Operational Systems for Observing the Lower Atmosphere: Importance of Data Sampling and Archival Procedures

Abstract

A brief field project was conducted during July 1988 to assess the potential for Next Generation Weather Radar (NEXRAD), 404-MHz radar wind profilers, and digital sounding systems to monitor the low-level wind field during clear-air conditions. The low-level jet was chosen as the phenomenon of interest because it is neither well sampled nor resolved by the current upper-air network, yet it is a common feature of mesoscale convective system and severe thunderstorm environments. Data were collected under quiescent synoptic conditions during several low-level jet events using a 10-cm NEXRAD-like Doppler radar and a digital sounding system colocated in Norman, Oklahoma. These data suggest that the areal-averaged horizontal winds calculated from the Doppler radar data using the Velocity Azimuth Display (VAD) technique are comparable with the winds observed using a digital sounding system, except under weak wind conditions. However, the vertical spacing of 304 m (1000 ft) between levels of horizontal VAD calculated winds, as currently proposed for NEXRAD, may not be of sufficient resolution to document the detailed wind structure of these events. The height of the maximum wind speed of the low-level jet on all days studied was below the planned lowest observation range gate of the 404-MHz radar wind profiler, indicating that a combination of NEXRAD and profiler data might be needed to sample the important wind field structure of the lower atmosphere. Lastly, the National Weather Service rawinsonde data processing software affects the vertical resolution of the low-level wind field in operational, and therefore archived, upper-air soundings. The procedure used to calculate NWS 1000 ft winds actually damps the wind speed profile and artificially increases the height of the level of maximum wind speed associated with the low-level jet. The appropriateness of these highly smoothed 1000 ft winds for input into sophisticated mesoscale weather prediction models should be considered.