References

ACP

Atmospheric Chemistry and Physics

ACP

1680-7324

Copernicus GmbH

Göttingen, Germany

10.5194/acp-11-2341-2011

An aircraft case study of the spatial transition from closed to open mesoscale cellular convection over the Southeast Pacific

Wood

¹ Bretherton

C. S.

¹ Leon

² Clarke

A. D.

³ Zuidema

⁴ Allen

⁵ Coe

⁵

Atmospheric Sciences, University of Washington, Seattle, USA

Atmospheric Science, University of Wyoming, Laramie, USA

Department of Oceanography, University of Hawaii, Honolulu, USA

Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, USA

School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK

15 03 2011

11 5 2341 2370

This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

This article is available from http://www.atmos-chem-phys.net/11/2341/2011/acp-11-2341-2011.html

The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/2341/2011/acp-11-2341-2011.pdf

Aircraft measurements are presented from the 27/28 October 2008 case study of the VOCALS Regional Experiment (REx) over the remote subtropical southeast Pacific (18° S, 80° W). Data from two aircraft that took measurements approximately twelve hours apart but in the same advected airmass are used to document a remarkably sharp spatial transition in marine boundary layer (MBL), cloud, and aerosol structure across the boundary between a well-mixed MBL containing overcast closed mesoscale cellular stratocumulus, and a pocket of open cells (POC) with significantly lower cloud cover. Long (~190–250 km) straight and level flight legs at three levels in the marine boundary layer and one level in the lower free troposphere permit sampling of the closed cells, the POC, and a 20–30 km wide transition zone with distinctly different structure from the two airmasses on either side. The POC region consists of intermittent active and strongly precipitating cumulus clouds rising and detraining into patches of drizzling but quiescent stratiform cloud which is optically thin especially toward its edges. Mean cloud-base precipitation rates inside the POC are several mm d−1, but rates in the closed cell region are not greatly lower than this. This latter finding suggests that precipitation is not a sufficient condition for POC formation from overcast stratocumulus. Despite similar cloud-base precipitation rates in the POC and overcast region, much of the precipitation (>90%) evaporates below cloud in the overcast region, while there is significant surface precipitation inside the POC. In the POC and transition region, although the majority of the condensate is in the form of drizzle, the integrated liquid water path is remarkably close to that expected for a moist adiabatic parcel rising from cloud base to top. The transition zone between the POC and the closed cells often consists of thick "boundary cell" clouds producing mean surface precipitation rates of 10–20 mm d−1, a divergent quasi-permanent cold/moist pool below cloud, a convergent inflow region at mid-levels in the MBL, and a divergent outflow near the top of the MBL. The stratiform clouds in the POC exist within an ultra-clean layer that is some 200–300 m thick. Aerosol concentrations (Na) measured by a PCASP in the diameter range 0.12–3.12 μm in the center of the ultra-clean layer are as low as 0.1–1 cm−3. This suggests that coalescence scavenging and sedimentation is extremely efficient, since Na in the subcloud layer, and droplet concentration Nd in the active cumuli are typically 20–60 cm−3. The droplet concentrations in the quiescent stratiform clouds are extremely low (typically 1–10 cm−3), and most of their liquid water is in the form of drizzle, which mainly evaporates before reaching the surface. The cloud droplet concentration in the overcast region decreases strongly as the transition region is approached, as do subcloud accumulation mode aerosol concentrations, suggesting that coalescence scavenging is impacting regions in the overcast region as well as inside the POC. Both flights show lower accumulation mode aerosol concentration in the subcloud layer of the POC (Na &sim; 30 cm−3) compared with the overcast region (Na &sim; 100 cm−3), but elevated (and mostly volatile) total aerosol concentrations are observed in the POC at all levels around 20–50 km from the transition zone, perhaps associated with some prior nucleation event. Despite the large differences in cloud and MBL structure across the POC-overcast boundary, the MBL depth is almost the same in the two regions, and increases in concert over the 12 h period between the flights.

References 1

Ackerman, A S., Toon, O B., and Hobbs, P V.: Dissipation of marine stratiform clouds and collapse of the marine boundary-layer due to the depletion of cloud condensation nuclei by clouds, Science, 262, 226–229, 1993.

Ackerman, A S., Kirkpatrick, M P., Stevens, D E., and Toon, O B.: The impact of humidity above stratiform clouds on indirect aerosol climate forcing, Nature, 432, 1014–1017, 2004.

Ackerman, A S., vanZanten, M C., Stevens, B., Savic-Jovcic, V., Bretherton, C S., Chlond, A., Golaz, J.-C., Jiang, H., Khairoutdinov, M., Krueger, S K., Lewellen, D C., Lock, A., Moeng, C.-H., Nakamura, K., Petters, M D., Snider, J R., Weinbrecht, S., and Zulauf, M.: Large-eddy simulations of a drizzling, stratocumulus-topped marine boundary layer, Mon. Weather Rev., 137, 1083–1110, 2009.

Albrecht, B A.: Aerosols, cloud microphysics, and fractional cloudiness, Science, 245, 1227–1230, 1989.

Austin, P., Siems, S., and Wang, Y.: Constraints on droplet growth in radiatively cooled stratocumulus clouds, J. Geophys. Res., 100, 14231–14242, 1995.

Berner, A., Bretherton, C. S., and Wood, R.: Large-eddy simulation of the VOCALS RF06 pocket of open cells: cloud macrostructure, entrainment and maintenance timescale of microphysical gradients, Atmos. Chem. Phys. Discuss., in preparation, 2011.

Brenguier, J.-L. and Wood, R.: Observational strategies from the micro to meso scale, from the Strüngmann Forum Report, Clouds in the Perturbed Climate System: Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation, edited by: Heintzenberg, J. and Charlson, R. J., MIT Press ISBN 978-0-262-01287-4, 2009.

Bretherton, C S., Austin, P., and Siems, S T.: Cloudiness and marine boundary layer dynamics in the ASTEX lagrangian experiments, Part 2: Cloudiness, drizzle, surface fluxes and entrainment, J. Atmos. Sci., 52, 2724–2735, 1995.

Bretherton, C S., Uttal, T., Fairall, C W., Yuter, S E., Weller, R A., Baumgardner, D., Comstock, K., and Wood, R.: The EPIC 2001 stratocumulus study, B. Am. Meteorol. Soc., 85, 967–977, 2004.

Bretherton, C S., Uchida, J., and Blossey, P N.: Slow manifolds and multiple equilibria in stratocumulus-capped boundary layers, J. Adv. Model. Earth Syst., 2, 14, 2010.

Bretherton, C. S., Wood, R., George, R. C., Leon, D., Allen, G., and Zheng, X.: Southeast Pacific stratocumulus clouds, precipitation and boundary layer structure sampled along 20° S during VOCALS-REx, Atmos. Chem. Phys., 10, 10639–10654, http://dx.doi.org/10.5194/acp-10-10639-2010doi:10.5194/acp-10-10639-2010, 2010.

Brost, R A., Wyngaard, J C., and Lenschow, D H.: Marine stratocumulus layers, Part 2: Turbulence budgets, J. Atmos. Sci., 39, 818–836, 1982.

Caldwell, P., Wood, R., and Bretherton, C S.: Mixed-layer budget analysis of the diurnal cycle of entrainment in SE Pacific stratocumulus, J. Atmos. Sci., 62, 3775–3791, 2005.

Clarke, A., McNaughton, C., Kapustin, V., Shinozuka, Y., Howell, S., Dibb, J., Zhou, J., Anderson, B., Brekhovskikh, V., Turner, H., and Pinkerton, M.: Biomass burning and pollution aerosol over North America: organic components and their influence on spectral optical properties and humidification response, J. Geophys. Res., 112, D12S18, http://dx.doi.org/10.1029/2006JD007777doi:10.1029/2006JD007777, 2007.

Comstock, K., Wood, R., Yuter, S., and Bretherton, C S.: Radar observations of precipitation in and below stratocumulus clouds, Q. J. Roy. Meteor. Soc., 130, 2891–2918, 2004.

Comstock, K., Bretherton, C S., and Yuter, S.: Mesoscale variability and drizzle in Southeast Pacific stratocumulus, J. Atmos. Sci., 62, 3792–3807, 2005.

Comstock, K., Yuter, S E., Wood, R., and Bretherton, C S.: The three dimensional structure and kinematics of drizzling stratocumulus, Mon. Weather Rev., 135, 3767–3784, 2007.

Feingold, G. and Siebert, H.: Cloud-aerosol interactions from the micro to the cloud scale, in: Perturbed Clouds in the Climate System, edited by: Heintzenberg, J. and Charlson, R. J., Ernst Strungmann Forum MIT Press, Cambridge, MA, 319–338, Frankfurt Institute for Advanced Study, 2009.

Feingold, G., Boers, R., Stevens, B., and Cotton, W R.: A modeling study of the effect of drizzle on cloud optical depth and susceptibility, J. Geophys. Res., 102, 13527–13534, 1997.

Frisch, A S., Fairall, C W., and Snider, J B.: Measurement of stratus cloud and drizzle parameters in ASTEX with a Ka band doppler radar and a microwave radiometer, J. Atmos. Sci., 52, 2788–2799, 1995.

Geoffroy, O., Brenguier, J.-L., and Sandu, I.: Relationship between drizzle rate, liquid water path and droplet concentration at the scale of a stratocumulus cloud system, Atmos. Chem. Phys., 8, 4641–4654, http://dx.doi.org/10.5194/acp-8-4641-2008doi:10.5194/acp-8-4641-2008, 2008.

Houze~Jr., R A.: Mesoscale convective systems, Rev. Geophys., 42, RG4003, http://dx.doi.org/10.1029/2004RG000150doi:10.1029/2004RG000150, 2004.

Jonker, H J J., Heus, T., and Sullivan, P P.: A refined view of vertical mass transport by cumulus convection, Geophys. Res. Lett., 35, L07810, http://dx.doi.org/10.1029/2007GL0326063doi:10.1029/2007GL0326063, 2008.

Kubar, T L., Hartmann, D L., and Wood, R.: Understanding the importance of microphysics and macrophysics for warm rain in marine low clouds: Part I. Satellite observations, J. Atmos. Sci., 66, 2953–2972, 2009.

Liou, K N. and Ou, S C.: The role of cloud microphysical processes in climate: an assessment from a one-dimensional perspective, J. Geophys. Res., 94, 8599–8607, 1989.

Martin, G M., Johnson, D W., and Spice, A.: The measurement and parameterization of effective radius of droplets in warm stratocumulus clouds, J. Atmos. Sci., 51, 1823–1842, 1994.

Mason, B J.: Production of rain and drizzle in stratiform clouds, Q. J. Roy. Meteor. Soc., 78, 377–386, 1952.

Mason, B J.: The physics of clouds, Clarendon Press, Oxford, 1957.

Mason, B J. and Howorth, B P.: Some characteristics of stratiform clouds over North Ireland in relation to their precipitation, Q. J. Roy. Meteor. Soc., 78, 226–230, 1952.

Miller, M A. and Albrecht, B A.: Surface-based observations of mesoscale cumulus-stratocumulus interaction during ASTEX, J. Atmos. Sci., 52, 2809–2826, 1995.

Nakajima, T., King, M D., Spinhirne, J D., and Radke, L F.: Determination of the optical thickness and effective particle radius of clouds from reflected solar radiation measurements: 2. Marine stratocumulus observations, J. Atmos. Sci., 48, 728–750, 1991.

Nicholls, S.: The dynamics of stratocumulus: aircraft observations and comparisons with a mixed layer model, Q. J. Roy. Meteor. Soc., 110, 783–820, 1984.

Nicholls, S. and Leighton, J.: An observational study of the structure of stratiform cloud sheets: Part 1: Structure, Q. J. Roy. Meteor. Soc., 112, 431–460, 1986.

Paluch, I R. and Lenschow, D H.: Stratiform cloud formation in the marine boundary layer, J. Atmos. Sci., 48, 2141–2158, 1991.

Pawlowska, H. and Brenguier, J L.: An observational study of drizzle formation in stratocumulus clouds for general circulation model (GCM) parameterizations, J. Geophys. Res., 108, 8630, http://dx.doi.org/10.1029/2002JD002679doi:10.1029/2002JD002679, 2003.

Petters, M. D., Snider, J. R., Stevens, B., Vali, G., Faloona, I., and Russell, L.: 2006, Accumulation mode aerosol, pockets of open cells, and particle nucleation in the remote subtropical pa- cific marine boundary layer, J. Geophys. Res., 111, D02206, doi:10.1029/2004JD005694

Pruppacher, H. R. and Klett, J. D.: Microphysics of Clouds and Precipitation, Series: Atmospheric and Oceanographic Sciences Library, Vol 18, Second Revised and Expanded Edition with an Introduction to Cloud Chemistry and Cloud Electricity, Hardcover, Kluwer Academic Publishers, 976~pp., ISBN: 978-0-7923-4211-3, online available at: http://www.springer.com/series/5669, 1997.

Reynolds, R W. and Smith, T M.: Improved global sea surface temperature analyses using optimum interpolation., J Climate, 7, 929–948, 1994.

Rosenfeld, D., Kaufman, Y. J., and Koren, I.: Switching cloud cover and dynamical regimes from open to closed Benard cells in response to the suppression of precipitation by aerosols, Atmos. Chem. Phys., 6, 2503–2511, http://dx.doi.org/10.5194/acp-6-2503-2006doi:10.5194/acp-6-2503-2006, 2006.

Savic-Jovcic, V. and Stevens, B.: The structure and mesoscale organization of precipitating stratocumulus, J. Atmos. Sci., 65, 1587–1605, 2008.

Seinfeld, J H. and Pandis, S N.: Atmospheric Chemistry and Physics, John Wiley and Sons, Toronto, 1996.

Sharon, T M., Albrecht, B A., Jonsson, H., Minnis, P., Khaiyer, M M., VanReken, T M., Seinfeld, J., and Flagan, R.: Aerosol and cloud microphysical characteristics of rifts and gradients in maritime stratocumulus clouds, J. Atmos. Sci., 63, 983–997, 2006.

Simpson, G C.: On the formation of cloud and rain, Q. J. Roy. Meteor. Soc., 67, 99–133, 1941.

Singleton, F.: Aircraft observations of rain and drizzle from layer clouds, Q. J. Roy. Meteor. Soc., 86, 195–204, 1960.

Sorooshian, A., Feingold, G., Lebsock, M., Jiang, H., and Stephens, G.: On the precipitation susceptibility of clouds to aerosol perturbations, Geophys. Res. Lett., 36, L13803, http://dx.doi.org/10.1029/2009GL038993doi:10.1029/2009GL038993, 2009.

Squires, P.: The growth of cloud drops by condensation, 2: The formation of large drops, Aust. J. Sci. Res., 5, 473–499, 1952.

Squires, P.: The microstructure and colloidal stability of warm clouds: 1. The relation between structure and stability, Tellus, 10, 256–261, 1958a.

Squires, P.: The microstructure and colloidal stability of warm clouds: 2. The causes of the variations in microstructure, Tellus, 10, 262–271, 1958b.

Stevens, B. and Feingold, G.: Untangling aerosol effects on clouds and precipitation in a buffered system, Nature, 461, 607–613, 2009.

Stevens, B., Cotton, W R., Feingold, G., and Moeng, C.-H.: Large-eddy simulations of strongly precipitating, shallow, stratocumulus-topped boundary layers, J. Atmos. Sci., 55, 3616–3638, 1998.

Stevens, B., Vali, G., Comstock, K., Wood, R., VanZanten, M., Austin, P H., Bretherton, C S., and Lenschow, D H.: Pockets of Open Cells (POCs) and drizzle in marine stratocumulus, B. Am. Meteorol. Soc., 86, 51–57, 2005.

Tomlinson, J., Li, R., and Collins, D R.: Physical and chemical properties of the aerosol within the southeastern Pacific marine boundary layer, J. Geophys. Res., 111, D12211, http://dx.doi.org/10.1029/2006JD007771doi:10.1029/2006JD007771, 2007.

Toniazzo, T., Abel, S. J., Wood, R., Mechoso, C. R., Allen, G., and Shaffrey, L. C.: Large-scale and synoptic meteorology in the South-East Pacific during the observations campaign VOCALS-REx in Spring 2008, Atmos. Chem. Phys. Discuss., 11, 225–288, http://dx.doi.org/10.5194/acpd-11-225-2011doi:10.5194/acpd-11-225-2011, 2011.

Vali, G., Kelly, R D., French, J. S. H., Leon, D., and McIntosh, R E., and~Pazmany, A.: Finescale structure and microphysics of coastal stratus, J. Atmos. Sci., 55, 3540–3564, 1998.

Van~Zanten, M C. and Stevens, B.: Observations of the structure of heavily precipitating marine stratocumulus, J. Atmos. Sci., 62, 4327–4342, 2005.

Van~Zanten, M C., Stevens, B., Vali, G., and Lenschow, D.: Observations of drizzle in nocturnal marine stratocumulus, J. Atmos. Sci., 62, 88–106, 2005.

Wang, H. and Feingold, G.: Modeling mesoscale cellular structures and drizzle in marine stratocumulus, Part 2: The microphysics and dynamics of the boundary region between open and closed cells, J. Atmos. Sci., 66, 3257–3275, 2009.

Wang, S., Albrecht, B A., and Minnis, P.: A regional simulation of marine boundary layer clouds, J. Atmos. Sci., 50, 4022–4043, 1993.

Wendisch, M., Garrett, T J., and Strapp, J W.: Wind tunnel tests of the airborne PVM-100A response to large droplets, J. Atmos. Ocean. Tech., 19, 1577–1584, 2002.

Wood, R.: Parametrization of the effect of drizzle upon the droplet effective radius in stratocumulus clouds, Q. J. Roy. Meteor. Soc., 126, 3309–3324, 2000.

Wood, R.: Drizzle in stratiform boundary layer clouds: Part I. Vertical and horizontal structure, J. Atmos. Sci., 62, 3011–3033, 2005.

Wood, R.: The rate of loss of cloud droplets by coalescence in warm clouds, J. Geophys. Res., 111, D21205, http://dx.doi.org/10.1029/2006JD007553doi:10.1029/2006JD007553, 2006.

Wood, R.: Cancellation of aerosol indirect effects in marine stratocumulus through cloud thinning, J. Atmos. Sci., 64, 2657–2669, 2007.

Wood, R. and Bretherton, C S.: Boundary layer depth, entrainment and decoupling in the cloud-capped subtropical and tropical marine boundary layer, J Climate, 17, 3576–3588, 2004.

Wood, R. and Hartmann, D L.: Spatial variability of liquid water path in marine boundary layer clouds: The importance of mesoscale cellular convection, J Climate, 19, 1748–1764, 2006.

Wood, R. and Taylor, J P.: Liquid water path variability in unbroken marine stratocumulus, Q. J. Roy. Meteor. Soc., 127, 2635–2662, 2001.

Wood, R., Comstock, K K., Bretherton, C S., Cornish, C., Tomlinson, J., Collins, D R., and Fairall, C.: Open cellular structure in marine stratocumulus sheets, J. Geophys. Res., 113, D12207, http://dx.doi.org/10.1029/2007JD009596doi:10.1029/2007JD009596, 2008.

Wood, R., Köhler, M., Bennartz, R., and O'Dell, C.: The diurnal cycle of surface divergence over the global oceans, Q. J. Roy. Meteor. Soc., 135, 1484–1493, 2009a.

Wood, R., Kubar, T., and Hartmann, D L.: Understanding the importance of microphysics and macrophysics for warm rain in marine low clouds: Part 2. Heuristic models of rain formation, J. Atmos. Sci., 66, 2973–2990, 2009b.

Wood, R., Mechoso, C. R., Bretherton, C. S., Weller, R. A., Huebert, B., Straneo, F., Albrecht, B. A., Coe, H., Allen, G., Vaughan, G., Daum, P., Fairall, C., Chand, D., Gallardo Klenner, L., Garreaud, R., Grados, C., Covert, D. S., Bates, T. S., Krejci, R., Russell, L. M., de Szoeke, S., Brewer, A., Yuter, S. E., Springston, S. R., Chaigneau, A., Toniazzo, T., Minnis, P., Palikonda, R., Abel, S. J., Brown, W. O. J., Williams, S., Fochesatto, J., Brioude, J., and Bower, K. N.: The VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx): goals, platforms, and field operations, Atmos. Chem. Phys., 11, 627–654, doi:10.5194/acp-11-627-2011, 2011.

Wood, R., Bretherton, C. S., Mechoso, C. R., Weller, R. A., and Huebert, B. and coauthors: The VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx): goals, platforms, field operations, and meteorological context, Atmos. Chem. Phys. Discuss., in preparation, 2011.

Xue, Y., Wang, L P., and Grabowski, W W.: Growth of cloud droplets by turbulent collision-coalescence, J. Atmos. Sci., 65, 331–356, 2008.

Zuidema, P., Leon, D., Pazmany, A., and Cadeddu, M.: Aircraft millimeter-wavelength retrievals of cloud liquid water path during VOCALS, Atmos. Chem Phys. Discuss., in preparation, 2011.