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Michael J. Brennan* and Norman W. (Wes) Junker**

Observational Analysis and Numerical Simulation of the Inland Evolution of Tropical Storm Erin (2007). Michael J. Brennan* and Norman W. (Wes) Junker** NOAA/NWS/NCEP Hydrometeorological Prediction Center, Camp Springs, MD * Soon to be NHC ** Retired (for good this time)

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Michael J. Brennan* and Norman W. (Wes) Junker**

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  1. Observational Analysis and Numerical Simulation of the Inland Evolution of Tropical Storm Erin (2007) Michael J. Brennan* and Norman W. (Wes) Junker** NOAA/NWS/NCEP Hydrometeorological Prediction Center, Camp Springs, MD *Soon to be NHC **Retired (for good this time) 14th Cyclone Workshop Sainte-Adèle, Quebec 24 September 2008

  2. Motivation • Erin, made landfall on 16 August as tropical depression, but had its most significant impacts inland • Multiple heavy rainfall episodes near the core and well removed from the center (including 3 PREs) • Inland intensification with sustained TS force winds over OK • This event was marked by a complex inland evolution of the TC and poor forecasts by the operational NWP model suite • HPC responsible for QPF and inland advisories for tropical depressions, so this directly impacted two of our more visible operational products Adapted from Knabb (2008) 96-h precip through 12Z 20 Aug. Three core rainfall events with maxima ~ 10 in.

  3. Research Questions • How does the inland evolution of Erin compare to that of a continental MCC/MCV? • How sensitive is the early inland evolution of Erin to variations in model physics and initial conditions?

  4. Satellite Evolution – 16-17 AugGOES-12 IR 00 UTC 17 Aug 03 UTC 17 Aug 06 UTC 17 Aug 09 UTC 17 Aug 15 UTC 17 Aug 12 UTC 17 Aug Late afternoon/early evening maximum 16-17 Aug in Texas hill country followed by morning minimum

  5. Satellite Evolution – 17-18 AugGOES-12 IR 18 UTC 17 Aug 21 UTC 17 Aug 00 UTC 18 Aug 03 UTC 18 Aug 06 UTC 18 Aug 09 UTC 18 Aug Convective maximum around 06Z 18 Aug. but displaced southeast of low level center

  6. Satellite Evolution – 18-19 AugGOES-12 IR 18 UTC 18 Aug 00 UTC 19 Aug 03 UTC 19 Aug 06 UTC 19 Aug 09 UTC 19 Aug 12 UTC 19 Aug Nocturnal convective maximum coincident with maximum of low-level jet near low-level center

  7. Erin’s StructureGFS/NAM Analyses 06Z 19 Aug. 2007 GFS analysis 850 moisture flux, winds, 900-700 mb PV 06Z 19 Aug. 2007 NAM analyzed sounding at 34.43°N 98.55°W Similar to that found by Schumacher and Johnson (2008) (in review) Low level jet, mid level PV max, MAUL or near MAUL, high RH, modest CAPE, weak mid level winds Coat-hanger shape to hodograph

  8. Interaction with Upper Level Shortwave During Re-intensification GFS Analysis at 00Z 19 Aug. 200-300 mb PV 250-mb wind 250-mb positive PV advection 850-mb absolute vorticity Upper level positive PV advection impinges on eastern flank of low-level Erin vortex

  9. GFS QPFEnding 12Z 20 Aug. 00Z 17 Aug. run 12Z 17 Aug. run GFS suggested precip max near core of Erin on 18 Aug., but did not show good run-to-run continuity with final core precip maximum early on 19 Aug. 00Z 18 Aug. run 12Z 18 Aug. run

  10. NAM QPFEnding 12Z 20 Aug. 00Z 17 Aug. run 12Z 17 Aug. run NAM was too light with precip amounts near core of Erin on 18 and 19 Aug. Did show hints of nocturnal precip max in runs initialized at 00Z. 00Z 18 Aug. run 12Z 18 Aug. run

  11. Model Experiments • Utilized Workstation WRF • Forecasts initialized at 00 UTC 17 Aug. • Run 72 h through 00 UTC 20 Aug. • Forecasts run with 12-km horizontal grid spacing (similar to NAM) • Tested sensitivity to • ICs, CP scheme, microphysics, radiation, fluxes, and PBL scheme • Examine structure of Erin remnant in various model configurations

  12. Control Forecast • Control Forecast • ARW core • GFS ICs/BCs • KF CP scheme • Lin et al. microphysics • YSU PBL and surface fluxes • Realistic results for 3-day forecast considering how poorly operational models fared • Showed nocturnal enhancement of convection near center • Eye-like feature seen in simulated reflectivity field • Developed vigorous low level vortex – (but without interaction with upper-level shortwave) Simulated Reflectivity 03Z 17 Aug. through 00Z 20 Aug. 850-mb rel. vort, wind, and MSLP06Z 19 Aug.

  13. Sensitivity to Microphysics850-mb rel. vorticity, wind, MSLP KF/Ferrier KF/Lin • Tests showed large sensitivity to grid scale microphysics and CP scheme BMJ/Ferrier BMJ/Lin

  14. Sensitivity to ICs03Z 19 Aug. Control • Using NAM initial conditions, Erin tracks WNW after landfall and dissipates by 24 h NAM ICs 925 mb θe (shaded)10-m wind (barbs, kt)10-m isotachs (pink, > 25 kt)

  15. Summary • Structure of Erin after landfall resembled that of MCS/MCV • Intensification early on 19 Aug. coincided with interaction with upper-level shortwave • Control WRF forecast showed robust deepening with less shortwave interaction • Operational model forecast from NAM likely hindered by initial conditions and combination of BMJ CP scheme and Ferrier microphysics that produced poorest representation of Erin with WRF

  16. Summary • Model experiments show greater sensitivity to microphysics than CP scheme • Most precipitation generated on model grid scale at 12-km, likely due to small instability near center of Erin and lack of CP scheme activation • Large sensitivity to initial conditions • Forecasts improved markedly when using GFS ICs relative to NAM • Consistent with recent findings at EMC that NAM performance improves when cases re-run using GFS ICs

  17. Future Work • Further investigate role of shortwave in intensification • Quantify impact of nocturnally enhanced Plains LLJ in diurnal cycle of vortex intensity and precip distribution • Increase forecaster awareness of potential interaction of remnant TC vortex with upper-level shortwave in moisture-rich environment with moderate CAPE for potential intensification

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