Iossif Lozovatsky

Mixing efficiency in natural flows.

It is argued that the mixing efficiency of naturally occurring stratified shear flows, γ=Rf/(1−Rf), where Rf is the flux Richardson number, is dependent on at least two governing parameters: the gradient Richardson number Ri and the buoyancy Reynolds number Reb=ϵ/vN2. It is found that, in the range approximately 0.03<Ri<0.4, which spans 104<Reb<106, the mixing efficiency obtained via direct measurements of fluxes and property gradients in the stable atmospheric boundary layer and homogeneous/stationary balance equations of turbulent kinetic energy (TKE) is nominally similar to that evaluated using the scalar balance equations. Outside these Ri and Reb ranges, the commonly used flux-estimation methodology based on homogeneity and stationarity of TKE equations breaks down (e.g. buoyancy effects are unimportant, energy flux divergence is significant or flow is non-stationary). In a wide range, 0.002<Ri<1, the mixing efficiency increases with Ri, but decreases with Reb. When Ri is in the proximity of Ricr∼0.1–0.25, γ can be considered a constant γ≈0.16–0.2. The results shed light on the wide variability of γ noted in previous studies.

OCEANIC MICROSTRUCTURE MEASUREMENTS BY BAKLAN AND GRIF

The authors describe instruments and their calibration techniques developed at the Atlantic Branch of the P. P. Shizshov Institute of Oceanology, Russian Academy of Sciences and used for microstructure measurements by Russian oceanographers during the last decade. The vertical profiler BAKLAN and the towing body GRIF carried among other sensors a new microconductivity capillary probe with spectral resolution O(100 cpm). This sensor was used to calculate the scalar dissipation rate x with noise level of 3 3 10210 K2 s21. The kinetic energy dissipation rate « was measured by an airfoil-type sensor with a noise level less than 8 3 10210 Wk g 21. Results of the measurements are illustrated by the data obtained near the California coast and in the western equatorial Pacific. A parameterization of equatorial turbulence in terms of turbulent diffusivities and the gradient Richardson numbers is considered. A relationship between horizontal patchiness of conductivity microstructure in the coastal zone and local thermohaline fronts was detected.

Upper pycnocline turbulence in the northern South China Sea

The first regional mapping of the averaged turbulent kinetic energy dissipation rate 〈ɛp 〉 in the upper pycnocline of the northern South China Sea is presented and discussed. At φ = 20°N and to the north of this latitude, 〈 ɛp 〉 appears to be more than two times larger than that to the south of 20°N. It is suggested that this asymmetry is associated with the predominant northwestward propagation and dissipation of the internal waves originated in the Luzon Strait area. An approximately linear relationship between 〈ɛp 〉 and the available potential energy of the waves PIW, suggests a characteristic time of the PIW dissipation of about 6 h.

Intermittency of near‐bottom turbulence in tidal flow on a shallow shelf

U.S. Office of Naval Research [N00014-05-1-0245]; Spanish Ministry of Education and Science [FIS2008-03608]; Major State Program of China for Basic Research [2006CB400602]; Catalan Institute for Water Research (ICRA)

Variations of thermohaline structure and turbulent mixing on the Black Sea shelf at the beginning of autumn cooling

Abstract Microstructure measurements taken prior and after some strong atmospheric events on the shallow Black Sea shelf allowed to track the formation and evolution of the thermohaline structure caused first by wind-induced mixing and local convection, later by a storm surge, and finally by intense heating during a short period of the ‘Indian summer’. It was found that even a day long mild surge can decrease the temperature by 2.5°C and increase salinity by 2 psu over the whole 20–25 m quasi-homogeneous upper layer, which was formed by previous intense vertical storm-induced mixing. During the following period of upwelling-favorable winds, the near-bottom temperature decreased by 8–9°C. The upwelling was accompanied by a series of thermohaline intrusions overlaying the inclined boundary of the sharp near-bottom thermocline. Restratification in the upper 10 m layer in the form of a series of quasi-homogeneous steps was successfully reproduced by a numerical model of wind-induced daytime mixing, followed by nighttime convection, using the measured sea-surface fluxes as background conditions. The vertical turbulence structure was depicted by the logarithm of the averaged kinetic energy dissipation, which showed a parabolic decrease from the boundary layers to the midpoint of internal weakly-stratified part of the water column. Intermittent turbulent patches were superimposed at this background profile, which closely coincided with mean structure of the vertical shear. A correlation between the averaged vertical profiles of the turbulent buoyancy Reynolds number Re b , mixing activity  G , and the gradient Richardson number Ri was found. The  G cannot be solely determined by Re b , it also strongly depends on Ri at all depths. Analytical relations betweeen these variables that may be used for mixing parameterization on the shelf are proposed.

ASIRI : an ocean–atmosphere initiative for Bay of Bengal

AbstractAir–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange...

Turbulence decay in stratified and homogeneous marine layers

A spectral approach is applied to shear-induced turbulence in stratified layers. A system of spectral equations for stationary balance of turbulent energy and temperature variances was deduced in the vicinity of the local shear scale LU = (ϵ/UZ3)12. At wavenumbers between the inertial-convective (k−53) and wak turbulence (k−3) subranges, additional narrow spectral intervals—‘production’ subranges—may appear (E ∽ k−1, ET ∽ k−2). The upper boundary of these subranges is determined as LU, and the lower boundaries as LR ∽ (ϵ/UZN2)12(χ/TZ2). It is shown that the scale LU is a unique spectral scale that is uniform up to a constant value for every hydrophysical field. It appears that the spectral scale LU is equivalent to the Thorpe scale LTh for the active turbulence model. Therefore, if turbulent patches are generated in a background of permanent mean shear, a linear relation between temperature and mass diffusivities exists. In spectral terms, the fossil turbulence model corresponds to the regime of the Boldgiano-Obukhov buoyancy subrange (E ∽ k−115, ET ∽ k−75). During decay the buoyancy subrange is expanded to lower and higher wavenumbers. At lower wavenumbers the buoyancy subrange is bounded by L∗∗ = 3(χ12/N12TZ), which is equivalent to the Thorpe scale LTh. In such a transition regime only, when the viscous dissipation rate is removed from the set of main turbulence parameters, the Thorpe scale does not correlate with the buoyancy scale LN ∽ ϵ12/N32 and fossil turbulence is realized. Oceanic turbulence measurements in the equatorial Pacific near Baker Island confirm the main ideas of the active and fossil turbulence models.

Internal waves in a summer pycnocline of the East China Sea

The bottom-mounted acoustic Doppler current profiler (ADCP) data obtained over a 70-h period in the East China Sea, about 80 miles to the southwest of the Jeju Island, are used to study internal waves in relation to tidal cycle. The spectral densities of the horizontal velocities followed Eh ∼ f− 1 between f0 ∼ 0.8 cph and the buoyancy frequency, where f0 is a transition point connecting the f− 1 subrange and the steep falloff of energy from the semidiurnal spectral maximum at fM2 toward higher frequencies. Manifestation of large-amplitude internal waves in the pycnocline has been mostly observed during a transition phase from high to low barotropic tide.

Bottom Turbulence in Stratified Enclosed Seas

Publisher Summary This chapter reviews bottom turbulence in stratified enclosed seas. Except for localized areas and limited periods of time, the turbulence extends to the bottom and the existence of an upper wind-mixed layer, separated from the water below by a thermocline, is not an essential feature of the North Sea Hydrodynamics. In the absence of wind, the water is smoothly and regularly stratified from the surface to the bottom. When the wind starts blowing, it exerts a stress on the water surface, momentum is transferred to the sea, and a turbulent layer develops that extends downwards by entraining water from below. The lower boundary of the turbulent mixed layer is then marked by a sharp density gradient usually referred to as the thermocline because temperature is, in most cases, the essential factor affecting density. The chapter also explains vertical structure of turbulence in the Baltic.

CASPER: Coupled Air-Sea Processes and Electromagnetic (EM) ducting Research

CapsuleCASPER objective is to improve our capability to characterize the propagation of radio frequency (RF) signals through the marine atmosphere with coordinated efforts in data collection, data analyses, and modeling of the air-sea interaction processes, refractive environment, and RF propagation.