Field Observations of Nocturnal Drainage Winds Blowing Down to the Fukuoka Plains

Abstract

The vertical structures of nocturnal drainage winds that blow onto the Fukuoka Plains from mountains behind were examined on the basis of echo intensity and wind velocity obtained by using a Doppler sodar. Air and soil surface temperatures, wind velocity and net radiation near the ground surface were also measured to estimate the heat budget between the air and the ground. Though the nocturnal drainage winds have the depth of 250 m to 300 m, the shape of the fronts varies with the sea breeze and the surrounding flow. Twice in the nighttime, the occurrence of the horizontal wind with the maximum speed of 4 to 6 ms-1 was observed at the height about 150 m above the ground surface. Though the ascending currents with the speed of 0.2 to 0.4 ms-1 were also seen twice with the occurrence, the second current ascended higher than the first one. These phenomena may be due to the intermitted supply of cold air mass from a basin being in the upstream side. After the first occurrence, the air temperature near the ground surface reaches to the air temperature at a height of 30m. This reason is that the cold air near the ground surface formed by the radiational cooling is replaced with the upper warm air by the advection of the drainage winds. After then, the decreasing rate of the air temperature becomes smaller because the heat fluxes from the air toward the ground suppress the radiational cooling.