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Frank D. Marks Jr.

Abstract

Radar played an important role in studies of tropical cyclones since it was developed in the 1940s. In the last 15 years, technological improvements such as the U.S. National Oceanic and Atmospheric Administration (NOAA) WP-3D tail airborne Doppler radar, the operational Weather Service Radar 1988-Doppler (WSR-88D) radar network, portable Doppler radars, and the first spaceborne radar system on the National Aeronautics and Space Administration Tropical Rainfall Measuring Mission (NASA TRMM) satellite have produced a new generation of tropical cyclone data whose analysis has given scientists an unprecedented opportunity to document the dynamics and rainfall of tropical cyclones, and has led to improved understanding of these devastating storms.

The NOAA WP-3D airborne Doppler datasets led to improved understanding of the symmetric vortex and the major asymmetries. The addition of a second airborne Doppler radar on the other WP-3D enabled true dual-Doppler analyses and the ability to study the temporal evolution of the Kinematic structure over 3–6 h. The advent of the WSR-88D Doppler radar network, and the construction of portable Doppler radars that can be moved to a location near tropical cyclone landfall, has also generated new and unique datasets enabling improved understanding of 1) severe weather events associated with landfalling tropical cyclones, 2) boundary layer wind structure as the storm moves from over the sea to over land, and 3) spatial and temporal changes in the storm rain distribution. The WP-3D airborne Doppler and WSR-88D data have also been instrumental in developing a suite of operational single Doppler radar algorithms to objectively analyze a tropical cyclone's wind field by determining the storm location and defining the primary, secondary, and major asymmetric circulations. These algorithms are used operationally on the WP-3D aircraft and on the ground at NOAA's Tropical Prediction Center/National Hurricane Center.

The WSR-88D rainfall data, together with new satellite microwave passive and active sensors on the NASA TRMM satellite, are proving useful in studies of the temporal and spatial variability of rain in tropical cyclones. The instantaneous satellite snapshots provide rain estimates to improve our understanding of tropical cyclone rain distributions globally, providing estimates from one instrument and common algorithms in each basin, while the WSR-88D provides high-temporal-resolution rain estimates (1 h), to improve our understanding of the temporal variability of the rain as the storm makes landfall.

While these new datasets have led to improved understanding, they have also led to a number of new challenges that the radar meteorology community must face by transferring the understanding gained into new applications and improved numerical weather prediction. These challenges will drive our science well into the next century.

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Frank D. Marks Jr.

Abstract

Reflectivity data from the airborne radar systems on board the three NOAA aircraft were gathered during six consecutive days in Hurricane Allen of 1980. The data have been used to specify the horizontal and vertical precipitation distribution within 111 km radius of the hurricane center. The evolution of the structure and intensity of the precipitation in the storm is described from representative time composite radar maps for seven research flights made during the 6-day period.

The eyewall was characterized by a narrow ring (12–15 km wide) of intense reflectivity (42–47 dBZ) surrounding the center of the storm at a radius that varied in time from 12–40 km. The eyewall had steep radial gradients of reflectivity (4–5 dB km−1) and tilted radially outward in height. The rainbands were characterized by areas of enhanced reflectivity embedded in a region of stratiform rainfall that contained a distinct bright band at the height of the 0°C isotherm.

The most striking changes in structure during the 6-day period were the rapid contraction in eyewall radius and the development of a secondary ring of intense reflectivity 80–100 km from the storm center. These changes in eye radius appeared to be related to the vortex evolution, as discussed by Willoughby and others.

Changes in storm intensity, coincident with the eyewall radius changes, seemed to have little effect on the total storm rainfall or latent heat release. The maximum storm rainfall occurred when the storm had a double eyewall structure. After the period of the double eyewall, the mean rain rate in the eyewall increased as the storm approached maximum intensity. However, coincident with the increase in eyewall rain rate, the eyewall area decreased, resulting in little change in the total storm rainfall.

The sequence of time composites provided the first opportunity to describe, quantitatively, the precipitation distribution within 111 km of the center of a mature hurricane that was away from land influences. The rainfall analysis showed that the mean rain rates in the eyewall were a factor of 6 greater than those outside the eyewall (11.3 mm h−1 versus 1.8 mm h−1), but because the eyewall region encompassed such a small area, it only contributed 40% of the total rainfall within a radius of 1° latitude of the storm center. The precipitation distribution around the storm was asymmetric; more rainfall occurred ahead of the storm than behind. In general, the maximum precipitation in the eyewall region was within 15–20° of the storm track. The maximum rainfall in the rainband region was 40–50° to the right of that in the eyewall.

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Frank Roux
and
Frank D. Marks Jr.

Abstract

The authors present an improved version of the velocity track display (VTD) method, proposed by Lee et al., to deduce the primary vortex circulation in hurricanes from airborne Doppler radar data obtained during straightline legs through the storm center. VTD allows the derivation of one projection of the mean horizontal wind, the wavenumber 0, 1, and 2 components of the tangential wind and one projection of the radial wind, in a series of concentric rings centered on the storm circulation center. The extended VTD (EVTD) algorithm determines additional information through a combination of data collected during successive legs: the Cartesian components of the mean horizontal wind; the wavenumber 0, 1, and 2 components of the tangential wind; and the wavenumber 0 and 1 components of the radial wind.

Application of EVTD to airborne Doppler data collected on 17 September 1989 in Hurricane Hugo is discussed. Comparisons between the EVTD-derived winds, the flight-level measurements, and winds deduced from “pseudo-dual-Doppler” analyses show qualitatively good agreement. These results reveal the asymmetric structure of the storm and show that it was in a deepening stage, with increasing tangential wind, inflow, and upward velocity. Further applications are finally discussed.

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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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George Andrew Soukup
and
Frank D. Marks

Abstract

To determine how well a low-order wavenumber representation describes a hurricane wind speed field, given its natural variability in space and time, low-order wavenumber representations were calculated for hourly “snapshots” of the 10-m wind speed field generated by the current operational hurricane model. Two distinct periods were examined: the first when the storm is in a reasonably steady state over 7–8 h and the second where the storm is changing its internal structure over a similar time interval. Observing system sensitivity experiments were also performed using wind speed field time series obtained from interpolation of the model snapshots for each of the two periods. The time series were sampled along the flight legs of a typical “figure four” aircraft flight pattern to simulate the surface wind data collection process to ascertain the effects of the wind speed field’s temporal and spatial variability upon the low-order wavenumber analyses.

The comparison between the model wind speed field at any time and the wavenumber representations during the “steady state” period shows that the essential features of the wind speed field are captured by wavenumbers 0 and 1 and that including up to wavenumber 3 practically reproduces the model field. However, in the “nonsteady” period the wavenumber 0 and 1 representation is frequently unable to capture the essential characteristics of the wind speed field. The observing system sensitivity experiments suggest that when the primary circulation is rapidly changing in amplitude and/or structure during the data collection period, the low-order wavenumbers analysis of the wind speed field will only represent the temporal mean structure.

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Xiaomin Chen
and
Frank D. Marks

Abstract

Development of accurate planetary boundary layer (PBL) parameterizations in high-wind conditions is crucial for improving tropical cyclone (TC) forecasts. Given that eddy-diffusivity mass-flux (EDMF)-type PBL schemes are designed for nonhurricane boundary layers, this study examines the uncertainty of MF parameterizations in hurricane conditions by performing three-dimensional idealized simulations. Results show that the surface-driven MF plays a dominant role in the nonlocal turbulent fluxes and is comparable to the magnitude of downgradient momentum fluxes in the middle portion of TC boundary layers outside the radius of maximum wind (RMW); in contrast, the stratocumulus-top-driven MF is comparably negligible and exerts a marginal impact on TC simulations. To represent the impact of vertical wind shear on damping rising thermal plumes, a new approach of tapering surface-driven MF based on the surface stability parameter is proposed, aiming to retain the surface-driven MF only in unstable boundary layers. Compared to a traditional approach of MF tapering based on 10-m wind speeds, the new approach is physically more appealing as both shear and buoyancy forcings are considered and the width of the effective zone responds to diurnal variations of surface buoyancy forcing. Compared to the experiments retaining the original MF components, using either approach of MF tapering can lead to a stronger and more compact inner core due to enhanced boundary layer inflow outside the RMW; nevertheless, the radius of gale-force wind and inflow layer depth are only notably reduced using the new approach. Comparison to observations and further discussions on MF parameterizations in high-wind conditions are provided.

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John F. Gamache
,
Frank D. Marks Jr.
, and
Frank Roux

Abstract

Three different airborne Doppler radar sampling strategies were tested in Hurricane Gustav (1990) on 29 August 1990. The two new strategies were the fore-aft scanning technique (FAST) and airborne dual-platform Doppler sampling. FAST employs radar mans in cones pointing alternately fore and aft of the vertical plane that is perpendicular to the flight track. The airborne dual-platform sampling uses two Doppler radars, each aboard a separate aircraft. The Doppler radars scan strictly in the vertical plant normal to the flight track. The aircraft fly simultaneously along different, preferably perpendicular, tracks. The third strategy tested in Hurricane Gustav was single-platform sampling, which uses one Doppler radar on one aircraft that flies two consecutive, usually orthogonal, flight tracks. The antenna scans in the plane normal to the flight track. The third technique had been used previously in hurricanes and other disturbed weather.

The rms differences between the aircraft in situ winds and the Doppler winds derived near the aircraft by single-platform sampling, dual-platform sampling, and FAST are found to be 7.8, 5.1, and 2.5 m s−1, respectively. These results suggest that in hurricanes dual-platform flat-plane sampling and FAST both enable substantial improvements in the accuracy and temporal resolution of airborne Doppler wind fields over those obtained from single-platform, fiat-plane scanning. The FAST results should be applicable to dual-beam sampling, which began in 1991. The actual rms errors of Doppler winds far from the flight tracks, at levels well above flight level, and in highly sheared environments may be significantly higher than the above differences.

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Wen-Chau Lee
and
Frank D. Marks Jr.

Abstract

This paper is the second of a series and focuses on developing an algorithm to objectively identify tropical cyclone (TC) vorticity centers using single-Doppler radar data. The first paper dealt with the formulation of a single-Doppler radar TC wind retrieval technique, the ground-based velocity-track-display (GBVTD), and the results are verified using analytical TCs. It has been acknowledged that the quality of the GBVTD-retrieved TC circulation strongly depends on accurately knowing its center position. However, existing single-Doppler radar center finding algorithms are limited to estimate centers for axisymmetric TCs. The proposed algorithm uses a simplex method to objectively estimate the TC vorticity center by maximizing GBVTD-retrieved mean tangential wind.

When tested with a number of axisymmetric and asymmetric analytical TCs, the accuracy of the TC centers estimated by the GBVTD-simplex algorithm is ≈340 m from the true center. When adding 5 m s−1 random noise to the Doppler velocities, the accuracy of the TC centers is nearly unchanged at 350 m. When applying the GBVTD-simplex algorithm to Typhoon Alex (1987), the estimated uncertainty varies between 0.1 and 2 km. When the overall velocity gradient is weak, the uncertainties in the retrieved TC centers are usually large. The GBVTD-simplex algorithm sometimes has problems finding a solution when a large sector of Doppler radar data is missing in conjunction with weak velocity gradients.

The GBVTD-simplex algorithm significantly reduces the uncertainties in estimating TC center position compared with existing methods and improves the quality of the GBVTD-retrieved TC circulation. The GBVTD-simplex algorithm is computationally efficient and can be easily adapted for real-time applications.

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Frank D. Marks Jr.
and
Robert A. Houze Jr.

Abstract

Airborne Doppler radar measurements are used to determine the horizontal winds, vertical air motions, radar reflectivity and hydrometer fallspeeds over much of the inner-core region (within 40 km of the eye) of Hurricane Alicia (1983). The reconstructed flow field is more complete and detailed than any obtained previously. The data show both the primary (azimuthal) and secondary (radial-height) circulations. The primary circulation was characterized by an outward sloping maximum of tangential wind. The secondary circulation was characterized by a deep layer of radial inflow in the lower troposphere and a layer of intense outflow above 10 km altitude. The rising branch of the secondary circulation was located in the eyewall and sloped radially outward. Discrete convective-scale bubbles of more intense upward motion were superimposed on this mean rising current, and convective-scale downdrafts were located throughout and below the core of maximum precipitation in the eyewall.

Precipitation particles in the eyewall rainshaft circulated 18–20 km downwind as they fell, consistent with the typical upwind slope with increasing altitude of eyewall precipitation cores Outside the eyewall, the precipitation was predominantly stratiform. A radar bright band was evident at the melting level. Above the melting level, ice particles were advected into the stratiform region from the upper levels of the eyewall and drifted downward through a mesoscale region of ascent. Hypothetical precipitation particle trajectories showed that as these particles fell slowly through the mesoscale updraft toward the melting level, they were carried azimuthally as many as 1½ times around the storm. During this spiraling descent, the particles evidently grew vigorously. The amount of water condensed by the ambient mesoscale ascent exceeded that transported into the stratiform region by the eyewall outflow by a factor of 3. As the particles fell into the lower troposphere, they entered a mesoscale region of subsidence, the top of which coincided with the radar bright band.

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Frank D. Marks Jr.
and
Robert A. Houze Jr.

A pulse-Doppler radar on board a National Oceanic and Atmospheric Administration (NOAA) WP-3D research aircraft has been used to map the wind field in the vicinity of the developing eye wall of Hurricane Debby, which occurred in 1982. The Doppler-derived winds in the eye wall region compare favorably with winds measured aboard the aircraft. The Doppler radar allowed the wind field to be documented in much more detail than has been possible in previous hurricane studies. The maximum winds were found radially, just inward of the band of maximum radar reflectivity, and were concentrated in two mesoscale maxima. A mesoscale trough associated with the developing eye wall sloped upwind and radially outward through the 1–5 km layer. The trough was best defined at 2–3 km, where it contained a closed mesocyclonic circulation.

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