Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison
Inertial Frequency, Inertial Walls, and Tropical Plumes
Room 811 AOSS, February 27, 2017, 3:30 PM
Low PV outflow from convective systems ranging from individual storms to MCSs, TCs and even the ITCZ require energy to push mass away from or downward from the convection outflow source region against the confining forces of inertial resistance and static stability. For example, increasing inertial resistance with latitude, gives rise to the "inertial wall", which traps outflow from the ITCZ in tropical latitudes. In a number of investigations, inertial resistance is shown to be associated with inertial stability while the lack of inertial resistance is associated with inertial instability, which is thought to lead to rapid acceleration of unbalanced down-pressure gradient flow.
In this talk, we focus on the parameter, "I", which is the "inertial" frequency as it appears in the dynamic balance equation. "I" quantitatively measures the contribution to the structure of the pressure field, resulting from the apparent force of inertia compared to the response created by static stability ("Brunt Vasallai Frequency") and other forces such as friction. Matching the pressure forcing of divergence with that of the inertial frequency leads to "dynamic balance", such as geostrophic, gradient and cyclostrophic balance. When matched with pressure, inertial frequency can grow to large values, exceeding all other competing frequencies in the balance equation. The magnitude of I determines the inertial "stiffness" of flow, and so the level of inertial resistance to perturbations. Loss of inertial frequency is associated with diminishing inertial stability or inertial instability. Moreover, we assert that inertial frequency, I, is the true and only correct measure of the magnitude of Galilean-independent absolute curvature vorticity.
The hypothesis, on which we focus here, is that we can understand the efficacy of the inertial wall by measuring the inertial frequency along the wall. We can further predict the breakdown of the inertia wall leading to outflow jets from convective systems and TCs and even the eruption of tropical plumes flowing from the tropical potential energy bubble poleward into the extratropics by monitoring features that impact inertial frequency. This suggests that the eruption of a tropical plume across that inertial wall may be a predictable event.