Convection-resolving simulations of the environment associated with widespread turbulence within the upper-level outflow of a large mesoscale convective system

Widespread moderate turbulence was encountered on three commercial airline flights over northern Kansas near the northern edge of the extensive cirrus anvil of a nocturnal mesoscale convective system (MCS) on 17 June 2005. A noteworthy aspect of the turbulence was its location several hundred km from the active deep convection regions of the MCS. In this presentation we use convection resolving simulations with the WRF model to examine how the environment in this location of the MCS evolves in a manner that supports the development of turbulence.

Large vertical shears (DeltaV > 30 m/s over 1.5 km) are found at flight levels near the northern edge of the asymmetric upper-level outflow. The vertical shear in this location helps reduce the Richardson Number to values near Ri ~ 0.25, which permits turbulence generation in the real atmosphere through dynamical instabilities. The vertical shear plays an additional role in turbulence generation by contributing to reductions in static stability. Here, adiabatic cooling resulting from the mesoscale updraft near the center of the anvil cloud produces horizontal temperature gradients that are differentially advected by vertical shear associated with outflow jet, leading to a steepening of the isentropes downstream closer to northern anvil edge. This thermodynamic destabilization process supports turbulence generation by both lowering the vertical shear threshold necessary for dynamic instability and by producing localized regions of static instability where isentropes overturn.

Comparison of a control simulation to one in which the temperature tendency due to latent heating was eliminated indicates that increases in vertical shear in the location of the observed turbulence was dominated by the MCS-induced outflow component. The northern portion of the MCS-outflow may be a particularly favored region for widespread turbulence, since its southwesterly vertical shear is often in the same sense as the upper-level environmental vertical shear, resulting in particularly large values of total vertical shear.