We formulate an expression for the turbulent kinetic energy dissipation rate,
Here we have determined the nature of turbulent flow associated with oceanic nonbreaking waves, which are on average much more prevalent than breaking waves in most wind conditions. We found this flow to be characterized by a low turbulence microscale Reynolds number of
- NSF-PAR ID:
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Medium: X
- Sponsoring Org:
- National Science Foundation
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We formulate an expression for the turbulent kinetic energy dissipation rate,
ϵ, associated with shear‐generated turbulence in terms of quantities in the ocean or atmosphere that, depending on the situation, may be measurable or resolved in models. The expression depends on the turbulent vertical length scale, ℓ v, the inverse time scale N, and the Richardson number Ri= N2/ S2, where Sis the vertical shear, with ℓ vscaled in a way consistent with theories and observations of stratified turbulence. Unlike previous studies, the focus is not so much on the functional form of Ri, but the vertical variation of the length scale ℓ v. Using data from two ∼7‐day time series in the western equatorial Pacific, the scaling is compared with the observed ϵ. The scaling works well with the estimated ϵcapturing the differences in amplitude and vertical distribution of the observed ϵbetween the two times series. Much of those differences are attributable to changes in the vertical distribution of the length scale ℓ v, and in particular the associated turbulent velocity scale, u t. We relate u tto a measure of the fine‐scale variations in velocity, . Our study highlights the need to consider the length scale and its estimation in environmental flows. The implications for the vertical variation of the associated turbulent diffusivity are discussed.
This paper reports the first measurement of the relationship between turbulent velocity and cloud size in the diffuse circumgalactic medium (CGM) in typical galaxy halos at redshift
z≈ 0.4–1. Through spectrally resolved absorption profiles of a suite of ionic transitions paired with careful ionization analyses of individual components, cool clumps of size as small as lcl∼ 1 pc and density lower than nH= 10−3cm−3are identified in galaxy halos. In addition, comparing the line widths between different elements for kinematically matched components provides robust empirical constraints on the thermal temperature Tand the nonthermal motions bNT, independent of the ionization models. On average, bNTis found to increase with lclfollowing over three decades in spatial scale from lcl≈ 1 pc to lcl≈ 1 kpc. Attributing the observed bNTto turbulent motions internal to the clumps, the best-fit bNT– lclrelation shows that the turbulence is consistent with Kolmogorov at <1 kpc with a roughly constant energy transfer rate per unit mass of ϵ≈ 0.003 cm2s−3and a dissipation timescale of ≲100 Myr. No significant difference is found between massive quiescent and star-forming halos in the sample on scales less than 1 kpc. While the inferred ϵis comparable to what is found in C ivabsorbers at high redshift, it is considerably smaller than observed in star-forming gas or in extended line-emitting nebulae around distant quasars. A brief discussion of possible sources to drive the observed turbulence in the cool CGM is presented.
Though the Kelvin‐Helmholtz instability (KHI) has been extensively observed in the mesosphere, where breaking gravity waves produce the conditions required for instability, little has been done to describe quantitatively this phenomenon in detail in the mesopause and lower thermosphere, which are associated with the long‐lived shears at the base of this statically stable region. Using trimethylaluminum (TMA) released from two sounding rockets launched on 26 January 2018, from Poker Flat Research Range in Alaska, the KHI was observed in great detail above 100 km. Two sets of rocket measurements, made 30 min apart, show strong winds (predominantly meridional and up to 150 ms
−1) and large total shears (90 ms −1km −1). The geomagnetic activity was low in the hours before the launches, confirming that the enhanced shears that triggered the KHI are not a result of the E‐region auroral jets. The four‐dimensional (three‐dimensional plus time) estimation of KHI billow features resulted in a wavelength, eddy diameter, and vertical length scale of 9.8, 5.2, and 3.8 km, respectively, centered at 102‐km altitude. The vertical and horizontal root‐mean‐square velocities measured 29.2 and 42.5 ms −1, respectively. Although the wind structure persisted, the KHI structure changed significantly with time over the interval separating the two launches, being present only in the first launch. The rapid dispersal of the TMA cloud in the instability region was evidence of enhanced turbulent mixing. The analysis of the Reynolds and Froude numbers ( = 7.2 R e ×10 3and = 0.29, respectively) illustrates the presence of turbulence and weak stratification of the flow. F r
In this study, we report on turbulent mixing observed during the annual stratification cycle in the hypolimnetic waters of Lake Michigan (USA), highlighting stratified, convective, and transitional mixing periods. Measurements were collected using a combination of moored instruments and microstructure profiles. Observations during the stratified summer showed a shallow, wind‐driven surface mixed layer (SML) with locally elevated dissipation rates in the thermocline (
) potentially associated with internal wave shear. Below the thermocline, turbulence was weak ( ) and buoyancy‐suppressed ( < 8.5), with low hypolimnetic mixing rates ( ) limiting benthic particle delivery. During the convective winter period, a diurnal cycle of radiative convection was observed over each day of measurement, where temperature overturns were directly correlated with elevated turbulence levels throughout the water column ( ; ). A transitional mixing period was observed for spring conditions when surface temperatures were near the temperature of maximum density ( TMD 3.98 ) and the water column began to stably stratify. While small temperature gradients allowed strong mixing over the transitional period ( ), hypolimnetic velocity shear was overwhelmed by weakly stable stratification ( ; ), limiting the development of the SML. These results highlight the importance of radiative convection for breaking down weak hypolimnetic stratification and driving energetic, full water column mixing during a substantial portion of the year (>100 days at our sample site). Ongoing surface water warming in the Laurentian Great Lakes is significantly reducing the annual impact of convective mixing, with important consequences for nutrient cycling, primary production, and benthic‐pelagic coupling.
In the Arctic Ocean, limited measurements indicate that the strongest mixing below the atmospherically forced surface mixed layer occurs where tidal currents are strong. However, mechanisms of energy conversion from tides to turbulence and the overall contribution of tidally driven mixing to Arctic Ocean state are poorly understood. We present measurements from the shelf north of Svalbard that show abrupt isopycnal vertical displacements of 10–50 m and intense dissipation associated with cross‐isobath diurnal tidal currents of
∼0.15 m s −1. Energy from the barotropic tide accumulated in a trapped baroclinic lee wave during maximum downslope flow and was released around slack water. During a 6‐hr turbulent event, high‐frequency internal waves were present, the full 300‐m depth water column became turbulent, dissipation rates increased by a factor of 100, and turbulent heat flux averaged 15 W m −2compared with the background rate of 1 W m −2.