Wednesday, 12 February 2003: 1:30 PM
LES modeling of the cross-equatorial trade boundary layer along 95W
Using a large eddy simulation (LES) forced with typical conditions observed in the East Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System (EPIC) experiment during September and October of 2001, we simulate the evolution of a Lagrangian parcel in the atmospheric boundary layer as it traverses the equator northward along 95°W longitude. The LES models a 3-km cubic domain for 5 days at a resolution of ~50 meters in both horizontal dimensions and 25 meters in the vertical. During this time the modeled parcel travels northward from 8°S, in the stratus regime, to 5°N, where deep convection begins. The simulation matches the observations well. The modeled stratus-topped boundary layer shallows from 8°S to the equator in response to a cooling sea surface temperature (SST) and large-scale subsidence from above. Over the coldest water at the equatorial cold tongue, the LES reproduces the dramatic decrease in the southerly surface winds and vertical shear of 0.01 s-1 in the lowest ~400 meters of the atmosphere. The rapid remixing and corresponding increase in surface wind as the parcel crosses the warm SST front (at ~0.5°N during September 2001) is also reproduced by the LES. Two mechanisms have been proposed explain the coupling between SST and surface wind in the tropics. Lindzen and Nigam (1987) proposed that warm SST induces hydrostatic low surface pressure, and winds converge down the pressure gradient toward warmer SST, while Wallace et al (1989) proposed that relatively stable stratification over cold SST supports more shear and lower surface winds, and unstable stratification over warm SST mixes more momentum down to the surface. SST-induced pressure gradients drive the large-scale southerlies that advect the model domain. However, since the modeled length scales are too small to adjust to SST-induced pressure gradients, the fact that the model reproduces the meridional surface wind divergence at the SST front is attributed to low-altitude wind shear maintained by internal boundary-layer stability on the cold side of the SST front, and meridional momentum flux driven by surface instability on the warm side.