Hurricane Research Division
FY-2001 Publications
(October 1, 2000-September 30, 2001)
(names of HRD authors appear in capital letters)
	[1]	ABERSON, S.D.  The ensemble of tropical cyclone track forecasting models in the North Atlantic Basin (1976-2000).  Bulletin of the American Meteorological Society, 82(9):1895-1904 (2001).
The suite of tropical cyclone track forecast models in the Atlantic basin from the 1976 to 2000 hurricane seasons are treated as a forecast ensemble.  The 12-h ensemble mean forecast, adjusted for forecast difficulty, has improved at a rate of just under 1% per year, and the improvement rate increases to almost 2.4% per year for the 72-h forecasts.  The average size of the 72-h (48-h) error in 1976 is less than the average size of the 48-h (36-h) error in 2000.  The average 36-h forecast error in 2000 is comparable to the 24-h forecast error in 1976.  The ensemble currently spans the true path of the tropical cyclone in the cross-track direction more than 90% of the time and in the alongtrack direction between 60% and 90% of the time depending on the forecast lead time.  The ensemble spread is unable to provide estimates of individual forecast reliability, likely making probabilistic landfall forecasts from this ensemble unreliable. The reliability of the spread in the cross-track direction suggests the possibility of limiting hurricane watch and warning regions depending upon the ensemble spread at landfall.
	[2]	GOLDENBERG, S.B., C.W. LANDSEA, A.M. Mestas-Nuñez, and W.M. Gray.  The recent increase in Atlantic hurricane activity. Causes and implications.  Science, 293:474-479 (2001).
The years 1995 to 2000 experienced the highest level of North Atlantic hurricane activity in the reliable record.  Compared with the generally low activity of the previous 24 years (1971 to 1994), the last six years have seen a doubling of overall activity for the whole basin, a 2.5-fold increase in major hurricanes ($50 m/s), and a fivefold increase in hurricanes affecting the Caribbean.  The greater activity is caused by simultaneous increases in North Atlantic sea-surface temperatures and decreases in vertical wind shear, both of which are known to favor hurricane formation.  Because these changes exhibit a multidecadal time scale, the present high level of hurricane activity is likely to persist for an additional -10 to 40 years.  The shift in climate calls for a reevaluation of preparedness and mitigation strategies.
	[3]	KAPLAN, J., and M. DeMaria.  On the decay of tropical winds after landfall in the New England area.  Journal of Applied Meteorology, 40(2):280-286 (2001).
A version of the Kaplan and DeMaria empirical model for predicting the decay of tropical cyclone 1-min maximum sustained surface winds after landfall is developed for the New England region. The original model was developed from the National Hurricane Center (NHC) best-track wind estimates for storms that made landfall in the United States south of 37EN from 1967 to 1993. In this note, a similar model is developed for U.S. storms north of 37EN, which primarily made landfall in New York or Rhode Island and then moved across New England. Because of the less frequent occurrence of New England tropical cyclones, it was necessary to include cases back to 1938 to obtain a reasonable sample size. In addition, because of the faster translational speed and the fairly rapid extratropical transition of the higher-latitude cases, it was necessary to estimate the wind speeds at 2-h intervals after landfall, rather than every 6 h, as in the NHC best track. For the model development, the estimates of the maximum sustained surface winds of nine landfalling storms (seven hurricanes and two tropical storms) at 2-h intervals were determined by an analysis of all available surface data. The wind observations were adjusted to account for variations in anemometer heights, averaging times, and exposures.  Results show that the winds in the northern model decayed more (less) rapidly than those of the southern model, when the winds just after landfall are greater (less) than 33 knots. It is hypothesized that this faster rate of decay is due to the higher terrain near the coast for the northern sample and to the more hostile environmental conditions (e.g., higher vertical wind shear).  The slower decay rate when the winds fall below 33 knots in the northern model might be due to the availability of a baroclinic energy source as the storms undergo extratropical transition.
	[4]	Katzberg, S.J., R.A. Walker, J.H. Roles, T. Lynch, and P.G. BLACK.  First GPS signals reflected from the interior of a tropical storm: Preliminary results from Hurricane Michael.  Geophysical Research Letters, 28(10):1981-1984 (2001).
Using GPS signals reflected from the ocean surface is developing into a simple technique for measuring sea-state and inferring surface wind speeds. Theoretical models have been developed which are considered valid to approximately 24 meters per second.  The GPS reflection technique has an obvious extension to extremely high sea states, cyclones, and extra-tropical storms.  In October 2000, a GPS system mounted in a NOAA Hurricane Hunter research aircraft was flown into Hurricane Michael off the South Carolina coast.  The first acquisition of GPS signals reflected from the sea surface inside tropical cyclones was accomplished.  This paper presents some examples of the data sets, as well as early wind speed retrieval results using direct extensions of current models.  Data from the GPS wind speed retrievals, as well as from direct aircraft measurements, are compared and discussed.
	[5]	Kollias, P., B.A. Albrecht, and F.D. MARKS.  Raindrop sorting induced by vertical drafts in convective clouds.  Geophysical Research Letters, 28(14):2787-2790 (2001).
Evidence of raindrop sorting by a convective updraft is presented.  Using a vertically pointing 94-GHz Doppler radar (λ = 3.2 mm) and capitalizing on the resonant nature of the backscattering cross-section as a function of the raindrop size (Mie scattering), the vertical air motions to an accuracy of 0.1 m s-1, and the shape of the raindrop size distribution are retrieved from the Doppler spectra.  The interaction of vertical drafts and raindrops is documented for the first time by high resolution radar data.  The updraft structure clearly causes horizontal and vertical sorting of the raindrops. In the updraft core, small raindrops (D # 1.7 mm) that have terminal velocities less than the updraft velocities (6-7 m s-1) and a clear absence of drops $3 mm are observed. Towards the updraft periphery, a gradual increase in the raindrop sizes is documented where large raindrops (D $3 mm) are observed. The observations demonstrate the importance of updrafts in distributing th raindrops in space.
	[6]	LANDSEA, C.W.  Comment on “Changes in the rates of North Atlantic major hurricane activity during the 20th century.”  Geophysical Research Letters, 28(14):2871-2872 (2001).
No abstract.
	[7]	LANDSEA, C.W.  El Niño-Southern Oscillation and the seasonal predictability of tropical cyclones.  In El Niño and the Southern Oscillation: Multiscale Variability and Global and Regional Impacts, H.F. Diaz and V. Markgraf (eds). Cambridge University Press, 149-181 (ISBN 0521621380) (2000).
Perhaps the most dramatic effect that El Niño has upon the climate system is in changing tropical cyclone characteristics around the world. This chapter reviews how tropical cyclone frequency, intensity, and areas of occurrence are altered in all of the cyclone basins by the phases of El Niño-Southern Oscillation (ENSO). In addition to ENSO, other global (such as the stratospheric Quasi-Biennial Oscillation) and local factors (such as sea surface temperature, monsoon intensity and rainfall, sea level pressures, and tropospheric vertical shear) can also help modulate tropical cyclone variability. Understanding how these various factors relate to tropical cyclone activity can be challenging due to the fairly short (on the scale of only tens of years) record of reliable data. Despite this limitation, many of the factors that have been linked to tropical cyclones, the foremost of which being ENSO, have substantial lead relationships and can be utilized to provide seasonal forecasts of tropical cyclones. Details of methodologies that have been developed for the North Atlantic, northwest Pacific, south Pacific and Australian basin tropical cyclones are presented, as well as the real-time forecasting performance of Atlantic hurricanes as issued by Professor William Gray. 
	[8]	Lee, W.-C., B. J.-D. Jou, P.-L. Chang, and F.D. MARKS.  Tropical cyclone kinematic structure retrieved from single-Doppler radar observations.  Part III: Evolution and structures of Typhoon Alex (1987).  Monthly Weather Review, 128(12):3982-4001 (2000).
This paper is the third of a series that focuses on the applications of the ground-based velocity track display (GBVTD) technique and the GBVTD-simplex center finding algorithm developed in the previous two papers to a real tropical cyclone (TC). The evolution and structure of Typhoon Alex (1987), including full tangential winds, mean radial winds, one component of the mean flow, and their derived axisymmetric angular momentum and perturbation pressure fields are reconstructed from 16 volume scans (6.5 h of data with a 2-h gap) from the Civil Aeronautic Administration (CAA) Doppler radar while Typhoon Alex moved across the mountainous area in northern Taiwan. This analysis retrieves a plausible and physically consistent three-dimensional primary circulation of a landfalling TC using a single ground-based Doppler radar. Highly asymmetric wind structures were resolved by the GBVTD technique where the maximum relative tangential wind at z = 2 km evolved from 52 m s-1 (before landfall), to less than 40 m s-1 (after landfall), to less than 35 m s-1 (entering the East China Sea). Alex’s eye began to fill with precipitation while its intensity decreased rapidly after landfall, a characteristic of circulations disrupted by terrain. The mean radial wind field revealed a layer of low-level inflow in agreement with past TC observations. The outward slope of the eyewall reflectivity maximum was consistent with the constant angular momentum contours within the eyewall. After Alex entered the East China Sea, its circulation became more axisymmetric. The axisymmetric perturbation pressure field was retrieved using the gradient wind approximation, which, when used in conjunction with one or more surface pressure measurements within the analysis domain, can estimate the central pressure. The retrieved perturbation pressure fields at two time periods were compared with surface pressures reported in northern Taiwan.  Considering the assumptions involved and the influence of terrain, good agreement (only 1–2-mb deviation) was found between them. This agreement indicates the relative quality of the GBVTD-retrieved axisymmetric circulation and suggests GBVTD-retrieved quantities can be useful in operational and research applications.
	[9]	Michaels, M., M. Shepard, S.D. ABERSON, H.A. FRIEDMAN, and K. Murphy.  Survey results of Society membership: The face of our profession at the threshold of the new millennium.  Bulletin of the American Meteorological Society, 82(7):1331-1352 (2001).
In the spring of 1999, the American Meteorological Society surveyed its membership in order to update demographic information on the Society and to gain a more detailed perspective on the workplace.  The survey was sent out with the dues statement and was solicited on a separate form returned independently to protect privacy and maintain anonymity.  The responses were captured in a newly employed, machine-readable format to provide an ease of statistical analysis and data compilation not available in prior survey analysis.  This data collection and subsequent demographic analysis represents the first attempt to update information regarding the membership since the 1993 survey results were published by Zevin and Seitter.  The format of the 1999 survey was designed to logically follow and expand upon the historical data of the membership collected at varying intervals since 1975.  The 1999 survey was broken into six parts.  The sections on demographics, education, and current employment closely followed the previous surveys from 1993 and 1990 to facilitate direct comparisons between historical datasets whenever possible.  The last three sections were reworked to elicit more declarative responses regarding personal circumstances, workplace circumstances, and additional issues concerning career choice and AMS membership, respectively.  An additional space was provided for narrative comments regarding opportunities for women and minorities in the AMS-related sciences.  Some 10,000 members were sent the 1999 dues statement and enclosed survey questionnaire.  A total of 4,669 members responded.  The following is a detailed analysis of the data collected from the 1999 membership survey.
	[10]	OOYAMA, K.V.  A dynamic and thermodynamic foundation for modeling the moist atmosphere with parameterized microphyics.  Journal of the Atmospheric Sciences, 58(15):2073-2102 (2001).
Moist convection is an exquisite yet powerful participant in the creation of weather on our planet. To facilitate numerical modeling of weather systems in a moist atmosphere, a direct and consistent application of dynamic and thermodynamic principles, in conjunction with parameterized microphysics, is proposed. An earlier formulation of reversible thermodynamics, in terms of the mass of air and water substance and the total entropy, is now extended to include the irreversible process of precipitation through parameterized microphysics. The dynamic equations are also formulated to account consistently for the mass and momentum of precipitation.  The theoretical proposal is tested with a two-dimensional model that utilizes a versatile and accurate spectral method based on a cubic-spline representation of the spatial fields. In order to allow a wide range of scale interactions, the model is configured on multiply nested domains of outwardly decreasing resolution, with noise-free, two-way interfaces. The semi-implicit method provides efficient time integration for the nested spectral model.  The tests performed are the simulation of the growth of single-cell clouds and also the generation of self-sustaining multicell squall lines, and the effects of various resolutions on the simulations are examined. The results favorably compare with similar results found in the literature, but also offer new insights into the interplay between dynamics and precipitation.
	[11]	Phoebus, P.A., D.R. Smith, P.J. Croft, H.A. FRIEDMAN, M.C. Hayes, K.A. Murphy, M.K. Ramamurthy, B. Watkins, and J.W. Zeitler.  Meeting summary: Ninth AMS symposium of education.  Bulletin of the American Meteorological Society, 82(2):295-303 (2001).
The American Meteorological Society held its Ninth Symposium on Education in conjunction with the 80th Annual Meeting in Long Beach, California.  The theme of this year’s symposium was “Atmospheric and Oceanographic Education-Expanding our Vision for the New Millennium.” Thirty-five oral presentations and 53 poster presentations summarized a variety of educational programs or examined educational issues for both the precollege and university levels.  There was a special session reporting on a recent survey conducted by the Board on Women and Minorities as well as a special session on the educational applications of satellite meteorology and oceanography.  Over 200 people representing a wide spectrum of the Society attended one or more of the sessions in this two-day conference.  The program for the Ninth Symposium on Education can be viewed in the October 1999 issue of the Bulletin.
	[12]	REASOR, P.D., and M.T. Montgomery.  Three-dimensional alignment and corotation of weak, TC-like vortices via linear vortex Rossby waves. Journal of the Atmospheric Sciences, 58(16):2306-2330 (2001).
The vertical alignment of an initially tilted geostrophic vortex is shown here to be captured by linear vortex Rossby wave dynamics when the vortex cores at upper and lower levels overlap. The vortex beta Rossby number, defined as the ratio of nonlinear advection in the potential vorticity equation to linear radial advection, is less than unity in this case. A useful means of characterizing a tilted vortex flow in this parameter regime is through a wave-mean flow decomposition. From this perspective, the alignment mechanism is elucidated using a quasigeostrophic model in both its complete and linear equivalent barotropic forms. Attention is focused on basic-state vortices with continuous and monotonically decreasing potential vorticity profiles. For internal Rossby deformation radii larger than the horizontal scale of the tilted vortex, an azimuthal wavenumber 1 quasi mode exists. The quasi mode is characterized by its steady cyclonic propagation, long lifetime, and resistance to differential rotation, behaving much like a discrete vortex Rossby wave. The quasi mode traps disturbance energy, causing the vortex to precess, or corotate, and thus prevents alignment.  For internal deformation radii smaller than the horizontal vortex scale, the quasi mode disappears into the continuous spectrum of vortex Rossby waves. Alignment then proceeds through the irreversible redistribution of potential vorticity by the sheared vortex Rossby waves.  Further decreases in the internal deformation radius result in a decreased dependence of vortex evolution on initial tilt magnitude, consistent with a reduction of the vortex beta Rossby number.  These results are believed to have relevance to the problem of tropical cyclone (TC) genesis. Cyclogenesis initiated through the merger and alignment of low-level convectively generated positive potential vorticity within a weak incipient vortex is captured by quasi-linear dynamics. A potential dynamical barrier to TC development in which the quasi mode frustrates vertical alignment can be identified using the linear alignment theory in this case.
	[13]	ROGERS, R.F., and J.M. Fritsch.  Surface cyclogenesis from convectively-driven amplification of mid-level mesoscale convective vortices.  Monthly Weather Review, 129(4):605-637 (2001).
Mesoscale convective vortices (MCVs) are mid-tropospheric warm-core cyclonic circulations that often develop in the stratiform region of mesoscale convective systems.   Typically, divergent, anticyclonically-circulating, mesoscale cold anomalies appear both above and below the MCV.  The upper level cold anomaly is usually found near the tropopause while the low-level anomaly is surface-based and exhibits locally higher pressure.  One aspect of MCVs that has received much attention recently is the role that they may play in tropical cyclogenesis.  Of special interest is how an MCV amplifies when deep convection redevelops within the borders of its mid-level cyclonic circulation and how the amplified MCV transforms the divergent surface-based cold pool with anomalously high surface pressure into a convergent cyclonic circulation with anomalously low pressure. The Pennsylvania State University/National Center for Atmospheric Research mesoscale model MM5 is used to simulate an MCV that was instrumental in initiating, within the borders of the mid-level vortex's circulation, several successive cycles of convective development and decay over a two-day period.  After each cycle of convection, both the horizontal size of the cyclonic circulation and the magnitude of the potential vorticity associated with the vortex were observed to increase.  The simulation reproduces the development and evolution of the MCV and associated convective cycles.  Mesoscale features responsible for the initiation of convection within the circulation of the vortex and the impact of this convection on the structure and evolution of the vortex are investigated.  A conceptual model is presented to explain how convective redevelopment within the MCV causes low-level heights to fall and cyclonic vorticity to grow downward to the surface.  Applying this conceptual model to a tropical marine environment is also considered.
	[14]	Testud, J., S. Oury, R.A. BLACK, P. Amayenc, and X. Dou.  The concept of “normalized” distribution to describe raindrop spectra: A tool for cloud physics and cloud remote sensing.  Journal of Applied Meteorology, 40(6):1118-1140 (2001).
The shape of the drop size distribution (DSD) reflects the physics of rain. The DSD is the result of the microphysical processes that transform the condensed water into rain. The question of the DSD is also central in radar meteorology, because it rules the relationships between the radar reflectivity and the rainfall rate R. Normalizing raindrop spectra is the only way to identify the shape of the distribution. The concept of normalization of DSD developed in this paper is founded upon two reference variables, the liquid water content LWC and the mean volume diameter Dm. It is shown mathematically that it is appropriate to normalize by N0* % LWC/Dm4 with respect to particle concentration and by Dm with respect to drop diameter.  Also, N0* may be defined as the intercept parameter that would have an exponential DSD with the same LWC and Dm as the real one. The major point of the authors’ approach is that it is totally free of any assumption about the shape of the DSD. This new normalization has been applied to the airborne microphysical data of the Tropical Ocean and Global Atmosphere-Coupled Ocean Atmosphere Response Experiment (TOGA-COARE) collected by the National Center for Atmospheric Research Electra aircraft. The classification of the TOGA-COARE raindrop spectra into four categories (one stratiform, and three convective [0-10, 10-30, and 30-100 mm h-1]) allowed the following features to be identified. (1) There is a distinct behavior of N0* between stratiform and convective rains; typical values are 2.2 × 106 m4 for stratiform and 2 × 107 m4 for convective.  (2) In convective rain, there is a clear trend for Dm to increase with R, but there is no correlation between N0* and R.  (3) The “average” normalized shape of the DSD is remarkably stable among the four rain categories. This normalized shape departs from the exponential, but also from all the analytical shapes considered up to now (e.g., gamma, lognormal, modified gamma).  The stability of the normalized DSD shape and the physical variability of N0* and Dm are discussed in respect to the equilibrium theory of List et al. The stability of the shape implies that two parameters (and only two) are needed to describe the DSD. This stability supports the robustness of rain relations parameterized by N0*. The same TOGA-COARE dataset is used to check that the rain relations parameterized by N0* are much less dispersed than the classical ones, even after rain-type classification.
	[15]	WILLOUGHBY, H.E., and R.W. JONES.  Nonlinear motion of a barotropic vortex in still air and in an environmental zonal flow.  Journal of the Atmospheric Sciences, 58(14):1907-1923 (2001).
This study employs a Vortex Tracking Semispectral (VTSS) model cast in cylindrical coordinates that move with the vortex.  Variables are represented spectrally in azimuth only, so that the model becomes a set of linear equations for each azimuthal wavenumber component, forced by the environmental flow and coupled by wave-wave interactions that account for all of the nonlinearity. The vortex is advected by the surrounding wind and propagates when potential vorticity (PV) gradients due to the surrounding flow or the β effect force wavenumber one (WN1) asymmetries. Nonlinearity generally plays a dissipative role. Although propagation is faster in stronger PV gradients, nonlinear interactions cause the motions due to superposed PV gradients to be slower than the sum of their individual motions. In still air or uniform wind on a β plane, the wave energy spectrum falls off rapidly with wavenumber. For most situations, the calculations converge for truncation at WN6 on a 4000-km domain. In an anticyclonically sheared environmental zonal flow, the spectrum of asymmetric energy narrows because the WN2 asymmetry is forced directly by the environmental deformation. The deformation-induced asymmetry interferes destructively with WN2 due to internal wave-wave interaction. In a cyclonically sheared zonal flow, the deformation-induced and nonlinearly-induced asymmetries interfere constructively, resulting in a broader spectrum. Energy cascades from WN2 to wavenumbers $2.  A reverse cascade also carries energy to WN1, changing the β gyres and the motion. Consequent perturbation of WN1 leads to slow convergence of the predicted vortex position after 10 simulated days with increasing spectral resolution. When imposed mass sources and sinks are used to supply energy directly to the asymmetries in the middle of the spectrum, similar wave-wave interactions force WN1, leading to a trochoidal vortex track.
	[16]	Wright, C.W., E.J. Walsh, D. Vandemark, W.B. Krabill, A.W. Garcia, S.H. Houston, M.D. POWELL, P.G. BLACK, and F.D. MARKS.  Hurricane directional wave spectrum spatial variation in the open ocean.  Journal of Physical Oceanography, 31(8):2472-2488 (2001).
The sea surface directional wave spectrum was measured for the first time in all quadrants of a hurricane’s inner core over open water. The NASA airborne Scanning Radar Altimeter (SRA) carried aboard one of the NOAA WP-3D hurricane research aircraft at 1.5-km height acquired the open-ocean data on 24 August 1998 when Bonnie, a large hurricane with 1-min sustained surface winds of nearly 50 m s-1, was about 400 km east of Abaco Island, Bahamas. The NOAA aircraft spent more than five hours within 180 km of the eye and made five eye penetrations. Grayscale coded images of Hurricane Bonnie wave topography include individual waves as high as 19 m peak to trough. The dominant waves generally propagated at significant angles to the downwind direction. At some positions, three different wave fields of comparable energy crossed each other. Partitioning the SRA directional wave spectra enabled determination of the characteristics of the various components of the hurricane wave field and mapping of their spatial variation. A simple model was developed to predict the dominant wave propagation direction.