Albedo susceptibility of marine low-clouds: the role of covarying meteorological conditions

Quantification of the radiative adjustment of marine low-clouds to aerosol perturbations, regionally and globally, remains the largest source of uncertainty in assessing current and future climate. An important step towards quantifying the role of aerosol in modifying cloud radiative properties is to quantify the susceptibility of cloud albedo and liquid water path (LWP) to perturbations in cloud droplet number concentration (Nd). We use long-term space-borne observations of marine low-cloud properties and radiative fluxes at top-of-atmosphere, to quantify the albedo susceptibility of marine low-clouds to Nd perturbations, with a focus on the subtropical stratocumulus regions. For the Northeastern Pacific stratocumulus, we find a low-cloud brightening potential of 20.8 ± 0.96 W m-2 ln(Nd)-1, despite an overall negative LWP adjustment for non-precipitating marine stratocumulus, owing to the high occurrence (37% of the time) of thin non-precipitating clouds (LWP < 55 g m-2) that exhibit brightening. In addition, we identify two more susceptibility regimes, the entrainment-darkening regime (36% of the time), corresponding to a negative LWP adjustment, and the precipitating-brightening regime (22% of the time), corresponding to precipitation suppression. The influence of large-scale meteorological conditions, obtained from the ERA5 reanalysis, on the occurrence-weighted radiative susceptibility is shown to be primarily through governing the states of the clouds, e.g. LWP and Nd, which in turn determine the occurrence of the albedo susceptibility regimes. These analyses for the northeast Pacific are extended to global marine warm clouds, to quantify the occurrence of each susceptibility regime and the radiative susceptibilities for different cloud regimes. The radiative susceptibility is then converted to a global low-cloud radiative effect/forcing, for given estimates of aerosol perturbations, or Nd changes, from the preindustrial era to present day. Influences of meteorological factors, as well as their variabilities within different cloud regimes, are quantified.