Impact of swell on the wind-sea and resulting modulation of stress

Charles L. Vincent, Jim Thomson, Hans C. Graber, Clarence O. Collins

Research output: Contribution to journalArticle

Abstract

Investigation of 37,106 ocean surface wave spectra from the Pacific, Atlantic Ocean, and Gulf of Mexico demonstrate that swell modulates the energy level of the high frequency tail of the wind-sea wave spectrum, altering sea surface roughness. With a mixture of sea and swell, the wind-sea part of spectra follows the well-known f-4 (equilibrium range) and f-5 (saturation range) power laws. Swell modulates the energy levels but does not change the power-law structure. For swell with minimal winds, the spectra follow the −4, −5 power-law paradigm, but energy correlates to swell steepness not wind speed. Swell shifts the transition between the two sub-ranges towards lower frequencies. For sea-swell mixtures, a modulation factor λ is proposed that depends on wind speed and swell steepness which allows parameterization of the spectral tail. Comparison of large swell with little wind to wind-sea spectra of same height and period, indicates that there is little difference in spectral shape and suggests that the Hasselmann Snl source term is likely the mechanism by which energy is transferred into the wind-sea tail causing the modulation. Analysis of 33,000+ directional spectra at Ocean Station Papa shows that the mean direction for the wind-sea high frequency tail is strongly correlated to wind direction, no matter the swell direction or steepness or level of swell dominance. An equation for the friction velocity of a sea state with swell (u*s) is developed, u∗s1/2u∗o where u∗o is the friction velocity in the absence of swell, by neglect of the direct swell impact. Noting that this is only a partial estimate of the total measured stress, the prediction is evaluated for 3,000+ observed spectra yielding a correlation of 0.91 suggesting that it may be of consequence. Observations of u/u∗o suggest a dependence with swell steepness that is similar to that predicted by λ1/2. At low winds, λ1/2 overestimates the stress, but noting that it was derived absent the components from the swell frequencies. In the tail, the momentum transport is downward, while in the swell the transport is predominantly upward, suggests a possible correction for λ1/2. The case of a swell generated wind is discussed.

Original languageEnglish (US)
Article number102164
JournalProgress in Oceanography
Volume178
DOIs
StatePublished - Nov 2019

Fingerprint

swell
tail
wind direction
energy
friction
wind speed
oceans
sea
surface roughness
momentum
power law
Gulf of Mexico
wave spectrum
Atlantic Ocean
sea surface
wind velocity
prediction
sea state
ocean wave
ocean

ASJC Scopus subject areas

  • Aquatic Science
  • Geology

Cite this

Impact of swell on the wind-sea and resulting modulation of stress. / Vincent, Charles L.; Thomson, Jim; Graber, Hans C.; Collins, Clarence O.

In: Progress in Oceanography, Vol. 178, 102164, 11.2019.

Research output: Contribution to journalArticle

Vincent, Charles L. ; Thomson, Jim ; Graber, Hans C. ; Collins, Clarence O. / Impact of swell on the wind-sea and resulting modulation of stress. In: Progress in Oceanography. 2019 ; Vol. 178.
@article{90ed01e443fd4a2d9529abf56387413e,
title = "Impact of swell on the wind-sea and resulting modulation of stress",
abstract = "Investigation of 37,106 ocean surface wave spectra from the Pacific, Atlantic Ocean, and Gulf of Mexico demonstrate that swell modulates the energy level of the high frequency tail of the wind-sea wave spectrum, altering sea surface roughness. With a mixture of sea and swell, the wind-sea part of spectra follows the well-known f-4 (equilibrium range) and f-5 (saturation range) power laws. Swell modulates the energy levels but does not change the power-law structure. For swell with minimal winds, the spectra follow the −4, −5 power-law paradigm, but energy correlates to swell steepness not wind speed. Swell shifts the transition between the two sub-ranges towards lower frequencies. For sea-swell mixtures, a modulation factor λ is proposed that depends on wind speed and swell steepness which allows parameterization of the spectral tail. Comparison of large swell with little wind to wind-sea spectra of same height and period, indicates that there is little difference in spectral shape and suggests that the Hasselmann Snl source term is likely the mechanism by which energy is transferred into the wind-sea tail causing the modulation. Analysis of 33,000+ directional spectra at Ocean Station Papa shows that the mean direction for the wind-sea high frequency tail is strongly correlated to wind direction, no matter the swell direction or steepness or level of swell dominance. An equation for the friction velocity of a sea state with swell (u*s) is developed, u∗s=λ1/2u∗o where u∗o is the friction velocity in the absence of swell, by neglect of the direct swell impact. Noting that this is only a partial estimate of the total measured stress, the prediction is evaluated for 3,000+ observed spectra yielding a correlation of 0.91 suggesting that it may be of consequence. Observations of u∗/u∗o suggest a dependence with swell steepness that is similar to that predicted by λ1/2. At low winds, λ1/2 overestimates the stress, but noting that it was derived absent the components from the swell frequencies. In the tail, the momentum transport is downward, while in the swell the transport is predominantly upward, suggests a possible correction for λ1/2. The case of a swell generated wind is discussed.",
author = "Vincent, {Charles L.} and Jim Thomson and Graber, {Hans C.} and Collins, {Clarence O.}",
year = "2019",
month = "11",
doi = "10.1016/j.pocean.2019.102164",
language = "English (US)",
volume = "178",
journal = "Progress in Oceanography",
issn = "0079-6611",
publisher = "Elsevier Limited",

}

TY - JOUR

T1 - Impact of swell on the wind-sea and resulting modulation of stress

AU - Vincent, Charles L.

AU - Thomson, Jim

AU - Graber, Hans C.

AU - Collins, Clarence O.

PY - 2019/11

Y1 - 2019/11

N2 - Investigation of 37,106 ocean surface wave spectra from the Pacific, Atlantic Ocean, and Gulf of Mexico demonstrate that swell modulates the energy level of the high frequency tail of the wind-sea wave spectrum, altering sea surface roughness. With a mixture of sea and swell, the wind-sea part of spectra follows the well-known f-4 (equilibrium range) and f-5 (saturation range) power laws. Swell modulates the energy levels but does not change the power-law structure. For swell with minimal winds, the spectra follow the −4, −5 power-law paradigm, but energy correlates to swell steepness not wind speed. Swell shifts the transition between the two sub-ranges towards lower frequencies. For sea-swell mixtures, a modulation factor λ is proposed that depends on wind speed and swell steepness which allows parameterization of the spectral tail. Comparison of large swell with little wind to wind-sea spectra of same height and period, indicates that there is little difference in spectral shape and suggests that the Hasselmann Snl source term is likely the mechanism by which energy is transferred into the wind-sea tail causing the modulation. Analysis of 33,000+ directional spectra at Ocean Station Papa shows that the mean direction for the wind-sea high frequency tail is strongly correlated to wind direction, no matter the swell direction or steepness or level of swell dominance. An equation for the friction velocity of a sea state with swell (u*s) is developed, u∗s=λ1/2u∗o where u∗o is the friction velocity in the absence of swell, by neglect of the direct swell impact. Noting that this is only a partial estimate of the total measured stress, the prediction is evaluated for 3,000+ observed spectra yielding a correlation of 0.91 suggesting that it may be of consequence. Observations of u∗/u∗o suggest a dependence with swell steepness that is similar to that predicted by λ1/2. At low winds, λ1/2 overestimates the stress, but noting that it was derived absent the components from the swell frequencies. In the tail, the momentum transport is downward, while in the swell the transport is predominantly upward, suggests a possible correction for λ1/2. The case of a swell generated wind is discussed.

AB - Investigation of 37,106 ocean surface wave spectra from the Pacific, Atlantic Ocean, and Gulf of Mexico demonstrate that swell modulates the energy level of the high frequency tail of the wind-sea wave spectrum, altering sea surface roughness. With a mixture of sea and swell, the wind-sea part of spectra follows the well-known f-4 (equilibrium range) and f-5 (saturation range) power laws. Swell modulates the energy levels but does not change the power-law structure. For swell with minimal winds, the spectra follow the −4, −5 power-law paradigm, but energy correlates to swell steepness not wind speed. Swell shifts the transition between the two sub-ranges towards lower frequencies. For sea-swell mixtures, a modulation factor λ is proposed that depends on wind speed and swell steepness which allows parameterization of the spectral tail. Comparison of large swell with little wind to wind-sea spectra of same height and period, indicates that there is little difference in spectral shape and suggests that the Hasselmann Snl source term is likely the mechanism by which energy is transferred into the wind-sea tail causing the modulation. Analysis of 33,000+ directional spectra at Ocean Station Papa shows that the mean direction for the wind-sea high frequency tail is strongly correlated to wind direction, no matter the swell direction or steepness or level of swell dominance. An equation for the friction velocity of a sea state with swell (u*s) is developed, u∗s=λ1/2u∗o where u∗o is the friction velocity in the absence of swell, by neglect of the direct swell impact. Noting that this is only a partial estimate of the total measured stress, the prediction is evaluated for 3,000+ observed spectra yielding a correlation of 0.91 suggesting that it may be of consequence. Observations of u∗/u∗o suggest a dependence with swell steepness that is similar to that predicted by λ1/2. At low winds, λ1/2 overestimates the stress, but noting that it was derived absent the components from the swell frequencies. In the tail, the momentum transport is downward, while in the swell the transport is predominantly upward, suggests a possible correction for λ1/2. The case of a swell generated wind is discussed.

UR - http://www.scopus.com/inward/record.url?scp=85072692712&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85072692712&partnerID=8YFLogxK

U2 - 10.1016/j.pocean.2019.102164

DO - 10.1016/j.pocean.2019.102164

M3 - Article

AN - SCOPUS:85072692712

VL - 178

JO - Progress in Oceanography

JF - Progress in Oceanography

SN - 0079-6611

M1 - 102164

ER -