The 27–28 October 1986 FIRE IFO Cirrus Case Study: Cloud Optical Properties Determined by High Spectral Resolution Lidar

Abstract

During the FIRE cirrus IFO, the High Spectral Resolution Lidar (HSRL) was operated from a roof top site on the University of Wisconsin?Madison campus. Because the HSRL technique separately measures the molecular and cloud particle backscatter components of the lidar return, the optical thickness is determined independent of particle backscatter. This is accomplished by comparing the known molecular density distribution to the observed decrease in molecular backscatter signal with altitude. The particle to molecular backscatter ratio yields calibrated measurements of backscatter cross section that can be plotted to reveal cloud morphology without distortion due to attenuation. Changes in cloud particle size shape and phase affect the backscatter to extinction ratio (backscatter-phase function). The HSRL independently measures cloud particle backscatter phase function. This paper presents a quantitative analysis of the HSRL cirrus cloud data acquired over an ?33 hour period of continuous near-zenith observations. Correlations between small-scale wind structure and cirrus cloud morphology have been observed. These correlations can bias the mite averaging inherent in wind profiling lidars of modest vertical resolution, leading to increased measurement errors at cirrus altitudes. Extended periods of low intensity backscatter were noted between more strongly organized cirrus cloud activity. Optical thicknesses ranging from 0.01?1.4, backscatter-phase functions between 0.02?0.065 sr?1, and backscatter cross sections spanning 4 orders of magnitude were observed. The altitude relationship between cloud top and bottom boundaries and the cloud optical center altitude was dependent on the type of formation observed. Cirrus features were observed with characteristic wind drift estimated horizontal sizes of 5 km?400 km. The clouds frequently exhibited cellular structure with vertical to horizontal dimension ratios of 1:5?1:1.