A Sharper View of Fuzzy Objects: Warm Clouds and their Role in the Climate System

as seen by Satellite

Ralf BennartzUniversity of Wisconsin – Madison

Outline

• What do we want to know?

• Cloud microphysical parameters

• Cloud fraction and albedo

• Conclusions and outlook

Clouds are the single-most important factor modulating climate sensitivity…..

… and we know very little about forcings and feedbacks

IPCC 2007

IPCC 2007

21 climate models- one scenario

How can observations help narrow down these uncertainties?

What do we need to know?

• Cloud droplet number concentration (N)

• Liquid water path (LWP)

• Drizzle/rain? Rain water path (RWP)

• Cloud fraction

• Albedo

(Sub) adiabatic stratiform clouds

LWC

• LWP from VIS/NIR: Factor f depends on stratification of cloud (e.g. f=5/9 for adiabatic, f=2/3 for vertically uniform)

• N for an adiabatic cloud related to optical depth and LWP. C depends weakly on temperature and width of droplet spectrum

Basic physical relations

• LWP from VIS/NIR: Factor f depends on stratification of cloud (e.g. f=5/9 for adiabatic, f=2/3 for vertically uniform)

• N for an adiabatic cloud related to optical depth and LWP. C depends weakly on temperature and width of droplet spectrum

• MW: Very direct measurement. Mass absorption coefficient depends slightly on temperature, σr depends on rain water content too.

Basic physical relations

Basic physical relations

Bennartz et al. (2010)

Basic physical relations

Bennartz et al. (2010)

Three input variables (τVIS reff τMW)

Three output variables (N,LWP,RWP)

Basic physical relations

Three input variables (τVIS reff τMW)

Three output variables (N,LWP,RWP)

Auxiliary data, assumptions:

• Cloud top height• Cloud top temperature• Drizzle/rain particle size distribution• Width of cloud droplet spectrum

Basic physical relations

• Direct physical relationship between MW/VNIR optical properties and cloud physical properties.

• Errors and uncertainties due to input and auxiliary parameters can be specified and dependencies can be explicitly spelled out.

• Validity of assumptions can be assessed from observations.

• No unknown unknowns (though a lot of known unknowns).

Basic physical relations

Bennartz et al. (2010), Bennartz (2007), Rausch et al. (2010)

Products

Cloud droplet number concentration

Cloud droplet number concentration

Cloud droplet number concentration

Bennartz et al. (2011)

Cloud droplet number concentration

Bennartz et al. (2011)

Application example: Constraining cloud-aerosol interaction processes in GCMs

Storelvmo, Lohmann, Bennartz (2009)

Differences in cloud droplet activation account for about 65 % (1.3 W/m2) of the total spread in shortwave forcing (2 W/m2) in IPCC AR4 models

• Deviations from adiabaticity valid ‘targets’ can be identified by checking multi-spectral agreement of reff with adiabatic assumption.

Open issues & proposed solutions

Effective Radius

• Deviations from adiabaticity valid ‘targets’ can be identified by checking multi-spectral agreement of reff with adiabatic assumption.

Open issues & proposed solutions

• Deviations from adiabaticity valid ‘targets’ can be identified by checking multi-spectral agreement of reff with adiabatic assumption.

• Physical explanation for differences in reff still outstanding: partly cloudy pixels, subscale variability, rain, 3D…. Probably all of them to some extend

Open issues & proposed solutions

Validating models: LWP observations against iRAM

Lauer et al. (2009)

Special Sensor Microwave/Imager

Liquid Water Path [kg/m2]

Local Time [hours]

Liquid Water Path [kg/m2]

Local Time [hours]

Liquid Water Path [kg/m2]

Local Time [hours]

Liquid Water Path [kg/m2]

Local Time [hours]

Liquid Water Path [kg/m2]

Local Time [hours]

Conclusions

• Satellite observations provide a wealth of information about cloud microphysical and radiative properties.

• Simple physically-based cloud models provide an excellent framework to interpret satellite observations and brigde gap between observations and GCMs

• Caveats, uncertainties, and the limits of our current understanding need to be conveyed.

• Working closely with GCM/RCM community.

Acknowledgements

Collaborations: A. Lauer (U Hawaii), T. Storelvmo (Yale), U. Lohmann (ETH), J. Rausch (UW-AOS), L. Borg, (UW-AOS), A. Heidinger, (NOAA), M. Foster (CIMSS), R. Leung (PNNL), J. Fan (PNNL), C. O’Dell (CSU), R. Wood (UW)

Funding: NASA MODIS Science Team, NOAA/Joint Center for Satellite Data Assimilation, DOE Regional Climate Modeling