Entrainment is a phenomenon in the atmosphere that occurs when a turbulent flow captures a non-turbulent flow. It is typically used to refer to the capture of a wind flow with high moisture content or in the case of

tropical cyclone A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its locat ...

s, the capture of drier air. Detrainment is the opposite effect, when the air from a convective cloud, usually at its top, is injected into the environment.

Theory

Entrainment is mixing environmental air into a preexisting air current or cloud so that the environmental air becomes part of the current or cloud. The ''entrainment coefficient'' in

clouds In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may c ...

is one of the most sensitive variables causing uncertainty in

climate models Numerical climate models (or climate system models) are mathematical models that can simulate the interactions of important drivers of climate. These drivers are the atmosphere, oceans, land surface and ice. Scientists use climate models to stu ...

. Homogeneous mixing is a model that assumes that the timescale for the mixing within a cloud is short compared to the evaporation timescale. This would imply that the dry, unsaturated, environmental air would be entrained throughout the cloud before it evaporates the cloud droplets. The entrainment mixing resulting from this model manifests as partial evaporation of all the drops within the cloud but no change in the number of cloud drops.Jonas, P.R., (1996). Turbulence and cloud microphysics. ''Atmospheric Research'', 40(2-4), 283-306, .Lu C., Y. Liu, S. Niu, S. Krueger, and T. Wagner, 2013

J. Geophys. Res., 118, 185-194. A contrasting model of entrainment is inhomogeneous mixing. This model assumes that the time it takes to evaporate cloud drops is short compared to the mixing timescales. Therefore, the saturated air that mixes with the unsaturated environment air would completely evaporate the cloud drops within the entrained area, reducing the total number of cloud drops. The major difference between the two models is how they affect the shape of the cloud drop spectrum. Homogeneous mixing changes the spectrum's shape because the supersaturation differs over large and minor drops. Exposing a cloud to homogeneously mixed entrained air will result in a narrower cloud drop spectrum, while inhomogeneous mixing does not change the spectrum.

Entrainment rate

Cumulus cloud Cumulus clouds are clouds that have flat cloud base, bases and are often described as puffy, cotton-like, or fluffy in appearance. Their name derives from the Latin , meaning "heap" or "pile". Cumulus clouds are low-level clouds, generally less ...

s significantly impact energy and water vapor transport and influence precipitation and climate. In large-scale models, cumulus clouds need to be parameterized. The entrainment rate is a key parameter in cumulus parameterization.

Henry Stommel Henry Melson Stommel (September 27, 1920 – January 17, 1992) was a major contributor to the field of physical oceanography. Beginning in the 1940s, he advanced theories about global ocean circulation patterns and the behavior of the Gulf Strea ...

was the first to study the entrainment rate in cumulus clouds.Stommel, H. Entrainment of air into a cumulus cloud. Journal of the Atmospheric Sciences, 1947, 4: 91-94.

References

* Lu C., S. Niu, Y. Liu, A. Vogelmann, 2013

Geophys. Res. Lett., 40, 2333–2338. * Lu C., Y. Liu, and S. Niu, 2013

Chin. Sci. Bull., 58, 545–551. * Lu C., Y. Liu, S. Niu, A. Vogelmann, 2012

Geophys. Res. Lett., 39, L20812. * Lu C., Y. Liu, S. Yum, S. Niu, S. Endo, 2012

Geophys. Res. Lett., 39, L04802. * Lu C., Y. Liu, and S. Niu, 2014

Chinese Sci. Bull., 59(9), 896–903. * Lu C., Y. Liu, and S. Niu, 2011
{{fontcolor, blue, Examination of turbulent entrainment-mixing mechanisms using a combined approach
J. Geophys. Res., 116, D20207. * Burnet, F., and J.L. Brenguier, 2007: Observational Study of the Entrainment-Mixing Process in Warm Convective Clouds. J. Atmos. Sci., 64, 1995–2011. * Chosson, F., J.L. Brenguier, and L. Schüller, 2007: Entrainment-Mixing and Radiative Transfer Simulation in Boundary Layer Clouds. J. Atmos. Sci., 64, 2670–2682. * Hill, A.A., G. Feingold, and H. Jiang, 2009: The Influence of Entrainment and Mixing Assumption on Aerosol–Cloud Interactions in Marine Stratocumulus. J. Atmos. Sci., 66, 1450–1464. * Hicks, E., C. Pontikis, and A. Rigaud, 1990: Entrainment and Mixing Processes as Related to Droplet Growth in Warm Midlatitude and Tropical Clouds. J. Atmos. Sci., 47, 1589–1618. * Pontikis, C.A., and E.M. Hicks, 1993: Droplet Activation as Related to Entrainment and Mixing in Warm Tropical Maritime Clouds. J. Atmos. Sci., 50, 1888–1896. * Baker, B.A., 1992: Turbulent Entrainment and Mixing in Clouds: A New Observational Approach. J. Atmos. Sci., 49, 387–404. * Paluch, I.R., 1979: The Entrainment Mechanism in Colorado Cumuli. J. Atmos. Sci., 36, 2467–2478. * Baker, M., and J. Latham, 1979: The Evolution of Droplet Spectra and the Rate of Production of Embryonic Raindrops in Small Cumulus Clouds. J. Atmos. Sci., 36, 1612–1615. * Baker, M.B., R.G. Corbin, and J. Latham, 1980, The influence of entrainment on the evolution of cloud droplet spectra: I. A model of inhomogeneous mixing. Quarterly Journal of the Royal Meteorological Society, 106, 581–598. Cloud and fog physics

Theory

Entrainment rate

References

Further reading