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Fig. 9. Cloud-top-boundary layer on 28 July, 2004 (summer).

Fig. 10. Cloud-top-boundary layer on 25 February, 2005 (winter).

a cloud-top-boundary layer exhibit high relative humidity, with a diurnal minimum value of over 60%.

6.5. Frontal passage

In the case of frontal passage, one can expect the change of wind direction, gradual change of cloud height, and the starts of precipitation. However, various features are found in the lidar return signals, presumably due to the various types of frontal passage. Some typical cases are shown below.

Evaporation Low Boundary Layer Clouds

Fig. 11. Frontal passage on 6 June, 2004. (a) Lidar data. During the rainfall, the cloud base height is low. (b) Diurnal variation of precipitation, temperature, relative humidity, water vapor pressure, and wind velocity. (c) Weather map at 9:00 (JST). The front stays offshore of the Chiba prefecture (east of Tokyo).

Fig. 11. Frontal passage on 6 June, 2004. (a) Lidar data. During the rainfall, the cloud base height is low. (b) Diurnal variation of precipitation, temperature, relative humidity, water vapor pressure, and wind velocity. (c) Weather map at 9:00 (JST). The front stays offshore of the Chiba prefecture (east of Tokyo).

Fig. 12. Frontal passage on 4 January, 2005. (a) Lidar data. In the morning, cloud height sharply drops from 10 to 3 km. After 22:00, a cold front passes over the Chiba area. (b) Diurnal variation of precipitation, temperature, relative humidity, water vapor pressure, and wind speed. The maximum temperature appears around 15:00 due to the insolation in the daytime. (c) Weather map at 9:00 (JST). The front stays over the Chiba prefecture.

Fig. 12. Frontal passage on 4 January, 2005. (a) Lidar data. In the morning, cloud height sharply drops from 10 to 3 km. After 22:00, a cold front passes over the Chiba area. (b) Diurnal variation of precipitation, temperature, relative humidity, water vapor pressure, and wind speed. The maximum temperature appears around 15:00 due to the insolation in the daytime. (c) Weather map at 9:00 (JST). The front stays over the Chiba prefecture.

6.5.1. A summer case

A summer case on 6 June, 2004 is shown in Figs. 11(a)-11(c). On this day the precipitation was as much as 61 mm, and the onset of the rainy season was announced. From Fig. 11(b), a large amount of precipitation occurred during 16:00 and 18:00 with low cloud height. The rain stopped after 19:00. The wind direction changed from SE ^ E ^ NE ^ N. There was observed frequent jumps of the cloud bottom height.

6.5.2. A winter case

As an example in winter, a case in 4 January, 2005 is shown in Fig. 12. In Fig. 12(a), the vertical profile up to 10km is shown. In the morning, cloud decreases the altitude suddenly from 10 to 1 km; then the cloud disappears and sunshine appears. Around 16:00, the wind direction changes from SW to NNE, and the temperature lowered sharply. Also, the relative humidity decreased from 80% to 30% due to the intrusion of cold, dry air. During 22:00 to 23:00, it is found that the raindrops evaporate even before reaching the ground.

6.6. Sudden change of cloud height

An example of sudden decrease of cloud height has already been mentioned in association with the frontal passage (Sec. 6.5). Similar phenomena are seen under different circumstances. Figure 13 shows a case in winter

0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 Time (JST)

Fig. 13. An example of sudden change of cloud height observed on 1 February, 2005.

0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 Time (JST)

Fig. 13. An example of sudden change of cloud height observed on 1 February, 2005.

(1 February, 2005). There was no precipitation with the sunshine duration of 7.6 h, and wind direction changed from SW to NE with the speed of 3-12 m/s. The relative humidity was as low as 20% to 50%. The relation with the front is not clear.

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