High-resolution Observations and Model Simulations of the Life Cycle

of an Intense Mesoscale Snowband over the Northeastern United States

Reporter: Prudence Yi-Yun Chien

Reference:

Novak, D. R., B. A. Colle, and S. E. Yuter, 2008: High-resolution Observations and Model Simulations of the Life Cycle of an Intense Mesoscale Snowband over the Northeastern United States. Mon. Wea. Rev., 136, 1433–1456.

2

Outline

• Key words

• Introduction

• Datasets and methodology

• Case overview

• Band life cycle

• Moisture availability

• Summary and discussion

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3

Key words

yy

v

xy

u

yyx

v

xx

u

x

dt

dF D

1

2

2D form of frontogenesis equation (Miller, 1948)

Assess frontal forcing for ascent F2D > 0

3

4

Key words

esgEPV Saturation equivalent potential vorticity (EPV)

θes surface slope > M surface slope

EPV<0Saturation environment

0

0

EPVfzgyMf

yMf

eses

es

1→2

Conditional Symmetric instability

CSI condition:

4

0

0

y

Mzes Conditional stable

Inertial stable, M = fy-ug

5

Introduction

• Motivation:Advance cool-season QPF=> Structural and dynamical evolution of a cool-season mesoscale snowband

• Using high-resolution obs. & model

• Case period: 2002/12/25~ 2002/12/26

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6

Datasets and methodology

Dual-Doppler synthesis

1. Upper-air obs2. Doppler radar3. Wind profiler4. ground-based integrated

precipitable water vapor (IPW)5. Conventional surface obs6. Commercial aircraft

Observation

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7

MM5 V3.4

Cumulus Grell (1993)

Microphysics Dudhia (1989)

PBL MRF

IC NCEP - EDAS analysis (0000UTC 25 Dec)

BC NCEP – Eta forcast (0000UTC 25 Dec)

SST US Navy OTIS

Model configurations

31 sigma levels

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8

Case overview

• Synoptic-scale evolution

• Mesoscale evolution

• QPF

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9

Synoptic-scale evolution

500-hPa Φ(solid)

300-hPa wind (barb)

IR brightness temperature (shaded)

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10

Synoptic-scale evolution

isobars (solid)

isotherms (gray)

wind (barb) 10

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Mesoscale evolution

700-hPa Φ (thick solid)

F2D (thin solid)

reflectivity (shaded)

MM51800UTC 12/25

2760m2790m

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Mesoscale evolution

700-hPa θ (thick solid)

F2D (shaded)

wind (barb)

MM5 700-hPa Φ (thick solid)

F2D (thin solid)

reflectivity (shaded)

2100UTC 12/25

2700m 2760m

12

13

Mesoscale evolution

700-hPa Φ (thick solid)

F2D (thin solid)

reflectivity (shaded)

MM50000UTC 12/26

2670m 2728m

13

14

QPF 24-h accumulated precipitation (liquid equivalent)

1200UTC 12/25 ~ 1200UTC 12/26

Max = 76mm

Max = 53mm

underpredict~ 30%

Max = 59mm

underpredict~ 22%

Max = 46mm

underpredict~ 40%

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Band life cycle

• Band formation

• Band maturity

• Band dissipation

• Model time series

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Band formation

Conv.

R1

R2

Conv.

R1*

R2*

Conv.

R2Defm.

Conv.

R2*Defm.

1802UTC WSR-88D

1929UTC WSR-88D

1800UTC MM5

1930UTC MM5

F2D (shaded)

θ (solid)

Reflectivity, 1km (shaded)

wind, 3km (barb)

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Reflectivity (shaded)

Ascent (dot)

F2D (solid)

EPV (shaded) RH=100% (thick solid)

Negative η (dot)

θes (gray solid)

II & CICI

II & CI

snowfall

F2D max

Ascent max

1802UTC WSR-88D

1800UTC MM5

II

F2D max

Ascent max

II

CI:conditional instability

II: inertial instability

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Band maturity2101UTC WSR-88D 2100UTC MM5

2100UTC MM5

R2R2*

F2D (shaded)

θ (solid)

Reflectivity, 1km (shaded)

wind, 3km (barb)

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Reflectivity (shaded)

Ascent (dot)

F2D (solid)

EPV (shaded) RH=100% (thick solid)

Negative η (dot)

θes (gray solid)

2101UTC WSR-88D

2100UTC MM5

CI

Weak CSI

II

II

F2D max

Ascent max

F2D max

Ascent maxCS

Weak CSI

CS:conditional stability

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2300UTC MM52359UTC WSR-88D

2300UTC MM5

R2

R2*

Band dissipation

F2D (shaded)

θ (solid)

Reflectivity, 1km (shaded)

wind, 3km (barb)

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2300UTC MM5

2359UTC WSR-88D

Reflectivity (shaded)

Ascent (dot)

F2D (solid)

EPV (shaded) RH=100% (thick solid)

Negative η (dot)

θes (gray solid)

II

CI

II

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Model time series

Cross section A-B

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Moisture availability

Obs: IPW

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Band formation (1930UTC 12/25 MM5)

Band dissipation (2200UTC 12/25 MM5)

1930UTC

2200UTC

1930UTC

2200UTC

24

2525

Compare QPF between 2100UTC and 2200UTC

Ascent profile Θe profile (QPF2200-QPF 2100)/QPF 2100

same -6%

same -23.5%

=> Changes in ascent dominated changes in moisture

MD

-0.14

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• Band evolution=> forcing, stability, and moisture

• Forcing (MM5 results)(1) Band formation deepening of a midlevel trough Increase in deformation, convergence, and frontogenesis

(2 )Band dissipation midlevel trough less defined, and frontogenesis weakened Conditional stability led to weak frontal circulation

Summary and discussion

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Summary and discussion

• StabilityConventional thinking: CI and SI increase => band formationThis study: CI occurred before band formation~1.5h band formation: CI decrease & F2D increase

• W max & F2D maxPrevious study: W max locate on the warm side of F2D max ~50-200km This study: W max and F2D max are nearly coincident

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Summary and discussion

• Moisturesource: Atlantic Ocean (band formation)change of ascent => change of moisture

• QPFMM5 underforecast ~30% max preci. (Δx=4km)axis of heaviest preci. ~50km to the SE of the observation locationHigher horizontal model resolution => improvement QPF much better improvement on F2D, stability, and moisture

Thanks for your listening.&

Questions?

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Conditional instability (CI)

0

zes

0

zes

(Holton, 2004: An Introduction to Dynamic Meteorology (4 th Ed.), Fig9.10, p.294)

(2) Air parcel reach the LFC

0

zes

(1)

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Inertial instability (II)

Absolute momentum: M = fy - ug

(Holton, 2004: An Introduction to Dynamic Meteorology (4th Ed.), p.205)

In this study, set M = fy – u u=ug+u’

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Fig.7

Fig.8

Band formation

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