Intermediate Wind Speeds Research Articles

Meteorological controls of gas exchange at a small English lake

The relationships between gas transfer velocity, k600, wind speed, wind direction, rainfall, and relative humidity were examined using measurements of SF6 evasion from Coatenhill Reservoir, a small (0.017 km2), shallow (1.9 ± 0.1 m), man‐made lake in northeast England characterized by predominantly low to intermediate wind speeds ~1–10 m s-1. A graphical method was used to estimate the wind speed at a standard height of 10 m, U10, from wind speed measurements at 2 m and 3.8 m. Derived values of U10 normalized to remove thermal stability effects (U10‐n) were significantly correlated with k600 (R2 = 0.71, p < 0.001). A detailed analysis of surface roughness lengths estimated with this procedure showed the k600 versus U10‐n relationship to be sensitive to wind speed modification by the meteorological mast and sensors. With normalization to remove these effects, the correlation between k600 and U10‐n substantially improved (R2 = 0.86, p < 0.001). In contrast to previous laboratory findings, relative humidity was not significantly correlated with k600 and rainfall rate (Rn) was only weakly correlated with k600, possibly as a consequence of the effects of these variables being largely masked by the time scales of data averaging and SF6 sampling. Similarly, k600 was not correlated with wind direction (i.e., fetch). An empirical gas exchange model accounted for 88% of the total variance in k600 (p = 0.01) at Coatenhill, with U10‐n and Rn accounting for 86% and 2%, respectively. Future field investigations of the meteorological controls of k600 will require careful experimental design to allow for more detailed sampling than hitherto has been possible.

An updated analysis of the ocean surface wind direction signal in passive microwave brightness temperatures

We analyze the wind direction signal for vertically (v) and horizontally (h) polarized microwave radiation at 37 GHz, 19 GHz, and 11 GHz; and an Earth incidence angle of 53/spl deg/. We use brightness temperatures from SSM/I and TMI and wind vectors from buoys and the QUIKSCAT scatterometer. The wind vectors are space and time collocated with the radiometer measurements. Water vapor, cloud water and sea surface temperature are obtained from independent measurements and are uncorrelated with the wind direction. We find a wind direction signal that is noticeably smaller at low and moderate wind speeds than a previous analysis had indicated. We attribute the discrepancy to errors in the atmospheric parameters that were present in the data set of the earlier study. We show that the polarization combination 2v-h is almost insensitive to atmospheric changes and agrees with the earlier results. The strength of our new signals agrees well with JPL aircraft radiometer measurements. It is significantly smaller than the prediction of the two-scale sea surface emission model for low and intermediate wind speeds.

Impingement of rain drops on a tall building

Soiling on the walls of limestone buildings can be washed off when the surface erodes due to rain impingement. In this study, the delivery of rain to the 42-story Cathedral of Learning in Pittsburgh, Pennsylvania, represented by a 30 m×30 m×160 m rectangular block, was modeled using the RNG K– ε model for turbulence and Lagrangian trajectory calculations for individual rain drops. Local Effect Factors (LEF) for the rectangular block compared well with earlier work in the literature. LEFs increased with wind speed, raindrop size, and height along the block. Wind speed, direction, and rain intensity were measured continuously over a seven-week period and provided input parameters for modeling rain fluxes to the Cathedral of Learning. Model results suggested that sections of the building receiving larger amounts of rain corresponded to white areas, indicating that rain fluxes have a significant effect on the soiling patterns. Intermediate wind speeds (2.5 and 5 m s −1) resulted in high rain fluxes. Although less frequent, high wind speeds also resulted in high rain fluxes. Much of the rain was delivered to the block as 1.25 and 2.5 mm drops with 5 mm drops having a smaller effect. Consideration of wind incidence angles other than 0° was shown to be important for future modeling efforts.

Wind speed dependence of acoustic backscattering

The results from acoustic scattering measurements conducted in the North Sea during the Synthetic Aperture Radar and X Band Ocean Nonlinearities (SAXON)‐Forschungsplattform Nordsee (FPN) experiment are presented. Scattering strength values were obtained for selected frequencies (5–70 kHz) and for wind speeds up to 20 m/s. The grazing angle dependence was measured for 50 kHz at three different sea state conditions. Two grazing angle regimes were identified (above and below 60°) where the wind speed dependence of acoustic backscattering shows a different behavior. One grazing angle from each regime (normal incidence and 45°) was investigated to explain the physical processes that dominate the backscattering in the different wind speed regimes. In the low grazing angle regime the backscattering strength is caused by the small‐scale surface roughness at low wind speeds and by subsurface bubbles at high wind speeds. There is only weak dependence of backscattering in the intermediate wind speed regime. Scattering and absorption of a random swarm of interacting bubbles inside the propagation range lead to undefined acoustic intensities in front of the surface. Thus scattering from a boundary layer instead of sea surface backscattering has to be considered. The presented acoustic data set is a base for further correlation with radar measurements that were conducted simultaneously. Good correlation between electromagnetic and acoustic backscattering is expected for the wind speed regime in which the scattering mechanisms are the same.

In situ measurements of capillary‐gravity wave spectra using a scanning laser slope gauge and microwave radars

Capillary‐gravity wave spectra are measured using a scanning laser slope gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanographic Institution at Woods Hole, Massachusetts. Wave spectral densities estimated from the radar measurements using the Bragg theory agree with those measured using the SLSG at the Bragg wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of capillary‐gravity waves is in reasonable agreement with measurements by Jähne and Riemer (1990) obtained from measurements in a large wind‐wave flume at intermediate wind speeds, but our data indicate a higher degree of saturation at very low wind speeds. The rate at which the slope‐frequency spectrum falls off, however, is much lower in the field than in laboratories, even at moderate winds, suggesting long waves are responsible for a large Doppler shift of capillary‐gravity waves. Close examination of combined wavenumber‐frequency slope spectra also reveals significant smearing of the spectra in the frequency domain due to long waves. These observations confirm that spatial measurements (wavenumber spectra measurements) are essential for characterizing short capillary‐gravity waves, since this strong Doppler shift will dramatically change apparent frequency spectra.

Prediction of Raingun Application Patterns in Windy Conditions

A semi-empirical model has been developed for predicting the distortion by wind of raingun application patterns. The model can be calibrated from three full-circle wetting pattern field tests and used to predict wetting patterns at any intermediate wind speed and for any sector angle, thus providing data for raingun evaluation. For large sprinklers and rainguns, the water jet induces flow of the surrounding air mass, reducing air resistance and enhancing throw. The model assumes that the distortion of the irrigation pattern by wind is due to the combination of wind drift and range shortening due to disruption of the induced air flow, and that the disruption of the air flow is proportional to the component of the wind velocity perpendicular to the jet near the nozzle. The model was tested against 18 other measured patterns and, except in one case, achieved a reasonable degree of fit with standard deviations of the errors between 1·4 and 3·4 mm/h compared to peak application rates between 23·8 and 30·6 mm/h and a typical standard deviation in the field measurements of 1·55 mm/h due to non-symmetry.

Relationship between gas exchange, wind speed, and radar backscatter in a large wind‐wave tank

Gas transfer velocities across the air‐water interface are correlated with wind speed, friction velocity, and radar backscatter from the surface in a large wind‐wave tank in Delft, the Netherlands. The rates of sulfur hexafluoride and nitrous oxide exchange were measured at wind speeds ranging from 3.5 m/s to 20 m/s, and with mechanically generated waves in the 100 m long, 8 m wide, and 0.7 m deep tank. Gas transfer velocities were related to wind speed with a power law dependence. Gas transfer showed a linear dependence with friction velocity. An exponential relationship between gas transfer and average radar backscatter cross section fits the wind tunnel data well for microwave units operating at 13.5 and 35 GHz in VV and HH polarization. The relative rates of gas exchange of the two gases confirm an inverse square root dependence on the Schmidt number at intermediate wind speeds. A slight enhancement of sulfur hexafluoride exchange compared to nitrous oxide exchange was evident when breaking waves were present.

Estimating Estuarine Reaeration Rates

Estuarine reaeration rates induced by wind and bottom stresses are compared through calculation of air-water-mass-transfer coefficients. For typical tidal velocities and depths, the transfer coefficient (and hence reaeration) induced by wind speed greater than 5 m/s exceeds the transfer coefficient generated by bottom stress. For wind speed less than 1.5 m/s, wind influence on the transfer coefficient is negligible relative to bottom stress. For intermediate wind speeds, the transfer coefficient may be dominated by wind or by bottom stress.

Wind dispersal of conidia of Botrytis spp. pathogenic to Vicia faba

Increasing numbers of conidia were blown from sporulating cultures of Botrytis fabae and B. cinerea as windspeed increased up to 10 m/s. Partial drying of cultures increased the number of spores blown away at low and intermediate windspeeds. Release of spores at a constant windspeed was sustained over a prolonged period. Different patterns of release from colonies of the two species when windspeed was gradually increased, or at a constant windspeed. may be related to differences in spore size affecting the drying rate. Many conidia of both species were released as clumps. A higher proportion of B. cinerea than B. fabae conidia were clumped, partly because the mean number of spores per clump was greater. Individual conidia fell more slowly in still air than did clumps. The humidity in a bean crop was more favourable to development of Botrytis lesions than that above the crop. Low windspeeds measured within crops may restrict dispersal of conidia and may result in uneven distribution of chocolate spot lesions.

Flue gas discharge from cooling towers. Wind tunnel investigation of building downwash effects on ground-level concentrations

German power plants are required to meet new emission standards which limit the maximum sulfur dioxide (SOs) concentration in flue gas discharges to 400 mg m −3. To achieve this level of reduction in SO 2 concentration, wet scrubbing is necessary for large plants using lignite or hard coal. Wet scrubbing results in a significant reduction in the flue gas temperature leading to low effective stack heights. Instead of using stack gas reheating to achieve the plume rise necessary to satisfy local environmental standards, it was proposed to discharge the scrubbed flue gas from the existing natural-draft cooling towers (NDCT). This method should be effective in reducing local ground-level concentrations since NDCT-plumes are typically very buoyant (densimetric Froude number below 1 ) and normally reach considerable heights of rise. Only under strong wind conditions does the situation reverse itself. For such strong winds, the NDCT-plume is subject to tower and building downwash with the possibility of unacceptably high ground-level concentrations. For a 2700 MW e lignite-fired power plant near Cologne, a wind tunnel study was carried out to investigate the effects of tower and building downwash effects on the ground-level concentrations of SO 2 produced by discharging the scrubbed flue gas from the natural-draft cooling towers. Also, a comparison was made between the ground-level concentrations produced by the cooling tower discharge method and those produced by a traditional stack. It was found that for low and intermediate wind speeds, the groundlevel concentrations are lower for the case of the cooling tower discharge. Only for strong winds, which occur only very rarely at most German sites, did the conventional stack discharge appear to be superior.