Orographic Cirrus and Its Radiative Forcing in NCAR CAM6
Résumé
Introduction Cirrus clouds, covering over 30% of the Earth's surface, play an essential role in climate (Liou, 1986). Composed of ice crystals, they can reflect solar radiation with a cooling effect, absorb and reemit longwave radiation with a warming effect, and therefore strongly influence the Earth's radiative budgets. The net radiative effect of cirrus clouds and their feedback on global warming is still uncertain (Ceppi et al., 2017; Stephens, 2005; Wetherald & Manabe, 1988). Current global climate models (GCMs) produce a large spread in cirrus feedback, ranging from weakly negative to strongly positive feedback (−0.13-1.24 W m −2 K −1) (Ceppi et al., 2017). Vertical velocity is a critical factor in triggering the formation of ice crystals in cirrus clouds. A larger vertical velocity can generate a stronger adiabatic cooling, decrease the parcel's temperature, increase the relative humidity with respect to ice, activate more aerosols, and thus form more ice crystals (Kärcher & Lohmann, 2002). Previous studies show that, when vertical velocity increases from 10 to 100 cm s −1 , the number of newly-formed ice particles increases by more than one order of magnitude (Kärcher & Lohmann, 2002). The vertical velocity on smaller scales is more important to the nucleation of ice crystals in cirrus clouds than the synoptic-scale vertical velocity (
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