Analysis of an Ensemble of High-Resolution WRF Simulations for the Rapid Intensification of Super-Typhoon Rammasun (2014)

Xun Li¹, Noel E. Davidson² 

1. China Meteorological Administration, China, Haikou, HAINAN PROVINCE, China
2. Research and Development Branch, Bureau of Meteorology, Melbourne, Vitoria, Australia

Diagnostics are presented from an ensemble of high resolution forecasts that differed markedly in their predictions of the rapid intensification (RI) of typhoon Rammasun. We show that the basic difference stems from subtle differences in initializations of (a) 500–850 hPa environmental winds, and (b) mid-level moisture and ventilation. We then describe how these differences impact on the evolving storm structure, convective organization and the timing of RI. 

The evolution of vortex cloudiness from the member that best forecasts the RI is similar to the actual imagery, with the development of an inner cloud band wrapping inwards to form the eyewall. We illustrate that this structure is related to the tilt and associated dynamics of the developing inner-core in shear. For the most accurate ensemble member : (a) initially in easterly shear, inhibition of ascent and a reduction in convection over the up-shear sector allows moistening of the boundary layer air, which is transported to the down-shear sector to feed a developing convective asymmetry; (b) minimal ventilation provides favorable conditions for development of the convective asymmetry; (c) undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud; (d) this process seems related to a critical down-shear tilt of the vortex structure from mid-levels, and the vertical synchronization of low- and mid-level circulations over up-shear quadrants; and (e) the eyewall cloud continues to develop episodically via these processes. For the member that forecasts a much-delayed RI, these processes are inhibited by stronger shear, lack of vertical coherence of the circulation, lesser moisture and larger ventilation.

Analysis suggests that ensemble prediction for RI is needed to account for the sensitivity to a relatively narrow range of environmental wind shear, moisture and vortex inner-structure.

An operational long-range cyclone forecasting system for the Bureau of Meteorology

Paul A Gregory¹, Katrina Bigelow¹, Joanne Camp², Andrew Brown¹ 

1. Bureau of Meteorology, Docklands, VICTORIA, Australia
2. Hadley Centre, UK Met Office, Exeter, United Kingdom

Earlier work by Camp et. al (2018) on the preliminary hindcast of the Bureau's new seasonal model ACCESS-S1 showed it had multi-week skill in forecasting cyclone formation in the Southern Hemisphere. This was attributed to the model correctly simulating large scale changes in the atmosphere with the phase of the MJO. Continuing this work on the full hindcast showed monthly variation in forecast cyclone biases during the cyclone season. Further investigation from 2017-18 season involved the use of monthly bias corrections and lagged ensembles. Ten day forecasts generated by the numerical weather prediction model ACCESS-GE over the same period also showed good skill in predicting cyclone formation.  An operational system combining both ACCESS-S1 and GE ran in real-time during the 2018-19 season. Results for this season will be presented along with case studies, customer feedback and user experiences.

1. J Camp, MC Wheeler, HH Hendon, P Gregory, AG Marshall, KJ Tory, AB Watkins, C MacLachlan, Y Kuleshov, Skilful multiweek tropical cyclone prediction in ACCESS‐S1 and the role of the MJO, Quarterly Journal of the Royal Meteorological Society, February 2018, https://doi.org/10.1002/qj.3260

The 4km ACCESS-TC3 Tropical Cyclone Numerical Weather Prediction System

Jim Fraser¹, Yi Xiao², Thomas Coleman², Hongyan Zhu¹, Peter Steinle¹, Gary Dietachmayer¹, Jeff Kepert¹, Robin Bowen¹ 

1. Environment and Research Division, Bureau of Meteorology, Docklands, VIC, Australia
2. National Operations Centre, Bureau of Meteorology, Docklands, VIC, Australia

A new 4km resolution APS3 version of the ACCESS-TC tropical cyclone numerical weather prediction system has been developed by the Bureau of Meteorology and is scheduled for operational implementation in mid-2019.  This version (commonly known as ACCESS-TC3) will replace the existing 12km resolution ACCESS-TC2 system.  The general framework of the new ACCESS-TC3 system retains many similarities with the previous system, i.e. it remains a deterministic analysis and prediction system that produces 3-day forecasts for up to 3 concurrent TCs on relocatable 33°x33° domains within the Greater Asian tropics covering the Western Pacific and Eastern Indian Oceans.  Data assimilation is performed in five 6-hourly assimilation cycles over the previous 24-hours.  Unlike the previous system though, ACCESS-TC3 is run in convection permitting mode rather than using a parameterised convection scheme. The model physics configuration is based on the RA1-T tropical model settings developed by the UK Met Office as part of the Regional Model Evaluation and Development (RMED) process. The enhanced horizontal resolution allows the ACCESS-TC3 model to develop much deeper and more realistic central pressures than the previous system.  Verification of the ACCESS-TC3 forecast performance will be presented and compared against the performance of a version using simple downscaled initial conditions (i.e. without any observational data assimilation).