Tropical low formation during the Australian monsoon: the events of January 2013 (paper updated July 2016)

et al. 2016

An analysis of numerical simulations of tropical low intensification over land is presented. The simulations are carried out using the MM5 mesoscale model with initial and boundary conditions provided by ECMWF analyses. Seven simulations are discussed: a control simulation, five sensitivity simulations in which the initial moisture availability is varied, and one simulation in which the coupling between moisture availability is suppressed. Changing the initial moisture availability adds a stochastic element to the development of deep convection. The results are interpreted in terms of the classical axisymmetric paradigm for tropical cyclone intensification with recent modifications. Spin‐up over land is favoured by the development of deep convection near the centre of the low circulation. As for tropical cyclones over sea, this convection leads to an overturning circulation that draws absolute angular momentum surfaces inwards in the lower troposphere leading to spin‐up of the tangential winds above the boundary layer. The intensification takes place within a moist monsoonal environment, which appears to be sufficient to support sporadic deep convection. A moisture budget for two mesoscale columns of air encompassing the storm shows that the horizontal import of moisture is roughly equal to the moisture lost by precipitation. Overall, surface moisture fluxes make a small quantitative contribution to the budget although, near the circulation centre, these fluxes appear to play an important role in generating local conditional instability. Suppressing the effect of rainfall on the moisture availability has little effect on the evolution of the low, presumably because, at any one time, deep convection is not sufficiently widespread.

Section: Sensitivity Simulationssupporting

confidence: 94%

Numerical study of the spin‐up of a tropical low over land during the Australian monsoon

Tang

et al. 2016

“…Recently, the occurrence of deep convection close to the centre of an existing circulation has been highlighted as an important feature in the development of incipient tropical disturbances into cyclones (e.g. Smith et al , 2015b; Tang et al , 2016; Kilroy et al , 2016b). This preferred location is not, as frequently supposed, because deep convection is more ‘efficient’ in this location on account of the higher inertial stability there (Smith and Montgomery, 2016b).…”

Section: Vortex Evolution With and Without Frictionmentioning

confidence: 99%

The role of boundary‐layer friction on tropical cyclogenesis and subsequent intensification

Kilroy

2017

A recent idealized, high‐resolution, numerical model simulation of tropical cyclogenesis is compared with a simulation in which the surface drag is set to zero. It is shown that, while spin‐up occurs in both simulations, the vortex in the one without surface drag takes over twice as long to reach its intensification begin time. When surface friction is not included, the inner core size of the simulated vortex is considerably larger and the subsequent vortex intensity is significantly weaker than in the case with friction. In the absence of surface drag, the convection eventually develops without any systematic organization and lies often outside the radius of azimuthally averaged maximum tangential winds. The results underscore the crucial role of friction in organizing deep convection in the inner core of the nascent vortex and raise the possibility that the timing of tropical cyclogenesis in numerical models may have an important dependence on the boundary‐layer parametrization scheme used in the model.

“…If the convection is located outside the radius of maximum v , it will induce outflow at that radius and the maximum tangential wind above the boundary layer will tend to spin down as the M surfaces are drawn outwards. This argument is supported by the results of case studies of tropical lows in the Australian monsoon regime, including ones that intensified over the Australian continent (Kilroy et al , ; Smith et al , ; Tang et al , ). These studies highlighted the importance, in general, of deep convection occurring close to the centre of an existing circulation for intensification.…”

Section: Some Concerns/remarksmentioning

confidence: 86%

The efficiency of diabatic heating and tropical cyclone intensification

2016

Widely held arguments attributing the increasingly rapid intensification of tropical cyclones to the increasing ‘efficiency’ of diabatic heating in the cyclone's inner core region associated with deep convection are examined. The efficiency, in essence the amount of temperature warming compared with the amount of latent heat released, is argued to increase as the vortex strengthens on account of the strengthening inertial stability. Another aspect of the efficiency ideas concerns the location of the heating in relation to the radius of maximum tangential wind speed, with heating inside this radius seen to be more efficient in rapidly developing a warm‐core thermal structure and, presumably, a rapid increase in the tangential wind. A more direct interpretation of the increased spin‐up rate is offered when the diabatic heating is located inside the radius of maximum tangential wind speed. Further, we draw attention to the limitations of assuming a fixed diabatic heating rate as the vortex intensifies and offer reasons, on these grounds alone, as to why it is questionable to apply the efficiency argument to interpret the results of observations or numerical model simulations of tropical cyclones. Moreover, since the spin‐up of the maximum tangential winds in a tropical cyclone takes place in the boundary layer and the spin‐up of the eyewall is a result of the vertical advection of high angular momentum from the boundary layer, it is questionable also whether deductions about efficiency in theories that neglect the boundary‐layer dynamics and thermodynamics are relevant to reality.