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 th 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)

Figs. 4b and 5b from Nicosia and Grumm (1999) 700 hPa Geo. height 700 hPa Frontogenesis 6-h 40-km Meso Eta forecast valid at 1800 UTC 4 Feb 1995 Frontogenesis A B

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. height 750–650 hPa Frontogenesis 750–650 hPa Deformation 700 hPa Geo. height 750–650 hPa Frontogenesis 750–650 hPa Warm-air advection A B A B A B EPV* Frontogenesis  RH

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 (V g ) and full wind (V) with the nondivergent wind (V nd )

Motivation Use of V nd in place of V g 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.

1.0  GFS 0.5  GFS Effect of Resolution Increase 700 hPa h (dam),  (K), Q (arrows > 2.5  10  10 K m  1 s  1 ),  Q (10  14 K m  2 s  1 ) 1800 UTC 14 Feb

Novak et al. (2006, p. 19) discussion of EPV* for the 25 December 2002 snowband case: We suggest that in curved flow V nd better represents the balanced wind than V g or V Calculation of EPV*

Use of V nd 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 V g, V nd, and V

Datasets 0.5  NCEP GFS analyses NCDC WSR-88D radar archive

Diagnostics nondivergent wind geostrophic wind full wind Wind definitions

Diagnostics Petterssen frontogenesis    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)

L L L L L L L L L 12Z 00Z/15 00Z/14 12Z 00Z/13 00Z/12 12Z 00Z/15 00Z/14 12Z 00Z/14 00Z/13 00Z/12 00Z/15 14 February 2007 Position of key synoptic features marked every 12 h L primary cyclone; L secondary cyclone upper-level PV anomaly

Source:

dBZ 1200 UTC1500 UTC 1800 UTC2100 UTC NY MA CTRI VT NH ME PA 14 February 2007 WSR-88D base reflectivity mosaic

approximate band position SLP (hPa), 1000–500 hPa thickness (dam) 1800 UTC 14 Feb hPa h (dam),  (K), Q (arrows > 2.5  10  10 K m  1 s  1 ),  Q (10  14 K m  2 s  1 ) 10

approximate band 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 (10  5 s  1 ) 1800 UTC 14 Feb 2007

Frontogenesis [K (100 km)  1 (3 h)  1 ], EPV* (PVU),  es (K) 1800 UTC 14 Feb 2007 RH (%),  (10  3 hPa s  1 )

12Z 00Z/12 00Z/17 00Z/16 00Z/15 00Z/14 00Z/17 00Z/16 00Z/15 00Z/14 00Z/13 L L L L L 00Z/17 L L 00Z/16 00Z/15 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:

dBZ 2100 UTC0000 UTC 0300 UTC0600 UTC NY MA CTRI VT NH ME PA 15–16 April 2007 WSR-88D base reflectivity mosaic

approximate band position SLP (hPa), 1000–500 hPa thickness (dam) 0000 UTC 16 Apr hPa h (dam),  (K), Q (arrows > 2.5  10  10 K m  1 s  1 ),  Q (10  14 K m  2 s  1 ) 10

SLP (hPa), 1000–500 hPa thickness (dam) approximate band position 0000 UTC 16 Apr hPa h (dam), 750–650 hPa frontogenesis [K (100 km)  1 (3 h)  1 ], 750–650 hPa E (10  5 s  1 )

Frontogenesis [K (100 km)  1 (3 h)  1 ], EPV* (PVU),  es (K) 0000 UTC 16 Apr 2007 RH (%),  (10  3 hPa s  1 )

Case Summary Schematics Novak et al. (2004) conceptual model L 14 Feb 2007 L 16 Apr 2007 Streamlines Deformation Frontogenesis 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 V g and V Use of V nd in place of V g and V was hypothesized to produce cleaner and more coherent diagnostic signatures of mesoscale circulation systems

Concluding Remarks Use of V nd in place of V g 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