Abstract
Using a UV Raman Lidar for aerosol extinction, and combining Microwave Radiometer derived Liquid Water Path (LWP) with Multifilter Rotating Shadowband Radiometer derived Cloud Optical depth, to get cloud effective radius (R eff ), we observe under certain specialized conditions, clear signatures of the Twomey Aerosol Indirect effect on cloud droplet properties which are consistent with the theoretical bounds. We also show that the measurement is very sensitive to how far the aerosol layer is from the cloud base and demonstrate that surface PM25 is far less useful. Measurements from both the DOE ARM site and new results at CCNY are presented.
Highlights
One of the outstanding issues regarding the earth’s energy balance and resultant forcing on the climate are the indirect effects that aerosols have on cloud radiative properties
The relationship between optical depth and particle number depends on the microphysical distribution models and does not discriminate between aerosol types below cloud base that impact cloud micro-physics and aerosol particles located higher than cloud top, that only interact with the radiative transfer above clouds
We show the relationship between the effective radius and aerosol extinction where we consider both the surface extinction as well as the extinction 150 m below the cloud base
Summary
One of the outstanding issues regarding the earth’s energy balance and resultant forcing on the climate are the indirect effects that aerosols have on cloud radiative properties. Unlike direct cloud effects on the climate, the aerosol-cloud indirect interactions are very hard to measure from satellites directly since simultaneous aerosol loadings (especially below cloud base) and cloud properties such as COD and/or Reff are not possible and efforts at quantifying these effects are limited to statistical trend studied over large domains. While such statistical studies have provided some useful information, poor temporal statistics makes it difficult to establish underlying causal mechanisms. Within the measurement cycle, the exact nature of the cloud within the cloud lifecycle including general meso-scale processes is less important for these short term inter-cloud measurements
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