© Crown copyright Met Office Examining changes in tropical cyclones over Vietnam using a five member RCM ensemble Kuala Lumpur, Malaysia, 8 th – 11 th November 2012 Grace Redmond 1, David Hein 1, Kevin Hodges 2, Met Office Hadley Centre 1, The University of Reading 2

© Crown copyright Met Office Contents Introduction to RCM’s and tracking method used Are present day cyclones in the RCM ensemble realistic? What are the projected future changes?

© Crown copyright Met Office Providing REgional Climates for Impacts Studies The Hadley Centre regional modelling system PRECIS was run over Vietnam ( ) with sres scenario A1B A PPE ensemble consisting of five GCM’s (HadCM3Q) driving one RCM (HadRM3P.) This kind of PPE storm tracking has not been done before Ensemble projections particularly useful in the case of extreme events like TCs due to their high natural variability. PRECIS runs

© Crown copyright Met Office Introduction to TRACK TRACK – object based tracking where it identifies TC’s from an instantaneous scalar field (we have used 6hrly relative vorticity on four pressure levels.) TC’s occur as maxima (in NH). It tracks maxima coherently in space and time to produce feature trajectories for analysis. Written and maintained by Kevin Hodges (University of Reading, UK.)

© Crown copyright Met Office Vietnam, climate change and tropical cyclones Vietnam has a coast line of 3,260 km. 55% of its population lives in coastal areas < 10m above sea level (IPCC AR4.) If sea level rises by 1m, 12% of land would be lost and 17 million people displaced – as of 2007 (IPCC AR4). Vietnam is at the end of the West North Pacific cyclone track, none of its coastline is ‘cyclone free.’ Typhoon Ketsana 2009: 163 people dead, displayed, $785 million damage (Direct relief international.) en.18dao.net Direct relief international

Observations vs models Observation set IBTrACS ( ) – best track archive using data from various centres. Things to bear in mind: Different criteria of detection (models relative vorticity based, observations wind speed and MSLP based.) Max 10m wind speed in observations based on 10 min sustained wind speed at point. Max 10m wind speed in models maximum at 2.5 min time step over 25x25km. Qualitative comparison © Crown copyright Met Office

Present day results © Crown copyright Met Office

Present day tracks

© Crown copyright Met Office Present day track density JJAS OND Reasonably good summer/winter spatial distribution Under simulation in JJAS

© Crown copyright Met Office Overly strong monsoonal flow, wind shear is too high over TC genesis region in JJAS

© Crown copyright Met Office Validating ensemble members: intensity and annual cycle Q3 appears to have reasonably comparable annual cycle All other members underestimate, July cyclones particularly low. Overall not bad, can’t replicate most extreme All models tend to tail off steeply. Except Q10, all over simulate no. of ms-1 TC’s.

Cyclone structure Few observations however, pressure and wind speed drop over eye. Max wind speed at ~850 hPa. Warm core All models have similar structure. Q0 shows only small drop in wind speed and pressure over eye. © Crown copyright Met Office

Future Projections © Crown copyright Met Office

Future changes: minus Wind speed distribution shows distinct shift towards more >35ms-1 cyclones (except Q10) General decreases in mid-strength TC’s. Agrees with previous studies Consistent decrease in no. of TC’s (3-19%.) Consistent increase in TC precip (8-16%.) 0-9% increase in no, of TC’s with >35m/s max wind speed.

Changes in annual cycle No consensus on monthly change © Crown copyright Met Office

Spatial changes: track density and mean strength Track density Mean strength Consistent decrease in most areas, except Q0, increase in SE of domain. Same sign of change, different spatial patterns Different spatial patterns, overall increases, except Q10.

© Crown copyright Met Office Are tropical cyclones changes part of a long term trend? Very high natural variability - IOD, MJO, ENSO.... Statistical significance of changes in annual TC number 30-95% confidence

Continuing work Extend model runs out to 2100 to examine robustness of changes Consider physical mechanisms behind changes. Look at changes in land falling TC’s Run TRACK with ERA-interim driven PRECIS data. Possibly look at ENSO relationship © Crown copyright Met Office

Conclusions PRECIS does a good job at realistically representing TC’s (Q3 most realistic). Others show summer dip (strong monsoonal flow.) TC’s originating outside domain represented well. Changes, whilst not robust show a consistent decrease in number of TC’s across all members (3-19%) Members show an increase in more intense TC’s. Robust increases in mean precip associated with TCs (8-16%.)

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© Crown copyright Met Office