Instabilities of longitudinal vortex rolls in katabatic Prandtl slope flows

Stationary counter-rotating longitudinal vortex pairs emerge from one-dimensional Prandtl slope flows under katabatic as well as anabatic conditions due to a linear instability when the imposed surface heat flux magnitude is sufficiently strong relative to the stable ambient stratification. For anabatic flows, these vortices have already been identified to exhibit an unique topology that bears a striking resemblance to speaker-wires since they stay coherent as a single unit without the presence of another vortex pair. Under katabatic conditions and at a constant Prandtl number, we find that the longitudinal vortices emerging at a range of different slope angles possess the similar topology as their anabatic counterparts. We determine the existence of both fundamental and subharmonic secondary instabilities depending on the slope angle for the most likely transverse base flow wavelength. Our results indicate that the most dominant instability shifts from a fundamental to subharmonic mode with increasing slope angle. At shallow slopes, this dynamic contrast with the speaker-wire vortices in anabatic slope flows at the same angle which for which the subharmonic instability is clearly dominant. These modes are responsible for the bending and movement of single or multiple speaker-wire vortices, which may merge or reconnect to lead to dynamically more unstable states, eventually leading to transition towards turbulence. We demonstrate that at sufficiently steep slopes, the dynamics of these vortex pairs are dominated by long-wave reconnections or two-dimensional mergers between adjacent pairs.