Southeast Pacific stratocumulus clouds, precipitation and boundary layer structure sampled along 20° S during VOCALS-REx
Southeast Pacific stratocumulus clouds, precipitation and boundary layer structure sampled along 20° S during VOCALS-REx
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DOI：
10.5194/acp-10-10639-2010
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发表时间：
2010-07
影响因子：
6.3
通讯作者：
C. Bretherton;R. Wood;R. George;D. Leon;G. Allen;X. Zheng
中科院分区：
文献类型：
--
作者：
C. Bretherton;R. Wood;R. George;D. Leon;G. Allen;X. Zheng
抽象的。 2008年10月16日至11月15日VOCALS区域实验（REx）期间的多平台机载、船基和陆基观测被用来记录东南太平洋层积云顶部边界层和低层自由对流层的典型结构，该结构位于智利北部海岸和离岸1500公里浮标之间沿南纬20°的横断面上。云、降水和垂直结构的强系统梯度受到天气和昼夜驱动的变化的调节。边界层通常覆盖着强烈的（10-12 K）、急剧的反转。在沿海地区，边界层通常深 1 公里，混合得相当好，顶部是薄的、无毛毛雨的层积云，累积模式气溶胶和云滴浓度超过 200 cm−3。离岸较远的地方，边界层深度通常更深（1600 m）且变化更大，并且垂直结构通常是解耦的。近海层积云通常具有强大的中尺度组织、更高的峰值液态水路径、广泛的毛毛雨和低于 100 cm−3 的云滴浓度，有时还嵌入了具有较低滴浓度的开放细胞袋。海岸附近没有毛毛雨不仅仅是对高液滴浓度的微物理反应；与离岸较远的地方相比，较小的云层深度和液态水路径显得相当重要。潮湿的边界层空气沿着安第斯山坡被加热并混合，然后从邻近沿海海洋区域上方的边界层顶部平流出去。在离岸较远的地方，较低的自由对流层通常要干燥得多。这促进了强烈的云顶辐射冷却和近海云层中更强的湍流。与稍冷的自由对流层相结合，这可能会促进更强的夹带，从而维持近海更深的边界层。 ECMWF 和 NCEP 业务分析得出的风与边界层中并置的机载腿平均观测值的均方根差异仅为 1 m s−1，而边界层上方的风则为 2 m s−1。这支持使用轨迹分析来解释 REx 观测结果。南纬 20° 断面的两天回溯轨迹表明，在西经 75° 以东，边界层（通常是自由对流层）空气通常暴露于南美沿海气溶胶源，而在西经 85° 时，边界层或自由对流层空气通常都没有接触过这种接触。
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.