An airborne study of vertical structure and microphysical variability within a small cumulus

Abstract

SUMMARYOn 27 July 1981, as part of the CCOPE experiment, the University of Wyoming King Air research aeroplane made penetrations at six levels ranging from 590mb (-4.9"C) to 770mb (7.2"C) through a nonprecipitating, unglaciated cumulus cloud whose top was at about 560mb (-7°C) and base about 780mb (8°C). The entire flight sequence-from the top downwards-took about six minutes. Principal measurements were of droplet size distribution N ( d ) , number concentration N and liquid water content L , measured at 10Hz (-10m spatial resolution); drop-count N' (50Hz, -2m); temperature T. Also we calculated the fraction F of cloud base air in the mixture sampled ( l H z , -100m). The cloud was substantially subadiabatic throughout the volume studied. Paluch analyses, the observed air motions near cloud top, buoyancy profiles and the observed distribution of L and F with altitude demonstrated that the cloud was diluted principally by entrainment from cloud top. Considerable scatter was found in all microphysical and thermodynamic variables on all spatial scales. Large variations in L were primarily attributable to fluctuations in N and N' . Bimodal spectra were sometimes found at the interface between statistically smooth and highly variable regions. The breadth of the droplet spectra and the size of the largest droplets in the condensate spectrum were not systematically related to the degree of dilution. Calculations showed that a small fraction of the droplets (-100litres-') exhibited superadiabatic growth of up to 4 pm diameter at the intermediate levels, but not near cloud base or cloud top. Support for the idea that droplets in lower concentrations (-10 litre-') grew even faster was provided by 1-D probe measurements of droplets of up to 75 pm diameter in the central and upper regions of the cloud.

Droplet Activation and Mixing in Large-Eddy Simulation of a Shallow Cumulus Field

“…The simulated approximately constant-with-height mean cloud droplet concentration is broadly consistent with many cloud observations, including observations of Montana cumuli reported in Blyth and Latham [1985 (their Table 2 and Fig. 7), 1991 (their Table 1)] and RICO trade-wind cumuli discussed in Gerber et al [2008 (see their Table 3 in particular)] and in Arabas et al (2009, their Fig.…”

Vertical Velocity Statistics in Fair-Weather Cumuli at the ARM TWP Nauru Climate Research Facility

“…Observations of vertical air motions in shallow clouds are typically an order of magnitude higher (e.g., Warner 1977;Blyth and Latham 1985;French et al 1999;Kollias et al 2001;Damiani et al 2008). As a result, we can use the cloud droplets as a tracer of the vertical air motion in fair-weather cumuli.…”

Cloud microphysical relationships and their implication on entrainment and mixing mechanism for the stratocumulus clouds measured during the VOCALS project

“…Yum and Hudson [] also convincingly showed that not only in the SCMS cumulus clouds but also in the stratocumulus clouds measured during several other aircraft campaigns, the correlation between droplet concentration and mean diameter was positive in a large portions of the data sets. This apparent lack of evidence for so‐called superadiabatic droplets [ Blyth and Latham , ] in diluted cloud parcels seems to suggest that mixing of entrained air may not generally promote droplet growth.…”