Determining the light spectral absorption in the sea always encounters difficulties in collecting at the receiving device simul taneously both the rays that have traveled a certain distance in the medium after absorption and all the photons scattered along this path. In the currently used methods, it is not possible to collect all the scattered rays, so it is necessary to take into account the effect of lost photons on the absorption value through theoretical modeling and subsequent correction of the values. A new approach to determining the seawaters spectral absorption of light is proposed by passing a diverging beam through the measured medium placed in a double-walled conical reflecting cuvette with an integrating sphere as a light-receiving collector. By using a diverging beam in a conical quartz cuvette, it was possible to extend it and, thus, ensure the determination of spectral absorption of light in a larger range of seawaters transparency. To compensate the influence of the diffuse radiation of the sphere returning to the double-walled cuvette on the absorption value, a two-beam scheme with normalization to the reference flux was used. It is shown that the new approach makes it possible to redirect almost all scattered light to the receiver and thus minimize errors in determining light absorption in a weakly absorbing medium. To quantify the advantages of the new method, calculations were carried out of the geometric parameters of the scattered light propagation for a double-walled conical quartz cuvette.

The results of statistical modeling of the backscatter signal in lidars when probing the water column with pulses with internal modulation by complex frequency modulated signals and their matched processing in the receiving path of the lidar are presented. The simulation results are compared with analytical calculations in the small-angle approximation. It is shown that the photons spread along their paths, associated with multiple scattering in the medium, does not prevent effective compression of the complex signal, and the small-angle approximation well describes the energy-carrying part of the signal backscattered by the water column. A comparison was made of the levels of backscatter signals when probing water with a short pulse and a complexly modulated pulse. It is shown that the use of complexly modulated illumination pulses makes it possible to reduce the power emitted by the source while maintaining the level of the backscattering signal in the lidar and its range resolution. Calculations of the levels of the signal backscattered by a localized diffusely reflecting object are performed and it is shown that at a delay value corresponding  to the arrival time of ballistic photons, the compressed pulse is not distorted. At long delay times, a pulse tail is formed due to the spread of photons along their paths. An example of calculating the backscattering pulse in the presence of a non-reflecting object in the water is given.

A series of lidar measurements were conducted at stations with a homogeneous vertical distribution of hydrooptical char acteristics in the near-surface layer using a two-channel shipborne polarization lidar PLD-1. Lidar sounding was accompanied by synchronous contact measurements of a number of hydrooptical characteristics. A large dataset of measurement data was obtained in waters where hydrooptical characteristics varied widely. As a result of the statistical processing of these data, regression relationships were obtained linking the seawater beam attenuation coefficient c, absorption coefficient a, and diffuse attenuation coefficient Kd to the lidar attenuation coefficients of the co- and cross-polarized components. In most cases, a linear relationship between hydrooptical characteristics and the lidar attenuation coefficients of the polarized components is observed. These rela tionships are characterized by high values of the coefficient of determination — from 0.8 to 0.95. An exception is the relationship between the seawater beam attenuation coefficient c and the lidar attenuation coefficient of the cross-polarized component, where a second-degree polynomial is used to describe this relationship (coefficient of determination is 0.88). Data on the hydrooptical characteristics obtained using the cross-polarized component of the lidar echo signal mostly duplicate the data of the co-polarized component. However, the use of a two-channel optical receiving system increases the reliability and accuracy of the obtained data and provides the possibility of controlling the homogeneity of the underwater section of the sounding path.

The Sun glints that are observed on the water surface have different radiances, shapes, and color shades, depending on the positions of the Sun and the observer, the water surface statement, and the transparency of the water and atmosphere. The texture, radiance, and color characteristics of Sun glints carry information about the state of the water and atmosphere. Therefore, Sun glints play an important role in remote sensing of seas and oceans, being a useful signal in some problems and noise in others. Bright stripes (underwater caustics) on the bottom of the water basin that are caused by the waves are also an indicator of the water surface shape. The application of the wave theory of light is used to study the intensity distribution in the vicinity of the caustic (i. e., where the geometric-optical approximation is inapplicable), which arises when light is refracted on a wavy water surface.

A formula, which determines the parameters of the waves from the width of the caustic zone is obtained. The correctness of the method has been verified by experiments carried out in the pool. Also, here we give a brief overview of our investigations related to sun glints and underwater caustics.

Using numerical methods for solving the radiation transfer equation, ocean albedo values were calculated for a set of bio-op tical characteristics corresponding to situations with different chlorophyll concentrations (1 μg/L and 10 μg/L) and the case of intense coccolithophore bloom (8–12 million cells/L). Calculations were carried out in the spectral range of 280–2800 nm for cases of cloudless sky at various wind speeds and atmospheric transmission. It has been shown that for Case 1 waters, a change in chlorophyll concentration from 1 to 10 μg/L does not lead to changes in albedo. In the case of intense coccolithophore blooms, the ocean albedo can increase more than threefold. Calculation of average monthly albedo values for selected points in the Barents and Black seas showed that the presence of intense coccolithophore blooms significantly increases average monthly albedo values. The calculation of the values of radiation absorbed in the seawater column depending on the time of day, carried out for these points, demonstrated that the presence of blooms significantly reduces the values of absorbed radiation. It is shown that the contribution to the albedo of radiation emerging from water used in the state-of-the-art NEMO numerical ocean model, amounting to 0.005 ± ± 0.0005, corresponds only to Case 1 waters. Intense coccolithophore blooms can increase this contribution by more than 14 times. A simple formula is proposed for correcting albedo values taking into account the influence of bio-optical characteristics.

The vertical attenuation coefficient of downward irradiance is one of the hydro-optical characteristics that determine the parameters of the light field in the sea. Information from the vertical attenuation coefficient of downward irradiance (or the diffuse attenuation coefficient) is necessary when determining an important biological and ecological parameter of water basin — the thickness of the euphotic layer. A possible method for estimating the vertical attenuation coefficient of downward irradiance (or the diffuse attenuation coefficient) from measurements of the beam attenuation coefficient at a wavelength of 525 nm for the Black Sea waters is described. The Black Sea waters are characterized by an increased content of dissolved organic matter and belong to type II waters (according to the Morel classification). Examples of the application of the method are given, which showed a quite high correlation (R = 0.85) between the values of the vertical attenuation coefficient of downward irradiance determined from in situ measurements of underwater irradiance with a photometer and calculated from in situ measurements of the beam attenuation coefficient with a transparency meter. The standard deviation of the calculated values from the measured ones is 0.008 m^–1, the maximum deviation is 0.023 m^–1.

This work presents the optical methods for measurements of physical parameters on sea surface also covered by ice by using stereo cameras. New method of images with sea ice processing is offered. It can detect areas of open water and sharp edges on ice, which influence on radar signal refraction. This method is based on area of interest detection on optical images, statistical parameters for each area calculation, classification and local level definition to recognize needed structures. By using stereo system perspective distortions can be corrected and physical parameters of wind waves and sea ice can be calculated. This method was probe on stereo images obtained from 90th voyage of R/V «Akademik Mstislav Keldysh» in the Laptev Sea.

The results of ripple waves velocity measurements in case on open water by using development previously stereo method is presented. Comparison with Doppler shift of microwave signal is made.

The methods presented in this paper are of interest in field experiments simultaneously with the use of coherent radar stations for the quantitative interpretation of radar data, as well as for remote monitoring of the sea surface and ice conditions methods development.

The results of the hydro-optical model setup in the HydroLight software for three stations in the Kara Sea characterized by different vertical distributions of the values of bio-optical properties are presented. The selected distributions are typical for the areas of the Kara Sea influenced by the river runoff. The stations are located inside, outside and at the boundary of the surface desalinated layer. The model reproduces the spectra of remote sensing reflectance, vertical profiles of light attenuation coefficient of sea water and photosynthetically available radiation measured in expeditions with good accuracy (average relative error less than 10 %). The results obtained made it possible to evaluate the accuracy of different algorithms for determining chlorophyll-a concentration in the range of values typical for the Kara Sea. For the same purpose, similar calculations were performed for different values of the absorption coefficient of colored dissolved organic matter. It is shown that the IO RAS regional algorithm allows estimation of chlorophyll-a concentration in the range of values exceeding 0.5 mg/m3 with a significantly smaller relative error (less than 50 %) than the semi-analytical GIOP algorithm (more than 100 %). At the same time, variations in the yellow matter absorption coefficient have a significantly smaller impact on the results of the regional algorithm. The significant influence of the initial approximation of chlorophyll-a concentration values on the results of the semi-analytical GIOP algorithm makes it unsuitable for use in the area of influence of river runoff in the Kara Sea. A numerical method for determining the thickness of the layer forming 90 % of the water-leaving radiance was implemented, which will allow a more detailed assessment of the influence of surface layer parameters on the accuracy of chlorophyll-a concentration estimation in the Kara Sea based on satellite ocean color data in the future.

The features of the empirical bio-optical algorithm operation in the waters around the Antarctic Peninsula are analyzed based on a comparison of calibrated data from the shipborne flow fluorimeter and satellite data from the OLCI radiometer on Senti nel-3A and Sentinel-3B satellites during the Antarctic summers of January-February 2020 and 2022. It is shown that the standard OC4 bio-optical algorithm significantly underestimates satellite estimates of Chl-a concentration from ~1.5 to ~9 times (on aver age by a factor of ~3.1). The known regional OC4-SO algorithm provides acceptable errors of Chl-a concentration estimates and can be used for studies related to the analysis of Chl-a concentration in the waters around the Antarctic Peninsula. The developed in this work new regional algorithm OC4-AP has significantly lower error in comparison with the known standard and regional algorithms. It can be used if it is necessary to obtain a remote estimate of the concentration of Chl-a, as close as possible to the accumulated world experience in determining this value by standard extract spectrophotometric and fluorimetric methods. The observed underestimation of satellite estimates of Chl-a concentration using the standard empirical bio-optical OC4 algorithm can be attributed to at least three reasons typical for the studied water area: low relative CDOM content, high phycoerythrin content, and stronger effect of pigment packing in phytoplankton cells compared to the average values in the World Ocean.

Data on seasonal and long-term dynamics of hydrophysical characteristics in the water area of Lake Manzheroksky (Altai Republic) after dredging in 2017–2018 are presented. The expedition studies were conducted during the ice age in April 2021, as well as the summer warming of the lake in July 2021 and 2023. The spectral index of light attenuation in the range of 400–800 nm, the relative transparency of water over a white disk, the velocity of water movement in surface and bottom horizons, as well as temperature stratification are studied. It was found that the discovered bottom groundwater sources contributed to the erosion and dissolution of the not completely removed silt layer, which determined high values of the spectral index of light attenuation and low relative transparency of the water. The research results in July 2023 showed significant changes in hydrophysical characteristics compared to 2021, indicating an improvement in the ecological condition of the lake in a long-term aspect for the period from 2018 to 2023.

A comparison of spring and summer data on the dynamics of the spectral index of light attenuation at different depths for Lake Manzheroksky with other freshwater low-flow lakes showed that this hydro-optical parameter for the studied lake is maximum during the ice age, while for other lakes its minimum is noted in this phase of the annual limnic cycle.