1 Impacts of frontal SST gradient on the formation of axially 2 asymmetric thermal structure of a tropical cyclone: 3 A case study of a typhoon in the East China Sea 4 5 Fukiko Takehi, * Hisashi Nakamura, † and Takafumi Miyasaka 6 Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan 7 and 8 Mayumi K. Yoshioka 9 Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan 10 11 Submitted to Monthly Weather Review in April 2015, as a potential contribution to 12 the Special Collection “Climate Implications of Frontal-Scale Air-Sea Interaction” 13 Revised in September, 2015 14 15 16 17 *Corresponding author address: Fukiko Takehi, Research Center for Advanced Science and 18 Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo, 153-8904, Japan. 19 E-mail: [email protected] 20 Current affiliation: Office of Observation Systems Operation, Japan Meteorological Agency, 21 Tokyo, Japan 22 †Additional affiliation: APL, JAMSTEC, Yokohama, Japan 23 1 24 Abstract 25 A tropical cyclone (TC) is known to undergo substantial modifications in its thermal 26 structure as it approaches a deep baroclinic zone associated with a midlatitude westerly jet, 27 which is often collocated with an oceanic frontal zone (OFZ) with sharp meridional gradient in 28 sea-surface temperature (SST). This collocation often makes it difficult to isolate an influence, if 29 any, of the frontal SST gradient from that of a jet-associated free-tropospheric baroclinic zone 30 on the formation of axially asymmetric thermal structure of a TC in its extratropical transition. 31 The present study makes the first attempt to isolate the former influence by focusing on a 32 particular typhoon (Songda) that approached a prominent OFZ in the southern East China Sea 33 located far south of a midlatitude westerly jet. An investigation based on high-resolution 34 regional atmospheric analysis reveals that axially asymmetric thermal structure first emerged in 35 the planetary boundary layer as the typhoon approached the OFZ well before the corresponding 36 structure reached the mid-troposphere around the westerly jet. Thermodynamic analysis 37 indicates that a near-surface cool anomaly that constituted the axial asymmetry was generated to 38 the west of the typhoon center through cold advection largely by the strong northerlies across a 39 near-surface baroclinic zone along the frontal SST gradient. A set of experiments with a 40 cloud-resolving atmospheric model with different intensities of SST gradient confirms the 41 importance of the frontal SST gradient in enhancing the near-surface cold advection. 2 42 1. Introduction 43 It is well known that a tropical cyclone (TC) tends to transform itself into a 44 midlatitude weather system, as it approaches a midlatitude baroclinic zone associated with a 45 westerly jet. In this process called “extratropical transition (ET)” (Jones et al. 2003; Harr 2010; 46 Kitabatake 2012), a TC undergoes structural changes, including the formation of surface fronts 47 and associated heavy precipitation systems, in addition to expansion of the area of storm-force 48 winds and localized gusts (Kitabatake and Bessho 2008; Kitabatake 2012). In fact, some of the 49 decaying TCs evolve into rapidly developing extratropical cyclones (Jones et al. 2003; Harr 50 2010). In addition to its usefulness for disaster mitigation and prevention, understanding the ET 51 process itself is scientifically intriguing as a transformation process from a tropical weather 52 system into a midlatitude one. 53 Previous studies focused primarily on particular ET processes occurring around a deep 54 baroclinic zone associated with an upper-tropospheric westerly jet (Klein et al. 2000; Ritchie 55 and Elsberry 2001; Kitabatake et al. 2007; Kitabatake 2008). Through analysis of satellite 56 imageries and output data from the Navy Operational Global Atmospheric Prediction System, 57 Klein et al. (2000) proposed a conceptual model for ET interactions influenced by both a 58 near-surface baroclinic zone and a midlatitude westerly jet, which were then verified by Ritchie 59 and Elsberry (2001) through idealized experiments. Operationally, ET is defined rather 3 60 subjectively by using satellite imagery and other observations available (Kitabatake 2008). In 61 fact, Jones et al. (2003) pointed out that there is no universal definition of ET. Evans and Hart 62 (2003) nevertheless proposed an objective parameter for the ET onset that measures axial 63 asymmetries in thermal structure of a storm. This parameter was defined as the asymmetry of 64 thickness between the 900 and 600-hPa levels measured in the direction perpendicular to the 65 instantaneous storm motion, and the value of this parameter is supposed to increase during ET. 66 In the evaluation of this parameter, however, thermal asymmetry in the near-surface layer below 67 the 900-hPa level is not included, despite the formation of 925-hPa thermal asymmetry tends to 68 precede that at the 600-hPa level in the composite of 274 TCs observed over the western North 69 Pacific (WNP) by Kitabatake (2011). 70 It is also known that high sea-surface temperature (SST) over 26°C is necessary for the 71 generation and maintenance of TCs (e.g., Gray 1975; Emanuel 1986), and a role of SST on TCs 72 in the ET process has been examined. For example, a numerical experiment by Ritchie and 73 Elsberry (2001) with idealized SST distribution elucidates how the lowering of SST into the 74 midlatitudes affects a TC at the initiation of its ET. Specifically, reduction in heat and moisture 75 supply from the ocean leads to the weakening of deep convection within the inner core of a TC. 76 Its warm core thus weakened becomes tilted under the vertically sheared westerlies, enhancing 77 axial asymmetries in convective precipitation. On the basis of global reanalysis data, Kitabatake 4 78 (2011) found that ET of a TC over the WNP (i.e., typhoon) is often completed within a 79 midlatitude baroclinic zone characterized by strong westerly shear between the 925 and 200-hPa 80 levels, including a warm maritime region where SST exceeds 24°C. 81 In the midlatitude ocean there are regions referred to as “oceanic frontal zones 82 (OFZs)”, where warm and cool currents are confluent to enhance SST gradient locally. From a 83 potential-vorticity perspective (Hoskins et al. 1985), near-surface baroclinicity associated, for 84 example, with surface air temperature (SAT) gradient is essential for baroclinic development of 85 extratropical cyclones. Recent studies have revealed that a sharp decline in sensible heat supply 86 from the ocean across an OFZ efficiently restores SAT gradient against the relaxing effect by 87 extratropical cyclones to allow their recurrent development (Nakamura et al. 2004; Taguchi et al. 88 2009; Hotta and Nakamura 2011). For individual cyclones the sensible heat exchange with the 89 ocean acts as thermal damping, while moisture supply from the warm current is important for 90 their growth (Nakamura et al. 2004). Therefore, sharp SST gradient across an OFZ can 91 influence a TC during its ET and its redevelopment as an extratropical cyclone. In fact, it has 92 been pointed out that frontal SST gradient is one of the environmental factors that can affect 93 structural transformation of a TC during its ET (Fig. 11 in Jones et al. 2003). Through numerical 94 experiments with different SST conditions around the Kuroshio Extension (KE) east of Japan, 95 Wada et al. (2013) suggested the influence of an OFZ that modulates the thermal structure of 5 96 Typhoon “Choi-wan” in the course of its ET. They found the influence of the OFZ within the 97 planetary boundary layer (PBL) outside of the inner-core region of the TC, in addition to 98 possible influence of a nearby stationary rain front. Still, specific impacts of frontal SST 99 gradients have not been fully clarified, because most of the previous studies investigated the ET 100 process under a baroclinic zone associated with a westerly jet. 101 This study attempts to identify the impacts of a frontal SST gradient through the 102 investigation of a particular TC that approached a well-defined OFZ, located far south of a 103 westerly jet. There is a tendency for an eddy-driven westerly jet to be collocated with a 104 midlatitude OFZ (Nakamura et al. 2004), which makes it difficult to distinguish the role of a 105 near-surface baroclinicity associated with an OFZ in the ET process from that of 106 free-tropospheric baroclinicity associated with the westerlies. In the KE region, for example, the 107 collocation of the westerlies with a prominent OFZ frequently occurs except in August (Sampe 108 and Xie 2010). Thus it is not often that a typhoon encounters frontal SST gradient associated 109 with the KE prior to its interaction with a westerly jet. In fact, when the typhoon Choiwan 110 reached the KE region on September 20, 2009, 200-hPa westerly wind speed exceeded 25 m s–1 111 (not shown). Although the August situation appears to be suited for our purpose, high-resolution 112 data necessary for capturing the structure of TCs away from landmasses are severely limited in 113 the KE region. A high-resolution data set provided by the Japan Meteorological Agency (JMA; 6 114 see section 2 for details) is not available east of 150°E. To avoid any serious influence from the 115 main island of Japan, we can select only those typhoons whose centers moved around 145°E for 116 our analysis. Unfortunately, no such typhoon was observed in August during the period since 117 2007 in which the particular data set is available. Furthermore, no operational radiosonde 118 observations are carried out in the KE.