Lidar monitoring of troposheric clouds and precipitation for the remote sensing of aircraft in-flight icing conditions

In-flight icing has long been recognized a serious hazard to general aviation, commuter planes and helicopters. Recently, a need has been expressed for the development of ground base or on-board remote sensors of cloud and precipitation conditions most susceptible to cause icing. A coordinated experimental program, sponsored by the FAA, the NASA Glenn Research Center and the Department of Defense, was put together and carried out during the month of April 1999 at Mount Washington, NH. The aim of the program was to characterize icing conditions and test potential remote sensors. The parameters of primary importance to in-flight icing are the temperature, the phase, the liquid water content and the droplet size of clouds and precipitation. We participated with our lidar and performed systematic elevation scans and time series measurements of cloud base and precipitation. We were located along with the other remote sensors, radars and microwave radiometers, at a site 3.9 km west of the mount Washington summit and 1000 m below the summit altitude. We recorded both the parallel and the perpendicular polarization components of the returns to map out the regions of liquid and crystal phases. In the stare mode, time series measurements, we further operated at multiple fields of view to obtain additional information on droplet size. We will present elevation scan and time series results that show detailed information on the structure of the crystal and liquid phase regions, on the spatial and temporal structure of the cloud base, on the existence and angular distribution of plate crystals, and in some instances on special flow patterns. Retrieved maps of liquid water content and average droplet diameter will also be presented and discussed. Depending on the state of the coordinated data analysis, these will be correlated to other sensor measurements. The results obtained thus far demonstrate the potential of lidars for systematic remote probing of the base structure and microphysics of the clouds of the troposphere in general and, in particular, of atmospheric conditions leading to hazardous aircraft in-flight icing.