A. L. M. Grant

A global perspective on Langmuir turbulence in the ocean surface boundary layer

The turbulent mixing in thin ocean surface boundary layers (OSBL), which occupy the upper 100 m or so of the ocean, control the exchange of heat and trace gases between the atmosphere and ocean. Here we show that current parameterizations of this turbulent mixing lead to systematic and substantial errors in the depth of the OSBL in global climate models, which then leads to biases in sea surface temperature. One reason, we argue, is that current parameterizations are missing key surface-wave processes that force Langmuir turbulence that deepens the OSBL more rapidly than steady wind forcing. Scaling arguments are presented to identify two dimensionless parameters that measure the importance of wave forcing against wind forcing, and against buoyancy forcing. A global perspective on the occurrence of wave-forced turbulence is developed using re-analysis data to compute these parameters globally. The diagnostic study developed here suggests that turbulent energy available for mixing the OSBL is under-estimated without forcing by surface waves. Wave-forcing and hence Langmuir turbulence could be important over wide areas of the ocean and in all seasons in the Southern Ocean. We conclude that surface-wave-forced Langmuir turbulence is an important process in the OSBL that requires parameterization. Citation: Belcher, S. E., et al. (2012), A global perspective on Langmuir turbulence in the ocean surface boundary layer, Geophys. Res. Lett., 39, L18605, doi: 10.1029/2012GL052932.

Characteristics of Langmuir Turbulence in the Ocean Mixed Layer

This study uses large-eddy simulation (LES) to investigate the characteristics of Langmuir turbulence through the turbulent kinetic energy (TKE) budget. Based on an analysis of the TKE budget a velocity scale for Langmuir turbulence is proposed. The velocity scale depends on both the friction velocity and the surface Stokes drift associated with the wave field. The scaling leads to unique profiles of nondimensional dissipation rate and velocity component variances when the Stokes drift of the wave field is sufficiently large compared to the surface friction velocity. The existence of such a scaling shows that Langmuir turbulence can be considered as a turbulence regime in its own right, rather than a modification of shear-driven turbulence. Comparisons are made between the LES results and observations, but the lack of information concerning the wave field means these are mainly restricted to comparing profile shapes. The shapes of the LES profiles are consistent with observed profiles. The dissipation length scale for Langmuir turbulence is found to be similar to the dissipation length scale in the shear-driven boundary layer. Beyond this it is not possible to test the proposed scaling directly using available data. Entrainment at the base of the mixed layer is shown to be significantly enhanced over that due to normal shear turbulence.

Wind-Driven Mixing below the Oceanic Mixed Layer

This study describes the turbulent processes in the upper ocean boundary layer forced by a constant surface stress in the absence of the Coriolis force using large-eddy simulation. The boundary layer that develops has a two-layer structure, a well-mixed layer above a stratified shear layer. The depth of the mixed layer is approximately constant, whereas the depth of the shear layer increases with time. The turbulent momentum flux varies approximately linearly from the surface to the base of the shear layer. There is a maximum in the production of turbulence through shear at the base of the mixed layer. The magnitude of the shear production increases with time. The increase is mainly a result of the increase in the turbulentmomentumfluxatthebaseofthemixedlayerduetotheincreasein thedepthoftheboundarylayer. The length scale for the shear turbulence is the boundary layer depth. A simple scaling is proposed for the magnitude of the shear production that depends on the surface forcing and the average mixed layer current. The scaling can be interpreted in terms of the divergence of a mean kinetic energy flux. A simple bulk model of the boundary layer is developed to obtain equations describing the variation of the mixed layer and boundary layer depths with time. The model shows that the rate at which the boundary layer deepens does not depend on the stratification of the thermocline. The bulk model shows that the variation in the mixed layer depth is small as long as the surface buoyancy flux is small.

Comparisons of aircraft, ship, and buoy radiation and SST measurements from TOGA COARE

Mean radiative fluxes and sea surface temperature measured by the five Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) boundary layer research aircraft were compared with each other and with surface measurements from moored buoys and ships. The basic data-processing techniques for radiative flux and sea surface temperature (SST) measurements from an aircraft were reviewed, and an empirical optimization method to calibrate an Eppley pyrgeometer was introduced. On the basis of aircraft wingtip-to-wingtip comparison periods, the processed aircraft downwelling shortwave and longwave irradiance and SST measurements were found to agree to 28±18 W m−2, 9±4 W m−2, and 0.7±0.4°C, respectively. By using the same comparison periods, empirical corrections that removed systematic errors in the aircraft data were determined. Application of these corrections improved the wingtip comparison accuracy to 3±16 W m−2, 1±4 W m−2, and 0.1±0.3°C, respectively. Comparisons between the (fully corrected) aircraft and the surface platform measurements revealed the aircraft data to be slightly greater for all three parameters. The agreement was around 3±37 W m−2, 3±6 W m−2, and 0.3±0.5°C for shortwave irradiance, longwave irradiance, and SST, respectively. (Detailed comparison results were provided for each individual ship and buoy.) After applying the aircraft empirical corrections the level of accuracy was near the COARE objectives.

Volcanic ash layer depth: Processes and mechanisms

The long duration of the 2010 Eyjafjallajokull eruption provided a unique opportunity to measure a widely dispersed volcanic ash cloud. Layers of volcanic ash were observed by the European Aerosol Research Lidar Network with a mean depth of 1.2 km and standard deviation of 0.9 km. In this paper we evaluate the ability of the Met Offices Numerical Atmospheric-dispersion Modelling Environment (NAME) to simulate the observed ash layers and examine the processes controlling their depth. NAME simulates distal ash layer depths exceptionally well with a mean depth of 1.2 km and standard deviation of 0.7 km. The dominant process determining the depth of ash layers over Europe is the balance between the vertical wind shear (which acts to reduce the depth of the ash layers) and vertical turbulent mixing (which acts to deepen the layers). Interestingly, differential sedimentation of ash particles and the volcano vertical emission profile play relatively minor roles.

Comparisons of aircraft, ship, and buoy meteorological measurements from TOGA COARE

Comparisons of mean ambient temperature, specific humidity, static pressure, and horizontal wind from the five Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) boundary layer aircraft were obtained from 38 two- and three-aircraft, close-formation, level runs. These, together with consideration of surface measurements from buoys and ships, led to proposed empirical corrections for the aircrafts temperature, humidity, and pressure measurements, minimizing the systematic errors between the aircraft data sets. The aircraft-measured winds were also compared. The TOGA COARE bulk flux algorithm was used to extrapolate the low-level aircraft data to the individual ship and buoy sensor heights for 267 overflight comparisons. In addition, all low-level aircraft data and corresponding ship and buoy data from boundary layer missions were extracted and adjusted to a 10-m reference height. The recommended aircraft corrections bring the aircraft-ship-buoy data sets into better agreement, resulting in a consistent data set for air-sea interaction analyses. Frequency distributions of the 10-m aircraft, ship, and buoy data from the boundary layer missions also agree.

Langmuir Turbulence and Surface Heating in the Ocean Surface Boundary Layer

This study uses large-eddy simulation to investigate the structure of the ocean surface boundary layer (OSBL) in the presence of Langmuir turbulence and stabilizing surface heat fluxes. The OSBL consists of a weakly stratified layer, despite a surface heat flux, above a stratified thermocline. The weakly stratified (mixed) layer is maintained by a combination of a turbulent heat flux produced by the wave-driven Stokes drift and downgradient turbulent diffusion. The scaling of turbulence statistics, such as dissipation and vertical velocity variance, is only affected by the surface heat flux through changes in the mixed layer depth. Diagnostic models are proposed for the equilibrium boundary layer and mixed layer depths in the presence of surface heating. The models are a function of the initial mixed layer depth before heating is imposed and the Langmuir stability length. In the presence of radiative heating, the models are extended to account for the depth profile of the heating.

Reply to “Comments on ‘Langmuir Turbulence and Surface Heating in the Ocean Surface Boundary Layer’”

AbstractThe differences between the conclusions of Noh and Choi and of Pearson et al., which are largely a result of defining different length scales based on different quantities, are discussed. This study shows that the layer over which Langmuir turbulence mixes (nominally hTKE) under a stabilizing surface buoyancy flux should be scaled by a combination of the Langmuir stability length LL and initial/nocturnal boundary layer depth h0 rather than by the Zilitinkevich length.