# Presentation: 803489

## A case study of tropical extratropical interaction 

-  **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 

**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

**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

**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

## MSLP animation

**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

**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 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

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

## Global PQPF animation - NCEP Global Ensemble 

**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 

**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?**