Cloud Boundary Statistics during FIRE II

An 8-mm wavelength radar, 3-mm wavelength radar, and 10.6-µm wavelength lidar operated side by side in vertically pointing mode during the First ISCCP Regional Experiment (FIRE II). This data collection mode yielded detailed information on distribution of cloud and cloud boundaries as a function of altitude. Statistics on the location of cloud boundaries during the FIRE II experiment indicate that cloud bases tended to form at two discrete levels centered around 2.5 and 7.5 km, cirrus cloud tops formed most frequently at 9.5 km and cloud thickness were usually 2 km or less. The atmosphere had the highest incidence of cloudiness at 8.5 km AGL, with a secondary maximum at an altitude of 3.5 km AGL. The incidence of cloudiness fell off rapidly between 8 and 11 km; there was also a distinct minimum in cloudiness at 2 km AGL. The diurnal variation of upper-level cloud base and top heights was about 1.0 km AGL, with the highest bases and tops occurring at 0500 UTC and the lowest bases and tops occurring at 1500 UTC. Co-occurring cloud layers (two or more simultaneous layers) were common, with the condition of a single cloud layer accounting for only 40% of the observation period.