Vertical Structure of a Nonprecipitating Cold Frontal Head as Revealed by Raman Lidar and Wind Profiler Observations

Abstract

The vertical distributions of water vapor, aerosols and cloud backscattering, and three-dimensional wind were measured using a Raman lidar and a wind profiler during the passage ofa non-precipitating cold front during the night of 19-20 March 2002 over Tsukuba, Japan. The passage of the cold frontal head was identified by a sharp drop in the temperature at a height of 213 m, when the backscattering ratios sharply decreased below an altitude of approximately 2.7 km. The water vapor mixing ratios showed high values (∼2.5 g kg−1) around the frontal head. Arc clouds were observed in the watersaturated air above the frontal head in updraft regions (∼1 m s−1). The cold air mass showed the geometrical and dynamical characteristics of gravity (density) currents: nose and head identified by the lowbackscattering ratio region at the leading edge, billow-like patterns in the backscattering ratio behind the head, and feeder flows within the head (in a positive direction toward the frontal head). The observed height, and the propagation speed of the cold air mass, approximately agreed with those calculated from surface meteorological data using gravity current models, although there were uncertainties in the calculated values because of the lack of the vertical distributions of temperature and pressure data.