Towards retrieving CCN from satellites by using clouds as CCN chambers

The inability to use satellite retrievals for separating aerosols (CCN) and meteorology (updrafts) effects on cloud properties and radiative effects has been the main cause for the great uncertainty incurred by cloud aerosol interactions on climate forcing. A new conceptual approach to resolve this challenge is presented here. It is based on satellite retrievals of convective cloud base updrafts and drop concentrations in the fair weather convective clouds that grow from the boundary layer. This allows us to use these clouds as CCN chambers. CCN chambers measure the supersaturation (S) and number of activated aerosols into cloud drops (Na). Therefore, clouds can provide CCN(S) along the following steps:

1. Na is retrieved from the shape of satellite retrieved T-re (cloud top temperature – drop effective radius) relationships and satellite retrieved cloud base temperature, Tb.

2. S is calculated from the knowledge of Na and Wb (Cloud base updraft). Wb is retrieved from the difference between surface skin and air temperatures and the wind speed.

We have developed methodologies for satellite retrieval of all the necessary components: Tb, Na and Wb, and validated them against instrument measurements at the DOE/SGP site in Oklahoma. We applied these retrieved properties to retrieve CCN(S) and validated against CCN instrument measurements. It is the first time ever that clouds are used successfully as CCN chambers for measuring CCN from space. This became possible with the advent of the Imager of the NPP/VIIRS with 375 m resolution, which allowed us to retrieve cloud microstructure at this resolution. This allows resolving the small boundary layer clouds and using them as presented above.

This study builds on several other studies, each resulting in at least one publication, as numbered in the reference list, for: (1) using aircraft-measured T-re for retrieving Na; (2) satellite retrieval of cloud microstructure using NPP/VIIRS and constructing high resolution T-re; (3) satellite retrieved cloud base temperature, Tb; (4) Satellite retrieval of Na based on satellite retrieved T-re and Tb; (5) Satellite retrieved convective cloud base updraft, Wb; (6) Integration of all these retrieved properties to satellite retrieved CCN(S), as described above. (7 & 8) These ideas have been also the basis of a proposed satellite mission (CHASER).

References:

1. Freud E., D. Rosenfeld, and J. R. Kulkarni (2011), Resolving both entrainment-mixing and number of activated CCN in deep convective clouds. Atmos. Chem. Phys., 11, 12887-12900, doi:10.5194/acp-11-12887-2011.

2. Rosenfeld, D., G. Liu, X. Yu, Y. Zhu, J. Dai, X. Xu, and Z. Yue (2013), High resolution (375 m) cloud microstructure as seen from the NPP/VIIRS Satellite imager, Atmos. Chem. Phys. Discuss., 13, 29845-29894, doi:10.5194/acpd-13-29845-2013.

3. Zhu Y., D. Rosenfeld, X. Yu, G. Liu, J. Dai, X. Xu, 2014: Satellite retrieval of convective cloud base temperature based on the NPP/VIIRS Imager. J. Geophys. Res., 41, doi:10.1002/2013GL058970.

4. Rosenfeld D., B. Fishman, T. Goren, D. Giguzin, 2014: Combined satellite and radar retrievals of drop concentration and CCN at convective cloud base. Submitted.

5. Zheng Y., D. Rosenfeld, Z. Li, 2014: Retrieving updraft speeds of thermals by observed surface temperatures and winds. Submitted.

6. Rosenfeld D. et al., 2014: Towards retrieving CCN from satellites by using clouds as CCN chambers. In preparation.

7. Rosenfeld D., E.Williams, M. O. Andreae, E. Freud1, U. Pöschl, and N. O. Rennó (2012b), The scientific basis for a satellite mission to retrieve CCN concentrations and their impacts on convective clouds. Atmos. Meas. Tech., 5, 2039–2055, 2012, www.atmos-meas-tech.net/5/2039/2012/ doi:10.5194/amt-5-2039-2012.

8. Rennó N. O., E. Williams, D. Rosenfeld, D. G. Fischer, J. Fischer, T. Kremic, A. Agrawal, M. O. Andreae, R. Bierbaum, R. Blakeslee, A. Boerner, N. Bowles, H. Christian, A. Cox, J. Dunion, A. Horvath, X. Huang, A. Khain, S. Kinne, M. C. Lemos, J. E. Penner, U. Pöschl, J. Quaas, E. Seran, B. Stevens, T. Walati, T. Wagner (2013), CHASER: An Innovative Satellite Mission Concept to Measure the Effects of Aerosols on Clouds and Climate. Bull. Amer. Meteor. Soc, May 2013.