INCORPORATING HRD SURFACE WIND FIELDS INTO THE SLOSH MODEL

Principal Investigator: Samuel H. Houston
Collaborating scientist(s):
Dr. Wilson A. Shaffer (TDL)
Dr. Mark D. Powell
Dr. Jye Chen (TDL)
Objective: To compare the Hurricane Research Division's (HRD) surface wind fields with the input fields for the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model and then incorporate the HRD winds into the SLOSH model.
Rationale: HRD has developed new methods to produce surface winds fields in tropical cyclones based on all available surface wind observations, including aircraft flight-level observations adjusted to the surface. HRD and the Techniques Development Laboratory (TDL) evaluated the parametric wind model used as input to the National Weather Service's (NWS) SLOSH model through comparisons with HRD's surface wind fields in recent hurricanes (Houston et al. 1995). Previously, the SLOSH model was tested for hurricane landfalls along the U.S. coastlines and calculated storm surge heights were found to have an accuracy of 20% when the hurricane is adequately described.
Method: The HRD surface wind analyses are based on all available surface wind observations from buoys, Coastal-Marine Automated Network (C-MAN) platforms, ships, and surface facilities. Because these types of data are often sparse near tropical cyclones, especially over the ocean, aircraft flight-level observations near the storm are used to supplement the in situ surface measurements after adjustment to the surface with a planetary boundary layer model. All of the observations in the data set are adjusted to conform to a common framework for exposure (marine or open-terrain over-land), height (10 m), and averaging time (10 min average for comparison with SLOSH model winds). The values input to the SLOSH parametric model are the tropical cyclone's position (either observed or forecast), size, and intensity. Using these input parameters, SLOSH computes a wind field which is the primary forcing mechanism for the oceanographic processes.
Accomplishment: Comparisons of HRD and SLOSH wind fields for several recent hurricanes were completed. For example, the HRD surface winds (Figure 1) for Hurricane Bob (1991) when it was east of North Carolina indicated that the strongest winds were to the right of the storm (Bob was moving north-northeast). The SLOSH parametric wind model produced wind speeds slightly greater than the HRD wind speeds near the area of peak winds (> 44 m/s) and to the left of the storm (the observed and SLOSH computed storm surge for North Carolina were nearly the same). However, east of the area of peak winds, the SLOSH values decreased at a faster rate than the HRD winds. Another example of these comparisons was for Hurricane Emily (1993) in which the peak HRD wind speeds (Figure 3) were nearly the same strength on the west and east sides of the northward moving storm during its closest approach to North Carolina. This resulted in higher than forecast storm surge near Hatteras, North Carolina caused by strong winds across Pamlico Sound. The SLOSH model isotachs for Emily (Figure 4) indicate that the storm's peak winds were weaker and the strongest winds were only to the right of the storm unlike the HRD surface winds. Andrew (not shown) and Bob showed only small differences in areas impacted by significant storm surge (the differences between the observed and SLOSH computed storm surge values were small). For these two storms, the differences between the SLOSH and HRD peak winds are within the expected range of errors associated with the wind observations and the assimilation techniques used in the HRD wind analysis procedures. However, in the case of Emily, the SLOSH model could not capture the stronger winds to the left of the storm's track, resulting in an underestimate of the forecast storm surge on the Pamlico Sound side of the North Carolina Outer Banks near Hatteras. Incorporation of the HRD winds into the SLOSH model is currently underway and should prove useful for providing real-time storm surge forecasts.
Key reference:
S. H. Houston and M. D. Powell, 1994: Observed and modeled water level response from Tropical Storm Marco (1990). Wea. Forecasting, 9, 427-439.

S. H. Houston, W. A. Shaffer, M. D. Powell, and J. Chen, 1995: Comparisons of SLOSH parametric and HRD analyzed surface wind fields in recent hurricanes. A.M.S. 21st Conf. on Hur. and Trop. Met., 24-28 Apr. 1995, Miami FL, 619-621.

S. H. Houston, W. A. Shaffer, M. D. Powell, and J. Chen, 1995: Incorporating HRD surface wind fields into the SLOSH model. A.M.S. Conf. on Coastal Ocean. and Atmos. Pred., 29 Jan. - 2 Feb. 1996, Atlanta, GA.


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