David Novak

An Investigation of Model-Simulated Band Placement and Evolution in the 25 December 2002 Northeast

U.S. Banded Snowstorm

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

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

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

Daniel KeyserUniversity at Albany, State University of New York, Albany, New York

Previous WorkCompare Eta, MM5, and WRF forecasts to observations

– Models initialized with EDAS

at 0000 UTC 25 Dec 2002

– 36/12/4 km one-way nest

for MM5/WRF

Model SST Convection PBL Micro-physics

Eta Eta BMJ MYJ Ferrier

MM5 v3.4.0

Navy Grell MRF Simple Ice (3 class)

WRF v2.0.3

Navy Grell–Devenyi MRF WSM-3

MSLP Time Series

12Z 15Z 18Z 21Z 00Z 03Z 06Z

Time (UTC)

b) Obs

12 km MM5 12 km WRF

•Simulated Radar Reflectivity (shaded, dBZ)

•700-hPa height (thick solid, m)

•700-hPa 2D Miller Frontogenesis (thin solid, °C 100 km-1 h-1)

1800 UTC

12 km MM5 12 km WRF

2000 UTC

12 km MM5 12 km WRF

2200 UTC

12 km MM5 12 km WRF

0000 UTC

4 km MM5 4 km WRF

•700-hPa 2D Miller Frontogenesis (shaded, °C 100 km -1 h-1)

•700-hPa temperature (thick solid, C)

•700-hPa wind barbs

2000 UTC

Motivation• Why did the MM5 and WRF models forecast the

band too far to the southeast?– Is the deformation/frontogenesis farther northwest?

• Can the modeled sharp 700-hPa trough and attendant intense frontogenesis be verified?

• What accounts for the different band evolution forecasts in the WRF and MM5?– MM5: one single band that dissipates early– WRF: correct event length but two separate bands

Analyses and Observations

• RUC and EDAS used for analysis, with supplemental tropospheric observations

Datasource Variables Instrument Error

NOAA Profiles Wind 1 kt; 3 degrees

WSR-88D VAD Wind Situationally dependent

MDCRS Wind, Temp 3–5 kt, 5 degrees

Analysis Resolution Technique

RUC 20 km OI

EDAS 12 km 3-D VAR

Analyses and Observations18 UTC

700 mb Height (red, 15 m)

700 mb Temp (shaded, 2°C)

Analysis Winds (white barb)

Observed Winds (black barb)

RUC vs. EDAS18 UTC

RUCEDAS

700 mb Frontogenesis (red, °C 100 km-1 h-1)

RUC vs. EDAS00 UTC

RUCEDAS

MM5 and WRF19 UTC

WRFMM5

MM5 and WRF22 UTC

WRFMM5

MM5 and WRF01 UTC

WRFMM5

Features of Note

• Sharp 700-hPa trough, attendant winds and frontogenesis can be verified

• Trough and associated frontogenesis farther northwest than models forecast

• Easterly flow forecast in WRF run over CT was not observed

fgPV )(

Potential Vorticity

• High values of PV associated with– Cyclonic flow

– High static stability

– Low tropopause

– Upper trough

• Low values of PV associated with– Anticyclonic flow

– Low static stability

– High tropopause

– Upper ridge

• PV is the product of the – Absolute vorticity

– Static stability

Figures from Thorpe (1985) for Northern Hemisphere

Slide courtesy Dr. Mike Brennen (NCSU)

Dynamic Tropopause12 UTC

MM5 WRF

Pressure and winds on the PV=2 PVU surface (shaded)

MM5 WRF

• PV generated below level of maximum heating – Warming increases static stability– Pressure falls convergence increases absolute vorticity

PV+PV+

PV-PV-

PV and Latent HeatingPV and Latent Heating

• Opposite occurs above level of maximum heating where PV is reduced

• PV growth rate determined by vertical gradient of LHR

Slide courtesy Dr. Mike Brennen (NCSU)

12 UTCModel PV - Reflectivity Comparison

MM5 WRF

Pressure/winds on the DT (shaded) and reflectivity contoured > 32 dBZ

MM5 WRF

PV Cross Sections21 UTC

MM5 WRF

800-600 mb PV21 UTC

MM5 WRF

PV Findings• Model-simulated bands appear downwind of

PV filaments

• PV filaments appear to be created by diabatic processes occurring in southeast sector of cyclone

• Simulated band evolution was particularly sensitive to diabatically-generated lower-tropospheric PV anomaly over Long Island

Conclusions and Implications

•Southeast band position error appears to be due to a misplacement of the sharp 700-hPa trough and associated frontogenesis

•Although both the MM5 and WRF successfully predicted band formation, respective band evolution appears to be sensitive to convection occurring in the southeast sector of the cyclone

• Suggests the likelihood of banding may be more predictable than exact timing, location, and evolution

18 UTCRadar Observations

David Novak

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