Time Variation of Water Temperature in Lake Biwa-ko (I)
Abrupt change of temperature at the anchored station
1968 Volume 29 Issue 4 Pages 139-147
Details
1968 Volume 29 Issue 4 Pages 139-147
An automatic recorder which has six sensors (thermistors) for water temperature has been set up on board an anchored vessel. The arrangement of sensors for measurement is as follows : three of them are at the same depth of 20 meters below the lake surface, being detached from each other by 8 meters as illustrated in Fig. 2; and other three are in vertical at depths 0.3, 15 and 25 meters. A continuous recording was made during 24 hours from 16th to 17th September, 1967 at the anchored station 5 in Lake Biwa-ko, Honshu, Japan. Moreover, a temperature measurement was hourly performed at all depths from surface to bottom with the aid of another thermistor thermometer.
In the early morning of 17th the automatic recorder caught an abrupt change of temperature at 20 meter depth as illustrated in Fig. 3 : Sudden depression of temperature, which was accompanied with vigorous oscillation, began at 7 : 08 at point C, and soon afterwards same phenomena occurred at 7 : 14 at point B, and at 7 : 15 at point A. The temperature depression amounted to 2.5°C in the lapse of 12 minutes. The event suggests the passage of thermally discontinuous surface (or thermal front), whose travelling direction and velocity may be so determined as CD (cf. Fig. 2) and as 1.8cm/sec, owing to the phase difference of the occurrence among three points A, B and C. Upward movement of the isothermal surface can be computed as 3. 8 meters by the combination of a temperature depression (2.5°C) with a vertical gradient of temperature (0.66°C/m). As a result, the slope of the isothermal surface is calculated at 0.29. Considering that the slope of isotherms is in general 10-3 in Lake Biwa-ko, the thermal front mentioned above should be said to be inclined extremely.
Hourly sounding made by another thermometer has shown the oscillatory change of about 12 hours period. On the other hand, the transverse internal seiche with uninode is conceived to have nearly the same period (10.7 hours) on the assumption of two-layered system. The early morning of 17th when the sudden depression of temperature occurred at 20 meter depth corresponds to the critical time of maximum temperature in all water depths as illustrated in Fig.4.
The mechanism and the life time for development of such thermal front in the lake water may be unaccountable at present, although such disturbances of circumstance as a wind field variation or heavy rainfall have been able to be pointed out. A rapid change of water temperature, however, is not always considered to be unusual in Lake Biwa-ko as illustrated in Table 2. Therefore, further investigation should be promoted.
