The use of multiple thermodynamic and chemical tracers applied to entrainment analysis in warm cumulus bands

Entrainment analysis (Paluch, 1979; Betts, 1982) have traditionally used thermodunamic tracers to study the entrainment in cumulus. In warm, clouds there are oftern issues with temperature sensor wetting (Lemone, 1980), such that the calculation of conserved variables (e.g. wet equivalent potential temperature, Θ_q, and total water mixing ratio, Q) become erroneous. Other studies, e.g. Pearson and Weaver (1989) and Paluch, et al., (1995 ) have used chemical tracers (for instance ozone), that can be assumed to be conserved over short time scales. Faloona et al (2005) has used DMS, ozone and thermodynamic tracers to determine the entrainment flux through the top of stratocumulus clouds.

In marine cumulus clouds, such as the trade wind cumuli studied during the RICO project, the situation is difficult for determining the entrainment sources and amounts. This is because the cloud layer within the marine boundary-layer is continuously recirculating; that is, the difference between clouds and their environment in cumulus clouds is relatively small. The situation is compounded by the fact that they contain very large amounts of liquid water. The implication is that temperature sensor wetting may be a significant effect and that thermodynamic tracers, Θ_q and Q, cannot – in some cases – be considered to be conservative due to the fallout of rain water.

In this study we examine 5 band clouds; for these we have measurements of thermodynamic tracers (Θ_q and Q), fast ozone and DMS. Preliminary results show that adiabatic or near-adiabatic cores do exist in the RICO cumulus clouds, although the main character of the cloudy air shows significant dilution with entrained air. Using thermodynamic tracers, the near adiabatic cores can only be identified by considering both cloud and rain water, and by using the Lyman-alpha hygrometers and assuming saturation in cloudy air. The chemical tracers show promise, but the difference between the ozone concentration between the mixed-layer air and the cloud-layer air is small. DMS measurements show some promise, but the signal is also affected by some noise.