Ocean Waves and Surf Forecasting: Wave Climate and Forecasting

Transcription

1 Overview Ocean Waves and Surf Forecasting: Wave Climate and Forecasting Ocean regions Characterizing and describing ocean waves Wave theory, propagation, and dispersion Refraction, shadowing, and bathymetry Stuart H. Sweeney Department of Geography University of California, Santa Barbara Wave climate Forecasting Winter 27 Time-Scales & Spatial-Scales Motivation Surfer questions: When will waves arrive from a storm at a given location? How big will the waves be when they arrive? Where should I surf? Will the waves break at one of the local beaches/reefs? Alternative approaches to answering the questions: Long time scale (long-distance trip planning): wave climate Near term (surf next week? tomorrow?): forecasting -Experience / local knowledge - On-line materials (Wavewatch III, surface analysis, CDIP) - Forecasting hobbiest Wave Climate: global Wave Climate: Regional (pp , Surf Science) Wave climate: Western Europe: Variation in wave characteristics (period, height, direction) over seasons for a particular place. - mid-lat depressions - December-March - Ex-hurricanes reform Aug / Sep - Ireland / UK biggest (and messiest) - France, Portugal, Spain clean big swells

2 Wave Climate: Regional (pp , Surf Science) Western Indian Ocean: - Cyclones north of equator; May/June, Oct/Nov big surf to Arabian Sea and Bay of Bengal. - Cyclones south of equator; Nov/Feb Madagascar, Mauritius, Reunion. Wave Climate: Regional (pp , Surf Science) Also look at: North Pacific Western North America Mexico & Central America Wave Climate: measurement Remote: Radar altimetry - radiation pulse, time delay surface height - long / short period wave mixed together - no directionality - good height measures and global coverage - wave source not distinguished In-situ: Buoys: detailed directional spectrum / limited coverage. Wave Climate: local Wave Climate: local Wave Climate: local

3 Wave Climate: local Forecasting: Short Range Surface pressure charts: - wind speed and direction (wind barbs) - path of the storm (lower / upper interaction) - local winds and expected changes Wave Height & Period Contours: - NOAA Wavewatch III - mathematical model: inputs(obs, other model output), output (directional spectrum and derived summaries) Wave buoy data - CDIP nowcast / forecasts Forecasting: Short Range Wave model issues: Not primarily intended for surfers Attempting to show areas of rough seas (shipping) Errors in input data (wind model) will result in errors in wave data Systematic under / over prediction -low quality wind sea overestimate height - high quality ground swell in narrow period band underestimate height. Directionality is average of spectrum at that location. Motivation Surfer questions: When will waves arrive from a storm at a given location? How big will the waves be when they arrive? Where should I surf? Will the waves break at one of the local beaches/reefs? Alternative approaches to answering the questions: Long time scale (long-distance trip planning): wave climate Near term (surf next week? tomorrow?): forecasting -Experience / local knowledge - On-line materials (Wavewatch III, surface analysis, CDIP) - Forecasting hobbiest How big? Wind Wave Pierson-Moskowitz spectrum (height, period)=f(fetch, wind speed, duration) Beaufort Scale Simplified Tables in Cool (pp , Tables B-2 and B-4) Example: (see Tables) How big? Energy Loss: Distance decay factor (Cool, pp. 6, Fig. 28) Angular spreading (Cool, pp. 61) Example: Assume: Winds: kts Location: beach 45 o off storm heading, ~7 km (Swell Height)x(Decay factor)x(1-spreading Loss) 13 x.4 x (1-25) = 4.5

4 How big? Other factors: Some areas have greater than normal decay, which needs to be taken into account. Storms forming in the Tasman Sea located between Australia and New Zealand can be ideal for the local breaks on the east coast of Australia. However, swell-making systems moving north through this region will encounter an acute decay factor for other regions like California and Hawaii. Storms forming in the Tasman Sea face shallower ocean depths farther north and to the east due to the numerous South Pacific islands. These islands, including New Caledonia, Anuatu, Fiji, and Somoa have water depths around them ranging from only 2 to 2 meters, compared with deeper water depths of 4 meters of more in the lower portion of the Tasman Sea. These islands will soak up much of the energy before it can travel to California or Hawaii, causing greater than normal decay. Cool (23) pp Rate x Time = Distance Time = Distance / Rate Rate = Group Speed (c g ) Deep water wave speed: Wave speed ~ 1.56T m s -1 Group speed = c/2 ~ 8T m s -1 or 2.87T km per hour Example: (see Table) Rate x Time = Distance Time = Distance / Rate Distance = f(storm coordinates, reef/beach coordinates) Option 1: Azimuthal Equidistant Projection Option 2: Spherical Trigonometry Example: Storm: (-128,32) Campus Point: ( , ) Option 1: Measure distance on map d(storm,cp) = 97mm Use map scale to convert 12mm per 1km (from map scale) (1/12) x 97 = 88 km Arrival Time: (88 km / 28.1 kph) = 28.8 hours Example: Storm: (-128,32) Campus Point: ( , ) Option 2: see handout (from course website) Arrival Time: ( km / 28.1 kph) = hours Where should I surf? Swell Windows Use protractor and azimuthal equidistant map Read off or calculate swell windows Isochrones for given swell period Refraction Need bathymetric data Different wave rays and wave focusing for different swell directions. Not easy to calculate by hand

5 Table: Wind Wave Table: Wind Wave Wind Duration Height Period (T) (knots) (hours) (feet) (sec.) Wind Duration Height Period (T) (knots) (hours) (feet) (sec.) Table: Angular Spreading Loss Figure: Distance Decay Factor Angle Loss (degrees) Storm heading angle Your beach Figure: Distance Decay Factor Figure: Distance Decay Factor

6 Figure: Distance Decay Factor Table: Wind Wave Group Speed Wind Duration Height Period (T) Wave Swell Swell Speed (knots) (hours) (feet) (sec.) Speed (c) Speed kph mph If the storm is 7 km away and waves are 1 feet at the storm center, they will decay to 4 feet by the time they cover the distance to shore. Table: Wind Wave Group Speed Wind Duration Height Period (T) Wave Swell Swell Speed (knots) (hours) (feet) (sec.) Speed (c) Speed kph mph

7 36-(177-9)=273