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1.
Sonic anemometers are capable of measuring the wind speed in all three dimensions at high frequencies (10–50 Hz), and are relied upon to estimate eddy-covariance-based fluxes of mass and energy over a wide variety of surfaces and ecosystems. In this study, wind-velocity measurement errors from a three-dimensional sonic anemometer with a non-orthogonal transducer orientation were estimated for over 100 combinations of angle-of-attack and wind direction using a novel technique to measure the true angle-of-attack and wind speed within the turbulent atmospheric surface layer. Corrections to the vertical wind speed varied from −5 to 37% for all angles-of-attack and wind directions examined. When applied to eddy-covariance data from three NOAA flux sites, the wind-velocity corrections increased the magnitude of CO2 fluxes, sensible heat fluxes, and latent heat fluxes by ≈11%, with the actual magnitude of flux corrections dependent upon sonic anemometer, surface type, and scalar. A sonic anemometer that uses vertically aligned transducers to measure the vertical wind speed was also tested at four angles-of-attack, and corrections to the vertical wind speed measured using this anemometer were within ±1% of zero. Sensible heat fluxes over a forest canopy measured using this anemometer were 15% greater than sensible heat fluxes measured using a sonic anemometer with a non-orthogonal transducer orientation. These results indicate that sensors with a non-orthogonal transducer orientation, which includes the majority of the research-grade three-dimensional sonic anemometers currently in use, should be redesigned to minimize sine errors by measuring the vertical wind speed using one pair of vertically aligned transducers.  相似文献   

2.
Abstract

Airborne measurements in the atmospheric boundary layer (ABL) above the marginal ice zone (MIZ) on the Newfoundland Shelf reveal strong lateral variations in mean wind, temperature and the vertical fluxes of heat and momentum under conditions of cold, off‐ice wind. Flux measurements in (and near) the surface layer indicate that the neutral 10‐m drag coefficient depends on ice concentration, ranging from 2 × 10‐3 at 10% coverage to 5 × 10‐3 at 90%. Furthermore, cross‐ice‐edge transects consistently show increasing wind speed, temperature and heat flux in the off‐ice direction, but the momentum flux may either increase or decrease, depending on the relative importance of surface buoyancy flux and roughness. For the conditions encountered in this experiment, it appears surface wave maturity does not have a significant influence on the drag coefficient in fetch‐limited regimes near the ice edge.  相似文献   

3.
Abstract

Airborne measurements of mean wind velocity and turbulence in the atmospheric boundary layer under wintertime conditions of cold offshore advection suggest that at a height of 50 m the mean wind speed increases with offshore distance by roughly 20% over a horizontal scale of order 10 km. Similarly, the vertical gust velocity and turbulent kinetic energy decay on scales of order 3.5 km by factors of 1.5 and 3.2, respectively. The scale of cross‐shore variations in the vertical fluxes of heat and downwind momentum is also 10 km, and the momentum flux is found to be roughly constant to 300 m, whereas the heat flux decreases with height. The stability parameter, z/L (where z = 50 m and L is the local Monin‐Obukhov length), is generally small over land but may reach order one over the warm ocean. The magnitude and horizontal length scales associated with the offshore variations in wind speed and turbulence are reasonably consistent with model results for a simple roughness change, but a more sophisticated model is required to interpret the combined effects of surface roughness and heat flux contrasts between land and sea.

Comparisons between aircraft and profile‐adjusted surface measurements of wind speed indicate that Doppler biases of 1–2 m s?1 in the aircraft data caused by surface motions must be accounted for. In addition, the wind direction measurements of the Minimet anemometer buoy deployed in CASP are found to be in error by 25 ± 5°, possibly due to a misalignment of the anemometer vane. The vertical fluxes of heat and momentum show reasonably good agreement with surface estimates based on the Minimet data.  相似文献   

4.
An extensive meteorological observational dataset at Dome C, East Antarctic Plateau, enabled estimation of the sensitivity of surface momentum and sensible heat fluxes to aerodynamic roughness length and atmospheric stability in this region. Our study reveals that (1) because of the preferential orientation of snow micro-reliefs (sastrugi), the aerodynamic roughness length \(z_{0}\) varies by more than two orders of magnitude depending on the wind direction; consequently, estimating the turbulent fluxes with a realistic but constant \(z_{0}\) of 1 mm leads to a mean friction velocity bias of \(24\,\%\) in near-neutral conditions; (2) the dependence of the ratio of the roughness length for heat \(z_{0t}\) to \(z_{0}\) on the roughness Reynolds number is shown to be in reasonable agreement with previous models; (3) the wide range of atmospheric stability at Dome C makes the flux very sensitive to the choice of the stability functions; stability function models presumed to be suitable for stable conditions were evaluated and shown to generally underestimate the dimensionless vertical temperature gradient; as these models differ increasingly with increases in the stability parameter z / L, heat flux and friction velocity relative differences reached \(100\,\%\) when \(z/L > 1\); (4) the shallowness of the stable boundary layer is responsible for significant sensitivity to the height of the observed temperature and wind data used to estimate the fluxes. Consistent flux results were obtained with atmospheric measurements at heights up to 2 m. Our sensitivity study revealed the need to include a dynamical parametrization of roughness length over Antarctica in climate models and to develop new parametrizations of the surface fluxes in very stable conditions, accounting, for instance, for the divergence in both radiative and turbulent fluxes in the first few metres of the boundary layer.  相似文献   

5.
Land-surface heterogeneity effects on the subgrid scale of regional climate and numerical weather prediction models are of vital interest for the energy and mass exchange between the surface and the atmospheric boundary layer. High-resolution numerical model simulations can be used to quantify these effects, and are a tool used to obtain area-averaged surface fluxes over heterogeneous land surfaces. We present high-resolution model simulations for the LITFASS area near Berlin during the LITFASS-2003 experiment, which were carried out using the non-hydrostatic model FOOT3DK of the University of Köln with horizontal resolutions of 1 km and 250 m. The LITFASS-2003 experimental dataset is used for comparison. The screen level quantities show good quality for the simulated pressure, temperature, humidity and wind speed and direction. Averaged over the four week experimental period, simulated surface energy fluxes at land stations show a small bias for the turbulent heat fluxes and an underestimation of the net radiation caused by excessive cloudiness in the simulations. For eight selected days with low cloud amounts, the net radiation bias is close to zero, but the sensible heat flux shows a strong positive bias. Large differences are found for latent heat fluxes over a lake, which are partly due to local effects on the measurements, but an additional problem seems to be the overestimation of the turbulent exchange under stable conditions in the daytime internal boundary layer over the lake. In the area average over the LITFASS area of 20 ×  20 km2, again a strong positive bias of 70 W m?2 for the sensible heat is present. For the low soil moisture conditions during June 2003, the simulation of the turbulent heat fluxes is sensitive to variations in the soil type and its hydrological properties. Under these conditions, the supply of ground water to the lowest soil layer should be accounted for. Different area-averaging methods are tested. The experimental set-up of the LITFASS-2003 experiment is found to be well suited for the computation of area-averaged turbulent heat fluxes.  相似文献   

6.
The marine atmospheric boundary layer （MABL） plays a vital role in the transport of momentum and heat from the surface of the ocean into the atmosphere. A detailed study on the MABL characteristics was carried out using high-resolution surface-wind data as measured by the QuikSCAT （Quick scatterometer） satellite. Spatial variations in the surface wind, frictional velocity, roughness parameter and drag coefficient for the different seasons were studied. The surface wind was strong during the southwest monsoon season due to the modulation induced by the Low Level Jetstream. The drag coefficient was larger during this season, due to the strong winds and was lower during the winter months. The spatial variations in the frictional velocity over the seas was small during the post-monsoon season （-0.2 m s^-1）. The maximum spatial variation in the frictional velocity was found over the south Arabian Sea （0.3 to 0.5 m s^-1） during the southwest monsoon period, followed by the pre-monsoon over the Bay of Bengal （0.1 to 0.25 m s^-1）. The mean wind-stress curl during the winter was positive over the equatorial region, with a maximum value of 1.5×10^-7 N m^-3, but on either side of the equatorial belt, a negative wind-stress curl dominated. The area average of the frictional velocity and drag coefficient over the Arabian Sea and Bay of Bengal were also studied. The values of frictional velocity shows a variability that is similar to the intraseasonal oscillation （ISO） and this was confirmed via wavelet analysis. In the case of the drag coefficient, the prominent oscillations were ISO and quasi-biweekly mode （QBM）. The interrelationship between the drag coefficient and the frictional velocity with wind speed in both the Arabian Sea and the Bay of Bengal was also studied.  相似文献   

7.
The boundary layer in the warm sector of a moderately deepening winter cyclone during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) is studied near the cold front. Data from the National Center for Atmospheric Research Electra research aircraft are used to examine mean and turbulence quantities. The aircraft data and supplemental data from ships, drifting buoys and moored buoys reveal an equivalent-barotropic pressure field. The area is found to be dominated by gradients in temperature and in turbulent fluxes, with changes occurring over 100 km horizontally being comparable to changes over 350 m vertically. The horizontal components of the gradients are found to be a maximum in a direction perpendicular to the front. Cross-sections perpendicular to the front are used to illustrate boundary-layer structure. Profiles of wind speed, stress, wind direction and stress direction are estimated from an Ekman model that is modified to take into account the equivalent-barotropic pressure field. Comparison of profiles from the model to the aircraft-measured data show reasonable agreement far from the front (100 km) when the model uses a constant eddy viscosity of approximately 6 kg m–1 s–1. Near the front there is less agreement with the model. Profiles of turbulent fluxes of momentum, heat and latent heat are divergent, with along-wind momentum flux negative and decreasing upward, cross-wind momentum flux positive and increasing upward, and heat flux and latent heat flux small, positive and decreasing upward. Far from the front, the turbulent kinetic energy budget shows that dissipation balances shear production. However, near-front behavior has an imbalance at low altitude, with shear production appearing as a TKE sink.  相似文献   

8.
Wind-tunnel simulations of theatmospheric stable boundary layer (SBL) developedover a rough surface were conducted by using athermally stratified wind tunnel at the Research Institutefor Applied Mechanics (RIAM), Kyushu University. Thepresent experiment is a continuation of the workcarried out in a wind tunnel at Colorado StateUniversity (CSU), where the SBL flows were developed over asmooth surface. Stably stratified flows were createdby heating the wind-tunnel airflow to a temperature ofabout 40–50°and by cooling the test-section floor toa temperature of about 10°. To simulate therough surface, a chain roughness was placed over thetest-section floor. We have investigated the buoyancyeffect on the turbulent boundary layer developed overthis rough surface for a wide range of stability,particularly focusing on the turbulence structure andtransport process in the very stable boundary layer.The present experimental results broadly confirm theresults obtained in the CSU experiment with the smoothsurface, and emphasizes the following features: thevertical profiles of turbulence statistics exhibitdifferent behaviour in two distinct stability regimes with weak and strong stability,corresponding to the difference in the verticalprofiles of the local Richardson number. The tworegimes are separated by the critical Richardsonnumber. The magnitudes in turbulence intensities andturbulent fluxes for the weak stability regime aremuch greater than those of the CSU experiments becauseof the greater surface roughness. For the very stableboundary layer, the turbulent fluxes of momentum andheat tend to vanish and wave-like motions due to theKelvin–Helmholtz instability and the rolling up andbreaking of those waves can be observed. Furthermore,the appearance of internal gravity waves is suggestedfrom cross-spectrum analyses.  相似文献   

9.
The link between the sea-ice cover of the Amundsen Gulf and the overlying atmospheric boundary layer was explored on a weekly timestep from winter to summer 2008. The total sea-ice cover was around 97% (3% leads) from 7 January to 21 April. From 28 April to 12 May, the total sea-ice cover approached 100%. From May 19, the total sea-ice declined rapidly to its July minimum of 3%. During the winter, a turbulent internal boundary layer (IBL), attributed to the upward flux of sensible heat (mean = 46 W m−2), was present in most of the mean daily potential temperature profiles. The mean latent heat flux was 1.7 Wm−2. A turbulent IBL was also present in most of the mean daily profiles for early spring. Surface fluxes were not estimated. During late spring and early summer, a stable IBL, attributed to the downward flux of sensible heat (mean = −19 W m−2), was present in most of the potential temperature profiles. Both downward and upward fluxes of latent heat occurred in this period (means = −3.3 and 1.1 W m−2). The sensible heat flux estimates are consistent with the results of others; however, the latent heat flux estimates may be too small due to condensation/deposition within the IBL. The unconsolidated nature of the pack ice in the Amundsen Gulf, and the low sea-surface temperatures following break-up, were critical factors controlling the presence and type of IBL.  相似文献   

10.
During slightly unstable but still very close to neutral conditions new results from two previous investigations have shown a significant increase of sensible and latent heat fluxes over the sea. The vertical heat transport during these conditions is dominated by detached eddies originating at the top of the boundary layer, bringing relatively cold and dry air to the surface. This effect can be described in numerical models by either enhanced heat transfer coefficients for sensible and latent heat (Stanton and Dalton numbers respectively) or with an additional roughness length, added to the original roughness lengths for heat and humidity. Such new expressions are developed using turbulence measurements from the Baltic Sea valid for wind speeds up to 14 m s−1. The effect of including the increased heat fluxes is investigated using two different numerical models: a regional three-dimensional climate model covering northern Europe, and a process-oriented ocean model for the Baltic Sea. During periods of several days, the latent heat flux can be increased by as much as 100 W m−2. The increase in sensible heat flux is significantly smaller since the process is only of importance in the very near-neutral regime where the sensible heat flux is very small. The long-term average effect over the Baltic Sea is of the order of several W m−2.  相似文献   

11.
An analysis tool for the study of wind speed profiles over the water has been developed. The profiles are analysed using a modified dimensionless wind speed and dimensionless height, assuming that the sea surface roughness can be predicted by Charnock’s roughness length model. In this form, the roughness dependency on wind speed is extracted and the variations on the wind profile are due solely to atmospheric stability. The use of the Charnock’s non-dimensional wind profile is illustrated using data collected from a meteorological mast installed in the Danish North Sea. The best fit with the observed mean non-dimensional wind profile under neutral atmospheric conditions is found using a value of 1.2 × 10−2 for Charnock’s parameter. The stability correction on the neutral wind profile suggested by the Businger-Dyer relations was found to perform well over the sea.  相似文献   

12.
The severe bora case that lasted from 13 to 15 November 2004 has been selected for the analysis of the bora of Pag’s ribs, which occurs in the northern part of the eastern Adriatic coast over the Pag island area (Croatia). According to the measurements from automatic stations, the MM5 numerical model is successful in the 10-min mean wind speed prediction at 10-m height. The vertical analysis of the wind speed and potential temperature also gave satisfactory results. At the commencement of the bora the modelled wind had a magnitude of 20ms−1 at 10-m height in the Pag island area, which sharply attenuated in the cross direction and to the open sea. In this way the model has proved successful in predicting the characteristics of the bora of Pag’s ribs. Potential vorticity (PV) at 600m has lower values within PV banners than during the developed bora. The consequence is a strong jet emanating from the nearest gap. The vertical cross-sections through the centre of the gap point out a permanent hydraulic-like flow. At the time of the bora of Pag’s ribs the highest modelled turbulent kinetic energy is found in the jump-like region above the inversion and within the boundary layer along the lower boundary, ranging from 6–8m2 s−2. It is concluded that the dissipation in the hydraulic jumps and the wave breaking regions are the reasons for PV generation.  相似文献   

13.
We measured the surface energy budget of an Alpine grassland in highly complex terrain to explore possibilities and limitations for application of the eddy-covariance technique, also for CO2 flux measurements, at such non-ideal locations. This paper focuses on the influence of complex terrain on the turbulent energy measurements of a characteristic high Alpine grassland on Crap Alv (Alp Weissenstein) in the Swiss Alps during the growing season 2006. Measurements were carried out on a topographic terrace with a slope of 25◦ inclination. Flux data quality is assessed via the closure of the energy budget and the quality flag method used within the CarboEurope project. During 93% of the time the wind direction was along the main valley axis (43% upvalley and 50% downvalley directions). During the transition times of the typical twice daily wind direction changes in a mountain valley the fraction of high and good quality flux data reached a minimum of ≈50%, whereas during the early afternoon ≈70% of all records yielded good to highest quality (CarboEurope flags 0 and 1). The overall energy budget closure was 74 ± 2%. An angular correction for the shortwave energy input to the slope improved the energy budget closure slightly to 82 ± 2% for afternoon conditions. In the daily total, the measured turbulent energy fluxes are only underestimated by around 8% of net radiation. In summary, our results suggest that it is possible to yield realistic energy flux measurements under such conditions. We thus argue that the Crap Alv site and similar topographically complex locations with short-statured vegetation should be well suited also for CO2 flux measurements.  相似文献   

14.
A time series of microwave radiometric profiles over Arctic Canada’s Cape Bathurst (70°N, 124.5°W) flaw lead polynya region from 1 January to 30 June, 2008 was examined to determine the general characteristics of the atmospheric boundary layer in winter and spring. A surface based or elevated inversion was present on 97% of winter (January–March) days, and on 77% of spring (April–June) days. The inversion was the deepest in the first week of March (≈1100 m), and the shallowest in June (≈250 m). The mean temperature and absolute humidity from the surface to the top of the inversion averaged 250.1 K (−23.1°C), and 0.56 × 10−3 kg m−3 in winter, and in spring averaged 267.5 K (−5.6°C), and 2.77 × 10−3 kg m−3. The median winter atmospheric boundary-layer (ABL) potential temperature profile provided evidence of a shallow, weakly stable internal boundary layer (surface to 350 m) topped by an inversion (350–1,000 m). The median spring profile showed a shallow, near-neutral internal boundary layer (surface to 350 m) under an elevated inversion (600–800 m). The median ABL absolute humidity profiles were weakly positive in winter and negative in spring. Estimates of the convergence of sensible heat and water vapour from the surface that could have produced the turbulent internal boundary layers of the median profiles were 0.67 MJ m−2 and 13.1 × 10−3 kg m−2 for the winter season, and 0.66 MJ m−2 and 33.4 × 10−3 kg m−2 for the spring season. With fetches of 10–100 km, these accumulations may have resulted from a surface sensible heat flux of 15–185 W m−2, plus a surface moisture flux of 0.001–0.013 mm h−1 (or a latent heat flux of 0.7–8.8 W m−2) in winter, and 0.003–0.033 mm h−1 (or a latent heat flux of 2–22 W m−2) in spring.  相似文献   

15.
A three-dimensional numerical meteorological model is used to perform large-eddy simulations of the upslope flow circulation over a periodic ridge-valley terrain. The subgrid-scale quantities are modelled using a prognostic turbulence kinetic energy (TKE) scheme, with a grid that has a constant horizontal resolution of 50 m and is stretched along the vertical direction. To account for the grid anisotropy, a modified subgrid length scale is used. To allow for the response of the surface fluxes to the valley-flow circulation, the soil surface temperature is imposed and the surface heat and momentum fluxes are computed based on Monin–Obukhov similarity theory. The model is designed with a symmetrical geometry using periodic boundary conditions in both the x and y directions. Two cases are simulated to study the influence of along-valley geostrophic wind forcing with different intensities. The presence of the orography introduces numerous complexities both in the mean properties of the flow and in the turbulent features, even for the idealized symmetric geometry. Classical definitions for the height of the planetary boundary layer (PBL) are revisited and redefined to capture the complex structure of the boundary layer. Analysis of first- and second-moment statistics, along with TKE budget, highlights the different structure of the PBL at different regions of the domain.  相似文献   

16.
We simulate the microscale heterogeneities of turbulent variables observed at a complex site for different wind directions. The atmospheric computational fluid dynamics (CFD) results are compared with an ensemble of 36 months of data collected at the experimental site SIRTA “Site Instrumental de Recherche par Télédétection Atmosphérique”, located near Paris (France) in a semi-urban environment. The experimental data show that the normalized turbulent kinetic energy (TKE) k/U 2 (where k is TKE and U is the wind speed) at 10-m height, for two different locations, is highly dependent on wind direction and strongly influenced by trees. These measurements show a strong increase of the normalized TKE downstream of the forest canopies with a large variability within the 36-month period in part due to the variation of the tree foliage. The numerical simulations are carried out using the CFD code Code_Saturne with the standard k?ε closure, in neutral stratification. The buildings are taken into account explicitly in the mesh and the forested areas are modelled with two approaches: the classical roughness wall law and a drag porosity. A comparison has been performed between the calculated values and the median of measured values of the normalized TKE and the normalized friction velocity, for each wind sector of 10°. A very good agreement is obtained with the drag porosity model, whereas the classical roughness law leads to a strong underestimation downstream of the forested areas. However, this large improvement of the results using the drag porosity model can only be obtained with a refinement of the grid, especially in forested areas, and an accurate land-use map.  相似文献   

17.
Impact of Sea-Spray on the Atmospheric Surface Layer   总被引：1，自引：0，他引：1  
The feedback effects of sea-spray on the heat and momentum fluxes under equilibrium conditions associated with winds of tropical cyclones are investigated using a one-dimensional coupled sea-spray and atmospheric surface-layer (ASL) model. This model is capable of simulating the microphysical aspects of the evaporation of saline water droplets of various sizes and their dynamic and thermal interaction with the turbulence mixing that is simulated by the Mellor–Yamada 1.5-order closure scheme. Sea-spray droplet generation is described by a state-of-the-art parametrization that predicts the size spectrum of sea-spray droplets for a given surface forcing. The results from a series of simulations indicate the way in which evaporating droplets of various sizes modify the turbulence mixing near the surface, which in turn affects further droplet evaporation. All these results are direct consequences of the effects of sea-spray on the balance of turbulent kinetic energy in the spray-filled surface layer. In particular, the overall impact of sea-spray droplets on the mean wind depends on the wind speed at the level of sea-spray generation. When the wind speed is below 40 m s−1, the droplets are small in size and tend to evaporate substantially and thus cool the spray-filled layer, while for wind speeds above 50 m s−1, the size of the droplets is so large that they do not have enough time to evaporate much before falling back into the sea. The sensible heat carried by the droplets is released to the ambient air, increasing the buoyancy of the surface layer and enhancing the turbulent mixing. The suspension of sea-spray droplets reduces the buoyancy and makes the surface layer more stable, decreasing the friction velocity and the downward turbulent mixing of momentum. The results from the numerical experiments also suggest that, in order not to violate the constant flux assumption critical to the Monin–Obukhov similarity theory, a displacement equal to the mean wave height should be included in the logarithmic profiles of the wind and thermal fields.  相似文献   

18.
Profile and eddy-correlation (heights of 4 and 10 m) measurements performed on the Pasterze glacier (Austria) are used to study the characteristics of the stable boundary layer under conditions of katabatic and large-scale forcing. We consider cases where large-scale forcing results in a downslope (or following) ambient wind. The analysis of averaged spectra and cospectra reveals low frequency perturbations that have a large influence on the variances of temperature and horizontal wind components and also alter the cospectra of momentum and sensible heat flux. Only the spectrum of the vertical wind speed is comparable to universal spectra. The low frequency perturbations occur as brief intermittent events and result in downward entrainment of ambient air thereby producing enhanced downward sensible heat fluxes and downward as well as upward momentum fluxes with various magnitudes and timescales. After the variances were high pass filtered, the normalised standard deviations of wind speed and temperature compare favourably to findings in the literature within the range 0>z/L>0.5. For larger z/L they deviate as a result of an increased influence from low frequency perturbations and thus non-stationarity. In line with this, the turbulent kinetic energy budget (at 4 m height) indicates that production (shear) is in balance with destruction (buoyancy and dissipation) within the range 0>z/L>0.3. Non-dimensional gradients of wind speed within the range 0>z/L>0.3 have a slope of about 3.5. The scatter for the dimensionless temperature gradient is quite large, and the slope is comparable to that for wind speed gradients. For z/L>0.3 the imbalance in the turbulent kinetic energy budget grows and non-dimensional gradients for wind speed and temperature deviate considerably from accepted values as a result of increased non-stationarity. Average roughness lengths for momentum and sensible heat flux derived from wind speed and temperature profiles are respectively 1 × 10-3 m and 6 × 10-5 m, consistent with the literature. The ratio (z0h/z0m) compares to those predicted by surface renewal models. A variation of this ratio with the roughness Reynolds number is not indicated by our data.  相似文献   

19.
The Summer Surface Energy Balance of the High Antarctic Plateau   总被引：1，自引：0，他引：1  
The summertime surface energy balance (SEB) at Kohnen station, situated on the high Antarctic plateau (75°00′ S, 0°04′ E, 2892m above sea level) is presented for the period of 8 January to 9 February 2002. Shortwave and longwave radiation fluxes were measured directly; the former was corrected for problems associated with the cosine response of the instrument. Sensible and latent heat fluxes were calculated using the bulk method, and eddy-correlation measurements and the modified Bowen ratio method were used to verify these calculated fluxes. The calculated sub-surface heat flux was checked by comparing calculated to measured snow temperatures. Uncertainties in the measurements and energy-balance calculations are discussed. The general meteorological conditions were not extraordinary during the period of the experiment, with a mean 2-m air temperature of −27.5°C, specific humidity of 0.52×10−3kg kg−1 and wind speed of 4.1ms−1. The experiment covered the transition period from Antarctic summer (positive net radiation) to winter (negative net radiation), and as a result the period mean net radiation, sensible heat, latent heat and sub-surface heat fluxes were small with values of −1.1, 0.0, −1.0 and 0.7 Wm−2, respectively. Daily mean net radiation peaked on cloudy days (16 Wm−2) and was negative on clear-sky days (minimum of −19 W m−2). Daily mean sensible heat flux ranged from −8 to +10 Wm−2, latent heat flux from −4 to 0 Wm−2 and sub-surface heat flux from −8 to +7 Wm−2.  相似文献   

20.
A simple model of the atmospheric boundary layer over the ocean where the swell impact on the atmosphere is explicitly accounted for is suggested. The model is based on Ekman’s equations, where the stress in the wave boundary layer is split into two parts: the turbulent and wave-induced stress. The turbulent stress is parameterized traditionally via the eddy viscosity proportional to the generalized mixing length. The wave-induced stress directed upward (from swell to the atmosphere) is parameterized using the formalism of the wind-over-waves coupling theory. The model can be seen as an extension of the model by Kudryavtsev and Makin (J Phys Oceanogr 34:934–949, 2004) to the scale of the entire atmospheric boundary layer by including the Coriolis force into the momentum conservation equation and generalizing the definition of the mixing length. The regime of low winds for swell propagating along the wind direction is studied. It is shown that the impact of swell on the atmosphere is governed mainly by the swell parameter—the coupling parameter that is the product of the swell steepness and the growth rate coefficient. When the coupling parameter drops below − 1 the impact of swell becomes significant and affects the entire atmospheric boundary layer. The turbulent stress is enhanced near the surface as compared to the no-swell case, and becomes negative above the height of the inner region. The wind profile is characterized by a positive gradient near the surface and a negative gradient above the height of the inner region forming a characteristic bump at the height of the inner region. Results of the model agree at least qualitatively with observations performed in the atmosphere in presence of swell.  相似文献   

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