Entrainment (meteorology)

Phenomenon of the atmosphere

Air flows in and around a convective cloud

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 cyclones, 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

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Diagram showing the variation of temperature (T) with height in the environment and the effect of the entrainment rate ( λ {\displaystyle \lambda } ) on the top height of a cumulonimbus

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 is one of the most sensitive variables causing uncertainty in climate models.[1]

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.[2][3] 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.[2][3]

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.[2]

## Entrainment rate

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Cumulus clouds 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 was the first to study the entrainment rate in cumulus clouds.[4]

## References

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1. ^ Knight CG, Knight SHE, Massey N, Aina T, Christensen C, et al., (2007). Association of parameter, software, and hardware variation with large-scale behavior across 57,000 climate models. Proc. Natl. Acad. Sci. USA 104, 12259–64
2. ^ a b c Jonas, P.R., (1996). Turbulence and cloud microphysics. Atmospheric Research, 40(2-4), 283-306, doi:10.1016/0169-8095(95)00035-6.
3. ^ a b Lu C., Y. Liu, S. Niu, S. Krueger, and T. Wagner, 2013: Exploring parameterization for turbulent entrainment-mixing processes in clouds. J. Geophys. Res., 118, 185-194.
4. ^ Stommel, H. Entrainment of air into a cumulus cloud. Journal of the Atmospheric Sciences, 1947, 4: 91-94.

## Further reading

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* Lu C., S. Niu, Y. Liu, A. Vogelmann, 2013: Empirical relationship between entrainment rate and microphysics in cumulus clouds. Geophys. Res. Lett., 40, 2333–2338.
* Lu C., Y. Liu, and S. Niu, 2013: A method for distinguishing and linking turbulent entrainment mixing and collision-coalescence in stratocumulus clouds. Chin. Sci. Bull., 58, 545–551.
* Lu C., Y. Liu, S. Niu, A. Vogelmann, 2012: Lateral entrainment rate in shallow cumuli: Dependence on dry air sources and probability density functions. Geophys. Res. Lett., 39, L20812.
* Lu C., Y. Liu, S. Yum, S. Niu, S. Endo, 2012: A new approach for estimating entrainment rate in cumulus clouds. Geophys. Res. Lett., 39, L04802.
* Lu C., Y. Liu, and S. Niu, 2014: Entrainment mixing parameterization in shallow cumuli and effects of secondary mixing events. Chinese Sci. Bull., 59(9), 896–903.
* Lu C., Y. Liu, and S. Niu, 2011: 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.