Home Articles Observations of the daytime internal boundary layer in onshore flow

Observations of the daytime internal boundary layer in onshore flow

N. M. Saleh, M.Z. MatJafri and K. Abdullah
School of Physics, Universiti Sains Malaysia
11800 Penang, Malaysia
Telephone: 604-6577888ext3676 Fax: 604-6579150
E-mail: [email protected]

Abstract
One of the most important characteristics of the atmospheric environment in coastal regions is the internal boundary layer (IBL), formed when air flows across the surface discontinuity between land and water. Since the two surfaces rarely have the same temperature and almost always exhibit a difference in aerodynamic roughness, an interface is created. A study to investigate the development and behavior of the IBL by using the atmospheric data acquisition Tethersonde System in coastal area of Sufolk, during the periods of onshore wind flow. A data set from a 16m mast deployed near the coast to correlate with features observed in the tethersonde data. A new method to improved the utility of the measured tethersonde data has been developed which is shown to produce consistent results. A best-fit straight line to wind speed and potential temperature profiles are used to determine the evolution of the IBL at a distance of 850m from the shoreline. The data collected during periods of IBL development has shown that the atmosphere below 110m is divided into three main layers: The lower is the adjusted surface boundary layer; The middle is the transition layer and; The upper is s region of free atmosphere. It is also found that the IBL heights obtained lay the same population as Hsu’s (1988) observations, which ranged over fetch-lengths of 30m to 6000m.

1. Introduction
One of the principle areas of meteorology at the coastal sites, has been on the growth of the convective thermal internal boundary layer, mainly concerning the influence of this boundary layer on coastal pollution from industrial sites. This study considers the development of Internal Boundary Layer (IBL) which develops when air flows and the air above the ground surface. This horizontal thermal discontinuity principally arises since the two surfaces rarely have the same temperature and almost always differ in aerodynamic roughness, resulting in an interface.

In this research a boundary layer height (ZIBL) is determined by using the wind speed, potential temperature profile. The best fit straight line is introduced onto the wind speed and potential temperature profile to which the boundary layer is determined. The IBL determined is informatively integrated with the specific humidity and the atmospheric stability characteristic of the study area.

In this study a 16m mast is set up (120m from the coastline) to monitor the fresh onshore flow at the coastal areas. However, data collected from instruments supported on a cable of a tethered balloon, which can be systematically raised and lowered in the free atmosphere, is used to study the atmospheric behavior further inland at (about 850m from the coastline). Therefore, to used this kind of data will not be a straight forward process as a specific method has to be developed in order to present and interpret the data. As such the mast data (as shown in formula below) is used to correct the tethersonde data(formula below) and thus minimize the tethersonde data of large variation from the mean.

Uc = UT – U'(t)
Uc = The tethesonde wind speed
UT = The tethersonde correction wind speed
U’ = The correction factor, t is the time lag.

The internal boundary layer obtained from this study is compared with some works that had been conducted at different coastal areas.

2 Method to Determine the IBL Height.

2.1 Best Fit Straight Line.
The thickness of the IBL was defined as the height above the ground, where logarithmic profiles of wind speed and potential temperature were intersect and abrupted, that is, separated by a distinct change of vertical slope for both wind speed and potential temperature profile. In practice, the thickness of the IBL was determined by means of finding the intersection of the best fit straight lines on the wind speed and the potential temperature profile (refer figure 1).

Figure 1 : Schematic diagram of straght line drawn on. (a) wind speed and (b) potential temperature profile.
To put it briefly, the sketching of the straight line to form the slope from the profile points can be accomplished utilising two methods; the first takes into consideration the maximum points from the profile as the slope boundary between the lower and upper slope points and; secondly, if the maximum points are scattered below and far from the other points, especially to the lower profile, thereupon the maximum points that are scattered are not taken into consideration as the boundary for the sketching of the straight line.

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 (ZIBL) was mainly caused by the active marine air flow inland. The increased of potential temperature at this (ZIBL) 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 ZIBL 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 ZIBL 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 ZIBL 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 ZIBL 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 ZIBL obtained from the potential temperature was slightly higher compared to the ZIBL obtained from the wind speed profile.

3.2 Intercomparison of the Internal Boundary Layer Heights.
There are many other IBL studies that have been conducted along the coastal area. Interestingly some of these studies have been carried out at different fetches (X). It is the intention of this section to compare the ZIBL that was obtained in this study to those found in the literature. Three different studies (Ogawa & Ohara, (1985), Smedman & Hogstrom, (1983) and Druilhet et. al., (1982)).who their studies at different places and at a different fetches will be used as a guide to observed the ZIBL growth with fetch distance.

Table 3 highlight the ZIBL obtained from this study (30th. June 1994, as a function of stability) and studies that have being conducted by the people mentioned above.

Table 3 The Tabulation of the ZIBL against fetch (X)

X (m) ZIBL (m)
30 8a
90 11a
160 24a
850
(Our study)
74d
1000 79c
1500 76b
2000 105c
3000 123c
4000 138c
5000 151c
6000 162c

Note:
a = from Ogawa and Ohara, (1985)
b = from Smedman and Hogstrom, (1983)
c = From Druilhet et. al., (1982)
d = Obtained ZIBL from stability function (30th. June 1994)

From table 3 it can be observed that the ZIBL obtained as a function of stability on the 30th. June 1994 is significantly correlated to the ZIBL growth with fetch conducted from the other studies . The ZIBL growth presented statistically by Hsu (1988) when he formularised the boundary layer height from the three different studies mentioned above as ZIBL = (1.91 ± 2 x 0.38) X1/2

The ZIBL obtained as a function of stability on the 30th. of June was in the increasing slope of the Internal Boundary Layer height with fetch (lies in the range between 30m to 6000m downwind distance).

4. Discussion
Two methods had been used to obtained the thickness of the adjusted Surface Boundary Layer (SBL) and the Internal Boundary Layer (ZIBL); the first from the best fit straight line drawn on the wind speed and potential temperature profile and the SBL and ZIBL was obtained from which the two straight lines intersect and abstrupt and; the second was determined as a function of stability (Richardson number). The layer of separation between the bottom and the top layer can be seen when there is an abstrupt change on the profile behavior. The top of the ZIBL was determined when fresh incoming onshore flow on top of the land surface atmospheric layer. The corresponding potential temperature behavior at the top of ZIBL, which displayed an inversion characteristic gave evidence of this fresh onshore wind.

Data on 30/6 were examined in order to investigate the ZIBL growth. This was done by using both methods. The result explained showed the existence of the Internal Boundary Layer height on 30/6/94 during the summer of 1994. Comparatively the ZIBL that was obtained lay within the increase slope of the Internal Boundary Layer height proposed by Hsu (1988).

References

  • Druilhet, A., Herrada, A., Pages, J. P., and Saissac, J., (1982). Etude experimentale de la couche limite interne associee a la brize de mer. Boundary Layer Meteor., 22, 511-524.
  • Hsu, S. A. (1988). Coastal Meteorology. 260pp. ISBN 0-12-357955-4. Academic Press Inc. London.
  • Ogawa, Y. and Ohara, T., (1984). The Turbulent Structure of the Internal Boundary Layer near the coast, Boundary Layer Meteorol. 31, 369-384.
  • Smedman, A. S. and Hogstrom, U., (1983). Turbulent Characteristic of a shallow Convective Internal Boundary Layer. Boundary Layer Meteorology, 25, 271-287.