Orographic Cirrus and Its Radiative Forcing in NCAR CAM6

Abstract

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 (Mace et al., 2001;Quante & Starr, 2002). In the lower stratosphere and upper troposphere, mesoscale gravity waves are ubiquitous and usually generate strong disturbances (

Significant trend in vertical wind velocity variability over the last 40 years

“…Compared to traditional GCMs these simulations can better capture convection, vertical transport and cloud microphysics [16]. However, even at such scale GCRMs still misrepresent high frequency gravity waves [17] and turbulence, for which even finer resolution may be required [18][19][20]. Nevertheless, the GCRM simulations and the extensive data they produce represent a wealth of information that can be used to understand and model σ W .…”