First Ever Satellite Remote Sensing of Small Ice Crystals in Cirrus Clouds
David L. Mitchell1, Robert P. d'Entremont2, and
R. Paul Lawson3
1) Desert Research Institute, Reno, NV
2) Atmospheric and Environmental Research, Lexington, MA
3) SPEC, Inc., Boulder, CO
 
 
This research was supported by the Office of Science (BER),
U.S. Department of Energy, Grant No. DE-FG02-06ER64201.
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Examples of AER/DRI retrieval products using earlier methodologies. |
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Abstract
To date, in situ instruments have not been able to characterize the concentrations of small (D < 60 μm) ice crystals in cirrus clouds. Some instruments indicate their concentrations are orders of magnitude greater than the larger crystals in a particle size distribution (PSD) and that they account for about half of the cirrus optical depth, while other instruments indicate no anomaly and that they contribute little to optical depth. The viewpoint one adopts here will have a strong impact on the way cirrus clouds affect predictions of climate change in global climate models (GCMs). Therefore we have developed a method for detecting these small crystals from satellites.
Aircraft measurements of the PSD are most reliable for larger ice particles (D > 60 microns) and are highly uncertain for smaller ice crystals. On the other hand, certain thermal radiances are very sensitive to the concentrations of small ice crystals. To estimate the cirrus PSD, we first parameterize aircraft measurements of the PSD in terms of temperature and ice water content (IWC) and then use thermal radiances to adjust certain PSD parameters (that the radiation is sensitive to) so that the measured radiative properties are consistent with the PSD. This is made possible by using the Modified Anomalous Diffraction Approximation (MADA) to describe the cirrus optical properties, since the optical properties in MADA are expressed in terms of the PSD parameters. This makes it possible to adjust the PSD parameters and rapidly recalculate the optical properties to make the PSD consistent with the observed radiances.
Preliminary results indicate that the anomalously high concentrations of small ice crystals found in many in situ measurements are inconsistent with the cirrus radiative properties and may be artifacts due to the shattering of larger ice particles on the inlet of the sampling device aboard aircraft. |
Instruments aboard aircraft designed to measure cirrus ice particle size distributions (PSD) differ greatly regarding the measured concentrations of small crystals. A possible explanation for these differences is that some ice particles collide upon the rim of the inlet used to sample them, causing the ice particles to shatter into many smaller particles that are then detected and counted as natural ice crystals. Instruments without a sampling inlet (like the CDP) should be much less vulnerable to this shattering problem. |
PSD measurements from one probe (the CAS) having an inlet tube where ice particle shattering may occur is shown in purple, while another probe (the CDP) with an open path inlet design is shown in blue for cirrus anvils sampled north of Darwin Australia (McFarquhar et al. 2007, GRL). The two probes yield concentrations that differ by up to 2-3 orders of magnitude at small sizes.
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