V. G. Polnikov

Wind-Wave Characteristics and Climate Variability in the Indian Ocean Region Using Altimeter Data

The significant wave height and wind speed derived for the period 1993–2010 from altimeter data sets over the Arabian Sea, Bay of Bengal, and the Indian Ocean categorized as six zones has been analyzed. The average variation of both significant wave height and wind speed is found to be almost stable for the period of study. The study reveals that the average wind speed increases by about 6cm/sec/year during monsoon and post monsoon in the southern Indian Ocean. The distribution of wind and waves was studied in the context of seasonal variations. In addition, the average inter-annual and intra-annual variations along with the statistical parameters such as standard deviation, and root mean square wave height for the six zones are also reported in this paper.

Joint analysis of the wind and wave-field variability in the Indian Ocean area for 1998–2009

In this paper, a detailed statistical analysis of the wind and wave fields in the Indian Ocean (IO) for the period of 1998-2009 was performed based on using the wind fields taken from the site of the National Centers for Environmental Prediction and the National Oceanic and Atmospheric Administration (NCEP/NOAA) [1] and on the numerical wind-wave model WAM [2] modified with the source function proposed in [3]. The primary analysis of the fields includes mapping the wind and wave fields, as well as their energy fields, calculated with different scales of space-time averaging; the subsequent zoning of the IO area; and assessing the seasonal interannual variability of all the fields and their 12-years trends. Further analysis is carried out taking into account the zoning. This analysis includes a construction of the time series obtained with different scales of space-time averaging for all the fields, a spectral analysis of these series, finding and analyzing the spatial and temporal distribution of extrema of the wind and wave fields (accounting for the their sharing in the zones), and making histograms of the wind and wave fields and calculating their first four statistical moments (in the zones and in the ocean as a whole). The results allow us to evaluate a large set of statistical characteristics of the wind and wave fields in the IO area, scales of their variability, their long-term trends, and the features of distribution for these statistical characteristics in the ocean area as well.

An extended verification technique for solving problems of numerical modeling of wind waves

Based on different modifications of the source function in the WAM(C4) wind-wave model, a large series of verification calculations aimed at increasing the quality of the numerical model (with respect to the parameters of accuracy and performance) is performed. We propose a methodology allowing us to solve the following fundamental and practical problems of numerical modeling: (1) determining the minimum interval of verification of numerical wind-wave models, (2) finding a criterion for choosing the best model out of all models subjected to verification, and (3) formulating the accuracy requirement for specifying the input field necessary for the given accuracy of wind-wave field calculations. Particularly, we have found that (a) the minimum term of verification calculations for numerical wind-wave models is three months; (b) according to our criterion, the proposed modification of the WAM model impartially is “essentially preferable” to the original model; and (c) the relative errors (yielded by the proposed version of the WAM model) in the calculated wave heights ρHs and average periods ρTm for different levels of the relative error of the input wind-wave field ρW make it possible to solve the third problem mentioned above.

The role of wind waves in dynamics of the air-sea interface

Wind waves are considered as an intermediate small-scale dynamic process at the air-sea interface, which modulates radically middle-scale dynamic processes of the boundary layers in water and air. It is shown that with the aim of a quantitative description of the impact said, one can use the numerical wind wave models which are added with the blocks of the dynamic atmosphere boundary layer (DABL) and the dynamic water upper layer (DWUL). A mathematical formalization for the problem of energy and momentum transfer from the wind to the upper ocean is given on the basis of the well known mathematical representations for mechanisms of a wind wave spectrum evolution. The problem is solved quantitatively by means of introducing special system parameters: the relative rate of the wave energy input, IRE, and the relative rate of the wave energy dissipation, DRE. For two simple wave-origin situations, the certain estimations for values of IRE and DRE are found, and the examples of calculating an impact of a wind sea on the characteristics of both the boundary layer of atmosphere and the water upper layer are given. The results obtained permit to state that the models of wind waves of the new (fifth) generation, which are added with the blocks of the DABL and the DWUL, could be an essential chain of the general model describing the ocean-atmosphere circulation.

Comparative Study of Performance of Wind Wave Model: Wavewatch—Modified By New Source Function

Abstract With the aim of assessing the merits of the new source function proposed earlier in Polnikov (2005), it was tested and validated by means of the modification of the well known model WAVEWATCH-III. Assessment was done on the basis of comparing the wave simulation results found from both models against the buoy data obtained in three oceanic regions: eastern and western parts of the North Atlantic, and the Barenz Sea. First of all, incorporation of the new source function into the numerical codes of WAVEWATCH was done, and this modified version of WAVEWATCH was fitted and tested by standard tests. On the basis of these results some physical conclusions were drawn. Then, sophisticated fitting of the modified model was carried out, using the observation data of 19 buoys obtained in two regions of the North Atlantic for a period of 30 days at 1-hour time intervals. After this, the standard validation of both models was continued on the basis of the 8-month historical data, corresponding to 12 buoys located in the western part of the North Atlantic. Finally, comparative validation of the models was done against the wave observations obtained at one buoy in the Barenz Sea for a period of 3 years at 6-hour time intervals. Estimations of simulation accuracy were carried out for the three parameters of wind waves: significant wave height, Hs, peak wave period, Tp, and mean wave period, Tm. Comparative analysis of these estimations was carried out for the original and modified model WAVEWATCH. The advantage of the modified model was revealed, with an increase of simulation accuracy for Hs by 20 to 50% for more than 70% of the buoys considered. Additionally, it was found that the speed of calculation was increased by 15%.

On the wind profile near a wavy surface

The formation of the mean-wind profile near the rough surface of a fluid is discussed. Classical definitions of the profile’s characteristics are presented, the treatments of the parameters of a logarithmic profile are considered, and the meaning of major concept is refined. On the basis of the numerical model developed by Makin and Kudryavtsev for the dynamic boundary layer, the mean-wind profile is calculated for different stages of wind-wave evolution. Differences between the calculated profiles and the classical logarithmic profile are discussed. A treatment is given for the established differences.

Extended verification of the model of dynamic near-surface layer of the atmosphere

This paper formulates the most general principles for verifying models of the dynamic near-water layer of the atmosphere (DNWLA) and performs an advanced verification of the model proposed by the author earlier [6]. Based on empirical wave spectra from the studies by Donelan [15], Elfouhaily [14], and Kudryavtsev [13] and well-known empirical laws describing the wave-age dependence of the friction coefficient, we adjusted the original version of the model. It was shown that the improvement of model reliability is most dependent on the adequacy of the parameterization of the tangential portion of the total momentum flux to the wavy surface. Then the new version of the model was verified on the basis of field data from two different groups of authors. It was found that the new version of the model is consistent with empirical data with an error not exceeding the measurement error of near-water layer parameters.

A comparative analysis of models for dynamic boundary layer of atmosphere

This paper considers the currently available approaches to constructing numerical models describing the dependence of parameters of the atmospheric boundary layer on waving parameters (dynamic boundary layer). The models proposed in [1, 2] are characterized by detailed numerical algorithms, numerical calculations, and comparisons of the resistance coefficient Cd as functions of the parameters of the waving surface, the state of which is given by the model two-dimensional wave spectrum as represented in [3]. For the same spectrum, the calculation results obtained by different models are shown to yield estimates for the value of Cd with a more than twofold discrepancy; however, the trends in the dependence of Cd on wave age and wind strength are close to one another and to observational data. Possible shortcomings of both approaches are analyzed, and ways to eliminate them are proposed. The requirements for setting special experiments needed to verify theoretical models of the dynamic boundary layer are discussed.

The Role of Evolution Mechanisms in the Formation of a Wind-Wave Equilibrium Spectrum

The formation of a stationary (equilibrium) range in a wind-wave spectrum is investigated by numerical simulation. The equation of evolution of the wind-wave spectrum is solved using the exact calculation of the Hasselmann kinetic integral and involving various modifications of known parameterizations of the mechanisms of wave pumping by wind (In) and of wave dissipation (Dis). It is shown that it is these two mechanisms that are responsible for the shape of the stationary range of the wind-wave spectrum, whereas the nonlinear mechanism plays a stabilizing but subsidiary role. With an appropriate choice of mathematical representations for In and Dis, any known empirical shape of the stationary range of the spectrum can be obtained. During the calculations it is found that, for real wind waves, the known representations of In and Dis do not ensure the existence of the inertial interval required for Kolmogorov-type spectra formation due to the nonlinear interactions between waves.

Features of the atmospheric boundary layer block for three versions of the WAM wind wave model

The estimated characteristics of the atmospheric boundary layer, obtained by the simulation of wind wave fields using three versions of the WAM numerical model are compared with the well-known empirical dependences of drag coefficient Cd on wind speed U10 and wave age A, as well as with the dependence of dimensionless roughness height zn on inverse wave age u*/ср. Calculations carried out for several years in the areas of the Pacific and Indian oceans, based on the ERA-interim and CFSR wind reanalyses have shown good agreement between the model and empirical dependences Cd(U10) and Cd(A). The range of estimated variability for zn(u*/ср) has been found to be significantly less than empirical. It has been also found that estimated values of wind speed U10W(t) are overestimated from 5 to 10% in all versions of WAM models compared with the input wind reanalysis U10R(t) at the moments of appearance maximum values of wind U10R(t). The reasons for the established features of the WAM model and their dependence on the model version are discussed.