Spatial distribution of cloud droplet size properties from Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) measurements
<p>The global variability of clouds and their interactions with aerosol and radiation make them one of our largest sources of uncertainty related to global radiative forcing. The droplet size distribution (DSD) of clouds is an excellent proxy that connects cloud microphysical properties with r...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-04-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/13/1777/2020/amt-13-1777-2020.pdf |
| Summary: | <p>The global variability of clouds and their interactions
with aerosol and radiation make them one of our largest sources of
uncertainty related to global radiative forcing. The droplet size
distribution (DSD) of clouds is an excellent proxy that connects cloud
microphysical properties with radiative impacts on our climate. However,
traditional radiometric instruments are information-limited in their DSD
retrievals. Radiometric sensors can infer droplet effective radius directly
but not the distribution width, which is an important parameter tied to the
growth of a cloud field and to the onset of precipitation. DSD heterogeneity
hidden inside large pixels, a lack of angular information, and the absence of
polarization limit the amount of information these retrievals can provide.
Next-generation instruments that can measure at narrow resolutions with multiple
view angles on the same pixel, a broad swath, and sensitivity to
the intensity and polarization of light are best situated to retrieve DSDs at
the pixel level and over a wide spatial field. The Airborne Hyper-Angular
Rainbow Polarimeter (HARP) is a wide-field-of-view imaging polarimeter
instrument designed by the University of Maryland, Baltimore County (UMBC),
for retrievals of cloud droplet size distribution properties over a wide
swath, at narrow resolution, and at up to 60 unique, co-located view zenith
angles in the 670 <span class="inline-formula">nm</span> channel. The cloud droplet effective radius (CDR) and
variance (CDV) of a unimodal gamma size distribution are inferred
simultaneously by matching measurement to Mie polarized phase functions. For
all targets with appropriate geometry, a retrieval is possible, and
unprecedented spatial maps of CDR and CDV are made for cloud fields that
stretch both across the swath and along the entirety of a flight
observation. During the NASA Lake Michigan Ozone Study (LMOS) aircraft
campaign in May–June 2017, the Airborne HARP (AirHARP) instrument observed
a heterogeneous stratocumulus cloud field along the solar principal plane.
Our retrievals from this dataset show that cloud DSD heterogeneity can occur
at the 200 <span class="inline-formula">m</span> scale, much smaller than the 1–2 <span class="inline-formula">km</span> resolution of most spaceborne
sensors. This heterogeneity at the sub-pixel level can create artificial
broadening of the DSD in retrievals made at resolutions on the order of 0.5
to 1 <span class="inline-formula">km</span>. This study, which uses the AirHARP instrument and its data as a
proxy for upcoming HARP CubeSat and HARP2 spaceborne instruments,
demonstrates the viability of the HARP concept to make cloud measurements at
scales of individual clouds, with global coverage, and in a low-cost,
compact CubeSat-sized payload.</p> |
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| ISSN: | 1867-1381 1867-8548 |