VHF-ST Radar Observations of an Upper-Level Front Using Vertical and Oblique-Beam C2N Measurements

Abstract

Mesoscale aspects of an upper-level front that moved over Brittany (France) during the Mesoscale Frontal Dynamical Project 1987 experiment are investigated using very high frequency stratospheric?tropospheric (VHF-ST) radar data and European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. The synoptic study shows that the upper-level front moved without significant change over the radar during the observation period, from 0000 UTC 14 November 1987 to 0000 UTC 15 November 1987. This allows radar data height?time cross sections to be interpreted as representing the corresponding spatial data distributions. A radar technique of stable airmass detection using the measurement of the aspect ratio, that is, the vertical to oblique power ratio, and the turbulence, that is, the structure constant of the refractive index fluctuations C2N, is developed. This method is applied to the case study of the upper-level frontal passage, which allows the associated stable air masses above the radar site to be monitored when going from 9 km down to the lower troposphere in the 2?4-km altitude range. Mesoscale dynamical aspects of the upper-level frontal zone are described using time series of radar-derived vertical wind shear. The 0.3° slope of the upper-level front, evaluated from the radar data and a 20 m s?1 eastward propagation velocity of the large-scale baroclinic system, agrees well with the one derived from radiosounding data of the nearest synoptic stations. Along this slope, an enhancement of the turbulence and a maximum vertical wind shear of 20 m s?1 km?1 were found. At a larger scale (timescale of 6 h), the reliability of the VHF-ST radar measurements of the air vertical velocity in the vicinity of the frontal zone is supported by both a favorable cross comparison with the vertical velocity calculated from the ECMWF analyses and downward motions roughly deduced from the winds on the isentropic surfaces.