Principal Investigator: James L. Franklin
Collaborating scientist(s):
Stephen J. Lord (NOAA/NCEP)
Robert W. Burpee (NOAA/NHC)
Robert E. Tuleya (NOAA/GFDL)
Sim D. Aberson
Objective: To evaluate the impact of special environmental observations on the objective hurricane track forecast guidance available to the National Hurricane Center.
Rationale: Recent research with dynamical and statistical models has shown that improved prediction of hurricane motion requires more information on the hurricane's large-scale environmental wind and height patterns than is typically available. While satellites can provide some wind data using cloud-tracking procedures in the upper and lower troposphere, the middle levels are frequently almost void of observations. Operational models used for hurricane track forecasting need information from throughout the troposp here to be initialized. As a result, operational models may fail to predict important changes of storm speed or direction due to inadequate initial data, rather than inadequate physics of the prediction models.

The Hurricane Synoptic Flow Experiments use dropwindsondes to gather vertical profiles of wind, temperature, and humidity within 1,000 km of hurricanes. The experiment is 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. The dropwindsondes define the hurricane's surrounding large-scale flow, particularly in the 400-700 mb middle tropospheric layer - the layer most directly related to tropical cyclone motion.

From 1982-1996, an analog sonde using OMEGA navigation (ODW) was used in the experiments. Beginning in 1997, the ODW will be replaced by a completely new digital design of dropwindsonde based on GPS navigation.

Method: Using the historical sample of Synoptic Flow experiments conducted by HRD from 1982-1993, an assessment was made of the impact of ODW data on three dynamical hurricane track models providing operational guidance to the forecasters at NHC: HRD's barotropic VICBAR model, NCEP's global spectral model, and the GFDL hurricane model. The overall impact of the ODWs was obtained by averaging the forecasts from the three models to form a "consensus" forecast (CON3).
Accomplishment: In this historical sample of cases, the ODW observations accounted for statistically significant reductions in 12-60 h track errors in the CON3 forecasts. The error reductions, which ranged from 16%-30%, are at least as large as the accumulated improveme nt in operational forecasts attained over the last 20-25 years. (FIGURE 1) shows the mean forecast errors with and without the ODW data, relative to forecast errors from a climatology and persistence model (CLIPER). This work is described in the following publication:

Burpee, R. W., J. L. Franklin, S. J. Lord, R. E. Tuleya, and S. D. Aberson, 1996: The impact of Omega dropwindsondes on operational hurricane track forecast models. Bull. Amer. Meteor. Soc., 77, 925-933.

More recently, the Director of NHC requested additional Synoptic Flow experiments for Hurricane Felix on 15 August 1995, and for Hurricane Edouard on 30 August 1996. ODW data identifying key environmental features influencing the tracks of these storms w ere successfully gathered and transmitted to NHC/TPC and NCEP. There is some early indication that the ODWs may have helped reduce the scatter in the numerical forecast guidance for Edouard.

In 1997, NOAA will have a new Gulfstream IV (G-IV) jet aircraft available for operational dropwindsonde flights in the environments of potential landfalling hurricanes. HRD will have responsibility for designing the flight tracks for these operational mi ssions, and will be providing real-time quality-control of the dropwindsonde data on the G-IV.

Key references:
Franklin, J. L., and M. DeMaria, 1992: The impact of Omega dropwindsonde observations on barotropic hurricane track forecasts. Mon. Wea. Rev., 120, 381-391.

Last modified: 9/10/96