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James L. Franklin

Abstract

Omega dropwindsonde (ODW) observations from three synoptic-flow experiments in environment of Hurricane Josephine have been analyzed in a research mode using an objective analysis procedure. The nominal times of the analyses are 0000 UTC 10, 11, and 12 October 1984. The filtered, three-dimensional analyses have been used as a basis for several diagnostic and prognostic calculations relating to the motion of the hurricane.

Examination of Josephine's environment revealed a strong variability of the flow with distance from the storm center and with pressure. Josephine moved at right angles to the azimuthally averaged wind at 500 mb; the vortex motion was more consistent with the flow near 700 mb. Forecasts made with a barotropic forecast model showed a high sensitivity of the forecast track to the vertical layer used in the initial analysis. These results demonstrate the potential value of vertical sounding information from the ODWs, and show that single-level midtropospheric information is not always representative of a hurricane's environment flow.

On each of the three days, the motion of Josephine deviated significantly from its environmental “steering,” as measured by an azimuthal average of the 300–850 mb mean flow over the 5°–7° radial band. This deviation from steering (the so-called “propagation” vector) was oriented with components parallel and to the left of the gradient of absolute vorticity in the asymmetric wind field. The magnitude of the propagation was proportional to the strength of the absolute vorticity gradient. These results are consistent with many barotropic modeling studies.

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James L. Franklin and Mark DeMaria

Abstract

A scarcity of observations in the hurricane environment is one factor believed to be limiting the improvement in hurricane track forecast accuracy. Since 1982, the Hurricane Research Division (HRD) of the NOAA Atlantic Oceanographic and Meteorological Laboratory has conducted 14 experiments to determine the wind and thermodynamic fields within about 1000 km of tropical cyclones in the Atlantic basin. During these synoptic-flow experiments, Omega dropwindsondes (ODWs) are released from the two NOAA WP-3D research aircraft over a 9–10-h period in the hurricane environment. The ODWs measure pressure, temperature, humidity, and wind as they descend from flight level (about 400 mb) to the surface. These data are then transmitted in real time to the National Hurricane Center (NHC) and the National Meteorological Center (NMC).

Recently, a barotropic, nested, spectral hurricane track forecasting model, VICBAR, has been developed at HRD and tested quasi-operationally during the 1989 and 1990 hurricane seasons. Forecasts from this model have compared favorably with other models run at NHC and NMC. In this study, the VICBAR model is used to evaluate the impact of ODW data on track forecast error for the 14 HRD synoptic-flow experiments.

The ODW data produced highly consistent reductions in track forecast errors in this sample of cases. Forecast improvements due to single-level midtropospheric (aircraft) data were significantly smaller than those due to the ODWs. At the important verification times of 24–36 h (prior to landfall), when the decision to issue a hurricane warning is being made, the ODWs reduced the model mean forecast error by 12%–16%. These improvements, statistically significant at the 99% level, are comparable to the total improvement in normalized NHC official 24-h forecast error occurring over the, past 20–25 years.

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John L. Beven II and James L. Franklin

Abstract

The 1999 hurricane season in the eastern North Pacific is summarized, and individual tropical storms and hurricanes are described. Producing only nine named storms, the season tied 1996 as the second least active on record. Hurricane Dora was the strongest and longest-lived cyclone of the season. Hurricane Greg, the only cyclone to make landfall during the season, weakened to a tropical storm just before moving ashore in Baja California, Mexico. Fifteen deaths resulted from the tropical cyclones.

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James L. Franklin and Paul R. Julian

Abstract

The major sources of error in Omega-derived wind estimates are examined and illustrated. Sample dropwindsondes and local Omega signals are used to illustrate the effects of several types of phase propagation anomalies. A stationary test sonde and synthetic Omega phases are used to determine the accuracy of three Omega phase-smoothing algorithms and their associated error estimates and to determine the impact of base station motion for sondes released from aircraft.

Omega windfinding errors can be classified as either “internal” or ”external” errors. Internal errors are associated with signal quality and transmitter-sonde geometry, while external errors are caused by anomalous phase propagation. Estimates of wind error (wind uncertainties) are provided by the equations of Omega windfinding. These uncertainties, however, estimate only the effects of internal errors. Precise assessment of errors caused by anomalous phase propagation requires the measurement of phase data by a stationary receiver. Such measurements show that errors from external sources range from about 1 m s−1 for diurnal changes in ionospheric height to 20–30 m s−1 for sudden ionospheric disturbances. Methods for dealing with these problems in sonde postprocessing are described.

Data from a stationary test sonde show that the effect of aircraft maneuvers on real-time Omega wind estimates is substantial; during turns, errors in real-time wind estimates increase by over 50%. The comparison of phase-smoothing algorithms shows that cubic-spline smoothing produces wind estimates 20–50% more accurate than those obtained with other methods. Hence, it is recommended that this smoothing algorithm be used in dropwindsonde postprocessing. It is estimated that such postprocessing will reduce errors by 60% during aircraft turns and by 30% at other times.

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Michael T. Montgomery and James L. Franklin

Abstract

The validity of the traditional balance approximation for the asymmetric flow above the boundary layer generally in hurricanes is examined here. Scaling considerations of the divergence equation show that the validity of the balance approximation hinges on the smallness of the nondimensional product (δn/ζn)×(n υ/η r). The first term represents the ratio of asymmetric horizontal divergence to asymmetric vertical vorticity for azimuthal wavenumber n, while the second term represents a Rossby number based upon the azimuthal mean tangential wind and absolute vertical vorticity of the hurricane vortex. Wind observations of Hurricane Gloria (1985) indicate that this product is not at all small in the near-vortex region (several hundred kilometers beyond the radius of maximum tangential winds) where asymmetric convergence forced by surface friction and cumulus convection is typically large. Although the Gloria observations represent only a single case, there are dynamical reasons to expect this product to be O(1) just above the hurricane boundary layer in steadily translating hurricanes. The meteorological relevance of these results to the problem of balance dynamics in hurricanes is briefly discussed.

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Lloyd J. Shapiro and James L. Franklin

Abstract

A set of nine synoptic-flow cases, incorporating Omega dropwindsonde observations for six tropical storms and hurricanes, is used to deduce the three-dimensional distribution of potential vorticity (PV) that contributed to the deep-layer mean (DLM) wind that steered the cyclones. A piecewise inversion technique, the same as that previously applied by Shapiro to Hurricane Gloria of 1985, is used to derive the DLM wind induced by pieces of anomalous PV restricted to cylinders of different radii centered on each cyclone. The cylinder of PV that induces a DLM wind that best matches the observed DLM wind near the center of each cyclone is evaluated.

It is found that the results can be loosely placed into two categories describing the spatial scale of the PV anomalies that influenced the cyclone’s motion. Four of the cases, including Hurricane Gloria, had “local” control, with a good match (to within ∼40%) between the observed DLM wind near the cyclone center and the DLM wind attributable to a cylinder of PV with a given radius ⩽1500 km. Further decomposition of the PV anomaly into upper (400 mb and above) and lower levels (500 mb and below) indicates the dominance of upper-level features in steering two of the cyclones (Hurricanes Gloria of 1985 and Andrew of 1992), while Hurricane Debby of 1982 was steered by more barotropic features. These results supplement those found in other studies.

Five of the cases, by contrast, had “large-scale” control, with no cylinder of radius ⩽2000 km having a good match between the induced and observed DLM wind. Hurricanes Emily of 1987 and 1993 fell into this category, as did Hurricane Josephine of 1984. Implications of the results for guiding in situ wind measurements to improve hurricane track forecasts are discussed.

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Christopher W. Landsea and James L. Franklin

Abstract

“Best tracks” are National Hurricane Center (NHC) poststorm analyses of the intensity, central pressure, position, and size of Atlantic and eastern North Pacific basin tropical and subtropical cyclones. This paper estimates the uncertainty (average error) for Atlantic basin best track parameters through a survey of the NHC Hurricane Specialists who maintain and update the Atlantic hurricane database. A comparison is then made with a survey conducted over a decade ago to qualitatively assess changes in the uncertainties. Finally, the implications of the uncertainty estimates for NHC analysis and forecast products as well as for the prediction goals of the Hurricane Forecast Improvement Program are discussed.

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Sim D. Aberson and James L. Franklin

In 1997, the Tropical Prediction Center (TPC) began operational Gulfstream-IV jet aircraft missions to improve the numerical guidance for hurricanes threatening the continental United States, Puerto Rico, and the Virgin Islands. During these missions, the new generation of Global Positioning System dropwindsondes were released from the aircraft at 150–200-km intervals along the flight track in the environment of the tropical cyclone to obtain profiles of wind, temperature, and humidity from flight level to the surface. The observations were ingested into the global model at the National Centers for Environmental Prediction, which subsequently serves as initial and boundary conditions to other numerical tropical cyclone models. Because of a lack of tropical cyclone activity in the Atlantic basin, only five such missions were conducted during the inaugural 1997 hurricane season.

Due to logistical constraints, sampling in all quadrants of the storm environment was accomplished in only one of the five cases during 1997. Nonetheless, the dropwindsonde observations improved mean track forecasts from the Geophysical Fluid Dynamics Laboratory hurricane model by as much as 32%, and the intensity forecasts by as much as 20% during the hurricane watch period (within 48 h of projected landfall). Forecasts from another dynamical tropical cyclone model (VICBAR) also showed modest improvements with the dropwindsonde observations. These improvements, if confirmed by a larger sample, represent a large step toward the forecast accuracy goals of TPC. The forecast track improvements are as large as those accumulated over the past 20–25 years, and those for forecast intensity provide further evidence that better synoptic-scale data can lead to more skillful dynamical tropical cyclone intensity forecasts.

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James L. Franklin and Stephen J. Lord

Abstract

Synoptic-wale thermodynamic fields in the environment of Hurricane Debby (1982) determined from two sets of VAS soundings (VAS1, VAS2) are compared with those obtained from in-situ data (INS). VAS1 sounding were derived from an iterative solution of the radiative transfer equation with manual quality control. VAS2 soundings, which represent the present state-of-the-art, were derived from a simultaneous solution of the transfer equation with objective quality control. In situ data were obtained primarily from Omega dropwindsondes. Comparisons are made for 0000 UTC 16 September 1982 at the mandatory pressure levels up to 400 mb. The integrated effect of VAS-INS differences is estimated by comparing 400 mb geopotential height fields and their associated gradient winds.

The comparisons show that the VAS1-INS temperature differences are not spatially uniform at most levels, due largely to the influence of moisture. The quality of the VAS2 data is much improved over VAS1; the effect of moisture is not noticeable. However, the VAS2 analyses still show spatially nonuniform differences from INS at some levels. Thus, VAS gradient data may be of irregular quality on the synoptic scale. Geopotential height fields at 400 mb imply gradient wind differences from INS of up to 12 m s−1 for VAS 1 and 6 m s−1 for VAS2. The VAS2 sounding set could be improved further by the use of manual data editing, and a more accurate first-guess of the surface temperature analysis.

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