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: nasirun@usm.my
Keywords: Internal Boundary Layer, Mast, Wind Speed, Potential Temperature
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) U
c = The tethesonde wind speed
U
T = 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.
