Coincidence Errors in a Cloud Droplet Probe (CDP) and a Cloud and Aerosol Spectrometer (CAS), and the Improved Performance of a Modified CDP

Abstract

Abstract Central to the aerosol indirect effect on climate is the relationship between cloud droplet concentrations Nd and cloud condensation nuclei (CCN) concentrations. There are valid reasons to expect a sublinear relationship between measured Nd and CCN, and such relationships have been observed for clouds in a variety of locations. However, a measurement artifact known as “coincidence” can also produce a sublinear trend. The current paper shows that two commonly used instruments, the cloud droplet probe (CDP) and the cloud and aerosol spectrometer (CAS), can be subject to significantly greater coincidence errors than are typically recognized, with an undercounting bias of at least 27% and an oversizing bias of 20%–30% on average at Nd = 500 cm−3, and with an undercounting bias of as much as 44% at Nd = 1000 cm−3. This type of systematic error may have serious implications for interpretation of in situ cloud observations. It is shown that a simple optical modification of the CDP dramatically reduces oversizing and undercounting biases due to coincidence. Guidance is provided for diagnosing coincidence errors in CAS and CDP instruments.