wind area. Since the objective H*Wind radii are large compared to the NHC Best Track – and yet underestimate the area of tropical storm force winds – this argues for further examination of the methods used to ascertain these significant wind radii.

The gradual loss and episodic creation of Palm Cockatoo (Probosciger aterrimus) nest-trees in a fire- and cyclone-prone habitat

We present information regarding the rate of loss of nest-hollows over a long period in the tropics, and a dramatic episode of nest-hollow creation following two tropical cyclones. We followed the fate of 61 Palm Cockatoo (Probosciger aterrimus) nest-trees over a period of 6 years (1999 to 2005). On average, 48 trees were monitored each year. During this time 16 (26.2%) were rendered unusable for Palm Cockatoos; their final cause of loss was fire (n = 10; 62.5%), wind (n = 4; 25%), and intense decay where the tree was left standing but unusable (n = 2; 12.5%). On average, 5.2% of nest-trees were lost each year. Dead trees were more likely to be destroyed than living trees. Individual Palm Cockatoos build nests in multiple hollows, a subset of which are used for actual breeding whereas the rest are used for display purposes only (inactive nest-trees). Inactive nest-trees were also more likely to be destroyed than active trees, suggesting Palm Cockatoos actually breed in nest-trees that are more durable. We revisited our study area in July 2006, following two severe tropical cyclones. Most of the known nest-trees that we visited after the cyclones were still standing. The first cyclone created ∼0.02 new nesting opportunities per hectare, whereas the second created 0.19. The differences between the two cyclones can be explained by their different paths and different radii of destructive winds in relation to our study area. While the loss of nest-trees owing to fire is significant, and should be monitored given that they are a limiting resource (Murphy et al. 2003), our results suggest that fire management should focus more on maintaining adequate recruitment of large trees, which could be affected by excessively frequent burning.

Improvement of Advanced Microwave Sounding Unit Tropical Cyclone Intensity and Size Estimation Algorithms

Abstract Previous work, in which Advanced Microwave Sounding Unit (AMSU) data from the Atlantic Ocean and east Pacific Ocean basins during 1999–2001 were used to provide objective estimates of 1-min maximum sustained surface winds, minimum sea level pressure, and the radii of 34-, 50-, and 64-kt (1 kt ≡ 0.5144 m s−1) winds in the northeast, southeast, southwest, and northwest quadrants of tropical cyclones, is updated to reflect larger datasets, improved statistical analysis techniques, and improved estimation through dependent variable transforms. A multiple regression approach, which utilizes best-subset predictor selection and cross validation, is employed to develop the estimation models, where the dependent data (i.e., maximum sustained winds, minimum pressure, wind radii) are from the extended best track and the independent data consist of AMSU-derived parameters that give information about retrieved pressure, winds, temperature, moisture, and satellite resolution. The developmental regression models result in mean absolute errors (MAE) of 10.8 kt and 7.8 hPa for estimating maximum winds and minimum pressure, respectively. The MAE for the 34-, 50-, and 64-kt azimuthally averaged wind radii are 16.9, 13.3, and 6.8 n mi (1 n mi ≡ 1852 m), respectively.

North Atlantic Hurricanes 1977–99: Surface Hurricane-Force Wind Radii

Abstract The Hurricane Research Division (NOAA/AMOL/HRD) flight-level data archive of 1977–99 North Atlantic hurricanes is used to create radial profiles of earth-relative surface wind speeds. Radii of hurricane-force winds are determined directly from these generated profiles. The distributions of hurricane-force wind radii stratified by minimum central pressure and quadrant, relative to the direction of storm motion, indicate marked variability among storms. However, a model which uses storm parameters to predict the radius of hurricane-force winds in a given quadrant of a hurricane does seem feasible. Roughly three-quarters of the variance in the hurricane- force wind radii of minimal hurricanes and about one-half of the variance in the hurricane-force wind radii of major hurricanes is explained by parameters of the storm, enough to make a predictive model of significant value. In addition, cumulative two-dimensional probability distributions of hurricane-force wind radii may have some utility for hurr...

Comparison and further development of parametric tropical cyclone models for storm surge modelling

Cyclone forced storm surges cause severe damage to coastal structures and loss of human lives and properties. In cyclone protection and warning it is important to be able to calculate the cyclone air pressure and wind field on the basis of rather limited information, such as cyclone position, pressure drop, maximum wind speed and radius to maximum wind speed. In the present work, parametric cyclone models based on such limited information are investigated and compared. Analytical expressions of the tangential and radial velocity distributions are derived from the governing momentum equations. It is found that the analytical models provide very similar air pressure and tangential wind speed distributions. Instead of using a simple formulation of the deflection angle, the derived analytical expression of the radial velocity distribution is used and compared to earlier numerical solutions. A procedure is formulated for estimation of the shape of the cyclone on the basis of maximum wind speed versus pressure drop relations. Finally, examples of comparisons of measured and calculated air pressures and wind speeds and directions are presented.

High-resolution spectroscopy of two LBV cycles of HR Car

Luminous Blue Variables (LBVs or S Dor Variables) are showing characteristic variability of very long timescales of a decade and more. During their expansion and contraction phases, they move across a wide part of the H-R diagram with dramatic changes of their stellar wind characteristics, radius and temperature. We present results of our long-term spectroscopic monitoring campaigns for the galactic star HR Car with optical spectra taken over 13 years and two minimum and maximum phases observed during this time.

An analysis of a pressure pattern in severe Typhoon Bart hitting the Japanese Islands in 1999 and a comparison of the gradient wind with the observed surface wind

Typhoon Bart (T9918) hit Kyushu in Japan on September 24 in 1999, and caused storm surge at Matsuai-area of Shiranui in Kumamoto Prefecture with 12 deaths. We analyzed the pressure pattern in Typhoon Bart by an objective method and computed the gradient wind from it. The computed wind was compared with the wind observed at the sites of the Network for Wind Measurement in Kyushu (NeWMeK) system. The ratio of the observed wind speed to the gradient wind speed was about 0.5 outside of the maximum wind radius, but it increased rapidly with the decrease of relative radius inside of it. In addition to wind data at six stations of the Japan Meteorological Agency, the wind ratio outside of the maximum wind radius increased with height in proportion to 0.33 power of the anemometer height.