ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-5223-2012 Strongly sheared stratocumulus convection: an observationally based large-eddy simulation study Wang S. 1 Zheng X. 2 Jiang Q. 1 Naval Research Laboratory, Monterey, CA, USA RSMAS, University of Miami, Miami, FL, USA 14 06 2012 12 11 5223 5235 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/12/5223/2012/acp-12-5223-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/5223/2012/acp-12-5223-2012.pdf

Unusually large wind shears across the inversion in the stratocumulus-topped marine boundary layer (MBL) were frequently observed during VOCALS-REx. To investigate the impact of wind shear on the MBL turbulence structure, a large-eddy simulation (LES) model is used to simulate the strongly sheared MBL observed from Twin-Otter RF 18 on 13 November 2008. The LES simulated turbulence statistics agree in general with those derived from the measurements, with the MBL exhibiting a decoupled structure characterized by an enhanced entrainment and a turbulence intensity minimum just below the clouds. Sensitivity simulations show that the shear forcing tends to reduce the dynamic stability of the inversion, characterized by the bulk (or gradient) Richardson number. This decrease enhances the entrainment mixing, leading to reduced cloud water. Consequently, the turbulence intensity in the MBL is significantly weakened by the intense wind shear. The inversion thickens considerably and the MBL top separates from the cloud top, creating a finite cloud-free sublayer of 10–50 m thickness within the inversion, depending on the Richardson number. The weakened inversion tends to enhance the turbulence buoyant consumption and simultaneously lead to a reduced buoyant production in the cloud layer due to less radiative cooling. These effects may result in a decoupling process that creates the different heating/moistening rates between the cloud and subcloud layer, leading to a two-layered structure in the strongly sheared stratocumulus-topped MBL.

 References % vor jede Referenz Bott, A.: A positive definite advection scheme obtained by nonlinear renormalization of the advective fluxes, J. Atmos. Sci., 117, 1006–1015, 1989. Bretherton, C. S. and Wyant M. C.: Moisture transport, lower-tropospheric stability, and decoupling of cloud-topped boundary layers, J. Atmos. Sci., 54, 148–167, 1997. 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 II: Turbulence budgets, J. Atmos. Sci., 39, 818–836, 1982. Chen, C. and Cotton, W. R.: The physics of the marine stratocumulus-capped mixed layer, J. Atmos. Sci., 44, 2940–2950, 1987. Conzemius, R. J. and Fedorovich, E.: Dynamics of sheared convective boundary layer entrainment. Part I: Methodological background and large-eddy simulations, J. Atmos. Sci., 63, 1151–1178, 2006. Conzemius, R. J. and Fedorovich, E.: Bulk models of the shear convective boundary layer: evaluation through large eddy simulations, J. Atmos. Sci., 64, 786–807, 2007. De Roode, R. S. and Wang, Q.: Do stratocumulus clouds detrain? FIRE I data revisited, Bound-Layer Meteor., 122, 479–491, 2007. Deardorff, J. W.: Stratocumulus-capped mixed layers derived from a three dimensional model, Bound.-Layer Meteorol., 18, 495–527, 1980. Fu, Q. and Liou, K.-H.: On the correlated k-distribution method for radiative transfer in nonhomogenous atmospheres, J. Atmos. Sci., 49, 2139–2156, 1992. Golaz, J.-C., Wang, S., Doyle, J. D., and Schmidt, J. M.: COAMPS$^\textTM$ LES: Model evaluation and analysis of second and third moment vertical velocity budgets, Bound.-Layer Meteorol., 116, 487–517, 2005. Grenier, H. and Bretherton, C. S.: A moist PBL parameterization for large-scale models and its application to subtropical cloud-topped marine boundary layers, Mon. Weather Rev., 129, 357–377, 2011. Katzwinkel, J., Siebert H., and Shaw, R. A.: Observation of a self-limiting, shear-induced turbulent inversion layer above marine stratocumulus, Bound.-Layer Meteorol., http://dx.doi.org/10.1007/s10546-011-9683-4doi:10.1007/s10546-011-9683-4, 2011. Kim, S.-W., Park, S.-H., and Moeng, C.-H.: Entrainment processes in the convective boundary layer with varying wind shear, Bound.-Lay. Meteorol., 108, 221–245, 2003. Lilly, D. K.: Models of cloud-topped mixed layers under a strong inversion, Q. J. Roy. Meteor. Soc., 94, 292–308, 1968. Moeng, C.-H., Stevens, B., and Sullivan P. P.: Where is the interface of stratocumulus-topped PBL?, J. Atmos. Sci., 62, 2626–2631, 2005. Nicholls, S.: The dynamics of stratocumulus: aircraft observations and comparisons with a mixed-layer model, Q. J. Roy. Meteor. Soc., 110, 784–820, 1984. Nicholls, S.: The structure of radiatively driven convection in stratocumulus, Q. J. Roy. Meteor. Soc., 115, 487–511, 1989. Galperin, B., Sukoriansky, S., and Anderson, Ph. S.: On the critical Richardson number in stably stratified turbulence, Atmos. Sci. Lett., 8, 65–69, http://dx.doi.org/10.1002/asl.153doi:10.1002/asl.153, 2007. Randall, D. A.: Buoyant production and consumption of turbulent kinetic energy in cloud-topped mixed layer, J. Atmos. Sci., 41, 402–413, 1984. Stevens, B.: Cloud transitions and decoupling in shear-free stratocumulus-topped boundary layers, Geophys. Res. Lett., 27, 2557–2560, http://dx.doi.org/10.1029/1999GL011257doi:10.1029/1999GL011257, 2000. Stevens, B., Moeng C.-H., Ackerman, A. S., Bretherton, C. S., Chlond, A., de Roode, S., Edwards, J., Golaz, J.-C., Jiang, H., Khairoutdinov, M., Kirkpatrick, M. P., Lewellen, D. C., Lock, A., Muller, F., Stevens, D. E., Whelan, E., and Zhu, P.:: Evaluation of large-eddy simulations via observations of nocturnal marine stratocumulus, Mon. Weather Rev., 133, 1443–1461, 2005. Sun, J., Mahrt, L., Banta, R. M., and Pichugina, Y. L.: Turbulence regimes and turbulence intermittency in the stable boundary layer during CASES-99, J. Atmos. Sci., 69, 338–350, 2011. Turner, J. S.: Buoyancy effects in fluids, Cambridge University Press, 368 pp., 1973. Turton, J. D. and Nicholls, S.: A study of the diurnal variation of stratocumulus using a multiple mixed layer model, Q. J. Roy. Meteorol. Soc., 113, 969–1009, 1987. Yamaguchi, R. and Randall D. A.: Cooling of entrained parcels in a large-eddy simulation, J. Atmos. Sci., 69, 1118–1136, 2012, Wang, S., Golaz, J.-C., and Wang, Q.: Effect of intense wind shear across the inversion on stratocumulus clouds, Geophys. Res. Lett., 35, L15814, http://dx.doi.org/10.1029/2008GL033865doi:10.1029/2008GL033865, 2008. 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, http://dx.doi.org/10.5194/acp-11-421-2011doi:10.5194/acp-11-421-2011, 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, doi:10.5194/acp-11-627-2011, 2011. Zheng, X., Albrecht, B., Jonsson, H. H., Khelif, D., Feingold, G., Minnis, P., Ayers, K., Chuang, P., Donaher, S., Rossiter, D., Ghate, V., Ruiz-Plancarte, J., and Sun-Mack, S.: Observations of the boundary layer, cloud, and aerosol variability in the southeast Pacific near-coastal marine stratocumulus during VOCALS-REx, Atmos. Chem. Phys., 11, 9943–9959, http://dx.doi.org/10.5194/acp-11-9943-2011doi:10.5194/acp-11-9943-2011, 2011.