|Year : 2016 | Volume
| Issue : 1 | Page : 20-24
Estimation of diurnal variation of surface layer at Tarapur site using ultrasonic anemometer
Vedesh K Varakhedkar, SV Vanave, A Baburajan, PM Ravi, RM Tripathi
Health Physics Division, BARC, Trombay, Mumbai, Maharashtra, India
|Date of Web Publication||1-Jul-2016|
Vedesh K Varakhedkar
Health Physics Division, BARC, Trombay, Mumbai - 400 085, Maharashtra
Source of Support: None, Conflict of Interest: None
Surface layer (SL) is defined by the region above the earth surface (50-100 m) wherein shearing stress is approximately constant with height, and the wind structure is primarily determined by the nature of the surface and the vertical gradient of temperature. During day time due to convection, variation in shear stress with height is less and hence the SL heights are higher and conversely during nights decrease in shear stress with height is higher and hence SL heights are lower. SL height is decreasing with increase in the stability and also SL heights are higher for lower values of for any stability category. Hourly average SL heights for Tarapur site varied from 40 to 142 m with = 0.1. Estimation of SL height for the site will be useful input parameter for the dispersion models.
Keywords: Dispersion models, surface layer, ultrasonic anemometer
|How to cite this article:|
Varakhedkar VK, Vanave S V, Baburajan A, Ravi P M, Tripathi R M. Estimation of diurnal variation of surface layer at Tarapur site using ultrasonic anemometer. Radiat Prot Environ 2016;39:20-4
|How to cite this URL:|
Varakhedkar VK, Vanave S V, Baburajan A, Ravi P M, Tripathi R M. Estimation of diurnal variation of surface layer at Tarapur site using ultrasonic anemometer. Radiat Prot Environ [serial online] 2016 [cited 2019 Sep 15];39:20-4. Available from: http://www.rpe.org.in/text.asp?2016/39/1/20/185159
| Introduction|| |
Sharma and Shirvaikar,  1976, as well as Lumley and Panofsky,  defined the surface layer (SL) height, as the height at which the shearing stress (τz ) decreases to 80% of the surface value. If h denotes this height, then τh = 0.8 τo . The value of h is taken variously as 50  or 100 m.  It is however evident that this height should be diurnally varying, depending on atmospheric stability category. The objective of this paper is to examine this aspect.
Adequate SL turbulence and flux quantification require the use of fast-response instrumentation that produce time-synchronized measurements of velocity components and temperature. The solid state sonic anemometer, with no moving parts and low maintenance requirements, is the instrument most readily able to provide the requisite measurements.
Fiedler and Panofsky  show that the vertical variation of friction velocity U* in the lower atmospheric layer is given by:
where the suffixes Z and 0 refer to height, B is a function of stability parameter μ,
where f is Coriolis parameter, k is Von Karman constant (=0.4), and L is the Monin-Obukhov Length scale. The value of B may be found using:
B (μ) = 1.0 μ ≤ −75
B (μ) = 0.00062 μ2 + 0.093 μ + 4.5 μ > −75
Equations 1 and 3.
And now replacing Z with h we get:
where, h* is nondimensional length scale of the SL. Thus, the variation of h* with time may be computed if the variation of L with time is known. Value of SL height "h" is:
Description of site
Tarapur nuclear site is situated on the west coast of India, about 100 km North of Mumbai. A well-equipped micrometeorological laboratory (MM Lab) with 30 m high met tower is located at 1.5 km fetch distance from the east of coastline.
The tower site is exposed to relatively open fetches consisting mostly of annual grasses in all directions. A range of 5-15 m high buildings are located over a fetch distance of about 500 m from the met tower. The 30 m high meteorological tower installed at MM Lab at 8 m MSL is a self-supporting, square type, hot dip galvanized steel structure, with three platforms at 10 m, 20 m, and 30 m, with access ladder and safety guard.
Experimental verification of SL height depending on the prevailing atmospheric stability and hence its verification with time has been made using three-dimensional ultrasonic anemometer with (1/L) Monin-Obukhov Stability (MOS) parameter. The ultrasonic anemometers are operated at heights 10 m and 30 m with 40 Hz frequency, which provides wind and turbulence data on a continuous basis. The present investigations use wind and turbulence data from ultrasonic anemometer installed at 10 m height for the year 2013 on hourly averaged basis.
Sonic anemometers provide hourly averaged data for three wind components (u, v, w) and their standard deviations (σu , σv , σw ) in m/s, wind speed in m/s, wind direction in degree, along with turbulence parameters such as friction velocity in m/s and characteristic temperature K, drag coefficient, MOS parameter in (m−1 ), vertical momentum flux in kg/ms 2 , vertical heat flux in W/m 2 , and atmospheric diffusion category.
Out of 8760 data hours, 123 h data are missing for the year 2013, and moreover MOS parameter in (m−1 ) i.e., 1/L (inverse of Monin-Obukhov length) is not properly defined for very low values of U*, data hours with low values of U* <0.1 have been discarded, i.e., for 1496 h U* is not properly defined. Therefore, total 7141 data hours during the year 2013 has been considered for the analysis and hourly MOS parameter (1/L) has been used to find μ and SL height "h."
| Results and discussions|| |
Average SL heights for = 0.1, 0.2, and 0.3 using 1/L has been estimated and grouped as per the stability classes evaluated by Turner method and shown in [Table 1]. It can be observed that value of SL height is decreasing with increase in the stability and also SL heights are higher for lower values of for any stability category.
|Table 1: Average surface layer height estimated for various stability classes|
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Diurnal variation of average SL heights for = 0.1, 0.2, and 0.3 has been estimated for all the months and also for the whole year and shown in [Table 2]. It can be observed that for the year 2013, SL heights for the night varied from 40 to 51 m, and for the daytime, the values ranged from 63 to 142 m. This confirms that during daytime convective regime, variation in shear stress with height is less and hence the SL heights are higher and conversely during nights decrease in shear stress with height is higher and hence SL heights are lower, this has been graphically shown in [Figure 1].
|Table 2: Month - wise diurnal variation of surface layer height (m) for the year 2013 at Tarapur Maharashtra Site, Tarapur|
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[Figure 2],[Figure 3] and [Figure 4] give the seasonal diurnal variation of SL height, and it is observed that during summer season values of SL heights are higher compared to winter and monsoon and in particularly maximum occurs in March, i.e., 196 m. Particularly for monsoon, the values of SL heights are smaller because of less intensity of convection.
|Figure 2: Diurnal variation of surface layer height (m) during winter for the year 2013|
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|Figure 3: Diurnal variation of surface layer height (m) during summer for the year 2013|
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|Figure 4: Diurnal variation of surface layer height (m) during monsoon for the year 2013|
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Under neutral stratification, the value of B = 4.5 leading to h* = 0.0178; then the height of SL h is given by h = 0.0178 /f. This may be compared with the H = 0.2 /f, where H is the height of the Ekman layer, indicating that under neutral conditions, the height of the SL is about 9% of that of the Ekman layer.
Had we put the equivalence of h and Zp , the height of the patching layer  given by Z p = 0.01 /f evaluated from Equations 4 and 5 will be lower by a factor of 1.8 and the variation in shearing stress magnitude will be about 11% instead of 20%. However, in the following computations, we have assumed the validity of equation τh = 0.8 τo at the top of the SL. For f of the order 1.0E-4 s−1 , and for reasonable value of varying from 0.1 to 0.3 m/s, h varies from about 40 to 142 m.
| Conclusions|| |
Parameterization of SL height varying diurnally and with atmospheric stability for the site will be a useful input for the latest realistic atmospheric dispersion models as fidelity of the output results of such models strongly dependent on the quality meteorological data input and site-specific physical parameterization.
During day time due to convection, variation in shear stress with height is less and hence the SL heights are higher and conversely during nights decrease in shear stress with height is higher and hence SL heights are lower.
SL height is decreasing with increase in the stability and also SL heights are higher for lower values of for any stability category.
Hourly average SL heights for Tarapur site varied from 40 to 142 m with = 0.1.
The authors like to thank Dr. K. S. Pradeepkumar, Associate Director, Health, Safety, and Environment Group, BARC for his keen interest and encouragement. Thanks are also due to authorities of Tarapur site, for their continuous support in carrying out the studies.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
| References|| |
Sharma LN, Shirvaikar VV. Diurnal variation of the surface layer. Boundary Layer Meteorol 1976;10:507-11.
Lumley JL, Panofsky HA. The Structure of Atmospheric Turbulence. New York: Interscience Publishers; 1964. p. 100.
Estoque MA. A numerical model of the atmospheric boundary layer. J Geophys Res 1963;68:1103-13.
Counihan J. Adiabatic atmospheric boundary layers. Atmos Environ 1975;9:875.
Fiedler F, Panofsky HA. The geostrophic drag coefficient and the effective roughness length. Q J R Meteorol Soc 1972;98:213-20.
Blackadar AK, Tennekes H. Asymptotic similarity in neutral barotropic planetary boundar layers. J Atmos Sci 1968;25:1015-20.
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]