Studying Entrainment in the Laboratory

Entrainment of dry air into clouds influences their optical properties, response to aerosol perturbations, and the likelihood of precipitation. In addition to theoretical work, which established the two extremes of homogeneous and inhomogeneous mixing, entrainment has been studied extensively in field campaigns and simulations, but rarely in a laboratory setting. Latham and Reed’s seminal 1977 paper is a notable exception. We recently modified the Michigan Tech Pi Chamber such that we could observe the effect of entrained dry air on the properties of a pre-existing, steady-state cloud. Clouds with well defined, steady microphysical properties (e.g. number concentration and size distribution of droplets) can be created and held in the chamber for hours at a time. We take advantage of the large scale circulation in the chamber to measure drop size distributions before and after dry air is mixed into the cloud. Measurements conducted immediately before and immediately after mixing always show the signature of inhomogeneous mixing. When we compare experiments across different steady state regimes, the signature is one of homogeneous mixing. These results imply that system-wide versus local sampling of clouds can lead to seemingly contradictory results for mixing, which informs the long-standing debate about the microphysical response to entrainment, and the parameterization of this process for coarse-resolution models.