Publication Abstracts

Sinclair et al. 2018, submitted

Sinclair, K., B. van Diedenhoven, B. Cairns, M. Alexandrov, R. Moore, E. Crosbie, and L. Ziemba, 2018: Polarimetric retrievals of cloud droplet number concentrations. Remote Sens. Environ., submitted.

Cloud droplet number concentration (Nd) is a key parameter of liquid clouds and is essential to understanding aerosol-cloud interactions. It couples boundary layer aerosol composition, size and concentration with cloud reflectivity. It impacts radiative forcing, cloud evolution, precipitation, global climate and, through observation, can be used to partially monitor the cloud albedo effect, or the first indirect effect.

With its unique combination of multi-wavelength, multi-angle, total and polarized reflectance measurements, the Research Scanning Polarimeter (RSP) retrieves Nd with relatively few assumptions. The approach involves measuring cloud optical thickness, mean droplet extinction cross-section and cloud physical thickness. Polarimetric observations are capable of measuring the effective variance, or width, of the droplet size distribution. Estimating cloud geometrical thickness is an important component of the polarimetric Nd retrieval, which is accomplished using polarimetric measurements in a water vapor absorption band to retrieve the amount of in-cloud water vapor and relating this to physical thickness. We highlight the unique abilities and quantify uncertainties of the polarimetric $pproach.

We validate the approach using observational data from the North Atlantic and Marine Ecosystems Study (NAAMES). NAAMES targets specific phases in the seasonal phytoplankton lifecycle and ocean-attnosphere linkages. This study provides an excellent opportunity for the RSP to evaluate its approach of sensing Nd over a range of concentrations and cloud types with in situ tneasurements from a Cloud Droplet Probe (CDP). The RSP and CDP, along with an array of other instruments, are flown on the NASA C-130 aircraft, which flies in situ and remote sensing legs in sequence.

Cloud base heights retrieved by the RSP compare well with those derived in situ (R=0.83) and by a ceilometer aboard the RIV Atlantis (R=0.79). Comparing geometric mean values from 12 science flights throughout the NAAMES-1 and NAAMES-2 campaigns, we find a strong correlation between Nd retrieved by the RSP and CDP (R=0.96). A linear least squares fit has a slope of 0.92 and an intercept of 0.3 cm-3. Inconsistencies in this comparison can be partially attributed to inhomogeneous clouds, and variations in spatial and temporal sampling. Radiometric uncertainties of the RSP measurements lead to biases on derived optical thickness and cloud physical thickness, but these biases largely cancel out when deriving Nd for most conditions and geometries. We find that a polarimetric approach to sensing Nd is viable and the RSP is capable of accurately retrieving Nd for a variety of cloud types and meteorological conditions.

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BibTeX Citation

  author={Sinclair, K. and van Diedenhoven, B. and Cairns, B. and Alexandrov, M. and Moore, R. and Crosbie, E. and Ziemba, L.},
  title={Polarimetric retrievals of cloud droplet number concentrations},
  journal={Remote Sens. Environ.},
  note={Manuscript submitted for publication}

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RIS Citation

ID  - si09200d
AU  - Sinclair, K.
AU  - van Diedenhoven, B.
AU  - Cairns, B.
AU  - Alexandrov, M.
AU  - Moore, R.
AU  - Crosbie, E.
AU  - Ziemba, L.
PY  - 2018
TI  - Polarimetric retrievals of cloud droplet number concentrations
JA  - Remote Sens. Environ.
ER  -

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