Observations of small cumuli with an airborne radar and instrumented aircraft.

摘要

Observations have been made in 6 small cumulus clouds using three instrumented aircraft, a ground-based radar, and an 95 GHz airborne Doppler radar. The clouds occurred on two days during the Small Cumulus Microphysics Study in east-central Florida, summer 1995. Cloud tops were below 3 km and in-cloud temperatures were warmer than 10°C.;Drizzle drops (drops with diameters greater than 50 mum) were found at all levels in cloud. The penetration-averaged concentration of such drops was normally less than 15 L-1, except for the upper-most levels in clouds on one of the days where drop concentrations exceeded 100 L-1. The formation of drizzle at low and mid-levels is attributed to collection by ultra-giant aerosols which act as coalescence nuclei. The development of drizzle at higher levels is consistent with models of droplet growth through condensation and stochastic collection.;The evolution of the kinematic structure of the observed clouds was tracked using measurements from both radars. High resolution cross-sections of reflectivity and vertical Doppler velocity from the airborne radar appear remarkably similar to fine-scale models of convection reported in the literature.;In general, each cloud resembled a collection of individual bubbles ascending through the boundary layer. During the growth phase of a bubble, a positive correlation existed between vertical velocity and reflectivity. As bubbles penetrated further into the inversion, entrainment/detrainment led to a weakening or, in some cases, a reversal of this correlation. Growth of subsequent bubbles ascending through remnants of earlier bubbles were aided by an increase in the amount of moisture in the environment resulting from earlier detrainment of cloudy air, and thus were able to achieve higher altitudes than their predecessors.;Droplet spectra in these clouds were generally bi-modal. Droplets within the larger mode resulted from growth through condensation. It remains unclear how droplets within the small mode formed as current hypotheses are unable to explain the constancy in size of the diameter of the small mode.