# Presentation: 638617

## Recent trends in dynamical medium-range tropical cyclone track prediction and the role of resolution v physics in the ECMWF model

**Mike Fiorino**
**michael.fiorino@noaa.gov**
**Assimilation and Modeling Branch**
**Global Systems Division**
**Earth System Research Laboratory**
**Boulder, CO**
- 4 March 2009

**Notes:**
acknowledge roger pierce and nhc and hfip test in 2008 and from analysis of gfs changes and fim

Study resulted from a conversation with acting NC XO Roger Pierce on the critical modeling factors in hurricane prediction, had found ECMWF did very after a physics change in november 2007 in WPAC and SHEM from that point forward, predicted a good 2008 season in the NHEM.

full disclosure – worked at ecmwf 1998-99 on era-40 reanalysis including TCs... dynamical TC NWP for 30+ y and have a WPAC/DoD bias – 72-h track

resolution – effective horizontal – “dynamics” or adiabatic primitive equations
physics – diabatic forcing – tropics phyiscs driven – not just convection – moisture and long-wave radiation

## Why medium-range (72-h) track?

***Operational – long-range warning is a medium-range track problem***
**no skill vis-à-vis CLIPER in early 1980s**
**high skill is required for using improved dynamical intensity guidance *****– ******I can’t believe a rapid intensification forecast if the track doesn’t make (physical) sense...***
***Modeling (& HFIP)***
**analysis v model error – 72 h is when *****model errors *****become *****dominant***
**multi-scale dynamics – *****synoptic scales dominate*****, but vortex scales can have large impacts**
**closely *****follows tropical wind score***
***MRT is the 500 mb 5-d Anomaly Correlation score of the tropics***

## The importance of modeling...

**POD of NHEM TC in reanalysis v**
**operations**

**improvement in 1989 from physics change, ditto for degradation in 1994...**

**TC POD**

**850 mb tropical wind score**
**time when the correlation of forecast and analysis vector wind drops to 70% **

**Notes:**
two measures of tropical skill:

POD of TC in analysis
850 mb

## OBS (not) critical to TC analysis?

***POD of “TC” in reanalysis v operations***
**ERA-15 uses model physics circa 1994 and achieves 1994 operational model POD *****throughout *****the 1979-1994 period**
***TC POD ******follows ******the tropical wind score******... and the ******tropical wind score****** follows the ******physics...***
**score improves from 1–7 d**
**when score reaches 3-4 d, global models began showing MRT skill**
***model of 1******st****** order importance in analysing the 1******st****** order TC vortex***

**Notes:**
Martin miller “TC”

## History of Atlantic MRT (72-h) MFE – Models v CON

***History of Atlantic MRT (72-h) MFE – Models v CON***

**CLIPER trend? better databases**
**error cut in half from ~ 280 ******** 130 nm 1990s-> 2008**
**BCON better than models**
**OFCL ~ BCON**

**medium-range track (72-h) mean forecast error – two models UKMO (global) and GFDL (limited-area) with long history v ‘best’ or baseline consensus (BCON) 1992-2008**

**higher skill in 2008 even though CLIPER error increased ******** **
**UKMO and GFDL models did very well in 2008...****.**

**Notes:**
Wpac similar, except ~ 10% higher error
all model output post processed in the same way (interpolate 6-h) for comps with ofcl and bcon
two models v bcon
2000 first year con appeared in operational adecks

## Atlantic %gain(+)/loss(-) v BCON 

***context for ECMWF results:***
**2008 a good year for the models in the LANT**
**models typically 20-25% worse than BCON in all basins, but more so in WPAC**

**Notes:**
setting context for ecmwf results in 2005-2008:
2008

cliper is a strawman aid to beat, the strong man is BCON

## ECMWF resolution v physics changes

## ALL TCs – % improvement over BCON

**MRT TC skill difference greater than change in tropical wind score**
**increased resolution (red v orange) improves 72-120 h error**
**physics change improves forecast at all times**
******skill relationship (physics > res)  consistent between basins, but stronger signal in WPAC and SHEM, more muted in LANT**

**ECMWF is 20% *****better *****than BCON when most models are 20% *****worse***
**at the medium range (72 h)**

## Summary

***ECMWF has shattered both TC MRT ******predictability limit ******from the 1990s (~130 nm); the skill of ******consensus;****** and the (old) ******PACOM ******(50, 100) 150 nm ******requirement***
***Updated results as of 20090303 in SHEM (TC activity ~ 54% below normal) 72-h MFE: ***
**CONW   – 158 nm**
**ECMWF – 108 nm**
***+32% gain over BCON*****; 61 cases  **

## My personal (biased) takeaways...

***How ECMWF achieved these results is of fundamental importance to HFIP...***
**improved TC v synoptic-scale forecast?**
***deterministic (hi-res) v ensembles***
**can ensembles work until the model(s) achieves the same skill as ECMWF? no,  physics 1****st**** **
***TC vortex analysis***
**1****st**** order structure (assimilate the “working best track”) 1****st****;detailed, hi-res obs 2****nd**
***global v limited-area models***
**when do errors the global-scales become significant?  ~ 36 h ******

**Notes:**
assimilation imperative is to working best track