A study about the initial stages of wind-wave growth in the presence of mixed sea state conditions

It is well known that the swell modifies the wind stress or wind wave properties, but it is not considered in studies of wind-generated wave growth. In most of the world's oceans, swell is present. During the early stages of wave development, swell plays an essential role in modulating the transfer of heat, momentum, and gases. During a measurement campaign in the Gulf of Mexico (GoM), continuous, high temporal resolution measurements of the directional spectrum of wave and turbulent Reynolds stresses were recorded with a platform moving with the free surface. Events were selected under cold front or northerly conditions. These events are nearly ideal for studying wave growth because the wind is nearly homogeneous and stationary with a predominant direction. Using Hanson and Phillips (2001) method, the swell is separated from the wave's directional spectrum to analyze the wind-sea's evolution in mixed conditions. The wind wave conditions are defined based on two swell criteria: the swell index (R = E_swell / E_tot) and the swell slope. The observations show that the swell dampens the energy of the young wind-sea. In the equilibrium region, the wind-sea energy is lower in the presence of the swell than in the absence. The physical process that explains this is that the swell reduces the surface roughness, i.e., the short waves have less slope, and therefore, their ability to extract momentum from the atmosphere is reduced. The swell modifies the Toba constant to a sub-saturated energy level. The spectral shape of the wind wave in the equilibrium region tends to have a more considerable spectral dip with a background swell. The transition frequency is shifted toward n times the spectral peak. There is no evidence of swell influence in the saturation spectral region of the wind-sea. It is concluded that the effect of the swell on the wind wave is a function of the direction of the swell, the ratio of the swell energy present in the spectrum, the slope of the swell, and the height of the swell. This research emphasizes the importance of swell inclusion in the analysis to better understand the physical processes of numerical wave models, the information processed by remote sensors, the modulation of swell on flow transfer, and the complexity of the wave field in hurricanes.