Gradient Richardson Number Ri Research Articles

High-Resolution Remote Sensing of the Gradient Richardson Number in a Megacity Boundary Layer

The Gradient Richardson Number (Ri) is an important parameter for appraising the stability and turbulence exchange at the atmospheric boundary layer (ABL). However, high-resolution measurements of Ri profiles are rarely reported, especially in megacities. In this study, a Doppler wind lidar and a microwave radiometer were simultaneously utilized to measure the 2 km Ri vertical profile in downtown Beijing. These measurements were verified to have high accuracy compared with observations from a 325 m meteorological tower, with root-mean-square errors (RMSEs) of less than 1.66 K, 7.9%, and 1.45 m/s for the temperature, relative humidity, and wind speed (WS) for all altitudes and corresponding Pearson correlation coefficients (R) of 0.97, 0.93, and 0.81. The inter-comparisons of different spatial (25 m, 50 m, 100 m) and temporal resolutions (1 min, 30 min, 1 h) form a 3 × 3 resolution matrix of Ri, in which the 1 h temporal resolution of Ri overestimates the intensity and active area of turbulence. The Ri value retrieved from the 100 m spatial resolution data overestimates these by half as it misidentifies the height of the stable area at the near surface. There are significant differences between the data with a 1 min temporal resolution and a 25 m spatial resolution (defined as the standard resolution of Ri), and the rest of the data in the resolution matrix (defined as data at other resolutions), with an RMSE > 1 and an R < 0.8. The difference between data at the standard resolution and data at other resolutions increases with elevations, which results from frequent weather processes or from water-vapor blocking at higher altitudes. The Ri profiles reveal that the atmospheric layer at altitudes from 100 m to 500 m in daytime is unstable, with Ri < 0, while it is neutral, with 0 < Ri < 0.25, at night-time from 200 m to 400 m. The atmosphere above the ABL in a megacity is rather stable, with Ri > 0.25, whereas below the ABL, it is neutral or unstable, which is due to drastic changes in the WS and temperature that are affected by the topography and surface friction.

Comparison of ADCP and ECCOv4r4 Currents in the Pacific Equatorial Undercurrent

Abstract The Pacific Equatorial Undercurrent (EUC) flows eastward across the Pacific at the equator in the thermocline. Its variability is related to El Niño. Moored acoustic Doppler current profiler (ADCP) measurements recorded at four widely separated sites along the equator in the EUC were compared to currents generated by version 4 release 4 of the Estimating the Circulation and Climate of the Ocean (ECCOv4r4) global model–data synthesis product. We are interested to learn how well ECCOv4r4 currents could complement sparse in situ current measurements. ADCP measurements were not assimilated in ECCOv4r4. Comparisons occurred at 5-m depth intervals at 165°E, 170°W, 140°W, and 110°W over time intervals of 10–14 years from 1995 to 2010. Hourly values of ECCOv4r4 and ADCP EUC core speeds were strongly correlated, similar for the EUC transport per unit width (TPUW). Correlations were substantially weaker at 110°W. Although we expected means and standard deviations of ECCOv4r4 currents to be smaller than ADCP values because of ECCOv4r4’s grid representation error, the large differences were unforeseen. The appearance of ECCOv4r4 diurnal-period current oscillations was surprising. As the EUC moved eastward from 170° to 140°W, the ECCOv4r4 TPUW exhibited a much smaller increase compared to the ADCP TPUW. A consequence of smaller ECCOv4r4 EUC core speeds was significantly fewer instances of gradient Richardson number (Ri) less than 1/4 above and below the depth of the core speed compared to Ri computed with ADCP observations. We present linear regression analyses to use monthly-mean ECCOv4r4 EUC core speeds and TPUWs as proxies for ADCP measurements. Significance Statement Hundreds of scientific papers have used ECCO data products generated with a continually evolving state-of-the-art ocean-model–data synthesis system. We ask, How representative is the latest version of ECCO equatorial ocean currents? We use independent in situ current measurements as the reference dataset to establish the accuracy of ECCO currents in the tropical Pacific. Attention is focused on the Pacific Equatorial Undercurrent (EUC) because it contributes to the formation of El Niño and La Niña events. ECCO EUC core speeds were smaller in magnitude and less variable in time compared to observations. As a consequence, ECCO currents generated smaller vertical mixing in the EUC compared to that inferred from current measurements. We developed a linear regression model to improve representation of monthly-mean ECCO currents.

Formation and transport of fluid mud triggered by typhoon events in front of the subaqueous Changjiang Delta

Fluid mud supported by waves can move downslope on a gentle slope under gravity to transport massive sediment across the continental shelf and cause significant geomorphological changes. In order to explore the formation and transportation of fluid mud under extreme conditions, we undertook in-situ measurements and observed fluid-mud-related processes triggered by a typhoon, Danas, in the subaqueous Changjiang Delta. Under the action of waves, firstly, the seabed was eroded slowly for ∼930 min caused by the wave-enhanced bed shear stress; next, rapid erosion occurred when the bed erodibility Me/ρB doubled the value during no waves. The great change in bed erodibility implied that bed liquefaction occurred due to waves. The fluid mud occurred concurrently with the strongest waves and went through five stages with an interval of 0.5–3.5 h between each stage in which waves weakened continuously. In addition, the five fluid mud stages all underwent three identical phases, i.e., formation, stabilization, and decay. In the formation and decay phases, the bottom friction force exceeded the buoyancy gravitational force, and the gradient Richardson number (Ri) was lower than the typical threshold of 0.25, indicating that fluid mud was unstable due to high turbulence energy enhanced by Kelvin-Helmholtz instabilities. During the stabilization phase, the buoyancy gravity force was almost balanced with the drag friction force, and Ri was close to or higher than 0.25, suggesting a laminar fluid mud status. Eventually, the fluid mud disappeared when significant wave height decreased to <1.39 m. Our observation results showed that bed erosion and liquefaction both contributed to the formation of the fluid mud, and the fluid mud played an important role in sediment transportation and bed landforms in subaqueous deltas during extreme events.

Turbulent Kinetic Energy Production in a Far‐Field River Plume Under Upwelling‐Favorable Winds

AbstractAn idealized three‐dimensional numerical model is used to investigate turbulent kinetic energy (TKE) production in a far‐field river plume under upwelling‐favorable winds. TKE production decreases over longer length scales as the river plume thickens. Maximum TKE production appears in the surface layer and is mainly generated by the alongshore component of the velocity shear. The large velocity shear and weak stratification in the surface layer result in a gradient Richardson number (Ri) of &lt;0.25, which corresponds to the locally high TKE production. We find that asymmetrical TKE production occurs at the two edges of the river plume, due to the opposite nonlinear interaction of the Ekman and geostrophic effects at the shoreward and seaward edges of the river plume. This asymmetrical TKE production combines with secondary upwelling circulation in the river plume under upwelling‐favorable winds, which may generate more intense biological activity at the shoreward edge of the river plume than at the seaward edge. Numerical model experiments are performed to examine the effects of wind, river discharge, and stratified conditions on TKE production in the river plume. Finally, we propose a conceptual model in which the depth‐averaged TKE production in the river plume is proportional to the alongshore wind stress () and inversely proportional to the cube of the surface boundary layer thickness (D3), which is consistent with the results of numerical experiments.

Threshold behavior of local gradient Richardson number in strongly stratified nonequilibrium turbulence

A nonequilibrium, strongly stratified turbulent flow is examined with a numerically simulated data set. We find self-organization of the flow around a critical state with a local gradient Richardson number (Ri) of about 1/4. The turbulence spontaneously organizes into layered, anisotropic flow structures within which the local Ri probability density function peaks around 1/4 for regions with the most significant dissipation rates and eddy viscosities. The results suggest that self-organization of the flow around Ri = 1/4 could be a property of layered anisotropic stratified turbulence and may not require the specific external forcing mechanisms previously reported.

Influence of Stability and Surface Roughness on Turbulence during the Stable Atmospheric Variability and Transport (SAVANT) Field Campaign

Abstract Harvesting of crops in a weakly sloping Midwestern field during the Stable Atmospheric Variability and Transport (SAVANT) observation campaign allowed for a systematic investigation of the influence of surface roughness and static stability magnitude on the applicability of the Monin–Obukhov similarity (MOST) and hockey-stick transition (HOST) theories during stable boundary layer periods. We analyze momentum flux and turbulent velocity scale VTKE in three regimes, defined using the gradient Richardson number Ri and flux Richardson number Rif as regime 1 (0 &lt; Ri ≤ 0.1 and 0 &lt; Rif ≤ 0.1), regime 2 (0.1 &lt; Ri ≤ 0.23 and 0.1 &lt; Rif ≤ 0.23), and regime 3 (both Ri and Rif &gt; 0.23). After harvest, in regime 1, stability varied from near-neutral to weakly stable and both MOST and HOST were applicable to estimate the momentum fluxes and VTKE as a function of mean wind speed. In regime 2, the momentum flux deviated from the MOST linear relationship as stability increased. In regimes 1 and 2, a HOST-defined threshold wind speed Vs was identified beyond which VTKE increased linearly with wind speed at a rate of 0.26 for all observation heights. Below this threshold wind speed, VTKE behaved independent of mean wind and observation heights. Alternatively, for preharvest periods, MOST was applicable in regimes 1 and 2 for all heights and HOST was applicable with reduced Vs for heights above the crop layer. Regime 3 during pre- and postharvest consisted of strongly stable periods and very weak to weak winds, where MOST was found to be invalid and VTKE remained low and independent of wind speed. The results suggest that roughness due to crops enhances the turbulence generation at lower wind speeds.

Scaling the Mixing Efficiency of Sediment‐Stratified Turbulence

AbstractThe flux Richardson number Rf, also called the mixing efficiency of stratified turbulence, is important in determining geophysical flow phenomena such as ocean circulation and air‐sea transports. Measuring Rf in the field is usually difficult, thus parameterization of Rf based on readily observed properties is essential. Here, estimates of Rf in a strongly turbulent, sediment‐stratified estuarine flow are obtained from measurements of covariance‐derived turbulent buoyancy fluxes (B) and spectrally fitted values of the dissipation rate of turbulent kinetic energy (ε). We test scalings for Rf in terms of the buoyancy Reynolds number (Reb), the gradient Richardson number (Ri), and turbulent Froude number (Frt). Neither the Reb‐based nor the Ri‐based scheme is able to describe the observed variations in Rf, but the Frt‐based parameterization works well. These findings support further use of the Frt ‐ based parameterization in turbulent oceanic and estuarine environments.

Turbulence Parameters in the Troposphere—Lower Stratosphere Observed by Beijing MST Radar

Based on three years of data observed by Beijing MST radar between 2012 and 2014, this study gives the distribution characteristics of the turbulence energy dissipation rate ε and the vertical turbulence diffusion coefficient Kz in the troposphere–lower stratosphere over Beijing based on the spectral width method. Additionally, from the perspective of atmospheric stability expressed by the Brunt–Väisälä frequency N and the gradient Richardson number Ri, the seasonal variations of turbulence parameters at different heights are analyzed. The results show that ε (Kz) is distributed in the range of 10−4 to 10−1.5 m2 s−3 (100 to 101 m2 s−1), with median values of 10−3 to 10−2.5 m2 s−3 (100.4 to 100.75 m2 s−1). In the height range of −1 km to about +1 km from the tropopause, the profiles of ε have no noticeable seasonal difference from October to next May, and ε increases sharply with the slope of linear regression ∇log(ε)/∇h = 0.25 (m2 s−3) km−1. The seasonal variation of turbulence parameters has noticeable differences at different atmospheric layers. Furthermore, the atmospheric static/dynamic stability and turbulence intensity are the influencing factors of turbulence parameters. From the tropopause to 14 km, the values of ε and N both have a peak from October to next May, and this peak layer disappears from July to August. Within the range of 2–4 km from the ground, Kz and the frequency of Ri &lt; 0.25 have a significant positive correlation, with a Pearson correlation coefficient greater than 0.5, and both have large value layers in March to May and September to October. The turbulence intensity above 15 km is likely the main factor affecting the turbulence dissipation rate and vertical turbulence diffusion coefficient.

Estimating the astronomical seeing above Dome A using Polar WRF based on the Tatarskii equation

A seeing model based on the Tatarskii equation, which uses wind shear and temperature gradient as inputs, has been proposed for Dome A in our previous study. It has been shown that the seeing was correctly reproduced statistically. In this paper, in order to investigate the ability of the Polar version of the Weather Research and Forecasting (PWRF) model to forecast the seeing, we have tried three different PWRF-simulated wind shear and temperature gradient combinations of two levels for the near-ground. The three combinations all give good performance with the correlation coefficients being higher than 0.75, when compared with the seeing measured by the Kunlun Differential Image Motion Monitor at Dome A during the summer of 2019. Besides this, the gradient Richardson number (Ri), which was also simulated by the PWRF model, basically behaved as expected as the Ri is generally large when the measured seeing is small. These results suggest that the PWRF model can be used to forecast the astronomical seeing above Dome A.

Fluid mud dynamics in a tide-dominated estuary: A case study from the Yangtze River

Fluid mud plays an important role in the maintenance of the estuarine turbidity maximum (ETM) and makes a significant contribution to estuarine sediment transport. A seabed tripod system and a shipboard vertical profile were deployed to explore the spatial-temporal variation and mechanism of fluid mud in the ETM of the Yangtze Estuary. The near-bed suspended sediment concentration (SSC) measured by OBS and ABS exceeded 50 kg/m3, confirming the existence of fluid mud. The fluid mud layer varied frequently in thickness during spring and intermediate tides, with a maximum of up to 0.6 m, but maintained stability during neap tides. The thickness rose rapidly during the larger ebb tide in semidiurnal tidal cycles of intermediate tides when the gradient Richardson number (Ri) increased to exceed the critical value of 0.25, and the velocity at 0.2 mab was in the range of 0.5–0.8 m/s. The fluid mud layer transported more than twice as much sediment as the low-concentration upper layer did. Waves and intense currents at the flood phase of tide enhanced vertical mixing within the water column, with Ri being reduced to lower than 0.25 and turbulent kinetic energy (TKE) exceeding 0.01 m2/s2, leading to the breakdown of the fluid mud layer. Sediment stratification damps turbulence close to the seabed and leads to gradual solidification of the bottom fluid mud. Approximately 0.2 m of fluid mud deposit was formed on the seabed within 148 h of the observation period.

Characterising the nocturnal surface and canopy heat islands in Oklahoma City, USA according to the atmospheric stability

This study attempts to explore the impacts of atmospheric stability on characterising the seasonal and spatial patterns of the nocturnal surface (sUHI) and canopy air (aUHI) heat islands in Oklahoma City (OKC), US. The urban meteorological network in OKC provides a unique opportunity to evaluate the impact of stability on UHI due to the routine collection of data at two heights. This enables the calculation of the gradient Richardson number (Ri), the utility of which can then be used to enhance approaches which use the more commonly used Pasquill-Gifford method for the calculation of atmospheric stability. Although, good correlations were generally found between the P-G scheme and the Ri, the more simplistic P-G scheme elicited a stronger relationship with UHI intensity (UHII). More specifically, it was found that the increased aUHI under more stable conditions highlighted the more superior controls of the atmospheric stability on the development of aUHI during both summer and transition seasons, comparing to the sUHI that is less affected by the atmospheric stability particularly during transition seasons. Overall, these findings have implications for the design of future studies as it highlights that the measurement of windspeed and temperature at a single height is sufficient to characterise atmospheric stability in UHI studies.

Off-Equatorial Deep-Cycle Turbulence Forced by Tropical Instability Waves in the Equatorial Pacific

AbstractThe equatorial Pacific cold tongue is a site of large heat absorption by the ocean. This heat uptake is enhanced by a daily cycle of shear turbulence beneath the mixed layer—“deep-cycle turbulence”—that removes heat from the sea surface and deposits it in the upper flank of the Equatorial Undercurrent. Deep-cycle turbulence results when turbulence is triggered daily in sheared and stratified flow that is marginally stable (gradient Richardson number Ri ≈ 0.25). Deep-cycle turbulence has been observed on numerous occasions in the cold tongue at 0°, 140°W, and may be modulated by tropical instability waves (TIWs). Here we use a primitive equation regional simulation of the cold tongue to show that deep-cycle turbulence may also occur off the equator within TIW cold cusps where the flow is marginally stable. In the cold cusp, preexisting equatorial zonal shear uz is enhanced by horizontal vortex stretching near the equator, and subsequently modified by horizontal vortex tilting terms to generate meridional shear υz off of the equator. Parameterized turbulence in the sheared flow of the cold cusp is triggered daily by the descent of the surface mixing layer associated with the weakening of the stabilizing surface buoyancy flux in the afternoon. Observational evidence for off-equatorial deep-cycle turbulence is restricted to a few CTD casts, which, when combined with shear from shipboard ADCP data, suggest the presence of marginally stable flow in TIW cold cusps. This study motivates further observational campaigns to characterize the modulation of deep-cycle turbulence by TIWs both on and off the equator.

Estimating fugitive particle emission from coal storage yard of thermal power plant using the flux-gradient method

In this paper, a 3D sonic anemometer and two optical particulate sensors were used to observe fugitive particulate matter 10 μm (PM10) emission from the coal storage yard of a thermal power plant for one month. The PM10 emission flux was calculated using the flux gradient method. The results show that the heat flux, convective velocity, and PM10 emission flux have obvious diurnal variation. The PM10 emission flux is positively correlated with heat flux, convective velocity, and frictional velocity, but negatively correlated with humidity. Based on the convective velocity, friction velocity and humidity as the independent variables, a multivariate regression model of PM10 emission flux was constructed. The prediction results of the model are in good agreement with the experimental values. The emission flux of PM10 can also be estimated quasi-quantitatively according to the value of gradient Richardson number (Ri). There are two time windows for PM10 emissions, namely the discharge window period (9:00–20:00) and the static window period (20:00–9:00 of next day).In order to reduce PM10 emissions more effectively, artificial climate intervention such as watering, spraying, and other measures should be carried out during the discharge window period. Studies have shown that the flux gradient method can be successfully used to accurately measure the PM10 emission flux of a coal storage yard. This study also provides useful recommendations for real-time monitoring and controlling PM10 emissions from coal storage sites.

COEFFICIENTS OF EDDY DIFFUSION OF MOMENTUM AND HEAT IN ATMOSPHERIC BOUNDARY LAYER: NUMERICAL STUDY

The changes in turbulent eddy mixing in the atmospheric boundary layer (ABL) are investigated with the use of the mesoscale RANS turbulence model with account for effects of internal gravitational waves, which support momentum transfer under condition of very stable stratification. A focus was put on analysis of behavior of the coefficients of vertical eddy diffusion of momentum and heat. The behavior of the turbulent eddy mixing parameters was found to be consistent with measurements in the laboratory and atmosphere. In particular, the flow Richardson number () during the transient flow to a strongly stable state can behave nonmonotonically, growing with increasing gradient Richardson number () to the state of saturation at a certain gradient Richardson number ( Ri@ 1 ), which separates two different turbulent regimes: strong mixing and weak mixing.

Шороон шуурга болох үеийн агаар мандлын тогтворшлын параметрүүд

Although many studies investigated the spatial distribution, duration and frequency of dust storm in Mongolia, there are a few studies were undertaken it in terms of physical process that underpinned the current study on the subject. The study examines whether it is possible to acquire the distribution of dust storms in a relatively simple way by estimating the atmospheric stability during a dust storm. In order to achieve the main purpose of the study, the atmospheric stability parameters such as Brunt-Vaisala frequency (N), Gradient Richardson Number (Ri), and Eady Growth Rate (σ_E) were evaluated on 3 different cases of heavy dust storms which are occurred from March to May 2020, covered almost all over Mongolia, using reanalysis data produced by the European Centre for Medium-Range Weather Forecasts and radiosonde data from “Dalanzadgad” an aerological station in Dalanzadgad, Umnugovi. Differential equations of N, Ri, and σ_E are solved using the finite difference method. The results of the study revealed that both Ri and σ_E stability parameters can be identified well the distribution of dust storm, while a linear relationship between a dust storm and N as well as the buoyancy component of Ri was not found. In addition, in the study, a simplified method to compute the Ri parameter based on power function was acquired by the wind shear. It was difficult to make statistical analyses in the results of the study based on few cases between March and May in 2020. Therefore, it is necessary to consider more cases in the future, to undertake statistical analysis.&#x0D; &#x0D; &#x0D;

Acoustic Observations of Kelvin‐Helmholtz Billows on an Estuarine Lutocline

AbstractKelvin‐Helmholtz (KH) instability plays an important role in turbulent mixing in deep oceans, coastal seas, and estuaries. Though widely observed and studied in thermohaline‐stratified waters, KH instability has only rarely been observed in sediment‐stratified environments. For the first time, we present direct observations of KH billows on an estuarine lutocline by combining echosounder images with velocity and density measurements. The interaction between velocity shear and the density stratification induced by suspended sediments initiated shear instabilities near the bed, indicated by gradient Richardson number (Ri) &lt; 0.25 in the early stages of the observed billows. Once formed, the instabilities enhanced the vertical mixing of momentum, reducing vertical shear and elevating Ri. Linear instability analysis using measured velocity and density profiles well predicts the vertical location and spatial characteristics of the observed billows. These instabilities are believed to contribute to the vertical mixing, entrainment, and transport of estuarine and coastal sediments.

ON THE TURBULENT PRANDTL NUMBER IN STABLY STRATIFIED TURBULENCE BY SECOND ORDER MODELS

The aim of this work is to investigate the behavior of the Turbulent Prandtl number by second order modeling of a stably stratified homogeneous sheared turbulence. By analytic solutions, we have confirmed the asymptotic equilibrium behavior of the turbulent Prandtl number. Then two between the most second order models of turbulence; the Classic Launder-Reece-Model and the sophisticated Craft Launder model are retained. A non dimensional form of transport equations have been obtained when non dimensional parameters are introduced to substitute second order moments. A numerical integration using the fourth order Runge kutta method has been conducted for different values of the gradient Richardson number Ri. In comparison with direct numerical simulation result’s of Shih et al. the obtained results by the Craft Launder model has shown for the turbulent Prandtl number the best agreement at moderate values of gradient Richardson number 0.15 &lt; Ri &lt; 0.28. The classic model has shown a great default for the different values of Ri. No any concordance with retained results of DNS has been obtained by this model. We show also that prediction of this model can be improved by introducing variation and optimization of model constants.

Estimation of Turbulence Parameters in the Lower Troposphere from ShUREX (2016–2017) UAV Data

Turbulence parameters in the lower troposphere (up to ~4.5 km) are estimated from measurements of high-resolution and fast-response cold-wire temperature and Pitot tube velocity from sensors onboard DataHawk Unmanned Aerial Vehicles (UAVs) operated at the Shigaraki Middle and Upper atmosphere (MU) Observatory during two ShUREX (Shigaraki UAV Radar Experiment) campaigns in 2016 and 2017. The practical processing methods used for estimating turbulence kinetic energy dissipation rate ε and temperature structure function parameter C T 2 from one-dimensional wind and temperature frequency spectra are first described in detail. Both are based on the identification of inertial (−5/3) subranges in respective spectra. Using a formulation relating ε and C T 2 valid for Kolmogorov turbulence in steady state, the flux Richardson number R f and the mixing efficiency χ m are then estimated. The statistical analysis confirms the variability of R f and χ m around ~ 0.13 − 0.14 and ~ 0.16 − 0.17 , respectively, values close to the canonical values found from some earlier experimental and theoretical studies of both the atmosphere and the oceans. The relevance of the interpretation of the inertial subranges in terms of Kolmogorov turbulence is confirmed by assessing the consistency of additional parameters, the Ozmidov length scale L O , the buoyancy Reynolds number R e b , and the gradient Richardson number Ri. Finally, a case study is presented showing altitude differences between the peaks of N 2 , C T 2 and ε , suggesting turbulent stirring at the margin of a stable temperature gradient sheet. The possible contribution of this sheet and layer structure on clear air radar backscattering mechanisms is examined.

Turbulence at the periphery of Sri Lanka dome

Measurements of kinetic energy dissipation rate ε and other oceanographic variables (temperature, salinity, density, and current profiles) were conducted at 20 stations along a 90 nautical mile section at the southwestern periphery of Sri Lanka Dome (SLD), which is a mesoscale cyclonic eddy that usually appears in Bay of Bengal to the east of Sri Lanka during summer monsoons. The broad aim was to document and interpret the structure of flow and turbulence in SLD and quantify turbulent mixing therein using parametric studies. A tendency of upward doming (upwelling) of the pycnocline toward the SLD center is observed, leading to substantial shallowing of the mixed layer therein, with the mixed layer depth decreasing from ~40 m outside the SLD to ~10–15 m inside the cyclonic eddy. The cumulative distribution of ε in the SLD pycnocline (below 40 m depth) is well approximated by the Burr probability distribution. A high level of ε in the pycnocline was observed at the inner periphery of SLD, concurrent with low values of gradient Richardson numberRi. A parameterization for eddy diffusivity in the SLD pycnocline as a function of Ri is suggested.

Quantifying Diapycnal Mixing in an Energetic Ocean

AbstractTurbulent diapycnal mixing controls global circulation and the distribution of tracers in the ocean. For turbulence in stratified shear flows, we introduce a new turbulent length scale dependent on χ. We show the flux Richardson number Rif is determined by the dimensionless ratio of three length scales: the Ozmidov scale LO, the Corrsin shear scale LS, and . This new model predicts that Rif varies from 0 to 0.5, which we test primarily against energetic field observations collected in 100 m of water on the Australian North West Shelf (NWS), in addition to laboratory observations. The field observations consisted of turbulence microstructure vertical profiles taken near moored temperature and velocity turbulence time series. Irrespective of the value of the gradient Richardson number Ri, both instruments yielded a median , while the observed Rif ranged from 0.01 to 0.50, in agreement with the predicted range of Rif. Using a Prandtl mixing length model, we show that diapycnal mixing can be predicted from and the background vertical shear S. Using field and laboratory observations, we show that where LE is the Ellison length scale. The diapycnal diffusivity can thus be calculated from . This prediction agrees very well with the diapycnal mixing estimates obtained from our moored turbulence instruments for observed diffusivities as large as m2s−1. Moorings with relatively low sampling rates can thus provide long time series estimates of diapycnal mixing rates, significantly increasing the number of diapycnal mixing estimates in the ocean.