Manipulating marine stratocumulus cloud amount and albedo: a process-modelling study of  aerosol-cloud-precipitation interactions in response to injection of cloud condensation nuclei

Abstract. We use a cloud-system-resolving model to study marine-cloud brightening. We examine how injected aerosol particles that act as cloud condensation nuclei (CCN) are transported within the marine boundary layer and how the additional particles in clouds impact cloud microphysical processes, and feedback on dynamics. Results show that the effectiveness of cloud brightening depends strongly on meteorological and background aerosol conditions. Cloud albedo enhancement is very effective in a weakly precipitating boundary layer and in CCN-limited conditions preceded by heavy and/or persistent precipitation. The additional CCN help sustain cloud water by weakening the precipitation substantially in the former case and preventing the boundary layer from collapse in the latter. For a given amount of injected CCN, the injection method (i.e., number and distribution of sprayers) is critical to the spatial distribution of these CCN. Both the areal coverage and the number concentration of injected particles are key players but neither one always emerges as more important than the other. The same amount of injected material is much less effective in either strongly precipitating clouds or polluted clouds, and it is ineffective in a relatively dry boundary layer that supports clouds of low liquid water path. In the polluted case and "dry" case, the CCN injection increases drop number concentration but lowers supersaturation and liquid water path. As a result, the cloud experiences very weak albedo enhancement, regardless of the injection method.

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Manipulating marine stratocumulus cloud amount and albedo: a process-modelling study of aerosol-cloud-precipitation interactions in response to injection of cloud condensation nuclei

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-4237-2011 ◽

2011 ◽

Vol 11 (9) ◽

pp. 4237-4249 ◽

Cited By ~ 51

Author(s):

H. Wang ◽

P. J. Rasch ◽

G. Feingold

Keyword(s):

Boundary Layer ◽

Liquid Water ◽

Marine Boundary Layer ◽

Cloud Condensation Nuclei ◽

Cloud Amount ◽

Number Concentration ◽

Condensation Nuclei ◽

Liquid Water Path ◽

Water Path ◽

Injection Method

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Evaluation of stratocumulus cloud prediction in the Met Office forecast model during VOCALS-REx

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-10541-2010 ◽

2010 ◽

Vol 10 (21) ◽

pp. 10541-10559 ◽

Cited By ~ 52

Author(s):

S. J. Abel ◽

D. N. Walters ◽

G. Allen

Keyword(s):

Boundary Layer ◽

Diurnal Cycle ◽

Liquid Water ◽

Large Scale ◽

Marine Boundary Layer ◽

Coastal Region ◽

Forecast Model ◽

Good Representation ◽

Liquid Water Path ◽

Water Path

Abstract. Observations in the subtropical southeast Pacific obtained during the VOCALS-REx field experiment are used to evaluate the representation of stratocumulus cloud in the Met Office forecast model and to identify key areas where model biases exist. Marked variations in the large scale structure of the cloud field were observed during the experiment on both day-to-day and on diurnal timescales. In the remote maritime region the model is shown to have a good representation of synoptically induced variability in both cloud cover and marine boundary layer depth. Satellite observations show a strong diurnal cycle in cloud fraction and liquid water path in the stratocumulus with enhanced clearances of the cloud deck along the Chilean and Peruvian coasts on certain days. The model accurately simulates the phase of the diurnal cycle but is unable to capture the coastal clearing of cloud. Observations along the 20° S latitude line show a gradual increase in the depth of the boundary layer away from the coast. This trend is well captured by the model (typical low bias of 200 m) although significant errors exist at the coast where the model marine boundary layer is too shallow and moist. Drizzle in the model responds to changes in liquid water path in a manner that is consistent with previous ship-borne observations in the region although the intensity of this drizzle is likely to be too high, particularly in the more polluted coastal region where higher cloud droplet number concentrations are typical. Another mode of variability in the cloud field that the model is unable to capture are regions of pockets of open cellular convection embedded in the overcast stratocumulus deck and an example of such a feature that was sampled during VOCALS-REx is shown.

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Ultra-clean and smoky marine boundary layers frequently occur in the same season over the southeast Atlantic

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-2341-2020 ◽

2020 ◽

Vol 20 (4) ◽

pp. 2341-2351 ◽

Cited By ~ 1

Author(s):

Sam Pennypacker ◽

Michael Diamond ◽

Robert Wood

Keyword(s):

Boundary Layer ◽

Boundary Layers ◽

Biomass Burning ◽

Liquid Water ◽

Marine Boundary Layer ◽

Liquid Water Path ◽

Accumulation Mode ◽

Water Path ◽

Aerosol Cloud ◽

Ascension Island

Abstract. We study 41 d with daily median surface accumulation mode aerosol particle concentrations below 50 cm−3 (ultra-clean conditions) observed at Ascension Island (ASI; 7.9∘ S, 14.4∘ W) between June 2016 and October 2017 as part of the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign. Interestingly, these days occur during a period of great relevance for aerosol–cloud–radiation interactions, the southeast Atlantic (SEATL) biomass-burning season (approximately June–October). That means that these critical months can feature both the highest surface aerosol numbers, from smoke intrusion into the marine boundary layer, as well as the lowest. While carbon monoxide and refractory black carbon concentrations on ultra-clean days do not approach those on days with heavy smoke, they also frequently exceed background concentrations calculated in the non-burning season from December 2016 to April 2017. This is evidence that even what become ultra-clean boundary layers can make contact with and entrain from an overlying SEATL smoke layer before undergoing a process of rapid aerosol removal. Because many ultra-clean and polluted boundary layers observed at Ascension Island during the biomass burning season follow similar isobaric back trajectories, the variability in this entrainment is likely more closely tied to the variability in the overlying smoke rather than large-scale horizontal circulation through the boundary layer. Since exceptionally low accumulation mode aerosol numbers at ASI do not necessarily indicate the relative lack of other trace pollutants, this suggests the importance of regional variations in what constitutes an “ultra-clean” marine boundary layer. Finally, surface drizzle rates, frequencies and accumulation – as well as retrievals of liquid water path – all consistently tend toward higher values on ultra-clean days. This implicates enhanced coalescence scavenging in low clouds as the key driver of ultra-clean events in the southeast Atlantic marine boundary layer. These enhancements occur against and are likely mediated by the backdrop of a seasonal increase in daily mean cloud fraction and daily median liquid water path over ASI, peaking in September and October in both LASIC years. Therefore the seasonality in ultra-clean day occurrence seems directly linked to the seasonality in SEATL cloud properties. These results highlight the importance of two-way aerosol–cloud interactions in the region.

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Observational Evidence That Enhanced Subsidence Reduces Subtropical Marine Boundary Layer Cloudiness

Journal of Climate ◽

10.1175/jcli-d-12-00736.1 ◽

2013 ◽

Vol 26 (19) ◽

pp. 7507-7524 ◽

Cited By ~ 91

Author(s):

Timothy A. Myers ◽

Joel R. Norris

Keyword(s):

Boundary Layer ◽

Liquid Water ◽

Marine Boundary Layer ◽

Cloud Fraction ◽

Liquid Water Path ◽

Water Path ◽

Cloud Data ◽

Marine Stratus ◽

Temperature Inversions ◽

Cloud Thickness

Abstract Conventional wisdom suggests that subsidence favors the presence of marine stratus and stratocumulus because regions of enhanced boundary layer cloudiness are observed to climatologically co-occur with regions of enhanced subsidence. Here it is argued that the climatological positive correlation between subsidence and cloudiness is not the result of a direct physical mechanism connecting the two. Instead, it arises because enhanced subsidence is typically associated with stronger temperature inversions capping the marine boundary layer, and stronger temperature inversions favor greater cloudiness. Through statistical analysis of satellite cloud data and meteorological reanalyses for the subsidence regime over tropical (30°S–30°N) oceans, it is shown that enhanced subsidence promotes reduced cloudiness for the same value of inversion strength and that a stronger inversion favors greater cloudiness for the same value of subsidence. Using a simple conceptual model, it is argued that enhanced subsidence leads to reduced cloud thickness, liquid water path, and cloud fraction by pushing down the top of the marine boundary layer. Moreover, a stronger inversion reduces entrainment drying and warming, thus leading to a more humid boundary layer and greater cloud thickness, liquid water path, and cloud fraction. These two mechanisms typically oppose each other for geographical and seasonal cloud variability because enhanced subsidence is usually associated with stronger inversions. If global warming results in stronger inversions but weaker subsidence, the two mechanisms could both favor increased subtropical low-level cloudiness.

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Ultra-clean and smoky marine boundary layers frequently occur in the same season over the southeast Atlantic

10.5194/acp-2019-628 ◽

2019 ◽

Cited By ~ 1

Author(s):

Sam Pennypacker ◽

Michael Diamond ◽

Robert Wood

Keyword(s):

Boundary Layer ◽

Boundary Layers ◽

Liquid Water ◽

Large Scale ◽

Marine Boundary Layer ◽

Median Surface ◽

Liquid Water Path ◽

Water Path ◽

Aerosol Cloud ◽

Ascension Island

Abstract. We study forty-one days with daily median surface accumulation mode aerosol particle concentrations below 50 cm−3 (ultra-clean conditions) observed at Ascension Island (7.9° S, 14.4° W) between June 2016 and October 2017 as part of the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign. Interestingly, these days occur during a period of great relevance for aerosol-cloud-radiation interactions, the southeast Atlantic (SEATL) biomass-burning season (approximately June–October). That means that these critical months can feature both the highest surface aerosol numbers, from smoke intrusion into the marine boundary layer, as well as the lowest. While carbon monoxide and refractory black carbon concentrations on ultra-clean days do not approach those on days with heavy smoke, they also frequently exceed background concentrations calculated in the non-burning season from December 2016–April 2017. This is evidence that even what become ultra-clean boundary layers can make contact with and entrain from an overlying SEATL smoke layer before undergoing a process of rapid aerosol removal. Because many ultra-clean and polluted boundary layers observed at Ascension Island follow similar isobaric back-trajectories, the variability in this entrainment is likely closely tied to the variability in the overlying smoke rather than large-scale horizontal circulation through the boundary layer. Finally, surface drizzle rates, frequencies and accumulation – as well as retrievals of liquid water path – all consistently tend toward higher values on ultra-clean days. This implicates enhanced coalescence scavenging in low clouds as the key driver of ultra-clean events in the southeast Atlantic marine boundary layer. These enhancements occur against and are likely mediated by the backdrop of a seasonal increase in daily mean cloud fraction and daily median liquid water path over ASI, peaking in September and October in both LASIC years. Therefore the seasonality in ultra-clean day occurrence seems directly linked to the seasonality in SEATL cloud properties. These results highlight the importance of two-way aerosol-cloud interactions in the region.

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An Algorithm for the Retrieval of Droplet Number Concentration and Geometrical Thickness of Stratiform Marine Boundary Layer Clouds Applied to MODIS Radiometric Observations

Journal of Applied Meteorology ◽

10.1175/jam-2185.1 ◽

2005 ◽

Vol 44 (1) ◽

pp. 28-38 ◽

Cited By ~ 22

Author(s):

Lothar Schüller ◽

Ralf Bennartz ◽

Jürgen Fischer ◽

Jean-Louis Brenguier

Keyword(s):

Boundary Layer ◽

Liquid Water ◽

Dispersion Model ◽

Cloud Model ◽

Number Concentration ◽

Trade Wind ◽

Liquid Water Path ◽

Water Path ◽

Airborne Measurements ◽

Boundary Layer Clouds

Abstract Algorithms are now currently used for the retrieval of cloud optical thickness and droplet effective radius from multispectral radiance measurements. This paper extends their application to the retrieval of cloud droplet number concentration, cloud geometrical thickness, and liquid water path in shallow convective clouds, using an algorithm that was previously tested with airborne measurements of cloud radiances and validated against in situ measurements of the same clouds. The retrieval is based on a stratified cloud model of liquid water content and droplet spectrum. Radiance measurements in visible and near-infrared channels of the Moderate Resolution Imaging Spectroradiometer (MODIS), which is operated from the NASA platforms Terra and Aqua, are analyzed. Because of uncertainties in the simulation of the continental surface reflectance, the algorithm is presently limited to the monitoring of the microphysical structure of boundary layer clouds over the ocean. Two MODIS scenes of extended cloud fields over the North Atlantic Ocean trade wind region are processed. A transport and dispersion model (the Hybrid Single-Particle Lagrangian Integrated Trajectory Model, HYSPLIT4) is also used to characterize the origin of the air masses and hence their aerosol regimes. One cloud field formed in an air mass that was advected from southern Europe and North Africa. It shows high values of the droplet concentration when compared with the second cloud system, which developed in a more pristine environment. The more pristine case also exhibits a higher geometrical thickness and, thus, liquid water path, which counterbalances the expected cloud albedo increase of the polluted case. Estimates of cloud liquid water path are then compared with retrievals from the Special Sensor Microwave Imager (SSM/I). SSM/I-derived liquid water paths are in good agreement with the MODIS-derived values.

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Evaluation of stratocumulus cloud prediction in the Met Office forecast model during VOCALS-REx

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-16797-2010 ◽

2010 ◽

Vol 10 (7) ◽

pp. 16797-16835 ◽

Cited By ~ 5

Author(s):

S. J. Abel ◽

D. N. Walters ◽

G. Allen

Keyword(s):

Boundary Layer ◽

Diurnal Cycle ◽

Liquid Water ◽

Large Scale ◽

Marine Boundary Layer ◽

Coastal Region ◽

Forecast Model ◽

Good Representation ◽

Liquid Water Path ◽

Water Path

Abstract. Observations in the subtropical south east Pacific obtained during the VOCALS-REx field experiment are used to evaluate the representation of stratocumulus cloud in the Met Office forecast model and to identify key areas where model biases exist. Marked variations in the large scale structure of the cloud field were observed during the experiment on both day to day and on diurnal timescales. In the remote maritime region the model is shown to have a good representation of synoptically induced variability in both cloud cover and marine boundary layer depth. Satellite observations show a strong diurnal cycle in cloud fraction and liquid water path in the stratocumulus with enhanced clearances of the cloud deck along the Chilean and Peruvian coasts on certain days. The model accurately simulates the phase of the diurnal cycle but is unable to capture the coastal clearing of cloud. Observations along the 20° S latitude line show a gradual increase in the depth of the boundary layer away from the coast. This trend is well captured by the model (typical low bias of 200 m) although significant errors exist at the coast where the model marine boundary layer is too shallow and moist. Drizzle in the model responds to changes in liquid water path in a manner that is consistent with previous ship-borne observations in the region although the intensity of this drizzle is likely to be too high, particularly in the more polluted coastal region where higher cloud droplet number concentrations are typical. Another mode of variability in the cloud field that the model is unable to capture are regions of pockets of open cellular convection embedded in the overcast stratocumulus deck and an example of such a feature that was sampled during VOCALS-REx is shown.

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MBL drizzle properties and their impact on cloud property retrievals

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-8-4307-2015 ◽

2015 ◽

Vol 8 (4) ◽

pp. 4307-4323

Author(s):

P. Wu ◽

X. Dong ◽

B. Xi

Keyword(s):

Liquid Water ◽

Cloud Droplet ◽

Number Concentration ◽

Effective Radius ◽

Liquid Water Path ◽

Water Path ◽

Cloud Property ◽

Annual Means ◽

Mean Differences ◽

The Impact

Abstract. In this study, we retrieve and document drizzle properties, and investigate the impact of drizzle on cloud property retrievals from ground-based measurements at the ARM Azores site from June 2009 to December 2010. For the selected cloud and drizzle samples, the drizzle occurrence is 42.6% with a maximum of 55.8% in winter and a minimum of 35.6% in summer. The annual means of drizzle liquid water path LWPd, effective radius rd, and number concentration Nd for the rain (virga) samples are 5.48 (1.29) g m−2, 68.7 (39.5) μm, and 0.14 (0.38) cm−3. The seasonal mean LWPd values are less than 4% of the MWR-retrieved LWP values. The annual mean differences in cloud-droplet effective radius with and without drizzle are 0.12 and 0.38 μm, respectively, for the virga and rain samples. Therefore, we conclude that the impact of drizzle on cloud property retrievals is insignificant at the ARM Azores site.

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Reconciling and Validating the Cloud Thickness and Liquid Water Path Tendencies Proposed by R. Wood and J. J. van der Dussen et al.

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0287.1 ◽

2015 ◽

Vol 72 (5) ◽

pp. 2033-2040 ◽

Cited By ~ 10

Author(s):

Mohamed S. Ghonima ◽

Joel R. Norris ◽

Thijs Heus ◽

Jan Kleissl

Keyword(s):

Boundary Layer ◽

Liquid Water ◽

Mixed Boundary ◽

Liquid Water Path ◽

Water Path ◽

Large Eddy ◽

Heat And Moisture ◽

Cloud Thickness ◽

Detailed Derivation ◽

Good Agreement

Abstract A detailed derivation of stratocumulus cloud thickness and liquid water path tendencies as a function of the well-mixed boundary layer mass, heat, and moisture budget equations is presented. The derivation corrects an error in the cloud thickness tendency equation derived by R. Wood to make it consistent with the liquid water path tendency equation derived by J. J. van der Dussen et al. The validity of the tendency equations is then tested against the output of large-eddy simulations of a typical stratocumulus-topped boundary layer case and is found to be in good agreement.

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