ICE-RF: Improving Cirrus Estimates of Radiative Forcing: Preliminary Laboratory Backscattering Results

Ice clouds make a major contribution to radiative forcing in the atmosphere, both trapping IR (terrestrial) radiation and reflecting solar radiation. The balance between these processes determines whether the cloud has a net warming or cooling effect. The range in climate sensitivity (i.e., the mean equilibrium surface temperature response to a doubling of CO2) reported across differing climate models in the last IPCC report was 2.1 – 4.7ºC with very high confidence that cloud feedback was the primary cause of the spread in results.
Whether absorption of longwave radiation or reflection of solar radiation dominates depends on the macrophysical and optical properties of the cloud, in particular the ice water content (IWC) and the cloud optical depth. However, inverting the well-known lidar equation to convert backscatter measurements to optical depths requires knowledge of the ice crystal exact backscatter properties. Currently, this conversion is problematic due to a lack of physical understanding of the precise backscatter properties of atmospheric ice. Moreover, previous work has shown that irregularities on ice crystal surfaces can affect their light scattering properties with implications on, for example, the asymmetry parameter of such particles.
To address the considerable uncertainty as to the nature of the ice crystal backscattering amplitude and to help constrain the various model predictions in the literature the core of the ICE-RF project is backscattering measurements of artificially produced ice crystals in the Manchester Ice Cloud Chamber (MICC).
This experimental facility has previously been used to measure backscattering from ice crystals and consists of a 10 m-high 1 m-diameter fall-tube contained within a set of stacked cold rooms, independently temperature controlled from room temperature to –55 °C. By varying relative humidity with respect to ice and temperature, a range of ice crystal habits can be produced similar to those observed in natural ice clouds in Earth’s atmosphere. At the base of the fall-tube we have designed and built a scattering enclosure enabling a laser beam to be directed horizontally across the cloud, configured to allow either exact backscatter measurements, or rotated from near the forward scatter peak to the backscatter position (180º). Initial results will focus on visible (532nm) wavelengths, though the addition of a 1064 nm laser in a dual channel configuration will make our results relevant to space-based observations from CALIOP, and, separately, a 355nm laser will assist with interpretation of future results from the EarthCARE instrument platform. Cloud microphysical properties are recorded with a combination of Cloud Particle Imaging probe and formvar replicas.
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This work was supported by the Natural Environment Research Council [grant number: NE/T00147X/1]