Contribution of different scales to integral moisture transport based on aircraft observations over the Sea of Japan

Abstract

Based on the Joint Research Agreement between the Central Aerological Observatory of the Russian Federal Service for Hydrometeorology and Environmental Monitoring (CAO), and the Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology Corporation (JST), a Russian research aircraft, IL-18, was employed to carry out investigations of the structure and formation/development mechanisms of mesoscale convective systems over the Sea of Japan area in January–February 2001. Vertical moisture transport was one of the subjects of the experiment. To study it, the instruments installed on board the aircraft measured the oscillations of the vertical wind component w′ and absolute air humidity ρ′. The vertical moisture transport was analyzed by an eddy correlation method using the formula Q= w′ρ′ . The technique was modified to determine the transport in a wider scale range, from 10 m to 50 km (0.1–500 s). In the course of the experiment, an inflow of cold dry air from the continent (the Siberian area) was observed. As the air mass was moving towards Japan, it was getting warmer and more humidified due to heat and moisture exchange with the open surface of the Sea of Japan. The cloud streets formed testify to the presence of roll circulation. The mean integral moisture transport has proved to be about 0.07 g/m 2 s. Spectral analysis has revealed the following three scales by their input to moisture transport: 10–1000 m (turbulence), 1000–3000 m (convective cell), 3–10 km (convective body). Larger scales do not make any considerable input. At a 100-m altitude, the contributions of all the three scales under consideration are equal, while at 500 m, the role of the largest one becomes more prominent. The share of turbulent flux is 1/3 and 1/6 at 100 and 500 m, respectively. In the space structure of moisture transport obtained for altitudes of 100 and 500 m, 25-km features were detected, which were associated with the position of cloud streets that had formed at an altitude of 1000–1500 m. These features can be accounted for by roll circulation (horizontal roll vortices, organized roll vortices). A comparison of heat and moisture transports (in terms of sensible and latent heat fluxes) has indicated that their ratio at a 100-m altitude is close to 2, and at 500 m to 1. The difference between the values of the transport at the two altitudes agrees with the estimates of horizontal heat and moisture advection.