Estimation of the Vertical Velocity Leading to the Formation of Cirrus Using Ultra-High Resolution Global Simulations

Abstract:

Cirrus clouds significantly impact the radiative and transport processes of the upper troposphere and the lower stratosphere. State-of-the-art global models parameterize the formation of cirrus explicitly linking ice nucleation events to the aerosol properties and the cloud-scale dynamics. However most GCMs cannot resolve the scale at which cloud formation occurs. Thus subgrid scale dynamics is typically parameterized by relating the vertical velocity variance, σ_w, to grid-scale fields. These parameterizations are typically validated against field campaign data for specific locations. However an assessment of the global spatial distribution of σ_w is lacking, limiting the ability of GCMs to describe cirrus formation. Here the non-hydrostatic version of the NASA Goddard Earth Observing System model (GEOS-5) is used to estimate the variance of vertical velocity in GCMs. GEOS-5 was run at cloud-resolving resolutions (~7 km), allowing the explicit calculation of σ_w. Our results indicate that σ_w is determined by orographic drag and local convection, and higher over the continents than over the ocean. A recently developed parameterization of σ_w is also evaluated. Compared to the model results the parameterization is able to reproduce the global distribution of σ_w for warm cirrus clouds but tends to overestimate σ_w near the tropopause. Our work provides for the first time an assessment of the global variability in the subgrid scale dynamics leading to the formation of cirrus.