Ground Based Approach to Direct Evaluation of Aerosol-Cloud Interaction

The quantification of the First Direct Aerosol Cloud Interaction Mechanism (ie. Twomey Effect) requires simultaneous observations of cloud water drop properties as well as aerosol properties below the cloud. The simultaneous measurement of both these properties is very difficult from space borne systems and efforts to develop ground remote sensing measurements is critical. To measure the cloud properties, we make use of an approach which combines a Microwave radiometer and a MFRSR radiometer for simultaneous Cloud Optical Depth (COD) and Liquid Water Path (LWP). From these measurements, effective droplet diameter can be estimated assuming the homogeneity of the . Unfortunately, COD measurements from the MFRSR are often underestimated due to strong forward scattering of radiation in the solar aureole region which is blocked and therefore not contained within the MFRSR diffuse measurements. In this presentation, we solve this problem by carefully calculating the fractional radiation within the shadow region as a function of cloud properties directly by suitable Gaussian Integration of the scattered downwelling radiance field over the band geometry using the Nakajima-Tanaka correction procedure. In particular, we find that for reasonable COD, significant errors occur when solar aureole contamination is not included. In addition, an AERONET sky radiometer combined with a Nd-YaG lidar will be used to estimate the aerosol loading below cloud base. While previous studies use the total column AOD, improvements are expected if some partition of the aerosols can be made. To this end, different approaches which use the AERONET SDA retrieval of fine mode aerosol and lidar data which allows us to partition the aerosol loading above and below cloud base will be used to isolate the most important correlations. Finally, efforts to include the cloud life cycle will be discussed.