"Retrospective simulation and analysis of changing SE

Asian high-resolution typhoon wind and wave

statistics"Hans von Storch

Institute for Coastal ResearchGKSS Research Center, Germany

AAMP8, session 1 modeling and prediction, 19 February 2007, Honolulu

Background – wind stats needed: decades,

homogeneous analyses• The assessment whether weather patterns are systematically

changing in the past decades requires long and homogeneous time series.

• Local time series representing wind speeds are usually not homogeneous, even for a few decades (sensitivity to instrumentation and surrounding).

• Problem solved for synoptic systems in N Europe in CoastDat@GKSS, using RCM spectrally nudged to NCEP.

• Similar problems with assessing changes of „typhoons“ and „polar lows“.

• Past detection of meso-scale features also not achieved in a homogeneous manner.

• Polar low project presently done at GKSS.• Typhoon project – more to come.

A comparative study on the retrospective simulation of typhoon

seasons in SE Asia• As a prerequisite to derive and assess changing statistics of typhoon

formation in SE Asia, simulation models should demonstrate their capability to simulate (“reconstruct”) year-to-year variations of typhoon statistics in SE Asia during the past 40-50 years.

• During this period, NCEP and ERA have constructed global-reanalyses describing the large scale atmospheric state with some fidelity, but spatial details are not that well described. Therefore regional atmospheric models, forced with global re-analyses, may be a suitable tool to reconstruct the past decades’ variability.

• A number of partners have agreed on a comparative study, namely to run their different regional atmospheric models for the storm-rich season of 1994 and the storm-poor season of 1998. The focus of the analysis will be on the number of storms, their tracks and their core pressure developments as well as on the mean circulation.

The modelling groups participating are

• GKSS, Dr. Frauke Feser, Geesthacht, Germany, with the spectrally nudged model CLM.

• National Taiwan University, Prof. Ming-Chin Wu, Taipei, Taiwan, with RSM

• National Central University, Prof. C. Sui, Jung-Li, Taiwan, with the MM5 model

• Seoul National University, Prof Dong-Kyou Lee, Seoul, S Korea, with the model REG-CM2 and a bogus-technique to introduce typhoons

Data for atmospheric conditions will be helped with by• Central Weather Bureau, Dr. Mong-Ming Lu, Taipei, Taiwan• Prof Tim Li, IPRC, with his model-based analysis of the 2004-season. 

Since analyses of wind on the sea are of limited reliability, a validation using derived wave conditions may be helpful. An agreement has been made that National Cheng Kung University, Prof. Chia Chuen Kao, Tainan, Taiwan will use the wind and pressure output of the regional models to run an ocean wave model and compare the output to ocean wave observations.

Downscaling SE Asian marine weather

• We now implement the dynamical downscaling approach for SE Asian marine weather.

• The key question is –will we master the description of typhoons?

• Presently: Case studies and seasons – formation of typhoons induced by large scale dynamics and NOT by initial values.

• Later – continuous 5-decade simulations constrained by NCEP or ERA global re-analyses.

• Similar study underway at GFDL (Tom Knutson) dealing with N Atlantic hurricanes

A case study:

Simulating tropical storm Winnie (August 1997)with a regional a climate model

-Sensitivity to spectral nudging

-Sensitivity to horizontal resolution

with regional atmospheric modelCLM; initiated at or close to1. August 1997.

Conventional RCM-simulations

with spectral nudging

NCEP

16.8.1997

Frauke Feser, pers. comm.

Dates refer to time of initialization of simulation

Effect of spectral nudging and different initial states

CLM run with 0.5º gridFrauke Feser, pers. comm.

Core pressure development

Frauke Feser, pers. comm.

Wind speed development

Frauke Feser, pers. comm.

Conclusions

• (Some) typhoons can be described by regional atmospheric models run in the climate model.

• Lateral boundary value control is insufficient to reconstruct number and track of typhoons

• Higher resolution, obtained through double nesting, gives better results.

• Results represented are preliminary.• We need better data for validation.