Evolution of the cross-equatorial atmospheric boundary layer in the east Pacific: observations and models
The NCAR C-130 research aircraft flew eight missions observing the atmospheric boundary layer (ABL) along 95°W, 1°S--12°N. The positive air-sea temperature difference over the equatorial cold tongue results in a shallow stable layer with reduced surface winds. Stratocumulus clouds at the ABL top tend to clear over the cold water, especially at times of enhanced humidity above the ABL. In the 0°--4°N ABL heat budget, cold advection and radiative cooling were balanced by surface and entrainment heating, where each of the four terms was about 30 W m-2. The humidity budget was a near balance between dry advection and surface evaporation (each about 150 W m-2). The entrainment rate estimated from the downstream-deepening of the inversion was 12 +/- 3 mm s -1.Principal component analysis of the sea-level pressure along 95°W, 1°S--12°N from daily TAO buoy observations and the eight flights shows that the principal mode of variability in the perturbation pressure explains 77% of the pressure variability. The pressure anomalies are the same at 1.6 km as at the surface. The timeseries of the first mode of the TAO observations shows that most of the variance is in the 2--7 day window. Low pressure at 12°N is associated with southerly and westerly surface wind anomalies, and enhanced convection in the ITCZ.A "quasi-Lagrangian" large-eddy simulation (LES) is used to model the ABL along 95°W from 8°S to 4°N. Large-scale tendencies are prescribed as a function of latitude. Surface stability accounts for the minimum in surface wind over the equatorial cold tongue and the maximum over the warm water to the north, in accordance with Wallace, Mitchell, and Deser (1989). Additional simulations show the robustness of the model ABL to changes in pressure gradients, zonal advection, free-tropospheric humidity, and initial conditions. Once formed at the southern edge of the cold tongue, modeled stratus clouds demonstrate a remarkable ability to maintain themselves over the cold tongue by radiative cooling at their tops.
- Atmospheric sciences