Jennifer D. Small

Airborne Phase Doppler Interferometry for Cloud Microphysical Measurements

Conducting accurate cloud microphysical measurements from airborne platforms poses a number of challenges. The technique of phase Doppler interferometry (PDI) confers numerous advantages relative to traditional light-scattering techniques for measurement of the cloud drop size distribution, and, in addition, yields drop velocity information. Here, we describe PDI for the purposes of aiding atmospheric scientists in understanding the technique fundamentals, advantages, and limitations in measuring cloud microphysical properties. The performance of the Artium Flight PDI, an instrument specifically designed for airborne cloud measurements, is studied. Drop size distributions, liquid water content, and velocity distributions are compared with those measured by other airborne instruments.

New Observations of Precipitation Initiation in Warm Cumulus Clouds

Abstract The mechanism responsible for formation of rain in warm clouds has been debated for over six decades. Here, the authors analyze new measurements of shallow cumulus made with a phase Doppler interferometer during the Rain in Cumulus over the Ocean (RICO) experiment. These observations show that drops sufficiently large (>55-μm diameter) to initiate precipitation (termed collision–coalescence initiators or CCIs) are found preferentially at cloud top, tend to cluster with each other, and are found in environments that are thermodynamically, dynamically, and microphysically distinct from those of smaller drops. The CCI environments exhibit cloud spectra that are shifted to larger sizes, with enhanced broadening toward larger drop sizes. Increased entrainment is also associated with CCIs, suggesting that it is an important process in CCI production. A simple model combining inhomogeneous mixing and condensation is inadequate to explain these observations. It is hypothesized that CCIs are produced in c...

Microphysical imprint of entrainment in warm cumulus

We analyse the cloud microphysical response to entrainment mixing in warm cumulus clouds observed from the CIRPAS Twin Otter during the GoMACCS field campaign near Houston, Texas, in summer 2006. Cloud drop size distributions and cloud liquid water contents from the Artium Flight phase-Doppler interferometer in conjunction with meteorological observations are used to investigate the degree to which inhomogeneous versus homogeneous mixing is preferred as a function of height above cloud base, distance from cloud edge and aerosol concentration. Using four complete days of data with 101 cloud penetrations (minimum 300 m in length), we find that inhomogeneous mixing primarily explains liquid water variability in these clouds. Furthermore, we show that there is a tendency for mixing to be more homogeneous towards the cloud top, which we attribute to the combination of increased turbulent kinetic energy and cloud drop size with altitude which together cause the Damköhler number to increase by a factor of between 10 and 30 from cloud base to cloud top. We also find that cloud edges appear to be air from cloud centres that have been diluted solely through inhomogeneous mixing. Theory predicts the potential for aerosol to affect mixing type via changes in drop size over the range of aerosol concentrations experienced (moderately polluted rural sites to highly polluted urban sites). However, the observations, while consistent with this hypothesis, do not show a statistically significant effect of aerosol on mixing type.