Evaluation of Temperature‐Dependent Complex Refractive Indices of Supercooled Liquid Water Using Downwelling Radiance and In‐Situ Cloud Measurements at South Pole. Issue 1 (4th January 2022)

• Record Type: Journal Article
• Authors: Rowe, Penny M.; Walden, Von P.; Brandt, Richard E.; Town, Michael S.; Hudson, Stephen R.; Neshyba, Steven
• Is Part Of: Journal of geophysical research. Volume 127:Issue 1(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-01-04
• Subjects: supercooled liquid cloud -- index of refraction -- temperature dependent -- South Pole -- infrared radiance -- in situ cloud measurements; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2021JD035182
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Clouds have a large effect on the radiation budget and represent a major source of uncertainty in climate models. Supercooled liquid clouds can exist at temperatures as low as 235 K, and the radiative effect of these clouds depends on the complex refractive index (CRI) of liquid water. Laboratory measurements have demonstrated that the liquid‐water CRI is temperature‐dependent, but corroboration with field measurements is difficult. Here we present measurements of the downwelling infrared radiance and in‐situ measurements of supercooled liquid water in a cloud at temperatures as low as 240 K, made at South Pole Station in 2001. These results demonstrate that including the temperature dependence of the liquid‐water CRI is essential for accurate calculations of radiative transfer through supercooled liquid clouds. Furthermore, we show that when cloud properties are retrieved from infrared radiances (using the spectral range 500–1, 200 cm −1 ) spurious ice may be retrieved if the 300 K CRI is used for cold liquid clouds (∼240 K). These results have implications for radiative transfer in climate models as well as for retrievals of cloud properties from infrared radiance spectra. Plain Language Summary: Clouds cause both a cooling effect, through reflecting sunlight away from the Earth, and a warming effect, through trapping infrared radiation (i.e., the greenhouse effect). "Supercooled" liquid droplets can exist in clouds at temperatures well below 0°C. Here we present images of liquid water in clouds over the South Pole, captured by flying a video camera on a balloon, at temperatures as low as −33°C. Prior laboratory measurements have indicated that the trapping efficiency of such cold clouds is different from that of warmer clouds due to changes in the index of refraction of liquid water with temperature. We provide further evidence for this temperature dependence by comparing measurements and simulations of the infrared radiation emitted by the cloud during the balloon flight. Key Points: In‐situ measurements of supercooled liquid water in a cloud at temperatures as low as 240 K are presented Polar measurements support laboratory results demonstrating that the liquid‐water complex refractive index is temperature dependent Radiative transfer through supercooled liquid cloud is sensitive to the temperature dependence of the liquid‐water complex refractive index