Transcript Use of the Nondivergent Wind for Diagnosing Banded Precipitation Systems Thomas J.

Use of the Nondivergent Wind for
Diagnosing Banded Precipitation Systems
Thomas J. Galarneau, Jr., and Daniel Keyser
Department of Earth and Atmospheric Sciences
University at Albany/SUNY
Albany, NY 12222
10th Northeast Regional Operational Workshop
NOAA/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)
6-h 40-km Meso Eta forecast valid at
1800 UTC 4 Feb 1995
B
A
700 hPa Geo. height
700 hPa Frontogenesis
Frontogenesis
es
EPVg*  0
Figs. 4b and 5b from Nicosia
and Grumm (1999)

0000 UTC 6 Feb 2001
WSR-88D Radar Mosaic
Fig. 2a from Novak et al. (2004)
80-km NCEP Eta analysis at 0000 UTC 6 Feb 2001
A
B
700 hPa Geo. height
750–650 hPa Frontogenesis
750–650 hPa Deformation
700 hPa Geo. height
750–650 hPa Frontogenesis
750–650 hPa Warm-air advection
es

EPV*
RH
Frontogenesis
A
B
A
Figs. 12c,d and 14a,b from Novak et al. (2004)
B
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 coldseason cyclones affecting the northeastern
U.S.
Effect of Resolution Increase
1800 UTC 14 Feb 2007
Vg
1.0
GFS
Vnd
10

Vg
0.5
GFS
Vnd

  Q(1014 K m2 s1)
700 hPa h (dam),  (K), Q (arrows > 2.5 1010 K m1 s1), Q
Calculation of EPV*
• 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
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
• Wind definitions
V  Vg  Vag  Vnd  Vir
Vnd  kˆ   p
1 ˆ
Vg  k   p 
f
full wind
nondivergent wind
geostrophic wind
Diagnostics
• Petterssen frontogenesis
d
1
 p    p D  E cos   
dt
2
D   p V



horizontal divergence
1
2
E  E st2  E sh2 
E st 
u v

x y
resultant deformation
E sh 
v u

x y
   angle between isentropes and axes of dilatation
Diagnostics
• Saturation equivalent potential vorticity
es v es u es 
EPV*  g p  f 
 g


p

p

x

p

y


• Q-vectors
 V

V
Q  
  p, 
  p 
y
 x


– Potential temperature in Q-vector calculation is
smoothed by a Gaussian filter (weight of 25)

14 February 2007
00Z/14
12Z
12Z
L
00Z/15
12Z
12Z
12Z
L
00Z/12
00Z/12
L
12Z
12Z
L
L
L
00Z/15
00Z/15 L
L
12Z
00Z/14
12Z
L
12Z
00Z/13
00Z/14
00Z/13 12Z 00Z/14
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
dBZ 1500 UTC
1200 UTC
VT
ME
NH
NY
MA
CT RI
PA
1800 UTC
2100 UTC
14 February 2007 WSR-88D base reflectivity mosaic
1800 UTC 14 Feb 2007
approximate
band position
Vg

SLP (hPa), 1000–500 hPa thickness (dam)
10
V

Vnd

  Q(1014 K m2 s1)
700 hPa h (dam),  (K), Q (arrows > 2.5 1010 K m1 s1), Q
1800 UTC 14 Feb 2007
approximate
band position
Vg

SLP (hPa), 1000–500 hPa thickness (dam)
V

Vnd

700 hPa h (dam), 750–650 hPa frontogenesis [K (100 km)1 (3 h)1], 750–650 hPa E (105 s1)
1800 UTC 14 Feb 2007
Vg

RH (%),  (103 hPa s1)
Vnd
V


Frontogenesis [K (100 km)1 (3 h)1], EPV* (PVU), es (K)
16 April12Z
2007
00Z/12
12Z
00Z/15
12Z
00Z/14
00Z/13
12Z
00Z/16
12Z
00Z/14
12Z
00Z/15
00Z/17
12ZLL 12Z
12Z
L00Z/17
00Z/16
L
00Z/15
00Z/17
L
L
12Z
12Z
00Z/16
L
12Z00Z/15
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
dBZ 0000 UTC
2100 UTC
VT
ME
NH
NY
MA
CT RI
PA
0300 UTC
0600 UTC
15–16 April 2007 WSR-88D base reflectivity mosaic
0000 UTC 16 Apr 2007
approximate
band position
Vg

SLP (hPa), 1000–500 hPa thickness (dam)
10
Vnd
V


  Q(1014 K m2 s1)
700 hPa h (dam),  (K), Q (arrows > 2.5 1010 K m1 s1), Q
0000 UTC 16 Apr 2007
approximate
band position
Vg

SLP (hPa), 1000–500 hPa thickness (dam)
V

Vnd

700 hPa h (dam), 750–650 hPa frontogenesis [K (100 km)1 (3 h)1], 750–650 hPa E (105 s1)
0000 UTC 16 Apr 2007
Vg

RH (%),  (103 hPa s1)
Vnd
V


Frontogenesis [K (100 km)1 (3 h)1], EPV* (PVU), es (K)
Case Summary Schematics
16 Apr 2007
14 Feb 2007
N
E
L
L
500 km
700 hPa
Novak et al. (2004)
conceptual model
Streamlines
Deformation
Frontogenesis
Upper-level jet
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