A case‐study of a monsoon low that formed over the sea and intensified over land as seen in ECMWF analyses

Abstract

A case‐study is presented of a tropical low that formed near Darwin, Australia, during the monsoon and subsequently intensified over land. The study is based on European Centre for Medium‐range Weather Forecast (ECMWF) analyses. Interpretations of the formation over the sea are given in terms of vorticity dynamics. The thermodynamic support for the intensification and maintenance of the low over land is investigated also. The analyses indicate that the intensification of the low depends on repeated bursts of deep convection occurring near the centre of the circulation that promote the further concentration of vorticity near the centre. This concentration of vorticity increases the local circulation about the centre, which amounts to increasing the local tangential wind speed and, through approximate gradient wind balance above the boundary layer, to a lowering of the central pressure. It is found that the horizontal transport of moisture into a mesoscale column centred on the low is approximately equal to the moisture lost by precipitation, so that total precipitable water levels are not rapidly depleted over land. While the contribution to the overall moisture budget by surface fluxes is comparatively small, these fluxes are necessary to maintain conditionally unstable conditions near the vortex centre so that deep convective bursts can continue to occur there, even when the system is located far inland.

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

“…The results add further support for the idea that a key requirement for the intensification of storms in general is the occurrence of deep convection near or at the existing centre of circulation. This result accords with the findings of Smith et al () and Kilroy et al (). The requirement transcends earlier ideas invoking the increased efficiency of diabatic heating in the high inertial stability region of the vortex core (e.g.…”

“…These are relatively large values, but typical of the Australian monsoon regime (e.g. Kilroy et al , ). For comparison, observed values found in the ‘pouch’ regions of pre‐genesis Atlantic and Carribean wave disturbances during the PREDICT experiment (Montgomery et al ) were generally around 60 kg m −2 near the sweet spot of the pouch (values for tropical storm Gaston are shown in Figures 2 and 3 of Smith and Montgomery, ).…”

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

“…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).…”

The efficiency of diabatic heating and tropical cyclone intensification

“…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.…”