Liquid Water Path Steady States in Stratocumulus: Insights from Process-Level Emulation and Mixed-Layer Theory

Hoffmann, Franziska; Glassmeier, Franziska; Yamaguchi, Takanobu; Feingold, Graham

doi:10.1175/jas-d-19-0241.1

Cited by 23 publications

(27 citation statements)

References 48 publications

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“…1, red circular outlines), after which A c increases with N d despite a continuous reduction in LWP, although more than halved in slope (dln(LWP)/dln(N d ) ∼ −0.38) compared to when LWP is above 55 g m −2 . This increase in A c with increasing N d after LWP drops below 55 g m −2 can be explained by a decrease in entrainment efficiency as LWP decreases (Hoffmann et al, 2020) and an enhanced Twomey effect for less reflective thin clouds (Platnick and Twomey, 1994). The framework for discussion is the commonly used approximation of cloud albedo response to aerosol perturbations (e.g., Bellouin et al, 2020),…”

Section: Meanmentioning

confidence: 99%

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

et al. 2022

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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.

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Section: Meanmentioning

confidence: 99%

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

et al. 2022

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“…The . This increase in A c with increasing N d after LWP drops below 55 g m −2 can be explained by a decrease in entrainment efficiency as LWP decreases (Hoffmann et al, 2020) and an enhanced Twomey effect for less reflective thin clouds (Platnick and Twomey, 1994). The framework for discussion is the commonly used approximation of cloud albedo response to aerosol perturbations (e.g.…”

Section: Methodsmentioning

confidence: 99%

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

Zhang

Zhou

Feingold

2021

Preprint

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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.

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“…In the LWP direction, individual ensemble members collectively evolve towards similar LWPs (along an approximately horizontal line in Figure 3). For these steady-state LWPs, there exists a balance in the contributions of different processes that are source and sink terms for LWP -in particular radiative cooling (source), and entrainment and precipitation drying (sink) [34]. The collective evolution in the N direction is structured around the critical radius for precipitation formation [35].…”

Section: Methodsmentioning

confidence: 99%

“…This means that for t → ∞ only the steady-state line is sampled and the LWP adjustment is quantified by the slope of this line. As the slope of the steadystate LWP line reflects the N -dependence of entrainment [34], the LWP-adjustment at t → ∞, d ln LWP ∞ /d ln N , is a direct quantification of N -or more generally aerosoleffects on cloud processes.…”

Section: Effect Of Cloud Field Evolution Towards Steady State On Adju...mentioning

confidence: 99%

Ship-track-based assessments overestimate the cooling effect of anthropogenic aerosol

Glassmeier,

Hoffmann,

Johnson

et al. 2020

Preprint

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The effect of anthropogenic aerosol on the reflectivity of stratocumulus cloud decks through changes in cloud amount is a major uncertainty in climate projections. The focus of this study is the frequently occurring non-precipitating stratocumulus. In this regime, cloud amount can decrease through aerosol-enhanced cloud-top mixing. The climatological relevance of this effect is debated because ship exhaust does not appear to generate significant change in the amount of these clouds. Through a novel analysis of detailed numerical simulations in comparison to satellite data, we show that results from ship-track studies cannot be generalized to estimate the climatological forcing of anthropogenic aerosol. We specifically find that the ship-track-derived sensitivity of the radiative effect of non-precipitating stratocumulus to aerosol overestimates their cooling effect by up to 200 %. This offsetting warming effect needs to be taken into account if we are to constrain the aerosol-cloud radiative forcing of stratocumulus.

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Liquid Water Path Steady States in Stratocumulus: Insights from Process-Level Emulation and Mixed-Layer Theory

Cited by 23 publications

References 48 publications

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

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

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

Ship-track-based assessments overestimate the cooling effect of anthropogenic aerosol

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