On the effect that the direction of geostrophic wind has on turbulence and quasiordered large-scale structures in the atmospheric boundary layer

The dependence that the structure and intensity of turbulent and large-scale quasiordered eddies in the atmospheric boundary layer (ABL) have on the direction of geostrophic wind has been studied on the basis of a series of numerical experiments with a three-dimensional nonstationary model of high spatial resolution. The presence of the meridional component of the angular velocity of the Earth’s rotation results in a significant intensification of velocity fluctuations in a neutrally stratified turbulent flow during the easterly and northeasterly winds and in their decay during the westerly and southwesterly winds. This, in turn, results in significant variations in the mean velocity profile. It is shown that these variations are associated with the largest scale fluctuations and are comparable (in scale) to the depth of Ekman’s turbulent layer. It is found that, in the neutrally stratified ABL bounded in height and under stable stratification inside the ABL, the wind-direction dependence significantly decreases. The possibilities of parameterizing these effects in locally one-dimensional ABL models are discussed.     

The “slip law” of the free surface

A “slip law” connects the excess velocity or “slip” of a wind-blown water surface, relative to the motion in the middle of the mixed layer, to the wind stress, the wind-wave field, and buoyancy flux. An inner layer-outer layer model of the turbulent shear flow in the mixed layer is appropriate, as for a turbulent boundary layer or Ekman layer over a solid surface, allowing, however, for turbulent kinetic energy transfer from the air-side via breaking waves, and for Stokes drift. Asymptotic matching of the velocity distributions in inner and outer portions of the mixed layer yields a slip law of logarithmic form, akin to the drag law of a turbulent boundary layer. The dominant independent variable is the ratio of water-side roughness length to mixed layer depth or turbulent Ekman depth. Convection due to surface cooling is also an important influence, reducing surface slip. Water-side roughness length is a wind-wave property, varying with wind speed similarly to air-side roughness. Slip velocity is typically 20 times water-side friction velocity or 3% of wind speed, varying within a range of about 2 to 4.5%. A linearized model of turbulent kinetic energy distribution shows much higher values near the surface than in a wall layer. Nondimensional dissipation peaks at a value of about eight, a short distance below the surface.     

Measurements of the atmospheric turbulence in the coastal zone of the sea during weak wind from a mountainous coast

Results of measurements of the atmospheric turbulence in the layer between 1.5 and 21 m above sea level and the drag coefficient of the sea surface as the wind blows from a 4-km-long mountainous slope with a mean inclination of 11° are presented. The measurements of wind-speed profiles and its fluctuations at several levels, waves, and the main meteorological parameters were carried out in autumn 2005 and 2008 from a stationary platform located in the Black Sea at a distance of approximately 1 km from the southern coast of Crimea. It is shown that during weak synoptic wind a low-level wind jet develops at night over the sea with a maximum velocity up to 5–6 m/s at a level of approximately 6 m over the sea induced by the katabatic wind over the coastal slope. According to the approximate estimates, the horizontal scale of the low-level jet can reach a few tens of kilometers. This flow is characterized by the dissipation rate of the turbulence energy independent of height and low-frequency velocity fluctuations related to the gravity waves and advection of turbulence from the coast. It is shown that the lower part of the boundary layer (up to a height of 3 m) is adjusted to the sea-surface roughness. The dependencies of the drag coefficient on the wind speed or wave age are steadier than in the data for the open sea. However, the age of the waves is not a universal parameter at long and short fetches.     

Modes of linear stationary response of the atmospheric boundary layer to a cold surge in the Black Sea

To describe the phenomenon of cold surges in the Black Sea in winter, we study the problem of atmospheric response to a local heat source on the surface in two simple formulations. In the shallow-water model, the planetary boundary layer of the atmosphere is homogeneous with variable upper bound. In the second model, the boundary layer has a constant thickness and its stratification is homogeneous. In the one-dimensional problem, for a constant wind blowing perpendicularly to the sea coast, the atmospheric response is determined by a single dimensionless parameter called the Froude number. Depending on its value, there are two possible different modes of the response. The range Fr < 1 (subcritical mode) corresponds to gentle winds, strong stratifications, thick boundary layers, and high velocities of inertial gravitational waves. The range Fr > 1 (supercritical mode) corresponds to strong winds, weak stratifications, thin boundary layers, and low wave velocities. In the two-dimensional problem for a round sea, there are four qualitatively different types of response depending on the combination of two dimensionless parameters: the Froude number and the ratio of the radius of the sea to the radius of deformation. Translated from Morskoi Gidrofizicheskii Zhurnal, No. 5, pp. 3–22, September–October, 2008.     

Field measurements of the vertical shears for a drift current

We describe the procedure of field experiments aimed at measuring the vertical profiles of the vectors of a drift current with the help of quasi-Lagrangian drifters. We present the data on the vertical shears of the current at depths of 0.5–5 m obtained under the conditions of neutral stratification in the upper 5-m layer of the sea in the presence of weak and moderate winds. The correspondence of the obtained data to the concept according to which the subsurface layer of the sea is regarded as a near-wall turbulent layer with Ekman current located below is analyzed. A conclusion is made that the results of measurements correspond, on the average, to the classical concepts demonstrating both the region of logarithmic sublayer and its transition into the Ekman spiral. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 1, pp. 32–44, January–February, 2007.     

Study of the Neutral Ekman Flow Using an Algebraic Reynolds Stress Model

A recently developed fully explicit algebraic model of Reynolds stress and turbulent heat flux in a thermally stratified planetary atmospheric boundary layer without stratification has been used for a numerical study of the Ekman turbulent boundary layer over a homogeneous rough surface for different dimensionless surface Rossby numbers. A comparative analysis has been conducted for a closure model of the transport term in the prognostic equation of turbulent kinetic energy dissipation including third-order moments. Dependences of the total wind rotation angle on the Rossby number have been obtained. The calculated vertical profiles of mean velocity, turbulent stress, turbulent kinetic energy, surface-friction velocity, and boundary-layer height agree satisfactorily with observational and earlier obtained LES data.     

Average velocity field of the air flow over the water surface in a laboratory modeling of storm and hurricane conditions in the ocean

Laboratory experiments on studying the structure of the turbulent air boundary layer over waves were carried out at the Wind-Wave Channel of the Institute of Applied Physics, Russian Academy of Sciences (IAP RAS), in conditions modeling the near-water boundary layer of the atmosphere under strong and hurricane winds and the equivalent wind velocities from 10 to 48 m/s at the standard height of 10 m. A modified technique of Particle Image Velocimetry (PIV) was used to obtain turbulent pulsation averaged velocity fields of the air flow over the water surface curved by a wave and average profiles of the wind velocity. The measurements showed that the logarithmic part of the velocity profile of the air flow in the channel was observed in the immediate vicinity from the water surface (at a distance of 30 mm) and could be detected only using remote methods (PIV). According to the measured velocity profiles, dependences of aerodynamic drag factors of the water surface on the wind velocity at a height of 10 m were retrieved; they were compared with results of contact measurements carried out earlier on the same setup. It is shown that they agree with an accuracy of up to 20%; at moderate and strong wind velocities the coincidence falls within the experimental accuracy.     

Turbulent structure of the subsurface layer of the sea according to the data of the <Emphasis Type="Italic">Sigma-1</Emphasis> measuring complex

We present the results of experimental investigations of the characteristics of turbulence in the layer of wave-induced mixing. The data on the fluctuations of velocity, temperature, and conductivity are obtained with the help of a Sigma-1 measuring complex. The computed values of the dissipation rate of turbulent energy are compared with different models proposed for the subsurface layer. It is shown that the available models fail to guarantee satisfactory agreement of the numerical results with the experimental data for the layer of active wave action and, in particular, in the presence of swell. This leads us to the conclusion concerning the necessity of parametrization and assimilation of more complete data on the state of the sea surface, the structure of currents, and the surface layer of the atmosphere in the models. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 2, pp. 15–28, March–April, 2007.     

The dependence of precipitation on wind direction over the Kuroshio front in the winter East China Sea

This study investigates atmospheric responses to the directions of surface wind over the Kuroshio front in the East China Sea, using wintertime satellite-derived data sets. Composite maps of sea surface temperature, wind speed, precipitation, turbulent heat flux, surface wind divergence, and the curl of wind vectors above the atmospheric boundary layer are depicted based on the classification of intense northeasterly (along the front) and northwesterly (across the front) winds over the East China Sea. When northeasterly winds prevail, considerable precipitation occurs on the offshore side of the Kuroshio front, in contrast to periods when northwesterly winds prevail. First, the northeasterly winds strengthen above the front because of the downward transfer of momentum from the fast-moving air at higher levels and/or an adjustment of sea level pressure over the oceanic front, although the process by which the influence of the Kuroshio penetrates beyond the marine atmospheric boundary layer remains unclear. Second, a cyclonic vortex forms above the marine atmospheric boundary layer (at 850-hPa height) on the offshore side of the front, and thereafter, surface wind convergence via Ekman suction (hence, enhanced precipitation) occurs over the East China Sea shelf breaks. The northeasterly winds blow over the East China Sea when the Aleutian Low retreats to the east and when high sea level pressure covers the northern Sea of Japan.     

Effects of Stokes production on summer ocean shelf dynamics

A two-dimensional numerical model,which is configured on the basis of Princeton ocean model（POM）,is used to study the effect of Stokes production（SP） of the turbulent kinetic energy on a density profile and Ekman transport in an idealized shelf region in summer.The energy input from SP is parameterized and included into the Mellor-Yamada turbulence closure submodel.Results reveal that the intensity of wind-driven upwelling fronts near the sea surface is weakened by the SP-associated turbulent kinetic energy input.The vertical eddy viscosity coefficient in the surface boundary layer is enhanced greatly owing to the impact of SP,which decreases the alongshore velocity and changes the distribution of upwelling.In addition,the SP-induced mixing easily suppresses the strong stratification and significantly increases the depth of the upper mixed layer（ML） under strong winds.     

Modeling a neutrally stratified turbulent air flow over a horizontal rough surface

The neutrally stratified boundary layer over a smooth rough surface is consider. The turbulent flow is simulated using a finite-difference eddy-resolving model of the atmospheric boundary layer (ABL). The model includes different turbulence closure schemes and numerical approximations for advection components of the momentum balance equation. We investigate the quality of reproduction of spectral characteristics of the turbulent flow and the model’s capabilities to reproduce the observed profile of mean wind velocity near the rough surface. It is shown that the best result is obtained by coupling a numerical scheme of higher order of accuracy with a mixed closure scheme based on an adaptive estimation of the mixing length for subgrid-scale fluctuations. Here, we are able to reproduce the asymptotics of the fluctuation spectrum of the longitudinal component of wind velocity near the surface and within the boundary layer as well as the logarithmic profile of mean velocity near the surface.     

Assessment of turbulence closure models for resonant inertial response in the oceanic mixed layer using a large eddy simulation model

Large eddy simulation (LES) of the resonant inertial response of the upper ocean to strong wind forcing is carried out; the results are used to evaluate the performance of each of the two second-order turbulence closure models presented by Mellor and Yamada (Rev Geophys Space Phys 20:851–875, 1982) (MY) and by Nakanishi and Niino (J Meteorol Soc Jpn 87:895–912, 2009) (NN). The major difference between MY and NN is in the formulation of the stability functions and the turbulent length scale, both strongly linked with turbulent fluxes; in particular, the turbulent length scale in NN, unlike that in MY, is allowed to decrease with increasing density stratification. We find that MY underestimates and NN overestimates the development of mixed layer features, for example, the strong entrainment at the base of the oceanic mixed layer and the accompanying decrease of sea surface temperature. Considering that the stability functions in NN perform better than those in MY in reproducing the vertical structure of turbulent heat flux, we slightly modify NN to find that the discrepancy between LES and NN can be reduced by more strongly restricting the turbulent length scale with increasing density stratification.     

Mesoscale structure of the fields of vertical velocity and vorticity of winds over the Black Sea

On the basis of a simple stationary linear model and a contemporary nonlinear model, we study the mesoscale characteristics of the fields of vertical velocity and vorticity of winds over the Black Sea. It is shown that, in the idealized case (without taking into account the coastal features), the temperature contrast between the land and the sea in both models leads to the formation of circulation cells in the coastal zone. However, the mountainous features prove to be the main factor affecting the process of circulation for the actual synoptic situations. We present the map of monthly average vorticity of the surface winds constructed according to the nonlinear mesoscale model with a resolution of 30 km for January 1996–2001. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 1, pp. 52–61, January–February, 2005.     

Comparing satellite and meteorological data on wind velocity over the Black Sea

Wind-velocity data obtained from in situ measurements at the Golitsyno-4 marine stationary platform have been compared with QuikSCAT scatterometer data; NCEP, MERRA, and ERA-Interim global reanalyses and MM5 regional atmospheric reanalysis. In order to adjust wind velocity measured at a height of 37 m above the sea surface to a standard height of 10 m with stratification taken into account, the Monin–Obukhov theory and regional atmospheric reanalysis data are used. Data obtained with the QuikSCAT scatterometer most adequately describe the real variability of wind over the Black Sea. Errors in reanalysis data are not high either: the regression coefficient varies from 0.98 to 1.06, the rms deviation of the velocity amplitude varies from 1.90 to 2.24 m/s, and the rms deviation of the direction angle varies from 26° to 36°. Errors in determining the velocity and direction of wind depend on its amplitude: under weak winds (<3 m/s), the velocity of wind is overestimated and errors significantly increase in determining its direction; under strong winds (>12 m/s), its velocity is underestimated. The influence of these errors on both spatial and temporal estimates of the characteristics of wind over the Black Sea is briefly considered.     

Field Study of the Space Uniformity of Meteorological and Wave Parameters in the Coastal Zone. On the Problem of Calibration of a Side-Looking Radar on the <Emphasis Type="Italic">Sich-1M</Emphasis> Satellite as an Instrument for Measuring Wind Velocity over the Sea

We study the problem of subsatellite calibration of a side-looking radar (SLR) of the Sich-1M satellite regarded as an instrument for the evaluation of the velocity of surface wind and discuss the possibility of application of the meteorological and wave parameters measured from the stationary oceanographic platform of the Experimental Department of the Marine Hydrophysical Institute (Ukrainian Academy of Sciences) for their evaluation in the remote regions of the sea. These regions can be observed with the help of the SLR without errors introduced by the appearance of the coast in elements of space resolution. To prove the indicated possibility experimentally, we perform simultaneous measurements of the water and air temperatures, wind velocity, and the characteristics of waves both from the platform and in the water area neighboring with the platform on the side of the sea at distances of up to 4.8 km. The quantitative estimates of the degree of space uniformity of the analyzed parameters are deduced under the conditions of stable inshore wind. A conclusion is made that the platform can be used for subsatellite measurements aimed at the validation and improvement of the algorithm of reconstruction of the velocity of surface wind by using the SLR of the Sich-1M satellite. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 3, pp. 31–43, May–June, 2005.     

青岛地区海雾分布及大气边界层条件分析

青岛地区海雾多发,观测表明海雾对沿海地区影响范围不尽相同,特别是海雾影响内陆的机理尚缺乏研究。本文利用观测资料及数值模式统计了青岛地区4月-7月海雾分布特征,并对不同影响范围海雾典型个例进行对比分析,结果表明:海雾发生日数自沿海向内陆递减。胶州湾沿岸雾日数比黄海沿岸明显减少,胶州湾东北部的雾日数要少于胶州湾西北部。海雾多发生于高空形势稳定,低层偏南流场的天气条件下。大气边界层内逆温层的的范围大致影响着海雾的分布。只影响沿海的海雾,地面为偏南风,风速在3~8 m/s之间,内陆风力减弱不明显。500 m以下大气边界层内风速切变大。湍流作用使海雾向内陆推进过程中倾斜抬升为低云,地面雾区减弱。能够影响内陆地区的海雾,多出现在地面风力较弱的情况之下,大部分在1~3 m/s之间。500 m以下大气边界层内风速切变小,大气边界层内湍流强度不强,使沿海到内陆的逆温层能够始终维持,沿海海雾在弱南风影响下延伸影响内陆地区。     

Air-Sea Interaction, Coastal Circulation and Primary Production in the Eastern Arabian Sea: A Review

Air-sea interaction, coastal circulation and primary production exhibit an annual cycle in the eastern Arabian Sea (AS). During June to September, strong southwesterly winds (4∼9 m s⁻¹) promote sea surface cooling through surface heat loss and vertical mixing in the central AS and force the West India Coastal Current equatorward. Positive wind stress curl induced by the Findlater jet facilitates Ekman pumping in the northern AS, and equatorward-directed alongshore wind stress induces upwelling which lowers sea surface temperature by about 2.5°C (compared to the offshore value) along the southwestern shelf of India and enhances phytoplankton concentration by more than 70% as compared to that in the central AS. During winter monsoon, from November to March, dry and weak northeasterly winds (2–6 m s⁻¹) from the Indo-China continent enhance convective cooling of the upper ocean and deepen the mixed layer by more than 80 m, thereby increasing the vertical flux of nutrients in the photic layer which promotes wintertime phytoplankton blooms in the northern AS. The primary production rate integrated for photic layer and surface chlorophyll-a estimated from the Coastal Zone Color Scanner, both averaged for the entire western India shelf, increases from winter to summer monsoon from 24 to 70 g C m⁻²month and from 9 to 24 mg m⁻², respectively. Remotely-forced coastal Kelvin waves from the Bay of Bengal propagate into the coastal AS, which modulate circulation pattern along the western India shelf; these Kelvin waves in turn radiate Rossby waves which reverse the circulation in the Lakshadweep Sea semiannually. This review leads us to the conclusion that seasonal monsoon forcing and remotely forced waves modulate the circulation and primary production in the eastern AS. This revised version was published online in July 2006 with corrections to the Cover Date.     

Seasonal variations in temperature,salinity and density in the southern coastal waters of the Caspian Sea

Variability in water temperature, salinity and density was investigated based on field measurements near Anzali Port, in the Southern Caspian Sea in 2008. Seasonal changes of seawater properties were mainly observed through the upper 100 m layer, while below this layer seasonal variations of the parameters were minor. Vertical structure of the temperature in the southern coastal waters of the Caspian Sea is characterized by a significant seasonal thermocline between 20–50 m depths with vertical variation in temperature about 16°C in midsummer (August). Decrease of the thermocline occurs with the general cooling of the air and sea surface water, and deepening of the mixed layer during late of autumn and winter. Seasonal averages of the salinity were estimated in a range of 12.27–12.37 PSU. The structure of thermocline and pycnocline indicated agreement between changes of temperature and density of seawater. Seasonal pycnocline was observed in position of the thermocline layer.     

The wind-field structure in a stably stratified atmospheric boundary layer over a rough surface

Both wind turning with height and ageostrophic flow in a stably stratified atmospheric boundary layer are analyzed using a three-parameter turbulence model. For a quasi-steady state of the boundary layer, the cross-isobaric flow is determined only by turbulent stress at the surface in the direction of geostrophic wind. The “operative” prediction models, in which the first-order turbulence closure schemes are used, tend to overestimate the boundary-layer depth and underestimate the angle between the surface and geostrophic winds when compared to “research” models (schemes of high-level turbulence closure). The true value of the angle between the surface and geostrophic winds is significant for the presentation of a large-scale flow. A nocturnal low-level jet is a mesoscale phenomenon reflected in data obtained from measurements in a stably stratified atmospheric boundary layer. It is found that such jets are of great importance in transporting humidity, momentum, and air pollution. In this study, the difference between jet flows over a homogeneous underlying surface and over a spatially localized large-scale aerodynamic roughness is shown.     

Experimental estimation of the coefficient of vertical turbulent exchange in a stratified layer of the black sea near the continental slope

On the basis of the data of field measurements, we present the results of numerical analysis of the intensity of vertical turbulent exchange in stratified layers of the Black Sea in the region of the shelf–continental-slope boundary depending on the local stratification. The experiments were carried out within the framework of the GEF/BSERP and Black Sea-2004 international projects. The data were obtained by using a probing version of the Sigma-1 measuring complex. In processing the data of measurements, we apply a procedure of evaluation of the coefficient of vertical turbulent diffusion depending on the external conditions based on the analysis of the spectra of the gradient of temperature fluctuations. For the two studied regions of the shelf, the coefficients of turbulent exchange turn out to be much higher (by about an order of magnitude) than for the open sea under similar conditions. This can be explained by the specific features of the bottom topography affecting the dynamics of quasiinertial waves playing to role of the main causes of small-scale mixing and vertical diffusion. Translated from Morskoi Gidrofizicheskii Zhurnal, No. 6, pp. 14–24, November–December, 2008.