A case study of tropical extratropical interaction

Yucheng Song and Zoltan Toth @EMC.NCEP.NWS.NOAA

Yucheng Song and Zoltan Toth

Environmental Modeling Center

National Centers of Environmental Prediction

National Weather Service

NOAA

A case study of tropical extratropical interaction


Outline – 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


On Oct 14, 12Z two low pressure systems developed


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


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


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 ECMWFThe NCEP ensemble spread shows tropical influence to the Gulf of Alaska low


MSLP animation(12Z forecast added to mimic analysis)

The animation on the left mimics how the two low pressure systems in North Pacific formed


Global IR animation – an example of tropical – extratropical interaction


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


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


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


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


Global PQPF animation - NCEP Global Ensemble

Animation shows how the first system near Gulf of Alaska formed by NCEP global ensemble forecast


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


Summary • 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 (continued)

• 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?

A case study of tropical extratropical interaction

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