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

Abstract

Abstract. Aircraft observations made off the coast of northern Chile in the Southeastern Pacific (20 • S, 72 • W; named Point Alpha) from 16 October to 13 November 2008 during the VAMOS Ocean-Cloud-Atmosphere-Land StudyRegional Experiment (VOCALS-REx), combined with meteorological reanalysis, satellite measurements, and radiosonde data, are used to investigate the boundary layer (BL) and aerosol-cloud-drizzle variations in this region. On days without predominately synoptic and meso-scale influences, the BL at Point Alpha was typical of a non-drizzling stratocumulus-topped BL. Entrainment rates calculated from the near cloud-top fluxes and turbulence in the BL at Point Alpha appeared to be weaker than those in the BL over the open ocean west of Point Alpha and the BL near the coast of the northeast Pacific. The cloud liquid water path (LWP) varied between 15 g m −2 and 160 g m −2 . The BL had a depth of 1140 ± 120 m, was generally well-mixed and capped by a sharp inversion without predominately synoptic and mesoscale influences. The wind direction generally switched from southerly within the BL to northerly above the inversion. On days when a synoptic system and related mesoscale costal circulations affected conditions at Point Alpha (29 OctoberCorrespondence to: X. Zheng (xzheng@rsmas.miami.edu) 4 November), a moist layer above the inversion moved over Point Alpha, and the total-water mixing ratio above the inversion was larger than that within the BL.The accumulation mode aerosol varied from 250 to 700 cm −3 within the BL, and CCN at 0.2 % supersaturation within the BL ranged between 150 and 550 cm −3 . The main aerosol source at Point Alpha was horizontal advection within the BL from south. The average cloud droplet number concentration ranged between 80 and 400 cm −3 . While the mean LWP retrieved from GOES was in good agreement with the in situ measurements, the GOES-derived cloud droplet effective radius tended to be larger than that from the aircraft in situ observations near cloud top. The aerosol and cloud LWP relationship reveals that during the typical wellmixed BL days the cloud LWP increased with the CCN concentrations. On the other hand, meteorological factors and the decoupling processes have large influences on the cloud LWP variation as well.

The first aerosol indirect effect quantified through airborne remote sensing during VOCALS-REx

“…. The estimate reported here is in better agreement with an independent aircraft-based study at 20 • S and 72 • W that reported ACI N = 0.71 based on averaged data collected in similar coastal daytime conditions (Zheng et al, 2011). The high values of ACI N reported here and in Zheng et al (2011) for the more polluted coastal regions also support the similar conclusions from the WRF-Chem simulation study of Yang et al (2012).…”

“…RF01, RF11 and RF12 favored the sampling of coastal regions that were more typically prone to anthropogenic influences (e.g. Bretherton et al, 2010;Painemal and Zuidema, 2010;Allen et al, 2011;Zheng et al, 2011), whereas RF02 sampled offshore clouds that, in this case, still had high N d values 500 km offshore.…”

“…More interesting is the case-to-case variability in winds associated with a coastal jet (Muñoz and Garreaud, 2005). For a high-N d day such as 18 October (RF02), the anticyclone and coastal winds are weaker and boundary layer shallower, than for a day with lower N d such as 9 November (RF11) (Painemal and Zuidema, 2010;Zheng et al, 2011).…”

Strong aerosol–cloud interaction in altocumulus during updraft periods: lidar observations over central Europe

“…The difference between the two aerosol number concentrations was then interpreted as the activated particle number concentration (and taken as a proxy for CDNC) in the ACI studies. This approach is corroborated by a study of Zheng et al (2011) in which a clear and strong dependence between measured CCN (for a relative humidity of 100.2 %) and CDNC was observed over the Pacific west of Chile.…”

Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean

“…Some modeling studies have shown that higher aerosol concentrations may lead to smaller liquid water paths through enhanced entrainment of dry air (Ackerman et al, 2004) or reduced drizzle-driven turbulence and smaller moisture flux from the ocean surface (Jiang et al, 2002). However, during a series of VOCALS measurements at a fixed near-shore location (20 • S, 72 • W), Zheng et al (2011) found that LWP actually was positively correlated with aerosol concentration for a well-mixed boundary layer. The response of LWP to aerosol particles may even depend on the cloud base height (Wood, 2007).…”