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

The Overview of TC LarryThe Overview of TC Larry

Townsville

BK

BF

(Bellenden Ker; 1593 m)

(Bartle Frere; 1622 m)

Complex TerrainComplex Terrain

Date & Time Description

0600 UTC 17 March About 1500km east of Cairns

1800 UTC 17 March Category 1 TC

Morning of 18 March Severe TC

1110 UTC 19 March The wind gust reach 59 m/s

2100 UTC 19 March Making landfall

Model MM5 V. 3.7

Domain D1:27 km ; D2: 9 km ; D3: 3 km ; D4: 1 km

Levels 46 half-sigma levels, extend vertically up to 50 hPa

Microphysics Resiner Mixing – Phase ( Reisner et al. 1998)

PBL The Eta Model Mellor-Yamada scheme

RadiationRapid Radiation Transfer Model (RRTM; Mlawer et al. 1997)

ICNCEP Final Analysis (FNL) 1 。 X 1 。 resolution

BC

Cumulus Betts-Miller (1986) ,only D1 & D2

Model Setting Model Setting

8

Terrain DataTerrain DataHigh 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

a. TC track and intensitya. TC track and intensity

RESULTS

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 Larry’s landfall

NOTOPOG TC is 18 hPa deeper than CTRL TC

SST is not different between two simulations

36h

Track & Central Pressure

b. TC structure during b. TC structure during landfalllandfall

RESULTS

Surface Wind Speed and Direction

The ‘‘surface wind’’ denotes the 10-m wind .

The TC’s 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

50-58 m/s

50 m/sBK

BF

77 m/s@500m

73 m/s@600m

CTRL TC

Surface Wind Speed and Direction

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

50-58 m/s

50 m/sBK

BF

NOTOPOG TC

60 m/s

67 m/s@250m

82 m/s@250m

CTRL

NOTOPOG

Equivalent Potential Temperature (contour)Cyclonic vertical vorticity (sading) at 1 km 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

CTRL TC NOTOPOG TC

Pentagonal-shaped Pentagonal-shaped eyewalleyewall

Observed Simulated (CTRL)

c. Boundary layer c. Boundary layer turbulenceturbulence

RESULTS

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

CTRL

NOTOPOG

Ocean:< 80 J/kgOcean:< 80 J/kg

Land:180 J/kgLand:180 J/kg

The contour is vertical shearThe contour is vertical shear

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

d. Influence of orography on d. Influence of orography on

TC windsTC winds

RESULTS

2100 UTC 18 MARCH 0100 UTC 19 MARCH

68 m/s

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

Mt. BK50-60 m/s

e. Downslope winds in the e. Downslope winds in the Port Douglas regionPort Douglas region

RESULTS

4-8 m/s 16-20

m/s

2300 UTC 18 MARCH 0030 UTC 19 MARCH

~24 m/s

~4 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)

f. Rainfallf. Rainfall

RESULTS

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.

12-h Accumulated Rainfall

CTRL

NOTOPOG

<5 m/s wind shear

CTRLNOTOPOG

200 mm

225 mm

75-100 mm2300UTC 19

0200 UTC 20

0500 UTC 20

0000 UTC 20

0300 UTC 20

0600 UTC 20

~70-100 mm

200 mm

175 mm

In despite of the NOTOPOG TC’s intensity is greater than CTRL TC, the accumulated rainfall of NOTOPOG TC’s is less than in the CTRL TC.NOTOPOG TC have more moisture content compared with the CTRL TC

Summary (I)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.

Summary (II) 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.

Summary (III) 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