Southeast Pacific stratocumulus clouds, precipitation and boundary layer structure sampled along 20° S during VOCALS-REx

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Multiplatform airborne, ship-based, and land-based observations from 16<br />October-15 November 2008 during the VOCALS Regional Experiment (REx) are<br />used to document the typical structure of the Southeast Pacific<br />stratocumulus-topped boundary layer and lower free troposphere on a<br />transect along 20 degrees S between the coast of Northern Chile and a<br />buoy 1500 km offshore. Strong systematic gradients in clouds,<br />precipitation and vertical structure are modulated by synoptically and<br />diurnally-driven variability. The boundary layer is generally capped by<br />a strong (10-12 K), sharp inversion. In the coastal zone, the boundary<br />layer is typically 1 km deep, fairly well mixed, and topped by thin,<br />nondrizzling stratocumulus with accumulation-mode aerosol and cloud<br />droplet concentrations exceeding 200 cm(-3). Far offshore, the boundary<br />layer depth is typically deeper (1600 m) and more variable, and the<br />vertical structure is usually decoupled. The offshore stratocumulus<br />typically have strong mesoscale organization, much higher peak liquid<br />water paths, extensive drizzle, and cloud droplet concentrations below<br />100 cm(-3), sometimes with embedded pockets of open cells with lower<br />droplet concentrations. The lack of drizzle near the coast is not just a<br />microphysical response to high droplet concentrations; smaller cloud<br />depth and liquid water path than further offshore appear comparably<br />important. Moist boundary layer air is heated and mixed up along the<br />Andean slopes, then advected out over the top of the boundary layer<br />above adjacent coastal ocean regions. Well offshore, the lower free<br />troposphere is typically much drier. This promotes strong cloud-top<br />radiative cooling and stronger turbulence in the clouds offshore. In<br />conjunction with a slightly cooler free troposphere, this may promote<br />stronger entrainment that maintains the deeper boundary layer seen<br />offshore.<br />Winds from ECMWF and NCEP operational analyses have an rms difference of<br />only 1 m s(-1) from collocated airborne leg-mean observations in the<br />boundary layer and 2 m s(-1) above the boundary layer. This supports the<br />use of trajectory analysis for interpreting REx observations. Two-day<br />back-trajectories from the 20 degrees S transect suggest that eastward<br />of 75 degrees W, boundary layer (and often free-tropospheric) air has<br />usually been exposed to South American coastal aerosol sources, while at<br />85 degrees W, neither boundary-layer or free-tropospheric air has<br />typically had such contact.




Bretherton, C. S., Wood, R., George, R. C., Leon, D., Allen, G., & Zheng, X. (2010). Southeast Pacific stratocumulus clouds, precipitation and boundary layer structure sampled along 20° S during VOCALS-REx. Atmospheric Chemistry and Physics, 10(21), 10639–10654.

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