Field Intercomparison of Ground-Based Cloud Physics Instruments at Whitetop Mountain, Virginia

Abstract

In May 1987 a two-week field intercomparison study of ground-based cloud liquid water content (LWC) and cloud detector instruments was performed at the Tennessee Valley Authority research station at the summit of Whitetop Mountain, Virginia. The objectives were to better understand the precision and accuracy of the methods and to assist instrument developers in assessing the field readiness of the instruments for operation in clouds impacting a mountain summit. About 80 h of cloud impaction occurred during the study providing ample opportunity for instrument testing. LWC instruments tested included four optical instruments (the Atmospheric Environment Service of Canada Fog Monitoring Device, the Engineering Design, Inc. Beeman Instrument, the Gerber Scientific PVM-100, and the, Particle Measuring Systems, Inc. FSSP-100), a psychrometric instrument (ASRC-McLaren), and a gravimetric instrument (TVA-Valente). Correlation coefficients for all pairings of properly operating LWC instruments ranged from 0.85?0.94 and linear regression slopes ranged from 0.90?1.02. Root-mean-square deviations from the mean LWC ranged from 0.026?045 g m?3 for the properly operating LWC instruments. Some of the LWC instruments experienced difficulty during their first exposure to field conditions and have been modified accordingly. Results for cloud detectors indicated that the ECN-Mallant optical cloud detector agreed with the reference reflectometer 98 and 92 percent of the time during cloud and nonclond periods, respectively. Relative humility sensors exhibited agreement ranging from 84 to 90 percent with the reference method provided that a carefully selected threshold was established after reviewing the data.