A Hypothesis for the Redevelopment of Warm-Core Cyclones over Northern Australia

Abstract

Tropical cyclones moving inland over northern Australia are occasionally observed to reintensify, even in the absence of well-defined extratropical systems. Unlike cases of classical extratropical rejuvenation, such reintensifying storms retain their warm-core structure, often redeveloping such features as eyes. It is here hypothesized that the intensification or reintensification of these systems, christened agukabams, is made possible by large vertical heat fluxes from a deep layer of very hot, sandy soil that has been wetted by the first rains of the approaching systems, significantly increasing its thermal diffusivity. To test this hypothesis, simulations are performed with a simple tropical cyclone model coupled to a one-dimensional soil model. These simulations suggest that warm-core cyclones can indeed intensify when the underlying soil is sufficiently warm and wet and are maintained by heat transfer from the soil. The simulations also suggest that when the storms are sufficiently isolated from their oceanic source of moisture, the rainfall they produce is insufficient to keep the soil wet enough to transfer significant quantities of heat, and the storms then decay rapidly.

Sensitivity in the Overland Reintensification of Tropical Cyclone Erin (2007) to Near-Surface Soil Moisture Characteristics

“…This suggests that strong heating of these soils ahead of the remnant TC, as described by Emanuel et al (2008), may allow for the generation of enhanced surface heat and moisture fluxes in the remnant TC's environment. From this, Emanuel (2008) posed that TC Erin (2007) experienced a TC-like reintensification early on 19 August 2007 as it passed over the resultant region of enhanced latent heat fluxes, subsequently decaying as it moved eastward away from the region of sandy soils and greater soil moisture content.…”

“…Another means by which a remnant TC or TC-like vortex can reintensify over land is described by Emanuel et al (2008) in a study of TC reintensification over the deserts of Australia. This overland reintensification mechanism, operating on time scales of hours to days, is driven by enhanced surface latent heat fluxes (e.g., Rotunno and Emanuel 1987).…”

“…Such desert soils are excellent natural heat and moisture conductors. Daytime heating ahead of the TC warms these soils, contributing to latent heat fluxes sufficiently large so as to allow for vortex reintensification over land in an otherwise favorable environment for TC development (e.g., Emanuel et al 2008;Montgomery et al 2009). Subsequent vortex decay occurs after environmental moisture transport that supports the soilmoistening rains is weakened or eliminated.…”

“…In a conference presentation, Emanuel (2008) extended this theory to TC Erin (2007), hypothesizing that a similar mechanism operating on longer time scales may be responsible for the overland reintensification of TC Erin. Enhanced rainfall during the anomalously wet MarchJuly 2007 period, as noted by Arndt et al (2009), resulted in relatively moist soils (cf.…”

“…The works of Emanuel et al (2008) and Emanuel (2008) each highlight how soil moisture content and accompanying land surface feedbacks on multiple time scales may increase the intensity of TC Erin (2007). These are hereafter referred to as the ''TC rainfall'' and ''seasonal'' signals, respectively.…”