Abstract
Abstract. In the second part of this work we study the day-to-day variability of the marine atmospheric boundary layer (MBL) over the subtropical southeast Pacific using primarily results from a numerical simulation that covered the whole VOCALS-REx period (October–November 2008). In situ and satellite-derived observations of the MBL height in the offshore region indicate rapid, significant variations (from 500 m to 1700 m a.s.l. over a few days) during October. These MBL changes are connected with the passage of midlatitude troughs that altered the large-scale environment over the VOCALS-REx region. In contrast, the synoptic forcing and MBL changes were less prominent during November. Modelled and observed MBL depth at Point Omega (20° S, 85° W) compare quite well during October (but the simulation is on average 200 m lower) while in November the simulation does not perform as well. In the prognostic local MBL height equation the height change, the horizontal MBL height advection, and the large scale vertical velocity at MBL top are calculated explicitly from the simulation. The entrainment velocity is calculated as the residual of the other terms in the equation. While the vertical velocity and residual terms are opposing and generally have the largest magnitude on average, it is the variability in the advection that explains most of the large changes in the MBL depth. Examination of several cases during VOCALS-REx suggests that the advective term is in turn largely controlled by changes in wind direction, driven by midlatitude activity, acting on a MBL that generally slopes down toward the coast. In one phase, the subtropical anticyclone is reinforced and extends toward the Chilean coast, leading to easterly wind that advects low MBL heights from the coast as far as Point Omega. The opposite phase occurs after the passage of an extratropical cyclone over southern Chile, leading to southwesterly wind that advects a deeper MBL towards subtropical latitudes.
Highlights
Large scale subsidence dominates over the subtropical southeast Pacific (SEP) resulting in a strong inversion above a well-mixed marine atmospheric boundary layer (MBL)
Local change in MBL depth can be attributed to the vertical velocity at the top of the MBL, entrainment velocity, and horizontal advection (Schubert et al, 1979)
In the present paper we focus on basic synoptic features and day-to-day changes of the MBL using the terms in the prognostic MBL depth equation
Summary
Large scale subsidence dominates over the subtropical southeast Pacific (SEP) resulting in a strong inversion above a well-mixed marine atmospheric boundary layer (MBL). For much larger scales, Wood and Bretherton (2004) examined the mean state of each of the terms in the prognostic MBL depth equation using a synthesis of satellite and reanalysis data over large regions in the northeast and southeast Pacific. The different data sets acquired during VOCALS-REx were complemented with a 2-month long meteorological simulation using the Weather Research and Forecasting model (WRF, Skamarock et al, 2005) Both the observed datasets and setup of WRF are described in Rahn and Garreaud (2009a). Documenting the day-to-day changes in the MBL structure provides context to more detailed (e.g., aerosol measurements) studies during VOCALS-REx. Examination of the individual processes occurring within the MBL (i.e., generation of turbulent kinetic energy from cloud top radiative cooling, airsea transfer, decoupling, etc.) are largely ignored in favor of characterizing the terms of the prognostic MBL height equation that drive changes in the local MBL depth.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
