This study presents the real-time performance of the United States (US) National Centers for Environmental Prediction (NCEP) operational Hurricane Weather Research and Forecast (HWRF) model in predicting rapid intensification (RI) of typhoons in the North Western Pacific (WPAC) basin in 2013. Examination of all RI cases in WPAC during 2013 shows that the HWRF model captures a consistent vortex structure at the onset of all RI as seen in previous idealized studies with HWRF. However, HWRF has issues with predicting RI when the model vortex is initialized with intensity greater than hurricane strength. Further verification of the probability of detection (POD) and the false alarm rate (FAR) of RI forecasts shows that the HWRF model outperforms all other models used by the US Navy’s Joint Typhoon Warning Center, possessing highest POD and lowest FAR in 2013. Examination of the intensity change forecasts at different forecast lead times also confirms that the HWRF model has superior performance, particularly at the 72-h lead time with the POD index ~0.91 and the FAR index ~0.33. Such unique performance of the HWRF model demonstrates its role in helping operational agencies improve their official intensity (and RI) forecasts for tropical cyclones in the WPAC basin.

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.

Despite decades of theoretical research and observational studies, a good understanding of tropical cyclone genesis (TCG) remains elusive. One school of theories proposes that TCG within an African Easterly Wave results from “bottom-up” development of cyclonic vorticity that is contingent upon favorable conditions in the lower-troposphere and boundary layer. Our observational study suggests that while lower-tropospheric forcing is a necessary condition for this type of TCG, it may not be sufficient in some cases, and that environmental conditions in the upper levels can have an influence. Specifically, we find evidence to suggest that pre-TCG upper-tropospheric flow patterns characterized by core-connecting outflow vents to the environment can in certain situations provide a modulating effect on Atlantic tropical disturbances trying to develop. Patterns of nearenvironment
upper-level inertial stability, divergence, outflow setup, and mass evacuation are identified and related to surface development. The study employs high-resolution satellite-derived wind data, aircraft GPS dropwindsondes, composite fields, multivariate objective analyses, and case studies to help identify conditions in the upper-level environment that can play a role in Atlantic TCG events.

The Hong Kong Observatory operates an in-house developed nowcasting system, namely “Short-range Warning of Intense Rainstorms in Localized Systems (SWIRLS)”, to support the operation of rainstorm and severe weather warnings as well as to provide rainfall nowcast services for the public and for special users in Hong Kong. Aiming to enhancing its performance in nowcast of rainfall brought by tropical cyclones, a new radar echo tracking scheme that separates the motion of the spiraling rain bands from the overall movement of tropical cyclone has been developed. Back-testing with historical cases in the past ten years reveals that the new scheme is more capable of preserving tropical cyclone rain band structures and can enhance forecast skills.

Analysis of tropical cyclone (TC) rainfall characteristics helps in understanding and improving quantitative precipitation forecasts of TCs. Based on the Tropical Rainfall Measuring Mission (TRMM) rainfall data for the period of 2000-2010, a TC rainfall database –Tropical Cyclone Rainfall Analysis for North Indian Ocean (TCRAIN) is developed to study the precipitation characteristics during various stages of life cycle of TCs of the North Indian Ocean (NIO). Three rainfall products, viz., frequency distribution of rain rate, azimuthally averaged radial profile of rain rate and quadrant mean rain rate with respect to the TC centre and the direction of motion of the TC are generated in a Lagrangian frame of coordinate system for 5 intensity stages of life cycle of each cyclone over NIO. Using this TCRAIN database, composites of frequency distribution of rain rates as well as quadrant mean rain rate for TCs of NIO are generated to bring out probable rain rates and asymmetric structure in rainfall distribution. Rainfall profiles for individual cases are likely to deviate from the climatological profiles under different environmental and oceanic conditions. TCRAIN database would serve as a useful tool for carrying out intensity based analytical studies on structure of rainfall associated with cyclones of NIO through ready depiction of TC rainfall profiles.