Ultra-clean Layers and Low Albedo Clouds in the Marine Boundary Layer

Robert Wood1 , Paquita Zuidema2, Chris Bretherton1,Bruce Albrecht2 Virendra Ghate3, Mampi Sarkar2,

Susanne Glienke4, Johannes Mohrmann1,Raymond Shaw4, and Jacob Fugal5

1Department of Atmospheric Sciences, University of Washington, Seattle, USA; 2University of Miami, USA; 3Argonne National Laboratory;

4Michigan Technological University, USA; 5Mainz University, Germany

ReferencesBaker, M B., and Robert J. Charlson. “Bistability of CCN Concentrations and Thermodynamics in the Cloud-Topped Boundary Layer.” Nature 345, no. 6271 (May 10, 1990): 142–45. doi:10.1038/345142a0.Leahy, L. V., R. Wood, R. J. Charlson, C. A. Hostetler, R. R. Rogers, M. A. Vaughan, and D. M. Winker, 2012: On the Nature and Extent of Optically Thin Marine Low Clouds. J. Geophys. Res., 117, doi:10.1029/2012-JD017929.Mann, G. W., K. S. Carslaw, C. L. Reddington, K. J. Pringle, M. Schulz, A. Asmi, D. V. Spracklen, et al. “Intercomparison and Evaluation of Global Aerosol Microphysical Properties among AeroCom Models of a Range of Complexity.” Atmos. Chem. Phys. 14, no. 9 (May 13, 2014): 4679–4713. doi:10.5194/acp-14-4679-2014.Petters, Markus D., Je�erson R. Snider, Bjorn Stevens, Gabor Vali, Ian Faloona, and Lynn M. Russell. “Accumulation Mode Aerosol, Pockets of Open Cells, and Particle Nucleation in the Remote Subtropical Paci�c Marine Boundary Layer.” Journal of Geophysical Research: Atmospheres 111, no. D2 (January 27, 2006): D02206. doi:10.1029/2004JD005694.Sharon, Tarah M., Bruce A. Albrecht, Ha�idi H. Jonsson, Patrick Minnis, Mandana M. Khaiyer, Timothy M. van Reken, John Seinfeld, and Rick Flagan. “Aerosol and Cloud Microphysical Characteristics of Rifts and Gradients in Maritime Stratocumulus Clouds.” Journal of the Atmospheric Sciences 63, no. 3 (2006): 983–97.Terai, C. R., C. S. Bretherton, R. Wood, and G. Painter. “Aircraft Observations of Aerosol, Cloud, Precipitation, and Boundary Layer Properties in Pockets of Open Cells over the Southeast Paci�c.” Atmospheric Chemistry and Physics 14, no. 15 (August 13, 2014): 8071–88. doi:10.5194/acp-14-8071-2014.Twomey, S. “The In�uence of Pollution on the Shortwave Albedo of Clouds.” Journal of the Atmospheric Sciences 34, no. 7 (July 1, 1977): 1149–52. doi:10.1175/1520-0469(1977)034<1149:TIOPOT>2.0.CO;2.Wood, R., C. S. Bretherton, D. Leon, A. D. Clarke, P. Zuidema, G. Allen, and H. Coe. “An Aircraft Case Study of the Spatial Transition from Closed to Open Mesoscale Cellular Convection over the Southeast Paci�c.” Atmos. Chem. Phys. 11, no. 5 (March 15, 2011): 2341–70. doi:10.5194/acp-11-2341-2011.Wood, R., C. R. Mechoso, C. S. Bretherton, R. A. Weller, B. Huebert, F. Straneo, B. A. Albrecht, et al. “The VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx): Goals, Platforms, and Field Operations.” Atmos. Chem. Phys. 11, no. 2 (January 21, 2011): 627–54. doi:10.5194/acp-11-627-2011.

Motivation and Background Gray clouds and UCLs• Cloud occurring in UCLs tend to be stratiform (Fig. 8a), geometrially thin (Fig. 8b), somewhat laminar when viewed from the limb, but possess small cumuliform ele-ments when seen from above (Fig. 8c). UCL clouds are distinct in structure from trade Cu (Fig. 8d).

• The term “gray clouds” stems from their rather low opti-cal thicknesses as seen in visible satellite imageryfrom GOES (Fig. 9), and MODIS (Fig. 10).

Ultraclean Layers over the subtropical NE Paci�c

Questions1. How do UCLs form? Are they the product of slow mesoscale ascent in regions of active trade Cu? Or are they formed in situ by radiative cooling at the tops of humid layers detrained from active Cu?

2. How do UCLs become so depleted of particles? Is cloud droplet sedimentation over relatively long periods of time responsible for the removal of most accumulation mode particles? The fall speed of r =15-20 µm drops is approximately 3 cm s-1, so typical UCL droplets would fall 100 m in an hour. Does coalescence play a role?

3. How do UCLs a�ect albedo susceptibility? Basic theory would suggest extremely strong susceptibility. However, UCLs are relatively quiescent, so they likely do not e�ectively entrain air and particles from the free troposphere. Surface aerosol must pass into the UCL via active trade Cu, many of which are precipitating, so many particles will likely be removed before entering.

Ultra-clean layer near ubiquity in CSET• Figure shows cross sections of accumulation

mode aerosol concentration (d>100 nm, UHSAS) as a function of longitude

RF02 RF04 RF06

RF03 RF05 RF07

RF08 RF10 RF12

RF09 RF11 RF13

RF14

RF15

Figure 5: Very low aerosol concen-trations often occur in the upper

MBL demonstrating near-ubiquity of UCLs over the subtropical NE

Pacific Ocean. Figure shows longi-tude-height cross sections from CSET flights showing concentration of aero-

sol particles larger than 0.1 µm (UHSAS) from all clear-air samples.

• Observations taken during the Cloud System Evolution in the Trades (CSET) field campaign (Ju-ly-Aug 2015) using the NSF/NCAR G-V aircraft

• 15 research flights sampling marine boundary layer and lower free-tropospheric air between Northern California and Hawaii (Fig. 4). Low-level sampling spanned 125-155o W

conc.[cm-3]

Figure 4: Flight tracks during CSET flown with the G-V (pictured right). Low-level sampling shown as colored lines.

• Classify all 1Hz samples into one of four categories: (i) cloudy UCL; (ii) clear UCL; (iii) cloudy non-UCL; (iv) clear non-UCL.

• UCLs hardly ever occur below 500 m and are infrequent below 1 km; UCLs are most commonly found at a height of 1.5-2 km, typically close to the top of the MBL (Fig. 6a).

• UCLs occur very infrequently east of 130o W (Fig 6b), i.e., within 500-600 km of the Californian coast. Most previous aircraft sampling oflow clouds occurred close to the Californian coast, so preva-lence of UCLs has not been previously noted.

• UCL coverage 0.4-0.6 between 135oW and 155oW. Similar chance of a cloudy column containing a UCL as a clear column.

• Sensitivity of cloud optical thickness to increases in aerosol loading are highly sensitive to aerosol and cloud droplet concentrations in the pre-industrial (unperturbed) environment [Twomey 1977].

• Low clouds with modest amounts of precipitation are able to significantly modify their aerosol environment through coalescence scavenging [Baker and Charlson 1990; Wood et al. 2012].

• Surface-measured aerosol climatologies are typically used as a target for climate model evaluation [e.g., Mann et al. 2014, Aerocom].

• Mounting evidence indicates that the upper marine boundary layer (MBL), where most marine stratiform clouds reside, is often extremely depleted in accumulation mode aerosol concentrations compared with near-surface or lower free-tropospheric air [Petters et al. 2006; Wood et al. 2011; Terai et al. 2014]. These layers were first documented in Sharon et al. [2006] and have been termed Ultra-Clean Layers (UCLs) by Bruce Albrecht.

• UCLs appear to be ubiquitous in observations of shallow open cellular convection in the subtropics/tropics [Terai et al. 2014]. How UCLs impact the sensitivity of albedo to anthropogenic aerosol is unclear.

Figure 1 (below): Top: MODIS visible image showing region of open cells and optically thin clouds in a pocket of open cells sampled on RF06. Bottom: Observationally-derived schematic showing ultra-clean layer and clouds in the MBL open cell environment. From NSF/NCAR C-130 Research Flight 6 of the VOCALS Regional Experiment [Mechoso et al. 2014].

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NCNNCN,hot

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freetroposphere

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} ultracleanlayer

Figure 2 (above): Profile of aerosol concentra-tions in a clear UCL between clouds in a pocket of open cells. Observations from VOCALS-REx (Wood et al. 2011) showing concentrations of aero-sols > 0.1 µm (black circles), total CN (red), and re-fractory (non-volatile) CN (dotted).

What is an Ultra-Clean Layer?• A horizontally-extensive layer that can contain either clear air, cloud, or both cloudy/clear air with very low con-centrations Na of accumulation mode aerosol particles or very low cloud droplet concentrations Nd (when the layer is cloud-filled).• Here, we define a UCL as having Na < 10 cm-3 (particles with diameters > 0.1 µm), or Nd < 10 cm-3.• In the VOCALS Regional Experiment, UCLs were found to be present near the top of the MBL in all of the cases where pockets of open cells (POCs) were sampled [Terai et al. 2014]. Figure 1 shows a conceptual dia-gram of the location of the UCL with respect to the clouds in the MBL. Figure 2 shows a typical profile of aero-sols through a clear UCL. Figure 2 indicates that Na can fall to values well below 1 cm-3 in parts of the UCL. Figure 3 shows profiles for each of the POC cases in VOCALS, demonstrating that UCL are a frequent occur-rence in shallow mesoscale open cellular convection over the tropical southeastern Pacific region.

•

Figure 3: Composite pro�les of PCASP aerosol concentration (d>0.1 µm) from the pocket of open cells (POC, blue) and overcast (red) regions in the VOCALS POC cases sampled with the NSF/NCAR C-130. Cyan (POC) and magenta (overcast) dots indicate individual data points from pro�les �own in the two regions. Concentrations < 0.1 cm−3 are set at 0.1 cm−3 to allow for easier plotting on the log-scale abscissa. The thicker pro�le line indicates the median taken from the pro�le data. PCASP data from level �ight legs are also included in the form of box-and-whisker plots. For reference, cloud-mean cloud droplet number concentrations from the cloud droplet probe (CDP) are shown as open triangles, mean inversion base heights calculated from pro�le measurements are shown as dashed lines, and mean lifting condensation levels calculated from subcloud �ight legs are shown as dotted lines.

• UCL clouds contain low cloud liquid water contents on average, but contain signi�cant amounts of drizzle-sized drop-lets, here de�ned as droplets with radii > 20 µm (Fig. 7a).

• UCLs at heights of 1.5-2 km have the lowest concentrations of both cloud drop-lets and accumulation mode aerosols (Fig. 7b). Profile of median Nd and Na are re-markably similar suggesting connection between the cloudy UCL microphysics and the clear UCL aerosols. This could be via activation of all accmulation mode aerosols, or return of all activated aerosols to the aerosol mode upon evaporation.

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Conditional probability (given cloud or clear) that column contains UCL at some level,

assuming random overlap

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cloudy columnsclear columns

(a) (b)

Figure 6: UCLs are a frequent occurrence in the summertime subtropical MBL over the northeastern Pacific. (a) Fraction of G-V cloudy or clear samples during CSET that are classified as UCLs, as a function of height (all longitudes); (b) Estimated UCL coverage (determined sepa-rately for clear and cloudy UCLs) determined by aggregating data flight-by-flight into 5 degree lon-gitude and 100 m altitude bins and taking level with maximum UCL frequency as the coverage on any given day.

(a)

Median aerosol or cloud droplet concentrations for UCL samples

Mean cloud qL,cld and drizzle qL,driz liquid water contents of cloudy UCLs

Figure 7: (a) Mean liquid water contents for cloud droplets (r<20 mm) and drizzle drops (r>20 mm). Cloud liquid water contents are shown for two different measurements (CDP and King hotwire probe); (b) Median concentrations of cloud droplets (for cloudy UCL samples) and accumulation mode aerosols (for clear UCL samples).

0.00 0.05 0.10 0.15 0.20Mean qL [g m-3]

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(a) Stratiform UCL cloud layer, RF11

(c) View angle dependenceof perceived laminarity ofUCL clouds, RF07

more laminar

more cumuliform

(b) Limb view of geometrically and optically thin UCL clouds, RF07

(d) UCL clouds and trade Cu, RF07

Figure 8: Photographs of UCL clouds during CSET

Figure 9: Laminar gray clouds seen in RF11. The aircraft posi-tion at the time of the photograph is shown superimposed on coin-cident GOES visible imagery at three scales (left: 3000 km; center: 500 km; right: 100 km)

Figure 10: Gray clouds seen in RF04. The aircraft posi-tion is at the time of the photograph is located at the center of the yellow boxes in the left and center satellite images (successive zooms).

Hawaii

California

Frequency of occurrence

Average properties of UCLs

Figure 11: Observations from a single profile through a UCL cloud in RF07. (a) cloud droplet size distribution showing mode at 15-20 µm radius; (b) cloud and drizzle droplet concentrations and cloud liquid water content during profile shown in highlighted circle; (c) HSRL lidar backscatter showing opitcally thick clouds from 19:10-19:12 UTC and then various layers of thin cloud after that.

(a)(b)

(c)

Figure 12: Satellite observations indicate preva-lence of optically thin clouds in regions of Sc-Cu transition and in trade Cu regions.