33 research outputs found
Structure of Turbulence in Katabatic Flows below and above the Wind-Speed Maximum
Measurements of small-scale turbulence made over the complex-terrain
atmospheric boundary layer during the MATERHORN Program are used to describe
the structure of turbulence in katabatic flows. Turbulent and mean
meteorological data were continuously measured at multiple levels at four
towers deployed along the East lower slope (2-4 deg) of Granite Mountain. The
multi-level observations made during a 30-day long MATERHORN-Fall field
campaign in September-October 2012 allowed studying of temporal and spatial
structure of katabatic flows in detail, and herein we report turbulence and
their variations in katabatic winds. Observed vertical profiles show steep
gradients near the surface, but in the layer above the slope jet the vertical
variability is smaller. It is found that the vertical (normal to the slope)
momentum flux and horizontal (along the slope) heat flux in a slope-following
coordinate system change their sign below and above the wind maximum of a
katabatic flow. The vertical momentum flux is directed downward (upward)
whereas the horizontal heat flux is downslope (upslope) below (above) the wind
maximum. Our study therefore suggests that the position of the jet-speed
maximum can be obtained by linear interpolation between positive and negative
values of the momentum flux (or the horizontal heat flux) to derive the height
where flux becomes zero. It is shown that the standard deviations of all wind
speed components (therefore the turbulent kinetic energy) and the dissipation
rate of turbulent kinetic energy have a local minimum, whereas the standard
deviation of air temperature has an absolute maximum at the height of
wind-speed maximum. We report several cases where the vertical and horizontal
heat fluxes are compensated. Turbulence above the wind-speed maximum is
decoupled from the surface, and follows the classical local z-less predictions
for stably stratified boundary layer.Comment: Manuscript submitted to Boundary-Layer Meteorology (05 December 2014
Annual Sedimentary Record From Lake Donguz-Orun (Central Caucasus) Constrained by High Resolution SR-XRF Analysis and Its Potential for Climate Reconstructions
Bottom sediments of the proglacial Lake Donguz-Orun situated at ∼2500 m a.s.l. in the Elbrus Region (Central Caucasus) reveal regular laminae, characteristic of proglacial varved lakes. This is the first laminated sediment sequence recorded in the region. However, visual counting of the layers was restricted due to partial indistinctness of the lamination. In order to confirm the annual sedimentary cyclicity and proceed with annually resolved data, in addition to the visual identification we used high-resolution geochemical markers. The upper 160 mm of the sediment core were scanned at 200 μm intervals using synchrotron radiation X-ray fluorescence analysis (SR-XRF). Additional ultrahigh resolution scanning at 30 μm increments was employed for the upper 20 mm of the core. The Rb/Sr and Zr/Rb ratios are interpreted to record annual changes in grain-size. Based on this geochemical assessment, we identified 88 annual layers covering the interval between 1922 and 2010, while visually we have been able to identify between 70 and 100 layers. The correctness of the geochemical results is confirmed by mean accumulation rates assessed by 137Cs and 210Pb dating. Cross-correlation between the ring width of local pine chronology and the layer thickness, identified as a distance between the annual Rb/Sr peaks, allowed for the accurate dating of the uppermost preserved year of the sediment sequence (AD 2010). Annually averaged elemental data were then compared with regional meteorological observations, glacier mass balance and tree-ring chronologies. The comparison revealed notable conformities: content of bromine is positively correlated with annual temperatures (r = 0.41, p < 0.01), content of terrigenous elements (major elements with the origin in watershed rocks) is positively correlated (up to r = 0.44, p < 0.01) with annual precipitation. A high statistically significant negative relationship is observed between the concentrations of terrigenous elements and tree-ring width of local pine chronology (up to r = -0.56, p < 0.01). Taken together, these data point to a common composite climatic signal in the two independent records (lake sediments and tree rings) and confirm that the laminae represent annual layers (i.e., varves). These findings open opportunities for high-resolution multiproxy climate reconstructions 300–350 years long using the longer sediment core and tree-ring records
The Critical Richardson Number and Limits of Applicability of Local Similarity Theory in the Stable Boundary Layer
Measurements of atmospheric turbulence made over the Arctic pack ice during
the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are used to
determine the limits of applicability of Monin-Obukhov similarity theory (in
the local scaling formulation) in the stable atmospheric boundary layer. Based
on the spectral analysis of wind velocity and air temperature fluctuations, it
is shown that, when both of the gradient Richardson number, Ri, and the flux
Richardson number, Rf, exceed a 'critical value' of about 0.20 - 0.25, the
inertial subrange associated with the Richardson-Kolmogorov cascade dies out
and vertical turbulent fluxes become small. Some small-scale turbulence
survives even in this supercritical regime, but this is non-Kolmogorov
turbulence, and it decays rapidly with further increasing stability. Similarity
theory is based on the turbulent fluxes in the high-frequency part of the
spectra that are associated with energy-containing/flux-carrying eddies.
Spectral densities in this high-frequency band diminish as the
Richardson-Kolmogorov energy cascade weakens; therefore, the applicability of
local Monin-Obukhov similarity theory in stable conditions is limited by the
inequalities Ri < Ri_cr and Rf < Rf_cr. However, it is found that Rf_cr = 0.20
- 0.25 is a primary threshold for applicability. Applying this prerequisite
shows that the data follow classical Monin-Obukhov local z-less predictions
after the irrelevant cases (turbulence without the Richardson-Kolmogorov
cascade) have been filtered out.Comment: Boundary-Layer Meteorology (Manuscript submitted: 16 February 2012;
Accepted: 10 September 2012
Vegetation type is an important predictor of the arctic summer land surface energy budget
Despite the importance of high-latitude surface energy budgets (SEBs) for land-climate interactions in the rapidly changing Arctic, uncertainties in their prediction persist. Here, we harmonize SEB observations across a network of vegetated and glaciated sites at circumpolar scale (1994-2021). Our variance-partitioning analysis identifies vegetation type as an important predictor for SEB-components during Arctic summer (June-August), compared to other SEB-drivers including climate, latitude and permafrost characteristics. Differences among vegetation types can be of similar magnitude as between vegetation and glacier surfaces and are especially high for summer sensible and latent heat fluxes. The timing of SEB-flux summer-regimes (when daily mean values exceed 0 Wm(-2)) relative to snow-free and -onset dates varies substantially depending on vegetation type, implying vegetation controls on snow-cover and SEB-flux seasonality. Our results indicate complex shifts in surface energy fluxes with land-cover transitions and a lengthening summer season, and highlight the potential for improving future Earth system models via a refined representation of Arctic vegetation types.An international team of researchers finds high potential for improving climate projections by a more comprehensive treatment of largely ignored Arctic vegetation types, underscoring the importance of Arctic energy exchange measuring stations.Peer reviewe
Vegetation type is an important predictor of the arctic summer land surface energy budget
Despite the importance of high-latitude surface energy budgets (SEBs) for land-climate interactions in the rapidly changing Arctic, uncertainties in their prediction persist. Here, we harmonize SEB observations across a network of vegetated and glaciated sites at circumpolar scale (1994–2021). Our variance-partitioning analysis identifies vegetation type as an important predictor for SEB-components during Arctic summer (June-August), compared to other SEB-drivers including climate, latitude and permafrost characteristics. Differences among vegetation types can be of similar magnitude as between vegetation and glacier surfaces and are especially high for summer sensible and latent heat fluxes. The timing of SEB-flux summer-regimes (when daily mean values exceed 0 Wm−2) relative to snow-free and -onset dates varies substantially depending on vegetation type, implying vegetation controls on snow-cover and SEB-flux seasonality. Our results indicate complex shifts in surface energy fluxes with land-cover transitions and a lengthening summer season, and highlight the potential for improving future Earth system models via a refined representation of Arctic vegetation types
Strain-Tuned Spin-Wave Interference in Micro- and Nanoscale Magnonic Interferometers
Here, we report on the experimental study of spin-wave propagation and interaction in the double-branched Mach–Zehnder interferometer (MZI) scheme. We show that the use of a piezoelectric plate (PP) with separated electrodes connected to each branch of the MZI leads to the tunable interference of the spin-wave signal at the output section. Using a finite element method, we carry out a physical investigation of the mechanisms of the impact of distributed deformations on the magnetic properties of YIG film. Micromagnetic simulations and finite-element modelling can explain the evolution of spin-wave interference patterns under strain induced via the application of an electric field to PP electrodes. We show how the multimode regime of spin-wave propagation is used in the interferometry scheme and how scaling to the nanometer size represents an important step towards a single-mode regime. Our findings provide a simple solution for the creation of tunable spin-wave interferometers for the magnonic logic paradigm
Strain-Tuned Spin-Wave Interference in Micro- and Nanoscale Magnonic Interferometers
Here, we report on the experimental study of spin-wave propagation and interaction in the double-branched Mach–Zehnder interferometer (MZI) scheme. We show that the use of a piezoelectric plate (PP) with separated electrodes connected to each branch of the MZI leads to the tunable interference of the spin-wave signal at the output section. Using a finite element method, we carry out a physical investigation of the mechanisms of the impact of distributed deformations on the magnetic properties of YIG film. Micromagnetic simulations and finite-element modelling can explain the evolution of spin-wave interference patterns under strain induced via the application of an electric field to PP electrodes. We show how the multimode regime of spin-wave propagation is used in the interferometry scheme and how scaling to the nanometer size represents an important step towards a single-mode regime. Our findings provide a simple solution for the creation of tunable spin-wave interferometers for the magnonic logic paradigm
Similarity theory based on the Dougherty-Ozmidov length scale
The article of record as published may be located at http://dx.doi.org/10.1002/qj.2488This article describes a local similarity theory for developed turbulence in the stably
stratified boundary layer that is based on the Brunt–V¨ais¨al¨a frequency and the dissipation
rate of turbulent kinetic energy instead of the turbulent fluxes used in the traditional
Monin–Obukhov similarity theory. Based on dimensional analysis (Pi theorem), it is
shown that any properly scaled statistics of the small-scale turbulence are universal
functions of a stability parameter defined as the ratio of a reference height z and the
Dougherty–Ozmidov length scale, which in the limit of z-less stratification is linearly
proportional to the Obukhov length scale.Measurements of atmospheric turbulence made
at five levels on a 20 m tower over the Arctic pack ice during the Surface Heat Budget
of the Arctic Ocean experiment (SHEBA) are used to examine the behaviour of different
similarity functions in the stable boundary layer. In the framework of this approach the
non-dimensional turbulent viscosity is equal to the gradient Richardson number, whereas
the non-dimensional turbulent thermal diffusivity is equal to the flux Richardson number.
These results are a consequence of the approximate local balance between production of
turbulence by shear in the mean flow and viscous dissipation. The turbulence framework
based on the Brunt–V¨ais¨al¨a frequency and the dissipation rate of turbulent kinetic energy
may have practical advantages for estimating turbulence when the fluxes are not directly
available
Parameterizing Turbulent Exchange over Sea Ice in Winter
The article of record as published may be located at http://dx.doi.org/10.1175/2009JHM1102.1The Surface Heat Budget of the Arctic Ocean (SHEBA) experiment produced 18 000 h of turbulence data
from the atmospheric surface layer over sea ice while the ice camp drifted for a year in the Beaufort Gyre.
Multiple sites instrumented during SHEBA suggest only two aerodynamic seasons over sea ice. In ‘‘winter’’
(October 1997 through 14 May 1998 and 15 September 1998 through the end of the SHEBA deployment in
early October 1998), the ice was compact and snow covered, and the snow was dry enough to drift and blow. In
‘‘summer’’ (15 May through 14 September 1998 in this dataset), the snow melted, and melt ponds and leads
appeared and covered as much as 40% of the surface with open water. This paper develops a bulk turbulent
flux algorithm to explain the winter data. This algorithm predicts the surface fluxes of momentum, and
sensible and latent heat from more readily measured or modeled quantities. A main result of the analysis is
that the roughness length for wind speed z0 does not depend on the friction velocity u* in the drifting snow
regime (u* $ 0.30 m s21) but, rather, is constant in the SHEBA dataset at about 2.3 3 1024 m. Previous
analyses that found z0 to increase with u* during drifting snow may have suffered from fictitious correlation
because u* also appears in z0. The present analysis mitigates this fictitious correlation by plotting measured z0
against the corresponding u* computed from the bulk flux algorithm. Such plots, created with data from six
different SHEBA sites, show z0 to be independent of the bulk u* for 0.15 , u* # 0.65 m s21. This study also
evaluates the roughness lengths for temperature zT and humidity zQ, incorporates new profile stratification
corrections for stable stratification, addresses the singularities that often occur in iterative flux algorithms in
very light winds, and includes an extensive analysis of whether atmospheric stratification affects z0, zT, and zQ.The U.S. National Science Foundation (NSF) supported our initial participation in SHEBA with awards to the U.S. Army Cold Regions Research and Engineering Laboratory, NOAA’s Environmental Technology Laboratory (now the Earth System Research Laboratory), the Naval Postgraduate School, and the Cooperative Institute for Research in Environmental Sciences. NSF also supported our use of the Flux-PAM stations from the facilities pool at the National Center for Atmospheric Research. Both NSF (Award 06-11942) and the National Aeronautics and Space Administration (Award NNX07AL77G) supported ELA at NorthWest Research Associates during the preparation of this manuscript