Regional Carbon Dioxide and Water Vapor Exchange Over Heterogeneous Terrain

In spite of setbacks due to forest fires, eviction after a change of landowners and unanticipated need to upgrade and replace much of the instrumentation, substantial progress has been made during the past three years, resulting in major new findings. Although most of the results are in manuscript form, three papers have been published and a fourth was recently submitted. The data has been subjected to extensive quality control. Extra attention has been devoted to the influence of tilt rotation and flux-calculation method, particularly with respect to nocturnal fluxes. Previous/standard methods for calculating nocturnal fluxes with moderate and strong stability are inadequate and lead to large random fluxes errors for individual records, due partly to inadvertent inclusion of mesoscale motions that strongly contaminant the estimation of fluxes by weak turbulence. Such large errors are serious for process studies requiring carbon dioxide fluxes for individual records, but are substantially reduced when averaging fluxes over longer periods as in calculation of annual NEE budgets. We have employed a superior method for estimating fluxes in stable conditions with a variable averaging width . Mesoscale fluxes are generally unimportant except for events and are generally not systematic or predictable. Mesoscale or regional models of our region are not able to reproduce important aspects of the diurnally varying wind fiel

On estimating the surface wind stress over the sea

Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 1533-1541, doi:10.1175/JPO-D-17-0267.1.Our study analyzes measurements primarily from two Floating Instrument Platform (FLIP) field programs and from the Air–Sea Interaction Tower (ASIT) site to examine the relationship between the wind and sea surface stress for contrasting conditions. The direct relationship of the surface momentum flux to U2 is found to be better posed than the relationship between and U, where U is the wind speed and is the friction velocity. Our datasets indicate that the stress magnitude often decreases significantly with height near the surface due to thin marine boundary layers and/or enhanced stress divergence close to the sea surface. Our study attempts to correct the surface stress estimated from traditional observational levels by using multiple observational levels near the surface and extrapolating to the surface. The effect of stability on the surface stress appears to be generally smaller than errors due to the stress divergence. Definite conclusions require more extensive measurements close to the sea surface.This work was supported by the U.S. Office of Naval Research through Award N00014-16-1-2600. We2019-01-1

Thermal Submeso Motions in the Nocturnal Stable Boundary Layer. Part 2: Generating Mechanisms and Implications

In the stable boundary layer, thermal submesofronts (TSFs) are detected during the Shallow Cold Pool experiment in the Colorado plains, Colorado, USA in 2012. The topography induces TSFs by forming two different air layers converging on the valley-side wall while being stacked vertically above the valley bottom. The warm-air layer is mechanically generated by lee turbulence that consistently elevates near-surface temperatures, while the cold-air layer is thermodynamically driven by radiative cooling and the corresponding cold-air drainage decreases near-surface temperatures. The semi-stationary TSFs can only be detected, tracked, and investigated in detail when using fibre-optic distributed sensing (FODS), as point observations miss TSFs most of the time. Neither the occurrence of TSFs nor the characteristics of each air layer are connected to a specific wind or thermal regime. However, each air layer is characterized by a specific relationship between the wind speed and the friction velocity. Accordingly, a single threshold separating different flow regimes within the boundary layer is an oversimplification, especially during the occurrence of TSFs. No local forcings or their combination could predict the occurrence of TSFs except that they are less likely to occur during stronger near-surface or synoptic-scale flow. While classical conceptualizations and techniques of the boundary layer fail in describing the formation of TSFs, the use of spatially continuous data obtained from FODS provide new insights. Future studies need to incorporate spatially continuous data in the horizontal and vertical planes, in addition to classic sensor networks of sonic anemometry and thermohygrometers to fully characterize and describe boundary-layer phenomena

Thermal Submesoscale Motions in the Nocturnal Stable Boundary Layer. Part 1: Detection and Mean Statistics

Submesoscale motions within the stable boundary layer were detected during the Shallow Cold Pool Experiment conducted in the Colorado plains, Colorado, U.S.A. in 2012. The submesoscale motion consisted of two air layers creating a well-defined front with a sharp temperature gradient, and further-on referred to as a thermal submesofront (TSF). The semi-stationary TSFs and their advective velocities are detected and determined by the fibre-optic distributed-sensing (FODS) technique. An objective detection algorithm utilizing FODS measurements is able to detect the TSF boundary, which enables a detailed investigation of its spatio–temporal statistics. The novel approach in data processing is to conditionally average any parameter depending on the distance between a TSF boundary and the measurement location. By doing this, a spatially-distributed feature like TSFs can be characterized by point observations and processes at the TSF boundary can be investigated. At the TSF boundary, the air layers converge, creating an updraft, strong static stability, and vigorous mixing. Further, the TSF advective velocity of TSFs is an order of magnitude lower than the mean wind speed. Despite being gentle, the topography plays an important role in TSF formation. Details on generating mechanisms and implications of TSFs on the stable boundary layer are discussed in Part 2

Observations of non-dimensional wind shear in the coastal zone

Vertical profiles of the time-averaged wind stress, wind speed and buoyancy flux from the off-shore tower site in the Risø Air Sea Experiment are used to evaluate similarity theory in the coastal zone. The observed dependence of the non-dimensional wind shear on stability is compared to the traditional parametrization. Relationships between the non-dimensional shear, development of internal boundary layers and wave state are explored. We find that the largest-scale turbulent eddies are suppressed in shallow convective internal boundary layers, leading to larger non-dimensional shear than that of the traditional prediction based only on stability. In shallow stable boundary layers, elevated generation of turbulence leads to smaller non-dimensional shear compared to the traditional prediction. Over young growing waves in stable stratification, the observed non-dimensional shear is less than that over older more mature waves in otherwise similar conditions. The non-dimensional shear is a function of wave state for stable conditions even though the observations are well above the wave boundary layer. We conclude that development of shallow internal boundary layers and young growing-wave fields, both of which are common in the coastal zone, can lead to substantial departures of the non-dimensional shear from the prediction based only on stability.Keywords: Turbulence, Similarity theory, Boundary laye

Vertically integrated sensible-heat budgets for stable nocturnal boundary layers

The stable nocturnal boundary layer is commonly viewed or modelled as a balance between the temperature tendency (cooling) and vertical heat-flux divergence. Sometimes the radiative-flux divergence is also included. This perspective has dictated the design of field experiments for investigating stable nocturnal boundary layers. Tower-based micrometeorological data from three field campaigns are analysed to evaluate the vertically integrated sensible-heat budget for nocturnal stable conditions. Our analysis indicates frequent occurrence of large imbalance between the temperature tendency and vertical heat-flux divergence terms. The values of the radiative-flux divergence are generally too small and sometimes of the wrong sign to explain the residual. An analysis of random flux errors and uncertainties in the tendency term indicate that such errors cannot explain large imbalances, suggesting the importance of advection of temperature or possibly the divergence of mesoscale fluxes. The implied role of advection is consistent with circumstantial evidence. Even weak surface heterogeneity can create significant horizontal gradients in stable boundary layers. However, it is shown that existing field data and observational strategy do not allow adequate evaluation of advection and mesoscale flux divergence terms

Moisture fluxes over snow with and without protruding vegetation

The sublimation of snow and evaporation of melted snow is contrasted between brush, grass and bare ground sites using eddy-correlation data. Averaged over the entire winter season, the evaporation/sublimation is about 20% greater over the brush site than the bare ground site, apparently due to greater supply of snow. Blowing snow collects in patches of brush, which protrude above the snow. This study evaluates commonly used simple formulations of the surface moisture flux including two versions of the bulk aerodynamic formula and the stability-dependent Penman and Penman–Monteith methods for predicting evaporation/sublimation. The evaluation is based on eddy-correlation data from the three sites with conditions ranging from snow-free to snow-covered ground surfaces. The water availability factors, required for these formations, increase with snow-fraction coverage, but less so with melting conditions, depending on site and formulation. Various influences on the sublimation/evaporation are examined and the performance and shortcomings of each formulation are assessed.Keywords: Penman, Snow melt, Sublimation, Bulk aerodynami

Evaluation of the air-sea bulk formula and sea-surface temperature variability from observations

Eddy‐correlation fluxes are compared to air‐sea fluxes predicted by a widely used bulk flux formulation without wave‐state effects. Systematic discrepancies are found. For example, the model approximately equates the roughness lengths for heat and moisture; however, the observed roughness length for heat (zoh) exceeds that for moisture (zoq) by an order of magnitude or more, except in the strongest wind‐speed conditions. This is apparently due to the dynamic nature of temperature, which dominates buoyancy generation of turbulence in these data sets. The observed correlation between temperature and vertical velocity fluctuations generally exceeds that for moisture. For 10‐m wind speeds above a threshold value of 12 m s−1, zoq exceeds zoh apparently owing to enhanced moisture flux associated with the onset of wave breaking coupled with advection of cold dry air from land. In near‐collapsed turbulence, the observed momentum flux is smaller than predicted, and there is no clear indication of a smooth flow viscous regime. The scatter between observed and bulk fluxes generally decreases with averaging the observed fluxes over greater length scales even with variations in sea‐surface temperature (SST). The reduction in random flux sampling errors more than compensates for capturing increased surface heterogeneity with increasing averaging scale. Since similarity theory does not apply to heterogeneous surfaces, the bulk model does break down in the extreme case where the averaging window includes a sharp SST front. The response of the flow to changes in SST is presented for different amplitudes of SST variability. The change in vertical structure and acceleration of the low‐level wind over warm pools is discussed

Along-shore variations of offshore flow

Using eddy correlation data collected by the LongEZ research aircraft, the adjustment of atmospheric flow downstream from a coastline is examined. Along-shore variation of the turbulence over the water is generated by the varying width of the upstream land strip between the sea and inland water. Over the coastal zone, the turbulence is strongest downstream from the widest part of land. The along-shore variation of the turbulence decreases with increasing sea fetch due to horizontal mixing. In other terms, the footprint of the flux farther offshore includes a larger width of upstream land. Beyond 5 km sea fetch, the along-shore variation becomes small

Dependence of surface exchange coefficients on averaging scale and grid size

The value of the effective exchange coefficient for area-averaged fluxes can depend significantly on the averaging scale. This dependence implies that the exchange coefficient in numerical models should depend on grid size. The main goal of this study is the assessment of the importance of such scale-dependence. When the large-scale flow is weak, the mesoscale motions on scales smaller than the averaging width (grid size) may generate significant turbulence which is not represented by the area-averaged wind vector (resolved flow). In this case, the effective exchange coefficient must be larger than that predicted by similarity theory in order to predict the grid-averaged turbulent flux. This study reviews different approaches for representation of spatially-averaged surface fluxes over heterogeneous surfaces. The scale-dependence of the effective exchange coefficient is posed in terms of spatially averaging the bulk aerodynamic relationship. This scale-dependence is evaluated in terms of observed fluxes from three different field programs. The effective drag coefficient is found to be more scale-dependent than the effective exchange coefficients for heat and momentum. The effective conductance is found to be less scale-dependent than the effective exchange coefficient and effective resistance. Except for the case of weak flow over strong surface heterogeneity, the exchange coefficient is nearly independent of grid size.Keywords: Surface fluxes, Grid-square averaging, Scale dependence, Exchange coefficients, Heterogeneous terrain, Boundary layerKeywords: Surface fluxes, Grid-square averaging, Scale dependence, Exchange coefficients, Heterogeneous terrain, Boundary laye