Feasibility of retrieving cloud condensation nucleus properties from lidar, Doppler cloud radar, and microwave radiometer

Nearly four decades ago Twomey explored the relationship between cloud condensation nucleus (CCN) parameters, the production of supersaturation due to adiabatic expansion in a vertically moving parcel, and the number of droplets activated from the CCN phase. The resulting relationship, called an activation spectrum, is of the form N _d=CS^k (1) where N_d is the number of droplets activated, S is the maximum supersaturation produced near cloud base, and C and k are parameters related to a power-law size distribution of aerosol particles. During the course of the intervening decades, equation (1) has become a standard in CCN measurements. The determination of C and k provides a critical link between aerosol and droplet microphysics and important information for studying the aerosol indirect effects whereby aerosol particles affect incoming solar radiation by allowing droplets to condense upon them. Values of C and k are derived from measurements in thermal gradient diffusion chambers or isothermal haze chambers. These measurements are performed in-situ either at the Earth´s surface or, onboard aircraft flying in the vicinity of cloud base. Unfortunately, the in-situ measurement of CCN is limited in that it provides only a point measurement with limited spatial resolution. On the other hand, a remote measurement, if successful, could supply long term data sets with fine temporal resolution, and valuable information for climate monitoring studies. This paper explores a technique that utilizes lidar, Doppler K_α-band radar and microwave radiometer to retrieve C and k or equivalent number and size information on CCN spectra