CSTAR Scientific Motivation

Predictability of High Impact Weather during the Cool Season: CSTAR Update

and the Development of a New Ensemble Sensitivity Tool for

the ForecasterBrian A. Colle, Edmund

Chang, and Minghua ZhengStony Brook University -

SUNY

CSTAR Scientific Motivation• Improve the understanding of Improve the understanding of

high impact weather high impact weather predictability during the cool predictability during the cool season through objective season through objective verification of cyclones and verification of cyclones and Rossby wave packets (RWPs) in Rossby wave packets (RWPs) in ensembles.ensembles.

• Integrate RWPs more in Integrate RWPs more in operations: Better understanding operations: Better understanding of RWP climatology, downstream of RWP climatology, downstream impact of targeted observations, impact of targeted observations, and linkage with extreme and linkage with extreme weather. weather.

• Better understanding of the Better understanding of the predictability of cyclones and predictability of cyclones and some mesoscale phenomena some mesoscale phenomena (e.g., snowbands).(e.g., snowbands).

• Better ensemble construction Better ensemble construction and post-processing.and post-processing.

From THORPEX International Science Plan (Shapiro and Thorpe, 2004)

Forecasts from 24 Dec 2010

Courtesy: Dan Petersen

Forecasts from 25 Dec 2010

Courtesy: Dan Petersen

NCEP SREF Cyclone Tracks

GDAS analysis 500 mb height/absolute vorticity 00z 24 Dec 2010







From Richard Grumm500 hPa difference between 06 UTC 24 Dec and 12 UTC 24 Dec runs starting at 18Z 24 Dec

Objective: Can ensemble sensitivity analysis (Torn and Hakim 2008; 2009) be used to enhance forecaster awareness on how upstream uncertainty is effecting a region of interest?* Ensemble sensitivity is a correlation between a

forecast metric at the final forecast time and any variable within the model state vector. It makes use of the different evolution of the forecasts among the different ensemble members to derive the sensitivity.

* For CSTAR: Want to use metrics useful to the forecaster: cyclone location and intensity, or why a shift in cyclone forecast between two model runs?

* Test for 26-27 Dec nor-easter and hurricane Irene

Initial time 12 Z 23 Dec, 84 hour ECMWF ensemble forecast

Dec 27 2010 00Z (84-h fcst)

Ensemble mean Variance

Sensitivity of EOF 1 (cyclone intensity) to 500 hPa height

0-lag

J is any forecast metric at the final forecast time- xi is any variable within the model state vector (500 hPa height in this example).

Here, J is the principal component (PC) of EOF. PCs are the projections of the dominant EOF patterns on each of the ensemble members.

EOF1 positive phase: weaker cyclone

)var(),()var(

),cov("" JxJcor

x

xJySensitivit i

i

i

“sensitivity” carries the dimension of the forecast metric and describes the change in the forecast metric corresponding to one standard deviation change in the initial condition uncertainties in the selected variable.

Projection of a pattern:

Pattern: pi Ensemble member anomaly: xi

Projection of Pattern onto Ensemble member: pi xi

Basically value of projection is large when the anomaly of the ensemble member resembles the pattern

Sensitivity of EOF 2 (SW-NE cyclone shift) to 500 hPa height

0-lag00z 27 Dec

EOF2 positive phase: southwest shift of cyclone

T-48 hr (25/00Z)Sensitivity

EOF1

EOF2

Why the shift in cyclone position between 24/00Z and 25/00z run cycles?

Using 50-member ECMWF, difference between MSLP ensemble mean forecast at 12z Dec 27 2010. Obtain pattern within red box.

Initial time 2010Dec2500Z (60hr) – 2010Dec2400Z (84hr)

Here, J is the projection of this “shift” pattern onto each ensemble member at final forecast time (Dec27 12Z)

positive areas: increasing z at that location is associated with enhancing the corresponding EOF patternnegative areas: decreasing z at that location is associated with enhancing the corresponding EOF pattern

Ensemble sensitivity calculations

forecast valid time (Dec27 12Z)

ECMWF (50 members): evolution of forecast difference ECMWF (50 members): evolution of forecast difference between DEC25Z00 and DEC24Z00between DEC25Z00 and DEC24Z00

MSLP (hPa) Z500 (dm)

Valid DEC2712Z (0 hr)

DEC2500Z

* Results from sensitivity analyses suggest that:– Cyclone E-W location at -0 hr (Dec 27 12Z) is

sensitive to conditions near the Gulf coast and over northern Canada at -48 to -60 hr

– Forecast errors from Dec 24 00Z forecast (comparing to Dec 25 00Z analysis and forecast) also show errors over those sensitive regions

– Can we assess the impacts of those short-range (24-hr) forecast errors?

– Here, we introduce an alternative way of ensemble sensitivity analysis to examine the “forward” impact of forecast errors at early time on forecasts at later time

Region near gulf coast at -60h

Z500 difference (m) between Dec2500Z analysis and the 24-hensemble mean forecast from Dec2400Z

Difference between Dec 25 and Dec 24 forecasts

Regression based on 500Z projection in black boxed region using forecast ensemble from Dec 24 00Z

MSLP(unit mb)

Note: Regression based on projection of the pattern on each ensemble member can be considered to be weighted mean of ensemble members that have anomalies resembling the chosen pattern

Hurricane IreneHurricane Irene

EOF 1: a deepening of cyclone; explains50.5% variance

EOF 2: slower movingCyclone; explains 24.1% variance

EOF 3: western shifting of cyclone; explains 11.0% variance

Target time: 2011082800(5-day forecast)Target region: LON is from 278E to 294E; LAT is from28N to 42N

Ensemble mean SLP and variance

Sensitivity of PC1 (deeper cyclone) to 500 hPa height for the target time of 20110828/00Z

Sensitivity of PC2 (slower cyclone) to 500 hPa height for the target time of 20110828/00Z

Sensitivity of PC3 (western cyclone shift) to 500 hPa height for the target time of 20110828/00Z

Real-time Sensitivity Real-time Sensitivity CalculationsCalculations

GET_BIN OUTPUT

VARIANCE

EOF

SENSITIVITY

MAIN_CSH

yyyymmddhh

• Code can run on any workstation (just need GNU Fortran and GrADS)

• Currently setting up at NCEP (so ECMWF ensemble can be combined with NCEP)

Default:• Target time: 6 days forecast initialized either on 00hr or 12hr• Target region: longitude is from 95W to 65W, and latitude is

from 30N to 50NOptional run:• target time: 0.5 days forecast to 6 days forecast with an

increment of 12h• target region: Forecaster Defined

Ensemble Members:• NCEP+CMC combined 40 members ensemble; however. it

can also compute sensitivity based on individual NCEP (20 members) or CMC (20members) ensemble. Will be setup at NCEP to include ECMWF ensemble.

Real-time Sensitivity Real-time Sensitivity CalculationsCalculations

1. There is a large spread is the forecast cyclone position in the ensembles. Forecaster asks: Why are some members closer to the coast and deeper than other members?

2. Forecaster defines his/her sensitivity box around the forecast cyclone in question (forecast hour of interest.

3. Using sensitivity analysis, Forecaster can get some idea of the upstream source of the cyclone spread (Pacific Rossby wave packet in medium range?; short-wave from more data sparse Canada?, …?)

4. If the next model cycle, the storm is closer to the coast, forecaster can confirm whether it was because of changes in upstream flow that sensi analysis suggested.

One ApproachOne Approach

Summary• Ensemble sensitivity approach has been constructed using

metrics useful for the forecaster (e.g., cyclone properties). Can be expanded to other features (300Z jet, snowband position, etc…).

• The ensemble sensitivity will be run at SBU or NCEP in real-time with results on the SBU CSTAR page: http://dendrite.somas.stonybrook.edu/CSTAR/cstar.html

• Results from 26-27 Dec 2010 sensitivity analyses suggest that:– Cyclone location at Dec 27 12Z sensitive to prior conditions (at -48hr to -

60hr) near the gulf coast as well as over northern Canada west of Hudson Bay

– Short range (24-hr) forecast errors over those locations from forecasts based on Dec 24 00Z apparently led to cyclone being forecast too far east

– Hypothesis: Forecasts based on Dec 25 00Z are better because of observations over those regions? Need to confirm with obs denial studies.

• Results from hurricane Irene suggest: -- The intensity and speed of Irene has sensitivity patterns suggesting relatively large regional influences from the cyclone itself. -- The western shift of the cyclone for day 5 h suggests some influences from the northern Pacific and downstream development.

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CSTAR Scientific Motivation