Daytime Mixed Layer over the Santiago Basin: Description of Two Years of Observations with a Lidar Ceilometer

Abstract

Two years of high-resolution backscatter profiles obtained with a commercial lidar ceilometer in Santiago Basin (33.5°S, 70.6°W) are analyzed. The generally large aerosol load in the Santiago atmospheric boundary layer (ABL) facilitates the use of these backscatter profiles for the retrieval of the daytime mixed layer height (MH), especially around midday. In winter mornings, however, MH retrievals are frequently confused by upper residual aerosol layers, while in summer afternoons very low aerosol concentrations often preclude them. Based on a database formed with successful MH retrievals over Santiago, the hourly, synoptic, and seasonal variability of clear-day MHs are documented. Daytime growth rates of MH show typical values of 50 m h?1 in winter and 100 m h?1 in summer. MHs at 1200 LT (UTC ? 4 h) present a fourfold change between the cold months (MH ?200 m) and the warm months (MH ?800 m). Interquartile ranges of the monthly distributions of MH are about 200 m, with little change along the seasons. This statistical description of MH data is supplemented with analysis of temperature, solar radiation, and wind data at selected stations located at the basin?s floor, at the basin?s entrance, and at an elevated location representative of a level about 400 m above the basin?s floor. Seasonal MH variability appears highly controlled by the surface energy budget, with about 30% of the top-of-the-atmosphere radiative energy being used in the warming and growth of the daytime MH. Advection of cool air from the marine boundary layer to the west of the basin also appears to be important in the basin?s ABL energy budget in some cases. Stability of the early morning temperature profile in the basin?s air mass is also a factor in the mixed layer growth. Under conditions of large bulk stability of the basin?s air mass, there exist cases of very shallow daytime mixed layers that appear to develop after nights in which the stability is highly enhanced near the surface. Results herein are a first step toward a better understanding of the dynamics of this complex terrain ABL.