Tropical Cyclone Winds at Landfall Experiment

This document is divided into FOUR sections:

Summary

This multi-option, single-aircraft experiment is designed to study the changes in TC near surface wind structure near and after landfall. An accurate description of the TC surface wind field near and after landfall in real-time is important for warning, preparedness, and recovery efforts. HRD is developing a real-time surface wind analysis system to aid the TPC/NHC in the preparation of warnings and advisories in TCs. The analyses could reduce uncertainties in the size of hurricane warning areas. Flight-level and Doppler wind data collected by a NOAA WP-3D will be transmitted to TPC/NHC where they could result in improved real-time and post-storm analyses. Doppler data collected near a WSR-88D would yield a time series of three-dimensional wind analyses showing the evolution of the inner core of TCs near and after landfall.

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Program Significance

An accurate real-time description of the TC surface wind field near and after landfall is important for warning, preparedness, and recovery efforts. During a hurricane threat, an average of 300 nmi (550 km) of coastline is placed under a hurricane warning , which costs about $50 million in preparation per event. The size of the warned area depends on the extent of hurricane and tropical storm force winds at the surface, evacuation lead-times, and the forecast of the storm's track. Research has helped reduce uncertainties in the track and landfall forecasts, but now there is an opportunity to improve the accuracy of the surface wind fields in TCs, especially near landfall.

HRD is developing a real-time surface wind analysis system to aid the TPC/NHC in the preparation of warnings and advisories in TCs. The real-time system was first tested in Hurricane Emily of 1993, but the system needs further testing before use in operational forecasts and warnings. The surface wind analyses could reduce uncertainties in the size of hurricane warning areas and could be used for post-storm damage assessment by emergency management officials. The surface wind analyses will also be useful for validation and calibration of an operational inland wind forecast model that HRD is developing under Federal Emergency Management Agency (FEMA) sponsorship. The operational storm surge model (SLOSH) could be run in real-time with initial data from the surface wind analysis.

As a TC approaches the coast, surface marine wind observations are normally only available in real-time from National Data Buoy Center (NDBC) moored buoys, C-MAN platforms, and a few ships. Surface wind estimates must therefore be based primarily on aircraft measurements. Low-level (<5,000 ft (1.5 km] altitude) NOAA and Air Force Reserve aircraft flight-level winds are adjusted to estimate surface winds. These adjusted winds, along with C-SCAT and SFMR wind estimates, are combined with actual surface observations to produce surface wind analyses. Such analyses were done after Hurricane Hugo's landfall in South Carolina and Hurricane Andrew's landfall in South Florida, as well as in real-time for Hurricane Emily's (1993) closest approach to the Outer Banks of North Carolina, and for the landfalls of Hurricanes Erin and Opal in 1995.

The surface wind analyses may be improved by incorporating airborne Doppler radar-derived winds for the lowest level available (~3,000 ft [1.0 km]). To analyze the Doppler data in real-time, it is necessary to use a Fourier estimation technique. The Velocity-Track Display (VTD) was developed to estimate the mean tangential and radial circulation in a vortex from a single pass through the eye. The technique was applied to Doppler data collected in Hurricane Gloria (1985) and found that the mean winds corresponded well with winds derived by pseudo-dual Doppler (PDD) analysis.The extended VTD (EVTD) was subsequently developed to combine data from several passes through the storm, resolving the vortex circulation up through the wave # 1 component. EVTD was used on data collected during six passes into Hurricane Hugo (1989) to show the development of mean tangential winds >100 kt (50 m s-1) over 7 h. EVTD analyses are computed quickly on the airborne HRD workstation and could be sent to TPC/NHC shortly after their computation. The wind estimates could then be incorporated into the real-time surface wind analyses.

Dual-Doppler analysis provides a more complete description of the wind field in the inner core. While these techniques are still too computationally intensive for real-time wind analysis, the data are quite useful for post-storm analysis. An observational study of Hurricane Norbert (1984), using a PDD analysis of airborne radar data to estimate the kinematic wind field in, found radial inflow at the front of the storm at low levels that switched to outflow at higher levels, indicative of the strong shear in the storm's environment. Another study used PDD data collected in Hurricane Hugo near landfall to compare the vertical variation of winds over water and land. The profiles showed that the strongest winds are often not measured directly by reconnaissance aircraft.

By 1989 both NOAA WP-3D aircraft were equipped with Doppler radars. A study of Eastern Pacific Hurricane Jimena (1991) utilizing several three-dimensional wind fields from true dual-Doppler data collected by two WP-3D's showed that a pulse of radial wind developed in the eyewall with a corresponding decrease in the tangential winds. By the fourth pass, however, the radial pulse was gone and the tangential winds had returned to their previous value. These results suggested that the maintenance of a mature storm may not be a steady-state process. Further study is necessary to understand the role of such oscillations in eyewall maintenance and evolution.

While collection of dual-Doppler radar data by aircraft alone requires two WP-3D aircraft flying in well coordinated patterns, a time series of dual-Doppler data sets could be collected by flying a single WP-3D toward or away from a ground-based Doppler radar. In that pattern, the aircraft Doppler radar rays are approximately orthogonal to the ground-based Doppler radar rays (Fig. 18), yielding true Dual-Doppler coverage.

By 1996, the Atlantic and Gulf coasts will be covered by a network of Doppler radars (WSR-88D) deployed by the National Weather Service (NWS), Department of Defense, and Federal Aviation Administration (Fig. 19). Each radar has a digital recorder to store the base data (Archive Level II). In precipitation or severe weather mode the radars will collect volume scans every 5-6 min.

If a hurricane or strong tropical storm ( i.e., one with sufficient radar scatterers to define the vortex) moves within 125 nmi (230 km) (Doppler range) of a coastal WSR-88D Doppler radar, a WP-3D will obtain Doppler radar data to be combined with data from the WSR-88D radar in dual-Doppler analyses. These analyses could resolve phenomena with time scales <10 min, the time spanned by two WSR-88D volume scans. This time series of dual-Doppler analyses will be used to describe the storm's inner core wind field and its evolution. The flight pattern for this experiment is designed to obtain dual-Doppler analyses at intervals of 10-20 min in the inner core. Unfortunately, these WSR-88D/aircraft dual-Doppler analyses will not be available in real-time, but the Doppler wind fields could be incorporated into post-storm surface wind analyses. The data set will also be useful for development and testing of TC algorithms for the WSR 88D. The Doppler data will be augmented by dropping new GPS-sondes near the coast, where knowledge of the boundary-layer structure is crucial for determining what happens to the wind field as a strong storm moves inland.

To augment the inner core analyses, dual-Doppler data can be collected in the outer portions of the storm (where the aircraft's drift angle is small) from a single aircraft using F/AST. The tail radar is tilted to point 20? forward and aft from the track during successive sweeps. The alternating forward and aft scans intersect at 40?, sufficient for dual-Doppler synthesis of winds.

Several studies indicate that loss of the oceanic moisture source is responsible for the decay of land falling TCs. These studies relied on surface observations that are usually sparse at landfall and require time-to-space compositing techniques that assume stationarity over relatively long time periods. More complete observations could help improve our knowledge of intensity change during and after landfall. With the exception of the U. S. Air Force monitoring of Hurricane Andrew's landfall in south east Florida for two passes after landfall, the decay of a TC over land has never been documented by aircraft observations. If the safety requirements can be met, the combination of WSR-88D observations with NOAA airborne Doppler radar and flight level measurements would allow detailed documentation of the thermodynamic and kinematic structural changes during landfall.

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Objectives

  1. Collect flight level wind data and make surface wind estimates to improve real-time and post-storm surface wind analyses in hurricanes.

  2. Collect single airborne Doppler radar data, analyze with EVTD, and send wind analyses in near real-time to TPC/NHC.

  3. Collect airborne Doppler radar to combine with WSR-88D radar data in post-storm three-dimensional wind analyses.

  4. Investigate the incorporation of EVTD wind fields into real-time surface wind analyses.

  5. Document thermodynamic and kinematic changes in the storm during and after landfall.

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Mission Description

This experiment will be flown with a single aircraft if a hurricane moves within 215 nmi (400 km) of the coast of the United States. If the storm moves slowly parallel to the coastline and resources permit, the experiment may be repeated with a second flight. The aircraft must have working lower fuselage and tail radars. The HRD workstation should be on board, so we can transmit radar images and an EVTD analysis back to TPC/NHC. Microphysical data should be collected, to compare rainfall rates with those used in the WSR-88D precipitation products. The SFMR should be operated, to provide estimates of wind speed at the surface. If the C-SCAT is on the aircraft then it should also be operated to provide another estimate of the surface winds. If the storm will be within 125 nmi (230 km) of a WSR-88D, arrangements must be made to ensure that Level II data are recorded.

The primary module of the experiment, the "real-time module", will support real-time and post-storm surface wind analyses. Two dual-Doppler options can be flown if the storm is near a WSR-88D radar. A coastal-survey option can be flown when the storm is too close to the coast to permit radial penetrations. The flight patterns will depend on the location of the storm relative to surface observing platforms and coastal radars.

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There are FIVE options associated with this experiment:

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