Use of the Nondivergent Wind for Diagnosing Banded Precipitation Systems

Thomas J. Galarneau, Jr., and Daniel KeyserDepartment of Earth and Atmospheric Sciences

University at Albany/SUNY

Albany, NY 12222

10th Northeast Regional Operational WorkshopNOAA/National Weather Service, Albany, NY

6 November 2008

Background• Mesoscale bands modulate the spatial distribution

and intensity of precipitation associated with cyclones– Cold-season examples include snowbands within coastal

extratropical cyclones– Warm-season examples include coastal fronts within

landfalling and transitioning tropical cyclones

• CSTAR pedigree for mesoscale substructure within cold- and warm-season cyclones affecting the northeastern U.S. (e.g., Novak et al. 2004, 2006; DeLuca 2004; Klein 2007)

Fig. 3 from Nicosia and Grumm (1999)

Figs. 4b and 5b from Nicosia and Grumm (1999)

700 hPa Geo. height

700 hPa Frontogenesis

6-h 40-km Meso Eta forecast valid at1800 UTC 4 Feb 1995

Frontogenesis

€

θes

A B

€

EPVg* < 0

Fig. 2a from Novak et al. (2004)

0000 UTC 6 Feb 2001

WSR-88D Radar Mosaic

80-km NCEP Eta analysis at 0000 UTC 6 Feb 2001

Figs. 12c,d and 14a,b from Novak et al. (2004)

700 hPa Geo. height750–650 hPa Frontogenesis750–650 hPa Deformation

700 hPa Geo. height750–650 hPa Frontogenesis750–650 hPa Warm-air advection

A

B

A BA B

EPV*

Frontogenesis

RH

€

θes

Conceptual Models

Single-banded event Nonbanded event

Fig. 15 from Novak et al. (2004)

Conceptual Models

Single-banded event

Fig. 2 from Novak et al. (2006)

Motivation• Continuing increases in the horizontal and

vertical resolution of global analyses are resulting in the improved representation of mesoscale circulation systems

• Extend applicability of balanced framework in diagnosing mesoscale circulation systems by replacing the geostrophic wind (Vg) and full wind (V) with the nondivergent wind (Vnd)

Motivation• Use of Vnd in place of Vg and V in a

balanced framework is hypothesized to produce cleaner and more coherent diagnostic signatures of mesoscale circulation systems

• This hypothesis is addressed here for mesoscale precipitation bands within cold-season cyclones affecting the northeastern U.S.

1.0GFS

0.5GFS

€

rV g

€

rV g

€

rV nd

€

rV nd

Effect of Resolution Increase

700 hPa h (dam), θ (K), Q (arrows > 2.5 1010 K m1 s1), Q (1014 K m2 s1)

1800 UTC 14 Feb 2007

€

rQ

€

∇•r Q

10

• Novak et al. (2006, p. 19) discussion of EPV* for the 25 December 2002 snowband case:

• We suggest that in curved flow Vnd better represents the balanced wind than Vg or V

Calculation of EPV*

• Use of Vnd in EPV* calculation is hypothesized to minimize the spatial extent of EPV* < 0, and the occurrence of localized regions of EPV* << 0 (i.e., EPV* bull’s-eyes)

• This modification to the EPV* calculation may lead to a more accurate assessment of the contribution of CSI to the formation and evolution of mesoscale precipitation bands

Calculation of EPV*

Goals

• Examine mesoscale precipitation bands for two northeast U.S. cyclones– 14 February 2007– 16 April 2007

• Compare structures shown by diagnostics using Vg, Vnd, and V

Datasets

• 0.5 NCEP GFS analyses

• NCDC WSR-88D radar archive

Diagnostics

€

rV =

r V g +

r V ag =

r V nd +

r V ir

€

rV nd = ˆ k ×∇ pψ

€

rV g =

1

fˆ k ×∇ pΦ

nondivergent wind

geostrophic wind

full wind

• Wind definitions

Diagnostics

• Petterssen frontogenesis

€

E = E st2 + E sh

2( )

1

2

€

E st =∂u

∂x−

∂v

∂y

€

E sh =∂v

∂x+

∂u

∂y

€

d

dt∇ pθ = −

1

2∇ pθ D − E cos δ −α( )[ ]

€

D =∇ p •r

V

angle between isentropes and axes of dilatation

resultant deformation

horizontal divergence

Diagnostics

• Saturation equivalent potential vorticity

• Q-vectors

– Potential temperature in Q-vector calculation is smoothed by a Gaussian filter (weight of 25)

€

EPV* = −g ζ p + f( )∂θes

∂p+ g

∂v

∂p

∂θes

∂x−

∂u

∂p

∂θes

∂y

⎛

⎝ ⎜

⎞

⎠ ⎟

€

rQ = −

∂r

V

∂x• ∇ pθ, −

∂r V

∂y• ∇ pθ

⎛

⎝ ⎜

⎞

⎠ ⎟

L

L

L

L

LL

LL

L

12Z

00Z/15

00Z/14

00Z/14

12Z12Z

00Z/13

00Z/12 12Z

12Z

12Z

12Z

12Z

12Z

12Z

00Z/15

00Z/14

12Z

00Z/1400Z/13

00Z/1200Z/15

14 February 2007

Position of key synoptic features marked every 12 h

L primary cyclone; L secondary cyclone upper-level PV anomaly

Source: http://www.erh.noaa.gov/er/aly/past.htm

dBZ1200 UTC 1500 UTC

1800 UTC 2100 UTC

NYMA

CT RI

VT

NH

ME

PA

14 February 2007 WSR-88D base reflectivity mosaic

approximateband position

SLP (hPa), 1000–500 hPa thickness (dam)

€

rV g

€

rV nd

1800 UTC 14 Feb 2007

€

rV

700 hPa h (dam), θ (K), Q (arrows > 2.5 1010 K m1 s1), Q (1014 K m2 s1)

€

rQ

€

∇•r Q

10

approximateband position

SLP (hPa), 1000–500 hPa thickness (dam)

700 hPa h (dam), 750–650 hPa frontogenesis [K (100 km)1 (3 h)1], 750–650 hPa E (105 s1)

€

rV g

€

rV nd

1800 UTC 14 Feb 2007

€

rV

Frontogenesis [K (100 km)1 (3 h)1], EPV* (PVU), θes (K)

€

rV g

€

rV nd

1800 UTC 14 Feb 2007

€

rV

RH (%), (103 hPa s1)

12Z12Z

12Z12Z

12Z

12Z

12Z

12Z

12Z

00Z/12

00Z/17

00Z/1600Z/15

00Z/1400Z/17

00Z/16

00Z/1500Z/14

00Z/13

L

L L

L

L

00Z/17LL

00Z/16

00Z/15

12Z

12Z

12Z

00Z/15

16 April 2007

Position of key synoptic features marked every 12 h

L primary cyclone; L secondary cyclone upper-level PV anomaly

Source: http://www.erh.noaa.gov/er/aly/past.htm

Source: http://www.erh.noaa.gov/er/aly/past.htm

dBZ2100 UTC 0000 UTC

0300 UTC 0600 UTC

NYMA

CT RI

VT

NH

ME

PA

15–16 April 2007 WSR-88D base reflectivity mosaic

approximateband position

SLP (hPa), 1000–500 hPa thickness (dam)

€

rV g

€

rV nd

0000 UTC 16 Apr 2007

€

rV

700 hPa h (dam), θ (K), Q (arrows > 2.5 1010 K m1 s1), Q (1014 K m2 s1)

€

rQ

€

∇•r Q

10

SLP (hPa), 1000–500 hPa thickness (dam)

approximateband position

€

rV g

€

rV nd

0000 UTC 16 Apr 2007

€

rV

700 hPa h (dam), 750–650 hPa frontogenesis [K (100 km)1 (3 h)1], 750–650 hPa E (105 s1)

Frontogenesis [K (100 km)1 (3 h)1], EPV* (PVU), θes (K)

€

rV g

€

rV nd

0000 UTC 16 Apr 2007

€

rV

RH (%), (103 hPa s1)

Case Summary Schematics

Novak et al. (2004)conceptual model

L

14 Feb 2007

L

16 Apr 2007

StreamlinesDeformationFrontogenesis

Upper-level jet

700 hPa

500 km

N

E

Concluding Remarks• Increases in horizontal and vertical resolution of

global analyses are leading to the improved representation of mesoscale circulation systems, but also are resulting in noisier diagnostics using Vg

and V

• Use of Vnd in place of Vg and V was hypothesized to produce cleaner and more coherent diagnostic signatures of mesoscale circulation systems

Concluding Remarks• Use of Vnd in place of Vg and V has been shown to

produce improved signatures of Q divergence, Petterssen frontogenesis, and moist symmetric stability within banded precipitation systems for two cold-season cyclone cases over the northeastern U.S.: 14 February and 16 April 2007

• sd

• Results for these two cases agree with previous work on mesoscale band formation– Deep-layer frontogenesis slopes toward colder air– Band forms on warm-air side of frontogenesis maximum

in presence of weak moist symmetric stability