The Effects of Complex Terrain on Severe Landfalling Tropical Cyclone Larry (2006) Over Northeast Australia

4334 MONTHLY WEATHER REVIEW VOLUME 136 The Effects of Complex Terrain on Severe Landfalling Tropical Cyclone Larry (2006) over Northeast Australia HAMISH A. RAMSAY School of Meteorology, and Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma LANCE M. LESLIE School of Meteorology, University of Oklahoma, Norman, Oklahoma (Manuscript received 19 October 2007, in final form 1 April 2008) ABSTRACT The interaction between complex terrain and a landfalling tropical cyclone (TC) over northeastern Australia is investigated using the fifth-generation Pennsylvania State University–National Center for At- mospheric Research (PSU–NCAR) Mesoscale Model (MM5). Severe TC Larry (in March 2006) made landfall over an area of steep coastal orography and caused extensive damage. The damage pattern suggested that the mountainous terrain had a large influence on the TC wind field, with highly variable damage across relatively small distances. The major aims in this study were to reproduce the observed features of TC Larry, including track, intensity, speed of movement, size, decay rate, and the three-dimensional wind field using realistic high-resolution terrain data and a nested grid with a horizontal spacing of 1 km for the finest domain (referred to as CTRL), and to assess how the above parameters change when the terrain height is set to zero (NOTOPOG). The TC track for CTRL, including the timing and location of landfall, was in close agreement with observation, with the model eye overlapping the location of the observed eye at landfall. Setting the terrain height to zero resulted in a more southerly track and a more intense storm at landfall. The orography in CTRL had a large impact on the TC’s 3D wind field, particularly in the boundary layer where locally very high wind speeds, up to 68 m sϪ1, coincided with topographic slopes and ridges. The orography also affected precipitation, with localized maxima in elevated regions matching observed rainfall rates. In contrast, the precipitation pattern for the NOTOPOG TC was more symmetric and rainfall totals decreased rapidly with distance from the storm’s center. Parameterized maximum surface wind gusts were located beneath strong boundary layer jets. Finally, small-scale banding features were evident in the surface wind field over land for the NOTOPOG TC, owing to the interaction between the TC boundary layer flow and land surface characteristics. 1. Introduction 2004). A “severe” TC is one that produces sustained winds of at least 118 km hϪ1 (33 m sϪ1), with gusts of Tropical cyclones are an annual threat to Australia 170–225 km hϪ1 (47–63 m sϪ1). An average of 12.5 TCs each year from November to April, often with devas- occur in the Australian region each year, with about 5 tating consequences. A tropical cyclone (TC) in the making landfall. Australian region (90°–160°E in the Southern Hemi- The far northeastern region of Australia (see loca- sphere) is defined by the Australian Bureau of Meteo- tion map Fig. 1a) is significant because the complex rology as “a nonfrontal, synoptic-scale cyclone that has terrain in close proximity to the coastline acts to en- developed over tropical waters, with a 10-min average hance the destructive potential of landfalling TCs. Be- V Ն Ϫ1 Ϫ1 wind speed, 63 km h [17 m s ] near the center tween one and two TCs make landfall on the northeast of the organized wind circulation” (Dare and Davidson Australian coast each year (Dare and Davidson 2004). While less prone to TCs than more sparsely populated northwestern Australia, TCs that impact the far north- Corresponding author address: Hamish A. Ramsay, CIMMS, University of Oklahoma, National Weather Center, Suite 2100, east coast pose an extreme threat to large cities such as 120 David L. Boren Blvd., Norman, OK 73072. Cairns and Townsville, Australia, smaller coastal com- E-mail: [email protected] munities, and the agricultural industry of the region. DOI: 10.1175/2008MWR2429.1 © 2008 American Meteorological Society Unauthenticated | Downloaded 10/05/21 10:04 PM UTC MWR2429 NOVEMBER 2008 RAMSAY AND LESLIE 4335 FIG. 1. (a) Location map showing the far northeastern region of Australia, delineated by the black rectangle. (b) Map showing the region of complex terrain affected by TC Larry, including the town of Innisfail where the eye crossed during landfall. Terrain contours are every 100 m. The tallest peaks of Mount Bartle Frere and Bellenden Ker are marked with the initials BF and BK, respectively. Severe TC Larry crossed the northeastern Australian caused by TC Agnes at Cairns was comparable to the coast near the town of Innisfail, Australia (Fig. 1b) on damage near where the eye crossed the coast at Towns- the morning of 20 March 2006, causing significant dam- ville. In 1986 TC Winifred, though smaller than Agnes age to infrastructure and crops, with an estimated cost in size, crossed the coast just south of Innisfail and of about $500 million Australian dollars. Analyses of caused scattered westerly wind damage to the Cairns damage to the region suggested that the local topogra- region. phy played a significant role in modifying Larry’s wind Previous studies of the influence of orography on TC field, as evidenced by varying degrees of damage across structure and intensity have focused mainly on ideal- relatively small distances. A preliminary damage inves- ized numerical simulations (e.g., Chang 1982; Bender et tigation by engineers at James Cook University, al. 1985), or on TCs that interact with the Central Townsville, Australia, found that wind speed up over Mountain Range (CMR) of Taiwan (e.g., Bender et al. topographic ridges resulted in a significant increase in 1987; Chang et al. 1993; Yeh and Elsberry 1993a,b; Lin damage to buildings, whereas those structures sheltered et al. 1999, 2002, 2005, 2006; Wu and Kuo 1999; Wu by topography suffered much less damage. The town of 2001; Wu et al. 2002). The Sierra Madre Mountains of Babinda, for example, to the northwest of Innisfail (Fig. Mexico have also been shown to influence TCs that 1b), sustained disproportionate damage relative to its approach from the Gulf of Mexico (e.g., Zehnder 1993; distance from Larry’s eye by the funneling of westerly Farfan and Zehnder 2001). Other mountainous regions downslope winds. Westerly downslope winds also affected by TCs include Luzon in the northern Philip- caused moderate damage in and around the Cairns re- pines (Brand and Blelloch 1973) and the Caribbean gion (Fig. 1b). Islands of Cuba, Hispaniola, and Puerto Rico (Bender Cairns is often affected by these damaging westerly et al. 1987). winds when TCs make landfall south of the region. Cal- The above studies have provided considerable under- laghan (2003) noted at least three historical TC events standing of how orography affects the track and struc- that resulted in substantial westerly wind damage in ture of TCs. While the importance of orographic influ- Cairns, the most extreme of which was TC Agnes in ences on landfalling TCs over northeastern Queensland 1956. TC Agnes crossed the coast near Townsville, has long been recognized (e.g., Whittingham 1964; about 280 km south of Cairns, with a central pressure of Harper 1999; Callaghan 2003), to our knowledge there 960 hPa. Despite its modest intensity at landfall, have been no comprehensive high-resolution numerical Agnes’s very large circulation resulted in extensive simulations that have explored the influence of orogra- westerly wind damage in the Cairns region, with wind phy on Australian TCs. speeds varying from nearly calm to 43 m sϪ1 over very Observations of TC Larry (more information avail- short time periods (Whittingham 1964). The damage able online at http://www.bom.gov.au/weather/qld/ Unauthenticated | Downloaded 10/05/21 10:04 PM UTC 4336 MONTHLY WEATHER REVIEW VOLUME 136 cyclone/tc_larry; Henderson et al. 2006) motivated this high-resolution numerical simulation. Our main objec- tive is to understand how the complex terrain of the northeastern Australian region affects TC track, winds, and precipitation, and to assess how different the impact of TC Larry would have been if the region was flat. Section 2 is a summary of the major observational aspects of TC Larry, including its track and its characteristics before, during, and after landfall. Section 3 de- scribes the fifth-generation Pennsylvania State Univer- sity–National Center for Atmospheric Research (PSU– NCAR) Mesoscale Model (MM5) model configuration and the proposed numerical experiments. The results are given in section 4, and section 5 summarizes the findings and suggests possible future work. 2. Observations of TC Larry FIG. 2. The quadruply nested grid configuration used for the TC Larry developed from a tropical low in the Coral simulations. The grid spacing for each domain is indicated next to Sea (see location in Fig. 1a), initially located about 1500 the domain number. km east of Cairns at 0600 UTC 17 March 2006. The low was upgraded to an Australian category 1 TC 12 h later duced extensive flooding in coastal rivers. As TC Larry (1800 UTC 17 March 2006) and continued to steadily moved inland toward the low-lying area south of the intensify as it moved westward, reaching the Australian Gulf of Carpentaria very heavy rain continued, with the severe TC classification during the morning of 18 highest recorded total of 436 mm at Gereta Station March 2006. Ten hours prior to landfall (1110 UTC 19 Ϫ (about 700 km inland) in the 24 h preceding 2300 UTC March 2006) a wind gust of 59 m s 1 was recorded at 21 March 2006. Flinders Reef, located about 260 km east of Innisfail. Larry was a small TC with destructive wind gusts of An analysis of high-resolution microwave satellite im- greater than 47 m sϪ1 extending no more than 50 km agery suggests that Larry may have briefly reached from the center.

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