History and Applications of Giga Large-Eddy Simulations

The first Giga LES (large-eddy simulation) was performed by Marat Khairoutdinov with SAM (System for Atmospheric Modeling) in 2008. The model domain (204.8 km x 204.8 km x 27 km) contained 1 billion grid points (2048 x 2048 x 256). The horizontal grid size was 100 m. The vertical grid size was 50 m below 1250 m, stretching to 100 m by 5000 m. The case was based on GATE (GARP Atlantic Tropical Experiment), and was 24 h in duration. The 100-m grid size allowed direct comparison to aircraft data. The agreement with the observed convective updraft and downdraft vertical velocity statistics was remarkably good. Because it was a LES, the large turbulent eddies in the boundary layer and the clouds were resolved. Therefore entrainment into the boundary layer and into convective updrafts and downdrafts was largely resolved. This is evident from the image which is a rendering of part of the domain from this simulation. In addition, convective circulations were resolved into sub-cloud scale drafts, which more realistically depicts the interaction between convective updraft and downdraft structures and precipitation. The second Giga LES using SAM was based on TWP-ICE (Tropical Warm Pool - International Cloud Experiment). It was a multiday simulation; it used time-varying large-scale forcing based on observations, double-moment microphysics, and interactive radiation, unlike the first Giga LES. Another new aspect of this simulation was collecting 146 million Lagrangian Parcel Trajectories (LPTs) over a two-hour period at 30-s intervals. We are still using these LPTs to analyze parcel properties in updrafts and downdrafts. Lagrangian analyses offer many advantags over Eulerian (grid-point) analyses. LES has also been applied to large-domain boundary-layer cloud systems. For example, we recently simulated an Arctic cold-air outbreak using a domain that moved with the mean wind over increasingly warmer SSTs for 15 h. The domain for this case was 64 km x 64 km x 5.5 km with 100-m horizontal grid size and 50-m vertical grid size, for a total of 45 million grid points.