Large-scale solid bodies on Earth such as volcanoes and man-made pyramids have been visualized with solid earth muography, and the recently invented technique, acqueous muography, has already demonstrated its capability to visualize ocean tides and tsunami. In this work, atmospheric muography, a technique to visualize and monitor the vertical profile of tropic cyclones (TCs) is presented for the first time. The density distribution and time-dependent behavior of several TCs which had approached Kagoshima, Japan, has been investigated with muography. The resultant time-sequential images captured their warm cores, and their movements were consistent with the TC trails and barometric pressure variations observed at meteorological stations. By combining multidirectional muographic images with barometric data, we anticipate that muography will become a useful tool to monitor the three-dimensional density distribution of a targeted mesoscale convective system.
Article
Atmospheric Muography for Imaging and Monitoring Mesoscale Convective Systems
https://doi.org/10.21203/rs.3.rs-1523711/v1
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Mesoscale convective systems (MCSs) are organized groupings of meteorological systems that involve the most massive convective storm types1. These are storms defined as thunderstorms in the tropics1,2,3 and mid-latitudes4. They can span thousands of square kilometers. Storms qualifying as MCSs are major contributors to extreme precipitation1 and meteotsunamis4. However, our knowledge of MCSs is limited. For example, the relationship between MCSs and precipitation is not quantitatively well understood; also, even though researchers have identified the organized deep convection patterns in tropical MCS, characterization of each event is complex, being linked to the surrounding environment and/or to the MCS’s internal dynamics and microphysics5. These factors influence the development of these deep convective systems6 in somewhat mysterious ways.
Both tropical and warmer midlatitude MCSs are generated in response to atmospheric convective instability coupled with widespread and heavy rainfall. Stable and slow-moving MCSs frequently cause flooding, hail, strong winds, and tornadoes. Although state-of-the-art models are not yet able to explicitly represent convective precipitation due to their coarse grid spacings (i.e. 12/100 km at regional scale, a new model based on a 4 km grid spacing can effectively represent MCSs1. There are two hypotheses for the creation of MCSs. The first hypothesis is that the MCS’s convective cloud is generated by the condensation that occurs in nonhydrostatic buoyant upward air currents, where parcels of warm and moist air are accelerated upward2. These air currents are thought to subsequently generate in-cloud turbulence as they rise, and after being decelerated, this motion will eventually stop and spread out laterally at a level of neutral buoyancy7. Another traditional hypothesis of MCS generation surmises that an upward air motion drives the layers of air to adapt a deep slanting, ascending motion8. According to this theory, the storm is characterized by a pressure reduction in the middle height of the MCS. Wind shear3–5 and density profile6 are the primary and essential parameters for buoyancy-driven convection of the atmosphere, and thus, understanding the pressure/density anomalies associated with MCSs can also be helpful in order to predict its mechanism. Regardless of which hypothesis about the formation of MCSs is ultimately found to be correct, it is evident that monitoring MCS pressure/density anomalies would bring researchers closer to creating accurate early warnings of MCS-associated hazards such as meteotsunamis4. Despite improvements in computer modelling of MCSs, the forecasting performances remain still inadequate7–9. Rapid sea level changes are produced by meteotsunamis11,12. Muography has been successfully tested in monitoring sea level changes within the Tokyo bay10, which could be an early warning in case of tsunamis or meteotsunamis.
Quantifying the mesoscale, and in particular, meso-beta (2-200 km) and meso-gamma (< 2 km) scale time-dependent pressure variations of the terrestrial atmosphere, would enable us to recognize the mechanisms of MCS, and muography has the capacity to be one of the solutions which could provide real-time mapping of the MCS associated pressure variations. Muography is similar to x-ray imagery, but instead of x-rays, it utilizes the strong penetration capability of high-energy muons (> a few tens of GeV) and their relativistic time-dilation effect. By detecting the number of muons that pass through gigantic bodies, like the mass of an entire or portion of an MCS, the internal spatial distribution of its density can be determined; this distribution can be mapped with muography by identifying where these muons passed through the object and subsequently creating a plot of the number of penetrating muons on a 2-dimensional plane. The origin of these high-energy muons is galactic cosmic rays (GCR) which are accelerated by supernovas and their remnants in our galaxy. The GCRs mainly consist of nuclei of hydrogen and helium, though other species are presented in the composition. These charged particles are generally accelerated to almost the speed of light (10− 1-1011 GeV), and spend millions of years in average in diffusive propagation in the interstellar magnetic field. This leads to the isotropic angular distribution of their momentum. In application to muography, it is reasonably safe to assume GCRs as isotropic at the entrance to the Earth’s atmosphere. Energetic GCRs are able to induce cascades of nuclear reactions in air, where multiple secondary particles are produced, generally classified as the hadronic, electromagnetic, muonic components and so-called atmospheric neutrinos [REFERENCE TO P.K.F. Grieder, 2010]. This work is focused at the muonic component (muons and antimuons) of cosmic-ray cascades in the atmosphere.
Muography takes advantage of the characteristics of the muon, particularly its penetrative nature and universality, for a wide variety of applications on Earth, including visualizing the internal structure of targeted gigantic solid earth and aqueous bodies9–27; its characteristics are also ideal for the application of visualizing the atmosphere. Solid muography (visualizing interiors of large-scale solid bodies on Earth) was invented by Luis Walter Alvarez and his group in 19689, aqueous muography (visualizing and monitoring large-scale liquid bodies on Earth such as seas and lakes) was invented by Hiroyuki K.M. Tanaka and his group in 202127. In this paper, the development of atmospheric muography (visualizing and monitoring large-scale gas bodies on Earth such as tropic cyclones) and its performance are presented. Ideas of this kind were proposed recently (e.g., Kozyrev et al., 2015) but not developed further. We report that a new muography imaging method could be used to two-dimensionally project density structures to render an image of TC (tropical cyclone) associated pressure anomaly variations.
Vertical muon flux versus atmospheric pressure. Cosmic-ray muons are produced in interactions of primary cosmic rays and ambient atoms in the Earth’s atmosphere, and most muons (both m+ and m-) are generated around the tropopause (an atmospheric depth of 100 gcm-2). Figure 1 shows the relationship between the vertical muon flux (N) at sea level and the sea-level atmospheric pressure (r) on Earth28. If the integral mass of matter (e.g., in g cm-2 or hPa) along the muon path increases, the muons lose more energy before reaching sea level and thus, the probability that either the muon stops or decays increases. This relationship can be approximated to be a linear relationship within the atmospheric pressure range between 990 and 1000 hPa. Generally, if the pressure drops by 1%, the muon flux increases by ~2%. For horizontally arriving muons, since they traversed the atmosphere with a thickness of 36,000 gcm-2, an increase in the average mass along the muon path by 1% increases the minimum muon energy required to arrive at the ground surface by 1 GeV (an increase from 106 GeV to 107 GeV)29. This reduces the horizontal muon flux from 4.5 m-2 s-1 sr-1 to 4.3 m-2 s-1 sr-1 (~5%). Consequently, if the barometric pressure drops uniformly by 10 hPa throughout the upper hemisphere, the muon flux increases 2-5% depending on the muon's arrival angles.
Observations. The observations were conducted in the city of Kagoshima, Japan. In Japan, Kagoshima is known to be a city prone to typhoons. A typhoon is defined as a TC when it develops in the Northwestern Pacific Basin, one of the most active TC areas in the world30. All types of typhoons are most common from July through to September in Kagoshima. During the current observations, muographic data were collected during the three previous typhoon seasons in 2016 (July 19-September 19, 2016), 2019 (July 10-October 10, 2016), and 2021 (July 10-October 10, 2016). During these observation periods, eight typhoons (T-1610, T-1612, T-1613, T-1616 in 2016, T-1908, T-1910, T-1917 in 2019, T-2109 in 2021) approached Kagoshima. In 2020, no typhoon landed in Japan. The recorded characteristics of the typhoons are summarized in Table 1. Figures 2, 3 and 4 respectively, show these typhoons' meteorological history in 2016, 2019, and 2021.
Table 1. Characteristics of the typhoons during which muographic data were collected. A strong wind area is defined as an area where the wind speed exceeds 15 ms-1, 31.
Muograph. A muograph is a device for recording the muographic data. The detection plane of the current muograph located in Kagoshima (Kagoshima Muograph: KM) is positioned vertically in relation to the ground surface. KM consists of three layers of segmented scintillation detectors (SSD) where the distance between the uppermost and lowermost stream detectors was set to be 1.5 m. In the KM 2016 model, each position-sensitive plane consisted of Nx=14 and Ny=14 adjacent scintillator strips, forming a segmented plane with 14 × 14 segments. The width of each scintillator strip was 10 cm. Therefore, at the beginning of the observation, each SSD had a 1.4 × 1.4 m2 active area with a spatial resolution of 10 cm, but these SSDs were eventually upgraded to 15 × 15 segments; hence it now has a 1.5 × 1.5 m2 active area. Each SSD was placed at the same interval, and a radiation shield with thicknesses of 10 cm lead and 2 cm stainless steel was inserted between each SSD interval. KM had an elevation viewing angle of 0o-45o and an azimuth viewing angle of ±45o, and it was pointed in the south by southwest (SSW) direction (sea direction); it had the capability to detect muons also arriving from the opposite direction (where the mountain is located), i.e., north by northeast (NNE), but since the viewing angle in this direction was partially covered by mountains, the NNE data were not considered in this work. Figures 2-5 indicate the vertical and horizontal viewing angles of KM that is located at an altitude of 146 m above sea level.
Most muons are generated at the height of 15-20 km above sea level. Therefore, the KM was designed to record the muons travelling through an atmosphere 20 km to 500 km thick; the length of the muon’s pathway (MPL) to the surface of the Earth depends on its arrival angle (Figure 5). Due to (a) the detector's geometric acceptance and (b) the zenith angular dependence of cosmic ray muons, 50% of the total muon events were recorded within the zenith and azimuth angular range between 60o and 80o and ±15o, respectively.
Figures 6-8 compare the muon counts collected every 4 hours with the atmospheric pressure values observed at three barometric pressure (BP) monitoring stations (Kagoshima, Yakushima and Naze as indicated in Figures 2-4) within the viewing angle of the KM 31. Yakushima is located 150 km south of Kagoshima, and Naze is located 380 km southwest of Kagoshima. There is a clear anti-correlation between muon counts and BP. Overall, the BP variations are in agreement among these three stations and the muon count rate increases by ~2% when the pressure drop (PD) had reached 10 hPa. By considering many of these are slanted muons, this level of the muon flux increase is in agreement with the relationship shown in Figure 1. As shown in Figure 6, the PD event, which started on August 28, 2016, was associated with the approach of T-1610; since T-1610 remained south of Kagoshima for a week, the PD steadily continued until the end of August when this typhoon transformed into an extratropical cyclone. The PD associated with the approach of T-1616 started on September 14, and accordingly, an increase of the muon count rate was observed; however, since the path of the typhoon was much closer to Kagoshima, this typhoon caused an electricity blackout, and data taking at the Kagoshima MOS was cut off at 20:00 on September 19th. Two other minor PD events observed within the periods between September 1-4 and between September 6-9 have been respectively associated with T-1612 and T-1613. As shown in Figure 7, the largest PD event observed within the period between August 10-17, 2019, was associated with T-1910, while the second-largest PD event was observed within the periods between September 20-23 was associated with T-1917. Another PD event observed on August 4-7 was associated with T-1908. In Figure 8, the largest PD event was observed within the period between August 4-10, 2021 and associated with T-2109.
As shown in Figs. 6, 7, and 8, the overall trend of the TC-driven PD events recorded at these weather stations matched except for those observed in the period between September 1–4, 2016 (T-1612 PD), and between August 4–7, 2019 (T-1908 PD). The common feature of these discrepancies can be summarized as follows: The PD negative spikes associated with T-1612 and T-1908 can be seen in the Kagoshima and Yakushima pressure data, but these cannot be seen in Naze data, and vice versa. Since the distance between Kagoshima and Yakushima is 130 km, and the distance between Kagoshima and Naze is 360 km, we expected meso-beta scale pressure anomalies during the aforementioned periods. We also anticipated that these anomalies could be captured with KM.
Figure 9 compares the high elevation angle (HEA) (0.4 < tan θ < 0.8; 25 km < MPL < 50 km at an altitude of 20 km ) (Fig. 9A) and low elevation angle (LEA) (0 < tan θ < 0.4; 50 km < MPL < 506 km at an altitude of 20 km) (Fig. 9B) components of the time-sequential muographic data for the 9 days during which T-1610 and T-1612 caused the local PD. The HEA component represents the muons that passed through a more local atmosphere, closer to KM, and the LEA component represents those muons that passed through a less local atmosphere, further from KM. In Fig. 9C, the time-dependent barometric variations observed at Yakushima and Naze stations31 are presented for reference. As can be seen in Figs. 9A and 9B, there are two muon count rate (MCR) increase events between August 27-September 1 and September 1–5. These MCR increase events were interpreted as PD events associated with T-1610 and T-1612. These events were labeled MCR1610 and MCR1612 here. In Figs. 9A and 9B, we find that the magnitude of HEA-MCR1610 is larger than LEA-MCR1610. Also, we find that the timing and duration are both different between HEA-MCR1612 and LEA-MCR1612. These results are consistent with the pressure variations observed at Yakushima (closer to KM) and Naze (further from KM) stations: PD smaller than that at Yakushima was observed at Naze for T-1610, and at Naze, T-1612 associated PD appeared earlier and lasted longer than Yakushima.
Figure 10. Time-sequential muographic images (A). The four images show the muon flux variations observed every 8 hours within the period between 00:00 on September 3, 2016, and 08:00 on September 4, 2016. The horizontal width of these muographic images corresponds to the shaded area indicated on the map on the right side. The meteorological history of T-1612 is also shown. Triangular symbols indicate the atmospheric pressure gauge stations (Yakushima Station is in the north, and Naze Station is in the south). The symbol KM indicates the location of Kagoshima Muograph. The red numbers and red hollow circles respectively indicate the date and the position of the T-1612's center. The filled red circles indicate the T-1612 positions at 08:00, 16:00 on September 3, and 08:00 on September 4. The red dashed circles indicate the storm area (wind speed > 25 ms− 1) at 00:00, 08:00, 16:00, and 24:00 on September 3, and the brown dashed circles indicate the strong wind area (wind speed > 15 ms− 1) at 00:00 and 24:00 on September 3. The time-dependent variations of the minimum pressure of T-1612 are also shown (B).
Figure 10 shows the time-sequential muographic images taken in the period between 00:00 on September 3, 2016, and 08:00 on September 4, 2016, to analyze the pressure variations caused by the passage of T-1612. In order to cancel the KM's geometrical acceptance and the muon's zenith angular dependence, the number of muon tracks in each pixel was divided by that averaged over a period of 6 months (including the period during the passage of T-1612). Moreover, each pixel was mapped within an elevation angular range between tangents 0.067 and 0.4, and an azimuthal angular range between tangents ± 0.17. This elevation angular range is respectively equivalent to the horizontal range between 300 km and 50 km at an altitude of 20 km. The overall feature is that the large-flux region indicated in the reddish pixels in Fig. 10 was shifted westwards within this period. This large flux-region was interpreted as the low barometric pressure (BP) region associated with the T-1612 passage, and these time-dependent BP variations are consistent with the trajectory of T-1612.
Figure 11 shows the schematic interpretation of the time-sequential muographic images shown in Fig. 10. The distance between KM and the center of T-1612 changed from 180 km to 120 km within the period between 00:00 and 08:00 on September 3, 2016. Therefore, the T-1612's lowest-density region where the warm core was located was captured at an elevation angle region between 70 mrad and 130 mrad. T-1612 continued to move northwards after 08:00 on September 3, and as a result, the distance between KM and the center of T-1612 was further shortened from 120 km (at 08:00) to 60 km (at 16:00). As a result, the T-1612’s warm core was captured at an elevation angle region between 130 mrad and 200 mrad. Figure 12 shows the resultant muographic image taken between 08:00 and 16:00 on September 3, 2016. In this figure, the image pixels in Fig. 10 were interpolated with polynomial functions. A low-pressure area that is warmer at its center than at its periphery (warm core) is visualized.
Figure 13 compares the eastern azimuthal angle (EAA) (0 < tan φ < 1.0) (Fig. 13A) and western azimuthal angle (WAA) (-1.0 < tan φ < 0) components of the time-sequential muographic data for the 21 days during which T-1908 affected the local atmospheric pressure. The EAA component represents the muons that passed through the atmosphere within the directional range between SSE-SSW, while the WAA component represents those muons that passed through the atmosphere within the direction range between SSW-SWW. In Fig. 13C, the time-dependent BP variations observed at Yakushima and Naze stations are presented for reference31. As illustrated in Fig. 13A, there is an MCR increase event during the period between August 3–8, 2019. This MCR increase event was interpreted as PD associated with T-1908 and is labeled MCR1908 in the current discussion. However, there is no MCR increase event in Fig. 13B between August 3–8. These results are consistent with that PD was observed at Yakushima (south of KM within the directional range between SSE-SSW), but it was not observed at Naze (southwest of KM within the direction range between SSW-SWW) stations.
Figure 14 shows the time-sequential muographic images taken between August 4, and August 8, 2016 to analyze the pressure variations caused by the passage of T-1908. Like previous time-sequential muographic images, the KM's geometrical acceptance and the muon's zenith angular dependence were corrected in this figure. These images were mapped within an elevation angular range between tangents 0–and 1.0, and an azimuthal angular range between tangents ± 1.0. This elevation angular range was respectively equivalent to the horizontal range between 506 km and 20 km at an altitude of 20 km. The overall feature was that the large muon flux region indicated in the reddish pixels in Fig. 13 appeared in the HEA-EAA region between 00:00 and 12:00 on August 6, 2019. This large muon flux region was interpreted as the low barometric pressure (BP) region associated with the T-1908 passage, and these time-dependent BP variations are consistent with the trajectory of T-1908.
In conclusion, gas muography can track meso-beta scale atmospheric density variations. Moreover, since muography measures the near-horizontally integrated density of the atmosphere, and BP stations measure the vertically integrated density of the atmosphere, a three-dimensional image of the atmospheric density distribution can be reconstructed with joint inversion of these techniques. Alternatively, the placement of multiple muographs (multidirectional muography) at dispersed locations could also be used to generate a three-dimensional image of the volume within the viewing angle of these muographs. Each muograph unit is low-cost and versatile, so it is possible to position these units nearly anywhere on land. We anticipate that real-time three-dimensional muographic monitoring of MCS can become practical and widespread in the near future.
Kagoshima Muograph (KM) originally consisted of 84 counter bars (3 layers of (Nx + Ny=14+14)), 1 muon readout module, 1 high-voltage power supply (Matsusada Precision HAR-2N300) and 2 radiation shields. Each counter bar consisted of a plastic scintillator strip (Bicron BC-408) connected to a photomultiplier tube (PMT; Hamamatsu H7724) via an acryl lightguide. Six counter bars were eventually added to KM to make 3 layers of (Nx + Ny=15+15). Each radiation shield consisted of a 10-cm thick lead plate covered by a 1-cm thick stainless-steel plate. The PMT signals were discriminated and sent to the field-programmable gate array (FPGA) for logical processing of these signals33. The muon track data were sent to a computer every 10 minutes and subsequently uploaded to the external server so that a remote computer could access the data in real-time.
Availability of Data and Materials
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.
Contributions
H.K.M.T. wrote the text. H.K.M.T. prepared the figures. All authors reviewed the manuscript.
Corresponding author
Correspondence to Hiroyuki K. M. Tanaka.
Competing interests
The authors declare no competing interests.
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No competing interests reported.