The Effects of Complex Terrain on Sever Landfalling Tropical Cyclone Larry 2006 over Northeast Austr

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The Effects of Complex Terrain on Sever
Landfalling Tropical Cyclone Larry (2006) over
Northeast Australia

• Hamish A. Ramsay and Lance M. Leslie,2008 The
  Effects of Complex Terrain on Sever Landfalling
  Tropical Cyclone Larry (2006) over Northeast
  Australia, Mon. Wea. Rev., 136,4334-4354

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• The focus is not only on the primary variable
  such as wind, pressure, and rainfall , but also
  on how complex terrain acts to modify the TC
  boundary layer
• The main objective is to understand how the
  complex terrain of the northeastern Australian
  region affects TC track, winds, and
  precipitation, and assess how different the
  impact of TC Larry would have been if the region
  was flat.

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The Overview of TC Larry
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Townsville
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Complex Terrain
BK
(Bellenden Ker 1593 m)
BF
(Bartle Frere 1622 m)
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• The highest official observed wind gust near time
  of landfall was 50 m/s recorded by the Automatic
  Weather Station(AWS)
• The highest unofficial measured wind gust was 82
  m/s at Bellenden Ker Tower near the peak of Mt.
  BK
• Heavy rainfall, with 3-h totals up to 139 mm ,
  produced expensive flooding in coastal rivers.
• The highest record in the 24-h accumulated
  rainfall is 436 mm at Gereta Station

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Model Setting
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Terrain Data

• High resolution terrain data with horizontal
  resolution of 900m was used in D4
• Mt. Bartle Frere
• nature 1622 m model 1600 m
• Mt. Bellenden Ker
• nature 1593 m model 1484 m

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a. TC track and intensity

• RESULTS

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Track Central Pressure

• The simulated TC track is in very good agreement
  with the observed track of TC Larry
• The simulated TC with topography crossed the
  coast about 2 h after the observed time of
  Larrys landfall
• NOTOPOG TC is 18 hPa deeper than CTRL TC
• SST is not different between two simulations

36h
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b. TC structure during landfall

• RESULTS

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Surface Wind Speed and Direction
CTRL TC

• The surface wind denotes the 10-m wind .
• The TCs tangential flow is accelerated by the
  orography
• Surface wind of up to 38m/s are evidence on their
  (Mt. BK and BF) southern slopes
• The surface winds on the sheltered lee sides of
  these mountains are significantly lower
• CTRL TC have more tilting (vertical wind shear)
  than NOTOPOG TC

50 m/s
BK
BF
50-58 m/s
77 m/s_at_500m
73 m/s_at_600m
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Surface Wind Speed and Direction
NOTOPOG TC

• The maximum surface winds are located in the
  eastern half of the circulation over water,
  collocated with a maximum in low-level cyclonic
  vorticity and very strong gradient of EPT

50 m/s
BK
60 m/s
BF
50-58 m/s
67 m/s_at_250m
82 m/s_at_250m
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Equivalent Potential Temperature
(contour) Cyclonic vertical vorticity (sading) at
1 km
CTRL

• NOTOPOG TC have a strong warm core with a maximum
  equivalent potential temperature of 380 K ( 5K
  higher than the 375 K for CTRL TC )near the
  center of eye

NOTOPOG
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CTRL TC
NOTOPOG TC
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Pentagonal-shaped eyewall
Simulated (CTRL)
Observed
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c. Boundary layer turbulence

• RESULTS

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NOTOPOG
CTRL
Land180 J/kg
Oceanlt 80 J/kg
The contour is vertical shear

• TKE maxima
• South of TC eye
• A narrow band west of the eye
• The windward slope
• The spatial distribution of the vertical shear in
  the lowest 100 m is in very close agreement with
  the spatial distribution of TKE

CTRL
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• The 50-m wind speed over the eastern slopes of
  Mt. BF (1200m)is 56 m/s whereas the surface wind
  is only 32 m/s
• Over the northern slopes of the mountain
  (960m)the 50-m wind speed is only 16 m/s
• Similar speedup/sheltering effects that coincide
  with distinct maxima and minima of TKE are also
  noted over and around Mt. BK father to the north

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d. Influence of orography on TC winds

• RESULTS

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2100 UTC 18 MARCH
0100 UTC 19 MARCH
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68 m/s
50-60 m/s
Mt. BK

• An observed westerly wind gust of 82 m/s was
  recorded at roughly the same location

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e. Downslope winds in the Port Douglas region

• RESULTS

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2300 UTC 18 MARCH
0030 UTC 19 MARCH
4 m/s
4-8 m/s
16-20 m/s
24 m/s

• Critical layers (10 km) have been shown to play
  an important role in the amplification of
  mountain waves and subsequent intensification of
  severe downslop windstorm (Clark and Peltier 1984)

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f. Rainfall

• RESULTS

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12-h Accumulated Rainfall

• Chen et al. (2006) show that for TCs in the
  Southern Hemisphere, enhanced precipitation is
  favored to the right of the deep-layer
  environmental shear
• For CTRL, analyses of the column-integration
  cloud liquid water content indicates maximum
  values occur generally in the front-left quadrant
  of the vortex, upstream of the heavy
  precipitation in the front-right /rear-right
  quadrants.

lt5 m/s wind shear
CTRL
NOTOPOG
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0000 UTC 20
CTRL NOTOPOG
200 mm
75-100 mm
2300UTC 19
200 mm
In despite of the NOTOPOG TCs intensity is
greater than CTRL TC, the accumulated rainfall of
NOTOPOG TCs is less than in the CTRL TC.
0300 UTC 20
225 mm
175 mm
0200 UTC 20
0600 UTC 20
NOTOPOG TC have more moisture content compared
with the CTRL TC
70-100 mm
0500 UTC 20
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Summary (I)

• These boundary layer jets produced strong
  low-level vertical wind shear (30 m /s)
• The shape of this range is well-suited for
  generating severe downslope winds, with its steep
  leeside slope and gentle windward rise.

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Summary (II)

• Rainfall amounts and patterns associated with TC
  Larry were reproduced well by the CTRL
  simulation, with 3-h totals in excess of 200 mm
  over the steep coastal orography.
• In contrast, the 3-h rainfall totals for the
  NOTOPOG TC were lower immediately following
  landfall, but increased relative to CTRL as the
  system moved farther inland.

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Summary (III)

• Small-scale banding features were evident in the
  surface wind field over land for the NOTOPOG TC,
  due to the interaction between the TC boundary
  layer flow and land surface characteristics

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