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

by C. S. Bretherton, R. Wood, R. C. George, D. Leon, G. Allen, X. Zheng
Atmospheric Chemistry and Physics ()
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Multiplatform airborne, ship-based, and land-based observations from 16\nOctober-15 November 2008 during the VOCALS Regional Experiment (REx) are\nused to document the typical structure of the Southeast Pacific\nstratocumulus-topped boundary layer and lower free troposphere on a\ntransect along 20 degrees S between the coast of Northern Chile and a\nbuoy 1500 km offshore. Strong systematic gradients in clouds,\nprecipitation and vertical structure are modulated by synoptically and\ndiurnally-driven variability. The boundary layer is generally capped by\na strong (10-12 K), sharp inversion. In the coastal zone, the boundary\nlayer is typically 1 km deep, fairly well mixed, and topped by thin,\nnondrizzling stratocumulus with accumulation-mode aerosol and cloud\ndroplet concentrations exceeding 200 cm(-3). Far offshore, the boundary\nlayer depth is typically deeper (1600 m) and more variable, and the\nvertical structure is usually decoupled. The offshore stratocumulus\ntypically have strong mesoscale organization, much higher peak liquid\nwater paths, extensive drizzle, and cloud droplet concentrations below\n100 cm(-3), sometimes with embedded pockets of open cells with lower\ndroplet concentrations. The lack of drizzle near the coast is not just a\nmicrophysical response to high droplet concentrations; smaller cloud\ndepth and liquid water path than further offshore appear comparably\nimportant. Moist boundary layer air is heated and mixed up along the\nAndean slopes, then advected out over the top of the boundary layer\nabove adjacent coastal ocean regions. Well offshore, the lower free\ntroposphere is typically much drier. This promotes strong cloud-top\nradiative cooling and stronger turbulence in the clouds offshore. In\nconjunction with a slightly cooler free troposphere, this may promote\nstronger entrainment that maintains the deeper boundary layer seen\noffshore.\nWinds from ECMWF and NCEP operational analyses have an rms difference of\nonly 1 m s(-1) from collocated airborne leg-mean observations in the\nboundary layer and 2 m s(-1) above the boundary layer. This supports the\nuse of trajectory analysis for interpreting REx observations. Two-day\nback-trajectories from the 20 degrees S transect suggest that eastward\nof 75 degrees W, boundary layer (and often free-tropospheric) air has\nusually been exposed to South American coastal aerosol sources, while at\n85 degrees W, neither boundary-layer or free-tropospheric air has\ntypically had such contact.

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