Observational Evidence That Enhanced Subsidence Reduces Subtropical Marine Boundary Layer Cloudiness

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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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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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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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 Discussions ◽

10.5194/acpd-11-885-2011 ◽

2011 ◽

Vol 11 (1) ◽

pp. 885-916 ◽

Cited By ~ 6

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

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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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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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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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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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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The Impacts of Atmospheric and Surface Parameters on Long-Term Variations in the Planetary Albedo

Journal of Climate ◽

10.1175/jcli-d-17-0848.1 ◽

2018 ◽

Vol 31 (21) ◽

pp. 8705-8718 ◽

Cited By ~ 3

Author(s):

Bida Jian ◽

Jiming Li ◽

Guoyin Wang ◽

Yongli He ◽

Ying Han ◽

...

Keyword(s):

Liquid Water ◽

Surface Albedo ◽

Cloud Fraction ◽

Liquid Water Path ◽

Water Path ◽

Ice Water Path ◽

Planetary Albedo ◽

Ice Water ◽

Long Term Variations

Planetary albedo (PA; shortwave broadband albedo) and its long-term variations, which are controlled in a complex way by various atmospheric and surface properties, play a key role in controlling the global and regional energy budget. This study investigates the contributions of different atmospheric and surface properties to the long-term variations of PA based on 13 years (2003–15) of albedo, cloud, and ice coverage datasets from the Clouds and the Earth’s Radiant Energy System (CERES) Single Scanner Footprint edition 4A product, vegetation product from Moderate Resolution Imaging Spectroradiometer (MODIS), and surface albedo product from the Cloud, Albedo, and Radiation dataset, version 2 (CLARA-A2). According to the temporal correlation analysis, statistical results indicate that variations in PA are closely related to the variations of cloud properties (e.g., cloud fraction, ice water path, and liquid water path) and surface parameters (e.g., ice/snow percent coverage and normalized difference vegetation index), but their temporal relationships vary among the different regions. Generally, the stepwise multiple linear regression models can capture the observed PA anomalies for most regions. Based on the contribution calculation, cloud fraction dominates the variability of PA in the mid- and low latitudes while ice/snow percent coverage (or surface albedo) dominates the variability in the mid- and high latitudes. Changes in cloud liquid water path and ice water path are the secondary dominant factor over most regions, whereas change in vegetation cover is the least important factor over land. These results verify the effects of atmospheric and surface factors on planetary albedo changes and thus may be of benefit for improving the parameterization of the PA and determining the climate feedbacks.

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Importance of Accurate Liquid Water Path for Estimation of Solar Radiation in Warm Boundary Layer Clouds: An Observational Study

Journal of Climate ◽

10.1175/1520-0442(2003)016<2997:ioalwp>2.0.co;2 ◽

2003 ◽

Vol 16 (18) ◽

pp. 2997-3009 ◽

Cited By ~ 23

Author(s):

Manajit Sengupta ◽

Eugene E. Clothiaux ◽

Thomas P. Ackerman ◽

Seiji Kato ◽

Qilong Min

Keyword(s):

Boundary Layer ◽

Solar Radiation ◽

Observational Study ◽

Liquid Water ◽

Liquid Water Path ◽

Water Path ◽

Boundary Layer Clouds

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Spatial Variability of Liquid Water Path in Marine Low Cloud: The Importance of Mesoscale Cellular Convection

Journal of Climate ◽

10.1175/jcli3702.1 ◽

2006 ◽

Vol 19 (9) ◽

pp. 1748-1764 ◽

Cited By ~ 233

Author(s):

Robert Wood ◽

Dennis L. Hartmann

Keyword(s):

Spatial Variability ◽

Liquid Water ◽

Large Scale ◽

Marine Boundary Layer ◽

Liquid Water Path ◽

Water Path ◽

Geographical Regions ◽

Low Cloud ◽

Moderate Resolution Imaging Spectroradiometer ◽

Subtropical Oceans

Abstract Liquid water path (LWP) mesoscale spatial variability in marine low cloud over the eastern subtropical oceans is examined using two months of daytime retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Terra satellite. Approximately 20 000 scenes of size 256 km × 256 km are used in the analysis. It is found that cloud fraction is strongly linked with the LWP variability in the cloudy fraction of the scene. It is shown here that in most cases LWP spatial variance is dominated by horizontal scales of 10–50 km, and increases as the variance-containing scale increases, indicating the importance of organized mesoscale cellular convection (MCC). A neural network technique is used to classify MODIS scenes by the spatial variability type (no MCC, closed MCC, open MCC, cellular but disorganized). It is shown how the different types tend to occupy distinct geographical regions and different physical regimes within the subtropics, although the results suggest considerable overlap of the large-scale meteorological conditions associated with each scene type. It is demonstrated that both the frequency of occurrence, and the variance-containing horizontal scale of the MCC increases as the marine boundary layer (MBL) depth increases. However, for the deepest MBLs, the MCC tends to be replaced by clouds containing cells but lacking organization. In regions where MCC is prevalent, a lack of sensitivity of the MCC type (open or closed) to the large-scale meteorology was found, suggesting a mechanism internal to the MBL may be important in determining MCC type. The results indicate that knowledge of the physics of MCC will be required to completely understand and predict low cloud coverage and variability in the subtropics.

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