Revisiting Upper Tropospheric Lows Revisiting Upper Tropospheric Lows and Their Interaction with the North and Their Interaction with the North American Monsoon: What is Next in American Monsoon: What is Next in Hypothesis Formulation and TestingHypothesis Formulation and Testing

……or in other wordsor in other words

What Are These Things and Why Do We What Are These Things and Why Do We Care?Care?

Erik Pytlak, SOONOAA/NWS Tucson, AZ

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What They Are Not…What They Are Not…

Easterly WavesEasterly Waves

• Tropical/Easterly WavesTropical/Easterly Waves

– About 25-30/yearAbout 25-30/year

– Vorticity max near 700mbVorticity max near 700mb

– Low latitude (S of 25N, but Low latitude (S of 25N, but usually south of 20N)usually south of 20N)

– Almost never observed Almost never observed near AZnear AZ

– Can trigger a Gulf Surge if Can trigger a Gulf Surge if far enough northfar enough north

– Slowing tropical waves => Slowing tropical waves => probable tropical cyclone probable tropical cyclone development development

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Upper Tropospheric LowsUpper Tropospheric Lows(Inverted Troughs)(Inverted Troughs)

• 12-20 of them a year affect North 12-20 of them a year affect North American Monsoon RegimeAmerican Monsoon Regime

• SubtropicalSubtropical

• ““IV season” Mid June - early IV season” Mid June - early SeptemberSeptember

• Relatively cold upper centersRelatively cold upper centers

• Peak vorticity near the Peak vorticity near the tropopausetropopause

• Peak vorticity advection at lower Peak vorticity advection at lower levels (700-400mb)levels (700-400mb)

• Stronger ones appear “inverted” Stronger ones appear “inverted” on 500mb and 700mb chartson 500mb and 700mb charts

• Vorticity arguments “inverted” Vorticity arguments “inverted”

• Strongly modulate monsoon-Strongly modulate monsoon-related MCS activity related MCS activity

• MCS outflows can MCS outflows can enhance/reinforce Gulf Surgesenhance/reinforce Gulf Surges

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Tracks 2003-2006Tracks 2003-2006

2003(average)

2004(strongsouth; weak north)

2005(strong)

2006(near

record)

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Origin PointsOrigin Points

Gulf of Mexico TUTT Monsoon Baroclinic Flank

MCVs Polar Trough Capture

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Original Conceptual ModelsOriginal Conceptual Models

Kelley and Mock 1982 Moore, Gall and Adang 1989

Whitfield and Lyons 1992

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Proposed Conceptual ModelProposed Conceptual Model

• Most active side is the east flank Most active side is the east flank

– Positive Differential Vorticity Advection Positive Differential Vorticity Advection

• West flank becomes activeWest flank becomes active

– Negative Differential Vorticity Advection Negative Differential Vorticity Advection (NDVA) weakens as low approaches SW flank (NDVA) weakens as low approaches SW flank of the upper high of the upper high

– Deformation lowers upper level static Deformation lowers upper level static stability (300-200mb Vorticity <=0)stability (300-200mb Vorticity <=0)

– 300mb divergence and upward vertical 300mb divergence and upward vertical motion results (or downward motion motion results (or downward motion weakens)weakens)

• Cooling aloft destabilizes the mid levels Cooling aloft destabilizes the mid levels

• Thermodynamics and orographics trump Thermodynamics and orographics trump otherwise hostile environmentotherwise hostile environment

• Usually quiet directly underneath the low Usually quiet directly underneath the low

• Orientation of the upper high critical for Orientation of the upper high critical for upper divergence/deformation upper divergence/deformation

– SW-NE aligned upper ridge will actually yield SW-NE aligned upper ridge will actually yield confluence aloftconfluence aloft

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NAME Cross SectionNAME Cross Section7/13/047/13/04

1°x1° res. Courtesy Paul Cieleski, Colorado State University

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Model Cross SectionModel Cross Section7/26/067/26/06

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Main Forecast ProblemsMain Forecast Problems

• Poor initializationsPoor initializations

– Lack of upper air dataLack of upper air data

– Model adjustments Model adjustments sometimes too slowsometimes too slow

• Moisture-dependent on west Moisture-dependent on west flanksflanks

– Cooling aloft but sinking Cooling aloft but sinking motion in coremotion in core

– Warmer cores may be more Warmer cores may be more active than colder ones active than colder ones (implies PW important)(implies PW important)

• Which flank will fire and when Which flank will fire and when (or if…)?(or if…)?

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What’s Next?What’s Next?

• Refine the hypothesisRefine the hypothesis

– University partners University partners

• Account for caveatsAccount for caveats

– Multiple upper lowsMultiple upper lows

– Warmer vs. colder coresWarmer vs. colder cores

– Moist vs. fairly dry Moist vs. fairly dry monsoon moisture monsoon moisture plumesplumes

• Teach/share the Teach/share the conceptual modelconceptual model