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,

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.

The Overview of TC Larry

Townsville

BK BF (Bellenden Ker; 1593 m) (Bartle Frere; 1622 m) Complex Terrain

Date & TimeDescription 0600 UTC 17 MarchAbout 1500km east of Cairns 1800 UTC 17 MarchCategory 1 TC Morning of 18 MarchSevere TC 1110 UTC 19 MarchThe wind gust reach 59 m/s 2100 UTC 19 MarchMaking landfall

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

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 Radiation Rapid Radiation Transfer Model (RRTM; Mlawer et al. 1997) IC NCEP Final Analysis (FNL) 1 。 X 1 。 resolution BC Cumulus Betts-Miller (1986),only D1 & D2 Model Setting 8

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

a. 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 landfall 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 m/s 50 m/s BK BF 77 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 m/s 50 m/s BK BF NOTOPOG TC 60 m/s 67

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 TCNOTOPOG TC

Pentagonal-shaped eyewall Observed Simulated (CTRL)

c. Boundary layer turbulence 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 NOTOPOG Ocean:< 80 J/kg Land:180 J/kg The 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 TC winds RESULTS

2100 UTC 18 MARCH0100 UTC 19 MARCH

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

e. Downslope winds in the Port Douglas region RESULTS

4-8 m/s m/s 2300 UTC 18 MARCH0030 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. 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

CTRL NOTOPOG 200 mm 225 mm mm 2300UTC UTC UTC UTC UTC UTC 20 ~ 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) 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) 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) 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