Oscillations in deep-open-cells during winter Mediterranean cyclones

Open cloud cells can be described in ideal form as connected clouds that surround spots of isolated clear skies in their centers. This cloud pattern is typically associated with marine stratocumulus (MSc) that form in the oceanic boundary layer. However, it can form in deeper convective clouds as well. Here, we focus on deep-open-cells (with tops reaching up to ~5–7 km) that form in the post-frontal regions of winter Mediterranean cyclones, and examine their properties and evolution. Using a Lagrangian analysis of satellite data, we show that deep-open-cells have a larger equivalent diameter (~58 ± 18 km) and oscillate with a longer periodicity (~3.5 ± 1 h) compared to shallow MSc. A numerical simulation of one Cyprus low event reveals that precipitation-generated convergence and divergence dynamic patterns are the main driver of the open cells’ organization and oscillations. Thus, our findings generalize the mechanism attributed to the behavior of shallow marine cells to deeper convective systems.

Impact of Improved Mellor–Yamada Turbulence Model on Tropical Cyclone-Induced Vertical Mixing in the Oceanic Boundary Layer

It has been identified that there are several limitations in the Mellor–Yamada (MY) turbulence model applied to the atmospheric mixed layer, and Nakanishi and Niino proposed an improved MY model using a database for large-eddy simulations. The improved MY model (Mellor–Yamada–Nakanishi–Niino model; MYNN model) is popular in atmospheric applications; however, it is rarely used in oceanic applications. In this study, the MY model and the MYNN model are compared to identify the efficiency of the MYNN model incorporated into an ocean general circulation model. To investigate the impact of the improved MY model on the vertical mixing in the oceanic boundary layer, the response of the East/Japan Sea to Typhoon Maemi in 2003 was simulated. After the typhoon event, the sea surface temperature obtained from the MYNN model showed better agreement with the satellite measurements than those obtained from the MY model. The MY model produced an extremely shallow mixed layer, and consequently, the surface temperatures were excessively warm. Furthermore, the near-inertial component of the velocity simulated using the MY model was larger than that simulated using the MYNN model at the surface layer. However, in the MYNN model, the near-inertial waves became larger than those simulated by the MY model at all depths except the surface layer. Comparatively, the MYNN model showed enhanced vertical propagation of the near-inertial activity from the mixed layer into the deep ocean, which results in a temperature decrease at the sea surface and a deepening of the mixed layer.

On the modelling of the dissipation rate of turbulent kinetic energy

We consider a relaxation equation for turbulence wavenumber for use in semi-empirical turbulence closures. It is shown that the well-known phenomenological equation for the dissipation rate of turbulent kinetic energy can be related to this relaxation equation as a close approximation of the latter for stably stratified quasi-stationary flows. The proposed approach allows for more physically found definition of the empirical constants and improvement of atmospheric and oceanic boundary layer turbulence closures by using direct numerical and large eddy simulation data to define equilibrium states and relaxation mechanisms.

Effects of wave breaking on the oceanic boundary layer in hurricane conditions

Purpose Wave breaking significantly affects the exchange process between ocean and atmosphere. This paper aims to simulate the upper ocean dynamics under the influence of wave breaking, which may help to figure out the transport of energy by these breakers. Design/methodology/approach The authors use a breaker-LES model to simulate the oceanic boundary layer in hurricane conditions, in which breakers become the main source of momentum and energy instead of traditional wind stress. Findings The mean horizontal velocities and energy increase rapidly with wind speed, reflecting that input from atmosphere dominates the coherent structure in the upper ocean. The penetration ability of a breaker limits its effective depth and thus the total turbulent kinetic energy (TKE) decreases sharply near the surface. Langmuir circulation is the main source of TKE in deeper water. The authors compared the dissipation rate (e) in the simulations with two estimates and found that the model tends to the scaling of ε∼z–3.4 at extreme wind speeds. Originality/value The probability distribution of breakers is also discussed based on the balance between the input from atmosphere and output by wave breaking. The authors considered the contribution of micro-scale breakers and revaluated the probability density function. The results show stability in hurricane conditions.

SENSIBLE HEAT TRANSFER ON ATMOSPHERIC-OCEANIC BOUNDARY IN THE OUTER AMBON BAY OF INDONESIA

Analysis of air-sea temperatures and sensible heat flux was conducted to investigate heat transfer process on the atmospheric-oceanic boundary for the outer Ambon Bay. The analysis used SST data derived from both satellite product and in situ measurement using linear regression method, as well as meteorological data such as air temperature and wind speed during daytime. The goals of the current work were to evaluate the relationship between SST and air temperature in the outer Ambon Bay, and to investigate the variation of sensible heat flux in association with seasonal variability of the bay. The major findings were: 1) SST was predominantly lower than airtemperature, resulting in the dominance of negative feedback process on the atmospheric-oceanic boundary layer of the bay; 2) the seasonal SST variability was influenced by land heating and upwelling in the Banda Sea; 3) land heating resulted in large gradient of air-sea temperatures, whereas cooler upwelled waters exerted an opposite effect.

Wave-turbulence scaling in the ocean mixed layer

Microstructure measurements were collected using an autonomous freely rising profiler under a variety of different atmospheric forcing and sea states in the open ocean. Here, profiles of turbulent kinetic energy dissipation rate, ε, are compared with various proposed scalings. In the oceanic boundary layer, the depth dependence of ε was found to be largely consistent with that expected for a shear-driven wall layer. This is in contrast with many recent studies which suggest higher rates of turbulent kinetic energy dissipation in the near surface of the ocean. However, some dissipation profiles appeared to scale with the sum of the wind and swell generated Stokes shear with this scaling extending beyond the mixed layer depth. Integrating ε in the mixed layer yielded results that 1% of the wind power referenced to 10 m is being dissipated here.

Wave-turbulence scaling in the ocean mixed layer

Microstructure measurements were collected using an autonomous freely rising profiler under a variety of different atmospheric forcing and sea states in the open ocean. Here, profiles of turbulent kinetic energy dissipation rate, ε, are compared with various proposed scalings. In the oceanic boundary layer, the depth dependence of ε was found to be consistent with that expected for a purely shear-driven wall layer. This is in contrast with many recent studies which suggest higher rates of turbulent kinetic energy dissipation in the near surface of the ocean. However, many dissipation profiles scaled with a Stokes drift-generated shear, suggesting there may be occasions where the shear in the mixed layer are dominated by wave-induced currents, which often causes turbulence to extend beyond the mixed layer depth. Integrating ε in the mixed layer yielded results that 1% of the wind power referenced to 10 m is being dissipated here.

On the Connection between Dissipation Enhancement in the Ocean Surface Layer and Langmuir Circulations

A mechanism for the enhancement of the viscous dissipation rate of turbulent kinetic energy (TKE) in the oceanic boundary layer (OBL) is proposed, based on insights gained from rapid-distortion theory (RDT). In this mechanism, which complements mechanisms purely based on wave breaking, preexisting TKE is amplified and subsequently dissipated by the joint action of a mean Eulerian wind-induced shear current and the Stokes drift of surface waves, the same elements thought to be responsible for the generation of Langmuir circulations. Assuming that the TKE dissipation rate ɛ saturates to its equilibrium value over a time of the order one eddy turnover time of the turbulence, a new scaling expression, dependent on the turbulent Langmuir number, is derived for ɛ. For reasonable values of the input parameters, the new expression predicts an increase of the dissipation rate near the surface by orders of magnitude compared with usual surface-layer scaling estimates, consistent with available OBL data. These results establish on firmer grounds a suspected connection between two central OBL phenomena: dissipation enhancement and Langmuir circulations.

A numerical investigation of the turbulent Stokes–Ekman bottom boundary layer

In the present paper turbulent mixing in the Stokes–Ekman bottom boundary layer is investigated analytically and by wall-resolving large-eddy simulations (LES). The analytical solution shows that when the Rossby number $\mathit{Ro}\ensuremath{\sim} 1$ oscillation and rotation interact with each other, this gives rise to a thickening of the boundary layer compared with the purely oscillating or the purely rotating case. The solution also shows the presence of elliptical patterns developing on the horizontal planes that shrink when approaching the low latitudes. In the turbulent regime, a LES was applied for an east–west tidal current, considered at three different latitudes in the Northern Hemisphere, namely, the polar case, the mid-latitude case ($4{5}^{\ensuremath{\circ} } $) and the quasi-equatorial case (${5}^{\ensuremath{\circ} } $). The Reynolds number of the simulation, based on the viscous penetration depth and the frequency of the purely oscillatory flow, was set equal to ${\mathit{Re}}_{\delta } = 1790$. The analysis suggests that rotation has two main effects on the flow field: in the polar case, rotation tends to delay the cyclic re-transition to turbulence and to narrow the turbulent phases of the cycle. Also, rotation suppresses vertical fluctuations of velocity and redistributes energy from the streamwise direction to the spanwise direction. It is noteworthy that the high-latitude effect makes the turbulent field substantially different from the reference Stokes boundary layer case, whereas the low-latitude effects appear to be of secondary importance, owing to the weakness of the rotation rate. Consequently, the study shows that the Stokes boundary layer may be representative of the oceanic bottom boundary layer in the low-latitude cases ($\mathit{Ro}\gt 1$ cases in our simulations). Conversely, it cannot be considered as archetypal of the oceanic boundary layer at high latitudes ($\mathit{Ro}\lt 1$ case of our study), where the vertical background vorticity profoundly modifies the turbulent field.