CLOUDEXT

A primary goal of the Atmospheric Radiation Measurement (ARM) Program is to improve the representation of clouds and related processes in Global Climate Models (GCM). Significant effort has been undertaken to improve the parameterization of extinction coefficient of ice and mixed-phase clouds based on measurements collected with airborne cloud particle size spectrometers. However, these techniques have a large uncertainty related to the accuracy of size-to-area conversion used in the calculation of extinction coefficient and more general issues related to the accuracy of particle sizing, measurements of particle concentration, and an unresolved problem with ice particle shattering on instrument inlets. Recently, a major field campaign coordinated by DOE, called the Indirect-Semi-Direct Aerosol Campaign (ISDAC), was conducted in Barrow, Alaska. During ISDAC a newly developed airborne cloud Extinction Probe was installed onboard the NRC Convair-580 research aircraft. This instrument provides measurements of cloud extinction coefficient from first principles based on the Beer-Bouguer law. An advantage of this instrument is the large distance between the transmitter/receiver and the retro-reflector. This makes its measurements practically insensitive to the shattering of ice particles. These new measurements may be used for validation of existing parameterizations of extinction coefficient and for helping to solve a long-standing problem related to the existence of small particles in ice clouds. We performed a set of laboratory and theoretical calibrations of the Extinction Probe. The calibrations were used to establish the correction function for forward scattering which becomes significant when large cloud particles are present. This correction was applied to extinction coefficients reported in ISDAC data. The corrected data was used for parameterization of extinction coefficient in Arctic ice and mixed-phase clouds and for drawing conclusions about the existence of small ice particle based on the closure of measured extinction coefficient and extinction coefficient derived from measurements of ice particle sizes.

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