Clouds play a key role in Earth's radiative balance. Radiative transfer through liquid clouds depends on the optical properties of liquid water. Although these optical properties are known to be temperature dependent, radiative transfer simulations typically use properties based on measurements made at 298 K, even for supercooled liquid clouds at temperatures as low as 240 K. Here we report temperature‐dependent complex refractive indices (CRIs) of liquid water at 240, 253, 263, and 273 K from 1 to 15,000 cm−1(0.7 to 10,000 μm). Imaginary parts of the CRI (
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.
more » « less- Award ID(s):
- 2127632
- NSF-PAR ID:
- 10447453
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 127
- Issue:
- 1
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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