arXiv subjects

Bethan L. Harris

Publications and source records attributed to Bethan L. Harris.

2 recordsLinked to original sources

arXiv2020

The generalised buoyancy/inertial forces and available energy of axisymmetric compressible stratified vortex motions

Adiabatic and inviscid axisymmetric perturbations to a stable reference vortex in gradient wind balance are known to experience two kinds of restoring forces: one that is proportional to both the perturbation density and the reference pressure gradient, and one that is purely radial and proportional to the squared angular momentum perturbation. We show that the work required to move a fluid parcel against such forces from its equilibrium to actual position is path-independent and formally equivalent to the available energy accounting for momentum constraints previously constructed by Andrews (2006) and Codoban and Shepherd (2006). Physically, this work represents the energy of the unbalanced part of the vortex and hence a form of eddy energy. It can be partitioned into available acoustic energy, slantwise available potential energy and centrifugal potential energy. We show that the conditions required for these energies to be positive definite correspond to the classical conditions for symmetric stability, valid for both small and finite-amplitude perturbations. The new available energy framework possesses various advantageous features over previous approaches that shed new light on how the thermodynamic and mechanical sinks/sources of energy control the intensification of the balanced and unbalanced parts of a warm core cyclonic vortex. These features should prove useful in the future to re-examine and clarify the links between the different existing paradigms of tropical cyclone (TC) intensification within a single unifying theoretical framework.

physics.flu-dyn↗

arXiv2017

Assessment of algorithms for computing moist available potential energy

Atmospheric moist available potential energy (MAPE) has been traditionally defined as the potential energy of a moist atmosphere relative to that of the adiabatically sorted reference state defining a global potential energy minimum. Finding such a reference state was recently shown to be a linear assignment problem, and therefore exactly solvable. However, this is computationally extremely expensive, so there has been much interest in developing heuristic methods for computing MAPE in practice. Comparisons of the accuracy of such approximate algorithms have so far been limited to a small number of test cases; this work provides an assessment of the algorithms' performance across a wide range of atmospheric soundings, in two different locations. We determine that the divide-and-conquer algorithm is the best suited to practical application, but suffers from the previously overlooked shortcoming that it can produce a reference state with higher potential energy than the actual state, resulting in a negative value of MAPE. Additionally, we show that it is possible to construct an algorithm exploiting a theoretical expression linking MAPE to Convective Available Potential Energy (CAPE) previously derived by Kerry Emanuel. This approach has a similar accuracy to existing approximate sorting algorithms, whilst providing greater insight into the physical source of MAPE. In light of these results, we discuss how to make progress towards constructing a satisfactory moist APE theory for the atmosphere. We also outline a method for vectorising the adiabatic lifting of moist air parcels, which increases the computational efficiency of algorithms for calculating MAPE, and could be used for other applications such as convection schemes.

physics.ao-ph↗