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Previous Studies

Repeating Patterns of Precipitation and Surface Pressure Evolution in Midlatitude Mesoscale Convective Vortices Eric James Colorado State University 17 August 2009. Previous Studies. 23-24 June 1985 OK PRE-STORM MCS

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Previous Studies

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  1. Repeating Patterns of Precipitationand Surface Pressure Evolutionin Midlatitude Mesoscale Convective VorticesEric JamesColorado State University17 August 2009

  2. Previous Studies • 23-24 June 1985 OK PRE-STORM MCS • “Onion-shaped” sounding at rear of MCS due to subsidence; heat bursts as subsidence locally penetrates shallow surface inversion • MCV forms within dissipating stratiform region • As stratiform precipitation dissipates, surface mesohigh rapidly transforms into strong mesolow, and vortex becomes visible in satellite imagery Johnson et al., 1989

  3. Previous Studies • 6-7 May 1985 OK PRE-STORM MCS • Cyclonic circulation forms within stratiform region, with westerly rear inflow jet to the south • “Onion-shaped” soundings within rear inflow, with low-level warm and dry air • Significant notch in reflectivity field, with strong mesolow at notch apex, associated with warming within rear inflow Brandes, 1990

  4. Objectives • Motivated by existence of dense surface observations by the Oklahoma Mesonet, we aim to: • Identify numerous MCV cases in this region during 2002-05 • Classify cases according to precipitation and surface pressure evolution • Propose some mechanisms for frequently-observed mesolows

  5. Case Selection • We run algorithm of Davis et al. (2002) on hourly RUC analyses in Oklahoma • Algorithm classifies gridpoints as vortices if all criteria are met • Can be run for each analysis time to derive vortex tracks; detected vortices are treated as one if track is continuous

  6. Classification • 45 vortices arise from MCSs; these MCV cases are examined in detail • MCV events grouped based on evolution of parent MCS, and surface pressure signatures • Five recurring types of MCVs are found: • Three types produce distinct surface mesolows: • Rear Inflow Jet MCV (19 of 45)‏ • Collapsing Stratiform Region MCV (8 of 45)‏ • Vertically Coherent MCV (1 of 45)‏ • Two types do not produce surface mesolows: • Remnant Circulation MCV (14 of 45)‏ • Cold Pool Dominated MCV (3 of 45) • Examples of mesolow-producing MCVs are presented, with hypotheses for their surface pressure effects

  7. Rear Inflow Jet MCV:24 May 2003 • Bow echo moves SE with stratiform rain to N • Small mesohigh on SW end of convective line • Rear inflow notch begins to develop at back edge of stratiform region, S of developing MCV • Intense mesolow forms near apex of rear inflow notch Oklahoma Mesonet (1400-1700 UTC)‏

  8. Rear Inflow Jet MCV:24 May 2003 Haskell Wind Profiler (0700-2000 UTC)‏ BAMEX Aircraft (~1930 UTC)‏ • Both analyses show low-level virtual warming of ~2.5 C, centered near 825 hPa, west of cool anomaly at similar height, and above ~700 hPa • Warming occurs near base of strong rear inflow according to profiler

  9. Collapsing Stratiform Region MCV:9 August 2004 • Small dissipating MCS moves into OK from N • Significant mesohigh on S edge of stratiform region • Large mesolow develops within dissipating stratiform region, collocated with developing MCV Oklahoma Mesonet (0800-1100 UTC)‏

  10. Collapsing Stratiform Region MCV:9 August 2004 ARM SGP Radiosonde (1136 UTC)‏ Purcell Wind Profiler (0800-1900 UTC)‏ • RASS and radiosonde observations show deep layer of warming collocated with mesolow, evidence of dry-adiabatic subsidence at low- to mid levels

  11. Vertically Coherent MCV:29 July 2004 • Large stratiform precipitation region moves NE into E OK • Cyclonic circulation of radar echoes evident in animated radar imagery • Well-defined meso-alpha scale low pressure center exists at center of circulation • Cyclonic circulation in mesonet winds Oklahoma Mesonet (1300-1600 UTC)‏

  12. Vertically Coherent MCV:29 July 2004 RUC analysis (1200 UTC)‏ • RUC cross-section of MCV shows deep, vertically coherent tower of PV directly over surface mesolow • PV tower resembles that documented in BAMEX MCV of 11 Jun 2003 • Both cases also have weak virtual temperature perturbations BAMEX Aircraft (~1730 UTC 11 Jun 2003)‏

  13. Rear Inflow Jet MCV:Conceptual Model • At MCS maturity (left), mesohigh centred behind strong convective line • At second stage (middle), right-hand portion of stratiform region begins to erode from rear, mesolow begins to form at back edge of precipitation, and midlevel vortex begins to develop • At final stage (right), MCS has become strongly asymmetric, with intense wake low at apex of rear inflow notch on right-hand side of system, and MCV is intensifying

  14. Collapsing Stratiform Region MCV:Conceptual Model • At MCS maturity (left), mesohigh centred in precipitation region • At second stage (middle), mesohigh shifts ahead of precipitation, mesolow begins to form due to subsidence warming in dissipating stratiform region, and midlevel vortex begins to develop • At final stage (right), mesohigh is weakening, mesolow has broadened and deepened within dissipating stratiform region, and MCV is intensifying

  15. Discussion • Rear Inflow Jet MCVs produce mesolows due to intense low-level subsidence warming within a rear inflow jet (i.e., the “wake low”)‏ • This mechanism has been documented by Johnson and Hamilton (1988) and Stumpf et al. (1991)‏ • The Brandes (1990) case appears to be in this category • Collapsing Stratiform Region MCVs produce surface mesolows due to broad-scale low- to midlevel subsidence warming within a dissipating stratiform region • The Johnson et al. (1989) case appears to be in this category • Vertically Coherent MCVs produce surface mesolows due to a deep warm core and a relatively weak surface-based cold pool • The 11 Jun 2003 BAMEX MCV appears to be in this category • Remnant Circulation MCVs have no precipitation in their vicinity and produce no surface pressure effects • Cold Pool Dominated MCVs have extensive precipitation but no mesolow

  16. Conclusions • The Vertically Coherent MCV documented here strongly resembles an incipient tropical cyclone; further study of these systems could help us understand tropical cyclogenesis • Identification of these distinct MCV types has implications for forecasting: • The formation of an MCV within an intense, asymmetric MCS could be a sign that wake low formation is likely, with associated high winds and low-level wind shear hazards • The appearance of a broad mesolow within a collapsing stratiform region could suggest that MCV formation is likely, which plays a role in subsequent convective initiation • Vertically Coherent MCVs moving off the coast over warm water should be monitored for possible tropical cyclogenesis (Bosart and Sanders 1981) • Future work will involve a composite analysis of the synoptic-scale environment of the MCVs, and modeling of some cases

  17. Hydrostatic pressure change associated with 1500-1700 UTC virtual warming in Haskell profile: -0.87 hPa • This agrees well with observed pressure drop at closest OM station (Okmulgee): -0.79 hPa • Tahlequah pressure drop: -3.62 hPa • This suggests core of warming is not sampled by Haskell profiler Tahlequah mesonet station (0700-2100 UTC 24 May 2003)‏

  18. Hydrostatic pressure change associated with 1100-1300 UTC virtual warming in Purcell profile: -1.75 hPa • This agrees well with observed pressure drop at closest OM station (Washington): -1.52 hPa Breckinridge mesonet station (0800-2000 UTC 9 Aug 2004)‏

  19. McAlester mesonet station (0600-2200 UTC 29 Jul 2004)‏ Purcell Wind Profiler (0600-2100 UTC 29 Jul 2004)‏

  20. Type # % Longevity (h) Radius (km) Vorticity (s-1)All 45 100 17 225 9.98x10-5RIJ 19 42 15 231 1.32x10-4***CSR 8 18 14 206* 6.75x10-5**VC 1 2 10 262 8.83x10-5RC 14 31 24* 225 8.21x10-5CPD 3 7 13 227 7.92x10-5*Significantly different from the mean of all the MCVs at the 90% level**Significantly different from the mean of all the MCVs at the 95% level***Significantly different from the mean of all the MCVs at the 99% level

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