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| Titel |
Cloud thermodynamic phase detection with polarimetrically sensitive passive sky radiometers |
| VerfasserIn |
K. Knobelspiesse, B. van Diedenhoven, A. Marshak, S. Dunagan, B. Holben, I. Slutsker |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 8, no. 3 ; Nr. 8, no. 3 (2015-03-24), S.1537-1554 |
| Datensatznummer |
250116231
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| Publikation (Nr.) |
 copernicus.org/amt-8-1537-2015.pdf |
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| Zusammenfassung |
The primary goal of this project has been to investigate if ground-based
visible and near-infrared passive radiometers that have polarization
sensitivity can determine the thermodynamic phase of overlying clouds, i.e.,
if they are comprised of liquid droplets or ice particles. While this
knowledge is important by itself for our understanding of the global climate,
it can also help improve cloud property retrieval algorithms that use total
(unpolarized) radiance to determine cloud optical depth (COD). This is a
potentially unexploited capability of some instruments in the NASA Aerosol
Robotic Network (AERONET), which, if practical, could expand the products of
that global instrument network at minimal additional cost.
We performed simulations that found, for zenith observations, that cloud
thermodynamic phase is often expressed in the sign of the Q component of
the Stokes polarization vector. We chose our reference frame as the plane
containing solar and observation vectors, so the sign of Q indicates the
polarization direction, parallel (positive) or perpendicular (parallel) to
that plane. Since the fraction of linearly polarized to total light is
inversely proportional to COD, optically thin clouds are most likely to
create a signal greater than instrument noise. Besides COD and instrument
accuracy, other important factors for the determination of cloud
thermodynamic phase are the solar and observation geometry (scattering angles
between 40 and 60° are best), and the properties of ice
particles (pristine particles may have halos or other features that make them
difficult to distinguish from water droplets at specific scattering angles,
while extreme ice crystal aspect ratios polarize more than compact
particles).
We tested the conclusions of our simulations using data from polarimetrically
sensitive versions of the Cimel 318 sun photometer/radiometer that compose a
portion of AERONET. Most algorithms that exploit Cimel polarized observations
use the degree of linear polarization (DoLP), not the individual Stokes
vector elements (such as Q). Ability to determine cloud thermodynamic phase
depends on Q measurement accuracy, which has not been rigorously assessed
for Cimel instruments. For this reason, we did not know if cloud phase could
be determined from Cimel observations successfully. Indeed, comparisons to
ceilometer observations with a single polarized spectral channel version of
the Cimel at a site in the Netherlands showed little correlation. Comparisons
to lidar observations with a more recently developed, multi-wavelength
polarized Cimel in Maryland, USA, show more promise. The lack of
well-characterized observations has prompted us to begin the development of a
small test instrument called the Sky Polarization Radiometric Instrument for
Test and Evaluation (SPRITE). This instrument is specifically devoted to the
accurate observation of Q, and the testing of calibration and uncertainty
assessment techniques, with the ultimate goal of understanding the practical
feasibility of these measurements. |
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