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Use of the Nondivergent Wind for Diagnosing Banded Precipitation Systems

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

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Use of the Nondivergent Wind for Diagnosing Banded Precipitation Systems

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  1. 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

  2. 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)

  3. Fig. 3 from Nicosia and Grumm (1999)

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

  5. 0000 UTC 6 Feb 2001 WSR-88D Radar Mosaic Fig. 2a from Novak et al. (2004)

  6. 80-km NCEP Eta analysis at 0000 UTC 6 Feb 2001 A B 700 hPa Geo. height 750–650 hPa Frontogenesis 750–650 hPa Warm-air advection 700 hPa Geo. height 750–650 hPa Frontogenesis 750–650 hPa Deformation EPV* Frontogenesis  RH B B A A Figs. 12c,d and 14a,b from Novak et al. (2004)

  7. Conceptual Models Single-banded event Nonbanded event Fig. 15 from Novak et al. (2004)

  8. Conceptual Models Single-banded event Fig. 2 from Novak et al. (2006)

  9. 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)

  10. 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.

  11. Effect of Resolution Increase 1800 UTC 14 Feb 2007 1.0 GFS 10 0.5 GFS 700 hPa h (dam),  (K), Q (arrows > 2.5 1010 K m1 s1), Q (1014 K m2 s1)

  12. 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

  13. 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

  14. 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

  15. Datasets • 0.5 NCEPGFS analyses • NCDC WSR-88D radar archive

  16. Diagnostics • Wind definitions full wind nondivergent wind geostrophic wind

  17. Diagnostics • Petterssen frontogenesis horizontal divergence resultant deformation    angle between isentropes and axes of dilatation

  18. Diagnostics • Saturation equivalent potential vorticity • Q-vectors • Potential temperature in Q-vector calculation is smoothed by a Gaussian filter (weight of 25)

  19. 14 February 2007 00Z/14 12Z 12Z L 00Z/15 12Z 12Z L 12Z 00Z/15 L 00Z/15 L 00Z/12 00Z/12 L 12Z 00Z/14 L 12Z L 12Z L L 12Z 12Z 00Z/13 00Z/14 00Z/14 00Z/13 12Z Position of key synoptic features marked every 12 h L primary cyclone; L secondary cyclone upper-level PV anomaly

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

  21. dBZ 1200 UTC 1500 UTC VT ME NH NY MA CT RI PA 1800 UTC 2100 UTC 14 February 2007 WSR-88D base reflectivity mosaic

  22. 1800 UTC 14 Feb 2007 approximate band position SLP (hPa), 1000–500 hPa thickness (dam) 10 700 hPa h (dam),  (K), Q (arrows > 2.5 1010 K m1 s1), Q (1014 K m2 s1)

  23. 1800 UTC 14 Feb 2007 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)

  24. 1800 UTC 14 Feb 2007 RH (%),  (103 hPa s1) Frontogenesis [K (100 km)1 (3 h)1], EPV* (PVU), es (K)

  25. 16 April 2007 12Z 00Z/12 00Z/15 12Z 00Z/14 00Z/13 12Z 12Z 00Z/16 00Z/17 L 12Z 12Z L 12Z 12Z 00Z/17 L L 00Z/16 00Z/14 00Z/15 00Z/17 L L 12Z 12Z 12Z 00Z/15 00Z/16 L 12Z 00Z/15 Position of key synoptic features marked every 12 h L primary cyclone; L secondary cyclone upper-level PV anomaly

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

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

  28. dBZ 2100 UTC 0000 UTC VT ME NH NY MA CT RI PA 0300 UTC 0600 UTC 15–16 April 2007 WSR-88D base reflectivity mosaic

  29. 0000 UTC 16 Apr 2007 approximate band position SLP (hPa), 1000–500 hPa thickness (dam) 10 700 hPa h (dam),  (K), Q (arrows > 2.5 1010 K m1 s1), Q (1014 K m2 s1)

  30. 0000 UTC 16 Apr 2007 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)

  31. 0000 UTC 16 Apr 2007 RH (%),  (103 hPa s1) Frontogenesis [K (100 km)1 (3 h)1], EPV* (PVU), es (K)

  32. 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

  33. 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

  34. 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

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