Mammatus‐Like Echo Structures Along the Base of Upper‐Tropospheric Outflow‐Layer Clouds of Typhoons Observed by Cloud Radar

Upper‐tropospheric clouds in the outflow layer of typhoons can affect the track of typhoons (tropical cyclones) through radiation effects. In this study, the microstructure of the outflow‐layer clouds of several typhoons was examined. Cloud radar observations of three typhoons around Japan revealed numerous protuberances in echoes along the base of the upper‐level clouds, which are referred to as mammatus‐like echoes. The horizontal and vertical scales of these mammatus‐like echoes were 0.5–3.0 and 0.3–1.5 km, respectively. Vertical observations revealed downward (upward) Doppler velocities in (between) the hanging echo regions. Upward and downward velocity maxima were estimated at 3 m s−1 around the mammatus‐like echoes. Neutral stratification developed in the dry layer beneath the cloud base in which the mammatus‐like echoes formed. These mammatus‐like structures may promote mixing along the cloud base that contributes to dissipation of the outflow‐layer clouds.

• Protuberances observed by cloud radar along the base of outflowlayer clouds of typhoons are termed mammatus-like echo structures • The mammatus-like echo structures are associated with vertical motions with velocity of several meters per second • Mammatus-like echoes occur in a layer with high potential for turbulence caused by sublimation of ice particles falling into a drier layer

Supporting Information:
Supporting Information may be found in the online version of this article.

Citation:
Ohigashi, T., Tsuboki, K., Shinoda, T., Minda, H., Kyushima which is very likely related to turbulence. One of the three layers, which was present at the cloud base near the edge of the central dense overcast clouds, was accompanied by a very dry underlying layer (relative humidity <40%). A similar arrangement of a layer of low i E R values with a very dry underlying layer was also observed in TCs studied by Braun et al. (2016), Kudo (2013), and Molinari et al. (2019). Kudo (2013) reported that many commercial aircraft encountered moderate turbulence below the upper-level clouds. This region of turbulence was found to extend more than 1,500 m below the sharp temperature inversion corresponding to the base of the upper-level clouds. Ohigashi et al. (2020) reported the presence of saw-like echoes along the base of TC outflow-layer clouds. However, because they used a C-band (5.5 GHz, 5.5 cm wavelength) precipitation radar, which can detect relatively large hydrometeors, the detailed structure of the saw-like echoes could not be elucidated.
The 8.6 mm wavelength) cloud radar of Nagoya University (Japan) has been used over several years to observe clouds associated with typhoons around Japan. In comparison with radars that target precipitation, this radar is more sensitive to smaller particles, such as cloud droplets, drizzle, and early ice crystals (Kollias et al., 2007;Maesaka, 2018), and has a narrower beam width that is suitable for observing the microstructure of outflow-layer clouds. This radar detected numerous protuberances in echoes along the base of upper-tropospheric outflow-layer clouds associated with three typhoons. In this study, we investigated the details of these echo structures and examined their formation processes.

Observations and Data
Cloud observations were conducted using the Ka-band cloud radar of Nagoya University. The specifications and settings of the radar are summarized in  (Bessho et al., 2016;Yamamoto et al., 2020) were used to determine the extent of cloud development around the radar observation ranges.
The location and pressure of the center of each typhoon were obtained from the Regional Specialized Meteorological Center Tokyo best track data.

Results
Figure 1 shows infrared images acquired by the Himawari-8 satellite, vertical cross-sections of the equivalent radar reflectivity ( e E Z ) obtained by RHI scans, and PPI images of e E Z at high elevation angles for Chaba (2016), Cimaron (2018), and Neoguri (2019). The PPI observations for Chaba (2016) are not shown because the PPI scan at the highest elevation angle (18.1°) did not reach the upper-level clouds within the observation range. The position and pressure of the typhoon centers, the distance between the typhoon centers and the cloud radar, and the minimum pressure during the lifetime of each typhoon are summarized in Table S2. These radar echo snapshots show the upper clouds of the typhoons at distances of several hundred kilometers from the centers of the typhoons. The clouds for Chaba (2016) were observed when the typhoon was in its most developed phase with central pressure of 905 hPa (Table S2). The cloud top temperatures around the cloud radar site were very low (−80°C to −70°C in Figure 1a). During the observation periods,  Table S2), and the cloud top temperatures around the radar sites were −50°C to −40°C (Figures 1c and 1f). Thus, the three typhoons were observed at different intensities.  Protuberance echoes were found along the base of the upper-level clouds. These protuberance echo structures were evident over a wide region, although they were not present at the base of all upper-level clouds. There were periods for Chaba (2016) when the protuberance echoes were seen throughout the entire range of RHI observations from 0° to 180° elevation (distance: ∼55 km). Additionally, including the period when the protuberance structures were seen only in part of the cloud bases, the protuberance structures were observed almost continuously during 04-20 UTC on October 3, 2016 for Chaba (2016). This corresponds to a distance of ∼290 km, given the distance that the center of the typhoon moved.
The PPI images at high elevation angles (Figures 1e and 1h) show a complicated echo structure in the innermost part of the upper-level clouds. These echoes appear as numerous projections rather than a wavelike (2-dimensional) structure with no variation in any one horizontal direction, and they appear similar to the form of mammatus clouds often observed visually along the base of anvil clouds extending from deep convective clouds (e.g., Schultz et al., 2006). As no visual observations were recorded in this study, these echoes are referred to as mammatus-like echoes.
Enlarged images of the mammatus-like echoes corresponding to In the layer of the mammatus-like echoes, downward (upward) Doppler velocities were present in (between) the hanging echo regions. This is similar to the characteristics of mammatus echoes (Kollias et al., 2005;Schultz et al., 2006). The range of d E V is from −4 to 2 m s −1 . Assuming that the maximum absolute values of upward and downward velocities produced in association with the mammatus-like echo structures were comparable, the maximum/minimum vertical air motion in these large variations can be estimated at ±3 m s −1 , while the sum of the terminal fall velocity of snow particles and the vertical air velocity of the mean field can be determined as −1 m s −1 . In 2016, five radiosondes were released into Chaba (2016) from NICT in Okinawa (blue "+" mark in Figure 1a) at the time when the mammatus-like echoes were seen. The observation results of the radiosonde released at 10:52 UTC on October 3, 2016 are illustrated in Figure 4. The horizontal position of this sonde (blue closed circle) at 11:30 UTC, when the sonde was located at a height of 10.6 km near the base of the outflow-layer clouds, and the cloud distribution at the same time are shown in Figure 1a. At heights below ∼10 km, the difference between the temperature ( E T ) and the dew-point temperature ( d E From the sounding observations, sublimation cooling of ice particles falling into a very dry layer below the cloud base is considered to cause the neutral layer ( 2 E N ≈ 0) just below the cooling layer with i E R < 0.25.
Convective motions within this layer are considered responsible for the formation of the mammatus-like echoes. Molinari et al. (2014) showed that the region with low i E R values was located in the same layer as the unstable layer ( 2 E N ≈ 0) associated with sublimation of snow below the base of the clouds, and at a different location from the strong vertical shear layer slightly above the base of the clouds. Through consideration of convection generation based on the Rayleigh number, Kudo (2013) used numerical simulations to show that convection cells below the base of the clouds were caused by Rayleigh-Bénard convection. In their sensitivity experiments, precipitation became stronger in higher air temperatures, which resulted in a larger temperature decrease attributable to enhanced sublimation cooling. Consequently, convection beneath the cloud base was intensified. In this study, when the cloud base temperature was higher in the order of Neoguri (2019), Cimaron (2018), and Chaba (2016), the maximum e E Z just above the cloud base and the depth of the mammatus-like echoes were larger in the same order. This result, which is consistent with the sensitivity experiments performed by Kudo (2013), supports the idea that buoyant convection in the unstable layer formed by sublimation cooling caused the mammatus-like echoes.
Among the sounding observations released in Chaba (2016), i E R did not reach the critical value below the cloud base for one profile (04:31 UTC, Figure S1b). This is because the vertical shear was too small, despite the weak static stability. This may explain the lack of mammatus-like echo structures in certain regions.

Conclusions
The present study used a Ka-band cloud radar to observe the outflow-layer clouds of three typhoons: Chaba (2016), Cimaron (2018), and Neoguri (2019). Cloud echoes in the upper levels of the typhoons were present in regions where the temperature was <0°C. Values of e E Z increased toward the lower levels and showed maxima at several hundred meters above the base of the clouds. An echo structure with numerous protuberances was observed along the base of the clouds, which was referred to as a "mammatus-like echo." The mammatus-like echoes were 0.5-3.0 km in width and 0. Cooling by sublimation of ice particles occurred just below the cloud base, corresponding to the upper part of the mammatus-like echoes. This cooling, which led to neutral stratification and the small values of i E R , caused vertical mixing evidenced by the mammatus-like echoes. In this study, the cloud radar clearly showed that mammatus-like echo structures with vertical motion of a few meters per second existed in the layer with low i E R values along the base of outflow-layer clouds of multiple typhoons. The mammatus-like structure leads to increased surface area. This may enhance mixing between the clouds and the underlying dry air and promote sublimation of the outflow-layer clouds which plays an important role in the process of dissipation and radiation of outflow-layer clouds of TCs. Numerical simulations are needed to reveal more accurately the horizontal extent and duration of the mammatus-like structure, and the cloud dissipation rate.