Albedo susceptibility of Northeastern Pacific stratocumulus: the role of covarying meteorological conditions

Abstract

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.

Addressing the difficulties in quantifying the Twomey effect for marine warm clouds from multi-sensor satellite observations and reanalysis

“…CC BY 4.0 License. via the combination of lower tropospheric stability and relative humidity near cloud top (Chen et al, 2014;Jia et al, 2019b) or the N d -LWP relationship at a certain phase relaxation time scale describing evaporation-entrainment feedback (Zhang et al, 2021), they are relatively rough approximations or qualitative differentiation. An updated approach for deriving measures of entrainment mixing at the global scale would be highly beneficial.…”

“…In such case, the change of N d is not necessarily related to actual aerosol perturbations (Chen et al, 2014) but rather to the intensity of cloud sink, thus in principle, should not be directly applied to infer ∆N d driven by anthropogenic emissions. However, due to the lack of simultaneous observations of precipitation and aerosol/cloud properties from passive satellite remote sensing alone, most of ACI estimates did not consider the influence of precipitation (Quaas et al, 2008;Ma et al, 2014;Gryspeerdt et al, 2017;Jia et al, 2021) or just roughly identify the occurrence of rain relying on some simplified metrics, such as the threshold of 14 µm cloud effective radius (CER) for rain initiation (Rosenfeld et al, 2019;Zhang et al, 2021) or the difference of CER between retrievals employing the bands of 2.1 and 3.7 µm (Saponaro et al, 2017;Jia et al, 2019b). Even though few studies have explicitly accounted for this by combining simultaneous precipitation observations from active remote sensing (Chen et al, 2014), how different treatments could influence the assessment of the Twomey effect remains unclear, which is helpful to reconcile the current diverse ACI estimates in order to achieve a more confident observational constraint.…”