Simulation of MCS anvil cloud bands associated with flight-level turbulence

Turbulence has frequently been reported in cirrus cloud bands that extend radially outward near edges of upper-level outflows within large mesoscale convective systems (MCSs). Notable aspects of these bands are their frequent occurrence in regions several hundred km removed from active deep convection (i. e., regions of large radar reflectivity) and in preferred quadrants of the MCS anvil edge. In this talk, results from a high resolution numerical simulation are used to illustrate mechanisms responsible for these "radial" cloud bands within an MCS that was associated with moderate or greater turbulence on three separate flights that traversed the central United States on 17 June 2005.

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 development of the near-neutral to moist statically unstable conditions supporting the anvil bands results from a combination of radiative and dynamical forcing. The vertical shear in the MCS outflow is important not only in influencing the orientation of the bands but also for its role, through differential temperature advection, in helping to thermodynamically destabilize the environment in which they originate.

Though high-resolution research numerical weather prediction models (dx, dy < 1 km) are required to explicitly simulate these bands, we discuss how satellite data, conceptual models, and analyses and forecasts from lower resolution operational numerical models might be used to delineate mesoscale regions susceptible to flight-level turbulence arising from this and similar mechanisms.->