Abstract
Abstract
Cloud attenuation due to cloud particles can be a critical issue for next-generation sub-terahertz (sub-THz), terahertz (THz), and free-space optical (FSO) satellite communications. Accurate estimation of cloud attenuation under various cloud conditions is essential for the design of satellite communication systems; accordingly, numerous cloud attenuation estimation models have been proposed. The widely used ITU-R model proposed up to 200 GHz is based on liquid water content and does not explicitly consider ice crystals. Previous studies have often assumed water droplets to be dominant, potentially leading to unrealistic cloud attenuation estimates, especially in multilayered cloud systems where ice crystals can exist over a wide temperature range. In addition, approaches based on reanalysis data and numerical weather prediction models rely on parameterized cloud representations and may involve substantial uncertainties. These limitations highlight the need for observationally grounded evaluations that explicitly account for both water droplets and ice crystals. In this study, cloud attenuation was estimated up to wavelengths of approximately 20
$$\mu \textrm{m}$$
(corresponding to about 10 THz) using ten cloud microphysical data obtained from cloud particle sensor (CPS) sondes observed in Okinawa, Japan, during the Baiu (East Asia rainy season) in 2016, 2017, and 2025. CPS sondes enable the vertical measurement of the number of cloud particles along the sonde flight path and allow for their rough classification into water droplets and ice crystals, making them particularly well-suited to addressing the aforementioned challenges. Cloud attenuation was estimated based on in situ observational data obtained using CPS sondes. To the best of our knowledge, this study is the first to estimate cloud attenuation for satellite communications using in situ observational data of cloud particles. The results demonstrate that cloud attenuation due to ice crystals becomes dominant, particularly at wavelengths shorter than 1.0 mm, challenging the conventional picture that cloud attenuation is dominated by water droplets. This finding was obtained by explicitly considering the size distribution of ice crystals. A key contribution of this study is the identification of limitations in conventional cloud attenuation models for satellite communications using in situ cloud particle observations. Expanding similar studies globally could lead to the development of more accurate and widely applicable cloud attenuation models.