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Diagnostics for an Extreme Rain Event near during the Landfall of Fitow (2013)

1 # 1 XUWEI BAO,* NOEL E. DAVIDSON, HUI YU,* MAI C. N. HANKINSON, ZHIAN SUN, 1 @ LAWRENCE J. RIKUS, JIANYONG LIU, ZIFENG YU,* AND DAN WU* * Shanghai Typhoon Institute, Meteorological Administration, Shanghai, China 1 Centre for Australian Weather and Climate Research,& Melbourne, Australia # Department of Mathematical Sciences, Monash University, Melbourne, Australia @ Meteorological Observatory, China Meteorological Administration, Ningbo, China

(Manuscript received 8 July 2014, in final form 13 May 2015)

ABSTRACT

Typhoon Fitow made landfall south of Shanghai, China, on 6 October 2013. During the following two days, 2 precipitation in excess of 300 mm day 1 occurred 400 km to the north of the typhoon center. The rain- producing systems included (i) outward-spiraling rainbands, which developed in the storm’s north sector in favorable environmental wind shear, and (ii) frontal cloud as a result of coastal frontogenesis. Over the rain area, in addition to enhanced ascent, there were increases in low-level moisture, convective instability, and midlevel relative vorticity. There is evidence of a preconditioning period prior to the rain when midlevel subsidence and boundary layer moistening occurred. From analysis of low-level equivalent potential temperature the following observations were made: (i) after landfall, a cold, dry airstream wrapped into Fitow’s circulation from the north, limiting the inner-core rainfall and producing a cold-air boundary, and (ii) an extended warm, moist airstream from the east converged with the cold-air intrusion over the rain area. The heavy rain occurred as the large-scale flow reorganized. Major anticyclones developed over China and the North Pacific. At upper levels, a large-amplitude trough relocated over central China with the entrance to a southwesterly jet positioned near Shanghai. Back trajectories from the rain area indicate that four en- vironmental interactions developed: (i) increasing midlevel injection of moist potential vorticity (PV) from Fitow’s circulation; (ii) low-level warm, moist inflow from the east; (iii) midlevel inflow from nearby Typhoon Danas; and (iv) decreasing mid- to upper-level injection of PV from the midlatitude trough. The authors propose that the resultant PV structure change provided a very favorable environment for the development of rain systems.

1. Introduction distribution and local intensity of rainfall, particularly heavy rain, is very challenging. This difficulty became At landfall, tropical cyclones (TCs) pose significant evident during the heavy rain event following the land- forecast problems because of the potential devastation fall of TC Fitow (2013), when most of the numerical to life and property from strong winds and heavy forecast guidance was poor. rain (Chen 2012). However, they can also provide After TCs make landfall, they normally weaken and much needed widespread precipitation, which supports rainfall correspondingly decreases. For the case ana- industry and farming. The task of forecasting the lyzed here, the TC circulation and inner-core rainfall did rapidly weaken at landfall, but the rainfall some 400 km to the north of the typhoon center rapidly and sub- & The Centre for Australian Weather and Climate Research is a stantially increased. When there are new sources of partnership between the Australian Bureau of Meteorology and CSIRO. energy available to TCs, such as baroclinic potential energy from westerly troughs or latent heat from mon- soon surges, the remnant circulation can revive and even Corresponding author address: Noel E. Davidson, CAWCR, Bureau of Meteorology, P.O. Box 1289, Melbourne, VIC 3001, produce ‘‘rainfall reinforcement associated with land- Australia. falling tropical cyclones’’ (RRLTCs; Dong et al. 2010; E-mail: [email protected] Bosart and Lackmann 1995). There are many studies on

DOI: 10.1175/MWR-D-14-00241.1

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FIG. 1. CPS diagrams based on Hart (2003) for Typhoon Fitow from 1200 UTC 5 Oct to 0600 UTC 7 Oct 2013. CMA best track and ACCESS-G analysis data have been used for the calculations. The final analysis point is the last time that the Fitow low-level, synoptic-scale circulation could be identified. the interaction between TCs and midlatitude baroclinic Choi 2010; Baek et al. 2013; Moore et al. 2013). Fitow environments. They interestingly focus on either the was not an ET event, since there is no evidence that the extratropical transition (ET) of TCs (e.g., Klein et al. circulation transitioned into an extratropical cyclone. 2000; Harr and Elsberry 2000; Jones et al. 2003; Ritchie Figure 1 shows cyclone phase space (CPS) diagrams, and Elsberry 2003, 2007; Anwender et al. 2008) or the based on Hart (2003), for Fitow. They indicate that antecedent rainfall well ahead of TCs (Stohl et al. 2008; Fitow did not undergo ET and retained its warm-core, Srock and Bosart 2009; Galarneau et al. 2010; Lee and symmetric structure until rapid dissipation occurred and

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TC tracking ceased. There is no evidence of a low-level, warm core, and promoted the development of shallow synoptic-scale circulation when tracking ceased (not warm and cold fronts that were prominent during the shown). The CPS diagrams and the dissipation of the landfall phase. This is similar to the Fitow event de- surface circulation also suggest that Fitow did not evolve scribed here. A case study of heavy rain distant from a into a warm seclusion (e.g., Shapiro and Keyser 1990; hurricane’s center was also presented by Srock and Schultz et al. 1998). Rainfall mostly occurred following Bosart (2009), who demonstrated that heavy rain during the landfall of Fitow and so it is not an antecedent rain the landfall of Marco was associated with cold-air event to the storm, but may have been a predecessor rain damming and coastal frontogenesis. Tropical Storm event (PRE; Galarneau et al. 2010) to Danas, which was Marco did not experience ET during this time period. located approximately 800 km to the east of the rain Interestingly, two other TCs were also active in the near event. There are also similarities to the squall lines pre- vicinity, which the authors claim to have had an effect on ceding landfalling tropical cyclones described in Meng the rain event. For the rain event considered here, Fitow and Zhang (2012). Indeed, although the rainbands here also did not experience ET, TC Danas was located to its may not have been squall lines, the environmental con- east, and there is evidence presented in section 7 that ditions for the Fitow event are somewhat similar to those Danas may have acted as a source of moisture and po- documented in Meng and Zhang (2012). There are thus tential vorticity for the rain systems. The case of TC few studies on heavy rain events like that of Fitow, which Fitow thus bears some similarities to these previous are remote from the typhoon center, associated with a studies, but contains some important differences and postlandfall TC, occur in combination with the intrusion additions that we will discuss. A major difference is that of cold air from the north, and with another TC located the extreme rainfall occurred in the vicinity of, but some well to the east. These unique aspects of the Fitow rain 400 km away from, the center of landfalling TC Fitow, event are analyzed in this study. In particular, we are and was associated with (i) propagating outer rainbands interested in the influence of the evolving large-scale flow from that storm and (ii) a cold-air intrusion from the and the roles of Fitow and Danas during the heavy rain. north. These unique aspects, including the impact of Although most TC rainfall may be associated with the Fitow on the event, are discussed in the manuscript. landfall of the inner circulation, extreme rain can some- Galarneau et al. (2013) present evidence that the in- times occur at large radial distance from the center. tensification of Hurricane Sandy (2012) occurred as Galarneau et al. (2010) define PREs as ‘‘coherent meso- ‘‘cold continental air encircled the warm core vortex’’ scale regions of heavy rainfall, with rainfall rates $100 mm and ‘‘occurred in response to shallow low-level conver- 2 (24 h) 1, that can occur approximately 1000 km poleward gence below 850 hPa that was consistent with the of recurving tropical cyclones.’’ Cote (2007) and Bosart Sawyer–Eliassen solution for the secondary circulation et al. (2012) document a number of prelandfall cases where that accompanied the increased baroclinicity in the ra- heavy rain occurred at some distance from storm centers. dial direction.’’ This scenario is also somewhat similar to According to these studies, PREs were typically located the processes we describe for the heavy rain associated ;1000 km ahead of the parent tropical system, occur with postlandfall Fitow, even though, unlike Sandy, it 1–2 days prior to the arrival of the TC, and last for ;12 h. A remained warm-core symmetric (Fig. 1), and the rain mid- and upper-level jet-entrance-region confluence zone, occurred well to the north of Fitow’s center and fol- well downstream of an approaching TC, was a favored lowing its dramatic weakening. Based on numerical synoptic location for development. The orientation of sensitivity simulations, Niu et al. (2005) also suggested midlatitude troughs and ridges lying poleward of TCs that when cold air encircled a TC making landfall in modulated whether, where, and when a PRE formed, while eastern China, the precipitation to the north of the TC mesoscale surface boundaries acted as a focus for PRE would be enhanced via frontogenesis associated with the heavy rainfall (Gao et al. 2009; Galarneau et al. 2010; Dong interaction between cold air from the north and warm et al. 2010; Lee and Choi 2010; Schumacher et al. 2011; air from the TC circulation, producing an inverted ty- Baek et al. 2013; Milrad et al. 2013, Moore et al. 2013). phoon trough pattern. In addition, if part of the cold air These studies implied that the upstream TC is an important wrapped into the typhoon’s inner core, the inner con- source of moisture for PREs. vection would be suppressed. Similarities between the Evidence of coastal frontogenesis as a major process study here and previous work include that the heavy with respect to enhanced precipitation was first de- rain event seems to have been a result of an in- scribed in Bosart et al. (1972). Knupp et al. (2006) teraction, whatever that might be, between Fitow and documented the case of Hurricane Gabrielle when east- an evolving and favorable environmental flow, with northeasterly surface flow transported cool air off the (i) an upper-level jet entrance region very near the rain west coast of Florida, toward Gabrielle’s convergent area, (ii) a tropical storm to the east of Fitow, and

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(iii) coastal frontogenesis with its associated secondary (TRMM) rainfall estimates (Joyce et al. 2004; Huffman circulation. In addition we will discuss the role that et al. 2007; Ebert et al. 2007). Objective analyses are Fitow played in the event, even though the rain oc- obtained from the ECMWF interim reanalysis (ERA- curred well to the north of its circulation center and it Interim; Dee et al. 2011) at 1.58 resolution and the was dissipating as the rain developed. Australian Community Climate and Earth System In the following sections we will expand upon these Simulator–Global (ACCESS-G; Puri et al. 2013) at 0.58 issues, introduce some additional synoptic- and meso- resolution. Operational, high-resolution forecasts are scale aspects, analyze some back trajectories from the derived from ACCESS-TC (Davidson et al. 2014). rain area to evaluate the influence of environmental flow Rainfall data from the 3-hourly, 0.25830.258 CMORPH changes and neighboring weather systems (in particular are used in this study. Comparisons of CMORPH and re- TCs), and discuss the relevance of these to the pro- analysis datasets has been carried out by Ruane and Roads duction of heavy rain. The major objectives of this study (2007), Ebert et al. (2007),andSapiano and Arkin (2009). are to (i) document the origin and evolution of the An evaluation of CMORPH data during mesoscale systems that produced the heavy rain and events can be found in Yu et al. (2009). Assessment of (ii) provide evidence that the large-scale circulation TRMM/TMPA data for tropical cyclone events is provided changes influenced the efficiency of the rain systems by by Chen et al. (2013a,b). The above studies show that both producing a highly favorable thermodynamic and kine- TMPA 3B42 and CMORPH are suitable for rainfall matic environment. An overview of the TC Fitow event analyses of TCs as they are highly correlated, with generally is provided in Yu et al. (2014), who describe details of low biases when compared against observations, even the track, intensity, and rainfall. though they both tend to underestimate heavy rain. The manuscript is organized as follows. Section 2 de- ACCESS-TC was developed for operational and re- scribes the sources of data used in the study. Section 3 search applications on tropical cyclones. For the appli- documents observational aspects of the rainfall and the cation here, after verification, operational forecasts mesoscale rain systems that produced the heavy rain. from the system for Fitow are used for model-based Section 4 illustrates the evolving synoptic-scale flow diagnostics. A detailed description of the ACCESS-TC during the event, including the evolution of the envi- system is contained in Davidson et al. (2014). The base ronmental wind shear and frontogenesis processes. system runs at a resolution of 0.118 across 50 levels. The Section 5 describes the use of mesoscale observations to domain is relocatable and nested in coarser-resolution (i) document the mesoscale characteristics of the rain ACCESS forecasts. Initialization consists of five cycles systems and (ii) verify high-resolution forecasts. Section 6 of 4DVAR assimilation over 24 h prior to the initial time illustrates the thermodynamic and kinematic changes and forecasts to 72 h are made. Without vortex specifi- that occurred over the local rain area, based on validated cation, initial conditions usually contain a weak and high-resolution forecasts. Section 7 shows results from misplaced circulation. Significant effort has been de- back-trajectory calculations, used to illustrate possible voted to building physically based, synthetic inner-core interactions with the storm’s environment and surround- structures, validated using historical dropsonde data and ing weather systems. Section 8 offers the summary and our surface analyses from the Atlantic. Based on estimates conclusions. of central pressure and storm size, vortex specification is used to filter the analyzed circulation from the original analysis, construct the inner core of the storm, locate it 2. Data sources to the observed position, and merge it with the large- In this study, extensive use is made of high-density scale analysis at outer radii. local and international datasets for both describing ob- Using all available conventional observations and served characteristics of the rain event, and for verifi- only synthetic surface pressure observations from the cation of operational, high-resolution forecasts. The idealized vortex to correct the initial location and observational datasets include high-resolution surface structure of the storm, the 4DVAR builds a balanced, and rain gauge observations over eastern China; com- intense 3D vortex with a well-developed boundary layer posite radar reflectivities from the observation sites at and secondary circulation. Mean track and intensity Shanghai, Ningbo, , and Hangzhou (see Fig. 2 errors for the Australian region and northwest Pacific for locations of Hangzhou Bay and the radar sites, rel- storms have been encouraging, as are results from the ative to the region of heavy rain); Multifunctional operational model running at the Australian National Transport Satellite (MTSAT) infrared imagery; and Meteorological and Oceanographic Centre. The system Climate Prediction Center (CPC) morphing technique became fully operational in November 2011 after ex- (CMORPH) Tropical Rainfall Measuring Mission tensive verification (Davidson et al. 2014).

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FIG. 2. Observed composite radar reflectivity every 3 h from (a) 1200 UTC 6 Oct to (n) 0300 UTC 8 Oct 2013. RBs 1, 2, and 3 indicate rainbands associated with heavy rain. The locations of the four radar stations (Wenzhou, WZ; Ningbo, NB; Hangzhou, HZ; and Shanghai, SH) are shown in (a).

The back trajectories shown here are calculated using during Fitow’s landfall. The locations of these radar sites the HYSPLIT Lagrangian trajectory system developed and other relevant place names are shown in Fig. 2 (and at NOAA (Draxler and Hess 1998) and implemented later in Fig. 11). The radars’ field of view covers the at the Australian Bureau of Meteorology (http:// entire domain plotted in Fig. 2. Fitow’s landfall occurred www.wmo.int/pages/prog/www/DPS/WMOTDNO778/ near 1800 UTC 6 October 2013. Figure 2 illustrates the rsmc-melbourne-a.htm). The trajectories are based development of the rainbands (RB1, RB2, and RB3) upon ERA-Interim analyses every 6 h. that correspond with the outer rainbands to Fitow’s north that produced the heavy rainfall, as well as the inner-core convection. Note how RB2 intensifies as it 3. Observational aspects of rainfall moves to the north. RB3 was very active at landfall but then rapidly weakened within 12 h. The reflectivities Figure 2 shows radar reflectivities, composited from also suggest the development of a frontal-like rainband, radars at WenZhou, Ningbo, Hangzhou, and Shanghai which appeared to evolve from a secondary outer

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FIG. 3. The 3-hourly satellite rainfall estimates from CMORPH (mm; contour interval is 1 mm) from (a) 0600 UTC 6 Oct to (p) 0300 UTC 8 Oct 2013. rainband (RB2; Figs. 2b–g) and was transformed into a have been three mesoscale rain systems that affected coastal rainband (RB2; Figs. 2i–m) as the inner-core the Hangzhou Bay area, just south of Shanghai near cloud (RB3) rapidly weakened. That is, this band 308N, 1218E. An initial outer rainband (RB1 in Fig. 2) seemed to commence as an outer rainband from the TC developed and propagated northwestward across that transforms and develops as the environment in the Hangzhou Bay area during the period 0600– which it is embedded evolves. Later we will offer some 1500 UTC 6 October (top panels in Fig. 2). A second, physically based speculation on the reasons for the de- more active, outer rainband (RB2) developed a few velopment and northward propagation of the rainbands. hours later to the south of the first band, propagated to Figure 3 displays 3-hourly CMORPH rainfall estimates the north, and intensified. It became quasi stationary from 0600 UTC 6 October to 0300 UTC 8 October 2013. near northern , just south of Shanghai. RB2 Although there may be some underestimation of rainfall then appeared to rotate into a mostly north–south- from CMORPH over land, we have used these estimates orientated line of rainfall, through Shanghai by 0300 UTC mostly to assess rainfall over the ocean, where the esti- 7 October. This rainband fluctuated in strength but mates are reasonably reliable (Chen et al. 2013b). These extended southward and had a peak in the vicinity of independent data support the description of the cloud Shanghai and northern Zhejiang around 1800 UTC features indicated from the radar data. There appear to 7 October. Inner-core rainfall (RB3) was very active at

Unauthenticated | Downloaded 10/11/21 11:17 AM UTC SEPTEMBER 2015 B A O E T A L . 3383 landfall but rapidly weakened and dissipated within and Fitow circulations. Note as well how Fitow’s 9h(Figs. 2d–g). midlevel circulation appears to be undergoing cap- Note that the two outer rainbands strengthened as ture by the midlatitude trough to the north as they moved northward. They became mostly separated northwest-to-westerly flows wrap around the Fitow from the parent Fitow circulation, intensified, and circulation. We will illustrate that, facilitated by the seemed to take on lives of their own. So what processes evolving environment, the TC systems during this maintained and enhanced the rain activity from these capture may have provided a source of midlevel cloud bands? The inner-core rainfall associated with potential vorticity and moisture for the rain system. Fitow was active at landfall, but very rapidly decreased 5) At upper levels, the large-amplitude trough, while in the 9 h between 1800 UTC 6 October and 0300 UTC progressive at high latitudes appeared to retrogress at 7 October. RB1 appeared to have some charac- lower latitudes from just east of China to be located teristics of a warm front, while RB2 seemed to evolve over central China. There was also an intensification into coastal frontal cloud. Similar characteristics have and westward extension of the mid- to upper- been described previously for transitioning storms by tropospheric, geopotential ridge toward the coast of Harr and Elsberry (2000), Jones et al. (2003), and China. This change repositioned the entrance to a Kitabatake (2008). This pattern of evolution is consis- southwesterly jet to near Hangzhou Bay. tent with the environmental flow changes, described in 6) The changes in the synoptic-scale flows produced a section 4, which favored a region of high rainfall near the change in the vertical structure of the horizontal Hangzhou Bay area (low-level moistening and large- wind field over the rain area. A situation favorable scale ascent), the intrusion of cold, dry air from the north for heavy rain of low-level easterlies overlain by into Fitow’s inner core, and the establishment of a upper-level southwesterlies, suggesting moist isen- frontal boundary separating cold, dry air from the north tropic ascent (or warm air advection), had developed and warm, moist air from the east. In section 5b we will near Hangzhou Bay and likely played a role in discuss observed and forecast rainfall amounts at spe- helping to force ascent. cific observing sites around Hangzhou Bay. These rain- Based on an analysis of the synoptic-scale flow fall observations also support the categories of rain changes, a favorable situation for very heavy rain was systems discussed above. developing in the vicinity of Hangzhou Bay. The precise location of the rainfall and the mesoscale circulations, 4. Synoptic-scale flow changes discussed above, that would produce the rainfall are considered in more detail in the following sections. a. Flow fields b. Environmental vertical wind shear The remarkable changes in the synoptic-scale flows that occurred prior to and during the heavy rain event Wind shear can influence the location of convective are illustrated in Fig. 4. From top to bottom the panels in asymmetries in TCs (Frank and Ritchie 1999; Corbosiero Fig. 4 show 200- and 500-hPa wind fields, and the mean and Molinari 2002), as well as the organization of sea level pressure (MSLP) analysis, at 0000 UTC 3 mesoscale convective systems (e.g., Houze 2004 and October (Figs. 4a–c, well prior to the heavy rain), references therein). We have calculated wind shear 0000 UTC 5 October (Figs. 4d–f, just prior to the heavy relative to the Fitow circulation by computing the rain), and at 1800 UTC 6 October 2013 (Figs. 4g–i, just at mean wind at various pressure levels over a 300–500-km the landfall of Fitow and the start of the heavy rain). The annulus centered on the storm’s location. We have following main points are noted: made these calculations based on ACCESS-G analyses 1) There was a significant reorganization of the large- and the ACCESS-TC forecast. Figure 5 shows a time– scale flow over a very broad area. height series of mean environmental wind calculated 2) Between the first and third times illustrated, major in this way from these datasets. Note that the opera- surface anticyclones developed over China and the tional track for Fitow ended, with a central pressure of northwest Pacific with a long, over-the-sea trajectory of 1000 hPa, at 0600 UTC 7 October and thus this is the low-level easterly winds flowing toward the China coast. final time in the cross section. Wind shear can be 3) TC Danas developed during the period and was readily inferred from Fig. 5. Comparison between ob- located in relatively close proximity to Fitow, just served and forecast environmental winds that were about 1500 km to its east-southeast. directly influencing the Fitow circulation suggests that 4) Through the midlevels, deep easterly flow is clearly ACCESS-TC was forecasting the near environment of evident and eventually seems to connect the Danas the storm quite well, although the turning of the winds

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FIG. 4. For 0000 UTC 3 Oct 2013, (a) the 200-hPa geopotential height (contour interval is 100 m) and horizontal wind (full barb is 2 2 10 m s 1, shading denotes wind speed with a contour interval of 10 m s 1); (b) as in (a), but at 500-hPa geopotential height with a contour 2 interval of 30 m and wind speed with a contour interval of 5 m s 1; and (c) MSLP (contour interval is 2 hPa). (d)–(f),(g)–(i) As in (a)–(c), but at 0000 UTC 5 Oct and 1800 UTC 6 Oct 2013, respectively. with height is confined to a shallower layer in the fore- as they moved into a location favorable for large-scale cast. An interesting aspect in the figures is the trend ascent. We thus suggest that the evolution of Fitow’s toward southwesterly wind shear just prior to and during environment favored rainband development to its north landfall. This can be seen as upper-level southerlies and this played a major role in the rain event. The overlay low-level easterlies as the storm approaches presence of Fitow in a weakly sheared environment landfall along the coast of China. The shear is not large, generated the outer rainbands that were to eventually 2 being of the order of 10 m s 1, and so should not be too produce some of the observed heavy rain. Thus, even detrimental to the storm’s intensity (Frank and Ritchie though heavy rainfall occurred well to the north of 1999), thus indicating it should not be a major factor in Fitow’s center, its presence in favorable vertical shear Fitow’s rapid weakening after landfall. However, the was critical to the development of the rainbands that shear would have had the effect of favoring convective were to eventually produce the heavy rain. development over the north-to-northwest sector of the c. Environmental conditions over the heavy rain area storm just prior to landfall. As can be seen from Figs. 2 and 3, multiple outer rainbands were observed to de- Figure 6 shows time–height series over a 18 latitude– velop over this sector. Some of these intensified as they longitude box centered over the heavy rain area, of propagated northward and contributed to the heavy rain mean wind, vertical velocity, and potential vorticity

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21 FIG. 5. Time–height plot of the environmental wind (full barb is 10 m s ) from 1200 UTC 5 Oct to 0600 UTC 7 Oct 2013, which is defined by the average wind inside an annulus between 300 and 500 km from typhoon center. The filled triangle denotes the time of landfall. (a) ACCESS-G analysis data (time interval is 6 h). (b) ACCESS-TC forecast data (time interval is 3 h) from base time (1200 UTC 5 Oct 2013).

(PV) from ACCESS-G analyses (6-h intervals) as well as stronger southerly winds in the forecasts. At mid- to from the ACCESS-TC forecast (3-h intervals). Again, upper levels high PV existed over the region and in- the comparison between the evolution of these analyzed creased to a maximum at the time of maximum ascent. and forecast fields is quite consistent. There is clearly The vertical structure of the PV and ascent fields, with correspondence between the timing of the ascent max- maxima through midlevels, suggests that neither vertical ima in the analyzed and forecast fields. Note that the advection nor horizontal convergence were dominant ascent maxima coincide with the timing of the low-level processes in enhancing the midlevel vorticity. The evi- winds turning to the east and the upper-level winds dence suggests that horizontal advection was playing a turning to the southwest (i.e., anticyclonic turning with significant role in increasing the midlevel cyclonic vor- height). However, there is a much deeper layer of ticity over the rain area.

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21 FIG. 6. Time–height series of area-averaged wind (full barb is 10 m s ), vertical velocity 2 2 (#20.05 Pa s 1; contour interval is 0.05 Pa s 1), and PV (shading; PVU) over the heavy rainfall area (29.58–30.58N, 120.58–121.58E) from 1200 UTC 5 Oct to 1200 UTC 8 Oct 2013. (a) ACCESS-G analysis data (time interval is 6 h). (b) ACCESS-TC forecast data (time interval is 3 h) from base time (1200 UTC 5 Oct 2013).

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21 21 21 21 FIG. 7. Frontogenesis function at 975 hPa [$0.2 K h (100 km) , contour interval is 0.2 K h (100 km) ], 10-m winds (full barb is 2 10 m s 1), and 850-hPa PV (shading; PVU) from ACCESS-G analysis data at (a) 1800 UTC 6 Oct, (b) 0000 and (c) 1200 UTC 7 Oct, and (d) 0000 UTC 8 Oct 2013. d. Frontogenesis processes by the presence of moist convection, the previous dis- cussion suggests that the environmental flow changes Examination of the environmental fields suggests that were also playing an important role in (i) the estab- frontogenesis with its associated secondary circulation lishment of the large potential temperature gradients was active at times during the rain event. Following and (ii) low-level convergence. Further evidence of this Srock and Bosart (2009), we have quantified this process is provided in Fig. 8, which shows the evolution of po- using the 2D frontogenesis function: tential temperature and wind via west–east and south–   north cross sections through the rain area (;30.58N, › ›y › › 5 1 u 1 j$ uj $ 5 1 121.58E). A preexisting potential temperature gradient Fdiv h , where h , 2 ›x ›y ›x ›y existed over the rain area near the time of landfall of TC Fitow. But during the following 24 h this gradient where u and y are zonal and meridional wind compo- concentrated over the rain area in both the zonal and nents and u is potential temperature. Note that the meridional directions. As pointed out by a reviewer, function is the product of horizontal mass convergence there was a buckling in the isentropes around 1188– and the magnitude of the horizontal potential tem- 1208E and this development of warm- and cold-air perature gradient. From ACCESS-G, Fig. 7 shows an- boundaries was a sign of the warm-air advection from alyzed fields of the frontogenesis function at selected the east, and cold-air advection from the north. Evi- times during Fitow’s landfall and the rain event. Note dence thus indicates that frontogenesis was a contrib- that large values of the function are diagnosed over the uting factor to the heavy rain, particularly for the heavy rain area and not elsewhere. Notwithstanding evolution of RB2. The process is very similar to that the fact that horizontal convergence may be influenced originally documented in Bosart et al. (1972).

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FIG. 8. (a)–(d) Zonal height cross sections along 308N (1218E) of potential temperature 2 (contour interval is 1 K) and horizontal wind (full barb is 10 m s 1), from ACCESS-G analysis data. (e)–(h) As in (a)–(d), but for meridional height cross sections. The times chosen are the same as in Fig. 7 (from top to bottom): 1800 UTC 6 Oct (landfall time), 0000 and 1200 UTC 7 Oct, and 0000 UTC 8 Oct 2013. The vertical lines mark the region of heavy rain (288–328N, 1198–1238E; see Fig. 3).

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5. Mesoscale features and verification of We additionally note that observed or forecast heavy ACCESS-TC rainfall was located on the windward slope of the terrain (Figs. 10c,f,i) in the low-level northeast and easterly flow In this section high-resolution observational datasets (Fig. 7). Also evident is the developing frontal boundary are used to further describe the mesoscale characteris- along the coast with convergence between northerlies tics of the rain event and to verify forecasts from over land and easterlies over the ocean. The coastal ACCESS-TC. front and surface convergence are collocated with the a. Verification of track and intensity forecast region of heaviest rainfall (RB2). It is quite possible that upslope flow in the developing and persistent onshore Figure 9 shows observed and 72-h forecast tracks and easterly winds acted to release convective instability to intensities from operational ACCESS-TC for Fitow and maintain the intense rainfall. Effects of the underlying Danas from base time 1200 UTC 5 October 2013. The terrain on rainfall maintenance and even reinforcement forecasts clearly show very useful skill, with track errors associated with the circulation of landfalling tropical for Fitow ranging from about 10 km at t 5 0 h to around cyclones can be found in Niu et al. (2005) and Dong et al. 150 km at t 5 36 h. Forecasts of intensity for Fitow are (2010). It is difficult to separate the effects of orographic also encouraging, with the observed and forecast spin- uplift and environmental influences on the rainfall. The down after landfall being quite consistent, even though analysis conducted here suggests a modulation of the the forecast timing of the landfall is in error by about 4 h. rainfall by the topography in an evolving environment For Danas the forecast is less skillful. However, we note highly conducive to heavy rainfall. It may be of interest that this system actually entered the ACCESS-TC do- to conduct some experiments on the sensitivity to model main through a lateral boundary and thus its location topography. and intensity on entry can be seen to have quite Detailed verification of the timing and intensity of large errors. rainfall is illustrated in Fig. 11, which shows 3-hourly Although the forecasts of Fitow’s track and intensity observed and forecast accumulated rainfall amounts are encouraging, this does not guarantee that its evolv- for six surface observing stations. The top panel in ing structure and rainfall have also been forecast well Fig. 11 showsthelocationsofcitiesusedfortheveri- (e.g., Davidson and Ma 2012). We demonstrate below fication. Of course, this point comparison represents a that the rainfall prediction from this forecast also ver- very difficult form of verification, but the forecast does ifies quite well and so we are confident that the forecast quite well. The first heavy rain episode occurred be- can be used to obtain detailed diagnostics to document tween about 1200 UTC 6 October and 0300 UTC and understand the kinematic and thermodynamic 7 October and was quite well predicted by the model processes over the rain area. (cf. black and gray banners). We suggest that this rain corresponded with the northward-propagating rain- b. Verification against rain gauge observations bands described earlier. The impact of these rainbands Figures 10a–c show analyzed, accumulated rainfall is evident mostly at Wenlin, Ningbo, , and from the dense rain gauge network over eastern China. Hangzhou, which are all located farther south than Figures 10d–f show the corresponding forecast rainfall Jiaxing and Shanghai. As can be seen in Figs. 2 and 3, from the 1200 UTC 5 October ACCESS-TC forecast. these stations were in the direct path of the outer Although there are clearly errors in the forecast, rainbands as they propagated northwest and in- particularly at the mesoscale, the forecasts show en- tensified. Rainfall from these rain systems at locations couraging skill in rainfall prediction for these time just slightly farther north was much less, but not in- scales. Comparison of the 2-day observed and forecast significant. A separate, local heavy rain event appears accumulations, which of course eliminates some of the to occur between about 1800 UTC 7 October and timing errors in the forecast rainfall, is particularly en- 0300 UTC 8 October, 1 day after landfall. This rain was couraging. There is perhaps some indication of the me- associated with the north–south-orientated, coastal soscale rain features described earlier even in these rainband that developed a little later (as shown by RB2 diagrams. The inner-core rainfall (RB3 in Fig. 2), the as it shifted from a northwest–southeast orientation first rainband (RB1 in Fig. 2), the second rainband to a north–south direction). Based on the evidence (RB2), and the later coastal rainfall (southern part of presented in section 4d of the presence of active RB2 in Fig. 2) are indicated in these forecast accumu- frontogenetic processes, we suggest that coastal lations by the large values initially around Hangzhou frontogenesis (e.g., Bosart et al. 1972) played an im- Bay and the development of large values along the coast portant role in this rainfall. The rain feature is very at a slightly later time. distinct from other previously documented rainfall

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FIG. 9. Operational tracks and intensities issued by CMA (black lines) and forecasted by ACCESS-TC (red lines) for Fitow and Danas. (a) Tracks, with filled circles (open squares) denoting Fitow’s (Danas’s) positions every 6 h from 1200 UTC 5 Oct to 1200 UTC 8 Oct 2013 (Note that Fitow’s track report ended at 0600 UTC 7 Oct 2013). (b) Comparison of Fitow’s MSLP (solid line) and maximum surface wind (dashed line). (c) As in (b), but for Danas.

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FIG. 10. Observed rainfall accumulated from (a) 0000 UTC 6 Oct to 0000 UTC 7 Oct (24 h), (b) 0000 UTC 7 Oct to 0000 UTC 8 Oct (24 h), and (c) 0000 UTC 6 Oct to 0000 UTC 8 Oct 2013 (48 h). (d)–(f) As in (a)–(c), but forecasted by ACCESS-TC. The black (red) line (or marker) denotes the operational track from CMA (ACCESS-TC), and the terrain height ($100 m; contour interval is 200 m) is combined in (f). The terrain height (shading; m) and horizontal wind vectors at 10-m height from ACCESS-TC forecasts at (g) 1800 UTC 6 Oct (landfall time), (h) 0600 UTC 7 Oct (end of TC tracking), and (i) 1800 UTC 7 Oct 2013. types like the inverted typhoon trough discussed by c. Verification against satellite imagery and Niu et al. (2005). The forecast does not predict this CMORPH data point rainfall at this time particularlywell,butwenote that the forecast had small timing and location errors, Figure 12 shows a comparison between synthetic which caused poor verification based on this metric. infrared satellite cloud imagery from ACCESS-TC The forecast did produce rain at this time, but it was and actual infrared imagery from MTSAT at 1500 UTC not quite heavy enough and occurred at a place and 5 October, 1800 UTC 6 October, and 0300 and 1200 UTC time that were slightly different than the observed. It is 7 October. Base time of the ACCESS-TC forecast is of interest though that the model did still try to gen- 1200 UTC 5 October 2013. The synthetic imagery sys- erate some coastal rainfall during the period (see tem is based on Sun and Rikus (2004). This diagram Figs. 10c,f and 13). illustrates large-scale aspects of the evolving observed

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FIG. 11. The 3-hourly accumulated rainfall (mm) recorded by six surface stations from 1500 UTC 6 Oct to 0300 UTC 8 Oct 2013. The black (gray) banners denote 3-h observed (forecasted) rainfall by rain gauges (by ACCESS-TC). The locations of the six surface stations are shown as red triangles in the top panel. The stations are Wenlin, WL; Ningbo, NB; Yuyao, YY; Hangzhou, HZ; Jiaxing, JX; and Shanghai, SH. Hangzhou Bay separates SH to the north and NB to the south.

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FIG. 12. Synthetic infrared satellite cloud imagery from ACCESS-TC and actual infrared imagery from MTSAT at 1500 UTC 5 Oct, 1800 UTC 6 Oct, and 0300 and 1200 UTC 7 Oct 2013. Base time of ACCESS-TC forecast is 1200 UTC 5 Oct 2013. and forecast cloud fields in which the mesoscale rain sys- shows estimated CMORPH rainfall at 3-h intervals, Fig. 13 tems were embedded. There is clearly encouraging co- shows 3-hourly forecast rainfall accumulations from the herence between the synthetic and actual imagery. A ACCESS-TC forecast. Interesting aspects of the forecast, striking feature of the imagery is the developing extra- which bear many similarities to the observed rainfall de- tropical interaction, best indicated here by the cloud scribed earlier, include the following points: streaming out of the storm in an extended outflow channel (i) Rainfall asymmetries develop to the north and andinassociationwithasouthwesterly jet streak, de- northeast of Fitow, consistent with the diagnosed scribed in section 4. Such midlatitude trough interactions environmental southwesterly shear acting on the and outflow channels are regularly associated with in- storm’s circulation (Figs. 13a–d). tensifying or transitioning TCs (e.g., Chen and Gray 1985; (ii) There is development of multiple active outer Klein et al. 2000) and may also occur with other heavy rain rainbands in the northeast sector of the Fitow events, like PREs (Cote 2007; Dong et al. 2010). So why circulation that propagate north or northwestward did Fitow not undergo ET? As indicated by Ritchie and (Figs. 13e–h). Elsberry (2007) and Atallah et al. (2007), the upper trough (iii) There is a very rapid decrease in inner-core rainfall may have been too distant from Fitow for it to influence an after landfall (Figs. 13e–l). ET. The rapid decay of the inner-core rainfall is also evi- (iv) There is evidence of coastal rainfall, although not dent, as is the presence and persistence of cold clouds over as intense as observed, along the coast of Zhejiang the rain area near 308N, 1208E. The forecast for Danas is province, south of Hangzhou Bay (Figs. 13i–p). not particularly skillful, since the forecast is slow in moving the storm to the north. Its intensity, as suggested by the The operational forecast provided useful, but far forming in the actual imagery, is also not well forecast. from perfect guidance on the impending heavy rain It is also interesting to study the evolution of the forecast event. In subsequent sections we will use the fore- rainfall at higher time resolutions. Similar to Fig. 3,which cast to further explore possible dynamical and

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FIG. 13. (a)–(p) The 3-hourly forecasted rainfall (color shading; mm) by ACCESS-TC at base time (1200 UTC 5 Oct 2013), from 0600 UTC 6 Oct to 0300 UTC 8 Oct 2013. thermodynamic reasons for the heavy rain and ACCESS-TC at various times during the rain event. The Fitow’s rapid dissipation. RMS errors at each analysis time are shown in Fig. 14g. The diagram and the quantitative verification indicate d. Verification against surface observations that there are reasonable comparisons to be made be- Boundary layer wind and moisture are critical to the tween the observed and predicted winds and dewpoint development of convection and thus we have used these temperatures. The mean RMS errors over the forecast for 2 variables as metrics of forecast quality. Figure 14 vector wind and dewpoint are about 5 m s 1 and 1.5 K, shows a comparison between observed winds from the respectively. These errors are only marginally larger than surface observing network and forecast 10-m winds from the observational measurement and scale-dependent

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FIG. 14. (a)–(f) Observed winds from surface stations (blue 2 wind barb; full barb is 10 m s 1) and dewpoint temperature (blue; K), and ACCESS-TC-forecasted 10-m wind (red wind barb) and surface dewpoint temperature (red value) every 6 h from 1200 UTC 6 Oct to 1800 UTC 7 Oct 2013. (g) The RMSE of surface wind and RMSE of dewpoint temperature are indicated by solid (dashed) lines from 1500 UTC 5 Oct to 1200 UTC 8 Oct 2013. The maximum, minimum, and mean errors are given at the bottom.

errors. This quite good comparison, together with other the rain event and so we now use the forecast to analyze verification, suggests that the model is representing the possible thermodynamic and kinematic changes changes in the thermodynamics and kinematics of the within the local rain area. Note that because of its rel- real atmosphere over the rain area reasonably well. atively high time and space resolution, more realistic ascent field, and the implied dynamical balance inherent in the forecast model, we believe that use of the model 6. Thermodynamic and kinematic changes over the output adds value to that available in global analyses. local rain area Critical to the evolution of an efficient rain system are The verification above indicates that the ACCESS-TC the availability of moisture, the production and main- forecast may be representing many observed aspects of tenance of convective instability (here monitored using

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21 FIG. 15. (a)–(i) ACCESS-TC 975-hPa equivalent potential temperature (color shading) and winds (full barb is 10 m s ), and 3-hourly accumulated rainfall (green; contour interval is 20 mm) every 6 h from 0600 UTC 6 Oct to 0600 UTC 8 Oct 2013. the vertical gradient in equivalent potential tempera- discussed above, the 3-hourly accumulated rainfall shows ture), vertical wind shear, the presence of conditions many of the observed features of the rainfall. These favorable for large-scale ascent, and possibly inertial features include the model’s representation of (i) the stability (vorticity) (since this may influence the ease inner-core rainfall and (ii) the active, northward- with which horizontal flows can transport moisture to propagating rainbands during landfall. There are some the rain area). An ingredients-approach based on lift, indications of the frontal rainband (Fig. 15e), but the model moisture, and instability is discussed in, for example, has underforecast the rainfall associated with this feature. Doswell et al. (1996). Changes in some of these pa- High theta-e air begins to enter Hangzhou Bay and sur- rameters have been discussed earlier in this article. rounding areas around 1200 UTC 6 October as southeast- Figure 15 shows forecasts every 6 h leading up to the to-northeast winds (Fig. 4) enter the region and start to heavy rain event of equivalent potential temperature converge. From this time, the outer rainbands begin to and wind at 975 hPa obtained from the ACCESS-TC form. Low theta-e air, initially located to the west of forecast from base time 1200 UTC 5 October 2013. As Shanghai and TC Fitow, begins to wrap into the

Unauthenticated | Downloaded 10/11/21 11:17 AM UTC SEPTEMBER 2015 B A O E T A L . 3397 circulation at landfall and the inner-core rainfall and 90%. The rapid moistening in the lead up to the event circulation rapidly spin down as this occurs. At the same indicates the presence of large convective instability. time, a boundary between low and high theta-e air is 6) Low-level vorticity within the rain area also begins to established near the coast, with high theta-e air in the increase prior to the heavy rain. Like convergence, it easterly flow over the ocean and low theta-e air in the deepens during the actual rain event and then again wraparound flow from the north. These conditions are mostly becomes confined to low levels after the rain associated with a combination of warm (cold) and moist subsides. As above, it remains unclear if this is (dry) advection from the east (north) and are very sug- related to the presence of rain clouds and their gestive of frontogenetic processes. heating profiles or are some aspect of the environmen- These characteristics bear similarities to classical ET of tal influence. We currently speculate that it may be at TCs (e.g., Klein et al. 2000). However, for TC Fitow and least partially influenced by the PV changes associated based on Fig. 15, there is no evidence that the circulation with environmental interactions (see section 7). transitioned into an extratropical cyclone. To examine the 7) The major differences between the failed and success- structural changes over the heavy rain area, Fig. 16 shows ful rain forecasts are (i) understandably, the ascent time–height series, over a 28328 boxcenteredontherain field over the rain area and (ii) the much deeper and area, of vertical motion (omega), equivalent potential stronger vorticity tower that exists in the successful temperature, relative vorticity, and relative humidity from forecast between 1200 UTC 6 October and 1200 UTC the ACCESS-TC forecast, base times 1200 UTC 4 and 7 October. These two differences may be connected, 5 October 2013 (Figs. 16a–c and Figs. 16d–f, respectively). but we speculate that the vorticity tower may be These base times are chosen because although they show related to the injection of moist PV from Fitow many common characteristics, the earlier forecast did not (section 7), which could alter the vertical PV structure predict the heavy rain as well as the later forecast, and it is of the rain system and thus the ascent field. of interest to try and find the differences in the two fore- casts as a means of isolating some critical aspects of the An important aspect of the rain systems is that they heavy rain event. The following main points are noted: developed as intensifying outer rainbands within the Fitow circulation. The development and northward 1) Both forecasts predict ascent at approximately the propagation of the outer rainbands require more study right time, although the later forecast has stronger and will be the subject of ongoing work. Our preliminary and more enduring ascent. diagnostics suggest that they formed to the northeast of 2) Low-level moistening occurs prior to the heavy Fitow’s center in environmental southwesterly wind rain and this coincides with diagnosed large-scale shear, which would favor convective development over upper- to midlevel subsidence. We suggest that this is this sector. The reasons for the outward propagation still an important preconditioning phase that temporarily remain unclear and will be the subject of ongoing study. inhibits the development of deep convection, while allowing the boundary layer to moisten for the later 7. Back trajectories from the rain area heavy rain. 3) Large increases in low-level theta-e occur prior Changes in the vertical structure of a weather system to the heavy rain, in association with the moisten- can influence the vertical motion field associated with ing. As a consequence, the convective instability the circulation. Previously, we have shown that moist increases substantially (decreasing theta-e with isentropic ascent (warm-air advection), a quasigeo- height) prior to the rain and is maintained through strophic (QG) ascent mechanism, was prominent during most of the event. the event. Another mechanism suggested by QG theory 4) Low-level convergence (not shown) and ascent begin is differential vorticity advection that can be diagnos- to increase prior to the heavy rain and seem also to be tically related to the vertical motion field (Holton aspects of the preconditioning. This continues during 1979; Hoskins et al. 1985). This could also be viewed and even after the rain event. The unique aspect of as the weather system’s attempt to rebalance its three- the convergence field is the deepening in this param- dimensional circulation after the balance is disrupted by eter during the heavy rain. It remains unclear if this is changes in its vertical structure. For the Fitow event related to the presence of rain clouds and their there appeared to be interactions with the environment heating profiles or some other effect like environ- that influenced the evolution of the rain-producing mental flow changes. weather system. To quantify these interactions, back 5) Low-level moistening begins prior to the onset of heavy trajectories from the rain area were calculated using the rain. Relative humidity reaches values of greater than HYSPLIT system developed at NOAA (Draxler and

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21 FIG. 16. (a) Time–height plot of area-averaged wind (full barb is 10 m s ), vertical velocity [omega, with red 2 2 2 (blue) indicating ascent (descent); Pa s 1], and relative vorticity (positive contour interval is 10 3 10 6 s 1, 2 2 negative contour interval is 20 3 10 6 s 1); (b) equivalent potential temperature (contour interval is 3 K), with the values greater than 342 K indicated by red contours; and (c) relative humidity (contour interval is 5%), with the values greater than 70% indicated by red contours. Averaging box is over the heavy rainfall area (29.58–31.58N, 1208–1228E) from ACCESS-TC at base time 1200 UTC 4 Oct 2013. (d)–(f) As in (a)–(c), but from ACCESS-TC at base time 1200 UTC 5 Oct 2013. Note that time increases from right to left. Filled triangle denotes landfall time (1800 UTC 6 Oct 2013).

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3) midlevel (3 km) injection of enhanced PV from the east-southeast, which originated from the Danas circulation; and 4) mid- to upper-level injection of PV from the west, which originated from the midlatitude, high-amplitude trough.

It is clear from the previous discussion that changes in the synoptic-scale circulation produced the flows that resulted in these back trajectories. We suggest that in- trusion of environmental PV and moisture into the rain area, brought about by environmental flow changes, was critical to the development of the heavy rain event. We suggest that this exchange of PV and moisture is one way to interpret the interaction between weather systems and their environments. Because of the limited domain size, we have not confirmed at this stage whether the ACCESS-TC forecasts reproduced these analyzed back FIG. 17. HYSPLIT back trajectories at 7, 3, and 1 km, based on trajectories. This will be part of our ongoing work and 6-hourly ERA-Interim analyses. The legend in the top-right corner should establish if these environmental injections of PV indicates the symbols corresponding to these levels. Back trajectories were critical to the success or otherwise of the rain are started from points surrounding the heavy rain area, indicated by forecasts. the boldface plus sign (1), and from 0600 UTC 7 Oct 2013. To illustrate how and when these environmental flow changes and interactions occurred, we have calculated back Hess 1998) and implemented at the Australian Bureau trajectories from the rain area at 7 km (the level where of Meteorology (http://www.wmo.int/pages/prog/www/ large interaction with the environment seemed to be oc- DPS/WMOTDNO778/rsmc-melbourne-a.htm). The tra- curring, as defined above) at times leading up to the rain jectories are based upon ERA-Interim analyses (Dee et al. event. Figure 18 shows back trajectories from 0000 UTC 5 2011) available every 6 h at 1.58 horizontal resolution. October (well prior to the rain), 0000 UTC 6 October (just A set of grid points (197 at each level, 0.58 apart), over prior to the rain and landfall), and 0000 UTC 7 October an area enclosed by a radius of 48, located within a three- (during the heavy rain). In addition, at each level we keep dimensional volume centered on the rain system, are track of the area influenced by air coming from the envi- specified from which back trajectories are calculated. ronment (beyond 600 km from the center of the rain area, We have chosen levels at 1, 3, and 7 km to make the back 308N, 120.58E) and also the percentage of PV that comes trajectories. These were selected based on examination of from the environment. Figures 18d and 18f show times the wind analyses at various levels (Fig. 4).Thetimeat series of ‘‘A,’’ the percent area of air from the environment, which the back trajectories were initially calculated is and ‘‘PV,’’ the percent PV coming from the environment. 0600 UTC 7 October 2013: the approximate time of the To relate the trajectories back to the wind fields, we also start of the heavy rain and following Fitow’s landfall. show wind analyses at 500 hPa, with the relative vorticity Figure 17 shows a representation of the three-dimensional overlaid, and 700 hPa at the corresponding times. back trajectories. The legend in the top right corner in The trajectories show that well prior to the rain event, Fig. 17 shows the symbols used to indicate the level the area where rain developed was only influenced by value. The colors on the trajectories indicate values of trajectories that came from the west. This is confirmed PV along each trajectory. Figure 17 clearly shows four from the corresponding wind analysis. As the event substantial environmental interactions that were influ- began to unfold, in addition to the trajectories from the encing the circulation and its vertical structure over the west, a large amount of high-PV air was entering the rain rain area: area from the Fitow circulation. The wind analysis sug- 1) low-level inflow at 1 km from the east, which was gests that this air was exiting the storm circulation over warm and moist; Fitow’s north and northeast sectors and entering the rain 2) mid- to upper-level inflow from the southeast with area from the east. A lobe of cyclonic relative vorticity is relatively high PV, in a streamer with generally seen to start exiting from the Fitow circulation from its constant PV, which originated from the Fitow north-northeast sector at 0000 UTC 6 October, and circulation; this lobe increases in size and strength over the next 24 h

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FIG. 18. Back trajectories at 7 km started from points surrounding the heavy rain area at (a) 0000 UTC 5 Oct, (b) 0000 UTC 6 Oct, and (c) 0000 UTC 7 Oct 2013. (d),(e) Time series of environmental PV entering the rain area from the east and northwest. Here, A represents the percent area increase over the rain area that enters from the environment. Sum PV represents the percent PV increase over the rain 2 area that enters from the environment. Analyzed wind fields at 500 hPa (full barb is 10 m s 1), overlaid with relative vorticity (contour 2 2 interval is 1 3 10 5 s 1), at (f) 0000 UTC 5 Oct, (g) 0000 UTC 6 Oct, and (h) 0000 UTC 7 Oct 2013. (i)–(k) As in (f)–(h), but at 700 hPa (full 2 2 barb is 10 m s 1; shading denotes wind speed with contour interval of 5 m s 1). The solid blue line in (f)–(h) marks the approximate axis of the environmental trough that impinges on Fitow’s circulation. as it rotates toward the rain area. We suggest that the Fitow into the rain area—the lobe of cyclonic vorticity in large-scale flow changes had opened up the Fitow cir- Fig. 18. An anonymous reviewer suggested that an al- culation via the developing environmental trough ternative interpretation could be that increased de- (marked in Fig. 18), which had developed to the north- formation in the environment strained/elongated the west. This ‘‘wound’’ then ‘‘bled’’ PV and moisture from Fitow vorticity into an open wave structure rather than a

Unauthenticated | Downloaded 10/11/21 11:17 AM UTC SEPTEMBER 2015 B A O E T A L . 3401 closed circulation pattern, and that there appears to be a meteorological observations, global analyses, and high- reorganization of vorticity from a curvature-dominated resolution forecasts. We have used composite radar flow to a shear-dominated flow. We hope to investigate reflectivities, CMORPH rainfall estimates, and surface this further using a form of the Okubo–Weiss parameter observations to show that the rain was associated (Dunkerton et al. 2009; Tory et al. 2013). The de- with (i) an initial outer rainband from Fitow, which formation interaction between the trough and Fitow developed and propagated northwestward across the seems crucial to the rain event. It is quite possible that, Hangzhou Bay area (just south of Shanghai) during the apart from the inflow of cold, dry air, the drainage of period from 0600 to 1500 UTC 6 October; (ii) a second, moisture and PV from Fitow may have also contributed more active outer rainband that developed a few hours to its rapid demise and resulted in it being unable to later to the south of the first band, propagated to the transition into an extratropical cyclone. At the later north, and intensified; and (iii) a southwest–northeast- time, there was also inflow coming from Danas via the orientated line of frontal-like cloud, which appeared to developing large-scale easterly winds to the east of the evolve out of the second, more active rainband, which rain area. We suggest that the resulting differential PV organized and fluctuated in strength as it extended structure change over the rain area was important in the southward, and which eventually produced a severe city development of ascent and rainfall. flood in Shanghai. This latter development appears to Finally, to further confirm the role of Fitow and Danas be a different and unique feature of the Fitow event that in supplying the rain system with moisture via the en- distinguishes it from other TC cases discussed in the vironmental flow, Fig. 19 shows moisture flux at 6-h in- literature. The outer rainbands developed in the north- tervals during the rain event obtained from east sector of Fitow’s circulation, which was the favored region for convective asymmetries in the environmental 5 $ southwesterly wind shear. The presence of Fitow in fa- Q h (qVh/g), vorable shear was thus a very critical ingredient in the where Q is the horizontal water vapor flux, g is the development of the rain systems that produced the gravitational acceleration, Vh is the horizontal wind heavy rain. These provided the rain-producing focus for vector, and q is the mixing ratio of water vapor. the large-scale forcing. Figure 19 shows three contributions to the moisture We have shown that the heavy rain occurred as the flux affecting the rain area, similar to the back trajec- large-scale flow reorganized. Major surface anticy- tories. These are from the environment to the east of clones developed over China and the North Pacific, the rain area, from TC Fitow, and from TC Danas. The resulting in the development of long, low-level, over- analysis suggests that Fitow and Danas supplied both the-sea trajectories into the Hangzhou Bay area. At moisture and midlevel PV to the rain system. Detailed upper levels, a high-amplitude trough retrogressed numerical experimentation and diagnostics are re- over low latitudes to be located over central China quired to further clarify the roles of Fitow and Danas with the entrance to a southwesterly jet positioned on the rain event. near Hangzhou Bay and Shanghai. The vertical struc- ture of the horizontal wind field was thus upper- level southwesterlies overlaying moist, low- to midlevel 8. Summary and conclusions easterlies. This is a very favorable structure for Late on 6 October 2013, Tropical Cyclone Fitow made large-scale vertical motion associated with moist is- landfall south of Shanghai, China, near the border of entropic ascent or warm-air advection (e.g., Hoskins Zhejiang and Provinces. During the event, rain- et al. 1985). 2 fall in excess of 300 mm day 1 was observed over a re- On the synoptic scale, based on back trajectories, gion 400 km to the north of the typhoon center. Fitow we have demonstrated that four environmental inter- did not undergo extratropical transition, since there is actions associated with the evolving large-scale flow no evidence of it transitioning into an extratropical occurred that likely influenced the development of the cyclone. The heavy precipitation occurred in the days heavy rain. These included (i) increasing injection of following landfall and so the rain was not a predecessor midlevel moist potential vorticity (PV) from the Fitow rain event to Fitow. With TC Danas located some circulation; (ii) low-level warm, moist inflow from the 1000 km to the east, it has similarities to predecessor east; (iii) midlevel inflow from nearby Typhoon Danas; rain events described, for example, by Galarneau and (iv) decreasing injection of mid- to upper-level PV et al. (2010). from the midlatitude trough. We propose that the re- Major features of the heavy rain have been investi- sultant change in PV structure over the rain area was a gated using satellite data, radar reflectivity, standard major influence on the rain event. Analysis of the back

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2 21 FIG. 19. (a)–(f) Horizontal water vapor flux distribution [shading; g (hPa m s) ] and horizontal wind vector (full 2 barb is 10 m s 1) at 850 hPa from ACCESS-TC at base time 1200 UTC 5 Oct, every 6 h from 1200 UTC 5 Oct to 0000 UTC 8 Oct 2013. The filled and open typhoon symbols denote TCs Fitow and Danas, respectively. trajectories suggests that the environmental flow changes potential temperature, convective instability, and mid- may have opened up the Fitow circulation through the level relative vorticity. There is evidence of a pre- midlevels and drained moisture and PV from the storm conditioning period prior to the heavy rain during which into the rain area. This may also have contributed to time large-scale midlevel subsidence and boundary layer Fitow’s rapid demise and prevented it from transitioning moistening occurred. Based on an analysis of low-level into an extratropical cyclone. equivalent potential temperature, we have shown that At the mesoscale, and consistent with the trajectory (i) after landfall, a cold, dry airstream wrapped into the calculations, there were large increases over the rain Fitow circulation from the north and west, limiting the area in diagnosed ascent, low-level moisture, equivalent inner-core rainfall and producing a cold-air boundary,

Unauthenticated | Downloaded 10/11/21 11:17 AM UTC SEPTEMBER 2015 B A O E T A L . 3403 and (ii) a warm, moist airstream from the east converged the eastern United States. Mon. Wea. Rev., 135, 2185–2206, with the cold-air intrusion over the rain area. This doi:10.1175/MWR3382.1. frontogenesis and the associated secondary circulation Baek, E. H., J. H. Kim, J. S. Kug, and G. H. Lim, 2013: Fa- vorable versus unfavorable synoptic backgrounds for in- contributed significantly to the enhanced rainfall in the direct precipitation events ahead of tropical cyclones Shanghai area. approaching the Korean Peninsula: A comparison of two In this study we have (i) documented the mesoscale cases. Asia-Pac.J.Atmos.Sci.,49, 333–346, doi:10.1007/ rain systems that produced the heavy rain and (ii) pro- s13143-013-0032-z. vided evidence of how the environmental flow chan- Bosart, L. F., and G. M. 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