1. Anomalous Reintensification of (Sub)Tropical Storm Allison (2001)
over Land
Kwan-yin Kong and Stanley David Gedzelman
EAS Department and NOAA CREST Center
City College of New York

2. Seminar Outline
1. Allison’s Legacy
2. Classical Hurricane Structure
3. IR Satellite Imagery
4. Radar Imagery
5. Structure and Evolution
6. Causes of Intensification
7. MM5 Simulations

3. Allison’s Legacy
Allison (2001) was a weak, mostly disorganized system that twice reached Tropical Storm Status. It formed over the Gulf of Mexico just south of Houston on 04 June. On 06 June Allison moved inland over Houston, where it produced up to 80 cm of rain in 5 days. It then moved back over the Gulf and wandered eastward, making a second landfall shortly after 00 UTC on 11 June. From 00 to 12 UTC on the 11th, Allison intensified and became more organized. It developed a squall line or rain band on its eastern side that wrapped around the center to produce a feature that resembled an eye. Rain began in southern Louisiana and Mississippi days before landfall there, transforming the lowlands into a watery world. This may have contributed to the post landfall intensification.
Here we analyze Allison and use MM5 simulations to help diagnose its anomalous intensification over land.

4. Track of Allison (2001)
Swamp-fall 0400 UTC 11 June

6. 00Z 06 JUN

7. 00Z 07 JUN

8. 12Z 11 JUN

9. 21Z 11 JUN

10. Sequence of Radar Charts
Features
1. Convective lines form around 0200 and 0800 to SE of center.
2. Second line wraps around center to encircle eyelike feature.
3. Dry slot advected around southern side of storm

11. 00Z 11 JUN

12. 02Z 11 JUN

13. 10Z 11 JUN

14. 12Z 11 JUN

15. Major Causes of Tropical Cyclone Intensification
1. Warm water eddy Opal (1995), Bret (1999)
2. Vorticity advection aloft David (1979)
3. Increasing baroclinicity Hazel (1953)
4. Convective bursts in Eye Wall. Hortense (1996)

16. 00Z 11 June 2001

17. 09Z 11 June 2001

18. 12Z 11 June 2001

19. MM5 Simulations
Offer choice of resolution + several parameterizations each for
1. Cloud Microphysics (mPH)
2. Convective Adjustment (CNV)
3. Boundary Layer Dynamics (BDL)
Results: Runs not very sensitive to mPH. Only the combination of Grell CNV + MRF BDL captured the intensification. Higher resolution (15 km) produced inferior results! Adding more levels near tropopause moved Allison eastward faster (an improvement).
Why??? Grell + MRF was the only combinaton to reproduce the convective burst that formed near the storm center - others produced competing convection far from center that never got entrained in the storm’s circulation . Grell + MRF also produced the greatest vapor flux convergence into the storm’s central region.

20. Domain setup for Allison
81km
27km

21. MM5 physics options Precipitation Cumulus PBL Anthes-Kuo Blackadar
Warm rain
Anthes-Kuo
Blackadar
Simple ice
Grell
Burk-Thompson
Mixed-phase
Arakawa-Suhubert
Eta
Goddard
Fritsch-Chappell
MRF
Reisner-graupel
Kain-Fritsch (KF)
Gayno-Seaman (GS)
Schultz
Betts-Miller
Pleim-Chang

25. Actual track of Allison versus MM5 runs using Grell cumulus + five different PBL
536
532,533,534
535
435 L
12Z 6/12 L L
00Z 6/12 L L
12Z 6/11 L L L L
12Z 6/10
00Z 6/11

26. Features of the MM5 Simulation Aloft
Pseudo-eye formation was due mainly to advection of dry air with a secondary contribution from sinking ( cm s-1 at 300 hPa level). Simulated intensification and eye formation were linked but occurred several hours later than observations.
Allison simulations did not contain a consistent warm core and did not contain a dry slot of sinking stratospheric air.

27. 400 hPa 16 UTC 11 JUN RH

28. 300 hPa 16 UTC 11 JUN RH

29. MM5 Cross Section RH Analysis 1400 UTC 11 June 2001

30. Next Allison MM5 Runs
No MM5 run fully captured Allison’s deepening. Because SST’s were warmer than the MM5 analysis indicated and because so much rain converted the swampy lowlands to a largely watery world, we will try two more experiments
1. Warm SST’s north of 29 N to K
2. Change the land use categories of several pixels to water. Nocturnal intensification may have been aided by thermal inertia of flooded land.

31. MM5 Sea Surface Temperature Analysis 10-13 June 2001
299.8
Buoys
299.9
299.8
MM5 Sea Surface Temperature Analysis
10-13 June 2001

33. Conclusions
The most exciting result is that MM5 was able to capture some critical features of Allison’s evolution and intensification over land, adding strong evidence to the finding that intensification of tropical cyclones often follows flareups of convection in or near the eye wall that get swirled around the storm to form a more symmetric and steady but often less convective eyewall.