Diurnal to Intraseasonal Precipitation Variation Around the Northwestern Coast of Luzon Island: Results From YMC‐BSM 2018 Field Campaign.

This study investigates multiscale variability of summer‐time precipitation around the northwestern coast of Luzon Island, the Philippines, by analyzing in situ observation data of a field campaign in summer 2018 conducted under the Years of the Maritime Continent‐Boreal Summer Monsoon study in 2018 (YMC‐BSM 2018) project. It is found that boreal summer intraseasonal oscillation modulates activity of precipitation diurnal cycle, with active diurnal cycle observed only under easterly (offshore) wind conditions in the free troposphere. On the other hand, in the period of westerly winds, quasi‐biweekly fluctuation is observed, with two heavy precipitation events. These events are associated with further intensified westerly winds caused by westward‐propagating quasi‐biweekly oscillations. Day‐to‐day precipitation variability over land during the two events is associated with the strength of monsoon winds blowing toward the mountain range, which is modulated cooperatively by the quasi‐biweekly oscillations and tropical depressions over the South China Sea. Plain Language Summary: In the Asian monsoon domain, including the Philippine Archipelago, Indochina Peninsula, and Indian Subcontinent, summer‐time monsoon circulation causes large amount of precipitation. Its temporal variation is caused by various large‐scale atmospheric phenomena, such as insolation diurnal cycle, tropical depressions, tropical cyclones, westward‐propagating cyclonic disturbances with quasi‐biweekly (10–20‐day) time scales, and so‐called boreal summer intraseasonal oscillations (BSISOs) that have 30–60‐day time scales and are characterized by northward‐propagating zonal wind anomalies. To deepen our understanding of the precipitation variability around the northwestern coast of Luzon Island, the Philippines, we conducted an intensive observation campaign in summer 2018. Analyses of weather radar, upper‐air sounding, and automatic weather station data reveal that BSISO considerably modulated diurnal cycle. The active diurnal cycle characterized by afternoon land precipitation and subsequent offshore migration of precipitation areas was observed only during the period of easterly winds in the free troposphere associated with BSISO. We also present an example of the cooperative influence of quasi‐biweekly oscillations and tropical depressions over the South China Sea. They modified the strength of monsoon winds blowing toward the mountain range and caused day‐to‐day precipitation variability over land during two heavy precipitation events that occurred in the period of westerly winds associated with BSISO. Key Points: The observation campaign in northwestern Luzon Island in summer 2018 captured precipitation variability involving multiscale interactionThe diurnal cycle with evening offshore migration of precipitation areas occurred only under the condition of offshore windsQuasi‐biweekly oscillation and tropical depressions cooperatively modified monsoon winds, contributing to variation of land precipitation [ABSTRACT FROM AUTHOR]