Aerosol and cloud microphysics covariability in the northeast Pacific boundary layer estimated with ship-based and satellite remote sensing observations

摘要

Ship measurements collected over the northeast Pacific along transects between the port of Los Angeles (33.7°N, 118.2°W) and Honolulu (21.3°N, 157.8°W) during May to August 2013 were utilized to investigate the covariability between marine low cloud microphysical and aerosol properties. Ship-based retrievals of cloud optical depth (τ) from a Sun photometer and liquid water path (LWP) from a microwave radiometer were combined to derive cloud droplet number concentration N_d and compute a cloud-aerosol interaction (ACI) metric defined as ACI_(CCN) = ? ln(N_d)/? ln(CCN), with CCN denoting the cloud condensation nuclei concentration measured at 0.4 (CCN_(0.4)) and 0.3 (CCN_(0.3)) supersaturation. Analysis of CCN_(0.4), accumulation mode aerosol concentration (N_a), and extinction coefficient (σ_(ext)) indicates that N_a and σ_(ext) can be used as CCN_(0.4) proxies for estimating ACI. ACI_(CCN) derived from 10 min averaged N_d and CCN_(0.4) and CCN_(0.3), and CCN_(0.4) regressions using N_a and σ_(ext), produce high ACI_(CCN): near 1.0, that is, a fractional change in aerosols is associated with an equivalent fractional change in N_d. ACI_(CCN) computed in deep boundary layers was small (ACI_(CCN) = 0.60), indicating that surface aerosol measurements inadequately represent the aerosol variability below clouds. Satellite cloud retrievals from MODerate-resolution Imaging Spectroradiometer and GOES-15 data were compared against ship-based retrievals and further analyzed to compute a satellite-based ACI_(CCN). Satellite data correlated well with their ship-based counterparts with linear correlation coefficients equal to or greater than 0.78. Combined satellite N_d and ship-based CCN_(0.4) and N_a yielded a maximum ACI_(CCN) = 0.88–0.92, a value slightly less than the ship-based ACI_(CCN), but still consistent with aircraft-based studies in the eastern Pacific.