Quasi-Lagrangian Large Eddy Simulations of Cross-Equatorial Flow in the East Pacific Atmospheric Boundary Layer

Abstract

Using a large eddy simulation (LES), the atmospheric boundary layer (ABL) is numerically modeled along 95°W from 8°S to 4°N during boreal autumn, and compared to observations from the East Pacific Investigation of Climate Processes in the Coupled Ocean?Atmosphere System (EPIC) 2001. Since the local ABL winds are predominantly southerly in this season, a ?quasi-Lagrangian? forcing is used in which the ABL air column is forced as if it were advecting northward with the mean September?October 2001 meridional wind across the equatorial cold tongue and the rapidly warming SSTs to the north. Pressure gradients and large-scale zonal advective tendencies are prescribed as a function of latitude. Where possible, observations from the EPIC 2001 experiment are used for forcing and for comparison with model results. The ABL's modeled vertical structure accords with the conceptual model of Wallace et al. and agrees well with observations. Surface stability accounts for the minimum in surface wind over the equatorial cold tongue and the maximum over the warm water to the north. Stability of the lower ABL over the cold tongue allows a jet to accelerate at about 500-m height, relatively uncoupled to the frictional surface layer. Vertical mixing over the warm water to the north distributes this momentum to the surface. Additional simulations were performed to explore the modeled ABL's sensitivity to pressure gradients, zonal advection, free-tropospheric humidity, and initial conditions. The model ABL was robust: changing the forcings resulted in little change in the modeled structure. The strongest sensitivity was of stratocumulus clouds over the cold tongue to cloud-top radiative cooling. Once formed at the southern edge of the cold tongue, modeled stratocumulus clouds demonstrate a remarkable ability to maintain themselves over the cold tongue in the absence of surface fluxes by radiative cooling at their tops. The persistence of thin stratocumulus clouds in this Lagrangian model suggests that horizontal advection of condensate might be an important process in determining cloudiness over the cold tongue.