Parametrization of turbulent fluxes over inhomogeneous landscapes

Reasons for the nonclosure of the heat balance in the atmospheric boundary layers over natural land surfaces are analyzed. Results of measuring the heat-balance components over different land surfaces are used. The Cabauw (Netherlands) data (obtained throughout 1996 over a grass surface with intermittent shrubs and single trees) and the data from the Anchor station in Germany (measured over coniferous forest in 2000–2001) are analyzed. In all, the analysis involves about fifty thousand independent values of the heat-balance components measured in the experiments, which should be indicative of the reliability of the results obtained in the paper. The data have shown that the heat balance is not closed and the imbalance is 50–250 W/m². The sum of the latent and sensible heat fluxes λE + H = STF is found to be systematically smaller than the difference between the net radiation and the heat flux into the ground R _n ? G. It is shown that the main cause of a systematic heat imbalance in the atmospheric boundary layers over inhomogeneous land surfaces is that the methods of surface-flux measurement and estimation are based on the theory that requires the hypothesis of stationarity and horizontal homogeneity. Direct data analysis has shown that the heat imbalance increases with landscape inhomogeneity. In the paper, a parametrization of the heat imbalance is carried out and the coefficient k _f (z ₀ ^ef /L ^ef ) is introduced as a measure of inhomogeneity. For this, data from the experiments FIFE, KUREX, TARTEX, SADE, etc., are also used. Empirical formulas are presented to refine the results of direct measurements and calculations of surface fluxes over natural (inhomogeneous) land surfaces from profile and standard (using bulk parametrizations) data. These formulas can also be used to determine surface fluxes over inhomogeneous underlying land surfaces in order to take into account so-called subgrid-scale effects in constructing prediction models.     

On the Development of Models for Height Profiles of the Wind Speed in the Atmospheric Surface Layer

The reliability of the known models of a height profile of the wind speed V(h) in the atmospheric boundary layer (ABL) and near-surface layer (NSL) is analyzed using the data of long-term ABL measurements accumulated in Russia in the state network of meteorological and aerological stations and the data of multilevel measurements at mast wind-measuring complexes. A new multilayer semiempirical model of V(h) is proposed which is based on aerodynamic and physical representations of the ABL vertical structure and relies on the hypothesis that wind-speed profiles providing the minimum wind friction on the ground and satisfying the conditions of profile smoothness are feasible in the ABL. This model ensures the best agreement with the data of meteorological, aerological, and mast wind measurements.     

A note on a critical wind speed for air–sea boundary processes

Wind and wind-generated waves were measured in a wind-wave tank. A clear transition was found in the relation between the wind speed U ₁₀ and the wind friction velocity u _* near u _* = 0.2 m/s, where U ₁₀ is the wind speed at 10 m height extrapolated from the measured wind profile in a logarithmic layer, and u _* = 0.2 m/s corresponds roughly to U ₁₀ = 8 m/s in the present measurement. Quite a similar transition was found in the relation between the spectral density of high frequency wind waves and u _*. These results suggest the existence of the critical wind speed for air–sea boundary processes, which was proposed by Munk (J Marine Res 6:203–218, 1947) more than half a century ago. His original idea of the critical wind speed was based on the discontinuities in such phenomena as white caps, wind stress, and evaporation, which commonly appear at a wind speed near 7 m/s. On the basis of the results of our present study and those of earlier studies, we discuss the phenomena which are relevant to the critical wind speed for the air–sea boundary processes. The conclusion is that the critical wind speed exists and it is attributed to the start of wave breaking rather than the Kelvin–Helmholtz instability, but the air–sea boundary processes are not discontinuous at a particular wind speed; because of the stochastic nature of breaking waves, the changes occur over a range of wind speeds. Detailed discussions are presented on the dynamical processes associated with the critical wind speed such as wind-induced change of sea surface roughness and high frequency wave spectrum. Future studies are required, however, to clarify the dynamical processes quantitatively. In particular, there is a need to further examine the gradual change of breaking patterns of wind waves with the increase of wind speed, and the associated change of the structure of the wind over wind waves, such as separation of the airflow at the crest of wind waves, the turbulent stress, and wave-induced stress. Studies on the dynamical structure of the high frequency wave spectrum are also needed.     

Features of the atmospheric boundary layer block for three versions of the WAM wind wave model

The estimated characteristics of the atmospheric boundary layer, obtained by the simulation of wind wave fields using three versions of the WAM numerical model are compared with the well-known empirical dependences of drag coefficient C _d on wind speed U ₁₀ and wave age A, as well as with the dependence of dimensionless roughness height z _n on inverse wave age u*/с р. Calculations carried out for several years in the areas of the Pacific and Indian oceans, based on the ERA-interim and CFSR wind reanalyses have shown good agreement between the model and empirical dependences C _d (U ₁₀) and C _d (A). The range of estimated variability for z _n (u*/с _р ) has been found to be significantly less than empirical. It has been also found that estimated values of wind speed U _10W (t) are overestimated from 5 to 10% in all versions of WAM models compared with the input wind reanalysis U _10R (t) at the moments of appearance maximum values of wind U _10R (t). The reasons for the established features of the WAM model and their dependence on the model version are discussed.     

Upper ocean heat content and atmospheric anomaly fields in the off-equatorial North Pacific related to ENSO

We have investigated interannual-scale variations of oceanic and atmospheric anomaly fields, such as upper ocean heat content (OHC), sea surface temperature (SST), latent heat flux (LHF) through the sea surface, sea level pressure (SLP) and wind stress curl (WSC) in the tropical Pacific and their relationships to El Niño/Southern Oscillation (ENSO) events. The results reported here show that the OHC and SST anomalies are almost in phase and lead LHF anomalies in the western tropical Pacific (WTP) region, which are preferable to the generation of subsequent atmospheric anomalies in the WTP. We also describe linear relationships between the amplitudes of these variables in the WTP. In addition, the results show that the both WSC and LHF anomalies are in phase with the temporal trend of OHC anomalies in the WTP, and suggest a combined effect of the local WSC and LHF anomaly in the WTP and ENSO-related, off-equatorial, westward propagating OHC anomaly to generate a large OHC anomaly in the WTP. In contrast to the WTP, OHC and SST anomalies are not in phase to the east of the WTP. The results also indicate that OHC anomalies in the WTP have a potential effect on the generation of an equatorial OHC anomaly via both a reflection of waves at the western boundary and atmospheric variations, which force the enhancement of western equatorial OHC anomaly. Therefore, the WTP is a key region where ENSO events are significantly modulated, and OHC anomalies in the WTP play an important role in the subsequent ENSO event.     

Influence of Feedbacks in the Climate–Energetics System on the Intensity of an Urban Heat Island

Both horizontal and vertical heat exchanges and feedbacks between air temperature and anthropogenic heat fluxes significantly affect the characteristics of the urban heat island (UHI). The UHI intensity depends, in particular, on the ratio between the scales L_A (area of anthropogenic forcing) and L_γ (distance passed by an air particle of the oncoming stably stratified flow before its temperature approaches air temperature within the UHI). Both advection and feedback effects may be estimated based on the equation for the local heat balance of the underlying surface. In this case, heat advection is taken into account by calculating temperatures individually for the atmospheric boundary layer and the surface of the urban canopy layer. The estimates show that the asymptotics of strong advection is more characteristic of a typical city. However, under weak winds, with consideration for the feedback between air temperature and anthropogenic heat flux, some deviations from this asymptotics are probable.     

Effect of air-sea temperature difference on the momentum exchange between air and sea for fetch-limited cases

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南极西北威德尔海上层海洋湍流混合研究

Turbulent mixing in the upper ocean(30-200 m) of the northwestern Weddell Sea is investigated based on profiles of temperature,salinity and microstructure data obtained during February 2014.Vertical thermohaline structures are distinct due to geographic features and sea ice distribution,resulting in that turbulent dissipation rates(ε) and turbulent diffusivity(K) are vertically and spatially non-uniform.On the shelf north of Antarctic Peninsula and Philip Ridge,with a relatively homogeneous vertical structure of temperature and salinity through the entire water column in the upper 200 m,both ε and K show significantly enhanced values in the order of O(10~(-7))-O(10~(-6)) W/kg and O(10~(-3))-O(10~(-2)) m~2/s respectively,about two or three orders of magnitude higher than those in the open ocean.Mixing intensities tend to be mild due to strong stratification in the Powell Basin and South Orkney Plateau,where s decreases with depth from O(10~(-8)) to O(10~(-9)) W/kg,while K changes vertically in an inverse direction relative to s from O(10~(-6)) to O(10~(-5)) m~2/s.In the marginal ice zone,K is vertically stable with the order of10~(-4) m~2/s although both intense dissipation and strong stratification occur at depth of 50-100 m below a cold freshened mixed layer.Though previous studies indentify wind work and tides as the primary energy sources for turbulent mixing in coastal regions,our results indicate weak relationship between K and wind stress or tidal kinetic energy.Instead,intensified mixing occurs with large bottom roughness,demonstrating that only when internal waves generated by wind and tide impinge on steep topography can the energy dissipate to support mixing.In addition,geostrophic current flowing out of the Weddell Sea through the gap west of Philip Passage is another energy source contributing to the local intense mixing.     

Evaluation of spatial distribution of turbulent mixing in the central Pacific

A long-term mean turbulent mixing in the depth range of 200–1000 m produced by breaking of internal waves across the middle and low latitudes (40°S–40°N) of the Pacific between 160°W and 140°W is examined by applying fine-scale parameterization depending on strain variance to 8-year (2005–2012) Argo float data. Results show that elevated turbulent dissipation rate (ε) is related to significant topographic regions, along the equator, and on the northern side of 20°N spanning to 24°N throughout the depth range. Two patterns of latitudinal variations of ε and the corresponding diffusivity (K_ρ) for different depth ranges are confirmed: One is for 200–450 m with significant larger ε and K_ρ, and the maximum values are obtained between 4°N and 6°N, where eddy kinetic energy also reaches its maximum; The other is for 350–1000 m with smaller ε and K_ρ, and the maximum values are obtained near the equator, and between 18°S and 12°S in the southern hemisphere, 20°N and 22°N in the northern hemisphere. Most elevated turbulent dissipation in the depth range of 350–1000 m relates to rough bottom roughness (correlation coefficient?=?0.63), excluding the equatorial area. In the temporal mean field, energy flux from surface wind stress to inertial motions is not significant enough to account for the relatively intensified turbulent mixing in the upper layer.     

Mesoscale wind stress–SST coupling in the Kuroshio extension and its effect on the ocean

Mesoscale perturbations (with a size of 100–1000 km) of wind stress magnitude, divergence and curl in the Kuroshio Extension (KE) are observed to tightly link to those of sea surface temperature (SST), and downwind and crosswind SST gradients, respectively. Based on long-term satellite observational data, their empirical relationships are established, which are further used to represent mesoscale wind stress–SST coupling in an ocean model that is based on the Regional Oceanic Modelling Systems (ROMS). The strength of mesoscale perturbations of wind stress and SST is observed to display a consistent seasonal variability, with the maximum appeared in winter while the minimum appeared in summer. This seasonal variability characteristic is also successfully simulated by ROMS with high resolution. Through comparing two experiments with and without the mesoscale wind stress–SST coupling, it is found that the mesoscale wind stress perturbation (τ _MS) has a negative feedback on SST perturbation (SST_MS). Analyses of sensitivity experiments suggest that the τ _MS acts to inhibit SST_MS mainly by means of surface heat flux. The τ _MS –SST_MS coupling also exerts influences on the ocean mean state and seasonal variability of SST in the KE. The effect of τ _MS on the SST is distinct in autumn and winter when the mesoscale perturbations are most active. Analyses of sensitivity experiments demonstrate that the τ _MS can affect the long term mean SST through either way of surface heat flux or momentum flux.     

Using the acoustic-pulse conservation law in estimating the energy of surface acoustic sources by remote sounding

The atmospheric effect on the characteristics of infrasonic signals from explosions has been studied. New methods have been proposed to remotely estimate the energy of explosions using the data of infrasonic wave registration. One method is based on the law of conservation of acoustic pulse I, which is equal to the product of the wave profile area S/2 of the studied infrasonic signal and the distance to the source E_I [kt] = 1.38 × 10^–10 (I [kg/s])^1.482. The second method is based on the relationship between the explosion energy and the dominant period T of the recorded signal, EТ [kt] =1.02 × (Т [s]²/σ)^3/2, where σ is a dimensionless distance used for determining the degree of manifestation of nonlinear effects in the propagation of sound along ray trajectories. When compared to the conventional E_W (Whitaker’s) relation, the advantage of the EI relation is that it can be used for pulsed sources located at an arbitrary height over the land surface and having an arbitrary form of the initial-pulse profile and for any type of infrasonic arrivals. A distinctive feature of the expression for E_Т is that the atmospheric effect on the characteristics of recorded infrasonic signals is explicitly taken into account. These methods have been tested using infrasonic data recorded at a distance of 322 km from the sources (30 explosions caused by a fire that occurred at the Pugachevo armory in Udmurtia on June 2, 2011). For the same explosion, empirical relations have been found between energy values obtained by different methods: E_I = 1.107 × E _W , E _Т = 2.201 × E _I .     

引入拖曳系数参数化的海冰自由漂流模拟研究

Many interesting characteristics of sea ice drift depend on the atmospheric drag coefficient(C_a) and oceanic drag coefficient(C_w).Parameterizations of drag coefficients rather than constant values provide us a way to look insight into the dependence of these characteristics on sea ice conditions.In the present study,the parameterized ice drag coefficients are included into a free-drift sea ice dynamic model,and the wind factor α and the deflection angle θ between sea ice drift and wind velocity as well as the ratio of C_a to C_w are studied to investigate their dependence on the impact factors such as local drag coefficients,floe and ridge geometry.The results reveal that in an idealized steady ocean,C_a/C_w increases obviously with the increasing ice concentration for small ice floes in the marginal ice zone,while it remains at a steady level(0.2-0.25) for large floes in the central ice zone.The wind factor α increases rapidly at first and approaches a steady level of 0.018 when A is greater than 20%.And the deflection angle θ drops rapidly from an initial value of approximate 80° and decreases slowly as A is greater than20%without a steady level like α.The values of these parameters agree well with the previously reported observations in Arctic.The ridging intensity is an important parameter to determine the dominant contribution of the ratio of skin friction drag coefficient(C_s' /C_s) and the ratio of ridge form drag coefficient(C_r'/C_r) to the value of C_a/C_w,α,and θ,because of the dominance of ridge form drag for large ridging intensity and skin friction for small ridging intensity among the total drag forces.Parameterization of sea ice drag coefficients has the potential to be embedded into ice dynamic models to better account for the variability of sea ice in the transient Arctic Ocean.     

Analysis of optimal conditions for recording signals when studying the atmospheric boundary layer with the acoustic method of partial reflection

Equations for the coefficient of partial reflection K from stratified inhomogeneities in the atmospheric boundary layer have been derived on the basis of the Epstein transition and symmetrical layer models as functions of three dimensionless parameters, i.e., the relative layer altitude, its relative thickness, and the relative variations in the effective sound speed in a layer. The equations have been obtained for the relative layer altitude at which the total internal reflection appears; the behavior of the function K is studied at close altitudes. Significant weakening of the dependence of coefficient K on the relative layer thickness in these conditions is shown, which makes it possible to record partially reflected signals in a wide range of wave-lengths or frequencies of the sounding signal. In other cases, the coefficient of partial reflection K strongly depends on the layer thickness. According to experimental data on variations in the amplitude of received acoustic signals with an increase in the source-detector distance, a technique for the parameterization of the additional impedance attenuation of sound that propagates over the earth’s surface has been developed, and these parameters have been experimentally estimated for different stratification conditions and sounding signal frequencies. Many records of background acoustic noises typical for one or another measurement sites have been distinguished and classified, a technique for estimating the minimum signal amplitude distinguishable against noises has been developed, and the corresponding estimates have been made. Based on these data and the specifications of three different industrial acoustic sources, the parameter limits provided by these sources have been estimated for the method of partial reflection.     

Seasonal dynamics and estimation of the annual primary production of phytoplankton in the Black Sea

Seasonal variations in the surface chlorophyll a concentrations (Chl _s) and the integrated primary production (PP _inf) were investigated for ten regions of the Black Sea based on long term observations (1973–1997). Two or three maximums of both Chl _s and PP _inf were registered in most of the shelf regions (SR, <200 m), the continental slope (CS, 200–1500 m), and the deep regions (DSR >1500 m) in February–March, June–August, and October–November. Such a pattern suggests that the seasonal dynamics of PP _inf strongly depend on the Chl _s variability. The mean annual values of the PP _inf comprised 130–420, 130–150, and 140–150 g C m^?2 in the SR, CS, and DSR, respectively. These values are mainly typical of the eutrophic layer and the transition between the eutrophic and mesotrophic waters (SR) or for the upper boundary of the mesotrophic waters (CS and DSR). The maximal contribution of the wintertime (December–March) to the total PP _inf values (40–42%) was observed in the DSR. In the SR and the adjacent eastern CS areas, the proportion of the PP _inf summertime production (June–September) reaches 40–60% and is higher than the wintertime production. The lowest values of PP _inf (9–17%) were produced in the spring and autumn periods. The total annual values of PP _inf in the Black Sea are close to 50–70 Mt C.     

Internal flow structure of short wind waves

The minimum value of wind stress under which the flow velocity in short wind waves exceeds the phase speed is estimated by calculating the laminar boundary layer flow induced by the surface tangential stress with a dominant peak at the wave crest as observed in previous experiments. The minimum value of the wind stress is found to depend strongly on, the ratio of the flow velocity just below the boundary layer and the phase speed, but weakly onL, the wavelength. For wind waves previously studied (=0.5,L=10 cm), the excess flow appears when the air friction velocityu _* is larger than about 30 cm sec^–1. The present results confirm that the excess flow found in my previous experiments is associated with the local growth of a laminar boundary layer flow near the wave crest.     

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.     

Variability of Particulate Organic Phosphorus in the Northwestern Part of the Black Sea

Based on long-term (1985–1995) monitoring data, the paper considers the peculiarities of seasonal variability in the spatial and vertical distribution of particulate organic phosphorus (Р_POM) in the surface layer and in the photosynthetic zone in the northwestern Black Sea. Regression equations, experimental data, and satellite observations for the chlorophyll a concentration allowed us to evaluate the seasonal longterm (1979–1995) variability in Р_POM in the surface layer and photosynthesis zone. The ratios of the concentrations of particulate organic carbon, nitrogen, phosphorus, and chlorophyll a are calculated and statistical estimates of seasonal changes in the Р_POM in the areas with different degrees of influence of river runoff and water of open seas are obtained. The consistency of intra-annual changes in the concentrations of Р_POM, chlorophyll a, and phytoplankton biomass is shown, which indicates the role of phytoplankton in the formation of Р_POM and in its intra- and interannual variability in the northwestern part of the sea. It is shown that long-term seasonal variations in Р_POM and related changes in the concentration of chlorophyll a depend on the variability of bulk river runoff, the extent of its abundance in the northwestern shelf, and regional hydrometeorological conditions.     

Spatial Distribution and Seasonal Dynamics of the Chlorophyll <Emphasis Type="Italic">a</Emphasis> Concentration in the Sea of Azov Based on MERIS Images

The chlorophyll a concentration (Cchl a) in the Sea of Azov is estimated by the two-band NIR-red algorithm [34] from MERIS images for 2002–2012. The sea-truth spectrophotometric measurements and MERIS remote estimates of Cchl a are compared. The monthly average Cchl a values are mapped from MERIS data for its lifetime for the first time. The features of the spatiotemporal distribution of Cchl a are ascertained. Differences between the seasonal dynamics of Cchl a in the Sea of Azov according to the literature data and the dynamics derived from MERIS data are found, namely: the summer–autumn phytoplankton growth period is longer than the spring period and is characterized by higher Cchl a values throughout the water area.     

The Role of Evolution Mechanisms in the Formation of a Wind-Wave Equilibrium Spectrum

The formation of a stationary (equilibrium) range in a wind-wave spectrum is investigated by numerical simulation. The equation of evolution of the wind-wave spectrum is solved using the exact calculation of the Hasselmann kinetic integral and involving various modifications of known parameterizations of the mechanisms of wave pumping by wind (In) and of wave dissipation (Dis). It is shown that it is these two mechanisms that are responsible for the shape of the stationary range of the wind-wave spectrum, whereas the nonlinear mechanism plays a stabilizing but subsidiary role. With an appropriate choice of mathematical representations for In and Dis, any known empirical shape of the stationary range of the spectrum can be obtained. During the calculations it is found that, for real wind waves, the known representations of In and Dis do not ensure the existence of the inertial interval required for Kolmogorov-type spectra formation due to the nonlinear interactions between waves.