^{1}

^{*}

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The effect of the surface gravity waves over sea surface roughness length (
z
_{0}) is investigated from several idealized numerical experiments with the Wave-Watch-III (WW3) model. The WW3 model is combined with a simplified model to estimate
z
_{0},
C
_{D},
u
_{*} and
U
_{10} as function of the sea state. The impacts related to the presence of the ocean waves over
z
_{0} are obtained from conditions of growing (young waves) and mature seas (old waves). The wave spectrum is obtained from WW3 model for each idealized simulation under uniform wind conditions. Uniform wind experiments range from 15 to 45 m/s. The simplified algorithm determines
z
_{0},
C_{D},
u
_{*} and
U
_{10} for cases of young waves, old waves and by the Charnock method. The results show that when the ocean is characterized by young waves, both
z
_{0 }and
C_{D} (drag coefficient) increase while
U
_{10} is reduced. In Charnock case, the values of
z
_{0},
C_{D} and
U
_{10} have no dependence with the presence of gravity waves. Experiments using winds higher than 30 m/s result in young waves’
C_{D} values higher than the
C_{D} value for old waves. Even for young waves
C_{D} values are high for cases of strong winds. The results also show that in experiments using winds higher than 30 m/s the dependence between
C_{D} and wave age becomes stronger, which is in accordance with other studies.

Ocean surface waves are the result of forces acting on the ocean. The characteristics of the waves depend on the controlling forces. Tidal waves are generated by the response to gravity of the Moon and Sun and are rather large-scale waves. Capillary waves are dominated by surface tension in the water. Where the Earth’s gravity is the major determining factor we have the so-called gravity waves. Wind-generated gravity waves are almost always present at sea. These waves are generated by winds somewhere on the ocean. They are also very relevant in climate processes since they play a role in the heat, energy, gases and particles exchange between the oceans and atmosphere. Further, gravity waves can be classified into wind sea and swell. Wind seas are waves that constantly experience the action from the winds. Swells, however, are waves that propagate away from the wind forcing area.

To estimate wind stress over the ocean is one of the most important issues in atmospheric and ocean modeling, including extratropical and tropical cyclones forecasting. In the most operational numerical models the momentum flux at the air-sea interface is parameterized by a nondimensional parameter called surface roughness or Charnock coefficient (z_{ch}) [_{ch} must consider the state of the sea [

A direct dependence between surface roughness and the sea state is suggested in several papers [_{0}), a height in meters at which the wind speed becomes zero, occurs in the presence of young waves, while there is a low relationship between old waves and roughness length. [

From observed and experimental data, [_{0} and the waves varies in specific conditions. In addition, it was noted that z_{0} tends to diminish as the ratio between c_{p} and u_{*} (phase speed at the peak frequency and friction velocity, respectively) decreases. This ratio is a dependent relation between cause (winds) and effect (gravity waves) and is usually called wave age. The parameter wave age indicates the development state of the sea. When the nondimensional value of the wave age is less than 10 the sea is characterized by young waves [

[

In this context, the present study evolves a combination of an ocean wave numerical model (WW3) and a very simple algorithm to provide some insight on the effect of the waves on the behavior of some surface variables, including: z_{0}, sea surface drag (C_{D}), wind stress (u_{*}), and wind velocity at 10 meters (U_{10}). These effects are evaluated in conditions of growing and mature sea. A brief outline of the WW3, the simplified algorithm, and the experimental design, used here is introduced in Section 2. Section 3 describes results of the experiments with constant winds from 15 to 45 m/s to investigate roughness length behavior in mature and growing sea conditions. The summary and conclusions are given in Section 4.

The WW3 runs for all time integration. It outputs SWH, mean wave length (MWL), wave mean period, mean wave direction and peak frequency [_{0}, C_{D}, u_{*} and U_{10} from two different manner. In the first, the algorithm uses the Charnock relation (R_{BULK}), which computes z_{0} considering z_{ch} without dependence with waves. In the second case, the parameter wave age _{ch} (R_{WA}). In this case, z_{ch} takes into account the sea state. The simplified algorithm uses the complete wave spectrum from the WW3 model to compute R_{BULK} and R_{WA}.

The WW3 model solves the energy transfer equation for the wave spectrum. The equation describes the variation of the wave spectrum F in space and time due to the advection of energy and local interactions. The wave spectrum is locally modified by the input of energy from the wind, the redistribution of energy due to nonlinear interactions and energy dissipation due to wave breaking. These processes are represented by the source functions S_{in}, S_{nl}, and S_{ds}, respectively, according to Equation (1).

The three terms in the right hand side of Equation (1) are called source terms. The source terms of the WW3 use nonlinear wave-wave interaction using a discrete interaction approximation (DIA) modified by [

In most atmospheric, ocean and wave models z_{0} is determined from the Charnock relation [

where u_{*} is the friction velocity, g the gravity of the Earth, and z_{ch} is the Charnock constant with value of 0.0185, which is representative of a sea at equilibrium. Thus, the Charnock conventional formulation does not consider the evolution stage of the sea or its growth.

To include the wave growing effect z_{ch}, in simplified algorithm, is replaced by the coupled term wave age. Thus, z_{ch} becomes a function of the sea state. The effect of the sea development into z_{0} can be formulated by two particular ways: the first follows [

where _{w}, and c_{p} denotes, respectively, total stress, wave-induced stress, and the phase speed at the peak frequency. [

The 10 meters level drag coefficient under neutral conditions is given by

in which κ = 0.41 is the von Kármán constant and C_{D} is the drag coefficient.

where u_{*} is the friction velocity and U_{10} is the 10 meters wind speed given by

The simplified algorithm solves the Equations (3) to (6), in conventional (R_{BULK}) and nonconventional (R_{WA}) modes, from WW3 outputs. The use of the simplified algorithm allows us to evaluate the impact of the waves on the surface roughness length over the oceans.

The parameters of the WW3 used in this study are 1800 s (time step and wind input interval), 24 directions (directional resolution), 0.2˚ × 0.2˚ spatial grid resolution, and 4000 m (water depth). The grid is regularly spaced by a longitude-latitude grid. It extends 3000 km in the south-north direction and 1500 km in the east-west extent. The wave spectrum is discretized using 25 frequencies extending from 0.041 a 0.42 Hz with a logarithmic increment _{n} id the nth frequency. The WW3 provides output of two-dimensional directional wave spectra, as well as mean wave parameters such as significant wave heights, mean wave length, mean wave period, mean wave direction, peak frequency, and peak direction.

Seven experiments are performed for uniform wind conditions ranging from 15 to 45 m/s in all grid space. In the first experiment the uniform winds are 15 m/s, in the second they are of 20 m/s and so on until the last experiment were winds blow with 45 m/s. The simplified algorithm estimates the surface variables for R_{BULK} and R_{WA} for each experiment and in cases of growing and mature seas.

In this work, the effect of sea state on sea surface roughness length, drag coefficient, wind stress and wind velocity in 10 meters is evaluated. Two distinct cases were analyzed: mature and growing seas. Further, the results are divided into those which consider z_{ch} as a constant value of 0.0185 and those which use wave age in the place of z_{ch}.

_{ds} (Equation (1)).

Figures 3(a)-(d) show scatter plot of z_{0}, C_{D}, and U_{10} versus wave age for each experiment with uniform wind speed. At this time the sea is completely developed. For R_{WA} case (_{0} decreases. The ratio of z_{0} diminishing is more accentuated in cases of strong winds. In the second situation (wind velocity less than 30 m/s), different trends are noted. As the waves become older z_{0} values keep invariant. This means that old waves (or mature sea) produce slight or any effect on z_{0}, as observed in several studies. Results from R_{BULK} case (_{0} values are not modified by wave type. This occurs because z_{0} in R_{BULK} case is kept constant and it does not suffer any modification as the sea or waves growth. In R_{BULK} case the parameter z_{0} is just forced by wind speed. Because wind speed is it only forcing mechanism one can expected that z_{0} values are large for experiments with strong winds.

_{D} and wave ages for R_{WA} case. It is noted that the behavior of C_{D} is similar to the observed behavior of z_{0}. For strong wind experiments sea surface drag decreases as the waves become older. In experiments with wind velocity less than 30 m/s, C_{D} suffers slight changes with the sea state. The relation between CD and wave age are more significant for experiments with strong winds. As young waves yields more drag it is expected that wind speed diminish. Expressive reduction of the wind velocity at 10 meters level under strong wind speed and young sea conditions is observed in _{10} increases. The values of U_{10} are not substantially modified with the sea state in cases of experiments with wind speed weaker than 30 m/s. As can be noted, young waves produce more drag and weak winds. However, in experiments with uniform wind speed lower than 25 m/s the relation between U_{10} and wave ages weak until they become independent. Those results show that significant changes in the planetary boundary layer as well as momentum, heat and moisture fluxes can be expect by considering the sea state.

Figures 4(a)-(c) show the behavior of z_{0}, C_{D} and U_{10} in the reference point for each experiment with constant

wind speed. Further, surface parameters are put in perspective for developing (06 hours) and mature (72 hours) sea conditions (R_{WA}). It is also showed the results from R_{BULK} case. _{0}, while old waves are associated to low values of z_{0}. Lowest values of z_{0} are produced for cases without the sea state influence (R_{BULK}). More discrepancy between z_{0} values are observed for experiments with strong winds. Results from mature sea conditions and R_{BULK} case show similar z_{0} values for weak wind experiments (wind velocity less than 35 m/s). An important fact to note is that R_{BULK} case underestimates z_{0} values in comparison with young and old waves (R_{WA}) for all experiments. This results are in agreement with numerical modeling studies, including complex coupled experiments with atmospheric and ocean wave models [

_{D}) than old waves. Lower values of C_{D} are produced in R_{BULK} case. Differences between C_{D} values for mature and growing sea are more expressive in cases of strong wind conditions (wind velocity greater than 30 m/s). C_{D} values for mature sea conditions are similar to C_{D} values for R_{BULK }cases for weak wind experiments (wind velocity less than 30 m/s). It can be also noted that R_{BULK} case produces the lowest values of C_{D} for all experiments. This results are similar to those observed for z_{0} (

strong wind conditions. As young waves yield more drag and high values of z_{0}, it can be expected that in conditions of developing sea the wind velocity, near surface, diminish.

_{10} is lower at sea dominated by young waves than at developed sea. The wind velocity is reduced in developing sea because young waves produce more resistance (or drag) to the wind flow. According to [_{BULK} case, where sea is considered completely developed. U_{10} behaves as in the cases of z_{0} and C_{D} for experiments under strong wind conditions. However, under strong wind conditions young waves produces weak winds. Our results are in agreement with several works which have showed that when waves are considered to interact with the lower atmosphere they can produce significant impacts on wind field [

The effect of waves on sea surface roughness length, sea drag, and wind velocity at 10 meters has been investigated by combining the WW3 model with a simplified algorithm. The combined models estimate the behavior of z_{0}, C_{D}, and U_{10} for particular sea state situations. To evaluate the sea state effect the wave age coupling term (c_{p}/u_{*}) was included in the algorithm. Several numerical experiments were carried out for constant winds ranging from 15 to 45 m/s. For each numerical experiment, z_{0}, C_{D}, and U_{10} were computed in terms of mature and growing seas.

The results showed that there is a significant dependency between z_{0} and the sea state. This relation is strong in cases of developing sea under higher wind speed conditions (wind speed > 30 m/s). This dependency is slight weak in experiments with weak winds (wind speed less than 30 m/s). In addition, it was observed that young waves are related to high values of z_{0} while old waves are associated to low values of z_{0}. The behavior of C_{D} is very similar to those of z_{0} values. The wind velocity at 10 meters shows an inverse trend with relation to z_{0} and C_{D} patterns. As for instance, as the wave age increases, which usually occurs when the sea is reaching an equilibrium condition, U_{10} also increases. It means that, when the sea is young U_{10} must decrease because the sea is extremely rough. This pattern is consistent with earlier studies which point out that wind velocity near sea surface is reduced in regions dominated by young waves [

To evaluate the dependence of z_{0} on the sea state under mature and developing sea conditions, two cases were carried out. The first (R_{BULK}) considers z_{ch} constant in time, while the second one (R_{WA}) admits that z_{ch} changes according to the sea state. In R_{BULK} case z_{0} and C_{D} values present just a slight increase with wind speed in all experiments. However, in the case of R_{WA}, z_{0} and C_{D} present accentuated and expressive increase as the wind speed in the numerical experiments increases. Despite both z_{0} and C_{D} obtain a high ratio of increase in the case of R_{WA}, important and notable differences between developing and mature seas are observed. At developing seas z_{0} and C_{D} increase. In the case of mature sea z_{0} and C_{D} values are lower than those in the developing sea case. The difference between developing and mature seas increases for experiments with strong winds. This behavior is also valid for the variable U_{10}. However, U_{10} decreases more for developing sea conditions than for mature stage (R_{WA}). U_{10} in R_{BULK} cases does not suffer any modification in all experiments. This is due to the fact that the sea state condition does not affect rigorously the flow passing by developing seas, as it occurs in R_{WA} cases.

The present results show important aspects when the sea state is included in the computation of sea surface roughness length. These aspects are related to the change of values of drag and wind velocity near surface, which would bring substantial effects on the life cycle of atmospheric system moving on young sea. Despite the numerical experiments showed that wave state affects the patterns of z_{0}, C_{D} and U_{10}, they are very limited. This limitation is due to the fact that in real conditions the winds will not blow and support constant wind speed too long. Issuing to bring more reality to the results, the second part of this work includes experiments with idealized tropical storms with different intensity and translation speeds.

José Augusto P.Veiga,Mônica R.Queiroz, (2015) Impact of the Waves on the Sea Surface Roughness under Uniform Wind Conditions: Idealized Cases for Uniform Winds (Part I). Atmospheric and Climate Sciences,05,317-325. doi: 10.4236/acs.2015.53024