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60 Enclosure H Weather Analysis WIND AND WEATHER The BBRA report provides an analysis of winds from the National Oceanic and Atmospheric Administration (NOAA) National Data Buoy Center (NDBC) BUZM3 station at the mouth of Buzzards Bay (Figure G-1) and, specifically, the mean and standard deviation of the wind speeds versus month (Figure G-2, also included in the report) and the percentage above thresholds for winds greater than 25 knots, in 5-knot increments (the reportâs Table 4, as shown below). The data and monthly mean time series are available online at the NOAA NDBC website. Unfortunately, no data are presented for the associated wave conditions, which are also available at the NDBC website. Figures G-3 and G-4 show the corresponding significant wave heights and periods, respectively, versus month.
61 FIGURE G-1 Location of the NOAA NDBC BUZM3 station at the entrance to Buzzards Bay. FIGURE G-2 Average wind speed (knots) and standard deviation for winds versus time (months in the year) at NOAA NDBC BUZM3 station.
62 FIGURE G-3 Average significant wave height (m) and standard deviation versus time (months in the year) at NOAA NDBC BUZM3 station. FIGURE G-4 Average wave period (s) and standard deviation versus time (months in the year) at NOAA NDBC BUZM3 station. The authors broadly discuss wind directionality but ignore directionality for waves. The inherent assumption appears to be that winds and waves are closely aligned and hence that the
63 winds can be considered as a proxy for waves. Specifically, one can expect the strongest winds and waves to be in alignment. This is not the case for Buzzards Bay. The U.S. Army Corp of Engineers has performed a detailed hindcast of winds and waves at a series of stations along the coast of the United States in its Wave Information Study (WIS) program (http://wis.usace.army.mil/products.html?staid=63075&lat=41.25000&lon=-71.08000&dep=41). Figure G-5 shows the station locations in the vicinity of the entrance to Buzzards Bay. Station 63075 has been selected for the present analysis. (The figures and tables presented here are available at the WIS website.) Tables G-1 and G-2 show the wind speed and direction and wave height and period probability distributions, respectively, for the hindcast period. Wave information by direction (not shown here) is available at the WIS website (http://wis.usace.army.mil/atl/pcnt/wave/63075/ST63075_WAVE_allyrs.txt). Figures G-6 and G-7 show the wind and wave roses, respectively, for WIS 63075. Comparison clearly indicates that the wind speed and wave roses have substantially different patterns, with the most frequent winds from the southwest and the northwest, while the most frequent waves are from the southwest. The strongest winds and highest waves come from the northwest and southwest, respectively. The strongest winds are the result of winter storms, and the highest waves result from the unlimited fetch to the southwest. Consideration of both wind and waves in the analysis, including their directional dependence, would have been prudent, rather than only the wind speed.
64 FIGURE G-5 U.S. Army Corp of Engineers WIS hindcast sites in the vicinity of the entrance to Buzzards Bay. Hindcast period from 1980 to 1999. (http://wis.usace.army.mil/products.html?staid=63075&lat=41.25000&lon=- 71.08000&dep=41.) TABLE G-1 Probability Distribution of Wind Speed and Direction at WIS Site 63075
65 TABLE G-2 Probability Distribution of Wave Height and Period for All Directions at WIS Site 63075
66 FIGURE G-6 Wind rose at WIS Site 63075.
67 FIGURE G-7 Wave rose at WIS Site 63075. In summary, adverse weather constraints selected are never clearly stated. No analysis of waves was performed to assess the potential impact on adverse weather constraints. VISIBILITY Visibility data from the Meteorological Terminal Air Report recorded at the New Bedford Regional Airport were acquired from the National Climatic Data Center (NCDC) for October 2006 to October 2012 by the report authors. Figure G-8 shows the observation footprint for the New Bedford Regional Airport observations as well as for all nearby airports.
68 FIGURE G-8 Footprint of visibility observations from selected airports in the vicinity of Buzzards Bay, all airports (upper panel) and New Bedford Regional Airport (nominally 10-mile radius) from NOAA NCDC (lower panel).
69 Airport-based observations are the only source of routine observations of visibility available in the area, and the New Bedford airport station provides the best coverage for Buzzards Bay. Observations are made by the ferries operating from New Bedford to Cuttyhunk, Massachusetts, and from Woods Hole to Vineyard Haven, Massachusetts, but the measurements are episodic at best. Recent observations are available at the NOAA website http://forecast.weather.gov/product.php?site=NWS&format=CI&version=1&glossary=0&highlig ht=off&issuedby=BOX&product=OMR. The New Bedford airport data were analyzed to determine the number of days in each year when the visibility was 0.5 and 1.0 mile at any time during the day. The results are given in the BBRA report (Table 5 and Figure 8, reproduced above). [Hayes (2009) from the Gray, Maine, NOAA National Weather Service office used ¼ nautical mile or 0.3 mile to define the limit for low-visibility conditions in its analysis for northern New England waters.] The present analysis results in 20 percent of the days in a year (76/365) having visibility of 1 mile or less [13.4 percent (49/365) if the threshold is 0.5 mile]. As noted in the report, the visibility data are
70 hourly, and low-visibility events typically last only a few hours. Hence, for planning purposes the authors reduce the visibility-restricted days to 10 percent of those in the year for the 1-mile threshold and to 6.7 percent for the 0.5-mile threshold. This assumed reduction factor is at best ad hoc, and no argument supporting its selection is presented. Since the visibility data are available at hourly intervals at the airport, why the authors did not simply evaluate the number of hours per year (or over the record) when visibility was below some minimum that would serve as an adverse weather threshold is unclear. The data could also be analyzed to determine the duration of these low-visibility events. The assumption of 10 percent of days being affected by low visibility is at best arbitrary and likely overrepresents the risk. To support this observation, Figure G-9 shows the daily average visibility (average value over 24 hourly observations) versus time for 2012. If the daily average value is used as the evaluation metric, rather than the highest 1-hour value used by the report authors, the visibility is always greater than 2 miles. If attention is restricted to the month with the lowest visibility in 2012 (October), the longest duration for visibility below 1 mile is 12 hours, while the typical duration for the rest of month ranges from 2 to 3 hours. There are about 10 events during the month or about 32 hours (1.3 days) during which visibility is below this threshold. This represents approximately 4.3 percent of the hours in the month, less than half of the value used in the analysis. If the threshold is reduced to 0.5 mile, the number of hours decreases to 16 and the percentage of hours in the month with low visibility decreases to 2.15 percent. If the threshold is 0.3 mile, as used by Hayes (2009), the number of hours decreases to 5, or 0.68 percent.
71 FIGURE G-9 Mean visibility per day versus time for 2012 at New Bedford Regional Airport, from NOAA NCDC. (http://weatherspark.com/history/30212/2012/New-Bedford- Massachusetts-United-States.) The authors never address the suitability of the airport visibility data to infer conditions in Buzzards Bay proper. A quick review indicated that a detailed analysis comparing offshore observations of visibility with observations from coastal airports was performed for northern New England waters by Hayes (2009). Figure G-10 (Hayes 2009, Figure 4) shows the percentage of low visibility for the northern New England coastline from Massachusetts to the midcoast of Maine. This analysis is based on airport and offshore buoy observations of visibility (by the Northeastern Regional Association of Coastal and Ocean Observing Systems). In the southern end of the area, which has conditions most similar to those in Buzzards Bay, the percentage of time with low visibility is 0.5 to 1.5 percent. There is little variation between land- and ocean-based observations. This compares with the estimate made above for the 0.3-mile limit from the New Bedford airport of 0.67 percent. In the absence of additional data, the New Bedford airport data can be assumed to give estimates for conditions in Buzzards Bay that are reasonable and appropriate for a risk assessment analysis.
72 FIGURE G-10 Percentage of hourly observations with visibility below ¼ nautical mile (or 0.3 mile) from Hayes (2009) for northern New England waters. ADVERSE WEATHER SUMMARY FROM REPORT The following is quoted from the BBRA report: Adverse Weather Summary: The number of days with low visibility is of importance because in the escort tug activity logs provided by MassDEP, about 50 percent of the escort requests were associated with low visibility or other inclement weather conditions (developed further in Section 5). The data show that, on average, 20 percent of the days in a year can expect visibility conditions under 1 mile. These periods of low visibility typically do not last an entire day, but last rather a few hours. For planning purposes a notional value of 10 percent will be designated as days with sustained low visibility to account for trip-scheduling opportunities to avoid low visibility periods.
73 Low-visibility conditions (fog) and high winds do not occur at the same time, therefore the number of days with each of these conditions can be added together to determine a total frequency of hazardous weather conditions. With a notional value of sustained low-visibility conditions occurring 6 percent of the year (49 days/365 ï´ 50 percent to account for less than a full day) and winds greater than 25 knots occurring 9.5 percent of the days in a year, we can expect that there are hazardous weather conditions 15.5 percent, rounded to 16 percent, of the days in a year. The adverse weather thresholds used in the analysis are never clearly stated, and the presentation related to them is confusing. It appears that 25 knots is the threshold for wind speeds and that either 0.5 or 1 mile is the threshold for visibility. In the first paragraph of the adverse weather summary, a value of 1 mile is used, whereas in the last paragraph a value of 0.5 mile is assumed. The impact of wave conditions is not addressed. If a more reasonable correction is used for the duration of low-visibility events and the 0.5-mile threshold is assumed, the combined days of adverse weather is 9.5 percent for winds greater than 25 knots and 2.15 percent for visibility less than 0.5 mile, for a total of 11.65 percent, which rounds up to 12 percent. This analysis is based on the lowest-visibility month in 2012 to give an example of the impact on the results, but it would need to be carried out for the entire record period. REFERENCE Hayes, J. C. 2009. An Applied Climatology of Low Visibility over the Coastal Waters of New Hampshire and Southern Maine. Eastern Region Technical Attachment No. 2009-04. National Weather Service, National Oceanic and Atmospheric Administration, Gray, Maine, Dec.
