Structure and growth of the mixing layer over the Amazonian rain forest

Abstract

Measurements carried out over the rain forest of central Amazonia during the NASA Amazon Boundary Layer Experiment (ABLE 2A) are used to examine the structure and growth of the atmospheric mixed layer. Fluxes of sensible and latent heat were measured at the top of the canopy, and measurements of temperature, moisture, and horizontal wind were made in and above the mixed layer by means of a tethered balloon, rawinsonde, and aircraft. The mixing layer grows rapidly at 5–8 cm s−1 soon after sunrise to a mean maximum height of 1200 m by 1300 local standard time (LST). Mixed layer heights above 1000 m between 1000 and 1600 LST are common during undisturbed conditions. Fossil mixed layers, well mixed but no longer mixing, were found to 2000 m and at all times of the day and night. Airborne lidar measurements provided an estimate of the depth of the mixed layer over large distances. No horizontal inhomogeneities in the mixed layer structure or depth were found over large distances, suggesting that point measurements under undisturbed conditions can be used to obtain estimates of mixed layer structure and fluxes over large areas of the Amazon basin. Budget calculations using the averaged mixed layer depths and surface fluxes are made, which provide estimates of the entrainment flux of moisture across the mixed layer top of the order of 600 W m−2. The simple jump model, when initialized with an early‐morning sounding and forced with observed buoyancy fluxes, is compared with the budget calculation and observations. The entrainment flux calculations agree favorably. Flux divergence in the mixed layer leads to significant drying, which is observed at the top of the canopy.