Assessing the Predictability of Band Formation and Evolution during Three Recent Northeast U.S....

Assessing the Predictability of Band Formation and Evolution during Three Recent Northeast U.S. Snowstorms

David R. NovakNOAA/ NWS Eastern Region Headquarters, Scientific Services Division, Bohemia, New York

Stony Brook University, State University of New York, Stony Brook, New York

Brian A. ColleStony Brook University, State University of New York, Stony Brook, New York

© New York Times

MotivationHigh-resolution models are capable of simulating mesoscale snowbands

1800 UTC 25 Dec 2002 Dual Doppler 4 km MM5

•Radar Reflectivity (shaded, dBZ)

•3 km winds

1815 UTC

Dual Doppler 4 km MM5


•3 km winds

1830 UTC



•3 km winds

1845 UTC



•3 km winds

1900 UTC



•3 km winds

1915 UTC



•3 km winds

1930 UTC



•3 km winds

1945 UTC



•3 km winds

2000 UTC



•3 km winds

2015 UTC



•3 km winds

2030 UTC



•3 km winds

2045 UTC



•3 km winds

2100 UTC



•3 km winds

MotivationHigh-resolution models are capable of simulating mesoscale snowbands

An ensemble of high-resolution models may provide useful band predictability information

However, in the words of Rich Grumm: “high-resolution deterministic forecasts can be highly detailed, but highly inaccurate.”

Objectives

Demonstrate ability of a high-resolution ensemble to provide qualitative band predictability information during three recent snowstorms

Explore sources of band uncertainty

25 Dec 2002

12 Feb 2006

14 Feb 2007

Ensemble Design-multi-model (MM5 v3 /WRF-ARW v 2.2)

-multi-initial condition (GFS/NAM/SREF)

-multi-physics (Microphysics/Convective)

Member Model IC/BC Microphysics Convective PBLNAM-MM5 MM5 NAM Simple Grell MRFGFS-MM5 MM5 GFS Simple Grell MRFGFS-MM5-R2 MM5 GFS Reisner2 Grell MRFSREF_N1-MM5 MM5 SREF_N1 Simple Grell MRFSREF_N2-MM5 MM5 SREF_N2 Simple Grell MRFSREF_P1_MM5 MM5 SREF_P1 Simple Grell MRFSREF_P1- MM5-KF MM5 SREF_P1 Simple Kain Fritch MRFSREF_P2-MM5 MM5 SREF_P2 Simple Grell MRFNAM-WRF WRF NAM WSM-3 Grell MRFGFS-WRF WRF NAM WSM-3 Grell MRFGFS-WRF-Thom WRF GFS Thompson Grell MRFSREF_N1-WRF WRF SREF_N1 WSM-3 Grell MRFSREF_N2-WRF WRF SREF_N2 WSM-3 Grell MRFSREF_P1_WRF WRF SREF_P1 WSM-3 Grell MRFSREF_P1- WRF-KF WRF SREF_P1 WSM-3 Kain Fritch MRFSREF_P2-WRF WRF SREF_P2 WSM-3 Grell MRF

•Initialized 15-21 h prior to band formation

Band DefinitionModel band = simulated reflectivity feature which has an aspect ratio (length/width) of 4:1 or greater, with an

intensity of at least 30 dBZ, maintained for at least 2 h.

No BandBand

Feb 14 2007

14 Feb 2007 Surface Cyclone Depth

Observed Ensemble

Storm Total Precipitation

Band Occurrence

•16 of 16 members (100%) had bands at some time during event

Band Timing

Band formation among members ranged from 14 UTC to 00 UTC

Band dissipation among member ranged from 22 UTC to 3 UTC

Band LocationFormation

Band LocationMaturity

Band LocationDissipation

Feb 14 2007 Summary

Band Characteristic

Spread Confidence

Occurrence 16/16 members High

Timing ~ 8 h Moderate

Location ~100 km High

Dec 25 2002

25 Dec 2002 Surface Cyclone Depth

Observed Ensemble


Band Occurrence


Band Timing

Band formation among members ranged from 17 to 23 UTC

Band dissipation among members ranged from 20 UTC to 2 UTC

25 Dec 2002 Summary

Band Characteristic

Spread Confidence

Occurrence 15/16 members High

Timing ~6 h Moderate

Location ~250 km Moderate

12 Feb 2006

12 Feb 2006 Surface Cyclone Depth

Observed Ensemble


Band Occurrence


Band Timing

Band formation among members ranged from 10 to 20 UTC

Band dissipation among members ranged from 13 UTC to 0 UTC

Band LocationAfter Dissipation

12 Feb 2006 SummaryBand Characteristic

Spread Confidence

Occurrence 12/16 members Moderate

Timing ~9 h Low

Location ~400 km Low

•Band occurrence was favored in the ensemble for each case

•However the specific timing and location of the bands had considerable spread, especially in the 25 Dec 2002 and 12 Feb 2006 cases.

Why?

Location Difference (GFS errored left, NAM errored right)

14 Feb 2007

36 km

NAM

0000 UTC

GFS

NAM - GFSR

~475 – 250 mb PV

-1 0 1 2 3 4 PVU -1 0 1 2 3 4

-2 -1 0 1 PVU

36 km

NAM

0600 UTC

GFS

NAM - GFSR

~475 – 250 mb PV

-1 0 1 2 3 4 PVU

-1 0 1 2 3 4

-2 -1 0 1 PVU

36 km

NAM

1200 UTC

GFS

NAM - GFSR

~475 – 250 mb PV

-1 0 1 2 3 4 PVU

-1 0 1 2 3 4

-2 -1 0 1 2 PVU

36 km

NAM

1800 UTC

GFS

NAM - GFSR

~475 – 250 mb PV

-1 0 1 2 3 4 PVU -1 0 1 2 3 4

-2 -1 0 1 2 PVU

Location Difference (SREF_N1 errored left, GFSR errored right)

25 Dec 2002

36 km

GFSR

0000 UTC

SREF_N1

SREF_N1 - GFSR

~475 – 250 mb PV

-1 0 1 2 3 PVU -1 0 1 2 3 4

-2 -1 0 1 PVU

36 km

GFSR

0600 UTC

SREF_N1

SREF_N1 - GFSR

~475 – 250 mb PV

-1 0 1 2 3 PVU -1 0 1 2 3

-2 -1 0 1 PVU

36 km

GFSR

1200 UTC

SREF_N1

SREF_N1 - GFSR

~475 – 250 mb PV

-1 0 1 2 3 PVU -1 0 1 2 3

-2 -1 0 1 PVU

36 km

GFSR

1800 UTC

SREF_N1

SREF_N1 - GFSR

-1 0 1 2 3 PVU -1 0 1 2 3

-2 -1 0 1 2 PVU

Location Difference (GFSR errored left, SREF-N1 errored far right)

12 Feb 2007

36 km

SREF_N1

1200 UTC

GFS

~475 – 250 mb PV

SREF - GFSR

-1 0 1 2 3 4 PVU

-2 -1 0 1 2 PVU

-1 0 1 2 3 4

36 km

SREF_N1

1800 UTC

GFS

~475 – 250 mb PV

SREF - GFSR

-1 0 1 2 3 4 PVU

-2 -1 0 1 2 PVU

-1 0 1 2 3 4

36 km

SREF_N1

0000 UTC

GFS

~475 – 250 mb PV

SREF - GFSR

-1 0 1 2 3 4 PVU

-2 -1 0 1 2 3 PVU

-1 0 1 2 3 4

36 km

SREF_N1

0600 UTC

GFS

~475 – 250 mb PV

SREF - GFSR

-1 0 1 2 3 4 PVU

-2 -1 0 1 2 3PVU

-1 0 1 2 3 4

36 km1200 UTC

GFS

~475 – 250 mb PV

SREF_N1

-1 0 1 2 3 4 PVU

-2 -1 0 1 2 PVU

-1 0 1 2 3 4

SREF - GFSR

Summary•A simple 16-member 12-km multi-model,-initial condition, and -physics ensemble can identify favored time periods and corridors of band formation threat.

•Although band occurrence was favored in the ensemble for each case, the specific timing and location of the bands had considerable spread.

-suggests that answering whether a band will occur may be easier to answer than when or where it will occur, even at 12-24 h forecast projections.

•Spread of variables differed markedly amongst the three cases – appeared to be related primarily to IC uncertainty.

-suggests the largest improvements in band prediction may occur with targeted initial condition improvements.

Model Comparison

Spread

•Where is the spread coming from?

-Examine 24 h forecast MSLP spread (max MSLP minus min MSLP) for each sub ensemble

25 Dec 12 Feb 14 Feb Mean

IC 10.0 mb 8.5 mb 11.0 9.8 mb

Model 6.4 mb 2.1 mb 3.6 mb 4.0 mb

Physics 1.5 mb 0.3 mb 3.8 mb 1.8 mb

IC uncertainty dominates

MSLP Spread Method


IC Spread= (MSLP spread of MM5 members with simple ice and Grell physics) + (MSLP spread of WRF members with simple ice and Grell physics) / 2

Members involved in calculation are highlighted

MSLP Spread Method


Model Spread= [(NAM-MM5 minus NAM-WRF)+(GFS-MM5 minus GFS-WRF) + (GFS-MM5-R2 minus GFS-WRF-R2) + (SREF_N1-MM5 minus SREF_N1-WRF)+etc..] / 8


MSLP Spread Method


Physics Spread= [(GFS-MM5 minus GFS-MM5-R2)+(GFS-WRF minus GFS-WRF-Thom)+(SREF_P1-MM5 minus SREF_P1-KF)+(SREF_P1-WRF minus SREF_P1-WRF-KF)] / 4


Assessing the Predictability of Band Formation and Evolution during Three Recent Northeast U.S....

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