Search

The heat and mass transfer over the sea is considered in terms of the sea surface roughness lengths for scalars, z 0T for potential temperature u, and z 0q for specific humidity q, or alternatively, in terms of the roughness-layer scalar increments, du and dq. A new scaling reasoning is proposed in support of the familiar square root dependence of the above increments on the roughness Reynolds number, Re0u 5 z 0uu * /n, where z 0u is the sea surface aerodynamic roughness length, u * is the friction velocity, and n is the molecular viscosity of the air. Scaling predictions are validated using data from measurements made by the National Oceanic and Atmospheric Administration’s Environmental Technology Laboratory aboard the R/V Moana Wave in the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment in 1992‐93 and the R/PFLIP in the San Clemente Ocean Probing Experiment in September 1993. Data presented as the dimensionless scalar increments (or the ratios z 0u/z 0T and z 0u/z 0q) versus Re 0u show a good agreement with theoretical predictions, especially at Re0u . 2 (over stormy sea). The resulting roughness-length formulations are recommended for practical use in climate and mesoscale air‐sea interaction models.

This paper examines the interpolation betweenBusinger–Dyer (Kansas-type) formulae,ϕu = (1 -1 6ζ )-1/4 andϕt = (1 - 16ζ )-1/2, and free convection forms. Based on matching constraints, the constants, au and at, in the convective flux-gradient relations, ϕu = (1 - auζ )-1/3 and ϕt = (1 - atζ )-1/3, are determined. It isshown that au and at cannot be completely independent if convective forms are blended with theKansas formulae. In other words, these relationships already carryinformation about au and at. This follows because the Kansas relations cover a wide stability range (up to ζ = - 2), which includes a lower part of the convective sublayer (about 0.1 < - ζ < 2). Thus, there is a subrange where both Kansas and convective formulae are valid. Matching Kansas formulae and free convection relations within thesubrange 0.1 < -ζ < 2 and independently smoothing ofthe blending function are used to determine au and at. The values au = 10 for velocity and at = 34for scalars (temperature and humidity) give a good fit. This new approacheliminates the need for additional independent model constants and yields a`smooth' blending between Kansas and free-convection profileforms in the COARE bulk algorithm.

Based on the idea that free convection can be considered as a particular case of forced convection, where the gusts driven by the large-scale eddies are scaled with the Deardorff convective velocity scale, a new formulation for the neutral drag coefficient, CDn, in the convective boundary layer (CBL) is derived. It is shown that (i) a concept of CDn can still be used under strongly unstable conditions including a pure free-convection regime even when no logarithmic portion in the velocity profile exists; (ii) gustiness corrections must be applied for rational calculations of CDn; and (iii) the stratification Ψ function used in the derivation of CDn should satisfy the theoretical free-convection limit. The new formulation is compared with the traditional relationship for CDn, and data collected over the sea (during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) and the San Clemente Ocean Probing Experiment (SCOPE)) and over land (during the BOREX-95 experiment) are used to illustrate the difference between the new and traditional formulations. Compared to the new approach, the traditional formulation strongly overestimates CDn and zo in the CBL for mean wind speed less than about 2 m s-1. The new approach also clarifies several contradictory results from earlier works. Some aspects related to an alternate definition of the neutral drag coefficient and the wind speed and the stress averaging procedure are considered.

The Prandtl, Obukhov, and Monin andObukhov similarity theories are widely used todescribe the structure of turbulence in theatmospheric surface layer. Currently it isunderstood that in strong convection with no or veryweak mean wind the traditional theory breaks down.In particular, the traditional theory implies asingle-valued correspondence between localturbulence statistics and local properties ofthe flow. In very strong convection, this is nottrue because of large-scale (∼ 10 3 m) coherentstructures, embracing the entire convective boundarylayer (CBL). These structures produce random guststhat crucially affect surface-layer turbulence andmake it dependent on global properties of theflow, such as the CBL depth. In the present paperthe limits of validity of the traditional surface-layer similarity theory are determined and a revisedtheory of fair weather convection in the surface layeris developed by considering the effect of gustiness. It is shownthat the theoretical predictions are consistent withfield data from the TOGA COARE and SCOPEexperiments.

Recent measurements made onboard the R/P FLIP in the San Clemente Ocean Probing Experiment in September 1993 and onboard the R/V Moana Wave during Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment are used to evaluate the direct dependence between the Monin–Obukhov stability parameter (ratio of height to Obukhov length) ζ and the bulk Richardson number Rib derived from standard meteorological mean observations of water surface temperature, wind speed, air temperature, and humidity. It is found that in the unstable marine surface layer, ζ = C Rib(1 + Rib/Ribc)−1, where the numerical coefficient C and the saturation Richardson number Ribc are analytical functions of the standard bulk exchange coefficients. For measurements at a height of 10–15 m, C is about 10 and Ribc is about −4.5. Their values are insensitive to variations of Rib and ζ over three decades. Thus, a simple dependence between ζ and Rib has a much wider range of applicability than previously believed. The ...

The aerodynamic classification of the resistance laws above solid surfaces is based on the use of a so-called Reynolds roughness number ReΘs =hsu*/υ, wherehs is the effective roughness height, ν-viscosity,u*-friction velocity. The recent experimental studies reported by Toba and Ebuchi (1991), demonstrated that the observed “variability” of the sea roughness cannot be explained only on the basis of the classification of aerodynamic conditions of the sea surface proposed by Kitaigorodskii and Volkov (1965) and Kitaigorodskii (1968) even though the latter approach gains some support from recent experimental studies (see for example Geernaertet al. 1986). In this paper, an attempt is made to explain some of the recently observed features of the “variability” of surface roughness (Toba and Ebuchi, 1991; Donelanet al., 1993). The “fluctuating” regime of the sea surface roughness is also described. It is shown that the contribution from the dissipation subrange to the variability of the sea surface can be very important and by itself can explain Charnock's (1955) regime.

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., Boundary-Layer Meteorology (Manuscript submitted: 16 February 2012; Accepted: 10 September 2012)

Frequency spectra of atmospheric turbulenceSα(f) in the inertial subrange are considered in the free convection regime over the sea surface in a case of motionless instrument measurements (Eulerian frequency spectra). The frequency spectra formulaef*Sα(f)/σα2 =cα(f*/f)5/3 for wind velocity (α=1–3), temperature (α=t) and humidity (α=e) fluctuations are derived on the basis of similarity theory and the “−5/3 law”. These relations also can be derived from a consideration of convective large-scale advection of small eddies. The frequency scalef* = (N1β2/∈)1/2 ≈ (βH/z2)1/3 is the lower bound of the inertial subrange and it is of order 10−2 Hz.

[1] Air-sea/land turbulent fluxes of momentum, sensible heat, water vapor, carbon dioxide, and ozone are discussed on the basis of eddy covariance measurements made aboard the NOAA R/V Ronald H. Brown during the Texas Air Quality Study (TexAQS) in August–September 2006. The TexAQS 2006 field campaign focused on air pollution meteorology associated primarily with ozone and aerosol transport in the Houston/Galveston region and the nearby coastal zone. The ship-based complement of instrumentation was used for the boundary layer measurements over water (the Gulf of Mexico and various harbors/bay areas) and “over land” (specifically, 80 km inside the Houston Ship Channel). In this study we focus on direct comparisons of TexAQS 2006 flux observations with the Coupled Ocean-Atmosphere Response Experiment (COARE) bulk flux algorithm to investigate possible coastal and urban area influences. It is found that the average neutral drag coefficient can be about an order of magnitude larger over very rough urban areas than over the sea surface. However, a similar effect was not observed for the scalar transfer; that is, the neutral Stanton and Dalton numbers do not change significantly over different footprint surfaces. Our data suggest that the TexAQS 2006 region was generally a sink for surface ozone whether over water or over land. The turbulent flux of carbon dioxide was mostly negative (uptake by the surface) for measurements over waters of the Gulf of Mexico and some bays, but the flux becomes positive (release to the air) for inland regions. Both ozone and carbon dioxide turbulent fluxes above land were larger in magnitude compared to the over water measurements.

were consistently � 0.01 cm s � 1 . During summer, ozone fluxes displayed distinct diurnal cycles, and evidence for regular occurrences of bi-directional behavior. Summer, daytime vd ranged between � 0.01 to 0.07 cm s � 1 . Maximum summertime downward fluxes (ozone deposition) coincided with the hours of maximum solar radiation, i.e., noon– afternoon. During summer nighttime hours upward ozone fluxes were observed. These upward fluxes were interpreted as ozone production in a shallow layer near and above the snow surface with resulting upward ozone fluxes out of the shallow surface layer. Comparisons with published observations from temperate, midlatitude sites suggest different controls and behavior of ozone fluxes, and that ozone fluxes over snow depend on a myriad of parameters, including solar irradiance, snow chemical and physical properties, snowpack depth, and the type of substrate underneath the snow. Citation: Helmig, D., L. D. Cohen, F. Bocquet, S. Oltmans, A. Grachev, and W. Neff (2009), Spring and summertime diurnal surface ozone fluxes over the polar snow at Summit, Greenland, Geophys. Res. Lett., 36, L08809

In modern atmosphere-hydrosphere-biosphere model chains, convective boundary-layer models and parameterization packages represent the most important coupling agents, which essentially control the overall quality of predictions from coupled models. This paper focuses on the enhancement of turbulent mixing due to large-scale semi-organized eddies and interactions between large eddies and surface roughness elements up to very high obstacles such as buildings, rocks and hills. Large-scale structures in the shear-free convective boundary layers consist of strong plumes and wider but weaker downdraughts. Close to the surface they cause local “convective winds” blowing towards the plume axes. The latter generate turbulence, in addition to its generation by the buoyancy forces, and strongly contribute to the turbulent fluxes of heat and other scalars. This mechanism is especially important over very rough surfaces. The proposed model is validated against data from measurements over different sites and also through large-eddy simulation (LES) of convective boundary layers (CBLs) over a range of surfaces from very smooth to extremely rough. Excellent correspondence between model results, field observations and large-eddy simulations is achieved. The obtained resistance and heat/mass transfer laws are recommended for practical use inmeso-scale, weatherprediction, climate and other environmental models.

[1] In this paper, we examine observations of shallow, stable boundary layers in the cool waters of the Gulf of Maine between Cape Cod, Massachusetts, and Nova Scotia, obtained in the 2004 New England Air Quality Study (NEAQS-04), which was part of the International Consortium for Atmospheric Research into Transport and Transformation (ICARTT). The observations described herein were made from the NOAA Research Vessel Ronald H. Brown. The ship was instrumented for measurements of meteorological, gas-phase and aerosol atmospheric chemistry variables. Meteorological instrumentation included a Doppler lidar, a radar wind profiler, rawinsonde equipment, and a surface flux package. In this study, we focus on direct comparisons of the NEAQS-04 flux observations with the COARE bulk flux algorithm to investigate possible coastal influences on air-sea interactions. We found significant suppression of the transfer coefficients for momentum, sensible heat, and latent heat; the suppression was correlated with lighter winds, more stable surface layers, S-SE wind direction, and lower boundary layer heights. Analysis of the details shows the suppression is not a measurement, stability correction, or surface wave effect. The correlation with boundary layer height is consistent with an interpretation that our measurements at 18-m height do not realize the full surface flux in shallow boundary layers. We also find that a bulk Richardson number threshold of 0.1 gives a better estimate of boundary layer height than 0.25 or 0.5. Mean ozone deposition velocity is estimated as 0.44 mm s−1, corresponding to a boundary removal timescale of about 1 day.

Close to the surface large coherent eddies consisting of plumes and downdraughts cause convergent winds blowing towards the plume axes, which in turn cause wind shears and generation of turbulence. This mechanism strongly enhances the convective heat/mass transfer at the surface and, in contrast to the classical formulation, implies an important role of the surface roughness, In this context we introduce the stability-dependence of the roughness length. The latter is important over very rough surfaces, when the height of the roughness elements becomes comparable with the large-eddy Monin-Obukhov length. A consistent theoretical model covering convective regimes over all types of natural surfaces, from the smooth still sea to the very rough city of Athens, is developed; it is also comprehensively validated against data from measurements at different sites and also through the convective boundary layer. Good correspondence between model results, field observations and large-eddy simulation is achieved over a wide range of surface roughness lengths and convective boundary-layer heights. © Royal Meteorological Society, 2006.

During the last several decades the surface frictional processes in the shear-free convective boundary layer (CBL) are considered conceptually in the spirit of the Prandtl (1932) theory of free convection, implying the ideas of (i) universal chaotic turbulence and (ii) local correspondence between turbulent fluxes and mean gradients. Accordingly the fluxes of heat and water vapour in the atmospheric surface layer are parameterized disregarding gross features of the CBL. Conventional practical tools are either the Monin-Obukhov similarity theory or simple downgradient turbulence closure models.

During the last several decades the near-surface frictional processes in the convective boundary layer (CBL) were considered in the spirit of the Prandtl [49] theory of free convection, implying the concepts of (i) universal chaotic turbulence and (ii) local correspondence between turbulent fluxes and mean gradients. Accordingly the fluxes of heat and water vapour in the atmospheric surface layer are parameterized disregarding gross features of the CBL. Conventional practical tools are the Monin-Obukhov similarity theory and downgradient turbulence closure models. In strong convection, however, local-transport models break down. This is because of the buoyancy-driven semi-organised structures embracing the CBL, namely, comparatively narrow uprising plumes surrounded by wider downdraughts. The surface layer feeds the plumes through the CBL-scale convergence flow patterns. These can be treated as internal boundary layers of radial geometry strongly affected by the buoyancy forces. Generally, convergence flows superimpose on mean wind. In calm weather they yield their own velocity shears characterised by the “minimum friction velocity”. As a result, turbulent mixing and heat/mass transfer are facilitated depending on gross features of the CBL, such as the CBL depth and the surface roughness length. Only the first step has been taken in analysing the above mechanism. In the present paper an advanced theoretical model for the non-local frictional processes is proposed for the roughness length to the CBL depth ratios less than 0.0001. Theoretical predictions are compared with new experimental data. It is shown that data from large-eddy simulation (LES), which served as an “empirical” basis in early models, diverge drastically from field data. It is conceivable that LES, although efficient in other respects, has limited capability in treating surface roughness effects. The proposed model is in excellent agreement with atmospheric data.

The von Karman constant . This is, thus, the largest, most comprehensive atmospheric data set ever used to evaluate the von Karman constant.