Hurricane Synoptic-Flow Experiment

This document is divided into 4 sections:


With the arrival of the new NOAA Gulfstream IV-SP high-altitude jet (G-IV), the Hurricane Synoptic Flow Experiment makes the transition from a research program to operations. Beginning in 1996, the G-IV will conduct routine "hurricane surveillance" missio ns that are essentially HRD Synoptic Flow experiments. When coordinated with these operational G-IV flights, the HRD Synoptic Flow experiment now becomes a single-option, multi-aircraft experiment As in previous years, the experiment seeks to obtain accurate, high-density wind and thermodynamic data sets from the environment and vortex regions of hurricanes that are within 72 h of potential landfall. The availability of the G-IV, however, greatly increases the amount of environment that can be sample d. GPS-based dropwindsondes (GPS-sondes) will be dropped from the G-IV and the two NOAA/AOC WP-3D aircraft to obtain these data over the normally data-void oceanic regions at distances up to 810 nmi (1500 km) from the hurricane center. Mandatory and significant level GPS-sonde data will be transmitted in real time for use in the preparation of official forecasts at the Tropical Prediction Center/National Hurricane Center (TPC/NHC). These data will also be incorporated into objective statistical and d ynamical hurricane prediction models at TPC/NHC and the National Centers for Environmental Prediction (NCEP). In a research mode, these data will be used to help improve short and medium term (24-72 h) hurricane track prediction, study the influence of synoptic-sc ale fields on vortex track and intensity, and assess methods for obtaining satellite soundings.

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Program Significance

Hurricane Synoptic Flow experiments conducted prior to 1996 used the WP 3Ds and the previous Omega-based generation of dropwindsondes (ODWs) to gather vertical profiles of wind, temperature, and humidity within 540 nmi (1,000 km) of hurricanes. The experiment was typically conducted over the data-sparse oceanic regions of the western Atlantic or Gulf of Mexico roughly 48-72 hours before the projected landfall of a mature hurricane on the coast of the United States. While satellites typically pro vide wind data in the upper and lower troposphere (near 200 and 850 mb, respectively), the middle levels - the levels most directly related to TC motion - are frequently almost void of observations. As a result, operational models often fail to predict im portant changes of storm speed or direction due to inadequate initial data, rather than inadequate physics of the prediction models. During the Synoptic Flow experiments, ODWs released from the WP-3Ds defined the hurricane's surrounding large-scale flow, particularly in the critical 400-700 mb middle tropospheric layer.

Synoptic Flow experiments were conducted on 18 occasions from 1982-93. Recent research at HRD, NCEP, and GFDL with this sample of cases demonstrates conclusively that the ODW data produce significant improvements in the operational models that are the primary guidance for TPC/NHC's official track forecasts. For consensus (averaged) forecasts from the three primary operational dynamical models (HRD's barotropic VICBAR model, GFDL's nested grid model, and NCEP's global spectral model), the ODWs were responsible for statistically significant 12-60 hour track forecast improvements of 16%-30%. These improvements are at least as large as the accumulated improvement in operational forecasts achieved over the last 20-25 years.

The size of these improvements suggests that operational GPS-sonde missions will be a highly effective way to reduce the costs associated with overwarning. Hurricane warnings are usually issued 18-24 hours before landfall for a length of coastline averaging 300 nmi (555 km). The swath of damaging winds and tides caused by hurricanes that strike land, however, is generally <100 nmi (185 km). Thus, current forecasting skill results in an overwarning zone of ~200 nmi (370 km) that is a trade-off betwe en maximizing warning lead time and keeping the warning area as small as possible. In 1990, TPC/NHC estimated that the preparation costs alone incurred by the public placed under a hurricane warning exceed $90,000 per km of coastline. By comparison, the cost of a three-aircraft dropwindsonde mission using 70 GPS-sondes (at $400 apiece) and 27 hours of flight time (at $2,800 per hour) is about $104,000. If forecasters are able to reduce the over-warning area by only 5% (20 km (12 nmi)) by taking advantage of GPS-sonde-improved numerical guidance, the cost of obtaining the data will be well worth the expenditure.

In addition to direct operational benefits of the Synoptic Flow experiments, diagnostic case studies of the ODW observations have led to improvements in our basic understanding of hurricane motion. Analyses of the existing data sets have helped to docume nt the relationship between vortex motion and the environmental flow and have provided the first observational evidence of the beta-gyres commonly found in barotropic models. A multi-scale, nested analysis of the Gloria data set has also been completed. This analysis identified a "steering envelope" in the deep-layer-mean flow just outside Gloria's eyewall. The Gloria analyses have also been used to document, for the first time, the potential vorticity (PV) distribution in a hurricane's core and environment.

Current work involving the inversion of Gloria's PV distribution is expected to provide a powerful new tool for diagnosing the synoptic features responsible for a given hurricane's steering flow. Preliminary results indicate that upper level PV features may dominate, and may act from large distances from the hurricane's center. Synoptic Flow experiments using the G-IV and WP-3Ds simultaneously will offer an unprecedented opportunity to document these features.

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The ultimate objective of these experiments is the improvement of short- and medium-range (24-72 h) hurricane track prediction. The immediate requirement is the collection of one or two data sets of GPS-sonde wind and thermodynamic soundings within 810 nmi (1500 km) of hurricanes that are threatening the United States. These data will be used by TPC/NHC and NCEP to prepare real-time analyses and official forecasts and will be incorporated in the objective statistical and dynamical h urricane prediction models.

The ODW's have been shown to be capable of improving hurricane track forecasts; however, the optimal deployment strategy is unknown. The increased range and altitude capability of a three-aircraft coordinated pattern, coupled with the PV inversion tools currently being developed, will allow the determination of optimal deployment strategies. Other research, which is just under way, is the initialization of multi-level models with the dropwindsonde data. With their added complexity, the current sample of cases is probably not large enough to adequately study the behavior of these models. These data sets will also be used to study the influence of synoptic-scale fields on changes in vortex intensity and track and to assess satellite-derived products.

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Mission Description

To collect a relatively uniform distribution of GPS-sonde soundings within ~810 nmi (1500 km) of hurricanes over a minimum period of time, both NOAA/AOC WP-3D aircraft will operate simultaneously in regions within and surrounding the hurricane. The WP-3Ds will operate simultaneously and in coordination with operational surveillance missions of the G-IV. Specific flight tracks will vary depending on such factors as the location of the storm, relative both to potential bases of operation and to particular environmental meteorological features of interest, and the operational pattern being flown by the G-IV.

A sample mission is shown below.

The two WP-3D aircraft and the G-IV will begin their missions at the same time. Subject to safety and operational constraints, each WP-3D will climb to the 500-mb level (about FL 180) or above, then proceed, step-climbing, along the routes assigned durin g preflight. It is particularly important that both aircraft climb to and maintain the highest possible altitude as early into the mission as aircraft performance and circumstances allow, and attain additional altitude whenever possible during the mission . GPS-sondes are released in one of two modes. Beyond 40 nmi (75 km) from the storm center, drops are made at pre-assigned locations, generally every 25 min or 120 nmi (222 km). These drop locations are provided with the particular mission flight tracks 2 h before blockout. Within 40 nmi (75 km) of the hurricane's center, drop locations are specified relative to the center's position (e.g., 40 nmi (75 km) north of the eye). During in-storm portions of the mission, drops will be made with possible spacings < 8 min or 40 nmi (75 km). Efforts should be made to avoid making drops in heavy precipitation, unless necessary. Aircraft turns are not expected to affect the GPS-sonde wind accuracy, but we expect to continue the practice of making drops AFTER THE TURN IS COMPLETE.

Usually, one aircraft will fly through the hurricane center and execute a Doppler figure 4 pattern. This aircraft's Doppler radar should be set to scan perpendicular to the aircraft track. "Hard" center fixes are not desirable. On the downwind leg of the figure 4, the Doppler should be set to record forward and aft (F/AST) continuously. If both aircraft penetrate the storm, the figure 4 pattern will generally be executed by the second aircraft through the storm, and the first aircraft through will collect vertical incidence Doppler data. Coordination with potential USAF reconnaissance is necessary to ensure adequate aircraft separation. The in-storm portion of the missions is shown schematically in Fig. 2, although the actual orientation of these tracks may be rotated.

Of paramount importance is the transmission of the GPS-sonde data to NCEP and TPC/NHC for timely incorporation into operational analyses, models, forecasts, and warnings. Operational constraints dictate an 0600 or 1800 UTC blockout time, so that the GPS-sonde data will be included in the 1200 or 0000 UTC analysis cycle. Further, limiting the total block time to 9 h allows adequate preparation time for aircraft and crews to repeat the mission at 24-h intervals. These considerations will ensure a fixe d, daily real-time data collection sequence that is synchronized with NCEP and TPC/NHC's analysis and forecasting schedules. If the missions are not to be repeated, then requested block times may exceed 9 h. In addition to the GPS sonde data, three to four RECCO's per hour should be taken and transmitted during each mission.

Special Notes

Missions very similar to the Synoptic Flow missions may be flown in clear-air (non-hurricane) conditions to collect GPS-sonde data sets for satellite sounding evaluations. These clear-air missions differ from the normal experiment as follows:

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