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…or in other words What Are These Things and Why Do We Care?

Revisiting Upper Tropospheric Lows and Their Interaction with the North American Monsoon: What is Next in Hypothesis Formulation and Testing. …or in other words What Are These Things and Why Do We Care?. Erik Pytlak, SOO NOAA/NWS Tucson, AZ. What They Are Not… Easterly Waves.

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…or in other words What Are These Things and Why Do We Care?

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  1. Revisiting Upper Tropospheric Lows and Their Interaction with the North American Monsoon: What is Next in Hypothesis Formulation and Testing …or in other words What Are These Things and Why Do We Care? Erik Pytlak, SOO NOAA/NWS Tucson, AZ

  2. What They Are Not…Easterly Waves • Tropical/Easterly Waves • About 25-30/year • Vorticity max near 700mb • Low latitude (S of 25N, but usually south of 20N) • Almost never observed near AZ • Can trigger a Gulf Surge if far enough north • Slowing tropical waves => probable tropical cyclone development

  3. Upper Tropospheric Lows(Inverted Troughs) • 12-20 of them a year affect North American Monsoon Regime • Subtropical • “IV season” Mid June - early September • Relatively cold upper centers • Peak vorticity near the tropopause • Peak vorticity advection at lower levels (700-400mb) • Stronger ones appear “inverted” on 500mb and 700mb charts • Vorticity arguments “inverted” • Strongly modulate monsoon-related MCS activity • MCS outflows can enhance/reinforce Gulf Surges

  4. Tracks 2003-2006 2004 (strong south; weak north) 2003 (average) 2006 (near record) 2005 (strong)

  5. Origin Points Monsoon Baroclinic Flank Gulf of Mexico TUTT MCVs Polar Trough Capture

  6. Original Conceptual Models Whitfield and Lyons 1992 Kelley and Mock 1982 Moore, Gall and Adang 1989

  7. Proposed Conceptual Model • Most active side is the east flank • Positive Differential Vorticity Advection • West flank becomes active • Negative Differential Vorticity Advection (NDVA) weakens as low approaches SW flank of the upper high • Deformation lowers upper level static stability (300-200mb Vorticity <=0) • 300mb divergence and upward vertical motion results (or downward motion weakens) • Cooling aloft destabilizes the mid levels • Thermodynamics and orographics trump otherwise hostile environment • Usually quiet directly underneath the low • Orientation of the upper high critical for upper divergence/deformation • SW-NE aligned upper ridge will actually yield confluence aloft

  8. NAME Cross Section7/13/04 C 1°x1° res. Courtesy Paul Cieleski, Colorado State University

  9. Model Cross Section7/26/06

  10. Main Forecast Problems • Poor initializations • Lack of upper air data • Model adjustments sometimes too slow • Moisture-dependent on west flanks • Cooling aloft but sinking motion in core • Warmer cores may be more active than colder ones (implies PW important) • Which flank will fire and when (or if…)? C C

  11. What’s Next? • Refine the hypothesis • University partners • Account for caveats • Multiple upper lows • Warmer vs. colder cores • Moist vs. fairly dry monsoon moisture plumes • Teach/share the conceptual model

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