A case study of tropical extratropical interaction

1. A case study of tropical extratropical interaction Yucheng Song and Zoltan Toth Environmental Modeling Center National Centers of Environmental Prediction National Weather Service NOAA

2. Ensemble forecasts from NCEP and ECMWF showing mean and spread of mean sea level pressure The ECMWF ensemble forecast is initialized 12 hour earlier than NCEP Global IR image animation showing how the tropical convection interact with extra-tropical system Difference in NCEP and ECMWF analyses for the case Expected forecast error reduction in the verification region by NCEP and ECMWF ensembles using ETKF method Precipitation verification by observation and probability forecast from NCEP global ensemble forecast Animation of expected forecast error reduction by NCEP ensemble Summary and discussion Outline

3. On Oct 14, 12Z two low pressure systems developed

4. Forecasts made starting with different initial dates NCEP Ensemble initialized on Oct 10, 00z was able to predict the low pressure system near Gulf of Alaska on a 4.5 day lead time

5. A map taken from the previous slide shows the forecasts on a 4.5 day lead for NCEP forecast, 5 day lead for ECMWF forecast

6. Forecast loop – initial time Oct 10 00Z The animation on the left shows the forecast initialized on Oct 10, 00Z for NCEP, Oct 9, 12Z for ECMWF The NCEP ensemble spread shows tropical influence to the Gulf of Alaska low

7. MSLP animation (12Z forecast added to mimic analysis) The animation on the left mimics how the two low pressure systems in North Pacific formed

8. Global IR animation – an example of tropical – extratropical interaction

9. Comparison of initial conditions – NCEP vs ECMWF - Mean Sea Level Pressure on Oct 10 00z, 2005 Differences in NCEP and ECMWF analyses can be spotted at initial time

10. Difference of Mean Sea Level Pressure on Oct 11 00z, 2005 between NCEP vs ECMWF analysis Differences in NCEP and ECMWF analyses can be spotted at 24 hours later – the region corresponds well with expected forecast error reduction in verification region spotted by ETKF using NCEP ensemble well –denoted by square boxes in the two figures

11. Expected forecast error reduction in verification region by ETKF method using 19-member NCEP ensemble Expected forecast error reduction in verification region by ETKF method using 25-member ECMWF ensemble

12. Precipitation verification – heavy rainfall was produced by the first system near Gulf of Alaska

13. Global PQPF animation - NCEP Global Ensemble Animation shows how the first system near Gulf of Alaska formed by NCEP global ensemble forecast

14. The blob shows the center of forecast error reduction evolution with forecast lead time

15. Influence of organized tropical convection on extratropical circulation Tropical system picked up by westerly jet, wave evolves into strong low in Gulf of Alaska Strong downstream propagation of energy leads to explosive development of second low in front of original wave Ensemble forecast performance NCEP ensemble captures First wave from Oct 7 12Z on Second wave from Oct 8 12Z on NCEP ensemble mean gives accurate forecast from Oct 10 00Z ECMWF ensemble captures developments at shorter lead times Summary

16. ETKF targeting method with ensemble from NCEP - identified area of tropical convection as main source of initial value uncertainty ECMWF - no clear signal What limits predictability for the first and second waves to 7 & 6 days? Errors in Initial conditions Observations Quality Quantity Model representation of tropical convective processes Other model problems? Summary (continued)