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Observations of the boundary layer, cloud, and aerosol variability in the southeast Pacific near-coastal marine stratocumulus during VOCALS-REx

by X. Zheng, B. Albrecht, H. H. Jonsson, D. Khelif, G. Feingold, P. Minnis, K. Ayers, P. Chuang, S. Donaher, D. Rossiter, V. Ghate, J. Ruiz-Plancarte, S. Sun-Mack show all authors
Atmospheric Chemistry and Physics ()

Abstract

Aircraft observations made off the coast of northern Chile in the\nSoutheastern Pacific (20 degrees S, 72 degrees W; named Point Alpha)\nfrom 16 October to 13 November 2008 during the VAMOS\nOcean-Cloud-Atmosphere-Land Study-Regional Experiment (VOCALS-REx),\ncombined with meteorological reanalysis, satellite measurements, and\nradiosonde data, are used to investigate the boundary layer (BL) and\naerosol-cloud-drizzle variations in this region. On days without\npredominately synoptic and meso-scale influences, the BL at Point Alpha\nwas typical of a non-drizzling stratocumulus-topped BL. Entrainment\nrates calculated from the near cloud-top fluxes and turbulence in the BL\nat Point Alpha appeared to be weaker than those in the BL over the open\nocean west of Point Alpha and the BL near the coast of the northeast\nPacific. The cloud liquid water path (LWP) varied between 15 gm(-2) and\n160 gm(-2). The BL had a depth of 1140 +/- 120 m, was generally\nwell-mixed and capped by a sharp inversion without predominately\nsynoptic and mesoscale influences. The wind direction generally switched\nfrom southerly within the BL to northerly above the inversion. On days\nwhen a synoptic system and related mesoscale costal circulations\naffected conditions at Point Alpha (29 October-4 November), a moist\nlayer above the inversion moved over Point Alpha, and the total-water\nmixing ratio above the inversion was larger than that within the BL.\nThe accumulation mode aerosol varied from 250 to 700 cm(-3) within the\nBL, and CCN at 0.2% supersaturation within the BL ranged between 150\nand 550 cm(-3). The main aerosol source at Point Alpha was horizontal\nadvection within the BL from south. The average cloud droplet number\nconcentration ranged between 80 and 400 cm(-3). While the mean LWP\nretrieved from GOES was in good agreement with the in situ measurements,\nthe GOES-derived cloud droplet effective radius tended to be larger than\nthat from the aircraft in situ observations near cloud top. The aerosol\nand cloud LWP relationship reveals that during the typical well-mixed BL\ndays the cloud LWP increased with the CCN concentrations. On the other\nhand, meteorological factors and the decoupling processes have large\ninfluences on the cloud LWP variation as well.

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