Evaluation of Liquid Water Measuring Instruments in Cold Clouds Sampled during FIRE

Abstract

Liquid water measurements from the Rosemount icing detector (RICE), Particle Measuring Systems (PMS) forward scattering spectrometer probe (FSSP), and Johnson?williams and King hot-wire probes used on the NCAR King Air aircraft are evaluated for 10 flights into cold clouds during the First ISCCP Research Experiment (FIRE). The noise level of the RICE instrument has not previously been defined, and an upper limit is determined by analyzing data collected at temperatures below ?40°C, where laboratory measurements and theoretical analyses suggest that all water droplets are frozen, and therefore the liquid water content (LWC) should be zero. Similarly, FSSP spectra at temperatures below ?40°C are used to place upper bounds on the false signatures produced from contamination by ice particles. The RICE and FSSP ?noise? levels appear to apply to all temperatures between ? 40 and ?20°C as well. The RICE is calibrated for liquid water content measurement by comparing its voltage output with simultaneous FSSP measurements, after removing the noise levels. LWC measurements from the icing detector and the FSSP compare favorably with those from the hot-wire probes in the range where LWC is above the detection limits of the latter (about 0.02 g m?3). The hot-wire probes, which have been used in many past studies in high clouds, have detection thresholds about one order of magnitude higher than is possible with the RICE and FSSP instruments. It is shown that most of the LWC measured at temperatures below ?20°C during FIRE fell below the hot-wire probe detection thresholds. A compilation of data from the FIRE experiment indicates that LWC should be taken into consideration in cloud studies at temperatures down to at least ?35°C.