Abstract
Abstract. Multiplatform airborne, ship-based, and land-based observations from 16 October–15 November 2008 during the VOCALS Regional Experiment (REx) are used to document the typical structure of the Southeast Pacific stratocumulus-topped boundary layer and lower free troposphere on a~transect along 20° S between the coast of Northern Chile and a buoy 1500 km offshore. Strong systematic gradients in clouds, precipitation and vertical structure are modulated by synoptically and diurnally-driven variability. The boundary layer is generally capped by a strong (10–12 K), sharp inversion. In the coastal zone, the boundary layer is typically 1 km deep, fairly well mixed, and topped by thin, nondrizzling stratocumulus with accumulation-mode aerosol and cloud droplet concentrations exceeding 200 cm−3. Far offshore, the boundary layer depth is typically deeper (1600 m) and more variable, and the vertical structure is usually decoupled. The offshore stratocumulus typically have strong mesoscale organization, much higher peak liquid water paths, extensive drizzle, and cloud droplet concentrations below 100 cm−3, sometimes with embedded pockets of open cells with lower droplet concentrations. The lack of drizzle near the coast is not just a microphysical response to high droplet concentrations; smaller cloud depth and liquid water path than further offshore appear comparably important. Moist boundary layer air is heated and mixed up along the Andean slopes, then advected out over the top of the boundary layer above adjacent coastal ocean regions. Well offshore, the lower free troposphere is typically much drier. This promotes strong cloud-top radiative cooling and stronger turbulence in the clouds offshore. In conjunction with a slightly cooler free troposphere, this may promote stronger entrainment that maintains the deeper boundary layer seen offshore. Winds from ECMWF and NCEP operational analyses have an rms difference of only 1 m s−1 from collocated airborne leg-mean observations in the boundary layer and 2 m s−1 above the boundary layer. This supports the use of trajectory analysis for interpreting REx observations. Two-day back-trajectories from the 20° S transect suggest that eastward of 75° W, boundary layer (and often free-tropospheric) air has usually been exposed to South American coastal aerosol sources, while at 85° W, neither boundary-layer or free-tropospheric air has typically had such contact.
Highlights
The cool waters of the Southeast Pacific are blanketed by the world’s largest and most persistent subtropical stratocumulus regime, often extending 2000 km or more off the west coasts of Northern Chile, Peru and Ecuador
The focus of this paper is the mean structure of the boundary layer, clouds, and precipitation along this 20◦ S transect, as deduced from measurements taken on the two longrange Regional Experiment (REx) aircraft during numerous flights along 20◦ S supplemented by other relevant REx airborne, ship and surface observations
Other factors that may contribute include increased boundary-layer radiative cooling and possible slight reduction of mean subsidence further offshore. The latter is suggested by Quikscat satellite nearsurface divergence measurements shown in Fig. 8c of Rahn and Garreaud (2010a), but if those divergence estimates were integrated over the boundary layer depth, with no additional subsidence above, the subsidence above the inversion would be similar at all latitudes west of 72◦ W, consistent with short-range forecasts from several regional and largescale models shown in Fig. 3 of Wyant et al (2010)
Summary
The cool waters of the Southeast Pacific are blanketed by the world’s largest and most persistent subtropical stratocumulus regime, often extending 2000 km or more off the west coasts of Northern Chile, Peru and Ecuador. The focus of this paper is the mean structure of the boundary layer, clouds, and precipitation along this 20◦ S transect, as deduced from measurements taken on the two longrange REx aircraft during numerous flights along 20◦ S supplemented by other relevant REx airborne, ship and surface observations. This expands upon composite thermodynamic transects along 20◦ S based on REx data and prior ship observations by Rahn and Garreaud (2010a). For the REx period, these are more fully discussed and compared with a regional model by Rahn and Garreaud (2010b) and with a high-resolution global weather forecast model by Abel et al (2010)
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