Comparisons of in situ measurements of cirrus cloud ice water content

Abstract

Aircraft‐based in situ measurements of cirrus cloud ice water content (IWC) are important for cloud microphysical/radiative modeling and satellite validation studies, yet the measurements have proven challenging due, in part, to the large dynamic range of IWC values present in cirrus clouds. To date, three instruments designed for the measurement of IWC have been flown aboard the NASA WB‐57F research aircraft: the University of Colorado closed‐path laser hygrometer, the Droplet Measurement Technologies Cloud Spectrometer and Impactor, and the Harvard University Lyman‐α total water photofragment‐fluorescence hygrometer. This paper compares IWC measurements from these three instruments taken during the Midlatitude Cirrus Experiment (MidCiX) in 2004. At larger values of IWC (IWC > 10 mg m−3), the three instruments agree, on average, to within 20%, which is of the order of their estimated instrumental uncertainties. At smaller IWC values (<10 mg m−3), the agreement is worse, in part due to increasing instrument uncertainties. These results have implications for measurements in thin and subvisual cirrus. Particle sampling and evaporation, instrument background levels, and hysteresis are not found to be significant contributors to discrepancies among the measurements. For remote sensing validation studies where IWC data are vertically integrated to obtain ice water path, the agreement between the instruments is better than 20% for thick cirrus (τ > 1), implying that IWC measurements in thicker clouds are of sufficient accuracy for validation studies.