MM-Wave Radar Structure and Microphysical Characteristics of a Mixed Phase Altocumulus Cloud on 2 November 2001

The ninth Cloud Layer Experiment (CLEX-9), conducted over western Nebraska from 8 October to 4 November 2001 provided in-situ microphysical and radar measurements of mid-level clouds from the University of Wyoming King Air research aircraft (UWKA). Microphysical measurements include cloud liquid water (LWC) and ice water contents (IWC) from a suite of microphysical probes (e.g., PMS 2D-C, 2D-P, and DMT-100 Probe). The 95-GHz Wyoming Cloud Radar (WCR) was also deployed on the UWKA, providing radar reflectivity (Z) observations. Several surface-based instruments, including supplementary radiosonde launches, a dual-frequency radiometer, and a micropulse lidar, were located at the North Platte, Nebraska Regional Airport, providing supplementary information regarding the structure, microphysics, and environmental characteristics of the mixed phase clouds. Integration of the in-situ and radar data provides the opportunity to investigate the structure and bulk microphysical characteristics of the mixed-phase clouds sampled during 8 mission days. This paper specifically investigates the microphysical characteristics and radar structure of a dissipating mixed-phase altocumulus cloud on 2 November 2001 during CLEX-9 during a three hour period beginning shortly after sunrise. In-situ microphysical, mm-wave radar, and surface based observations are compared and contrasted at the early, mid, and dissipating phases of the observed life cycle. As expected, the LWC and cloud droplet concentration and mean diameter gradually decreased from early to dissipating phase. The IWC and mean ice particle size actually peaked during the mid-phase. Interestingly, the cloud did not glaciate during the dissipating phase. As a matter of fact, there was detectable LWC in the final stages of cloud dissipation even when there was little or no measurable IWC. This apparent “de-glaciation” is contrary to the classic conceptual model of the glaciation and subsequent dissipation of mixed-phase altocumulus clouds.