Wind Speed U10 Research Articles

A New Wave-State-Based Drag Coefficient Parameterization for Coastal Regions

Abstract The large scatter of the drag coefficient CD at a given wind speed and its discrepancy in coastal regions and open oceans have received increasing attention. However, the parameterization of CD is still an open question, especially in coastal regions. Therefore, this study systematically investigated the influence of surface waves on wind stress based on in situ observations of surface waves and air–sea fluxes on three coastal tower-based platforms in different regions. A formulation that is a function of only wind speed was established in the wind speed range of 1–20 m s−1, and when extended to 30 m s−1, it could predict the saturation of coastal CD at a 20 m s−1 wind speed and then the attenuation. However, this wind-based formulation does not simulate the scatter of CD in practice. By further analyzing the effect of wave states on wind stress, the parameters of wave age and directionality of wind and waves were incorporated into the wind-based formulation, and a new wave-state-based parameterization on CD was proposed, which can estimate the widely spread CD values to a large extent and the saturation of CD. The RMSE between this new parameterization and observations reduce approximately 20% and 9% relative to the COARE and wind-based formula. The applicability of this new parameterization was further demonstrated through a comparison between the newly parameterized CD and observed asymmetric CD in different quadrants of a tropical cyclone. The wave-state-based parameterization scheme requires three parameters, wind speed U10, wave age β, and wave off-wind angle θ, and it is expected to be applied to coastal regions. Significance Statement Wind stress over the ocean plays an important role in numerical simulations for both the atmosphere and ocean, which requires accurate parameterization. However, parameterization of wind stress or drag coefficient CD is still an open question due to the complexity of the potential factors behind wind stress, especially for coastal regions. This manuscript provided a new wave-state-based parameterization scheme at low to high wind speeds for coastal regions, based on field observations on three coastal towers. This new parameterization can predict the saturation of CD at a wind speed of 20 m s−1 and then the attenuation, agreeing well with the previous coastal observations, and simulate the large scatter of CD to a large extent. Furthermore, it can predict the asymmetric CD in different quadrants of a tropical cyclone, consistent with the observations. This parameterization scheme requires only three parameters, wind speed, wave age, and misalignment angle between wind and wave, which can be conveniently applied to the numerical models.

Probability-based wind-wave relation

In a fully developed sea, the significant wave height (Hs) and wind speed (U10) are conventionally related to a pure quadratic equation. This relation is often violated, since in the field the measured local Hs is often contaminated by the swell, which is propagated from distant places. Therefore, a swell partition is required before the establishment of the wind-wave relation. The Spectra Energy Partition (SEP) is regarded as the best way to isolate the swell and the wind wave relation: it is based on the identification of a separation frequency in the ocean wave spectrum. However, for most field observations, the wave spectra information is unavailable, and thus the SEP is inapplicable. This work proposes a probability-based algorithm to identify the averaged swell without knowing the wave spectrum a priori. The local wind-wave relation is established by either a linear or an energy-conserved decomposition. We also find that the local wind-wave relation is a power-law when the wind speed U10 is above 4 m/s. The proposed method is first validated by applying the SEP method to buoy collected wave spectra data. The global pattern of the swell and the local wind waves is retrieved by applying the proposed method to a 17-year wind and wave database from the JASON satellite. Strong seasonal and spatial variations are obtained. Finally, a prediction model based on the empirical wind-wave relation is shown to successfully retrieve the wave field when the wind field is available.

Serre-Green-Naghdi Dynamics under the Action of the Jeffreys’ Wind-Wave Interaction

We derive the anti dissipative Serre-Green-Naghdi (SGN) equations in the context of nonlinear dynamics of surface water waves under wind forcing, in finite depth. The anti-dissipation occurs du to the continuos transfer of wind energy to water surface wave. We find the solitary wave solution of the system, with an increasing amplitude under the wind action. This leads to the blow-up of surface wave in finite time for infinitely large asymptotic space. This dispersive, anti-dissipative and fully nonlinear phenomenon is equivalent to the linear instability at infinite time. The theoretical blow-up time is calculated based on real experimental data. Naturally, the wave breaking takes place before the blow-up time. However, the amplitude’s growth resulting in the blow-up could be observed. Our results show that, based on the particular type of wind-wave tank data used in this paper, for h=0.14m, the amplitude growth rate is of order 0.1 which experimentally, is at the measurability limit. But we think that by gradually increasing the wind speed U10, up to 10 m/s, it is possible to have the experimental confirmation of the present theory in existing experimental facilities.

On the Hyperbolicity of the Bulk Air–Sea Heat Flux Functions: Insights into the Efficiency of Air–Sea Moisture Disequilibrium for Tropical Cyclone Intensification

AbstractSea-to-air heat fluxes are the energy source for tropical cyclone (TC) development and maintenance. In the bulk aerodynamic formulas, these fluxes are a function of surface wind speed U10 and air–sea temperature and moisture disequilibrium (ΔT and Δq, respectively). Although many studies have explained TC intensification through the mutual dependence between increasing U10 and increasing sea-to-air heat fluxes, recent studies have found that TC intensification can occur through deep convective vortex structures that obtain their local buoyancy from sea-to-air moisture fluxes, even under conditions of relatively low wind. Herein, a new perspective on the bulk aerodynamic formulas is introduced to evaluate the relative contribution of wind-driven (U10) and thermodynamically driven (ΔT and Δq) ocean heat uptake. Previously unnoticed salient properties of these formulas, reported here, are as follows: 1) these functions are hyperbolic and 2) increasing Δq is an efficient mechanism for enhancing the fluxes. This new perspective was used to investigate surface heat fluxes in six TCs during phases of steady-state intensity (SS), slow intensification (SI), and rapid intensification (RI). A capping of wind-driven heat uptake was found during periods of SS, SI, and RI. Compensation by larger values of Δq > 5 g kg−1 at moderate values of U10 led to intense inner-core moisture fluxes of greater than 600 W m−2 during RI. Peak values in Δq preferentially occurred over oceanic regimes with higher sea surface temperature (SST) and upper-ocean heat content. Thus, increasing SST and Δq is a very effective way to increase surface heat fluxes—this can easily be achieved as a TC moves over deeper warm oceanic regimes.

Application of Landsat imagery for the investigation of wave breaking

An algorithm for retrieving the fraction of the sea surface covered by whitecaps (W) from Landsat-8 satellite optical reflectance measurements in the near-infrared channel is described. The distribution of W derived from approximately 100 Landsat-8 scenes was compared with quasi-synchronous scatterometer measurements of wind speed (u10), which allowed us to obtain the W(u10) relation for large whitecaps from high-resolution satellite optical measurements. Further, we demonstrate the impact of various phenomena, including- internal waves, river plumes, bottom topography, atmospheric stability, ocean fronts, and mesoscale currents on whitecap coverage and its spatial variation in different areas of the ocean. These data are analysed using theoretical models, suggesting that whitecap coverage is a proxy of wave energy dissipation and reflects disturbances in the wind-wave energy balance caused by wave-current interactions and variable wind forcing due to changes in atmospheric stratification over ocean temperature fronts and the movement of wind-waves by surface currents relative to the atmospheric boundary layer.

Momentum and heat transfer across the foam-covered air-sea interface in hurricanes

Procedures of formal averaging over the air-sea interface are applied to both momentum and enthalpy surface-transfer coefficients, CD and CK, in hurricane conditions. The transfer coefficients across the total area of the sea surface and water-covered portions of the sea surface, CD, CK, and CDw, CKw, are estimated by measurements in the open-sea and laboratory (foam-free) conditions, respectively, while the transfer coefficients across the foam-covered portion of the sea surface, CDf, Kf, which cannot be measured directly, are estimated using the splitting relations. Applying the Monin-Obukhov similarity theory at the neutral stability atmospheric conditions to the transfer coefficients (separately for the foam-free, foam-covered, and total sea surfaces) yields the roughness lengths ZDw, ZKw, ZDf, ZKf, and ZD, ZK. The study is aimed at the description of an anomalous, as compared with laboratory conditions, behavior of the momentum and enthalpy transfer coefficients in hurricane conditions with wind speed U10 by the effect of the foam slipping layer sandwiched between the atmosphere and the ocean.

Comparison of Wind Speed and Wave Height Trends from Twentieth-Century Models and Satellite Altimeters

AbstractThe trends in marine 10-m wind speed U10 and significant wave height Hs found in two century-long reanalyses are compared against a model-only integration. Reanalyses show spurious trends due to the assimilation of an increasing number of observations over time. The comparisons between model and reanalyses show that the areas where the discrepancies in U10 and Hs trends are greatest are also the areas where there is a marked increase in assimilated observations. Large differences in the yearly averages call into question the quality of the observations assimilated by the reanalyses, resulting in unreliable U10 and Hs trends before the 1950s. Four main regions of the world’s oceans are identified where the trends between model and reanalyses deviate strongly. These are the North Atlantic, the North Pacific, the Tasman Sea, and the western South Atlantic. The trends at +24-h lead time are markedly weaker and less correlated with the observation count. A 1985–2010 comparison with an extensive dataset of calibrated satellite altimeters shows contrasting results in Hs trends but similar U10 spatial trend distributions, with general agreement between model, reanalyses, and satellite altimeters on a broad increase in wind speed over the Southern Hemisphere.

Correlation between foam-bubble size and drag coefficient in hurricane conditions

Recently proposed model of foam impact on the air sea drag coefficient Cd has been employed for the estimation of the efficient foam-bubble radius Rb variation with wind speed U10 in hurricane conditions. The model relates Cd (U10) with the efficient roughness length Zeff (U10) represented as a sum of aerodynamic roughness lengths of the foam free and foam covered sea surfaces Zw (U10 ), and Zf (U10) weighted with the foam coverage coefficient. This relation is treated for known phenomenological distributions Cd (U10), Zw (U10) at strong wind speeds as an inverse problem for the efficient roughness parameter of foam-covered sea surface Zf (U10).

Evidence of Sea-State Dependence of Aerosol Concentration in the Marine Atmospheric Boundary Layer

AbstractSea spray aerosols represent a large fraction of the aerosols present in the maritime environment. Despite evidence of the importance of surface wave– and wave breaking–related processes in coupling the ocean with the atmosphere, sea spray source generation functions are traditionally parameterized by the 10-m wind speed U10 alone. It is clear that unless the wind and wave field are fully developed, the source function will be a function of both wind and wave parameters. This study reports primarily on the aerosol component of an air–sea interaction experiment, the phased-resolved High-Resolution Air–Sea Interaction Experiment (HIRES), conducted off the coast of northern California in June 2010. Detailed measurements of aerosol number concentration in the marine atmospheric boundary layer (MABL) at altitudes ranging from as low as 30 m up to 800 m above mean sea level (MSL) over a broad range of environmental conditions (significant wave height Hs of 2 to 4.5 m and U10 from 10 to 18 m s−1) collected from an instrumented research aircraft are presented. Aerosol number densities and volume are computed over a range of particle diameters from 0.1 to 200 μm, while the sea surface conditions, including Hs, moments of the breaker length distribution Λ(c), and wave breaking dissipation, were measured by a suite of electro-optical sensors that included the NASA Airborne Topographic Mapper (ATM). The sea-state dependence of the aerosol concentration in the MABL is evident, stressing the need to incorporate wave parameters in the spray source generation functions that are traditionally parameterized by surface winds alone.

Parameterization of oceanic whitecap fraction based on satellite observations

Abstract. In this study, the utility of satellite-based whitecap fraction (W) data for the prediction of sea spray aerosol (SSA) emission rates is explored. More specifically, the study aims at evaluating how an account for natural variability of whitecaps in the W parameterization would affect SSA mass flux predictions when using a sea spray source function (SSSF) based on the discrete whitecap method. The starting point is a data set containing W data for 2006 together with matching wind speed U10 and sea surface temperature (SST) T. Whitecap fraction W was estimated from observations of the ocean surface brightness temperature TB by satellite-borne radiometers at two frequencies (10 and 37 GHz). A global-scale assessment of the data set yielded approximately quadratic correlation between W and U10. A new global W(U10) parameterization was developed and used to evaluate an intrinsic correlation between W and U10 that could have been introduced while estimating W from TB. A regional-scale analysis over different seasons indicated significant differences of the coefficients of regional W(U10) relationships. The effect of SST on W is explicitly accounted for in a new W(U10, T) parameterization. The analysis of W values obtained with the new W(U10) and W(U10, T) parameterizations indicates that the influence of secondary factors on W is for the largest part embedded in the exponent of the wind speed dependence. In addition, the W(U10, T) parameterization is able to partially model the spread (or variability) of the satellite-based W data. The satellite-based parameterization W(U10, T) was applied in an SSSF to estimate the global SSA emission rate. The thus obtained SSA production rate for 2006 of 4.4 × 1012 kg year−1 is within previously reported estimates, however with distinctly different spatial distribution.

Features of the atmospheric boundary layer block for three versions of the WAM wind wave model

The estimated characteristics of the atmospheric boundary layer, obtained by the simulation of wind wave fields using three versions of the WAM numerical model are compared with the well-known empirical dependences of drag coefficient C d on wind speed U 10 and wave age A, as well as with the dependence of dimensionless roughness height z n on inverse wave age u*/с р. Calculations carried out for several years in the areas of the Pacific and Indian oceans, based on the ERA-interim and CFSR wind reanalyses have shown good agreement between the model and empirical dependences C d (U 10) and C d (A). The range of estimated variability for z n (u*/с р ) has been found to be significantly less than empirical. It has been also found that estimated values of wind speed U 10W (t) are overestimated from 5 to 10% in all versions of WAM models compared with the input wind reanalysis U 10R (t) at the moments of appearance maximum values of wind U 10R (t). The reasons for the established features of the WAM model and their dependence on the model version are discussed.

A note on a critical wind speed for air–sea boundary processes

Wind and wind-generated waves were measured in a wind-wave tank. A clear transition was found in the relation between the wind speed U10 and the wind friction velocity u* near u* = 0.2 m/s, where U10 is the wind speed at 10 m height extrapolated from the measured wind profile in a logarithmic layer, and u* = 0.2 m/s corresponds roughly to U10 = 8 m/s in the present measurement. Quite a similar transition was found in the relation between the spectral density of high frequency wind waves and u*. These results suggest the existence of the critical wind speed for air–sea boundary processes, which was proposed by Munk (J Marine Res 6:203–218, 1947) more than half a century ago. His original idea of the critical wind speed was based on the discontinuities in such phenomena as white caps, wind stress, and evaporation, which commonly appear at a wind speed near 7 m/s. On the basis of the results of our present study and those of earlier studies, we discuss the phenomena which are relevant to the critical wind speed for the air–sea boundary processes. The conclusion is that the critical wind speed exists and it is attributed to the start of wave breaking rather than the Kelvin–Helmholtz instability, but the air–sea boundary processes are not discontinuous at a particular wind speed; because of the stochastic nature of breaking waves, the changes occur over a range of wind speeds. Detailed discussions are presented on the dynamical processes associated with the critical wind speed such as wind-induced change of sea surface roughness and high frequency wave spectrum. Future studies are required, however, to clarify the dynamical processes quantitatively. In particular, there is a need to further examine the gradual change of breaking patterns of wind waves with the increase of wind speed, and the associated change of the structure of the wind over wind waves, such as separation of the airflow at the crest of wind waves, the turbulent stress, and wave-induced stress. Studies on the dynamical structure of the high frequency wave spectrum are also needed.

Ocean bottom pressure records from the Cascadia array and short surface gravity waves

AbstractThe ocean bottom pressure records from eight stations of the Cascadia array are used to investigate the properties of short surface gravity waves with frequencies ranging from 0.2 to 5 Hz. It is found that the pressure spectrum at all sites is a well‐defined function of the wind speed U10 and frequency f, with only a minor shift of a few dB from one site to another that can be attributed to variations in bottom properties. This observation can be combined with the theoretical prediction that the ocean bottom pressure spectrum is proportional to the surface gravity wave spectrum E(f) squared, times the overlap integral I(f) which is given by the directional wave spectrum at each frequency. This combination, using E(f) estimated from modeled spectra or parametric spectra, yields an overlap integral I(f) that is a function of the local wave age . This function is maximum for f∕fPM = 8 and decreases by 10 dB for f∕fPM = 2 and f∕fPM = 30. This shape of I(f) can be interpreted as a maximum width of the directional wave spectrum at f∕fPM = 8, possibly equivalent to an isotropic directional spectrum, and a narrower directional distribution toward both the dominant low frequencies and the higher capillary‐gravity wave frequencies.

Surface Stress over the Ocean in Swell-Dominated Conditions during Moderate Winds

Abstract Atmospheric and surface wave data from several oceanic experiments carried out on the Floating Instrument Platform (FLIP) and the Air–Sea Interaction Spar (ASIS) have been analyzed with the purpose of identifying swell-related effects on the surface momentum exchange during near-neutral atmospheric conditions and wind-following or crosswind seas. All data have a pronounced negative maximum in uw cospectra centered at the frequency of the dominant swell np, meaning a positive contribution to the stress. A similar contribution at this frequency is also obtained for the corresponding crosswind cospectrum. The magnitude of the cospectral maximum is shown to be linearly related to the square of the orbital motion, being equal to , where Hsd is the swell-significant wave height, the effect tentatively being due to strong correlation between the surface component of the orbital motion and the pattern of capillary waves over long swell waves. A model for prediction of the friction velocity from measurements of Hsd, np, and the 10-m wind speed U10 is formulated and tested against an independent dataset of ~400 half-hour measurements during swell, giving good result. The model predicts that the drag coefficient CD, which is traditionally modeled as a function of U10 alone (e.g., the COARE algorithm), becomes strongly dependent on the magnitude of the swell factor and that CD can attain values several times larger than predicted by wind speed–only models. According to maps of the global wave climate, conditions leading to large effects are likely to be widespread over the World Ocean.

Foam input into the drag coefficient in hurricane conditions

A semi-empirical model is proposed for the estimation of the foam impact on the variation of the effective drag coefficient, Cd, with the reference wind speed U10 in stormy and hurricane conditions. The proposed model treats the efficient air–sea aerodynamic roughness length as a sum of two weighted aerodynamic roughness lengths for the foam-free and foam-covered conditions. On the basis of available optical and radiometric measurements of the fractional foam coverage and partitioning of the ocean surface into foam-covered and foam-free areas, the present model yields the resulting dependence of Cd vs. U10 within the range from low to hurricane wind speeds. This dependence is in fair agreement with those obtained from both open-ocean and laboratory measurements of the vertical variation of the mean wind speed. The velocity value, at which the fractional foam coverage is saturated, is found to be responsible for the difference of Cd behavior in the laboratory and open-ocean conditions.

Dependence of drag coefficient on the directional spreading of ocean waves

Drag coefficient C10 is one of the main characteristics used for calculation of surface stress based on mean wind speed U10. Most of the dependences employ the sea drag as a function of this wind speed. It has been proposed, however, that for a given wind speed C10 can depend on a number of other properties in the air‐sea system. In the present paper, dependence of the drag coefficient on the directional spreading of surface waves is studied numerically. It is shown that such dependence can be significant. For a given wind speed, the sea drag can grow as much as 25% depending on the width of directional spectrum. The highest drag corresponds to the narrowest spectra, and for very narrow directional distribution it saturates. The largest impact of the sea‐drag directional dependence is observed for the highest winds. Accounting for the directional spread of surface waves is therefore essential to improve parameterizations of C10.

Deriving the effect of wind speed on clean marine aerosol optical properties using the A-Train satellites

Abstract. The relationship between "clean marine" aerosol optical properties and ocean surface wind speed is explored using remotely sensed data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on board the CALIPSO satellite and the Advanced Microwave Scanning Radiometer (AMSR-E) on board the AQUA satellite. Detailed data analyses are carried out over 15 regions selected to be representative of different areas of the global ocean for the time period from June 2006 to April 2011. Based on remotely sensed optical properties the CALIPSO algorithm is capable of discriminating "clean marine" aerosols from other types often present over the ocean (such as urban/industrial pollution, desert dust and biomass burning). The global mean optical depth of "clean marine" aerosol at 532 nm (AOD532) is found to be 0.052 ± 0.038 (mean plus or minus standard deviation). The mean layer integrated particulate depolarization ratio of marine aerosols is 0.02 ± 0.016. Integrated attenuated backscatter and color ratio of marine aerosols at 532 nm were found to be 0.003 ± 0.002 sr−1 and 0.530 ± 0.149, respectively. A logistic regression between AOD532 and 10-m surface wind speed (U10) revealed three distinct regimes. For U10 ≤ 4 m s−1 the mean CALIPSO-derived AOD532 is found to be 0.02 ± 0.003 with little dependency on the surface wind speed. For 4 < U10 ≤ 12 m s−1, representing the dominant fraction of all available data, marine aerosol optical depth is linearly correlated with the surface wind speed values, with a slope of 0.006 s m−1. In this intermediate wind speed region, the AOD532 vs. U10 regression slope derived here is comparable to previously reported values. At very high wind speed values (U10 > 18 m s−1), the AOD532-wind speed relationship showed a tendency toward leveling off, asymptotically approaching value of 0.15. The conclusions of this study regarding the aerosol extinction vs. wind speed relationship may have been influenced by the constant lidar ratio used for CALIPSO-derived AOD532. Nevertheless, active satellite sensor used in this study that allows separation of maritime wind induced component of AOD from the total AOD over the ocean could lead to improvements in optical properties of sea spray aerosols and their production mechanisms.

Large-Eddy Simulation of Langmuir Turbulence in Pure Wind Seas

Abstract The scaling of turbulent kinetic energy (TKE) and its vertical component (VKE) in the upper ocean boundary layer, forced by realistic wind stress and surface waves including the effects of Langmuir circulations, is investigated using large-eddy simulations (LESs). The interaction of waves and turbulence is modeled by the Craik–Leibovich vortex force. Horizontally uniform surface stress τ0 and Stokes drift profiles uS(z) are specified from the 10-m wind speed U10 and the surface wave age CP/U10, where CP is the spectral peak phase speed, using an empirical surface wave spectra and an associated wave age–dependent neutral drag coefficient CD. Wave-breaking effects are not otherwise included. Mixed layer depths HML vary from 30 to 120 m, with 0.6 ≤ CP/U10 ≤ 1.2 and 8 m s−1 < U10 < 70 m s−1, thereby addressing most possible oceanic conditions where TKE production is dominated by wind and wave forcing. The mixed layer–averaged “bulk” VKE 〈w2〉/u*2 is equally sensitive to the nondimensional Stokes e-folding depth D*S/HML and to the turbulent Langmuir number Lat = u*/US, where u* = |τ0|/ρw in water density ρw and US = |uS|z=0. Use of a D*S scale-equivalent monochromatic wave does not accurately reproduce the results using a full-surface wave spectrum with the same e-folding depth. The bulk VKE for both monochromatic and broadband spectra is accurately predicted using a surface layer (SL) Langmuir number LaSL = u*/〈uS〉SL, where 〈uS〉SL is the average Stokes drift in a surface layer 0 > z > − 0.2HML relative to that near the bottom of the mixed layer. In the wave-dominated limit LaSL → 0, turbulent vertical velocity scales as wrms ∼ u*La−2/3SL. The mean profile (z) of VKE is characterized by a subsurface peak, the depth of which increases with D*S/HML to a maximum near 0.22HML as its relative magnitude /〈w2〉 decreases. Modestly accurate scalings for these variations are presented. The magnitude of the crosswind velocity convergence scales differently from VKE. These results predict that for pure wind seas and HML ≅ 50 m, 〈w2〉/u*2 varies from less than 1 for young waves at U10 = 10 m s−1 to about 2 for mature seas at winds greater than U10 = 30 m s−1. Preliminary comparisons with Lagrangian float data account for invariance in 〈w2〉/u*2 measurements as resulting from an inverse relationship between U10 and CP/U10 in observed regimes.

Microclimate of a desert playa: evaluation of annual radiation, energy, and water budgets components

AbstractWe set up two automatic weather stations over a playa (the flat floor of an undrained desert basin that, at times, becomes a shallow lake), approximately 65 km east–west by 130 km north–south, located in Dugway (40° 08′N, 113° 27′W, 1124 m above mean sea level) in northwestern Utah, USA, in 1999. These stations measured the radiation budget components, namely: incoming Rsi and outgoing Rso solar or shortwave radiation, using two Kipp and Zonen pyranometers (one inverted), the incoming Rli (or atmospheric) and outgoing Rlo (or terrestrial) longwave radiation, using two Kipp and Zonen pyrgeometers (one inverted) during the year 2000. These sensors were ventilated throughout the year to prevent dew and frost formation. Summation of these components yields the net radiation Rn. We also measured the air temperatures and humidity at 1 and 2 m and the soil moisture and temperature (Campbell Sci., Inc., CSI) to evaluate the energy budget components (latent (LE), sensible (H), and the soil (Gsur) heat fluxes). The 10 m wind speed U10 and direction (R.M. Young wind monitor), precipitation (CSI), and the surface temperature (Radiation and Energy Balance Systems, REBS) were also measured during 2000. The measurements were taken every 2 s, averaged into 20 min, continuously, throughout the year 2000. The annual comparison of radiation budget components indicates that about 34% of the annual Rsi (6937.7 MJ m−2 year−1) was reflected back to the sky as Rso, with Rli and Rlo amounting to 9943.4 MJ m−2 year−1 and 12 789.7 MJ m−2 year−1 respectively. This yields about 1634.3 MJ m−2 year−1 as Rn, which is about 24% of the annual Rsi. Of the total 1634.3 MJ m−2 year−1 available energy, about 25% was used for the process of evaporation (LE) and 77% for heating the air (H). The annual heat contribution from the soil to the energy budget amounted to 2% during the experimental period. Our studies showed that the total annual measured precipitation amounted to 108.0 mm year−1 during the year 2000, but the total evaporation was 167.6 mm year−1, which means some water was extracted from the shallow water table (about 60 cm on the average depth during the year 2000). Copyright © 2003 Royal Meteorological Society.

MEASUREMENT AND MODELLING OF AMMONIA EMISSIONS AT WASTE TREATMENT LAGOON-ATMOSPHERIC INTERFACE

Global emissions of ammonia are approximately 75 Tg N/yr (1 Tg = 10 12 g). The major global source is excreta from domestic animals (∼ 32 Tg N −1 yr −1 ). Waste storage and treatment lagoons are used to treat the excreta of hogs in North Carolina (NC). Proteins and nitrogen rich compounds in the lagoon are converted to ammonia, through a series of biological and chemical transformations. The process of ammonia emission has been investigated using two different model approaches: (1) Coupled Mass Transfer with Chemical Reaction Model (Model I), and (2) Mass Transport without Chemical Reaction Model (Model II). A sensitivity analysis is performed with the models, and the model results are compared with ammonia emission experiments at a swine waste storage and treatment lagoon in NC using a dynamic emission flux chamber. Results of model predictions of emission flux indicate an exponential increase in ammonia flux with increasing lagoon temperature and pH, a linear increase with increasing lagoon total ammoni- acal nitrogen (TAN), and a secondary degree increase with the increasing wind speed. In addition, the fluxes predicted by Model I are consistently larger than fluxes predicted by Model II. Experimental values of flux agreed well with model predictions, with the experimental values lying in different po- sitions between the two model predictions under different physical and chemical conditions. Further, when compared to diurnal and seasonal experimental flux values, Model I corroborates the results in calm meteorological conditions (wind speed U10 = 1.5 m s −1 ). However, the observed results are better predicted by Model II during unstable conditions, when wind speeds are higher than 2.0 m s −1