W. L. Clark

A Field Study of Reflectivity and Z-R Relations Using Vertically Pointing Radars and Disdrometers

Abstract Observations from a 16-month field study using two vertically pointing radars and a disdrometer at Wallops Island are analyzed to examine the consistency of the multi-instrument observations with respect to reflectivity and Z–R relations. The vertically pointing radars were operated at S and K bands and had a very good agreement in reflectivity at a gate centered on 175 and 177 m above ground level over a variety of storms. This agreement occurred even though the sampling volumes were of different size and even though the S band measured the reflectivity factor directly, whereas the K-band radar deduced it from attenuated K-band measurements. Indeed, the radar agreement in reflectivity at the collocated range gates was superior to that between the disdrometer and either radar. This is attributed in large part to the spatial separation of the disdrometer and radar sample volumes, although the lesser agreement observed in a prior collocated disdrometer–disdrometer comparison suggests the larger siz...

Profiler Contributions to Tropical Rainfall Measuring Mission (TRMM) Ground Validation Field Campaigns

Abstract Doppler radar profilers are widely used for routine measurement of wind, especially in the lower troposphere. The same profilers with minor modifications are useful tools for precipitation research. Specifically, the profilers are now increasingly being used to explore the structure of precipitating cloud systems and to provide calibration and validation of other instruments used in precipitation research, including scanning radars and active and passive satellite-borne sensors. A vertically directed profiler is capable of resolving the vertical structure of precipitating cloud systems that pass overhead. Standard profiler measurements include reflectivity, reflectivity-weighted Doppler velocity, and spectral width. This paper presents profiler observations of precipitating cloud systems observed during Tropical Rainfall Measuring Mission (TRMM) Ground Validation field campaigns. The observations show similarities and differences between convective systems observed in Florida; Brazil; and Kwajale...

Evaluation of Three-Beam and Four-Beam Profiler Wind Measurement Techniques Using a Five-Beam Wind Profiler and Collocated Meteorological Tower

In this paper a five-beam wind profiler and a collocated meteorological tower are used to estimate the accuracy of four-beam and three-beam wind profiler techniques in measuring horizontal components of the wind. In the traditional three-beam technique, the horizontal components of wind are derived from two orthogonal oblique beams and the vertical beam. In the less used four-beam method, the horizontal winds are found from the radial velocities measured with two orthogonal sets of opposing coplanar beams. In this paper the observations derived from the two wind profiler techniques are compared with the tower measurements using data averaged over 30 min. Results show that, while the winds measured using both methods are in overall agreement with the tower measurements, some of the horizontal components of the three-beam-derived winds are clearly spurious when compared with the tower-measured winds or the winds derived from the four oblique beams. These outliers are partially responsible for a larger 30-min, threebeam standard deviation of the profiler/tower wind speed differences (2. 2ms 1 ), as opposed to that from the four-beam method (1.2 m s 1 ). It was also found that many of these outliers were associated with periods of transition between clear air and rain, suggesting that the three-beam technique is more sensitive to small-scale variability in the vertical Doppler velocity because of its reliance on the point measurement from the vertical beam, while the four-beam method is surprisingly robust. Even after the removal of the rain data, the standard deviation of the wind speed error from the three-beam method (1.5 m s 1 ) is still much larger than that from the four-beam method. Taken together, these results suggest that the spatial variability of the vertical airflow in nonrainy periods or hydrometeor fall velocities in rainy periods makes the vertical beam velocities significantly less representative over the area across the three beams, and decreases the precision of the three-beam method. It is concluded that profilers utilizing the four-beam wind profiler technique have better reliability than wind profilers that rely on the three-beam wind profiler technique.

Monitoring the Reflectivity Calibration of a Scanning Radar Using a Profiling Radar and a Disdrometer

Abstract This paper describes a method of absolutely calibrating and routinely monitoring the reflectivity calibration from a scanning weather radar using a vertically profiling radar that has been absolutely calibrated using a collocated surface disdrometer. The three instruments have different temporal and spatial resolutions, and the concept of upscaling is used to relate the small resolution volume disdrometer observations with the large resolution volume scanning radar observations. This study uses observations collected from a surface disdrometer, two profiling radars, and the National Weather Service (NWS) Weather Surveillance Radar-1988 Doppler (WSR-88D) scanning weather radar during the Texas–Florida Underflight-phase B (TEFLUN-B) ground validation field campaign held in central Florida during August and September 1998. The statistics from the 2062 matched profiling and scanning radar observations during this 2-month period indicate that the WSR-88D radar had a reflectivity 0.7 dBZ higher than th...

Case Studies of the Vertical Velocity Seen by the Flatland Radar Compared with Indirectly Computed Values

Abstract The hypothesis that temporal averages of vertical motions over a single radar station are representative of weather systems large enough to be resolved by the radiosonde network is tested using data from the Flatland VHF radar, located in the very flat terrain of central Illinois. Six-hourly means of radar data were compared with four separate estimates of the synoptic or subsynoptic-scale vertical motions computed using the dynamical equations with unsmoothed rawinsonde data and with NMC gridded analyses. Spring and fall cases of large upward and downward vertical motions were selected for study. During the course of this study it was found that contamination of the Doppler radar spectra by heavy or moderate precipitation must be taken into account during analyses of VHF radar data in the troposphere. The signs of the vertical-motion estimates from the indirect schemes in the extreme cases selected for study here nearly always agree, although the magnitudes often differ by a factor up to about 4...

Comparisons of Horizontal Winds Measured by Opposing Beams with the Flatland ST Radar and between Flatland Measurements and NMC Analyses

Abstract This study examines the consistency between VHF horizontal wind measurements and those interpolated from routine objective analyses. First, the agreement between the two U components and between the two V components measured on opposing beams (here referred to as the beam-to-beam intercomparison) by the Flatland 49.8-MHz wind profiler is examined to determine the beam-to-beam consistency and relative precision of this radar. This part of the study demonstrates the ability of this technique to detect system problems affecting only one radar beam and provides a benchmark for comparison with radar systems operating near the Front Range of the Rockies and for the comparison in the second part of this study. This second comparison is between the Flatland observations and the spatially smooth winds from the National Meteorological Centers (NMC) regional objective analysis for July through November 1990. The location of the Flatland profiler near Champaign-Urbana, Illinois, is free of significant orogr...

Measurement of Synoptic-Scale Vertical Velocities by Two Nearby VHF Doppler Radars in Very Flat Terrain

Abstract An experimental field campaign to measure synoptic-scale vertical velocities was conducted from 5 to 11 January 1991 in the Urbana-Champaign, Illinois, region, which is in very flat terrain far from mountains. Both the Flatland and the Urbana wind-profiling radars, which are separated by 23.1 km, participated in the campaign. Meteorological sounding balloons were also launched from the Flatland Observatory site. In this study, lime averages are compared of the vertical wind velocity measured directly by both radars in order to help verify the capability of wind-profiling radars to measure synoptic-scale vertical velocities. This comparison, of course, also provides an opportunity to evaluate the performance of both radars. The variance of the vertical velocity observed by the Flatland radar has been previously shown to be dominated by short-period fluctuations with most of the variance occurring at periods less than 6 h. Also, since March 1987 when the Flatland radar began operating nearly contin...