Estimation of surface parameters from a 30 metre micro-meteorological tower over a deep moist convective region

The estimation of u*, 0*, q*. and Obukov-length In the surface layer from micro-meteorological tower data still poses an important challange. In the present study a procedure for the parametric estimation has been developed which is consistent both with the similarity relation and the profile relation. The study has been done using both fast response and slow response tower data. Since similarity relations involve a particular level z. so inspite of starting from a layer, the parameters should be attributed to a relations involve a particular level only, It has been suggested that the convenient level is geometric mean height of the layer. The ratio of eddy diffusivities (KhKm.) has been estimated both for stable and unstable situation and this ratio is presented by a single expression which incidentally yields a new value of a constant involved.

Observations of Eddy-Modulated Turbulent Mixing in the Southern Bay of Bengal

Long-term measurements of turbulent kinetic-energy dissipation rate (ε), and turbulent temperature-variance dissipation rate (χT) in the thermocline, along with currents, temperature, and salinity were made at two subsurface moorings in the southern Bay of Bengal (BoB). This is a part of a major international program, conducted between July 2018 and June 2019, for investigating the role of the BoB on the monsoon intraseasonal oscillations. One mooring was located on the typical path of the Southwest Monsoon Current (SMC), and the other was in a region where the Sri Lanka Dome is typically found during the summer monsoon. Microstructure and finescale estimates of vertical diffusivity revealed the long-term subthermocline mixing patterns in the southern BoB. Enhanced turbulence and large eddy diffusivities were observed within the SMC during the passage of a subsurface-intensified anticyclonic eddy. During this time, background shear and strain appeared to influence high-frequency motions such as near-inertial waves and internal tides, leading to increased mixing. Near the Sri Lanka Dome, enhanced dissipation occurred at the margins of the cyclonic feature. Turbulent mixing was enhanced with the passage of Rossby waves and eddies. During these events, values of χT exceeding 10−4 °C2 s−1 were recorded concurrently with ε values exceeding 10−5 W kg −1. Inferred diffusivity peaked well above background values of 10−6 m2 s−1, leading to an annually-averaged diffusivity near 10−4 m2 s−1. Turbulence appeared low throughout much of the deployment period. Most of the mixing occurred in spurts during isolated events.

Observations of Flow Separation and Mixing around the Northern Palau Island/Ridge

Turbulent mixing adjacent to the Velasco Reef and Kyushu–Palau Ridge, off northern Palau in the western equatorial Pacific Ocean, is examined using shipboard and moored observations. The study focuses on a 9-day-long, ship-based microstructure and velocity survey, conducted in November–December 2016. Several sections (9–15 km in length) of microstructure, hydrographic, and velocity fields were acquired over and around the reef, where water depths ranged from 50 to 3000 m. Microstructure profiles were collected while steaming slowly either toward or away from the reef, and underway current surveys were conducted along quasi-rectangular boxes with side lengths of 5–10 km. Near the reef, both tidal and subtidal motions were important, while subtidal motions were stronger away from the reef. Vertical shears of currents and mixing were stronger on the northern and eastern flanks of the reef than on the western flanks. High turbulent kinetic energy dissipation rates, 10−6–10−4 W kg−1, and large values of eddy diffusivities, 10−4–10−2 m2 s−1, with strong turbulent heat fluxes, 100–500 W m−2, were found. Currents flowing along the eastern side separated at the northern tip of the reef and generated submesoscale cyclonic vorticity of about 2–4 times the planetary vorticity. The analysis suggests that a torque, imparted by the turbulent bottom stress, generated the cyclonic vorticity at the northern boundary. The northern reef is associated with high vertical transports resulting from both submesoscale flow convergences and energetic mixing. Even though the area around Palau represents a small footprint of the ocean, vertical velocities and mixing rates are several orders magnitude larger than in the open ocean.

Methane emissions from an oil sands tailings pond: A quantitative comparison of fluxes derived by different methods

Tailings ponds in the Alberta Oil Sands Region are significant sources of fugitive emissions of methane to the atmosphere, but detailed knowledge on spatial and temporal variabilities is lacking due to limitations of the methods deployed under current regulatory compliance monitoring programs. To develop more robust and representative methods for quantifying these emissions, three micrometeorological flux methods were applied along with traditional flux chambers to determine fluxes over a 5-week period. Eddy covariance flux measurements provided the benchmark. A method is presented to directly calculate stability-corrected eddy diffusivities that can be applied to vertical gas profiles for gradient flux estimation. Gradient fluxes were shown to agree with eddy covariance within 7 %, and inverse dispersion model fluxes within 11 %, with an overall uncertainty of 28 % for the calculated mean flux. Fluxes were shown to have only a minor diurnal cycle (18 % variability) and to be mostly independent of wind speed, air and water surface temperatures. Flux chambers underestimated the fluxes by a factor of 2 in this particular campaign. These measurements indicate that the larger footprint of micrometeorological measurements results in more robust emission estimates representing the whole pond.

A quasi-experimental coastal region eddy diffusivity applied in the APUGRID model

In this study, Taylor statistical diffusion theory and sonic anemometer measurements collected at 11 levels on a 140 m high tower located in a coastal region in southeastern Brazil have been employed to obtain quasi-empirical convective eddy diffusivity parameterizations in a planetary boundary layer (PBL). The derived algebraic formulations expressing the eddy diffusivities were introduced into an Eulerian dispersion model and validated with Copenhagen tracer experiments. The employed Eulerian model is based on the numerical solution of the diffusion–advection equation by the fractional step/locally one-dimensional (LOD) methods. Moreover, the semi-Lagrangian cubic-spline technique and Crank–Nicolson implicit scheme are considered to solve the advection and diffusive terms. The numerical simulation results indicate that the new approach, based on these quasi-experimental eddy diffusivities, is able to reproduce the Copenhagen concentration data. Therefore, the new turbulent dispersion parameterization can be applied in air pollution models.

Seasonality of surface stirring by geostrophic flows in the Bay of Bengal

<p>Stirring of passive tracers in the Bay of Bengal driven by altimetry derived daily geostrophic surface currents, is studied on subseasonal timescales. To begin with, Hovmöller plots, wavenumber-frequency diagrams and power spectra confirm the multiscale nature of the flow. Advection of latitudinal and longitudinal bands highlights the chaotic nature of stirring in the Bay via repeated straining and filamentation of the tracer field. An immediate finding is that stirring is local, i.e. of the scale of the eddies, and does not span the entire basin. Further, stirring rates are enhanced along the coast of the Bay and are relatively higher in the pre- and post-monsoonal seasons. Indeed, Finite Time Lyapunov Exponent (FTLE) and Finite Size Lyapunov Exponent (FSLE) maps in all the seasons are patchy with minima scattered through the interior of the Bay. Further, these maps bring out a seasonal cycle wherein rapid stirring progressively moves from the northern to southern Bay during pre- and post-monsoonal periods, respectively. The non-uniform stirring of the Bay is reflected in long tailed probability density functions of FTLEs, that become more stretched for longer time intervals. Quantitatively, advection for a week shows the mean FTLE lies between 0.13±0.07 day<sup>-1</sup>, while extremes reach almost 0.6 day<sup>-1</sup> . Averaged over the Bay, Relative dispersion initially grows exponentially, followed by a power-law at scales between approximately 100 and 250 km, which finally transitions to an eddy-diffusive regime. Quantitatively, below 250 km, a scale dependent diffusion coefficient is extracted that behaves as a power-law with cluster size, while above 250 km, eddy-diffusivities range from 6 × 10<sup>3</sup> - 1.6 × 10<sup>4  </sup> m<sup>2</sup>s<sup>-1</sup> in different regions of the Bay. These estimates provide a useful guide for resolution dependent diffusivities in numerical models that hope to properly represent surface stirring in the Bay.</p>

Sensitivities of the MJO Forecasts on Configurations of Physics in the ECMWF Global Model

Sensitivities of MJO forecasts to various different configurations of physics are examined with the ECMWF global model, IFS. A motivation behind this study is to explore a possibility of interpreting the MJO as a nonlinear free wave under active interactions with Rossby waves from and to higher latitudes. With this motivation in mind, various momentum dissipation terms as well as diabatic heating are selectively turned off over the tropics for the range of the latitudes 20° S–20° N, and it is examined how physical tendencies control the MJO dynamics. The former include eddy diffusivities as well as dissipations by both shallow and deep convection. The reduction of momentum dissipations tends to improve the MJO forecasts, but the effects are hardly additive, and their total removals rather lead to a rapid decay of the MJO, illustrating the complexity of interactions between the physics.