Wind waves are considered. Differences between extreme and freak waves are discussed. It is specified, that not any big wave (surge) is unusual. Such wave should have a set of certain properties and therefore can be considered as a multidimensional random variable. Generalization of the field measurements data has allowed to estimate a freak wave appearance probability. Abrupt change of wave spectral structure is one of the approaches to forecast the freak wave appearance.

Estimation and Classification of the Extreme Wind Waves on the Basis of Visual Data For the first time the extreme waves in the ocean are studied using the most complete database of visual wave observations, assimilated in the ICOADS (International Comprehensive Ocean Atmosphere Data Set) archive. A new methodology for the data quality control and criteria for the classification of extreme wind waves and swell was established. All visual observations reporting the wind wave height of 15 meters or more likely indicate either actual severe storms (about 20% of the total number of cases), or observational artifacts. A very small number of reported extreme swells can be potentially attributed to the freak waves.

A review of the physical mechanisms of internal freak wave formation in the oceans is presented. The theoretical models are based on the modified Gardner equation obtained within the asymptotic theory. The following mechanisms are analyzed: nonlinear dispersive focusing, modulation instability of nonlinear internal waves, and wave transformation at the so-called turning points.

Ocean turbulent mixing scheme based on similarity theory is considered. This scheme’s been modified for including in 3-D ocean circulation model. Experiments have been made with the Baltic Sea model in which above mentioned scheme and scheme based on kinetic turbulent energy equation were used. It’s shown that simulation of temperature and salinity vertical distribution were improved.

Analysis of the wind wave measurements in the north-western Black Sea from December 2000 to January 2001 allowed to select 67 cases of freak waves in a form of single waves and as a member of wave packets of non-freak waves. The waves have an appreciable asymmetry: their front slopes are generally steeper than the back slopes, and the previous and the following freak wave wave troughs vary in depth and width. Involvement of meteorological data has allowed to analyze the feasible relations between the formation of abnormal waves and wind flow. Two characteristic types of situations are found. In the first, the birth of freak waves is accompanied by substantial changes in the wind speed direction, in the second situation its formation is preceded by the wind of almost constant direction.

Results of the analysis of long time series of sea level for June – September, 2009 near the Aniva cape (Sakhalin), recorded by pressure bottom station placed on depth of 12 m are given. There are 394 abnormal big waves, waves satisfying to freak wave amplitude criterion (the wave height exceeds the significant height more then twice) recorded. The amplification during six events exceeds 2.5 times. The cumulative frequency of the abnormal wave appearance is described by the Poisson distribution as follows from the theory of extreme statistics. Freak waves occurred on the average twice a day which is in good comparison with the Raleigh prediction for narrow-band Gaussian wave field.

The properties of extreme wave storms in the Darss Sill area, SW Baltic Sea, are analysed based on waverider data for 1991–2010 and long-term numerical simulations. The long-term significant wave height is H_S ~0.7 m and the most frequent wave periods 2–4 s. The largest measured H_S is 4.46 m. The typical measured and modelled wave periods differ by up to 2 s. The annual maximum HS has notched behaviour, with an increase for 1958–1990 and since 1993, and a drastic decrease in 1991–1992. The measured annual average and maximum H_S have changed insignificantly in 1991–2010 but the threshold for the top 1% of waves has considerably decreased.

The lidar images of internal waves (the lidar echo signal as a function of lidar horizontal coordinates, the depth from which the signal arrives, and the parameters of internal waves) were simulate on the basis of field measurements of depth distributions of the hydrophysical and hydrooptical properties in the White, Barents and Kara Seas. It was shown that images of internal waves, depending on the depth distribution of hydrophysical and hydrooptical parameters and their combination, have rather complex and diverse structure.