D. Sergeev

Statistical Parameters of the Air Turbulent Boundary Layer over Steep Water Waves Measured by the PIV Technique

AbstractA turbulent airflow with a centerline velocity of 4 m s−1 above 2.5-Hz mechanically generated gravity waves of different amplitudes has been studied in experiments using the particle image velocimetry (PIV) technique. Direct measurements of the instantaneous flow velocity fields above a curvilinear interface demonstrating flow separation are presented. Because the airflow above the wavy water surface is turbulent and nonstationary, the individual vector fields are conditionally averaged sampled on the phase of the water elevation. The flow patterns of the phase-averaged fields are relatively smooth. Because the averaged flow does not show any strongly nonlinear effects, the quasi-linear approximation can be used. The parameters obtained by the flow averaging are compared with the theoretical results obtained within the theoretical quasi-linear model of a turbulent boundary layer above the wavy water surface. The wave-induced pressure disturbances in the airflow are calculated using the retrieved s...

Bag-breakup fragmentation as the dominant mechanism of sea-spray production in high winds

Showing the record strengths and growth-rates, recent hurricanes have highlighted needs for improving forecasts of tropical cyclone intensities most sensitive to models of the air-sea interaction. Especially challenging is the nature of sea-spray supposed to strongly affecting the momentum- and energy- air-sea fluxes at strong winds. Even the spray-generation mechanisms in extreme winds remained undetermined. Basing on high-speed video here we identify it as the bag-breakup mode of fragmentation of liquid in gaseous flows known in a different context. This regime is characterized by inflating and consequent bursting of the short-lived objects, bags, comprising sail-like water films surrounded by massive liquid rims then fragmented to giant droplets with sizes exceeding 500 micrometers. From first principles of statistical physics we develop statistical description of these phenomena and show that at extreme winds the bag-breakup is the dominant spray-production mechanism. These findings provide a new basis for understanding and modeling of the air-sea exchange processes at extreme winds.

Laboratory study of cross-polarized radar return under gale-force wind conditions

This paper presents data of laboratory experiments on a high-speed wind-wave flume of the Institute of Applied Physics (Nizhny Novgorod, Russian Federation), which are devoted to the investigation of the X-band co-polarized and de-polarized radar return in a wide range of high wind speeds (from 8 to 40 m s–1). Microwave measurements were accompanied by the measurements of airflow and wave field parameters. Experiments showed that alternatively to the co-polarized return, the dependency of the de-polarized return on the wind speed does not saturate, although the growth rate decreases at wind speed exceeding 30 m s–1. Comparison of the experimental data with the composite-surface Bragg scattering model for the measured parameters of the wind and waves showed that the model is in agreement with measurements of microwave co-polarized return, but fails to describe the de-polarized radar return. The obtained dependency of de-polarized radar return was compared with the empirical geophysical model function based on collocated airborne and satellite data.

Development of a regional model based on adapted WAVEWATCH III and WRF models for the prediction of surface wind waves on the reservoir and wind

A regional model for the prediction of surface wind waves on the Gorky Reservoir and of the wind over the water area is proposed. The wind was calculated in the WRF model with and without Large Eddy Simulation block. The WRF model wind calculation results were used as wind forcing of the WAVEWATCH model. A comparison of the mean wave parameters with the measured in the experiment at the Gorky Reservoir was made.

The “Bag Breakup” Spume Droplet Generation Mechanism at High Winds. Part I: Spray Generation Function

AbstractThis paper describes the results of an experimental and theoretical investigation into the mechanisms by which spume droplets are generated by high winds. The experiments were performed in ...

The Problem of Forecasting of Vertical Temperature Distribution in Inland Hydrophysical Objects with Experimental Data

The problem of computation of vertical temperature distribution in inland water objects is raised in the research, and the modified version of the program complex LAKE is proposed for its solving. Also the results of validation of the modified version in accordance with the parameters of lake part of Gorky water reservoir are provided. The modification caused changing the procedure of input data assignment, ensured the possibility of using the results from field measurements as starting profiles, led to the greater consideration of wind influence on mixing and allowed to reproduce general forms of distribution more accurately. The data from the experimental sites of Gorky water reservoir in combination with the files of global reanalysis and the data of hydrometeorological station were used for validation. Measurements of the vertical temperature profiles were carried out with freely sinking CTD-probe, which recorded the temperature values several times per second. Time and temperature dependences in control points, comparison of the forms of the profiles and standard deviation were analyzed to assess the results of numerical modeling. Numerical simulation with the modified model gave results which were in good agreement with the real seasonal variations of thermal regime in lake part of Gorky water reservoir. The best quantitative and qualitative agreement was obtained for the thermal regimes with strong mixing: the results of the forecasting reproduced the forms of distribution and the values of temperature extremely accurately. The results for the ones with strong stratification demonstrated less accuracy in the values of temperature in control points, particularly near the surface, but all specificities of the forms of vertical temperature distribution were correctly reproduced.

Laboratory modelling of cross-polarized radar return at severe wind conditions

Laboratory experiments directed to investigation of co-polarized and de-polarized X-band microwave radar return from the water surface at strong and hurricane wind were carried out in the high-speed wind-wave flume. Microwave measurements were accompanied by the measurements of air-flow and wave field parameters. Experiments showed that alternatively to the co-polarized return, the dependency of the de-polarized return on the wind speed do not saturate, although the growth rate decreases at wind speed exceeding 30 m/s. Comparison of the of the experimental data with the composite-surface Bragg scattering model for the measured parameters of the wind and waves showed, that the model is in agreement with measurements of microwave co-polarized return, but fails to describe the de-polarized radar return. The obtained dependency of de-polarized radar return was compared with the empirical geophysical model function based on collocated airborne and satellite data.