John Gyakum, Ron McTaggart-Cowan, Eyad Atallah, and Lance Bosart

Transcription

1 Hurricane Juan (2003): Forecasting and numerical simulation John Gyakum, Ron McTaggart-Cowan, Eyad Atallah, and Lance Bosart

2 Outline Case Background Operational Forecasts and NWP Improved NWP Guidance Summary/Discussion

3 Case Background Tropical depression on 24 Sept. ~300 n mi SE of Bermuda Classified as a hurricane by 26 Sept. Maximum sustained winds of 90 knots by 27 Sept. Landfall near Halifax at 0300 UTC 29th as a Category 2 hurricane NHC Best Track

4 Case Background 0000 UTC 29 September Accelerating rapidly towards Halifax (landfall 3h later) Minimum SLP 974 hpa with est kt winds

5 Forecasting and NWP Forecasters noted a bifurcation in model guidance from the 0000 UTC 28 th run Common features at mid-levels are used to produce a good forecast

6 Forecasting and NWP Over the past 5 years, the major American, British, and European operational models have all been upgraded to deal with tropical features So what caused this potentially dangerous split in the guidance?

7 Composite vapour/ir at 0000 UTC 28 th EM-R initial t 0000 UTC, 8 Sept HM initial t 0000 UTC, 8 Sept Forecasting and NWP Quickscat winds at 2300 UTC 28 th GEM SLP/wind forecast 0000 UTC 29 Sept. GHM SLP/wind forecast 0000 UTC 29 Sept.

8 Forecasting and NWP So what happened? Forecast models initialized without a strong enough vortex failed to develop or maintain a closed circulation and favoured the western trough as the weather-maker

9 Forecasting and NWP Weak initial vortex without tropospheredeep PV column Weak potential stability ahead of trough PV tower tilts downshear as development occurs over the Bay of Fundy Surface circulation tracks westward

10 Forecasting and NWP Who produced Good guidance for Hurricane Juan? Centre NCEP Model GFDL Vortex Initialization Grown balanced vortex most UKMO Navy NCEP CMC UK Met NOGAPS GFS, Eta GEM Synthetic observations Synthetic observations Vortex relocation None complexity McGill MC2 None least

11 Improved Guidance Dense observations to the N/W/S of the storm over the continent mean that upstream analysis should be good Quality of forecast depends on the strength/coherence of the initial vortex Can we improve an irrelevant forecast with a better initial vortex alone?

12 Improved Guidance McGill vortex initialization Insert a fully-developed JMA-type vortex at 0000 UTC 28 th Rerun 48h forecast without any other modifications to the initial or boundary conditions (supplied by GEM) Both 12km and 3km domains are rerun

13 Improved Guidance satellite satellite Modified initial conditions at 0000 UTC 28 th September

14 Improved Guidance satellite satellite Modified forecast SLP and winds fields valid 0000 UTC 29 th September

15 Improved Guidance Initial PV tower is welldefined throughout the troposphere Convective instability over the continent is not changed Vortex tube remains upright and wraps potential instability into the circulation Much improved track and structure

16 Improved Guidance DT-theta for 00-h forecast DT-theta for 00-h hindcast

17 Improved Guidance DT-theta for 24-h forecast DT-theta for 24-h hindcast

18 Improved Guidance Vorticity and steering (21-h forecast) Vorticity and steering (21-h hindcast)

19 Improved Guidance Latent heat fluxes (12-h forecast); maximum value is 400 W m -2 Latent heat fluxes (12-h hindcast); maximum value is 1200 W m -2

20 Summary The operational forecasts for Hurricane Juan were of very high quality Operational models without a realisticallybogused vortex gave poor sfc guidance Synthesis of a strong (Category 2) vortex in the initial conditions of one of the bust models results in a much-improved forecast

21 Discussion The NWP for Hurricane Juan reduces to an initial value problem involving the tropical vortex itself: A realistically-strong, balanced synthetic vortex was ALL that was required for accurate numerical prediction of Juan in real time