Empirical relationship between entrainment rate and microphysics in cumulus clouds

Abstract

1] The relationships between fractional entrainment rate and key microphysical quantities (e.g., liquid water content, droplet number concentration, volume mean radius, and standard deviation of cloud droplet size distributions) in shallow cumuli are empirically examined using in situ aircraft observations from the Routine Atmospheric Radiation Measurement Aerial Facility Clouds with Low Optical Water Depths Optical Radiative Observations (RACORO) field campaign over the Atmospheric Radiation Measurement Southern Great Plains site. The results show that the microphysical quantities examined generally exhibit strong relationships with entrainment rate and that the relationships collectively suggest the dominance of homogeneous entrainment mixing, which is unfavorable to the formation of large droplets and the initiation of warm rain in the clouds. The dominance of the homogeneous mixing mechanism is further substantiated by the dependency on entrainment rate of relationships among various microphysical variables and of cloud droplet size distributions. The dominance of this mechanism is also quantitatively confirmed by examining the degree of homogeneous mixing in the clouds. The dominance of homogeneous mixing may be an important reason why none of the cumulus clouds studied was drizzling.Citation: Lu, C., S. Niu, Y. Liu, and A. M. Vogelmann (2013), Empirical relationship between entrainment rate and microphysics in cumulus clouds, Geophys.

An Observational Study of Entrainment Rate in Deep Convection

“…Although the |r| is low, all correlations satisfy p < 0.01; thus, the results are still reliable. In addition, the weak negative correlations between entrainment rate and cloud microphysics are still similar to those found for shallow convection in other studies in spite of the noisy data [22,31]; the mechanism is that a larger entrainment rate results in more dilution and evaporation of cloud droplets.…”

“…Figure 5b reveals that the relationship between Rv and LWC is significantly positive and also indicates the potential dominance of homogeneous entrainment-mixing mechanism This is similar to the mechanism in the lower part of stratocumulus cloud in Lu et al [33] and cumulus cloud in Burnet and Brenguier [37]. Some other observational studies also found that homogeneous mechanism dominated in their analyzed clouds [22,36,38]. It is noteworthy that the entrainment of Cloud Condensation Nuclei (CCN) from environmental air and the activation of CCN can cause some uncertainties in the analyses on entrainment-mixing mechanisms, because the activation of entrained CCN can increase the number concentration of small cloud droplets.…”

“…The following criteria are used in this study. Liquid water content (LWC) must be larger than 0.001 g·m −3 and cloud droplet number concentration (N) must be greater than 10 cm −3 [22]. Given that the bulk-plume approach is only appropriate for developing clouds, the percentage of updraft within a cloud must exceed 80% [8] and the cloud penetration time must be longer than 5 s [11] to eliminate the clouds that are too small.…”

Variation in entrainment rate and relationship with cloud microphysical properties on the scale of 5 m

“…The same dataset from RACORO in Lu et al [35,38] [36] are examined. Comprehensive measurements of radiation, cloud, atmospheric state parameters, and aerosol were made by the Twin Otter aircraft from the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS).…”

Skewness of cloud droplet spectrum and an improved estimation for its relative dispersion

“…This instrument measures the aerosol and cloud droplets within a radius in the 0.29-25 lm range, performing 20 interval measurements for the aerosol and cloud particles. Only cloud particles larger than 1 lm of average radius are considered in this work and included to calculate the cloud microphysical quantities (Lu et al 2013). The experiment took more than 5 months and 260 flight hours, obtaining a large amount of data for the boundary layer cloud and aerosol properties.…”