1. 1 Cloud Droplet Size Retrievals from AERONET Cloud Mode Observations Christine Chiu Stefani Huang, Alexander Marshak, Tamas Várnai, Brent Holben, Warren Wiscombe University of Reading NASA Goddard Space Flight Center

2. Importance of cloud microphysical and optical properties crucial determinant of the Earth’s radiative energy balance crucial parameter for understanding and modelling aerosol-cloud-precipitation processes droplet size retrievals from satellites: – larger drops, lower reflectance retrievals from ground-based observations: – cloud radar, flux+MWR – radiance: Kikuchi et al (2006), McBride et al. (2011) 2

3. 3 AERONET has provided routine cloud observations since 2010 Normal aerosol mode (sun-seeking) Cloud mode (zenith-pointing) Chiu et al. (JGR, 2010) some AERONET sunphotometers take measurements at 1.64 μ m wavelength

4. Relationship between zenith radiance and cloud optical depth 1640 nm 870 nm 8  m Cloud optical depth Zenith radiance 16  m We build look-up tables: I 440 = I 440 ( ,A c,R eff ) I 870 = I 870 ( ,A c,R eff ) I 1640 = I 1640 ( ,A c,R eff ) 4 cloud optical depth cloud fraction cloud droplet radius

5. Radiative contribution at each layer imparted to the overall size determination 5 Similar to Fig. 3 Platnick, JGR, 2000 Weighting function Optical depth within clouds total optical depth 10 20 50 Unlike satellite retrievals, our retrieved droplet size is affected by the entire cloud layer

6. Results from simulations 6

7. Stratocumulus case from I3RC (Intercomparison of 3D Radiative Codes) Monte-Carlo model 7 369120 3 6 9 0

8. 8 Good agreement between retrieved and true liquid water path (LWP) 85% data points fall within 30 g/m 2 Mean bias ~ 16 g/m 2 RMS: 25 g/m 2 True (g/m 2 ) retrieved (g/m 2 ) Liquid water path

9. True Retrieved Effective radius True Retrieved Cloud optical depth 9 Performance in cloud optical depth and droplet size True Retrieved Liquid water path stronger scatter between true and retrieved effective radius

10. The majority of retrievals fall within the 25-75th percentile of droplet size distributions 10 X-index True effective radius Retrieved Range

11. Results from observations 11

12. Lidar attenuated backscatter signal Flux+MWR ARM Oklahoma site (2007.06.15) 12 Cloud mode Time (UTC) Cloud droplet size (microns) MODIS

13. Comparison to coincident MODIS retrievals for overcast water clouds 13 Cloud mode retrieval MODIS/Terra overpass index Range (microns)

14. All water clouds at ARM Oklahoma site (2007/05 – 2008/06) 14 Retrieved droplet radius (microns) Occurrence counts

15. Summary Our cloud droplet radius retrievals show promising agreement with those from flux, microwave and cloud radar measurements Unlike satellite retrievals, our retrievals are determined by droplets in all cloud layers Retrievals for other sites are available but we need to find ancillary datasets for further evaluation 15