 |
| Titel |
Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 2: Synoptic variability |
| VerfasserIn |
D. A. Rahn, R. Garreaud |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 10, no. 10 ; Nr. 10, no. 10 (2010-05-18), S.4507-4519 |
| Datensatznummer |
250008458
|
| Publikation (Nr.) |
 copernicus.org/acp-10-4507-2010.pdf |
|
|
|
|
|
| Zusammenfassung |
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. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|