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Title : Sensitivities of Arctic clouds to climate change
Name of the speaker : Xiyue (Sally) Zhang
Affiliation : Environmental Science and Engineering, California Institute of Technology, Pasadena, CA
Laboratory organizer : LMD
Date and time : 10-11-2017 11h00
Location : Ecole Normale Supérieure, rue d'Ulm, salle E314
Summary :

The Arctic has been experiencing rapid changes in recent decades. In addition to the prominent sea ice loss, Arctic amplification is a robust feature in climate models under greenhouse warming and has also been observed over the past decades. Cloud feedbacks and the trapping of heat under the stable inversion are thought to contribute to this Arctic amplification of global warming. However, the sign of any cloud feedback in high latitudes is uncertain. Understanding and constraining Arctic cloud feedback is difficult because of the ubiquitous temperature inversion in high latitudes and the presence of mixed-phase clouds, both of which are challenging to capture in global climate models (GCMs). Here we use high-resolution large-eddy simulations (LES) with a one-moment mixed-phase microphysics scheme to investigate how Arctic clouds respond to climate changes. To represent changing large-scale conditions such as meridional moisture advection in a realizable way, we drive the LES with output from a GCM: temperature and moisture tendencies from a grid cell at 82N are used as forcing terms in the LES. We find that low-cloud fraction decreases with increasing temperature across a wide range of climates. The cloud fraction changes are associated with a change in cloud regimes from stratocumulus in the coldest climate, to cumulus in the warmest climate. This can be understood from changes in the saturation deficit, the difference between specific humidity and saturation specific humidity, which increases with temperature because relative humidity changes in the boundary layer are modest. We also investigate the relationship between lower-tropospheric stability and cloud fraction. A sensitivity experiment that uniformly increases temperature in an atmospheric columns shows that decreased cloud fraction can be associated with increased lower tropospheric stability, contrary to common beliefs. Implications for cloud feedbacks in the Arctic are discussed.

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