HRD Seminar – Dr. Jeff Kepert, Bureau of Meteorology, Melbourne, Australia – 23 April 2018

Dr. Kepert presented a seminar titled “Bias correction of tropical cyclone size and structure in the ECMWF global ensemble prediction system”.

ABSTRACT:

Global EPS systems provide very valuable warning of severe weather risk, including tropical cyclones. One limitation arises because the spatial resolution of the EPS is necessarily too coarse to capture the fine details of the tropical cyclone’s core circulation, resulting in the predicted storms having systematic biases in structure and intensity. In particular, the cyclones are too weak and the radius to maximum winds is too large, with these biases increasing with forecast length. Ocean waves are a major impact mechanism of tropical cyclones, but these atmospheric biases result in errors in the predicted ocean wave field also. While these biases do not affect users who respond purely to the expected risk of a tropical cyclone in their vicinity, they are important for users who wish to make more quantitative use of the wind or wave predictions.

We have developed a method for bias correcting tropical cyclone structure and intensity in EPS systems, and applied it to cyclones off north west Australia in the ECMWF global EPS. The method diagnoses the properties of forecast storms within the EPS, and replaces them in the surface wind and pressure field with parametric vortices with the corrected intensity and structure. The correction scheme is statistical, based on regression analysis of historical errors. The bias-corrected wind field ensemble is then used to force a wave model ensemble. Verification, albeit on a limited data set, demonstrates that the corrected wind and wave forecasts are superior to the raw data.

This work was supported by Woodside, Shell, Inpex and Chevron, who operate in the waters off north west Australia.

A recording of the presentation is available on the anonymous ftp site: ftp://ftp.aoml.noaa.gov/hrd/pub/blog/seminars/2018/Kepert_HRD_Seminar_20180423.mp4