The Role of Free‐Tropospheric Moisture Convergence for Summertime Heavy Rainfall in Western Japan

• Published in: Geophysical research letters 2021-09, Vol.48 (18), p.n/a
• Main Authors: Tsuji, Hiroki, Takayabu, Yukari N., Shibuya, Ryosuke, Kamahori, Hirotaka, Yokoyama, Chie
• Format: Article
• Language: English
• Subjects: Atmosphere; Atmospheric precipitations; Boundary layers; Convergence; Heavy rainfall; Inflow; layer inflow; Mesoscale convective systems; Mesoscale phenomena; moist absolutely unstable layer; Moisture; Precipitation; Rain; Rainfall; Troposphere; Water vapor; Water vapor flux; Water vapour; Wetting
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL095030

The role of vertically integrated water vapor flux convergence (IVFC) in heavy rainfall events in western Japan is investigated by dividing the atmosphere into a boundary layer and free troposphere using the Japan Meteorological Agency's mesoscale gridded analysis product and reanalysis product. Rainfall events are defined by peaks of area‐averaged precipitation. The results show an increase in the free‐tropospheric IVFC more than 15 h before rainfall events, contributing to atmosphere moistening. This preceding moistening is favorable for mesoscale convective systems (MCSs) with slantwise ascending deep inflow layers (layer lifting) that produce large precipitation amounts. Appearance frequency of the moist absolutely unstable layer (MAUL) increases around the precipitation peak times, which is especially notable for larger peaks. These results indicate that the free‐tropospheric IVFC contributes to heavy rainfall in western Japan by providing environments favorable for producing and maintaining MAUL, enhancing the organization of MCSs. Plain Language Summary The role of moisture convergence for heavy rainfall events in western Japan is investigated by dividing the atmosphere into its lowest part (boundary layer) and layers above it (free troposphere). Our results indicate that free‐tropospheric moisture increases earlier than 15 h before peak rainfall, contributing to atmospheric moistening. This moistening in the free troposphere is favorable for developing MCSs with slantwise ascending deep inflow layers, which produce large amounts of precipitation. These results indicate the significance of free‐tropospheric moisture convergence to rainfall events over western Japan. Key Points Significant increases in the free‐tropospheric moisture convergence precede heavy rainfall events over western Japan Moist absolutely unstable layers in the mid‐troposphere are produced and maintained by the deep moist inflow in the prepared environment Such conditions are favorable for the development of mesoscale convective systems, bringing large amounts of precipitation