Composite Analyses of Coupled UpperLevel Jet Streaks East

Composite Analyses of Coupled Upper-Level Jet Streaks East of the Rocky Mountains Chad M Gravelle Saint Louis University Scott M Rochette State University of New York College at Brockport Thomas A Niziol National Weather Service Forecast Office Buffalo Charles E Graves Saint Louis University Annual Missouri Academy of Science Meeting Missouri Western State University, 21 April 2007

Coupled Jet Streaks • The term ‘coupled jet streaks’ refers to the presence of two separate jet streaks juxtaposed in such fashion that the ascending branches of the transverse circulations are collocated with one another, resulting in an enhanced area of upward vertical motion (e. g. , Uccellini and Kocin 1987). • This study will investigate coupled UL jet Uccellini and Kocin 1987 streak occurrences during the cool season (1 October to 31 March) east of the Rocky Mountains over 10 seasons (1993 – 2003).

Uccellini and Kocin 1987

Methodology • Preliminary examination using the North American Regional Reanalysis (NARR) dataset revealed 79 possible coupled jet streak occurrences during the period. • Using the General Meteorological Package (GEMPAK) with the NARR dataset, plan-view and cross-sectional analyses of the possible occurrences were analyzed to ensure the interaction of the jet streak circulations. • This revealed 39 coupled jet streak cases, which were then subdivided into weak dynamic (n=20) and strong dynamic (n=19) scenarios. • The weak dynamic cases were characterized by modest surface circulations (MSLP > 1000 h. Pa) and open mid-tropospheric waves. • The strong dynamic cases were characterized by strong surface circulations (MSLP < 1000 h. Pa) and closed mid-tropospheric waves.

Methodology • Center points between the jet streaks were then qualitatively determined by finding the midpoint on a line between the strongest common isotach for the initial coupling time, along with the prior 6 - and 12 -h time periods (see right). • A 117 x 117 grid with 32 km grid spacing was then extracted from the NARR dataset on the center points between the jet streaks. • The 20 cases were then averaged with a locally written compositing program utilizing the GEMPAK software.

Weak Dynamic Composites Locations of the center points for the 20 weak dynamic cases used in the composite analysis of the initial (t = 0 h) coupling time.

Weak Dynamic Composites Composite analysis of 250 -h. Pa isotachs (shaded, m s-1), 250 -h. Pa heights (brown, gpdkm), 250 -h. Pa ageostrophic wind vectors (red, m s-1), and 500 -h. Pa omega (blue, -μbar s-1) at t = -12 h.

Weak Dynamic Composites Composite analysis of 250 -h. Pa isotachs (shaded, m s-1), 250 -h. Pa heights (brown, gpdkm), 250 -h. Pa ageostrophic wind vectors (red, m s-1), and 500 -h. Pa omega (blue, -μbar s-1) at t = 0 h.

Weak Dynamic Composites Composite analysis of 250 -h. Pa isotachs (shaded, m s-1), 850 -h. Pa isotachs (green, m s-1), and 850 -h. Pa wind vectors (green, m s-1) at t = -12 h.

Weak Dynamic Composites Composite analysis of 250 -h. Pa isotachs (shaded, m s-1), 850 -h. Pa isotachs (green, m s-1), and 850 -h. Pa wind vectors (green, m s-1) at t = 0 h.

Weak Dynamic Composites Composite analysis of 250 -h. Pa isotachs (shaded, m s-1), 850 -h. Pa wind vectors (green, m s-1), and 850 -h. Pa theta-e advection (red, +K hr-1) at t = -12 h.

Weak Dynamic Composites Composite analysis of 250 -h. Pa isotachs (shaded, m s-1), 850 -h. Pa wind vectors (green, m s-1), and 850 -h. Pa theta-e advection (red, +K hr-1) at t = 0 h.

Weak Dynamic Composites Composite analysis of 250 -h. Pa isotachs (shaded, m s-1) and 850 -h. Pa frontogenesis (red, +K [100 km]-1 [3 h] -1) at t = -12 h.

Weak Dynamic Composites Composite analysis of 250 -h. Pa isotachs (shaded, m s-1) and 850 -h. Pa frontogenesis (red, +K [100 km]-1 [3 h] -1) at t = 0 h.

Weak Dynamic Composites Composite analysis of 250 -h. Pa isotachs (shaded, m s-1) and 1000 -h. Pa heights (brown, gpm) at t = -12 h.

Weak Dynamic Composites Composite analysis of 250 -h. Pa isotachs (shaded, m s-1) and 1000 -h. Pa heights (brown, gpm) at t = 0 h.

Weak Dynamic Composites Composite cross section through the composite coupled jet streaks showing isotachs (green, m s-1), RH (>70%, green shading), ageostrophic circulation (blue arrows, m s-1), omega (red, -μbar s-1), and θe (black, K) at at t = -12 h. Inset figure provides the orientation of the cross-section with respect to the isotach field.

Weak Dynamic Composites Composite cross section through the composite coupled jet streaks showing isotachs (green, m s-1), RH (>70%, green shading), ageostrophic circulation (blue arrows, m s-1), omega (red, -μbar s-1), and θe (black, K) at at t = 0 h. Inset figure provides the orientation of the cross-section with respect to the isotach field.

Weak Dynamic Composites Composite cross section through the composite coupled jet streaks showing isotachs (green, m s-1), ageostrophic circulation (blue arrows, m s-1), frontogenesis (red, +10 K [100 km] -1 [3 h]-1), and EPVs (shaded, < 0. 25 PVU) at t = -12 h.

Weak Dynamic Composites Composite cross section through the composite coupled jet streaks showing isotachs (green, m s-1), ageostrophic circulation (blue arrows, m s-1), frontogenesis (red, +10 K [100 km] -1 [3 h]-1), and EPVs (shaded, < 0. 25 PVU) at t = 0 h.

Weak Dynamic Case Study • 8 March 2002 • Regions of 8+ inches of snow • 80+ reports of thundersnow Isohyets of snowfall (in. ) for the 24 -h period ending 1200 UTC 8 Mar 2002, based upon cooperative station data from NCDC. WSI NOWrad mosaic of composite reflectivity at 0330 UTC 8 MAR 2002. • Significant inverted trough development at surface (w/o significant cyclone in vicinity) • Lake influence unlikely (SE winds throughout event)

Weak Dynamic Case Study 32 -km NARR isobars (red solid, h. Pa), 250 -h. Pa isotachs (black solid, kts), 5400 -gpm thickness contour (blue solid), and 3 -h accumulated precipitation (shaded > 0. 05 in) at 0000 UTC 8 Mar 2002.

Weak Dynamic Case Study 32 -km NARR isobars (red solid, h. Pa), 250 -h. Pa isotachs (black solid, kts), 5400 -gpm thickness contour (blue solid), and 3 -h accumulated precipitation (shaded > 0. 05 in) at 0900 UTC 8 Mar 2002.

Weak Dynamic Case Study 32 -km NARR 250 -h. Pa heights (brown solid, gpdkm), 250 -h. Pa isotachs (black solid, kts), 250 h. Pa ageostrophic wind barbs (red, kts), and 500 -h. Pa omega (blue solid, -µbar s-1) at 0000 UTC 8 Mar 2002.

Weak Dynamic Case Study 32 -km NARR 250 -h. Pa heights (brown solid, gpdkm), 250 -h. Pa isotachs (black solid, kts), 250 h. Pa ageostrophic wind barbs (red, kts), and 500 -h. Pa omega (blue solid, -µbar s-1) at 0900 UTC 8 Mar 2002.

Weak Dynamic Case Study 32 -km NARR 250 -h. Pa isotachs (black solid, kts), 850 -h. Pa isotachs (green solid, kts), 850 -h. Pa wind vectors (green, kts), and 850 -h. Pa theta-e advection (red solid [+10 -1 K hr-1], blue solid [-10 -1 K hr-1]) at 0000 UTC 8 Mar 2002.

Weak Dynamic Case Study 32 -km NARR 250 -h. Pa isotachs (black solid, kts), 850 -h. Pa isotachs (green solid, kts), 850 -h. Pa wind vectors (green, kts), and 850 -h. Pa theta-e advection (red solid [+10 -1 K hr-1], blue solid [-10 -1 K hr-1]) at 0900 UTC 8 Mar 2002.

Weak Dynamic Case Study 32 -km NARR 250 -h. Pa isotachs (black solid, kts), and 850 -h. Pa frontogenesis (red solid, +K [100 km] -1 [3 h]-1) at 0000 UTC 8 Mar 2002. .

Weak Dynamic Case Study 32 -km NARR 250 -h. Pa isotachs (black solid, kts), and 850 -h. Pa frontogenesis (red solid, +K [100 km] -1 [3 h]-1) at 0900 UTC 8 Mar 2002. .

Conclusions • 250 -h. Pa ageostrophic cross-contour flow strengthens during coupling period, resulting in increased/focused upper-level divergence and mid-tropospheric UVM in coupling region • 850 -h. Pa low-level jet and θe advection become stronger and better organized in coupling region over coupling period • 850 -h. Pa frontogenesis region elongates and strengthens underneath and to the poleward side of entrance region of northern jet during coupling period

Conclusions • Poleward/upward transport of warm/moist (high θe) air (i. e. ageostrophic circulation) becomes better defined over coupling period • Low-level front strengthens over coupling period • Mid/upper-tropospheric frontogenetical circulation works in concert with coupled jet circulation as frontogenesis extends upward over coupling period • Moist layer between jets deepens and narrows over coupling period

This research was made possible by COMET Partners Project Award S 05 -52248. Questions or Comments? gravelle@eas. slu. edu