DOI: 10.22499/3.6901.016 JSHESS early online view This article has been accepted for publication in the Journal of Southern Hemisphere Earth Systems Science and undergone full peer review. It has not been through the copy-editing, typesetting and pagination, which may lead to differences between this version and the final version. Journal of Southern Hemisphere Earth Systems Science (<<YEAR>>) <<VOL.>>:<<ISSUE>> 1 Environmental Interactions During the 2 Extreme Rain Event associated with ex- 3 Tropical Cyclone Oswald (2013) 4 Marie-Dominique Leroux1, Mai C. Nguyen-Hankinson2, Noel E. Davidson3, Jeffrey 5 Callaghan4, Kevin Tory5, Alan Wain6, Xinmei Huang6 6 7 8 1Laboratoire de l'Atmosphère et des Cyclones 9 (Unité Mixte 8105 CNRS/Météo-France/Université de La Réunion), Saint-Denis de la Réunion, France 10 2Department of Mathematical Sciences, Monash University, Melbourne, Australia 11 3Centre for Australian Weather and Climate Research, 12 A partnership between the Australian Bureau of Meteorology and CSIRO, Melbourne, Australia 13 4formerly Bureau of Meteorology, Queensland Regional Forecasting Centre 14 5Science and Innovation Group, Australian Bureau of Meteorology 15 6Bureau National Operations Centre, Bureau of Meteorology, Melbourne, Australia 16 (Manuscript received April 2019; accepted Month Year) 17 Tropical Cyclone (TC) Oswald made landfall over northeast Australia as a 18 minimal or Category 1 TC on the Australian scale on 21 January 2013. As it 19 moved southward, it intensified over land and produced extreme rainfall for 20 nearly seven days. Tornadoes were reported and confirmed. Tragically seven 21 people died and insurance estimates were around $1 billion. 22 It is demonstrated that the event was associated with an interaction between 23 the ex-Oswald circulation and an amplifying Rossby wave, which propagated 24 northeastward from high latitudes. Diagnoses show that as the wave ampli- 25 fied and broke, a potential vorticity anomaly (PVA) extended to midlevels, 26 moved equatorward, and merged with, or axisymmetrised the ex-Oswald cir- 27 culation through midlevels. Backward trajectories from locations regularly 28 scattered within the midlevel circulation illustrate that the storm transitioned 29 from an isolated vortex into a circulation which was strongly influenced by 30 its environment for at least 5 days. During this interaction, PV was advected 31 from the environment toward the storm through midlevels. The heavy rain 32 coincided with the commencement and maintenance of this PV injection. 33 The PV injection is quantified and shown to be consistent with PV advection 34 by the mean radial flow. In addition, eddy angular momentum convergence 35 in the mid- to upper levels coincided with an intensification of the circulation 36 through this region. This was first related to outward transport of anticyclonic 37 momentum by the asymmetric outflow at upper levels, followed by inward 38 transport of cyclonic momentum by the asymmetric inflow. It is shown that 39 the environmental interaction had an impact on vortex structure changes, 40 rainfall and tornado development. We propose that the environmental pro- 41 cesses influenced the ascent within the storm (a) via differential vorticity ad- 42 vection and baroclinic forcing, as the mid- to upper-level PVA approached 43 the circulation, and (b) by low- to mid-level warm air advection. DOI: 10.22499/3.6901.016 44 45 1 Introduction 46 At landfall, Tropical Cyclones (TCs) pose significant forecast problems because of the potential devastation to 47 life and property from strong winds and heavy rain (e.g., Chen 2012). The task of forecasting the distribution and 48 local intensity of rainfall, particularly heavy rain, is very challenging. While most of the numerical forecast guid- 49 ance for the event described here was of good quality, it is still very important to analyse the event to see if we 50 can understand the physical mechanisms that produced such a high impact rain event. 51 After TCs make landfall, they normally weaken and rainfall correspondingly decreases. For the case analysed 52 here, this did not happen and in fact the storm strengthened after it made landfall and as it moved southwards. 53 Other studies in the literature presented cases of TCs re-intensifying over land, especially over Northern Austral- 54 ia (e.g., Emanuel et al. 2008, Evans et al. 2011). Smith et al. (2015) present a vorticity perspective of the genesis 55 of TC Oswald over the Gulf of Carpentaria. They indicated that the storm developed on a monsoon shear line 56 and that intensification continued as the low performed a loop over land from 18 to 20 January. They showed 57 that moisture levels were high enough in the monsoon circulation to enable intensification over land following 58 periodic bursts of convection near the circulation centre that concentrated absolute vorticity. 59 When baroclinic potential energy is available from a westerly trough, the remnant circulation of a landfalling TC 60 can revive and even produce enhanced rainfall (Dong et al. 2010; Bosart and Lackmann, 1995). However, the 61 precise nature of the "interaction" does not seem to be well understood and so one aim of the current study is to 62 describe it more precisely. There are many studies on the "interaction" between TCs and midlatitude baroclinic 63 environments. Interestingly, they tend to focus on either the extratropical transition (ET, Evans et al. 2017) of 64 TCs (e.g., Klein et al. 2000; Harr and Elsberry 2000; Jones et al. 2003; Ritchie and Elsberry 2003, 2007; An- 65 wender et al. 2008, Keller et al. 2019, Pohorsky et al. 2019), or the antecedent rainfall well ahead of TCs (Cote 66 2007; Stohl et al. 2008; Srock and Bosart 2009; Galarneau et al. 2010; Lee and Choi 2010; Bosart et al. 2012; 67 Baek et al. 2013, Moore et al. 2013). Oswald was not a well-defined ET event, since the circulation had made 68 landfall and had weakened below TC intensity before any interaction took place. But it did have many character- 69 istics of TCs undergoing ET. We show that it was clearly an extratropical interaction and so it bears similarities 70 with ET events. Rainfall mostly occurred following landfall and in association with the circulation itself, so it 71 was not an antecedent rain event. 72 Leroux et al. (2016) give a comprehensive review of how upper-level troughs or baroclinic forcing can alter the 73 environment of TCs and have different impacts on storm intensity. Their numerical simulations of a realistic TC- 74 trough interaction scenario suggest that the TC intensity modulation may depend on the maturity, and hence the 75 vertical depth, of the storm. Vortices initially at tropical depression strength intensify much less than vortices at 76 tropical storm strength or greater; the latter being able to benefit from the presence of a trough to reach their 77 maximum potential intensity (Leroux et al. 2016). Observational analyses of TCs at tropical storm strength in the 78 North Atlantic (Fischer et al. 2017) and western North Pacific systems (Wei et al. 2016) consistently indicate 79 that, overall, upper-level troughs promote TC intensification. For rapid intensification cases, the potential vorti- 80 city (PV) anomalies associated with the troughs are stronger and deeper compared to neutral intensification or 81 slow intensification cases (Fischer et al. 2017). For the TC Dora (2007) case, Leroux et al (2013) showed that 82 PV injection at midlevels from a mid-latitude cut-off low or PV coherent structure could help rapid intensifica- 83 tion by increasing convection on the downshear side and sustaining the formation of an eyewall replacement 84 cycle (PV budget analysis). They also noticed that the general 850 to 200 hPa shear was not indicative of rapid 85 intensification and that it would be more appropriate to use the 850 to 500 hPa shear in some cases, which was 86 confirmed in the recent study of Colomb et al. (2019). 87 The impact of troughs or midlatitude baroclinic environments on TC events that did not intensify has not yet 88 been documented in the literature. We want to investigate here if rain enhancement, instead of TC intensifica- 89 tion, might be an outcome of such interactions for a TC making landfall. Environmental interaction will be diag- 90 nosed via PV superposition, eddy angular momentum convergence and eddy PV fluxes. These processes and 91 their associated variables are thoroughly described in the original study of Leroux et al. (2013) that investigates 92 the main pathway to Dora’s intensification under an upper-level trough forcing: “The wave activity of a trough DOI: 10.22499/3.6901.016 93 can be viewed as large-scale eddy transport of angular momentum, heat (Molinari and Vollaro 1989, 1990), and 94 potential vorticity (Molinari et al. 1995, 1998) that may vary in connection with the storm PV structure evolu- 95 tion. Its impact on the mean tangential flow acceleration can be assessed with Eliassen–Palm (E–P) fluxes 96 (Hartmann et al. 1984) computed in a storm-following cylindrical and isentropic framework for an adiabatic 97 frictionless f plane.” The relative eddy angular momentum flux convergence (REAMFC; Molinari and Vollaro 98 1989) is a simpler parameter that can be used to quantify the spin-up of a storm’s tangential circulation on a 99 given pressure level due to asymmetric eddies (from Rossby wave activity for example). Azimuthal eddy flux of 100 angular momentum shows import or inward flux of cyclonic momentum in the upper troposphere within rapid 101 intensification cases – the primary features that produce such fluxes are outflow jets and approaching troughs. 102 The REAMFC is usually computed at 200 hPa over a 300–600-km radial range around the TC centre. An inter- 103 action is generally said to occur when the REAMFC exceeds 10 m s-1 day-1 for at least two consecutive 12-h time 104 periods (DeMaria et al.