Analysis of Meteorological and Oceanic Conditions During Rip Current Events Along the Outer Banks of North Carolina During the 2006 and 2007 Summer Seasons
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Date
2008-11-06
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Abstract
Rip current events, as reported by lifesaving personnel along the North Carolina coast, from Duck to Nags Head, were examined during the 2006 and 2007 summer forecasting periods. Rip current events were analyzed in conjunction with various meteorological and oceanographic parameters that have been suggested to be important in the formation of rip currents elsewhere. It was found that the frequency of rip current rescues and reports increases most significantly for: 1) mean wave directions approaching shore normal 2) wind directions between 22.5 degrees and 90 degrees to shore 3) significant wave heights between 0.5 meters and 1.5 meters 4) dominant wave periods from 5-10 seconds. Surface weather maps were analyzed in an effort to assist forecasters in recognizing the weather patterns, which produce conditions that are favorable for rip current formation. The sources of ocean waves present along the NC coast during rip currents events were identified in order to determine, which types of weather systems, were most likely to produce waves that cause dangerous rips currents during the summer season. Waves generated by winds on the cold side of exiting frontal systems were found to be present on the most days with rip current reports. Storms in the South Atlantic, tropical cyclones, Atlantic high pressure systems, and prefrontal winds were responsible for generating the remainder of rip current producing waves during the summer season.
Examination of rip current events throughout the study region revealed several similarities to prior research conducted in other regions. However, some of the wave and wind conditions, and meteorological patterns, that may create conditions favorable for the generation of rip currents in the study region appear to be unique. Additionally, several simulations of wave conditions, across the study region and adjacent waters, were completed using the Simulating WAves Nearshore (SWAN) model. The simulations depict the important roll distantly generated swells play in the forcing of rip currents under certain conditions. A large variation in wave heights across a fairly small stretch of coastline is also shown by the SWAN simulations. This suggests, that separate sets of parameters and weather patterns may need to be used to predict rip currents along adjacent coastlines.
Several changes to the current rip current forecasting scheme, utilized by the National Weather Service Weather Forecasting Office in Morehead City, NC were made, in the hopes of improving forecast accuracy. A directional dependancy was placed on both wind speed and wave data, so that near shore normal directions result in higher weighting. A reduction in the overall wind factor was also placed into the forecast scheme. The swell height/period matrix was expanded to account for short and very long period waves. Finally, a strong offshore wind dissipation factor was added to the forecast scheme, which reduces the overall swell weighting if offshore winds greater than 10 knots are present. A unbiased performance evaluation of the present Morehead City rip current forecast scheme, and the newly adjusted scheme was completed. The newly adjusted rip current forecast scheme exhibited a significant improvement in forecast performance over the present rip current forecast scheme, predicting more rip current rescue and report days.
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Keywords
outer banks, Duck, Rip currents, SWAN
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Degree
MS
Discipline
Marine, Earth and Atmospheric Sciences
