Abstract
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 10 years of space-borne observations from the polar-orbiting Aqua satellite, to quantify the albedo susceptibility of marine low-clouds over the northeast (NE) Pacific stratocumulus region to Nd perturbations. Overall, 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 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 albedo susceptibility is also examined. Over the NE Pacific, clear seasonal covariabilities among meteorological factors related to the large-scale circulation are found to play an important role in grouping favorable conditions for each susceptibility regime. Our results indicate that, for the NE Pacific stratocumulus deck, the strongest positively susceptible cloud states occur most frequently for low cloud top height (CTH), the highest lower-tropospheric stability (LTS), low sea-surface temperature (SST), and the lowest free-tropospheric relative humidity (RHft) conditions, whereas cloud states that exhibit negative LWP adjustment occur most frequently under high CTH and intermediate LTS, SST, and RHft conditions. The warm rain suppression driven cloud brightening is found to preferably occur either under unstable atmospheric conditions (low LTS) or high RHft conditions that co-occur with warm SST. Mutual information analyses reveal a dominating control of LWP, Nd and CTH (cloud state indicators) on low-cloud albedo susceptibility, rather than of the meteorological factors that drive these cloud states.
Highlights
Changes in aerosol concentrations in the marine boundary layer, of either natural or anthropogenic origin, can lead to significant changes in the brightness of marine low-level clouds
This is done based on the two-stream 190 approximation (Meador and Weaver, 1980) with the scattering asymmetry parameter approximated as a constant of 0.85 (Sagan and Pollack, 1967; Hu and Stamnes, 1993), using Clouds and the Earth’s Radiant Energy Systems (CERES)-MODerate resolution Imaging Spectroradiometer (MODIS) measured solar zenith angle (SZA) and retrieved τ
Ac reaches a plateau of ∼0.32 when Sc transitions into the non-precipitating regime where negative liquid water path (LWP) adjustments to increasing Nd start to play a dominant role in changes in Ac. 200 For non-precipitating Sc, LWP decreases with increasing Nd, more markedly when the evaporation-entrainment feedback (EEF; Wang et al, 2003; Xue and Feingold, 2006) becomes more active
Summary
Changes in aerosol concentrations in the marine boundary layer, of either natural or anthropogenic origin, can lead to significant changes in the brightness of marine low-level clouds. Shiptracks – bright linear cloud features associated with particle emissions (Coakley et al, 1987) – have been used to improve our understanding of cloud responses to aerosol perturbations. The routine and frequent occurrence of global shipping traffic, and constant meteorological conditions in- and out-of-shiptrack make them a ‘natural laboratory’ to improve our understanding of cloud responses to aerosol perturbations. Studies based on satellite observations (e.g. Coakley and Walsh, 2002; Gryspeerdt et al, 2019b; Chen et al, 2012; Christensen and Stephens, 2011; Christensen et al, 2014) and idealized frameworks such as large-eddy simulations (e.g. Wang et al, 2011; Hill et al, 2009), have been used to quantify/constrain the global aerosol radiative effect (e.g. Diamond et al, 2020). To date, our ability to narrow down estimates of climate sensitivity is still limited by uncertainties related to quantifying the radiative adjustment of marine low-clouds to the anthropogenic aerosol (Boucher et al, 2013)
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