ARM

Instruments for Airborne Research Undergo Calibration Campaigns

In a laboratory at the University of Manchester, scientists run an experiment using a micromanipulator to test the depth-of-field of the Cloud Particle Imager. Researchers (front to back) include Chris Emersic and Paul Connolly from the University of Manchester, and Junshik Um from the University of Illinois. ARM file photo.
In a laboratory at the University of Manchester, scientists run an experiment using a micromanipulator to test the depth-of-field of the Cloud Particle Imager. Researchers (front to back) include Chris Emersic and Paul Connolly from the University of Manchester, and Junshik Um from the University of Illinois. ARM file photo.

This summer, ARM sponsored a series of calibration studies in the United Kingdom and Canada for two key instruments used for airborne cloud research—the cloud extinction probe and the Cloud Particle Imager (CPI). These instruments have been used in ARM field campaigns to support studies of both tropical and arctic clouds: the Tropical Warm Pool International Cloud Experiment in 2006 and the Indirect and Semi-Direct Aerosol Campaign in 2008, respectively. The CPI will also be used in the Small Particles in Cirrus field campaign starting in fall 2009. The current calibration efforts are underway to explore the potential for these instruments to provide additional valuable information to the data collected during these campaigns.

The CPI provides high-resolution images of ice crystals. In addition to these images, scientists at the University of Manchester believe the instrument can give information about the size distribution of ice crystals given sufficiently large averaging times. To determine these size distributions, each version of the instrument must undergo a calibration to derive a scaling factor. The appropriate scaling factor must then be applied to the relevant campaign data sets so that size distributions can be generated and added to the Data Archive. Using a facility at the University of Manchester in the United Kingdom, scientists from the University of Illinois and the University of Manchester performed this calibration in August. In addition, they used the university’s cloud chamber to compare the performance of three different versions of the CPI and conduct an aggregation experiment.

The cloud extinction probe is a prototype instrument that measures how light decreases in intensity—called attenuation—when it encounters specific media, such as liquid droplets, ice particles or aerosol particles, in this case. There are several ground-based instruments for measuring this variable, but this is the first airborne version of the instrument. Because no calibration standards exist yet for this version of the instrument, this field campaign involves the use of a laboratory calibration process to study the effect of particle size on measurements of attenuated light. Results will be used to develop a correction algorithm that will be applied to the measurements obtained by the instrument during the Arctic campaign in 2008. The calibration study for the extinction probe began in May and continues through October 2009 at Environment Canada.

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