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 continuously, the vertical velocity measurements showed a nearly constant downward mean value of several centimeters per second in the troposphere. After bandpass filtering, the time-series measurements of vertical velocity to obtain 6-b and 1-day means, the filtered signal is compared to similar measurements made by the newly constructed Urbana radar. Both the 6-b and 1-day time averages of vertical velocity measured by the radars displayed large variations in time and height. Variations of 1.0?1.5 cm s?1 occurred frequently, which are considerably larger than the expected measurement error. Good to excellent agreement is generally found in the shape of height profiles measured by the two radars. These results suggest that wind-profiling radars located in very flat terrain are capable of measuring synoptic-scale vertical velocity profiles with useful precision.