In Situ Lidar for Cloud and Aerosol Radiation Sciences

The largest error source in validation of NASA's EOS satellite cloud products is the mismatch in sampling volumes between the remote sensors and the in situ cloud probes. The sampling error from the extrapolation of the in situ cloud probe measurement of a few cm3 to the remote sensing measurement of millions of m3 is very large because of the extreme variability of cloud density. The new in situ lidar built in Phase I and tested in cloud on a mountaintop measures the volumetric extinction coefficient in cloud volumes ranging from millions to billions of m3. The in situ lidar differs from a regular lidar in that inside an optically thick cloud the laser pulse is multiply scattered by cloud droplets and the signal measured by the detector is the number of photons returned as a function of time. The amplitude and shape of the returned pulse contain information about the cloud volumetric extinction coefficient at different spatial scales around the instrument and the distance to the cloud boundaries. Phase I proof-of-concept tests conducted in cloud on a mountaintop demonstrated that the time-varying signal measured by the in situ lidar fit exactly the shape of the signal predicted by a physical model. The volumetric extinction coefficients measured over a period of several hours by the in situ lidar agreed in magnitude with local measurements using a cloud droplet probe, although as expected, there was considerable variability in the measurements. In Phase II we will build an airborne in situ lidar and install it on a Learjet research aircraft. The in situ lidar will be compared with microphysical measurements of cloud properties and cloud boundaries collected by the Learjet in various types of clouds.