Diurnal Variations of the Mixed Layer Characteristics at Tsukuba

Abstract

Hourly and annual variations of the mixed layer characteristics were obtained by coupling the temperature profile of routine aerological data to near-surface hourly temperature and dew-point temperature, using a practically modified enchroachment mixed-layer model. Net radiation changes sign from minus to plus one or two hours after sunrise. This time is regarded as the effective sunrise when the mixed layer starts to develop. In most months, the mixed layer develops slowly at the beginning and then increases linearly till around noon. The Bowen ratio is unity during daytime in May. A large proportion of the net radiation is consumed by the surface latent heat flux during the summer season. During the winter season, the surface latent heat flux is larger than the surface sensible heat flux in the morning, but decreases around noon. The convective velocity scale W∗ increases parabolically, and continues to increase up to early afternoon all the year.
Relative humidity has a peak value at the effective sunrise. On the other hand, specific humidity has a peak one or two hours after the effective sunrise, and after the peak it decreases gradually till 14h-15h local time due to development of the mixed layer. The diurnal patterns of the specific humidity are similar during each month, but decrease rate of the specific humidity in the daytime from July to September is small. By considering the water vapor budget in the mixed layer, the specific humidity profile in the upper stable layer at the effective sunrise estimated.