Anomalous Reintensification of SubTropical Storm Allison 2001

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. Allisons Legacy 2. Classical
Hurricane Structure 3. IR Satellite Imagery 4.
Radar Imagery 5. Structure and Evolution 6.
Causes of Intensification 7. MM5 Simulations
3
Allisons 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
5
(No Transcript)
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 storms circulation . Grell
MRF also produced the greatest vapor flux
convergence into the storms central region.
20
Domain setup for Allison
21
MM5 physics options
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
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 (5 -
10 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
Allisons deepening. Because SSTs 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 SSTs north of 29 N to 299.8
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
299.8
Buoys
299.9
299.8
MM5 Sea Surface Temperature Analysis 10-13 June
2001
32
(No Transcript)
33
Conclusions The most exciting result is that MM5
was able to capture some critical features of
Allisons 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.