Waves Waves for Weather for Weather

WeeniesWeenies

ATMO 456ATMO 456

Wave (and Marine Wind) Wave (and Marine Wind) ForecastingForecasting

• Purview of the Meteorologist Purview of the Meteorologist – National Weather ServiceNational Weather Service– Industry Industry

•Oil and GasOil and Gas

•Ship RoutingShip Routing

– NavyNavy

• Inseparable from Marine MeteorologyInseparable from Marine Meteorology

DefinitionsDefinitions

From Oceanography: An Introduction, Wadsworth ©1985

Wave PeriodWave Period

• The time it takes for successive wave The time it takes for successive wave crests to pass a fixed point.crests to pass a fixed point.

Orbital MotionsOrbital Motions

From Oceanography: An Introduction, Wadsworth ©1985

Sea and SwellSea and Swell

Waves experienced at a location are a combination Waves experienced at a location are a combination of the locally-generated wind waves (Sea) and the of the locally-generated wind waves (Sea) and the Swell propagating in from other source regions.Swell propagating in from other source regions.

L

L

v

Local Wind Wave spectrum

Measures of Wave HeightMeasures of Wave Height

• HHss - Significant Wave Height – Most widely - Significant Wave Height – Most widely used and what customer usually wantsused and what customer usually wants– Avg. of the Highest 1/3 of all waves (wind wave Avg. of the Highest 1/3 of all waves (wind wave

or swell). AKA Hor swell). AKA H1/31/3 or Combined Seas or Combined Seas

– SQRT(Sum of Squares of Sea and Swell) SQRT(Sum of Squares of Sea and Swell) approximatesapproximates

• HH1/101/10

• HHmax max and Hand Havgavg

How Wind Waves are MadeHow Wind Waves are Made

• IngredientsIngredients– Straight Winds (more or less)Straight Winds (more or less)

• Don’t forget – less friction, less cross-isobar flow and greater Don’t forget – less friction, less cross-isobar flow and greater speedsspeeds

– Over a distance (Fetch)Over a distance (Fetch)– For some period of timeFor some period of time

• Two governing equationsTwo governing equations– Depend on depth (shallow or deep)Depend on depth (shallow or deep)

• Shallow is depth of one half wavelength (“feel bottom”)Shallow is depth of one half wavelength (“feel bottom”)– Speed of Shallow water waves depends only on depthSpeed of Shallow water waves depends only on depth– Speed of deep waves depends on wavelengthSpeed of deep waves depends on wavelength– Swell waves are generally much longer wavelength/periodSwell waves are generally much longer wavelength/period– West Coast waves much longer and larger (why?)West Coast waves much longer and larger (why?)

Concept of fully arisen seaConcept of fully arisen sea

From Oceanography: An Introduction, Wadsworth ©1985

For a 30 Knot wind

Forecasting WavesForecasting Waves

• Inland Waters vs. OffshoreInland Waters vs. Offshore

• Wind wave fetch limited or notWind wave fetch limited or not

• Phasing of wind speed and direction Phasing of wind speed and direction changes (duration of straight winds)changes (duration of straight winds)

• Account for SwellAccount for Swell

Thankfully, there is a Thankfully, there is a model!model!• Spectral Ocean Wave ModelsSpectral Ocean Wave Models

• Today’s primary model:Today’s primary model:– Wavewatch IIIWavewatch III– Run by NCEP (OPC) and by NAVY Run by NCEP (OPC) and by NAVY

(FNMOC)(FNMOC)

• Of course, only as good as the model Of course, only as good as the model atmospheric winds driving itatmospheric winds driving it

Forecast ProductsForecast Products

• Coastal Marine Forecast/Inland Waters Forecast Coastal Marine Forecast/Inland Waters Forecast (and marine zones) (20nm out and 20 to 60nm) (and marine zones) (20nm out and 20 to 60nm)

• Offshore Waters ForecastOffshore Waters Forecast• Small Craft Advisories/WarningsSmall Craft Advisories/Warnings• Open Ocean: High Winds and Seas WarningsOpen Ocean: High Winds and Seas Warnings

– Gale (34 to 47 knots) and Storm (48 knots and up, Gale (34 to 47 knots) and Storm (48 knots and up, although 50 is sometimes used)although 50 is sometimes used)• Generally associated with Lows (“Gale” Low)Generally associated with Lows (“Gale” Low)

– High SeasHigh Seas•12 ft or higher, with 6 ft increment boxes (12,18,24)12 ft or higher, with 6 ft increment boxes (12,18,24)

• http://Weather.gov/marine

When Waves reach the When Waves reach the Shore…Shore…

• The term becomes SurfThe term becomes Surf

• Surf forecasting is usually a Surf forecasting is usually a specialized business for recreation specialized business for recreation and for military operationsand for military operations

• When waves break, mass is When waves break, mass is transportedtransported– Leads to a forecasting frontier:Leads to a forecasting frontier:– Rip CurrentsRip Currents

Rip Current (Runout)Rip Current (Runout)

•http://www.nws.noaa.gov/mdl/rip_current/