Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 6 2009 10.5194/acpd-9-26063-2009 http://www.atmos-chem-phys-discuss.net/9/26063/2009/ http://www.atmos-chem-phys-discuss.net/9/26063/2009/acpd-9-26063-2009.html http://www.atmos-chem-phys-discuss.net/9/26063/2009/acpd-9-26063-2009.pdf 26063 26094 2009-12-07 Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 2: Synoptic variability D. A. Rahn darahn@gmail.com R. D. Garreaud Departamento de Geofísica, Universidad de Chile, Santiago, Chile 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&deg; S, 85&deg; 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. <br><br> Each term in the prognostic local MBL height equation (horizontal MBL height advection, large scale vertical velocity at MBL top, and entrainment velocity) is calculated explicitly from the simulation except the entrainment velocity which 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. Barrett,~B., Garreaud,~R D., and Falvey,~M.: Impacts of the Andes cordillera on precipitation from a~midlatitude cold front, Mon. Weather Rev., 137, 3092–3109, 2009. 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