Toward autonomous surface-based infrared remote sensing of polar clouds: retrievals of cloud optical and microphysical properties
<p>Improvements to climate model results in polar regions require improved knowledge of cloud properties. Surface-based infrared (IR) radiance spectrometers have been used to retrieve cloud properties in polar regions, but measurements are sparse. Reductions in cost and power requirements to a...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-09-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/12/5071/2019/amt-12-5071-2019.pdf |
| Summary: | <p>Improvements to climate model results in polar regions require improved
knowledge of cloud properties. Surface-based infrared (IR) radiance
spectrometers have been used to retrieve cloud properties in polar regions,
but measurements are sparse. Reductions in cost and power requirements to
allow more widespread measurements could be aided by reducing instrument
resolution. Here we explore the effects of errors and instrument resolution
on cloud property retrievals from downwelling IR radiances for resolutions
of 0.1 to 20 cm<span class="inline-formula"><sup>−1</sup></span>. Retrievals are tested on 336 radiance simulations
characteristic of the Arctic, including mixed-phase, vertically
inhomogeneous, and liquid-topped clouds and a variety of ice habits.
Retrieval accuracy is found to be unaffected by resolution from 0.1 to 4 cm<span class="inline-formula"><sup>−1</sup></span>, after which it decreases slightly. When cloud heights are
retrieved, errors in retrieved cloud optical depth (COD) and ice fraction
are considerably smaller for clouds with bases below 2 km than for higher
clouds. For example, at a resolution of 4 cm<span class="inline-formula"><sup>−1</sup></span>, with errors imposed
(noise and radiation bias of 0.2 mW/(m<span class="inline-formula"><sup>2</sup></span> sr cm<span class="inline-formula"><sup>−1</sup></span>) and biases in
temperature of 0.2 K and in water vapor of <span class="inline-formula">−3</span> %), using retrieved cloud
heights, root-mean-square errors decrease from 1.1 to 0.15 for COD, 0.3 to
0.18 for ice fraction (<span class="inline-formula"><i>f</i><sub>ice</sub></span>), and 10 to 7 <span class="inline-formula">µ</span>m for ice
effective radius (errors remain at 2 <span class="inline-formula">µ</span>m for liquid effective radius).
These results indicate that a moderately low-resolution, surface-based IR
spectrometer could provide cloud property retrievals with accuracy
comparable to existing higher-resolution instruments and that such an
instrument would be particularly useful for low-level clouds.</p> |
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| ISSN: | 1867-1381 1867-8548 |