ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-12-3511-2012 Limited-are a modelling of stratocumulus over South-Eastern Pacific Andrejczuk M. 1 3 Grabowski W. W. 2 Gadian A. 1 Burton R. 1 School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK National Center for Atmospheric Research, Boulder, Colorado, USA now at: Department of Physics, University of Oxford, Oxford OX1 3PU, UK 11 04 2012 12 7 3511 3526 Copyright: © 2012 M. Andrejczuk et al. 2012 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/12/3511/2012/acp-12-3511-2012.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/12/3511/2012/acp-12-3511-2012.pdf

This paper presents application of the Weather Research and Forecasting (WRF) model to limited-area modeling of atmospheric processes over the subtropical south-eastern Pacific, with the emphasis on the stratocumulus-topped boundary layer. The simulations cover a domain from the VAMOS (Variability of the American Monsoon Systems) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) field project conducted in the subtropical south-eastern Pacific in October and November 2008. We focus on a day where the UK's BAe-146 research aircraft encountered Pockets of Open Cells (POCs) at the very western edge of its flight track, rather than on the entire campaign as investigated in previous limited-area modeling studies. Model results are compared to aircraft observations with the main conclusion that the simulated stratocumulus-topped boundary layer is significantly too shallow. This appears to be a combination of an already too shallow boundary layer in the dataset used to provide initial and lateral boundary conditions, and the inability of the WRF model to increase the boundary-layer height. Several sensitivity simulations, applying different subgrid-scale parameterizations available in the model, a larger computational domain and longer simulations, as well as a different dataset providing initial and lateral boundary conditions were all tried to improve the simulation. These changes appeared to have a rather small effect on the results. The model does simulate the formation of mesoscale cloud-free regions that one might consider similar to Pockets of Open Cells observed in nature. However, formation of these regions does not seem to be related to drizzle-induced transition from open- to closed-cell circulations as simulated by LES models. Instead, the cloud-free regions appear to result from mesoscale variations of the lower-tropspheric vertical velocity. Areas of negative vertical velocity with minima (a few cm s<sup>−1</sup>) near the boundary layer top seem to induce direct evaporation of the cloud layer. It remains to be seen in LES studies whether the mechanism seen in the model is realistic or if it is simply an artifact of interactions between resolved and parameterized processes.

 References Abel, S. J., Walters, D. N., and Allen, G.: Evaluation of stratocumulus cloud prediction in the Met Office forecast model during VOCALS-REx, Atmos. Chem. Phys., 10, 10541–10559, <a href="http://dx.doi.org/10.5194/acp-10-10541-2010">https://doi.org/10.5194/acp-10-10541-2010</a>, 2010. Allen, G., Vaughan, G., Coe, H., and Minnis, P.: Gravity waves observed as a causal mechanism for transition from closed to open cellular convection in the remote South East Pacific, to be submitted, 2011. Beljaars, A.: The parameterization of surface fluxes in large-scale models under free convection, Quart. J. Roy. Meteor. Soc., 121, 255–270, 1994. Chen, F. and Dudhia, J.: Coupling an advanced land-surface/ hydrology model with the Penn State/ NCAR MM5 modeling system. Part I: Model description and implementation, Mon. Weather Rev., 129, 569–585, 2001. Chou, M.-D. and Suarez, M.&nbsp;J.: An efficient thermal infrared radiation parameterization for use in general circulation models, NASA Tech. Memo., 104606, 3, 85&nbsp;pp., 1994. Dyer, A.&nbsp;J. and Hicks, B.&nbsp;B.: Flux-gradient relationships in the constant flux layer, Quart. J. Roy. Meteor. Soc., 96, 715–721, 1970. Hong, S., Noh, Y., and Dudhia, J.: A new vertical diffusion package with an explicit treatment of entrainment processes, Mon. Weather Rev., 134, 2318–2341, 2006. Janic, Z.&nbsp;I.: The step-mountain coordinate: physical package, Mon. Weather Rev., 118, 1429–1443, 1990. Janic, Z.&nbsp;I.: The step-mountain eta coordinate model: further developments of the convection, viscous sublayer and turbulence closure schemes, Mon. Weather Rev., 122, 927–945, 1994. Janic, Z.&nbsp;I.: The surface layer in the NCEP Eta Model, Norfolk, VA, 19–23 August, Amer. Meteor. Soc., Boston, MA, 1996. Janic, Z.&nbsp;I.: Nonsingular Implementation of the Mellor-Yamada Level 2.5 Scheme in the NCEP Meso model, NCEP Office Note, No.&nbsp;437, 61, 2002. Kessler, E.: On the distribution and continuity of water substance in atmospheric circulation, Amer. Meteor. Soc, 32, 84&nbsp;pp., 1969. Minnis, P., Sun-Mack, S., Young, D.&nbsp;F., Heck, P.&nbsp;W., Garber, D.&nbsp;P., Chen, Y., Spangenberg, D.&nbsp;A., Arduini, R.&nbsp;F., Trepte, Q.&nbsp;Z., Smith Jr., W.&nbsp;L., Ayers, J.&nbsp;K., Gibson, S.&nbsp;C., Miller, W.&nbsp;F., Chakrapani, V., Takano, Y., Liou, K.-N., Xie, Y., and Yang, P.: CERES Edition-2 cloud property retrievals using TRMM VIRS and Terra and Aqua MODIS data, Part {I}: Algorithms, IEEE Trans. Geosci. Remote Sens., in press, 2011. Mlawer, E.&nbsp;J., Taubman, S.&nbsp;J., Brown, P.&nbsp;D., Iacono, M.&nbsp;J., and Clough, S.&nbsp;A.: Radiative trans-fer for inhomogeneous atmosphere: RRTM, a validated correlated-k model for the long-wave., J. Geophys. Res., 102, 16663–16682, 1997. Morrison, H., Thompson, G., and Tatarskii, V.: Impact of cloud micrpohysics on the development of trailing stratiform precipitation in a s:1imulated squall line: Comparison of one- and two-moment schemes., Mon. Weather Rev., 137, 991–1007, 2009. Paulson, C.&nbsp;A.: The mathematical representation of wind speed and temperature profiles in the unstable atmospheric surface layer, J. Appl. Meteorol., 9, 857–861, 1970. Petters, M.&nbsp;D., Snider, J.&nbsp;R., Stevens, B., Vali, G., Faloona, I., and Russell, L.&nbsp;M.: Accumulation mode aerosol, pockets of open cells, and particle nucleation in the remote subtropical Pacific marine boundary layer, J. Geophys. Res., 111, D02206, <a href="http://dx.doi.org/10.1029/2004JD005694">https://doi.org/10.1029/2004JD005694</a>, 2006. Pleim, J.&nbsp;E.: A simple, efficient solution of flux-profile relationships in the atmospheric surface layer, J. Appl. Meteorol. Climatol., 45, 341–347, 2006. Pleim, J.&nbsp;E.: A combined local and nonlocal closure model for the atmospheric boundary layer. Part I: Model description and testing., J. Appl. Meteorol. Climatol., 46, 1383–1395, 2007. Rahn, D. A. and Garreaud, R.: Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 1: Mean structure and diurnal cycle, Atmos. Chem. Phys., 10, 4491–4506, <a href="http://dx.doi.org/10.5194/acp-10-4491-2010">https://doi.org/10.5194/acp-10-4491-2010</a>, 2010. Savic-Jovcic, V. and Stevens, B.: The structure and mesoscale organization of precipitating stratocumulus, J. Atmos. Sci., 65, 1587–1605, 2008. Sharon, T.&nbsp;M., Albrecht, B.&nbsp;A., Jonsson, H.&nbsp;H., Minnis, P., Khaiyer, M.&nbsp;M., van Reken, T.&nbsp;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. Skamarock, W.&nbsp;C., Klemp, J.&nbsp;B., Dudhia, J., Gill, D.&nbsp;O., Barker, D.&nbsp;M., Duda, M.&nbsp;G., Huang, X.-Y., Wang, W., and Powers, J.&nbsp;G.: A Description of the Advanced Research WRF Version 3, National Center for Atmospheric Research, 2008. Stevens, B., Vali, G., Comstock, K., Wood, R., VanZanten, M.&nbsp;C., Austin, P.&nbsp;H., Bretherton, C.&nbsp;S., and Lenschow, D.&nbsp;H.: Pockets of open cells and drizzle in marine stratocumulus, Bull. Amer. Meteorol. Soc., 86, 51–57, 2005. Thompson, G., Rasmussen, R.&nbsp;M., and Manning, K.: Explicit forecasts of winter precipitation using an improved bulk microphysics scheme. Part I: Description and sensitivity analysis., Mon. Weather Rev., 132, 519–542, 2004. 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, <a href="http://dx.doi.org/10.5194/acpd-11-225-2011">https://doi.org/10.5194/acpd-11-225-2011</a>, 2011. VanZanten, M.&nbsp;C. and Stevens, B.: Observations of the Structure of Heavily Precipitating Marine Stratocumulus, J. Atmos. Sci., 62, 4327–4342, 2005. Wang, H. and Feingold, G.: Modeling Mesoscale Cellular Structures and Drizzle in Marine Stratocumulus. Part I: Impact of Drizzle on the Formation and Evolution of Open Cells., J. Atmos. Sci., 66, 3237–3256, 2009{a}. Wang, H. and Feingold, G.: Modeling Mesoscale Cellular Structures and Drizzle in Marine Stratocumulus. Part I: Impact of Drizzle on the Formation and Evolution of Open Cells., J. Atmos. Sci., 66, 3237–3256, 2009{b}. Wang, H., Feingold, G., Wood, R., and Kazil, J.: Modelling microphysical and meteorological controls on precipitation and cloud cellular structures in Southeast Pacific stratocumulus, Atmos. Chem. Phys., 10, 6347–6362, <a href="http://dx.doi.org/10.5194/acp-10-6347-2010">https://doi.org/10.5194/acp-10-6347-2010</a>, 2010. Wang, S., O'Neill, L. W., Jiang, Q., de Szoeke, S. P., Hong, X., Jin, H., Thompson, W. T., and Zheng, X.: A regional real-time forecast of marine boundary layers during VOCALS-REx, Atmos. Chem. Phys., 11, 421–437, <a href="http://dx.doi.org/10.5194/acp-11-421-2011">https://doi.org/10.5194/acp-11-421-2011</a>, 2011. Webb, E.&nbsp;K.: Profile relationships: The log-linear range, and extension to strong stability, Quart. J. Roy. Meteor. Soc., 96, 67–90, 1970. Wood, R., Comstock, K.&nbsp;K., Bretherton, C.&nbsp;S., Cornish, C., Tomlinson, J., Collins, D.&nbsp;R., and Fairall, C.: Open cellular structure in marine stratocumulus sheets, J. Geophys. Res., 113, D12207, <a href="http://dx.doi.org/10.1029/2007JD009">https://doi.org/10.1029/2007JD009</a>,371, 2008. Wood, R., Bretherton, C. S., Leon, D., Clarke, A. D., Zuidema, P., Allen, G., and Coe, H.: An aircraft case study of the spatial transition from closed to open mesoscale cellular convection over the Southeast Pacific, Atmos. Chem. Phys., 11, 2341–2370, <a href="http://dx.doi.org/10.5194/acp-11-2341-2011">https://doi.org/10.5194/acp-11-2341-2011</a>, 2011. 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, <a href="http://dx.doi.org/10.5194/acp-11-627-2011">https://doi.org/10.5194/acp-11-627-2011</a>, 2011. Wyant, M. C., Wood, R., Bretherton, C. S., Mechoso, C. R., Bacmeister, J., Balmaseda, M. A., Barrett, B., Codron, F., Earnshaw, P., Fast, J., Hannay, C., Kaiser, J. W., Kitagawa, H., Klein, S. A., Köhler, M., Manganello, J., Pan, H.-L., Sun, F., Wang, S., and Wang, Y.: The PreVOCA experiment: modeling the lower troposphere in the Southeast Pacific, Atmos. Chem. Phys., 10, 4757–4774, <a href="http://dx.doi.org/10.5194/acp-10-4757-2010">https://doi.org/10.5194/acp-10-4757-2010</a>, 2010. Yang, Q., W. I. Gustafson Jr., Fast, J. D., Wang, H., Easter, R. C., Morrison, H., Lee, Y.-N., Chapman, E. G., Spak, S. N., and Mena-Carrasco, M. A.: Assessing regional scale predictions of aerosols, marine stratocumulus, and their interactions during VOCALS-REx using WRF-Chem, Atmos. Chem. Phys., 11, 11951–11975, <a href="http://dx.doi.org/10.5194/acp-11-11951-2011">https://doi.org/10.5194/acp-11-11951-2011</a>, 2011. Zhang, D.-L. and Anthes, R.: A high-resolution model of the planetary boundary layer- sensitivity tests and comparisons with SESAME-79 data, J. Appl. Meteorol., 21, 1594–1609, 1982.