[A-AS15] general meteorology

[AAS15-P14] The Relationship Between Tropospheric Winds and Star Seeing

*AKITOMO FUNAYAMA¹, Ryuki Matsubara¹, Keita Sumiya¹, Hidehiko Suzuki¹ (1.Meiji university)

Keywords:

Observation method,Jet stream,Atmospheric turbulence

When stars are observed from the ground, disturbances in the refractive index along the optical path through the atmosphere and within the telescope tube cause fluctuations in the star's apparent position and brightness over time. This effect is called "seeing" in astronomy. The dominant sources of these disturbances are turbulence generated by wind shear and the advection of turbulent structures along the optical path. In particular, the tropospheric jet near the tropopause is accompanied by strong wind shear and has long been suggested to be related to seeing degradation. However, the quantitative relationship remains unclear. The location and shape of the jet stream are closely associated with the development of extratropical cyclones and provide important information for weather forecasting [1]. In this study, we aim to establish a method for detecting the jet stream through seeing observations using a small, simple optical system. As an initial step, we quantitatively evaluated the influence of atmospheric wind structures on seeing. At the Ikuta Campus of Meiji University, we conducted star image observations at night using a small telescope with a 90-mm aperture and a CMOS sensor. We defined the standard deviation of temporal fluctuations in the centroid position of the star image as “Image Motion (IM)". Temporal variations in the total count values of a single star were also defined as “scintillation” (SCI) [2]. We used ERA-5 reanalysis data, provided by the European Centre for Medium-Range Weather Forecasts (ECMWF), as a reference for the background atmospheric fields and compared them with the seeing indices (IM and SCI). Seeing indices tend to increase as the distance between the line of sight and the jet axis decreases, as well as when the mean horizontal wind speed increases within altitudes of 0.25–10 km. These results suggest a close relationship between wind speed structures along the line of sight and temporal variations in seeing conditions. Numerical simulations further demonstrate that image motion increases when the disturbance layer is at a lower altitude. The maximum correlation between IM and horizontal wind occurs at an altitude of approximately 3 km, which suggests that IM may indirectly reflect the approach of the jet stream, thereby enhancing lower-tropospheric wind speed. However, since strong, jet-related winds do not necessarily extend to lower altitudes, IM alone is an unreliable indicator of the jet's presence. In contrast, SCI more directly reflects high-altitude turbulent structures near the tropopause. Therefore, we conclude that analyses focusing on SCI are more effective at capturing the approach of the jet stream and high-altitude turbulence. In this presentation, we discuss the validity of estimating upper-level winds using a seeing monitor with simple optics. We also present the design and observation plan for an SCI monitor module to be installed in a Portable Meteorological Station (POMS), which our research group is developing.

[1] James R. Holton, “An Introduction to Dynamic Meteorology (4th Edition)”, Elsevier Academic Press, Burlington, 2004.
[2] J. Osborn, D. Fohring, V. S. Dhillon, R. W. Wilson, “Atmospheric scintillation in astronomical photometry”, Monthly Notices of the Royal Astronomical Society, 452(2), 1707–1716, 2015.