Laboratory and Flight Tests of 2D Imaging Probes: Toward a Better Understanding of Instrument Performance and the Implications for In-situ Cloud Microphysical Data

Two-dimensional (2D) imaging probes, such as the 2D Stereo (2D-S) and Cloud Imaging Probe (CIP), are routinely used to provide in situ measurements of cloud particle properties. The basic measurement is shadow-graphs of water drops and ice particles from which particle size distributions (PSDs), and projected particle area. Bulk cloud parameters (concentration, water content, extinction, etc.) are derived from these particle by particle measurements. These data permeate data archives of domestic and foreign government agencies, universities and in the private sector. Archived 2D data sets are used widely by the atmospheric science community to, among other things, validate remote sensing retrievals; develop parametrizations for high resolution models, and as case studies to understand fundamental cloud processes. Here we present results from laboratory and flight tests on a Learjet research aircraft that give new insights into the performance of the 2D imaging probes, and how their performance may have impacted measurements collected in data archives.

This study focuses on the current generation of commercially available Optical Array Probes instrument families (CIP and 2D-S), and includes perhaps the first detailed laboratory comparison of these instruments with particle velocities approaching jet aircraft speeds (190 m s^-1). We demonstrate how electronic time response impacts small particle counting efficiency as a function of particle velocity, and how dead time affects cloud particle measurements. Our laboratory results are corroborated with in-situ cloud measurements of a CIP and 2D-S probe at airspeeds up to 170 m s^-1. We also present comparisons of the Depth of Field studies for these probes, suggesting that variations in sample volume calculations must be developed for newer instruments. Together these results highlight the need for the development of improved 2D imaging probe analysis techniques, and reconsideration of some archived cloud microphysics datasets. Improvements in the efficacy of 2D in-situ data will result in more representative cloud PSDs. Reducing uncertainties in observed PSDs is of critical importance as any errors become more pronounced when for calculating cloud radiative properties (2^nd moment) and cloud water content (3^rd moment).