Atmospheric turbidity of urban and desert areas of the Nile Basin in the aftermath of Mt. Pinatubo’s eruption

Summary  The Linke T _L , ?ngstr?m β and Unsworth-Monteith δ _a turbidity parameters are investigated for two sites in Egypt: Cairo, a densely populated urban area, and Aswan, an arid unpolluted area. These three turbidity parameters are calculated from broadband pyrheliometric measurements recorded hourly over the period 1992–96. Monthly averages of T _L , β and δ _a show relatively flat and identical seasonal variations with a marked main maxima during spring at both sites, due to Khamsin depressions coming from the Great Sahara. A secondary maximum is observed at Aswan in summer, due to dust haze which prevails during that season, and at Cairo in autumn, due to the northern extension of the Sudan monsoon trough, which is accompanied by small scale depressions with dust particles. Annual mean values of T _L , β and δ _a (5.59, 0.250 and 0.372, respectively) at Cairo are larger than at Aswan (3.89, 0.139 and 0.213, respectively). In the same way, the seasonal mean values of T _L , β and δ _a at Cairo are larger than at Aswan. More generally, the monthly and yearly average turbidity values are significantly larger in Cairo than in Aswan for the whole period 1992–96, which is attributable in part to the urbanization/industrialization effect of Cairo. An estimate of the corresponding overburden is obtained by comparison between the present data and older T _L data from 1922–27. It is also shown that turbidity over both sites is largest during 1992, just after the eruption of Mount Pinatubo in 1991. The dependence of β on some meteorological parameters such as wind speed and direction, precipitable water, relative humidity, temperature and visibility, is also analyzed. This reveals in particular that visibility is not a good predictor of turbidity at either site. Conversely, the wind direction and speed have a definite effect on turbidity, and consequently, largest turbidities occur when the wind carries aerosols from the main industrial particle source areas around Cairo. For any season of the year, the average turbidity at the latter site is larger than that at other big cities such as Athens, Rome, and Toronto, but is lower than at Dhahran, Saudi Arabia. Received February 3, 2000 Revised August 25, 2000     

Changes in integral and aerosol atmospheric turbidity in Trans-Baikal and Central Siberia

The analysis of time series of the Linke turbidity factors is performed for the atmosphere mass m = 2 (T ₂) and atmospheric aerosol optical thickness for the wavelength λ₀ = 0.55 μm (AOT) from the data of 14 actinometrical stations of Central Siberia and Trans-Baikal territory. It is shown that over the period from 1976 to 2006, the increased atmospheric transparency is observed in the region. Quantitative estimates of changes in multiyear mean annual variations of T ₂ and AOT at different periods of averaging and for different time periods are derived.     

Variability of the Linke and Unsworth-Monteith turbidity parameters in Athens, Greece

Summary  This work studies the variability of the Linke (T _L ) and Unsworth-Monteith (T _U ) turbidity parameters in the urban atmosphere of Athens. Beam irradiance observations performed at the Actinometric Station of the National Observatory of Athens are used in the period 1975–1995. This study examines (i) the inter-annual variation of T _L and T _U , (ii) their mean seasonal variability, (iii) their monthly average variation, and (iv) their mean daily variation. Also, for various air-mass origins, the mean seasonal variations of T _L and T _U are given. The frequency of occurrence of the parameters in various ranges is also shown. An innovative interpretation of the impact of the Saharan dust on the radiative properties in the atmosphere of Athens is attempted. Received May 19, 2000 Revised October 17, 2000     

横断山脉中段云岭白茫雪山的大气混浊度

本文根据1983和1984年横断山脉中段云岭白茫雪山东坡三个不同拔海高度(4292m、3760m和2025m)上测量的太阳直接辐射资料,计算并分析了云岭白茫雪山林克大气混浊因子T_P。T_P随拔海高度的升高而减小,不同拔海高度上T_p的年变化趋势基本一致,最大值出现在7、8月份,最小值在11月份。若将T_P分成气溶胶混浊因子T_R和水汽混浊因子T_W(T_P=1+T_R+T_W),水汽混浊因子T_W的年变化趋势与降水分布相吻合,气溶胶混浊因子T_R呈相反的变化趋势。地处干热河谷的奔子拦,由于局地气候的影响,其混浊度较大,接近海拔50m的北京,年平均值为3.09。     

Analysis and trends of atmospheric turbidity parameters over Cairo

Summary This work deals with the Linke turbidity factor, based on total spectrum observations of the direct solar beam and aerosol turbidity parametersa _a , , and based on observations in broad spectral bands. Diurnal and seasonal variations of these turbidity parameters were analyzed for the period 1975 to 1991.Annual variations of these parameters show low values in winter and high values in both spring and summer. The extinction coefficients decrease with increase of both wavelength and optical airmass. Trend analysis shows an increase in aerosol extinction coefficient below 0.63 m, and a slight decrease for longer wave-lengths.Linear regression relations are also constructed to estimate botha _a and whenT _L is available. The relations show thata _a can be estimated with errors below 20%. The relation with the parameter, may give better results when it is estimated by assigning a fixed value of .Nomenclature AV Monthly and total average of the measured parameter - a Atmospheric optical thicknes - a _a Aerosol optical thickness - a _r Mean value of optical thickness of. Rayleigh atmosphere over all wavelengths - a _o Ozone optical thickness - a _o Ozone absorption coefficient - a _w Water vapor optical thickness - COR Correlation coefficient of the linear relation in percentage - Ex₁, Ex₂, Ex₃ Aerosol extinction coefficients in the bands .2–.53, .53–.63, .63–.695, respectively - I _(o) Normal incident direct solar radiation, under clear sky condition - I _o Extraterrestrial insolation at normal incidence - m _r Relative (optical) air mass - NO Number of the observations used in either making the relation or the verification - RMSE Root mean square error of the linear relation - RMSE% Percentage value of the root mean square error relative to the average measured value AV - T _L Linke turbidity factor - T Dry bulb temperature in °C - u _o Ozone layer thickness, cm - Z Zenith angle - Ångström wavelength exponent - Ångström turbidity coefficient - Wavelength - Y The year number after 1975 With 5 Figures     

Spectral aerosol optical depth and Angstrom parameters in the polluted Athens atmosphere

Summary Using ground-based spectral solar extinction data taken in the Athens atmosphere during a field survey, experimental and modeled aerosol optical depths AOD have been retrieved. The Angstrom parameters α and β were derived through spectral extinction curves determined from a log–log plot fit in four spectral intervals, e.g. UV, VIS, UV-VIS and VIS-NIR. The results reveal that exponent α derived in the shorter wavebands, is correlated with β; in the VIS-NIR range the correlation is weaker, however. It has also found that α does not depend on the Angstrom law fits, while β does. Retrieved experimental AOD obtained at longer wavelengths exhibits a daily pattern similar to turbidity β; by contrast, AOD at shorter wavelengths follows the temporal variation of α.     

Localized stirring in a field of salt-fingers

In a series of laboratory experiments, a partially mixed patch was produced in thick linear concentration gradients favorable to salt-finger convection. Salt-fingers, which give rise to an up-gradient flux of buoyancy, can reduce and invert the density gradient in the initial imposed patch. This leads to overturning convection within the patch if (a) the ratio of ambient T and S gradients, R_ρ=αT_z/βS_z, is near one; (b) the initial imposed turbulence results in a nearly well-mixed patch; and (c) the patch thickness is large enough that convective eddies are able to transport T and S faster than salt-fingers. Once overturning occurs, subsequent turbulent entrainment can lead to growth of the patch thickness. Experimental results for one-dimensional patches (layers) agree well with the theoretical prediction. This thickening is in contrast to the collapse that a partially mixed three-dimensional patch would experience due to lateral intrusion in a wide tank.     

Approximation formulas for the microphysical properties of saline droplets

Microphysical theory has proven essential for explaining sea spray's role in transferring heat and moisture across the air–sea interface. But large-scale models of air–sea interaction, among other applications, cannot afford full microphysical modules for computing spray droplet evolution and, thus, how rapidly these droplets exchange heat and moisture with their environment. Fortunately, because the temperature and radius of saline droplets evolve almost exponentially when properly scaled, it is possible to approximate a droplet's evolution with just four microphysical endpoints: its equilibrium temperature, T_eq; the e-folding time to reach that temperature, τ_T; its equilibrium radius, r_eq; and the e-folding time to reach that radius, τ_r.Starting with microphysical theory, this paper derives quick approximation formulas for these microphysical quantities. These approximations are capable of treating saline droplets with initial radii between 0.5 and 500 μm that evolve under the following ambient conditions: initial droplet temperatures and air temperatures between 0 and 40 °C, ambient relative humidities between 75% and 99.5%, and initial droplet salinities between 1 and 40 psu.Estimating T_eq, τ_T, and τ_r requires only one-step calculations; finding r_eq is done recursively using Newton's method. The approximations for T_eq and τ_T are quite good when compared to similar quantities derived from a full microphysical model; T_eq is accurate to within 0.02 °C, and τ_T is typically accurate to within 5%. The estimate for equilibrium radius r_eq is also usually within 5% of the radius simulated with the full microphysical model. Finally, the estimate of radius e-folding time τ_r is accurate to within about 10% for typical oceanic conditions.     

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 L_U = (ε/U_Z³)^1/2. At wavenumbers between the inertial-convective (k^−5/3) and wak turbulence (k⁻³) subranges, additional narrow spectral intervals—‘production’ subranges—may appear (E k⁻¹, E_T k⁻²). The upper boundary of these subranges is determined as L_U, and the lower boundaries as L_R (ε/U_ZN²)^1/2(χ/T_Z²). It is shown that the scale L_U is a unique spectral scale that is uniform up to a constant value for every hydrophysical field. It appears that the spectral scale L_U is equivalent to the Thorpe scale L_Th 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^−11/5, E_T k^−7/5). During decay the buoyancy subrange is expanded to lower and higher wavenumbers. At lower wavenumbers the buoyancy subrange is bounded by L_** = 3(χ^1/2/N^1/2T_Z), which is equivalent to the Thorpe scale L_Th. 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 L_N ε^1/2/N^3/2 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.     

A Study of Linke Turbidity Factor over Qena / Egypt

Data on instantaneous atmospheric Linke turbidity factor TL (m) are reported for clear days at Qena/Egypt in the period from June 1992 to May 1993.TL(m) is determined using the values of irradiance of direct solar radiation (I),which are calculated from global (G) and diffuse (D) - solar radiation measurements.Monthly and seasonally variations of both diurnal and daily average values of TL (m) increases steadily in the direction of sunset in the months from June to December 1992 as well as Summer and Autumn seasons,while it falls generally in this direction for the months from January to March and Winter season.In April and May,TL (m) fluctuates obviously through the day hours,it is also shown that the average values of TL(m) are particularly large during Summer months compared to other months of the year.This behavior of TL(m) is discussed in view of the variations of some weather elements,which affect the content of water vapor and dust particle in the atmosphere of the study region.It seems t be of s     

On Transpiration and Soil Moisture Content Sensitivity to Soil Hydrophysical Data

Sensitivity of evapotranspiration E and root zone soil moisture content θ to the parameterization of soil water retention Ψ(θ) and soil water conductivity K(Ψ), as well as to the definition of field capacity soil moisture content, is investigated by comparing Psi1-PMSURF and Theta-PMSURF models. The core of PMSURF (Penman–Monteith Surface Fluxes) consists of a 3-layer soil moisture prediction module based on Richard’s equation in combination with the Penman–Monteith concept for estimating turbulent heat fluxes. Psi1- PMSURF and Theta-PMSURF differ only in the parameterization of the moisture availability function F_ma. In Psi1,F_ma is parameterized by using Ψ(θ) and K(Ψ) hydrophysical functions; in Theta, F_ma is parameterized by using hydrophysical parameters: the field capacity θ_f and wilting point θ_w soil moisture contents. Both Psi1 and Theta are based on using soil hydrophysical data, that is, there is no conceptual difference between them in the parameterization of E even if in Psi1F_ma depends on 12 parameters, while in Theta only on two soil/vegetation parameters. Sensitivity tests are performed using the Cabauw dataset. Three soil datasets are used: the vG (van Genuchten), CH/vG (Clapp and Hornberger/van Genuchten) and CH/PILPS (Clapp and Hornberger/Project for Intercomparison of Land-surface Parameterization Schemes) datasets. The vG dataset is used in van Genuchten’s parameterization, while in Clapp and Hornberger’s the CH/vG and CH/PILPS datasets are used. It is found that the consistency of soil hydrophysical data in the simulation of transpiration is quite important. The annual sum of E obtained by Psi1E^Psi1, differs from the annual sum of E obtained by Theta, E^Theta, because of the inconsistency between the fitting parameters of Ψ(θ) and K(Ψ) and the θ_f, and not because of the differencies in the parameterization of F_ma. Further, θ_f can be estimated not only on the basis of using soil hydrophysical functions (the θ_f so obtained is θ^Soil_f) but also on the basis of analysing the transpiration process (the θ_fso obtained is θ^tr_f). θ^tr_f values estimated from the condition E^Theta ≈ E^Psi1 are in acceptable accordance with the θ^Soil_f values proposed by Wösten and co-workers. The results are useful in optimizing the parameterization of transpiration in land-surface schemes.     

Measurements of turbulence and fossil turbulence near ampere seamount

Measurements of temperature and velocity microstructure near and downstream of a shallow seamount are used to compare fossil turbulence versus non-fossil turbulence models for the evolution of turbulence microstructure patches in the stratified ocean. According to non-fossil oceanic turbulence models, all overturn length scales L_T of the microstructure grow and collapse in constant proportion to each other and to the turbulence energy (Oboukov) scale L_O and the inertial buoyancy (Ozmidov) scale of the patches; that is, with L_Trms ≈1.2L_R and viscous dissipation rate ≈ ₀^*. According to the Gibson fossil turbulence model, all microstructure originates from completely active turbulence with ₀ ≈ 3L_T²N³(≈ 28₀^*) and L_{T/√6 ≈ LTrms}, but this rapidly decays into a more persistent active-fossil state with _0F ≈ 30vN², where N is the buoyancy frequency and v is the kinematic viscosity and, without further energy supply, finally reaches a completely fossil turbulence hydrodynamic state of internal wave motions, with _F. The last turbulence eddies, with ≈ _F, vanish at a buoyant-inertial-viscous (fossil Kolmogorov) scale L_KF that is much smaller than the remnant overturn scales L_T for large ₀/_F ratios. These density, temperature, and salinity overturns with L_T ≈ 0.6 L_R0 0.6 L_R persist as turbulence fossils (by retaining the memory of _o) and collapse very slowly. In the near wake below the summit depth of Ampere seamount, a much larger proportion of completely active turbulence patches was found than is usually found in the ocean interior away from sources. Dissipation rates and turbulence activity coefficients of microstructure patches were found to decrease downstream, suggesting that the active turbulence indicated by the patches with A_T 1 was caused by the presence of the seamount as a turbulence source. Therefore, the turbulence and mixing processes of ocean layers far away from turbulence sources probably have been undersampled by microstructure data sets lacking any A_T 1 patches. This is because large fractions of the mixing and viscous dissipation of the patches occur in short-lived active turbulence regimes that are too brief to be detected. Consequently, large underestimates of the true space-time average turbulence fluxes and turbulence and scalar dissipation rates may result if non-fossil turbulence models are assumed in ocean microstructure data interpretation.     

Rate constants for the reactions of the NO3 radical with HCOOH/HCOO− and CH3COOH/CH3COO− in aqueous solution between 278 and 328 K

The kinetics of the aqueous phase reactions of NO₃ radicals with HCOOH/HCOO^– and CH₃COOH/CH₃COO^– have been investigated using a laser photolysis/long-path laser absorption technique. NO₃ was produced via excimer laser photolysis of peroxodisulfate anions (S₂O ₈ ^2– ) at 351 nm followed by the reactions of sulfate radicals (SO ₄ ^– ) with excess nitrate. The time-resolved detection of NO₃ was achieved by long-path laser absorption at 632.8 nm. For the reactions of NO₃ with formic acid (1) and formate (2) rate coefficients ofk ₁=(3.3±1.0)×10⁵ l mol^–1 s^–1 andk ₂=(5.0±0.4)×10⁷ l mol^–1 s^–1 were found atT=298 K andI=0.19 mol/l. The following Arrhenius expressions were derived:k ₁(T)=(3.4±0.3)×10¹⁰ exp[–(3400±600)/T] l mol^–1 s^–1 andk ₂(T)=(8.2±0.8)×10¹⁰ exp[–(2200±700)/T] l mol^–1 s^–1. The rate coefficients for the reactions of NO₃ with acetic acid (3) and acetate (4) atT=298 K andI=0.19 mol/l were determined as:k ₃=(1.3±0.3)×10⁴ l mol^–1 s^–1 andk ₄=(2.3±0.4)×10⁶ l mol^–1 s^–1. The temperature dependences for these reactions are described by:k ₃(T)=(4.9±0.5)×10⁹ exp[–(3800±700)/T] l mol^–1 s^–1 andk ₄(T)=(1.0±0.2)×10¹² exp[–(3800±1200)/T] l mol^–1 s^–1. The differences in reactivity of the anions HCOO^– and CH₃COO^– compared to their corresponding acids HCOOH and CH₃COOH are explained by the higher reactivity of NO₃ in charge transfer processes compared to H atom abstraction. From a comparison of NO₃ reactions with various droplets constituents it is concluded that the reaction of NO₃ with HCOO^– may present a dominant loss reaction of NO₃ in atmospheric droplets.     

Indirect determination of broadband turbidity coefficients over Egypt

Long-term data from diffuse and global irradiances were used to calculate direct beam irradiance which was used to determine three atmospheric turbidity coefficients (Linke T _L , Ångström β and Unsworth–Monteith δ _a ) at seven sites in Egypt in the period from 1981 to 2000. Seven study sites (Barrani, Matruh, Arish, Cairo, Asyut, Aswan and Kharga) have been divided into three categories: Mediterranean climate (MC), desert Nile climate (DNC) and urban climate (UC, Cairo). The indirect method (i.e., global irradiance minus diffuse irradiance) used here allows to estimate the turbidity coefficients with an RMSE% ≤20 % (for β, δ _a and T _L ) and ~30 % (for β) if compared with those estimated by direct beam irradiance and sunphotometeric data, respectively. Monthly averages of T _L , β and δ _a show seasonal variations with mainly maxima in spring at all stations, due to Khamsin depressions coming from Sahara. Secondary maxima is observed in summer and autumn at DNC and MC (Barrani and Arish) stations in summer due to dust haze which prevails during that season and at UC (Cairo) in autumn, due to the northern extension of the Sudan monsoon trough, which is accompanied by small-scale depressions with dust particles. The mean annual values of β, δ _a , and T _L (0.216, 0.314, and 4.6, respectively) are larger in Cairo than at MC stations (0.146, 0.216, and 3.8, respectively) and DNC stations (0.153, 0.227, and 3.8, respectively). Both El-Chichon and Mt. Pinatubo eruptions were examined for all records data at MC, UC and DNC stations. The overburden caused by Mt. Pinatubo’s eruption was larger than El-Chichon’s eruption and overburden for β, and T _L at DNC stations (0.06, and 0.58 units, respectively) was more pronounced than that at MC (0.02, and 0.26, respectively) and UC (0.05 and 0.52 units, respectively) stations. The annual variations in wind speed and turbidity parameters show high values for both low and high wind speed at all stations. The wind directions have a clear effect on atmospheric turbidity, and consequently, largest turbidities occur when the wind carries aerosols from the main particle sources, such as industrial particle sources around Cairo or to some extent from the Sahara surrounding all study stations.     

The lower symmetric regime and its transition to waves in rotating annulus convection

Measurements of the temperature and zonal velocity fields which develop in a rotating annulus of fluid with an upper surface, differentially heated from the inner to outer cylinder, are described for the lower symmetric regime (small radial temperature differences). The temperature field is essentially conductive for moderate to large rotation rates, Ω (>1.0 sec⁻¹). The zonal velocity field is poorly approximated by the thermal wind equation.Measurements of the transition to waves from the lower symmetric regime at very large rotation rates are presented for positive and negative radial temperature differences. They suggest that the centrifugal buoyancy force and the free surface curvature may be important factors for the lower symmetric-wave transition at large Ω. By varying the stratification of the fluid over a range of 10³ independently of the radial temperature difference, Δr_wT, it is conclusively shown that several theories are correct in predicting that the lower symmetric transition is independent of the stratification at small Δr_wT > 0 for large enough Ω.     

Determination of rate constants for reactions of some hydrohaloalkanes with OH radicals and their atmospheric lifetimes

Rate constants have been measured for the gas-phase reactions of hydroxyl radical with partly halogenated alkanes using the discharge-flow-EPR technique over the temperature range 298–460 K. The following Arrhenius expressions have been derived (units 10^–13 cm³ molecule^–1 s^–1): (8.1 _–1.2 ^+1.5 ) exp{–(1516±53)/T} for CHF₂Cl (HCFC-22); (10.3 _–1.5 ^+1.8 ) exp{–(1588±52)/T} for CH₂FCF₃ (HFC-134a); (11.3 _–1.6 ^+2.1 ) exp{–(918±52)/T} for CHCl₂CF₂Cl (HCFC-122); (9.2 _–2.0 ^+2.5 ) exp{–(1281±85)/T} for CHFClCF₂Cl (HCFC-123a).The atmospheric lifetimes for the substances have been estimated to be 12.6, 12.9, 1.05, and 4.8 years, respectively, and the accuracy of the estimates is discussed.     

Variations in the atmospheric electric field at tropical station during 1930–1987

The variations noticed in the atmospheric electric field recorded at Pune (18o32’N, 73o51’E, 559 m ASL), a tropi-cal inland station located in Deccan Plateau, India, during the period 1930-1987, have been examined in relation to the variations observed in the Angstrom turbidity coefficient (β) and selected meteorological parameters. The month-ly and annual mean values of the atmospheric electric field, Angstrom turbidity coefficient (β), rainfall, temperature and relative humidity for the years 1930-1938, 1957-1958, 1964-1965, 1973-1974 and 1987 were considered in the study.The results of the above study indicated gradual increases in the atmospheric electric field over the period of study (1930-1987) which is statistically significant at less than 5% level. The increases noticed during different periods varied from 30 to 109 %. The increase noticed during the period (1930-1938) and (1973-1974) was maximum (109%). The Angstrom turbidity coefficient also showed systematic increases during the period of study, which is consistent. The diurnal curve of the atmospheric electric field at the station by and large, showed a double oscillation, which is generally observed in the continental environments.     

The sea surface temperature deviation and the heat flow at the sea-air interface

The deviation of the sea surface temperature from the water temperature below is calculated as a function of the heat flow through the air-sea interface, using wind tunnel determinations of the effective thermal diffusivity in a boundary layer. The influence ofQ, shortwave radiation, andH, latent and sensible heat transfer plus effective back radiation, and U, wind speed, can be described by:T ₀ –T _w =C ₁ ·H/U +C ₂ ·Q/U. The calculated coefficients vary slightly with reference depth, Tables II and III. They are in good agreement with independent observations.On leave at Department of Oceanography, Oregon State University, Corvallis, Oregon in 1969–70.     

Internal tide generation at a continental shelf/slope junction: A comparison between theory and a laboratory experiment

Observations of internal tide generation over continental slopes in a laboratory experiment have been carried out, with the objectives of making comparisons with linear generation theory and investigating its limitations. Both continuous and layered stratification have been considered. A measure of the amplitude of the barotropic tidal forcing (and hence of non-linearity) is given by the Froude Number F = u_sb/c_w, where u_sb is the maximum barotropic tidal velocity at the shelf break, and c_w is the long-wave speed of the lowest internal mode.For continuous stratification, good agreement was obtained for “steep” slopes (α/c > 1), where α is the slope at the continental slope and c is the slope of the internal wave rays of tidal frequency), even for quite large amplitude motions (F < 1.6), and the upper limit of its quantitative usefulness was not reached. For “flat” slopes (α/c < 1) reasonable agreement was also obtained, even up to quite amplitudes (F < 3.1), although some departure from linear theory was apparent.For two-layer flows the applicability of linear theory was much more restricted. For F 0.5 there was only qualitative agreement and for larger F (>1) significant differences were observed. The latter were due to the substantial advection and associated hydraulic jumps which occured seaward of the shelf-break during the ebb-phase of the barotropic tide. Shelf-break values of F > 1 are common in the ocean.     

Rapid vertical sampling in stably stratified turbulent shear flows

A new method for obtaining instantaneous vertical profiles of two components of velocity and temperature in thermally stratified turbulent shear flows is presented. In this report, the design and construction of the traversing system will be discussed and results to date will be presented. The method is based on rapid vertical sampling whereby probe sensors are moved vertically at a high speed such that the measurement is approximately instantaneous. The system is designed to collect many measurements for the calculation of statistics such as vertical wave number spectra, mean square vertical gradients, and Thorpe scales. Results are presented for vertical profiles of temperature and compared to vertical profiles measured by single-point Eulerian time averages. The quality of the vertical profiles is found to be good over many profiles. Some comparisons are made between vertical measurements and standard single-point Eulerian measurements for three cases of stably stratified turbulent shear flow in which the initial microscale Reynolds number, Re_λ≈30. In case 1, the mean conditions are characterized by a gradient Richardson number, Ri_g=0.015, for which the flow is “unstable”, meaning the spatially evolving turbulent kinetic energy (E_k) grows. In case 2, Ri_g=0.095, for which the evolving turbulent kinetic energy is almost constant. In case 3, the flow is highly stable, where Ri_g=0.25 and E_k decays with spatial evolution. The measurements indicate anisotropy in the small scales for all cases. In particular, it is found that the ratio grows initially to a maximum and then decays with further evolution. Maximum Thorpe displacements are measured and compared to single-point measures of the vertical scales. It is found that vertical length scales derived from single-point measurements, such as the Ozmidov scale, L_O=(ε/N³)^1/2 and the overturn scale, L_t=θ′/(dT/dz), do not represent well the wide range of overturning scales which are actually present in the turbulence.