Retrieving cloud optical depth and ice particle size using thermal infrared radiometry: Application to the monitoring of thin ice clouds in an arctic environment Yann Blanchard , Alain Royer, Norm O’Neill
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Yann Blanchard, Alain Royer, Norm O’Neill
Centre d’Applications et de Recherches en Télédétection, Université de Sherbrooke, QC, Canada
Abstract: The cloud optical depth (COD) and ice particle size of thin ice clouds are two key parameters whose variations could strongly influence radiative effects and climate in the Arctic environment. Our objective is to assess the potential of using multi-band thermal radiance measurements of zenith sky radiance for retrieving COD and effective particle diameter (Deff) of thin ice cloud (TIC) in the Arctic. We present preliminary retrievals applied to ground-based thermal infrared data acquired at the high-Arctic PEARL observatory in Eureka, Nunavut, Canada and validated using LIDAR and RADAR data. The results of the retrieval method were used to separate TIC into two types : TIC1 characterized by small crystals (Deff < 30 µm) and TIC2 by large ice crystals (Deff > 30 µm, up to 300 µm). Inversions were performed across two polar winters for 100 TICs.
The methodology is based on zenith radiance measurements in 6 channels (8.3, 8.7, 9.1, 10.8, 11.4 and 13µm, as is currently employed in the CIMEL CE-312 radiometer) extracted from band integrated P-AERI spectra (provided by NOAA).
The radiance in these bands are sensitive to the COD and ice particle Deff as shown in the MODTRAN brightness temperatures simulations below.
Results, Validation and Monitoring
We applied the retrieval algorithm to 100 TICs measured at Eureka and compared the results with the LIDAR and LIDAR/RADAR products. We also studied the occurrence of the TIC1 and TIC2 classes in association with the COD amplitudes, looking for trends in relation to SO42- concentration.
Overall accuracy = 82%
Our algorithm is based on a multi-spectral zenith radiance comparison between radiative transfer simulations and measurements.
Comparison with SO42-
Located on Ellesmere Island in the Canadian Arctic (80°N, 86°W), the Polar Environmental Research Laboratory (PEARL) hosts many atmospheric instruments facilitating the validation of our methodology as well as supplying critical supporting data.
Cloud Altitude and
thickness (from LIDAR)
(using MODTRAN 4)
Look-up tables built by
varying COD and Deff
of ice cloud model
P, T, Td
Conclusion: By exploiting the sensitivity of multi-band thermal IR radiometry to ice clouds, we showed it is possible to retrieve COD with relatively good accuracy (R²=0.91 and RMSE=0.24) and classify TIC1 / TIC2 clouds with moderate classification accuracy (82%). Results over two winters were validated with respect to LIDAR and RADAR products. Our retrievals (and hopefully future retrievals) permit the monitoring of the temporal evolution of TICs in the Arctic. Further studies are required to better understand the interaction between aerosols and TICs and subsequently to improve the TIC1/TIC2 classification accuracy. We will continue to investigate the correlation between our retrievals and key auxiliary measurements such as the sulfate concentrations made using the AMS.
We acknowledge the technical support of the CANDAC operators at PEARL
Contact e-mail : [email protected]