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Hurricane Arthur, July 2014: A case study in using disparate weather sources to reconstruct a weather event
October 22, 2015NWA MeetingOklahoma City, OK
Who are we at Shade Tree Meteorology, LLC ?
“A Full-Service Forensic Meteorology Firm Specializing in Severe Weather Event Reconstruction”
Majority of our business involves weather event reconstruction for cases in litigation
We also provide the following:Special event forecasts; weather risk assessment & planning; weather instrument siting
Hurricane Arthur: A brief review
▪ CAT2 landfall along NC coast
▪ Storm surge/flooding alongOuter Banks
▪ Continued northeastward but stayed offshore
▪ ‘minor impacts’ to southeastern Massachusetts
▪ Extratropical transition by 1200 UTC 5 July
http://www.nhc.noaa.gov/data/tcr/AL012014_Arthur.pdf
Key time for our case
0000 UTC Jul 5 Surface Map
0000 UTC Jul 5 500 mb
Case specifics:
▪ Located on north shore of Cape
▪ Located on a bluff (elevation 34 feet)
▪ 3.5 story house, windows facing Cape Cod Bay
Key Questions to answer:
▪ How much rain?
▪ Wind speed/direction?
▪ Was wind-driven rain occurring, how long, and how much?
Issues:
▪ Data sources▪ KBOX Radar (52 mi NW, beam elevation ~3934 ft/1.2 km) – only
inbound/outbound▪ KCHH Radiosonde (2x/day)▪ ASOS (KHYA, KPVC) – missing data at KPVC▪ Coop/CoCoRaHS
▪ Yarmouth (4 mi SW), Brewster (6 mi E)
▪ CWOP at Brewster (on a bluff elevation 39ft, few blocks in from shore)▪ On-site anemometer shown in picture provided by client (N/A)
▪ Building code
▪ Timing
▪ Bluff speedup effect
1: Rainfall
▪ Radar
▪ Nearby obs
2. Winds
▪ Key issue: onshore vs. offshore flow
▪ Initially storm to south/southeast▪ Wind from southeast
▪ As storm moves northeastward, wind shifts▪ Northeast, then north, then northwest
▪ Recall: bluff elevated ~34 ft above shoreline
Key data: radar base velocity
▪ 52 miles northwest of site
▪ Beam height ~4000 ft
▪ Compare with winds in KCHH sounding
Radar velocity comparison
5 July 8:43 p.m. 5 July 11:30 p.m.
Also important: CWOP at Brewster
▪ 5 miles east-northeast of site
▪ On a bluff (~39 ft)
▪ Compare with other ASOS obs (PVC and HYA)
▪ Use Durst curve to estimate wind gusts with known 2-min wind speed
End result:
▪ Cross compare all available observations of wind, both at the surface and aloft, to estimate wind speeds and directions at site
▪ Effective height of building is higher when wind is blowing from N than from S▪ Take this into effect using power law to estimate faster winds
aloft
The result:Date Hour ending Hourly Precip (inches)
light rain = light green, moderate rain = dark green)
Highest 2-minute average wind speed (mph) at 3.5 stories*direction is FROM which wind blows
Wind gust range at 3.5 stories (42 ft for SE/E/SSE/ESE, 76 ft for NE, NW, NNW, N, WNW)
7/4/2014 1:00 p.m. .1 SE at 7
2:00 p.m. .1 SSE at 17
3:00 p.m. .1 SE at 14 20-28
4:00 p.m. .1 E at 12 17
5:00 p.m. .1 ESE at 14 21
6:00 p.m. .1 NE at 12 19
7:00 p.m. .25 NE at 12 19-31
8:00 p.m. .3 N at 16 22-31
9:00 p.m. .3 NNW at 19 25-37
10:00 p.m. .25 N at 15 21-31
11:00 p.m. .25 N at 15 21-31
7/5/2014 12:00 a.m. .1 NNW at 19 17-31
1:00 a.m. .1 NW at 22 31-43
2:00 a.m. NW at 25 35-37
3:00 a.m. .1 WNW at 27 37-43
4:00 a.m. .1 WNW at 25 35-40
5:00 a.m. .1 WNW at 25 35-37
6:00 a.m. .25 NW at 27 37-43
7:00 a.m. .1 NW at 25 35-37
8:00 a.m. .1 NW at 20 27-40
9:00 a.m. .1 NW at 15 21-31
10:00 a.m. .1 WNW at 12 17-25
Total: 2.27
Building code: Must withstand 3 second gusts 42.46 mph
Conclusions:• Strongest winds did not
correlate with period of heaviest rain
• Fastest gusts were still at or below code level
• Question for client: window design?
• Client question for us: winds on ACK/MVY?
Last question from client: speedup effect
▪ When air flows across a bluff, a speedup occurs
▪ Does this effect matter in this case?
FINDINGS:
▪ Speedup effect largest at bluff heightat 1.27x sustained wind speed
▪ Speedup effect at 3.5 stories: 1.1xUAA Civil Engineering
The bottom line
▪ Used any and all available data to get information about observed wind speeds and gusts, both at the surface and aloft
▪ Carefully considered topography and shape of coastline, as well as known information about track of storm
▪ Considered the relationship between wind direction and ‘effective’ building height
▪ Investigated speed-up effect of winds blowing across a bluff
