Observations of the Daytime Internal Boundary Layer In Onshore Flow
3. Results
3.1 Internal Boundary Layer, 30th. June 1994.
Method as mention above was fitted to the wind speed profile for the data taken on the 30th. of June 1994. A total of seven hourly (from 11:06 until 18:00) average wind speed data profile were plotted. Two intersecting points were obtained representing the adjusted SBL and the top was the internal boundary layer height. Both height layers are tabulated in table 1.
| Time | adjusted surface
layer(m) | ZIBL (m) |
| 11:06-12:07 | 14 | @ |
| 12:08-13:o6 | 19 | 72 |
| 13:06-14:05 | 41 | 95 |
| 14:06-15:00 | 35 | 73 |
| 15:01-15:52 | 52 | 92 |
| 15:59-17:06 | 25 | 70 |
| 17:07-18:00 | 22 | 75 |
Note; @ no sign of wind speed variation on the profile.
Table 1: The Adjusted surface and Internal Boundary Layer
Height from the wind speed profile on 30th. June 1994.
The results showed that the adjusted layer height increased during the mid afternoon and decreased during the evening. No sign of wind speed variation at the upper level between time period of 11:06 to 12:07 was observed. This made marking the point of intersection between the lines difficult. However, the profile clearly revealed the existence of the adjusted layer near 17m at this time period. The adjusted layer increased in height with time until mid afternoon and it is at its maximum height (52m) during time period between 15:01 to 15:52 and but decreased later in the evening. The ZIBL was first obtained from the 12:08-13:06 wind speed profile at 72m height and increased to 95m for the data taken between 13:06 to 14:05. The wind speed profile varied greater during this time period. This corresponded to the lower level wind speed increase with time. The ZIBL decreased to 73m during time period 14:06 to 15:00 and increased to 92m high between 15:01 to 15:52. The wind speed increase with height at and above this level (Z
IBL) was mainly caused by the active marine air flow inland. The increased of potential temperature at this (Z
IBL) provided evidence of marine air flow activity.
Later in the day between the time period of 15:59 to 17:06 and 17:07 18:00 the Z
IBL decreased to 70m and 75m respectively. There was no clear evidence of the potential temperature change with height at this level. However, the specific humidity decreased sharply over small
DZ at this level .The air above the IBL was dry. Angevine et.al., (1994), explained that the synoptic subsidence appeared to be the primary controller of the strength of the potential temperature increase with height and the boundary layer height. The subsidence associated with high pressure near the surface caused significant drying above the boundary layer and strengthen the inversion, and also directly suppressds boundary layer growth.
The same method, the best fit straight line, was applied to the potential temperature profile to determine the IBL height. It shows the ZIBL is at 96m during the early hour (11:06-12:07) and which decreased during the mid afternoon. The ZIBL was at its maximum height (96m) between the time period 15:59 to 17:06. Table 2 shows the ZIBL variation with height on the 30th. of June 1994 from the potential temperature profile between the time period 12:08 to 13:06, showed no sign of an inversion layer corresponding to the Z
IBL layer.
| Time | adjusted
surface layer (m) | ZIBL (m) |
| 11:06-12:07 | 25 | 96 |
| 12:08-13:06 | 28 | @ |
| 13:06-14:05 | 36 | 95 |
| 14:06-15:00 | 46 | 94 |
| 15:01-15:52 | 57 | 85 |
| 15:59-17:06 | 37 | 96 |
| 17:07-18:00 | 28 | 82 |
@ no sign of ZIBL from the profile.
Table 2: The Adjusted surface Internal Boundary Layer
Height from the potential temperature profile
on 30th. June 1994.
The adjusted layer was observed to develop at 25m during the late morning hours (11:06-12:07) and increased by 3m one hour later. The height of this layer continued to increase during the mid afternoon. This was associate with the intense ground heating as high temperature reading was recorded at 6m mast level . The temperature variation displayed in the figure corresponded to the solar radiation which was effected by the present of the cumulus cloud. The adjusted surface layer revealed a further increase in height (by 21m) which indicated it's maximum height (57m) between 15:01 to 15:52.
The Z
IBL was marked to have occurred between highest (96m) during the morning hour (11:06 to 12:07) and lowest (82m) during the evening (17:07 to 18:00). No
significant Z
IBL height was observed for the potential temperature profile for the data between 12:09 to 13:06. It reflected a decreasing tendency at upper layer, except a 10m inversion layer was observed at approximately 60m height. The Z
IBL obtained from the potential temperature was slightly higher compared to the Z
IBL obtained from the wind speed profile.
