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 *Retired (again) 14th Cyclone Workshop Sainte-Adèle, Quebec 24 September 2008

Radar Composite 1004 UTC 19 Aug 2007 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 model forecasts, especially by the NAM HPC responsible for QPF and inland advisories for tropical depressions, so this directly impacted two of our more visible operational products Radar Composite 1004 UTC 19 Aug 2007 GOES-12 IR 0600 UTC 19 Aug 2007

Tropical Storm Erin NHC Best Track Knabb (2008)

Tropical Storm Erin NHC Best Track Intensity Landfall at San Jose Island, TX Adapted from Knabb (2008)

96-h Precipitation Through 12Z 20 Aug. 3 2 1 Three core rainfall events 4-km Gauge-Radar Stage IV QPE from NPVU

Research Questions How does the inland evolution of Erin compare to that of a continental MCS/MCV? How did operational models handle the inland evolution of Erin How sensitive is the early inland evolution of Erin to variations in model physics and initial conditions?

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

Satellite Evolution – 17-18 Aug GOES-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

Satellite Evolution – 18-19 Aug GOES-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 surface center

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

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

Also Note Similarities of Erin with Figures from Fritsch et al. (1994) PV min PV max Cold pool 31.0°N 101.4°W 38.4°N 94.6°W GFS analysis cross section valid 06Z 19 Aug. of PV, θ, ω

Operational Model Performance NAM and ECMWF had low bias and low TS for amounts over 1” GFS had high bias for amounts over 4” HPC showed improvement over models, especially for amounts over 1” with TS > 0.5 up to 5 in. threshold

Forecast vs. Observed Precip 24 h ending 1200 UTC 17 Aug. 2007 WGRFC QPE HPC QPF RFC QPF NGM NAM GFS

Forecast vs. Observed Precip 24 h ending 1200 UTC 18 Aug. 2007 WGRFC QPE HPC QPF RFC QPF NGM NAM GFS

Forecast vs. Observed Precip 24 h ending 1200 UTC 19 Aug. 2007 WGRFC QPE HPC QPF RFC QPF NGM NAM GFS

Model Experiments Workstation WRF ARW Core v 2.1.2 Forecasts initialized at 00 UTC 17 Aug. Run 72 h through 00 UTC 20 Aug. 12-km horizontal grid spacing (similar to NAM) Tested sensitivity to ICs, CP scheme, microphysics, fluxes, and PBL scheme Examine evolutions of Erin in various model configurations

Control Forecast Control Forecast ARW v2.1.2 GFS ICs/BCs KF CP scheme Lin et al. microphysics YSU PBL and MM5 surface layer physics Quite good results for 3-day forecast Nocturnal enhancement of convection near center Eye-like feature seen in simulated reflectivity field Low-low level vortex deepened significantly – but without interaction with upper-level shortwave which passed by to the north Simulated Reflectivity 03Z 17 Aug. through 00Z 20 Aug. 850-mb rel. vorticity, wind, and MSLP 06Z 19 Aug.

Sensitivity to Microphysics 850-mb rel. vorticity, wind, MSLP Tests showed large sensitivity to grid scale microphysics and CP scheme GFS analyzed CAPE 06Z 18 Aug. KF/Ferrier KF/Lin BMJ/Ferrier BMJ/Lin

Sensitivity to ICs 03Z 19 Aug. Control Using NAM initial and boundary conditions, Erin tracks WNW after landfall and dissipates by 24 h NAM ICs/BCs 925 mb θe (shaded) 10-m wind (barbs, kt) 10-m isotachs (> 25 kt)

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 NAM forecast likely hindered by ICs and particulars of Ferrier microphysics 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 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

Future Work Further investigate role of shortwave in intensification over OK Was it a necessary ingredient for deepening? Quantify impact of nocturnally Plains LLJ in diurnal cycle of vortex intensity and precip distribution Investigate sensitivity to grid scale microphysics in other inland decaying TCs Increase forecaster awareness of potential intensification when remnant TC vortex interacts with upper-level shortwave in moisture-rich environment with moderate CAPE