On the Use of a System-Scale Ascent/Descent Diagnostic for Short-Term Forecasting of Tropical Cyclone Development, Intensification and Decay

• Published in: Tropical Cyclone Research and Review Vol. 3; no. 2; pp. 78 - 90
• Main Authors: Jeff Callaghan, Kevin Tory
• Format: Journal Article
• Language: English
• Published: KeAi Communications Co., Ltd 01.05.2014
• ISSN: 2225-6032
• DOI: 10.6057/2014TCRR02.02

The Brisbane Tropical Cyclone Warning Centre has used two forms of a thermal advection diagnostic to identify relatively large areas of isentropic ascent and descent for many years. When the thermodynamic conditions are favourable the ascent regions are correlated with significant outbreaks of convection that produce heavy rainfall. The diagnostic is based on the relationship between geostrophic winds that turn with height and flow perpendicular to thickness contours. As the relationship is also valid for the more general case of gradient winds, the diagnostic, in theory, should be useful for most heavy-rain-bearing tropical systems. A climatology of rainfall rate with one form of the diagnostic is presented at two Queensland locations (one tropical and one subtropical) that demonstrates a clear relationship between the isentropic ascent wind distribution and heavy to extreme rainfall.The diagnostics applied to numerical weather prediction models are valuable forecast tools as they identify heavy rainfall threat regions within which the extreme rain is likely to fall, whereas the rainfall from the same models is often under predicted or has large location errors. Applied to tropical lows and tropical cyclones the diagnostics have been used successfully to forecast tropical cyclone formation and rapid intensification and decay. Examples of such intensification and decay from around the world are presented, as well as a climatology of the diagnostic applied to intensifying tropical cyclones in the Australian region. Keywords: shear, thermal advection, intensification, rainfall, isentropic ascent