Measuring the drizzle drop size spectrum using lidars

Stratus and stratocumulus are well known to exert a strong influence on the earth's radiation budget, and drizzle is believed to be an important component of this (Wood 2008). However, measuring drizzle rates is very difficult: aircraft observations suffer due to small sample volumes, while remote sensing using radar alone has large uncertainties. I have developed a new technique to measure drizzle rates, using lidar measurements at two different wavelengths (0.9 and 1.5 microns). This takes advantage of the appreciable absorption of light within drizzle drops at 1.5 microns, whilst absorption at 0.9 microns is negligible. This differential absorption increases exponentially with drop size, and can be used to estimate the average drop size in the distribution; from this + the backscatter profile, the drizzle rate and other moments can be derived, and we can then infer the corresponding fluxes and cooling rates below cloud base. See paper for details. Here's an example retrieval for a drizzling stratus layer:

These retrievals have been used by Steve Abel and Ian Boutle to improve the representation of warm rain drop size spectra in the Met Office operational NWP and climate models: see their paper for more info.