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R. L. Coulter and M. L. Wesely

Abstract

Measurements of acoustic backscatter in the lower planetary boundary layer and optical line-of-sight scintillation in the surface layer are each used to compute sensible heat fluxes in the unstable surface layer. Comparisons with simultaneous low-level point measurements by eddy correlation show good agreement, indicating that remote-sensing methods can be successful over less homogeneous terrain where conventional surface layer measurement techniques are less accurate. Corrections to take into account the effects of humidity fluctuations are found necessary in order to achieve accuracies within 10%. Free convection is assumed to permit interpretation of the sodar data, while either forced or free convection is assumed for the scintillation data. A systematic overestimate of heat fluxes is found from sodar measurements made during the morning, when the height of the convectively mixed layer is increasing rapidly.

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R. L. Coulter and K. H. Underwood

Abstract

Temperature and velocity fluctuations within a cooling tower plume in stable conditions at the Keystone power plant in Pennsylvania have been measured by use of a calibrated sodar. Monostatic and bistatic systems probed the plume at several positions 40 to 300 m downwind of the cooling tower. Comparison of the sodar estimates of the temperature and velocity structure parameters (CT2 and CV2) with those derived from measurements taken by aircraft at the same location shows acceptable agreement. Alternate methods of averaging profiles of CT2 through the plume are used to investigate single and relative dispersion coefficients. Both methods describe a linear increase of plume width with distance from the cooling tower. Combining values of temperature and velocity structure parameters leads to estimates of turbulence kinetic energy dissipation rate ε near 0.3 m2 s−3 and temperature fluctuation destruction rate N of 0.01–0.21 K2 s−1. The decrease in CT2 and CV2 is found to be exponential with horizontal distance from the tower; CT2 decreases more rapidly than CV2. Time-lapse photography used simultaneously with the sodar measurements indicates that the thermal turbulence plume is larger than the condensation plume by a factor of 2–5.

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R. L. Coulter, Monte Orgill, and William Porch

Abstract

Measurements in a tributary to Brush Creek Valley during the September and October 1984 ASCOT campaign with laser anemometers, tethersondes, a minisodar, and smoke release were used to calculate the contribution by tributaries to nocturnal drainage flow from the main valley. Four experimental nights with different mesoscale wind regimes were used in the study. It was found that a simple picture of mass flux proportional to drainage area is not sufficient to predict the relative contributions of drainage basins. The exposure of the slopes within the tributaries to the external wind regime was found to be a significant factor in the contribution of the upper regions of the tributary; but drainage from the well-protected lower region was found to be stronger when the external wind direction was opposite to the drainage direction than when the external wind was along the drainage direction. A circulation cell that introduces mass into the tributary system both from the main canyon flow and from the side opposite the main canyon flow is proposed to explain this effect.

Data from the morning hours after flow reversal has occurred in the main canyon show continued drainage in the tributary for up to two hours. The data are used to show that this is probably due to continued radiational cooling of the protected sidewalls in the tributary.

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M. S. Pekour and R. L. Coulter

Abstract

A method is described and evaluated for decreasing artifacts in radar wind profiler data resulting from overflying, migrating birds. The method processes the prerecorded, averaged spectral data of a wind profiler to derive hourly wind profiles during conditions of frequent backscattering from birds. Comparison with in situ measurements revealed a significant improvement over the “traditional,” online processing routine. When both the traditional method and the proposed new method are applied to an extended dataset, a practical procedure can be implemented to detect periods with significant bird-caused artifacts.

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R. Boers, E. W. Eloranta, and R. L. Coulter

Abstract

Ground based lidar measurements of the atmospheric mixed layer depth, the entrainment zone depth and the wind speed and wind direction were used to test various parameterized entrainment models of mixed layer growth rate. Six case studies under clear air convective conditions over flat terrain in central Illinois are presented. It is shown that surface heating alone accounts for a major portion of the rise of the mixed layer on all days. A new set of entrainment model constants was determined which optimized height predictions for the dataset. Under convective conditions, the shape of the mixed layer height prediction curves closely resembled the observed shapes. Under conditions when significant wind shear was present, the shape of the height prediction curve departed from the data suggesting deficiencies in the parameterization of shear production. Development of small cumulus clouds on top of the layer is shown to affect mixed layer depths in the afternoon growth phase.

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D. W. Thomson, R. L. Coulter, and Z. Warhaft

Abstract

Simultaneous measurements of C T2 and C V2 were made using a calibrated Doppler sodar and a research aircraft equipped with meteorological and turbulence sensors. In each experiment a region of specific interest was identified using the sodar and then the aircraft vectored into it using air-ground radio. Measurements were made in both “layers” (with and without detectable turbulence and “waves”) and in convective plumes. In each case the spectra of turbulent temperature and velocity fluctuations derived from the in situ observations showed a well-developed inertial subrange. Excellent agreement was found between the magnitude of the in situ aircraft C T2 and C V2 values and those derived from the sodar signals interpreted using the Tatarski scattering theory.

Examples are shown of how sodar may be used for real-time, quantitative estimates of the dissipation rate of turbulent kinetic energy ε, and the rate of destruction of temperature variance N. On the present Penn State sodar system the operator may select a display of C T2, C V2, ε, N or winds derived from signal Doppler shifts. Either time series at selected heights, vertical time sections on a color, digital video display, or conventional printed or graphical output may be produced.

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R. L. Coulter, T. J. Martin, and T. M. Weckwerth

Abstract

Measurements of raindrop fall velocity spectra have been made with a minisodar. Amplitude calibration of the system enables the calculation of drop size parameters such as number density, water density, and surface area using methods similar to those with Doppler radar studies. The acoustic measurements are at 10 m intervals within 200 m of the surface and benefit from an almost complete separation of droplet velocity spectra from atmospheric vertical velocity spectra. Comparison of parameters with those reported in the literature shows good agreement. It is shown that the chief difficulty with the method is atmospheric attenuation; however, excess attenuation due to scattering from droplets is found to be unimportant.

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J. Song, M. L. Wesely, R. L. Coulter, and E. A. Brandes

Abstract

A model framework for parameterized subgrid-scale surface fluxes (PASS) has been modified and applied as PASS1 to use satellite data, models, and limited surface observations to infer root-zone available moisture (RAM) content with high spatial resolution over large terrestrial areas. Data collected during the 1997 Cooperative Atmosphere–Surface Exchange Study field campaign at the Atmospheric Boundary Layer Experiments site in the Walnut River watershed in Kansas were used to evaluate applications of the PASS1 approach to infer soil moisture content at times of satellite overpasses during cloudless conditions. Data from Advanced Very High Resolution Radiometers on the NOAA-14 satellite were collected and then adjusted for atmospheric effects by using LOWTRAN7 and local atmospheric profile data from radiosondes. The input variables for PASS1 consisted of normalized difference vegetation index and surface radiant temperature, together with representative observations of downwelling solar irradiance, air temperature, relative humidity, and wind speed. Surface parameters, including roughness length, albedo, surface conductance for water vapor, and the ratio of soil heat flux to net radiation, were estimated with parameterizations suitable for the area using satellite data and land-use information;pixel-specific near-surface meteorological conditions such as air temperature, vapor pressure, and wind speed were adjusted according to local surface forcing; and RAM content was estimated using surface energy balance and aerodynamic methods. Comparisons with radar cumulative precipitation observations and in situ soil moisture estimates indicated that the spatial and temporal variations of RAM at the times of satellite overpasses were simulated reasonably well by PASS1.

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W. Gao, R. L. Coulter, B. M. Lesht, J. Qiu, and M. L. Wesely

Abstract

The authors compared methods for estimating surface fluxes under clear-sky conditions over a large heterogeneous area from a limited number of ground measurements and from satellite observations using data obtained from the southern Great Plains Cloud and Radiation Testbed (CART) site, an area of approximately 350 km × 400 km located in Kansas and Oklahoma. In situ measurements from 10 energy balance Bowen ratio (EBBR) stations showed large spatial variations in latent and sensible heat fluxes across the site because of differences in vegetation and soil conditions. This variation was reproduced by a model for parameterization of subgrid- scale (PASS) surface fluxes that was developed previously and extended in the present study to include a distribution of soil moisture inferred from combined visible and thermal infrared remote sensing data. In the framework of the PASS model, the satellite-derived normalized difference vegetation index and surface temperature were used to derive essential surface parameters including surface albedo, surface conductance, soil heat flux ratio, surface roughness length, and soil moisture, which were then used to calculate a surface energy budget at satellite-pixel scales with pixel-specific surface meteorological conditions appropriately distributed from their mean values using a distribution algorithm. Although the derived soil moisture may be influenced by various uncertainty factors involved in the satellite data and the model, spatial variations of soil moisture derived from the multichannel data from the Advanced Very High Resolution Radiometers on the NOAA-14 satellite appeared to have some correlation (the correlation coefficient is as large as 0.6) with the amount of accumulated previous rainfall measured at the 58 Oklahoma Mesonet stations located within the CART area. Surface net radiation, soil heat flux, and latent and sensible heat fluxes calculated at a spatial resolution of 1 km (the size of a satellite pixel) were evaluated directly by comparing with flux measurements from the EBBR stations and indirectly by comparing air temperature and humidity inferred from calculated sensible and latent heat fluxes with corresponding values measured at 1.5 m above the 58 meteorological stations. In calculating regional fluxes, biases caused by the sampling uncertainty associated with point measurements may be corrected by application of the satellite data.

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R. A. Ferrare, J. L. Schols, E. W. Eloranta, and R. Coulter

Abstract

Lidar observations of clear-air convection during the 1983 Boundary Layer Experiment (BLX83) reveal the presence of elongated, parallel regions of updrafts marked by enhanced aerosol backscattering. These linear (banded) aerosol structures were observed over a two-hour period during a cloud-free morning. During this period, the depth of the convective boundary layer (CBL) increased from 100 to 1300 m. Wind speeds averaged over the depth of the CBL varied between 0 and 2 m s−1, while the wind direction varied over a range of 160 deg. The CBL instability parameter, −Zi/L, increased from approximately 25 (weakly unstable) to 250 (strongly unstable). The spacings of the elongated, parallel plumes scaled with the CBL height. These findings suggest that secondary circulations in the form of horizontal roll vortices were present under conditions not normally associated with roll vortices. The lines of aerosol structures aligned much more closely (within 15 deg) with the direction of the vertical shear of the horizontal wind through the depth of the CBL than with either the surface wind, mean CBL wind, or the wind at an altitude of 1.1Zi.

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