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  • Author or Editor: Wayne M. Angevine x
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Wayne M. Angevine

Abstract

The accuracy of vertical velocities measured by UHF wind-profiling radars has been a matter of discussion for some time. This paper shows that there are significant errors in mean vertical velocities measured by the vertical beam of 915-MHz wind profilers. The erroneous velocities are 0.1–0.3 m s−1 downward in daytime convective boundary layers over two sites, flat farmland in Illinois and rolling forest in Wisconsin. Velocities at night are not affected, and different days have different erroneous velocities. The directly measured velocities are compared to vertical velocities calculated from the divergence of the horizontal wind to show that they are indeed in error. The erroneous velocities are not caused by detectable rain, by an error in the beam pointing direction, or by the skewness of the vertical velocity distribution. They are probably due to small targets (particulate scatterers) that have a small fall velocity and are detected by the radar. An online algorithm for removing intermittent contamination reduces the error, but does not eliminate it. The fluctuating component of the velocity is not affected by these errors since it is much larger in magnitude.

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Alison W. Grimsdell
and
Wayne M. Angevine

Abstract

The depth of the atmospheric boundary layer is of interest in several different areas, such as chemistry, pollutant studies, and global modeling. In this research the authors describe and compare several different measurements of boundary layer depth. First, the authors use the standard measurement from radiosondes to confirm the validity of wind-profiler measurements, which use humidity gradients to estimate the boundary layer depth. A method for obtaining meaningful cloud-base altitudes is then presented, and the results are compared to the wind-profiler boundary layer heights. The authors find good agreement between the different types of measurement but see that the profiler peak reflectivity is slightly raised above cloud base in the presence of boundary layer clouds. This may be due to increased humidity gradients at the top and edges of clouds or to increased turbulence within the cloud. Calculation of the boundary layer height using the bulk Richardson number is commonly used in computer models. Comparison with the authors’ profiler measurements indicates that the calculation overestimates the height of the boundary layer and that the agreement between the methods is poor.

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Wayne M. Angevine
and
W. L. Ecklund

Abstract

With the use of simultaneous correction for radial wind, the accuracy of radio acoustic sounding systems for the measurement of temperature has been substantially improved. The temperature accuracy can now be affected by a number of factors that have been considered negligible in previous work. This paper describes two types of errors, those due to atmospheric effects and those due to approximations in the temperature retrieval equation. The errors are examined in a set of convective boundary layer RASS and radiosonde data. In the category of atmospheric effects, two errors are computed. The first is caused by a range error due to the gradient of signal strength. This range error is newly proposed and is approximately 0.05°−0.1°C. The second is an error due to wind and turbulence of about 0.1°C. Commonly used approximations for factors in the retrieval equation contribute errors of a few tenths of a degree Celsius. A significant difference remains after these two corrections have been applied to the sample data.

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Wayne M. Angevine
and
J. Ian Macpherson

Abstract

In August 1993, a 915-MHz boundary layer wind-profiling radar was deployed at Chebogue Point, Nova Scotia, to provide wind, turbulence, and boundary layer structure information for the North Atlantic Regional Experiment summer 1993 intensive campaign. The National Research Council Canada Twin Otter atmospheric research aircraft was also part of that campaign. During the campaign, the Twin Otter flew 29 soundings over Chebogue Point. This paper describes a comparison of the wind speed and direction measured by the profiler and the aircraft. In the height range 300–2000 m above sea level, the random difference between the wind speed measurements is 0.9 m s−1, and the random difference between the wind direction measurements is 9°. There is a small systematic difference in the wind speeds (0.14 m s−1) that is probably due to uncertainty in the zenith angles of the radar beams and extremely good agreement (within 0.5°) in the wind direction. The Kalman filter-smoother technique used to remove drifts in the inertial navigation system is shown to be important in achieving these favorable results.

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Wayne M. Angevine
,
Peter S. Bakwin
, and
Kenneth J. Davis

Abstract

A 915-MHz boundary layer wind profiler with radio acoustic sounding system (RASS) was sited 8 km from a very tall (450 m) television transmitting tower in north-central Wisconsin during the spring, summer, and autumn of 1995. The profiler measured wind means and variances, and the RASS attachment measured virtual temperature. These quantities are compared to measurements from cup and sonic anemometers and a thermometer/hygrometer at 396 m above ground level on the tower. The precision of hour-averaged profiler winds is better than 1 m s−1, and the precision of the RASS virtual temperature is better than 0.9 K. Corrections to the virtual temperature measured by the RASS are discussed, and a new virtual temperature retrieval method is proposed. Vertical velocity variance correlation is similar to a previous study, and the fact that bias is small indicates that the calculation method used is reliable.

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Wayne M. Angevine
,
W. L. Ecklund
,
D. A. Carter
,
K. S. Gage
, and
K. P. Moran

Abstract

Improved radio acoustic sounding system (RASS) technology for use with radar wind profilers has been developed and applied to 915-MHz and 50-MHz profilers. The most important advance is the simultaneous measurement of the wind velocity to correct the acoustic velocity measurement for air motion. This eliminates the primary source of error in previous RASS measurements, especially on short time scales. Another improvement is the use of an acoustic source that is controlled by the same computer that controls the radar. The source can be programmed to produce either a swept frequency or a random hopped frequency signal. Optimum choices of the acoustic source parameters are explored for particular applications. Simultaneous measurement of acoustic and wind velocity enables the calculation of heat flux by eddy correlation. Preliminary heat flux measurements are presented and discussed. Results of the use of RASS with oblique beams are also reported.

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