The Numerical Simulation of Thunderstorm Outflow Dynamics

Two high-resolution, two-dimensional numerical models are developed and used to investigate the dynamics of thunderstorm outflows. The first model employs the set of unapproximated, inviscid, fully compressible hydrodynamical equations, while the second, more economical model is based on a simplified set of inviscid, "quasi-compressible" equations. Neither model includes moist microphysical processes. The outflow models are designed with the flexibility needed to address many aspects of outflow dynamics with a high degree of physical realism. An outflow may be initialized in the models as a purely horizontal flow issuing from a lateral boundary, as a cold vertical downdraft imposed at the upper boundary (which is assumed to represent cloud base), or as a downdraft parameterized by a heat sink. The sensitivity of modeled outflow properties to these initialization techniques is addressed. Turbulent mixing in outflows, manifest as Kelvin -Helmholtz shearing instability, is successfully simulated with the outflow models. Although this type of hydrodynamical instability has long been observed in laboratory density currents (the dynamical analog of outflows), this is the first time it has ever been reproduced in modeled outflows. The characteristics of the Kelvin-Helmholtz instability are compared with linear theory and laboratory results, and the sensitivity of the associated turbulent mixing to several physical and computational parameters is discussed. The characteristics of the ambient environment are found to play key roles in the dynamics of simulated outflows. Several vertical wind shear and static stability profiles are examined in the model, and their influence on outflow behavior is addressed. Surface frictional effects are also shown to significantly alter the internal flow structure of outflows. Results of simulations with and without surface friction are compared, and several features in the modeled flows are shown to be similar to structure observed in laboratory density currents and thunderstorm outflows.