Numerical Simulation of the Boundary-Layer Eddy Structureduring the Cold-Air Outbreak of GALE IOP 2

Abstract

The boundary-layer eddy structure under conditions similar to the cold-air outbreak of GALE IOP 2 is studied using numerical simulations. The simulations are run in two basic modes: a quasi-two-dimensional version that takes advantage of the observed ?cloud-street? character of the flow, and a fully three-dimensional, unsteady simulation on a limited domain where periodic conditions are assumed to prevail. The two-dimensional simulation exhibits a cloud structure similar to that observed when the surface fluxes agree with the aircraft measurements. This requires very different values of effective surface roughness for temperature and humidity, which is unlikely to have been assumed in the absence of data. The three-dimensional simulation reveals that even when the eddy structure on this severely limited domain does not exhibit a dominant two-dimensional roll structure, the average turbulent statistics are quite consistent with those from the two-dimensional simulation. It is argued that a larger domain than can be readily used is needed to see a distinct cloud street pattern.