1. 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

2. 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 UTC 19 Aug 2007
GOES-12 IR 0600 UTC 19 Aug 2007

3. Tropical Storm Erin NHC Best Track
Knabb (2008)

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

5. 96-h Precipitation Through 12Z 20 Aug.3 2 1
Three core rainfall events
4-km Gauge-Radar Stage IV QPE from NPVU

6. 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?

7. 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 Aug in Texas hill country followed by morning minimum

8. 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

9. 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

10. Interaction with Upper Level Shortwave During Re-intensification
GFS Analysis 00Z 19 Aug.
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

11. Erin’s Structure GFS/NAM Analyses
06Z 19 Aug GFS analysis 850-mb moisture flux, winds, mb PV, 700-mb vertical velocity
06Z 19 Aug NAM analyzed sounding at °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

12. 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

13. 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

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

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

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

17. 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

18. 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.

19. 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

20. 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)

21. 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

22. 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