Ship-based and satellite remote sensing cloud retrievals consistency and the quantification of aerosol-cloud interactions

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Description

We evaluate the consistency among different remotely sensed cloud properties and analyze their applicability to the quantification of aerosol-cloud interactions over the northeast Pacific. Ship observations were collected in summer 2013 as part of MAGIC field campaign, along a transect between the port of Los Angeles (33.7˚N, 118.2˚W) and Hawaii (21.3˚N, 157.8˚W). Ship-based cloud observations include liquid water path (LWP) from a 3-channel microwave radiometer, and cloud optical depth (tau) and effective radius (Re) from a sunphotometer. Satellite LWP, tau, and Re were retrieved from the Fifteenth Geostationary Operational Environmental Satellite (GOES 15) Imager and the MOderate resolution Imaging Spectroradiometer (MODIS). LWPs derived from satellites and the sunphotometer agree well with their microwave counterparts (correlation r >0.84). Tau retrievals are also consistent between satellite and sunphotometer (r>0.8) although large disagreements are observed for Re (r=0.45). Interestingly, when casting a satellite cloud droplet number concentration (Nd) in terms of Re and tau, and a ship-based Nd as a function of microwave LWP and sunphotometer tau, the logarithmic scale correlation is high (0.72) and the slope is nearly 1. Cloud properties and ship-based cloud condensation nuclei (CCN) were utilized to compute an aerosol-cloud interaction metric ACI, defined as the logarithmic slope between Nd and CCN, or ACI=dln(Nd)/dln(CCN). ACI calculated using satellite and ship-based cloud properties are highly consistent with values near 0.9 and 1 for GOES-15/MODIS and ship-based retrievals, respectively. These results suggest that ACI is high in subtropical cloud regimes, and satellite retrievals combined with ground-based CCN provide a reasonable alternative for the ACI quantification. Different aerosol proxies are also evaluated in the context of ACI. Aerosol scattering coefficients (ASC) derived from a nephelometer and from theoretical calculations using the Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) particle size distribution correlate well with CCN (logarithmic correlation >0.85). This indicates that ASC is a viable CCN surrogate for the investigation of the aerosol indirect effect, especially when ASC measurements are made for aerosol sizes smaller than 1 µm. Interestingly, aerosol optical depth and CCN are poorly correlated, in disagreement with previous studies that sampled broader ranges of aerosol concentration than that measured over the no