Aerosol Indirect Effect on Clouds: Climate Implications

Ground-based Remote Sensing of Indirect Effect

We are integrating satellite and ground-based remote sensors, supplemented by in situ measurements and modeling to explore the effect of aerosol on non-precipitating, ice-free clouds.

This so-called "first indirect effect" which addresses the effect of aerosols on cloud drop size and cloud albedo, is perhaps the single largest unknown in climate forcing.

The ground-based approach uses remote sensors (lidar, radar and microwave radiometer) to quantify the effect of sub-cloud aerosol extinction on the cloud droplet size in a single column of air and at a temporal resolution of ~20 s. This is done under conditions of equal liquid water path. (DOE/ARM project)

Satellite measurements of the indirect effect are based on MODIS measurements on the Terra satellite.

Modeling components include large eddy simulations with bin aerosol and cloud microphysics as well as parcel models

References

Feingold, G., W. L. Eberhard, D. E. Veron, and M. Previdi, 2003: First measurements of the Twomey aerosol indirect effect using ground-based remote sensors. Geophys. Res. Lett., in press.

Measurement of aerosol uptake of water vapor using a single wavelength backscatter lidar

This work addresses the extent to which aerosol particles have an affinity for water vapor. The size increase of aerosol particles resulting from uptake of water vapor has important implications for the direct scattering of radiation (the "direct effect"). It also has bearing on the ability of these particles to serve as cloud condensation nuclei (CCN) and, under the right circumstances, to form cloud droplets (the "indirect effect").

We use a single wavelength backscatter lidar to measure the water vapor uptake by aerosol particles in well-mixed, cloud-capped boundary layers. Under these conditions we can assume that any increase in lidar backscatter with increasing proximity to the cloud is due to water vapor uptake.

The lidar measures aerosol backscatter profiles beneath a cloud. In situ thermodynamic measurements are used to derive simultaneous profiles of relative humidity (RH) under the assumption that the boundary layer is well-mixed. The change in backscatter is derived as a function of relative humidity over the range of the surface RH and up to ~98.5 % RH.

In situ measurements of the aerosol size distribution and composition are used to calculate the expected enhancement in backscatter due to equilibrium uptake of water vapor. Comparison between lidar backscatter enhancement as a function of RH, and that derived from the in situ aerosol size distribution and composition measurements shows good agreement.

References

Feingold, G., and B. Morley, 2003: Aerosol hygroscopic properties as measured by lidar and comparison with in-situ measurements. J. Geophys. Res., in press.

Wulfmeyer, V., and G. Feingold, On the relationship between relative humidity and particle backscattering coefficient in the marine boundary layer determined with differential absorption lidar, J. Geophys. Res., 105, 4729--4741, 2000.