AMERICANMETEOROLOGICALSOCIETYJournal of the Atmospheric SciencesEARLY ONLINE RELEASEThis is a preliminary PDF of the author-producedmanuscript that has been peer-reviewed and accepted for publication. Since it is being postedso soon after acceptance, it has not yet beencopyedited, formatted, or processed by AMSPublications. This preliminary version of the manuscript may be downloaded, distributed, andcited, but please be aware that there will be visualdifferences and possibly some content differences between this version and the final published version.The DOI for this manuscript is doi: 10.1175/2010JAS3531.1The final published version of this manuscript will replacethe preliminary version at the above DOI once it is available.©2010 American Meteorological SocietyNumerical Simulation of Radial Cloud Bands within the Upper-Level Outflow of an Observed Mesoscale Convective SystemS. B. TRIER AND R. D. SHARMANNational Center for Atmospheric Research*, Boulder, COR. G. FOVELLDepartment of Oceanic and Atmospheric Sciences, University of California, Los Angeles, Los Angeles, CAR. G. FREHLICHCIRES, University of Colorado, Boulder, CO12 April 2010Revised 9 June 2010Submitted to Journal of Atmospheric SciencesCorresponding author address:Stanley B. TrierNational Center for Atmospheric ResearchP.O. Box 3000Boulder, CO, [email protected] * The National Center for Atmospheric Research is sponsored by the National Science Foundation.AbstractTurbulence affecting aircraft is frequently reported within bands of cirrus anvil cloud extending radially outward from upstream deep convection in mesoscale convective systems (MCSs). A high-resolution convection permitting model is used to simulate bands of this type observed on 17 June 2005. The timing, location, and orientation of these simulated bands are similar to those in satellite imagery for this case. The 10-20 km horizontal spacing between the bands is also similar to typical spacing found in a recent satellite-based climatology of MCS-induced radial outflow bands.The simulated bands result from shallow convection in the near neutral to weakly unstable MCS outer anvil. The weak stratification of the anvil, the ratio of band horizontal wavelength to the depth of the near neutral anvil layer (5:1 to 10:1), and band orientation approximately parallel to the vertical shear within the same layer are similar to corresponding aspects of horizontal convective rolls in the atmospheric boundary layer, which result from thermal instability. The vertical shear in the MCS outflow is important not only in influencing the orientation of the radial bands but also for its role, through differential temperature advection, in helping to thermodynamically destabilize the environment in which they originate. High frequency gravity waves emanating from the parent deep convection are trapped in a layer of strong static stability and vertical wind shear beneath the near neutral anvil and, consistent with satellite studies, are oriented approximately normal to the developing radial bands. The wave-generated vertical displacements near the anvil base may aid band formation in the layer above.21. IntroductionBands of anvil cirrus extending radially outward from regions of deep convection (Fig. 1) are a common cloud characteristic near the outer edge of divergent upper-level outflows of mesoscale convective systems (MCSs). Lenz et al. (2009, hereafter L09) document such banding events, lasting an average of 9 h, in approximately ½ of a sample of 136 large MCSs over the central United States during the 2006 warm season. L09 suggest the practical importance of these bands by noting that at least one observation of light (moderate) commerical aviation turbulence was recorded in their vicinity for 93% (44%) of their MCS cases.Knox et al. (2010) illustrate similar cirrus bands in a wide range of atmospheric phenomena that, in addition to MCSs, includes tropical cyclone outflows and jet stream cirrus in midlatitude cyclones. While environments of the these bands share common characteristics including strong vertical and horizontal shears, which can support various dynamic instabilities, Knox et al. (2010) point out that there is no current consensus regarding their formation mechanism.Both L09 and Knox et al. refer to these bands as “transverse bands” with discussion in the latter suggesting that the terminology may have arisen from observations of the bands being oriented approximately normal to jet stream winds. However, this is not always the case [e.g., Knox et al. (2010), their Fig. 11]. Within MCSs and tropical cyclones, band orientationapproximately locally outward from the main cirrus cloud shield appears to be more generic. Thus, in the current paper we refer to these banded cloud features as “radial bands”. Another aspect of these cirrus bands in MCS outflows revealed by L09 is their tendency to occur on only one side of individual MCSs (their Fig. 5). To the extent that environmentalvertical and/or lateral shears influence band formation, the above result may be related to the well-known aysmmetry in midlatitude MCS outflow jets. Fritsch and Maddox (1981) pointed out that the preference for these anticylonic jets in a given sector of an MCS resulted from a3similarity of the outflow direction with that of background flow at the detrainment level. Such sectorization of cirrus bands is less common in tropical cyclone outflows (e.g., Heymsfield et al. 2006, Knox et al. 2010), which could be a manifestation of weaker background flow. The 17 June 2005 MCS case (Fig. 1) is an example of a radial band event associated with numerous turbulence reports on the north side of the system (Trier and Sharman 2009, hereafter TS09). TS09 used a convection-permitting model to simulate the mesoscale environment in which turbulence during the 17 June MCS was observed. They noted that the gradient Richardson Number,22mRi Nz≡∂∂V , where the numerator is the moist static stability and the denominator depends on vertical wind shear, had strong regional variations within the MCS anvil. Within the northern part of the anvil, where outflow was not opposed by background westerlies, both dynamic (0 < Ri < 1) instabilities, such as Kelvin-Helmholtz instability (e.g., Nappo 2002, §6.2), and moist static (Ri < 0) instability were supported. However, the horizontal resolution in TS09 was inadequate to resolve the radial bands of Fig. 1.In the current study we extend the