Atmospheric Research 235 (2020) 104736

Contents lists available at ScienceDirect

Atmospheric Research

journal homepage: www.elsevier.com/locate/atmosres

Characteristics of upper-tropospheric outflow-layer clouds of T Francisco (2013) observed by hydrometeor videosonde ⁎ Tadayasu Ohigashia,b, , Kazuhisa Tsubokia, Yuto Suzukia, Hiroyuki Yamadac, Katsuhiro Nakagawad a Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, b National Research Institute for Earth Science and Disaster Resilience, Tsukuba, Japan c Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, Nishihara, Japan d National Institute of Information and Communications Technology, Koganei, Japan

ARTICLE INFO ABSTRACT

Keywords: Ice crystals in upper-tropospheric outflow layers of are an important factor in determining typhoon intensity and track through radiative cooling of the secondary circulation. Quantitative representation of ice Cloud crystal distribution is fundamental for accurate reproduction of typhoon intensity and track in cloud-resolving Ice crystal numerical models. This study used hydrometeor videosondes (HYVISs) released from Okinawa Island (Japan) to Upper-tropospheric outflow layer conduct microphysical observations of ice crystals in the outflow layer of Typhoon Francisco (2013). During the Videosonde approach of the typhoon center, seven HYVISs were released between 02:53 JST October 23, 2013 and 13:33 JST October 24, 2013 at distances of between 230 and 550 km from the typhoon center (r = 230 and 550 km). The observation site was located at the downshear left flank, and at least three HYVISs were released within 100km of active convection in the eyewall from which outflow-layer clouds extended. During the observations, the central pressure of the typhoon increased slightly from 945 to 960 hPa. Ice particles were observed at heights of 7–14 km. The numbers of ice particles tended to decrease with distance from the typhoon center. The maximum number concentrations of ice crystals were 180 and 20 L−1 at r = 230 and 550 km, respectively. Ice particles with diameter of a few tens of micrometers were found dominant. Large particles with diameter > 100 μm were restricted to just outside the eyewall and the inner-core precipitation region. Ice supersaturation layers with thickness > 2 km were present from r = 230 to 360 km, i.e., reasonably close to the typhoon center. In addition, supercooled liquid droplets were observed in areas with relative humidity > 95% in two profiles (r = 260 and 290 km). Conversely, there was no ice supersaturation layer in the outermost profile at r = 550 km. Saw-like radar echo patterns were common along the bottom of outflow-layer clouds, below which a considerably drier layer was evident. Saturation ratio minima relative to ice within the dry layer were 0.01–0.34 (for relative humidities of 1%–31%). It was considered that ice particles formed in the upper levels of near the typhoon center, and that they were sublimated in both the dry layer below the outflow layer and the ice sub- saturation area in the outermost region of the typhoon, which resulted in decreased number concentrations of ice particles.

1. Introduction 2015). It is necessary to predict accurately the intensity and track of typhoons to mitigate the effects of associated disasters. Ito (2016) in- Typhoons are associated with strong winds, heavy rain, and storm vestigated the forecast errors of tropical cyclone (TC) intensity and surge, which frequently cause disasters in the western North Pacific track in the western North Pacific. It was reported that the mean error region. It has been reported that the intensity of typhoons that have of TC track forecasts has decreased to less than half that of 30 years ago, made in the western North Pacific region has increased since whereas the forecast error of TC intensity has not reduced. In addition, the latter half of the 1970s (Mei and Xie, 2016). It has also been pre- estimates of typhoon intensity based on satellite observations remain dicted that the maximum intensity of super typhoons will increase in problematic (Ito et al., 2018). the future under conditions of global climate warming (Tsuboki et al., Previous numerical simulation studies have shown that cloud

⁎ Corresponding author at: National Research Institute for Earth Science and Disaster Resilience, 3-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan. E-mail address: [email protected] (T. Ohigashi). https://doi.org/10.1016/j.atmosres.2019.104736 Received 25 April 2019; Received in revised form 16 October 2019; Accepted 25 October 2019 Available online 30 October 2019 0169-8095/ © 2019 Elsevier B.V. All rights reserved. T. Ohigashi, et al. Atmospheric Research 235 (2020) 104736 microphysical processes affect the intensity and track of TCs. Forex- studies of various cloud systems (e.g., Murakami et al., 1992; Murakami ample, the inclusion of ice microphysical processes (Lord et al., 1984; et al., 1994; Orikasa et al., 2013; Oue et al., 2015; Orikasa and Tao et al., 2011), the prevention of graupel formation (Fovell et al., Murakami, 2015; Ohigashi et al., 2016). The Meisei Electric Co. 2009), and changes in graupel fall speeds (McFarquhar et al., 2006) can (Japan), which manufactures the HYVIS, upgraded some internal have significant influence on TC intensity and its evolution rate. components of the HYVIS system in 2012, which resulted in clearer Moreover, some studies have reported that differences in cloud mi- images, lighter weight, and reduced costs. For each release, a GPS crophysical processes affect cloud radiative forcing, which can result in radiosonde (RS-06G; Meisei Electric Co.), which measured temperature, differences in TC track (Fovell and Su, 2007; Fovell et al., 2009; Fovell relative humidity, wind direction, and wind velocity, was attached to et al., 2010; Cao et al., 2011; Hsu et al., 2013; Bu et al., 2014). Fovell the HYVIS. During its ascent, the height of the RS-06G was obtained et al. (2016) presented a review of a series of studies on the effects of directly by the GPS. The ascent rate of the HYVIS/radiosonde package cloud microphysical processes and cloud radiative forcing on TC mo- was approximately 6 m s−1. Therefore, once released, it took the sonde tion. From numerical simulations, it has been established that cloud approximately 40 min to rise to the height of 15 km. radiative forcing in the upper-tropospheric outflow layer of TCs extends The C-band (5340 MHz) polarimetric radar, referred to as the CRL the range of associated cloud development and secondary circulation. Okinawa Bistatic Polarimetric Radar (COBRA), of the National Institute As shown in Fiorino and Elsberry (1989), greater spread of the sec- of Information and Communications Technology was used in this study. ondary circulation increases the speed of TC translation and it changes The COBRA was operated on the Okinawa Main Island (26.59°N, the direction of TC movement to the west owing to the beta drift. Al- 128.06°E; 363 m above sea level) and it was located at an azimuthal though systematic results have not yet been obtained (Fovell et al., direction of 65.5° and at a distance of 24.1 km from the HYVIS release 2016), it is widely accepted that cloud radiative forcing can affect TC site. Range height indicator (RHI) scan data with grid spacing of 75 m intensity (e.g., Trabing et al., 2019). in the radial direction and an observational range of 40.3 km were used To clarify the role of radiative forcing associated with TC clouds, it in this research. Radar observation was performed independently of the is important to determine quantitatively the characteristics of the ice HYVIS observation, and the directions of RHI scans and HYVIS positions crystals present in the upper-tropospheric outflow layer, which covers a were not synchronized. Details of other specifications of the COBRA can large part of the cloud area of a TC. Some measurements of the mi- be found in Nakagawa et al. (2003) and Bringi et al. (2006). The crophysical quantities of clouds in convective regions near TC centers COBRA reflectivity data, Japan Meteorological Agency (JMA) radar- have been conducted using aircraft (e.g., Black and Hallett, 1986; derived precipitation intensity data, and Multifunctional Transport Houze et al., 1992; Black, 2003; McFarquhar and Black, 2004; Satellite-2 infrared images were used to determine the clouds and Heymsfield et al., 2006). However, there have been no in situ ob- precipitation distributions associated with Francisco. The best track servations of entire layers of outflow-layer clouds because outflow-layer data of the JMA were used to determine the track and central pressure clouds can extend above the height of 15 km and few aircraft can op- of the TC. erate at that height. Vertical shear of the typhoon was derived using isobaric field data This study performed hydrometeor videosonde (HYVIS) observa- of the Japanese 55-year Reanalysis (JRA-55; Kobayashi et al., 2015) tions of Typhoon Francisco (2013). Seven HYVISs were released from with horizontal grid spacing of 1.25° and a 6-h time interval. Before Okinawa Island (Japan) at distances of 230–550 km from the center of calculating the vertical shear, the typhoon center was defined in the Francisco to measure ice crystals in the outflow layer. The number JRA-55 as a grid with maximum relative vorticity after smoothing using density and particle size distribution of the ice crystals were derived a 400-km filter. Vertical shear was calculated as a wind vector differ- from analysis of images obtained by the HYVISs. This study also in- ence between 850 and 200 hPa of the wind fields averaged for a dis- vestigated the atmospheric thermodynamic structure using radiosonde tance of between 200 and 800 km from the typhoon center defined in profiles to examine the formation and dissipation of ice crystals inthe the JRA-55. We considered this horizontally averaged vertical shear as upper-outflow layer. representative of the environmental field.

2. Observations and analysis 2.2. Analysis of HYVIS images

2.1. Observations and data Particles were identified through visual inspection of the HYVIS images that were extracted at 0.1-s intervals. The number of particles In situ observations of ice crystals using HYVISs were conducted in and their maximum length were measured manually. With reference to October 2013 as Typhoon Francisco (2013) approached the Okinawa Magono and Lee (1966), particles were classified into six types: (a) Islands. Seven HYVISs attached to balloons were released from the Plate, (b) Column, (c) Supercooled water, (d) Frozen drop, (e) Ag- Okinawa Electromagnetic Technology Center of the National Institute gregate, and (f) Undetermined (Fig. 1). We separated supercooled water of Information and Communications Technology (26.50°N, 127.84°E, particles from frozen drops using one of the following two criteria. The 8 m above sea level) from 02:53 JST October 23, 2013 to 13:33 JST first criterion was that although both types of particle presented nearly October 24, 2013. spherical shapes, frozen drops had angular parts at their rim. For the The original HYVIS was developed by the Meteorological Research second criterion, we considered that supercooled water droplets would Institute of Japan (Murakami and Matsuo, 1990). To measure clouds freeze immediately after falling onto the transparent film and would not with low number concentrations, another version of HYVIS that in- move on the film, whereas frozen drops could occasionally change corporated a suction fan was developed to increase the volume of position on the transparent film of the HYVIS. Therefore, when apar- sampled air (Orikasa and Murakami, 1997). Video cameras in the ticle changed position on the film, we determined that the particle was HYVIS take images of particles that had fallen onto a transparent film. not a supercooled water droplet but an ice particle that included a The video images are sent to a ground-based receiver via 1680 MHz frozen drop. Undetermined particles were those that were difficult to radio waves. In the HYVIS, two video cameras with different magnifi- assign to another particle type because they were either very small or cations (close-up and microscopic) alternately acquire images. The shaped irregularly, although there was no criterion for size. fields of view of the close-up and microscopic cameras are 7×5.25and The sampling volume of air and the collection efficiency of particles 1.2 × 0.9 mm, respectively. Microscopic camera images can capture were determined by Orikasa and Murakami (1997). The collection ef- particles with diameters of approximately > 7 μm. To measure only ficiency of all particles was considered as unity. In this study,the smaller particles, this study used modified HYVISs that incorporated number concentrations of the particles were calculated at 250-m ver- only the microscopic camera. The HYVIS has been used previously in tical intervals based on the counted number of particles, sampling

2 T. Ohigashi, et al. Atmospheric Research 235 (2020) 104736

Fig. 1. Samples of particle types: (a) Plate, (b) Column, (c) Supercooled water, (d) Frozen drop, (e) Aggregate, and (f) Undetermined. Sonde number, heights, and temperatures where the particles were observed are also shown. All particles are shown to the same scale. volume, and collection efficiency. A particle size distribution was de- rived for each HYVIS release for all ice particles in the outflow layer, which ranged from 7 to 14 km in height.

3. Overview of Typhoon Francisco (2013)

Typhoon Francisco (2013) strengthened to tropical storm category with a 10-min averaged wind speed of 17 m s−1 at 15:00 JST on October 16, 2013, when it was located at 12.1°N, 144.4°E. The central pressure at that time was 1000 hPa. It then decreased to 920 hPa by 03:00 JST on October 19 (Fig. 2). The lowest central pressure was maintained until 09:00 JST on October 20. Initially, the typhoon moved toward the southwest, but it later moved toward the northwest on and after October 19, finally approaching the Okinawa Main Island during October 23–24 (black and red lines in Fig. 3). The central pressure in- creased slightly from 945 hPa at 03:00 JST on October 23 to 960 hPa at 03:00 JST on October 24. After 09:00 JST on October 24, the typhoon track was directed toward the northeast. Typhoon Francisco trans- Fig. 2. Temporal variation of central pressure of Typhoon Francisco (2013). formed to an with central pressure of 984 hPa at Analysis times of best track data shown by dots are at 6-h intervals from 21 JST 15:00 JST on October 26, 2013, when it was located at 33.0°N, 144.2°E. on October 15 to 21 JST on October 26, 2019 (03, 09, 15, and 21 JST), except between 15 JST on October 23 and 09 JST on October 25 and between 21 JST Typhoon Francisco showed the 30th lowest central pressure and the on October 25 and 15 JST on October 26 when they are at 3-h intervals. Blue 23rd longest lifetime of the 245 typhoons generated during the 10-year lines indicate the period during which the storm was categorized as a typhoon. period of 2009–2018. Among the 32 typhoons that occurred in October Seven numbered red lines represent HYVIS release times. (For interpretation of during the same period, Typhon Francisco had the 8th lowest central the references to color in this figure legend, the reader is referred to theweb pressure and 7th longest lifetime. version of this article.) Seven HYVIS releases were performed from Okinawa Main Island during October 23–24 (Fig. 2), when the typhoon center was located to station pressure at the HYVIS observation site decreased because of the the southeast (Fig. 3). Table 1 lists the release time, central pressure of approach of the typhoon center. There was almost no diurnal variation the best track data at the time closest to the release time, and distance in temperature and humidity. High relative humidity values of between the release point and the typhoon center for each HYVIS re- 88%–97% were maintained. lease. The distance between the HYIVS observation site and the ty- Vertical shear vectors of the typhoon environment during the HYVIS phoon center decreased from 550 km for the No. 1 HYVIS to 230 km for observations are shown in Fig. 3. The track of the typhoon center was the No. 7 HYVIS. Station pressure, temperature, and relative humidity defined using the JRA-55 (gray line), where the vertical shearwas at the HYVIS release site are also shown. During the HYVIS observa- calculated, and the best track of the JMA (black and red lines) can be tions, although the central pressure of Francisco increased slightly, the

3 T. Ohigashi, et al. Atmospheric Research 235 (2020) 104736

orange lines in Fig. 5h–n. In Fig. 4a/h and b/i, moderately high clouds with temperatures below −40 °C can be seen to cover a wider extent compared with later times. However, there is no region of organized precipitation or high-intensity precipitation evident around the HYVIS release site in Fig. 5a/h and b/i. After the release of No. 3 HYVIS, the horizontal extent of the upper-level clouds decreased, as shown by the area of cloud with temperatures below −40 °C. As the typhoon center approached the Okinawa Islands, upper-level clouds spread over the release site. On October 24, a broad region of precipitation to the west of the typhoon center approached the release site (Fig. 5e–g/l–n). The HYVIS observations at this time were performed in the outflow layer close to the broad region of precipitation. The final observation (No. 7 HYVIS) was performed when the typhoon center was at its closest to the release site. In Fig. 5g, an area of strong precipitation of > 16 mm h−1 can be seen on the downshear left flank of the typhoon center inthe eyewall. The location of this strong precipitation is consistent with previous research showing that convective activity is enhanced on the downshear left flank (Frank and Ritchie, 1999; Corbosiero and Molinari, 2002). Strong precipitation (> 16 mm h−1) can also be seen on the downshear left flank of the eyewall (Fig. 5e/l and f/m), although precipitation on the southeast side of the eyewall might not have been detected by the radar. During the No. 5–7 HYVIS observations, rea- sonably active convection was present within 100 km of the release site.

4. Characteristics of upper-tropospheric outflow-layer clouds

Fig. 3. Best track (black and red lines) of Typhoon Francisco (2013) derived by Vertical cross sections of radar reflectivity obtained by the RHI the Japan Meteorological Agency. The track during the HYVIS observation scans of the COBRA are shown in Fig. 6a–g. The azimuth angle of the period is shown by the red line, and the numbered red closed rectangles are the RHI scans was set at 41.2° (northeast). The range of the RHI scans locations of the typhoon center when the HYVISs were released. The orange “x” shown in Fig. 6a–g is represented by lines in Figs. 4h and 5h–n. The indicates the HYVIS release site. Arrows indicate differences of horizontal ve- COBRA observes relatively larger particles because of the transmitting locity (m s−1) between 850 and 200 hPa. The gray line shows the path of the frequency (5340 MHz); therefore, it cannot necessarily detect all hy- typhoon center derived from JRA-55 reanalysis data when estimating the ver- drometeor particles. The main precipitation echoes (> 10 dBZ) were tical shear of the typhoon environment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version ofthis found restricted to below the height of 5 km with the exception of those article.) in Fig. 6g. The height was lower than the 0 °C heights that were in the range of 4.9–5.7 km determined from radiosondes attached to the HY- VISs. Therefore, it is considered that the surface precipitation was seen as reasonably similar during the HYVIS observational period. formed mainly by processes without the ice phase. In some places, radar During the HYVIS observational period (between 03:00 JST on October echoes < 10 dBZ appeared above the height of 7 km (Fig. 6a–d). There 23 and 09:00 JST on October 24), vertical shear vectors were directed were no deep convective echoes near the upper-level echoes, indicating to between northeast and north-northeast (i.e., between 12.7° and 38.5° that the shallow convective echoes in the lower layer and the upper- clockwise from north) with magnitudes of 5.4–10.4 m s−1. Therefore, level echoes were separated. In Fig. 6g and in the region within 20 km the HYVIS release site was located at the downshear left flank of the of the radar in Fig. 6e, saw-like echo patterns with horizontal scale of a typhoon center. Previous studies have shown that inner-core convective few kilometers and vertical scale of 0.5–1.0 km can be seen along the activity is enhanced on the downshear left flank (Frank and Ritchie, bottom of the upper-level echoes. 1999; Corbosiero and Molinari, 2002). In an area further than 20 km from the radar in Fig. 6g, the radar Satellite-derived infrared images of cloud and JMA radar-estimated observed a vertical cross section of the inner extending to the precipitation patterns just after the HYVIS release times are shown in north from the western eyewall echo shown in Fig. 5g/n. It can be seen Figs. 4 and 5, respectively, in which panels a–g and h–n correspond to that the echoes of the inner rainband extend seamlessly from the No. 1–7 HYVIS, respectively. HYVIS trajectories are also shown by ground to the height of 9 km. In this region, precipitation processes

Table 1 Central pressure of Typhoon Francisco (hPa) and distance from the typhoon center (km) to the observation site at the time of HYVIS release. Station pressure (hPa), temperature (°C), and relative humidity (%) at the HYVIS observation site at the time of HYVIS release are also shown. HYVISs were released at 8 m above mean sea level.

No. Release time Central pressure (hPa) Distance from the typhoon center (km) HYVIS release site

Station pressure (hPa) Temperature (°C) Relative humidity (%)

1 0253 JST on 23 945 550 1001.4 23.8 94 2 0531 JST on 23 945 510 999.9 24.3 93 3 1538 JST on 23 955 360 995.2 24.9 97 4 1738 JST on 23 955 350 995.2 25.1 95 5 0059 JST on 24 955 290 992.9 25.4 94 6 0540 JST on 24 960 260 991.2 25.6 88 7 1333 JST on 24 960 230 988.1 25.4 90

4 T. Ohigashi, et al. Atmospheric Research 235 (2020) 104736

Fig. 4. Infrared brightness temperature (gray and color scales: °C) obtained by a geostationary satellite (Multifunctional Transport Satellite-2). Brightness tem- peratures below −30 °C are shown in color. Indicated times correspond to when the HYVISs ascended into outflow layers. In panels (a–g), the black and white rectangles indicate the areas shown in panels (h–n) and Fig. 5a–g; the orange “x” in (a–g) and (h) indicates the HYVIS release site; while the red closed circle and red straight line in (h) represent the location of the COBRA and ranges of RHI scans shown in Fig. 6, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) including the ice phase could have contributed to the surface pre- droplets) was approximately 10 L−1 and it did not exceed 50 L−1 in cipitation. The upper-level echoes within 20 km of the radar were found most regions. Because of the small size of the particles, it was difficult to connected horizontally to these deep echoes. Therefore, we consider distinguish particle type and thus all ice particles were classified as that the upper-level clouds observed by the seven sondes were formed Undetermined type for No. 1–6 HYVIS. The lower limit of height at mainly in the deep convective clouds associated with the eyewall and which particles were present decreased gradually from 10.5 km for No. inner rainband, and that they spread outward from the typhoon center. 1 HYVIS to 7 km for No. 7 HYVIS. In the first two HYVIS profiles, the The vertical distributions of particle number density obtained by the vertical thickness of the outflow layer was found limited to <2km, seven HYVIS observations are shown in Fig. 7. Except for No. 7 HYVIS, whereas it was approximately 5 km in the profiles of No. 3–7 HYVIS. the number density of ice particles (excluding supercooled water In No. 7 HYVIS, the averaged number density of particles was

5 T. Ohigashi, et al. Atmospheric Research 235 (2020) 104736

Fig. 5. Radar-estimated precipitation intensity (color scale: mm h−1) obtained by the Japan Meteorological Agency composite radar. Indicated times in (a–g) and (h–n) correspond to the times when the HYVISs were ascending in outflow layers. (a–g) Black rectangles indicate the area shown in (h–n). The orange “x”indicates the location of the HYVIS release site. (h–n) Red straight and orange lines represent ranges of RHI scans shown in Fig. 6 and HYVIS trajectories, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) around 50 L−1 between the heights of 7.5 and 10.25 km; the maximum particles with length of > 125 μm were not observed in the soundings number density was 180 L−1. These averaged and maximum number of No. 1–6 HYVIS (Fig. 8a–f). Relatively larger particles of 125–325 μm densities are considerably higher in comparison with other vertical were observed only in the sounding of No. 7 HYVIS (Fig. 8g), in which profiles. Particles classified into the Undetermined type remained the number density was considerably larger than the others (Fig. 7g). dominant because of the large number of small particles, while the total In the profiles of No. 5 and No. 6 HYVIS, supercooled water droplets number of particles classified as Plate, Column, Frozen Drop, and were observed. The thickness of the layer containing supercooled water Aggregate types was only 25 L−1 at most. droplets was 250 and 60 m for the No. 5 and No. 6 HYVIS, respectively, The particle size distribution observed by each HYVIS is shown in and the number density was in the range 10–30 L−1. This value is re- Fig. 8. The particle size distribution was created using all observed ice markably low in comparison with the number density of common liquid particles at heights of 7–14 km, and the number was calculated for 25- clouds (e.g. Pruppacher and Klett, 1996). Examples of the supercooled μm bins. Most particles had length of ≤100 μm in all soundings, and water droplets obtained by No. 5 and No. 6 HYVIS are shown in Fig. 9a

6 T. Ohigashi, et al. Atmospheric Research 235 (2020) 104736

Fig. 6. Vertical cross sections of radar reflectivity (color scale: dBZ) obtained by RHI scans at an azimuth angle of 41.2° (clockwise from north) usingtheCOBRA. Indicated times in (a–g) correspond to the times when the HYVISs were ascending in outflow layers. Radar observation was performed independently of theHYVIS observations, and the directions of the RHI scans (red lines in Figs. 4h and 5h–n) and HYVIS positions (orange lines in Fig. 5h–n) were not synchronized. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) and b, respectively. The supercooled water droplets in Fig. 9a were shown in Fig. 10f), around which supercooled water droplets existed. observed at a height of 10.1 km and at a temperature of −30.1 °C. The For all sounding profiles (Fig. 10a–g), Si largely decreased down- image shown in Fig. 9b was taken at a height of 10.2 km and at a ward just below the layer in which Si was found near unity (or larger) temperature of −30.7 °C. The droplet diameters were mostly ≤25 μm. and where ice particles were observed. Minima of Si in the dry layer just However, in the lowermost part of the layer of supercooled water below the outflow-layer clouds were 0.01–0.34 (for relative humidities droplets observed in the No. 5 HYVIS profile, the maximum diameter of of 1%–31%). In the layer in which Si decreased sharply downward, the the droplets reached 50 μm (e.g., middle panel of Fig. 1c). potential temperature was found almost constant, indicating nearly

The saturation ratio with respect to ice (Si) obtained by the radio- statically neutral stratification, especially closer to the typhoon center. sondes released with each of the HYVISs is shown in Fig. 10. When Si is This indicates that sublimation evaporation cooling occurred in the larger (less) than unity, ice particles grow (sublimate). It can be seen uppermost part of the dry layer as the ice particles fell, and that vertical that Si in profile No. 1 indicates unsaturation in all layers (Fig. 10a). For overturning resulted in the formation of the almost statically neutral profile No. 2, a thin ice saturated layer is evident at 10.0–10.5 km stratification. (Fig. 10b). This layer corresponds to the region in which ice particles The saw-like echo patterns that were seen along the bottom of the were present (Fig. 7b). In contrast, Si in the upper part of the profile outflow-layer clouds (Fig. 6e and g) were present above this dry layer. shows ice subsaturated conditions and the degree of subsaturation is The dry layer below the outflow-layer clouds and the statically neutral greater than in the other profiles. Therefore, ice crystals sublimated and stratification along the bottom of the outflow-layer clouds were features tended to dissipate in the outer regions corresponding to profiles No. 1 common in all soundings and they reflected general thermodynamical and No. 2. In the other profiles, a relatively thicker ice saturation layer structures in the outer region of Typhoon Francisco. Therefore, it can be is seen where Si is close to or exceeds unity (Fig. 10c–g). The thickness inferred that such temperature and humidity structures were formed of this layer was in the range of 2.5–4.5 km, which is in good agreement within the saw-like echo regions, although the positions of the radio- with the thickness of the layer in which the ice particles were observed. sonde and the RHI observations where the saw-like echo patterns were Therefore, in the thick neutral or supersaturated layer with respect to seen differed (Figs. 4 and 5). ice in profiles No. 3–7, ice particles could have been blown outward without significant sublimation. It was noted that a layer in which Si was very close to the water saturation line (blue broken line) had 5. Summary and conclusions thickness of approximately 1 km (extending from 8.9 to 10.2 km in profile No. 5 shown in Fig. 10e and from 9.8 to 10.8 km in profile No. 6 In October 2013, to observe ice particles in the upper-tropospheric outflow layer of a typhoon, seven HYVISs were released into Typhoon

7 T. Ohigashi, et al. Atmospheric Research 235 (2020) 104736

Fig. 7. Number concentration (L−1) of particles observed by the HYVISs. Colors represent particle type: Plate (red), Column (navy blue), Supercooled water (sky blue), Frozen drop (purple), Aggregate (yellow), and Undetermined (gray). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Francisco (2013) as it approached the Okinawa Islands in Japan. although their number concentration was low. The largest particle Typhoon Francisco showed the 30th lowest central pressure (920 hPa) diameter exceeded 300 μm. The number density of ice particles ob- and the 23rd longest lifetime of the 245 typhoons generated during the served by the HYVIS released into the outer part of the typhoon was 10-year period of 2009–2018. The observations were performed during around 10 L−1 with a maximum value of 50 L−1. The diameter of most October 23–24, 2013, when the central pressure of Francisco was in- particles was < 50 μm. creasing slightly from 945 to 960 hPa. The HYVIS observations were Several aircraft observations around eyewall regions have been conducted at distances of 230–550 km from the typhoon center. During conducted. Black and Hallett (1986; flight level = 6 km), Houze et al. the HYVIS observation period, the area below the outflow-layer clouds (1992; 6 km), and Heymsfield et al. (2006; 8.5–9.5 km) used the 2D-C was very dry, and the radar echo was separated between the lower and imaging probe of the Particle Measuring System to measure parti- upper levels except for near No. 7 HYVIS. Therefore, it is considered cles > 50 μm. They found that number densities were approximately that the outflow-layer clouds were formed in the upper levels oftall 200 L−1 (> 50 μm) within 5–10 km of the eyewall updraft maxima, convective clouds seen near the typhoon center and that they flowed however rapidly decreased to 50 L−1 on the outside. The HYVIS ob- outward without further convective influence. servations in this study were carried out further outside from the up- Observations from the HYVIS released when Francisco was at its draft. For example, No.7 HYVIS was released about 50 km away from closest to the release site revealed the average (maximum) number the rainband (Fig. 5n). Limiting our observations to particles of > 50 concentration of ice particles was approximately 50 L−1 (180 L−1). In μm yielded an average number density of < 5 L−1 for No. 7 HYVIS. addition, particles with diameter of ≥100 μm were also observed, These values were significantly lower than those measured in eyewall

8 T. Ohigashi, et al. Atmospheric Research 235 (2020) 104736

Fig. 8. Particle size distribution (L−1 mm−1) derived from HYVIS images. Horizontal axis represents maximum dimension of ice particles in μm.

Fig. 9. Images of supercooled water droplets: (a) image ac- quired at a height of 10.1 km at 01:24:41 JST on October 24, 2013 during ascent of No. 5 HYVIS, when the ambient tem- perature was −30.1 °C, and (b) image acquired at a height of 10.2 km at 06:05:09 JST on October 24, 2013 during ascent of No. 6 HYVIS, when the ambient temperature was −30.7 °C.

updrafts and surrounding regions in the previous studies. Further away considerably by the enhanced convective rainband on the downshear from the updraft (No. 1–6 HYVISs), the number densities of particles left flank and they showed significant deviation from an axisymmetric of > 50 μm were even lower (Fig. 8). These observations indicate that structure. most particles > 50 μm form in the strong updrafts associated with Along the bottom of the outflow-layer clouds, saw-like radar echo eyewalls and rainbands, and fall within 50–100 km of the updraft re- patterns were found with a very dry layer present beneath. Saturation gions; while smaller ice particles remain without falling and form the ratio minima relative to ice within the dry layer were 0.01–0.34. outflow-layer clouds. Statically neutral stratification was formed between the outflow layer The thickness of the layer containing the ice particles, in which the and the very dry layer below. The location of this statically neutral water vapor indicated near ice saturation conditions or more, was ap- region corresponds to one of the three layers showing a low bulk proximately 2.5–4.5 km at distances of 230–360 km from the typhoon Richardson number, which represents certain turbulence, in tropical center. In contrast, the two outermost HYVIS profiles (distances of 510 cyclones (Molinari et al., 2014). Kudo (2012) reported that commercial and 550 km from the typhoon center) indicated that the ice saturation aircraft have sometimes encountered turbulence beneath the outflow- layer was very thin or absent. This indicates that ice particles tended to layer clouds of typhoons. Using idealized simulations, he showed that disappear by sublimation evaporation in the outer part of the outflow falling snow sublimated below cloud base could cool the air, which layer. This process could occur axisymmetrically. Conversely, particles would create a layer of absolute instability. He concluded that this observed near the innermost part of the typhoon appeared influenced generated Rayleigh–Bénard convection. Therefore, it is inferred that ice

9 T. Ohigashi, et al. Atmospheric Research 235 (2020) 104736

Fig. 10. Potential temperature θ (red solid lines: K) and saturation ratio with respect to ice Si (blue solid lines). The Si at water saturation is shown by blue broken lines. Panels a–g correspond to No. 1–7 HYVISs, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to theweb version of this article.) particles falling from the outflow layer in this study disappeared role of cloud radiative forcing on typhoons, especially regarding the through sublimation evaporation in the dry layer below the outflow maximum strength and temporal change of intensity. The COBRA used layer, which could explain the appearance of the saw-like echo pat- in this study operates in the C-band wavelength, and it was impossible terns. Although the very dry layer below the saw-like echoes and the to observe the entire extent of the clouds within the outflow layer. statically neutral stratification along the bottom of the echoes were Recently, cloud radar with polarimetric function has been introduced in common structures, the saw-like echoes did not always appear, as Japan. It is expected that the average characteristics, magnitude of shown in Fig. 6f. To clarify the formation conditions and mechanism of fluctuations, and fine structure of outflow-layer clouds both spatially the saw-like echoes, it will be necessary to reveal the detailed cloud and temporally will be clarified in the future using dense cloud radar microphysical, dynamical, and thermodynamical structures around data. In addition, it is necessary to acquire further observations of cloud such features. microphysics for various typhoons using aircraft in addition to HYVISs. In two HYVIS profiles, supercooled water droplets were observed at low concentration (10−30 L−1) in thin layers (thickness of 60 and Declaration of competing interest 250 m) at a temperature of approximately −30 °C. Relative humidity close to water saturation was observed in a layer of approximately 1-km The authors declare that they have no known competing financial thickness. Around this layer, the number concentration of ice particles interests or personal relationships that could have appeared to influ- increased. Therefore, this layer might have contributed to the formation ence the work reported in this paper. of ice particles in the outflow layer. The spatial extent of supercooled water droplets and the formation processes of the rising motion pro- ducing the supercooled water droplets remain to be clarified in future Acknowledgments work. Based on observations, this study revealed quantitatively the dis- We thank A. Ichinose, Y. Ohwaki, T. Sakai, and Y. Ushita of the tribution of ice crystals in the outflow layer of a typhoon. Although the Meteorology Research Group of Nagoya University for conducting the estimation of radiative heating was beyond the scope of this study, it balloon releases during high winds. We are also grateful to J. Amagai, must be examined in future work using such observational data. In K. Sato, H. Iwai, and the staff of the Okinawa Electromagnetic addition, after conducting validation of outflow-layer clouds in simu- Technology Center, National Institute of Information and lations using the observational results, it will be necessary to clarify the Communications Technology for their cooperation in relation to the observations. Thanks are extended to T. Sawada and T. Matsunaga of

10 T. Ohigashi, et al. Atmospheric Research 235 (2020) 104736 the Meisei Electric Co. for their technical assistance with the observa- Kobayashi, S., Ota, Y., Harada, Y., Ebita, A., Moriya, M., Onoda, H., Onogi, K., Kamahori, tions. The valuable comments received from the reviewers helped H., Kobayashi, C., Endo, H., Miyaoka, K., Takahashi, K., 2015. The JRA-55 Reanalysis: general specifications and basic characteristics. J. Meteor. Soc. Japan. greatly in improving the manuscript. This study was supported jointly Ser. 93, 5–48. https://doi.org/10.2151/jmsj.2015-001. by the Japan Society for the Promotion of Science KAKENHI Grant Nos. Kudo, A., 2012. The generation of turbulence below midlevel cloud bases: the effect of 25287121 and 16H06311, Virtual Laboratory for the Earth's Climate cooling due to sublimation of snow. J. Appl. Meteor. Climatol. 52, 819–833. https:// doi.org/10.1175/JAMC-D-12-0232.1. Diagnostics Program, and Tropical cyclones-Pacific Asian Research Lord, S.J., Willoughby, H.E., Piotrowicz, J.M., 1984. Role of a parameterized ice-phase Campaign for Improvement of Intensity estimations/forecasts (T- microphysics in an axisymmetric, nonhydrostatic tropical cyclone model. J. Atmos. PARCII). Multifunctional Transport Satellite-2 data were provided by Sci. 41, 2836–2848. https://doi.org/10.1175/1520-0469(1984) the Center for Environmental Remote Sensing, Chiba University. 041<2836:Roapip>2.0.Co;2. Magono, C., Lee, C.W., 1966. Meteoprological classification of natural snow crystals. J. Fac. Sci., Univ. Ser. VII. 2, 321–335. References McFarquhar, G.M., Black, R.A., 2004. Observations of particle size and phase in tropical cyclones: Implications for mesoscale modeling of microphysical processes. J. Atmos. Sci. 61, 422–439. https://doi.org/10.1175/1520-0469(2004)061<0422:Oopsap>2. Black, R.A., 2003. Extra large particle images at 12 km in a hurricane eyewall: evidence of 0.Co;2. high-altitude supercooled water? Geophys. Res. Lett. 30. https://doi.org/10.1029/ McFarquhar, G.M., Zhang, H., Heymsfield, G., Halverson, J.B., Hood, R., Dudhia, J., 2003gl017864. Marks, F., 2006. Factors affecting the evolution of Hurricane Erin (2001) andthe Black, R.A., Hallett, J., 1986. Observations of the distribution of ice in hurricanes. J. distributions of hydrometeors: role of microphysical processes. J. Atmos. Sci. 63, Atmos. Sci. 43, 802–822. https://doi.org/10.1175/1520-0469(1986) 127–150. https://doi.org/10.1175/jas3590.1. 043<0802:Ootdoi>2.0.Co;2. Mei, W., Xie, S.-P., 2016. Intensification of landfalling typhoons over the northwest Bringi, V.N., Thurai, M., Nakagawa, K., Huang, G.J., Kobayashi, T., Adachi, A., Hanado, Pacific since the late 1970s. Nat. Geosci. 9, 753–757. https://doi.org/10.1038/ H., Sekizawa, S., 2006. Rainfall estimation from C-band polarimetric radar in ngeo2792. Okinawa, Japan: comparisons with 2D-video disdrometer and 400 MHz wind pro- Molinari, J., Duran, P., Vollaro, D., 2014. Low Richardson number in the tropical cyclone filer. J. Meteor. Soc. Japan. 84, 705–724. https://doi.org/10.2151/jmsj.84.705. outflow layer. J. Atmos. Sci. 71, 3164–3179. https://doi.org/10.1175/jas-d-14- Bu, Y.P., Fovell, R.G., Corbosiero, K.L., 2014. Influence of cloud-radiative forcing on 0005.1. tropical cyclone structure. J. Atmos. Sci. 71, 1644–1662. https://doi.org/10.1175/ Murakami, M., Matsuo, T., 1990. Development of the hydrometeor videosonde. J. Atmos. Jas-D-13-0265.1. Ocean. Technol. 7, 613–620. https://doi.org/10.1175/1520-0426(1990) Cao, Y., Fovell, R.G., Corbosiero, K.L., 2011. Tropical cyclone track and structure sensi- 007<0613:DOTHV>2.0.CO;2. tivity to initialization in idealized simulations: a preliminary study. Terr. Atmos. Murakami, M., Yamada, Y., Matsuo, T., Mizuno, H., Morikawa, K., 1992. Microphysical Ocean. Sci. 22, 559–578. https://doi.org/10.3319/Tao.2011.05.12.01(Tm). structures of warm-frontal clouds - the 20 June 1987 case study. J. Meteor. Soc. Corbosiero, K.L., Molinari, J., 2002. The effects of vertical wind shear on the distribution Japan 70, 877–895. https://doi.org/10.2151/jmsj1965.70.5_877. of convection in tropical cyclones. Mon. Weather Rev. 130, 2110–2123. https://doi. Murakami, M., Matsuo, T., Mizuno, H., Yamada, Y., 1994. Mesoscale and microscale org/10.1175/1520-0493(2002)130<2110:Teovws>2.0.Co;2. structures of snow clouds over the . Part I: evolution of microphysical Fiorino, M., Elsberry, R.L., 1989. Some aspects of vortex structure related to tropical structures in short-lived convective snow clouds. J. Meteor. Soc. Japan 72, 671–694. cyclone motion. J. Atmos. Sci. 46, 975–990. https://doi.org/10.1175/1520- https://doi.org/10.2151/jmsj1965.72.5_671. 0469(1989)046<0975:Saovsr>2.0.Co;2. Nakagawa, K., Hanado, H., Satoh, S., Takahashi, N., Iguchi, T., Fukutani, K., 2003. Fovell, R.G., Su, H., 2007. Impact of cloud microphysics on hurricane track forecasts. Development of a new C-band bistatic polarimetric radar and observation of typhoon Geophys. Res. Lett. 34. https://doi.org/10.1029/2007GL031723. events. 31st Conference on Radar Meteorology, Vols 1 and 2. Amer Meteorological Fovell, R.G., Corbosiero, K.L., Kuo, H.C., 2009. Cloud microphysics impact on hurricane Society, Boston, pp. 863–866. track as revealed in idealized experiments. J. Atmos. Sci. 66, 1764–1778. https://doi. Ohigashi, T., Tsuboki, K., Oue, M., 2016. Cloud-top supercooled liquid droplets in stra- org/10.1175/2008jas2874.1. tiform clouds observed during winter in inland Hokkaido, Japan. SOLA. 12, 140–145. Fovell, R.G., Corbosiero, K.L., Seifert, A., Liou, K.N., 2010. Impact of cloud-radiative https://doi.org/10.2151/sola.2016-030. processes on hurricane track. Geophys. Res. Lett. 37. https://doi.org/10.1029/ Orikasa, N., Murakami, M., 1997. A new version of hydrometeor videosonde for cirrus 2010GL042691. cloud observations. J. Meteor. Soc. Japan. 75, 1033–1039. https://doi.org/10.2151/ Fovell, R.G., Bu, Y.P., Corbosiero, K.L., Tung, W.-W., Cao, Y., Kuo, H.-C., Hsu, L.-H., Su, jmsj1965.75.6_1033. H., 2016. Influence of cloud microphysics and radiation on tropical cyclone structure Orikasa, N., Murakami, M., 2015. Ice crystal shapes in midlatitude cirrus clouds derived and motion. Meteor. Monogr. 56, 11.01–11.27. https://doi.org/10.1175/ from hydrometeor videosonde (HYVIS) observations. J. Meteor. Soc. Japan. 93, amsmonographs-d-15-0006.1. 143–155. https://doi.org/10.2151/jmsj.2015-004. Frank, W.M., Ritchie, E.A., 1999. Effects of environmental flow upon tropical cyclone Orikasa, N., Murakami, M., Heymsfield, A.J., 2013. Ice crystal concentration in mid- structure. Mon. Weather Rev. 127, 2044–2061. https://doi.org/10.1175/1520- latitude cirrus clouds: in situ measurements with the balloonborne hydrometeor vi- 0493(1999)127<2044:EOEFUT>2.0.CO;2. deosonde (HYVIS). J. Meteor. Soc. Japan. 91, 143–161. https://doi.org/10.2151/ Heymsfield, A.J., Bansemer, A., Durden, S.L., Herman, R.L., Bui, T.P., 2006. Icemicro- jmsj.2013-204. physics observations in Hurricane Humberto: comparison with non-hurricane-gen- Oue, M., Ohigashi, T., Tsuboki, K., Nakakita, E., 2015. Vertical distribution of pre- erated ice cloud layers. J. Atmos. Sci. 63, 288–308. https://doi.org/10.1175/ cipitation particles in Baiu frontal stratiform intense rainfall around Okinawa Island, jas3603.1. Japan. J. Geophys. Res. Atmos. 120, 5622–5637. https://doi.org/10.1002/ Houze, R.A., Marks, F.D., Black, R.A., 1992. Dual-aircraft investigation of the inner core 2014JD022712. of Hurricane Norbert. Part II: Mesoscale distribution of ice particles. J. Atmos. Sci. 49, Pruppacher, H.R., Klett, J.D., 1996. Microstructure of clouds. In: Microphysics of Clouds 943–963. https://doi.org/10.1175/1520-0469(1992)049<0943:Daioti>2.0.Co;2. and Precipitation, 2nd ed. Springer, Netherlands, pp. 15–24. Hsu, L.H., Kuo, H.C., Fovell, R.G., 2013. On the geographic asymmetry of typhoon Tao, W.-K., Shi, J.J., Chen, S.S., Lang, S., Lin, P.-L., Hong, S.-Y., Peters-Lidard, C., Hou, A., translation speed across the mountainous island of Taiwan. J. Atmos. Sci. 70, 2011. The impact of microphysical schemes on hurricane intensity and track. Asia- 1006–1022. https://doi.org/10.1175/Jas-D-12-0173.1. Pac. J. Atmos. Sci. 47, 1–16. https://doi.org/10.1007/s13143-011-1001-z. Ito, K., 2016. Errors in tropical cyclone intensity forecast by RSMC Tokyo and statistical Trabing, B.C., Bell, M.M., Brown, B.R., 2019. Impacts of radiation and upper-tropospheric correction using environmental parameters. SOLA. 12, 247–252. https://doi.org/10. temperatures on tropical cyclone structure and intensity. J. Atmos. Sci. 76, 135–153. 2151/sola.2016-049. https://doi.org/10.1175/jas-d-18-0165.1. Ito, K., Yamada, H., Yamaguchi, M., Nakazawa, T., Nagahama, N., Shimizu, K., Ohigashi, Tsuboki, K., Yoshioka, M.K., Shinoda, T., Kato, M., Kanada, S., Kitoh, A., 2015. Future T., Shinoda, T., Tsuboki, K., 2018. Analysis and forecast using dropsonde data from increase of supertyphoon intensity associated with climate change. Geophys. Res. the innercore region of Tropical Cyclone Lan (2017) obtained during the first aircraft Lett. 42, 646–652. https://doi.org/10.1002/2014GL061793. missions of T-PARCII. SOLA. 14, 105–110. https://doi.org/10.2151/sola.2018-018.

11