2010 Dec 26 Blizzard – Using ensemble sensitivity analyses to examine forecast jump between Dec 24 and Dec 25 Sensitivity calculations performed by Minghua

2010 Dec 26 Blizzard – Using ensemble sensitivity analyses to examine forecast jump between

Dec 24 and Dec 25

Sensitivity calculations performed by Minghua Zheng (SBU student) and

Edmund Chang

Forecasts from 24 Dec 2010

Courtesy: Dan Petersen

Short-Range Forecast Issues

Forecasts from 25 Dec 2010

Courtesy: Dan Petersen

• Data analyzed– From TIGGE– ECMWF ensemble forecast from Dec 24 00Z

and Dec 25 00Z• 50 members• MSLP and Z500

– Also examined combined NCEP+CMC+UKMO ensemble from same initial times

• 20+20+23 members• Results are similar and will not be shown

ECMWFDifference between MSLP ens mean f/c at 2010Dec2712ZInitial time 2010Dec2500Z (60hr) – 2010Dec2400Z (84hr)

)var(),()var(

),cov("" JxJcor

x

xJySensitivit i

i

i

Ensemble sensitivity calculations:

Recall:- J is any forecast metric at the final forecast time- xi is any variable within the model state vector

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

Based on ECMWF 84-hr forecast ensemble initialized at Dec 24 00Z

Note: In all plots, -0h is the forecast valid time (Dec27 12Z)

DEC 25 00 Z

DEC 27 12 Z

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

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

MSLP (hPa) Z500 (dm)DEC2712Z

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

• We believe that this way of analysis complements the usual way of ensemble sensitivity analysis in which we compute sensitivity backwards based on forecast anomalies at forecast validation time

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

MSLP (hPa) Z500 (dm)DEC2712Z

DEC2500Z

How do the forecast errors at -60 hr over these regions impact the forecast at -0 hr?

Region near gulf coast at -60hZ500 difference (m) between Dec2500Z analysis and Dec2400Z forecast

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

Regression based on Z500 projection in black boxed region using forecast ensemble from Dec 24 00Z

Difference between Dec 25 and Dec 24 forecasts

Z500(unit dm)



MSLP(unit mb)

Region over North Canada at -60hZ500 difference (m) between Dec2500Z analysis and Dec2400Z forecast



Z500(unit dm)



MSLP(unit mb)



MSLP(unit mb)

Comparing results from the 2 regions

Summary

• Results from 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 based on ECMWF ensemble (50 members) and (NCEP+CMC+UKMO) ensemble (63 members) are qualitatively consistent

Outlook for CSTAR

• Can something similar be done in near real time for CSTAR?– Can access GEFS data (20 members)– Access to Canadian ensemble data?– No access to ECMWF and UKMO (2-day delay for

TIGGE)– Perhaps combine 2 GEFS ensemble (40 members)

with 6-hr difference initial time?• Automation difficult since reference regions have

to be selected subjectively• However, once data are available and regions

have been selected, computation of sensitivity and regression are rather straightforward

Documents

2010 Dec 26 Blizzard – Using ensemble sensitivity analyses to examine forecast jump between Dec 24 and Dec 25 Sensitivity calculations performed by Minghua