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 WavesTropical/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) of them a year affect North American Monsoon Regime12-20 of them a year affect North American Monsoon Regime SubtropicalSubtropical “IV season” Mid June - early September“IV season” Mid June - early September Relatively cold upper centersRelatively cold upper centers Peak vorticity near the tropopausePeak vorticity near the tropopause Peak vorticity advection at lower levels ( mb)Peak vorticity advection at lower levels ( mb) Stronger ones appear “inverted” on 500mb and 700mb chartsStronger ones appear “inverted” on 500mb and 700mb charts Vorticity arguments “inverted”Vorticity arguments “inverted” Strongly modulate monsoon- related MCS activityStrongly modulate monsoon- related MCS activity MCS outflows can enhance/reinforce Gulf SurgesMCS outflows can enhance/reinforce Gulf Surges

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

5 Origin Points Gulf of Mexico TUTT Monsoon Baroclinic Flank MCVsPolar Trough Capture

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

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

8 NAME Cross Section 7/13/04 1°x1° res. Courtesy Paul Cieleski, Colorado State University C

9 Model Cross Section 7/26/06

10 Main Forecast Problems Poor initializationsPoor initializations –Lack of upper air data –Model adjustments sometimes too slow Moisture-dependent on west flanksMoisture-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…)?Which flank will fire and when (or if…)? C C

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