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. One of the important steps 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 spaceborne observations from the polar-orbiting Aqua satellite to quantify the albedo susceptibility of marine low clouds to Nd perturbations over the northeast (NE) Pacific stratocumulus (Sc) region. Mutual information analysis reveals a dominating control of cloud state (e.g., LWP and Nd) on low-cloud albedo susceptibility, relative to the meteorological states that drive these cloud states. Through a LWP–Nd space decomposition of albedo susceptibilities, we show clear separation among susceptibility regimes (brightening or darkening), consistent with previously established mechanisms through which aerosol modulates cloud properties. These regimes include (i) thin non-precipitating clouds (LWP < 55 g m−2) that exhibit brightening (occurring 37 % of the time), corresponding to the Twomey effect; (ii) thicker non-precipitating clouds, corresponding to entrainment-driven negative LWP adjustments that manifest as a darkening regime (36 % of the time); and (iii) another brightening regime (22 % of the time) consisting of mostly precipitating clouds, corresponding to precipitation-suppression LWP positive adjustments. Overall, we find an annual-mean regional low-cloud brightening potential of 20.8±2.68 W m−2 ln(Nd)−1, despite an overall negative LWP adjustment for non-precipitating marine stratocumulus, owing to the high occurrence of the Twomey–brightening regime. 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 conditions favorable for each susceptibility regime. When considering the covarying meteorological conditions, our results indicate that for the northeastern Pacific stratocumulus, clouds that exhibit the strongest brightening potential occur most frequently within shallow marine boundary layers over a cool ocean surface with a stable atmosphere and a dry free troposphere above. Clouds that exhibit a darkening potential associated with negative LWP adjustments occur most frequently within deep marine boundary layers in which the atmospheric instability and the ocean surface are not strong and warm enough to produce frequent precipitation. Cloud brightening associated with warm-rain suppression is found to preferably occur either under unstable atmospheric conditions or humid free-tropospheric conditions that co-occur with a warm ocean surface.

On the Importance of Sea Surface Temperature for Aerosol‐Induced Brightening of Marine Clouds and Implications for Cloud Feedback in a Future Warmer Climate

“…This is likely the reason why the controlling role of SST is not reflected in the NEP (J. Zhang et al, 2021) and other stratocumulus-dominant regions (Qu et al, 2015). Nevertheless, we expect clouds in these regions to respond similarly to those over the North Atlantic Ocean under global warming.…”

“…These comparable levels of turbulence translate to similar entrainment rates; therefore it is the enhanced entrainment drying, rather than entrainment rate, that hampers cloud brightening. Note that when CWP is very low ($$< $$50 ∼ 60 g m −2 ), negative cloud adjustment is more than overcome by the enhanced Twomey effect and therefore S 0 becomes notably positive (J. Zhang et al., 2021).…”

“…The wide spatial variability in SST over the North Atlantic Ocean has allowed us to examine the response of S 0 to varying SST environments that are less contaminated by the seasonal co‐variability between meteorological conditions. In regions with more homogeneous SST (e.g., the North Eastern Pacific Ocean (NEP); J. Zhang et al., 2021), the seasonal co‐variation of SST with free tropospheric absolute humidity is remarkable, in a way that high (low) SST correlates with more (less) humid overlying air in summer (winter) when solar radiation is stronger (weaker) and the Hadley circulation is weaker (stronger). The humid free tropospheric air at high SST prevents efficient cloud top evaporation and offsets to a large extent the response of S 0 to changing SST.…”

“…The humid free tropospheric air at high SST prevents efficient cloud top evaporation and offsets to a large extent the response of S 0 to changing SST. This is likely the reason why the controlling role of SST is not reflected in the NEP (J. Zhang et al., 2021) and other stratocumulus–dominant regions (Qu et al., 2015). Nevertheless, we expect clouds in these regions to respond similarly to those over the North Atlantic Ocean under global warming.…”

“…As a result, the clouds in this study are mostly precipitation free or lightly precipitating (92% of samples have rain rate $$< $$1 mm day −1 ), such that aerosol‐related increases in cloud water accompanying suppressed precipitation are less than cloud water losses associated with increased cloud‐top entrainment. Clouds with more enhanced precipitation (lower N d ) are found to strongly brighten with an increase in aerosol loading (Christensen & Stephens, 2012; J. Zhang et al., 2021).…”