Ship's Motion and Eddy Correlation Measurements of Surface Fluxes on the Small Research Ship <i>NIES</i> ' <i>94</i> in Lake Kasumigaura, Japan

Abstract Lake surface fluxes provide important information about the lake's thermal environment. To capture their spatial variations, a ship serves as an excellent platform for applying the eddy correlation (EC) method. Although ship‐based EC measurements have been conducted over the ocean, this has not been the case over lake surfaces. Ship‐based measurements in a lake differ from those over the ocean in terms of the freedom to select the ship, route, and operation, as well as the wave regime, creating measurement conditions that have not been addressed in ocean studies. Thus, 10‐day EC flux measurements on the highly maneuverable yet stable research ship NIES ' 94 were conducted in Lake Kasumigaura (surface area of 172 km 2 ), which facilitated extensive data analysis on the ship's motion and fluxes under various conditions. The results indicated that the ship's motion differs greatly depending on the ship's shape and dimensions, and that a larger fluctuation in roll and pitch angles propagates into a larger error of the vertical wind velocity measurements. The motion correction was found necessary for momentum fluxes, while it is preferable but may not be essential under favorable conditions for scalar fluxes. Comparisons between the fluxes obtained from the EC method and those from the bulk method showed that the ship's speed and direction and wave height have minimal impact on the agreement, reflecting the use of a stable ship and lower wave height in our study, leading to small ship motion in Lake Kasumigaura compared to the ocean.