P. K. Kunhikrishnan

On the structure of sea-breeze fronts observed near the coastline of Thumba, India

An investigation has been made of the structure of sea-breeze fronts observed at Thumba mostly during the months of December to April using data from a Doppler SODAR and in situ measurements of wind components, humidity and temperature. The study shows that the vertical wind structure observed in the SODAR height range provides a distinct signature of the passage of a front and that the intensity of the front is decided by the intensity and direction of the prevailing winds as well as the amount of rotation of the wind vector during the onset of the sea-breeze. Spectral analyses of vertical winds during the passage of the front reveal a dominant periodicity of about 6 min for strong sea-breeze fronts.

Some characteristics of atmospheric surface layer over a tropical inland region during southwest monsoon period

Abstract A surface layer experiment conducted at a site in the northwest part of India is described. Micrometeorological data collected from the intense field experiment conducted at a tropical inland station, Ahmedabad (23°N, 72.5°E), India during southwest monsoon period by deploying tower-based instruments were analyzed using eddy correlation method to study the characteristics of atmospheric surface layer (ASL). The normalized standard deviation of vertical wind, temperature and specific humidity were studied using Monin Obukhov (MO) similarity relations. The normalized standard deviation of vertical wind velocity ( φ w ) is found to be around 1.37 for near neutral condition and it is found to be proportional to ( z / L ) 1/3 for unstable condition and ( z / L ) 1 for stable conditions. The normalized standard deviation of temperature ( φ T ) and humidity ( φ q ) are studied as a function of z / L and it showed ( z / L ) −1/3 dependence for unstable condition and ( z / L ) −1 dependence for stable condition. The value of φ T for near neutral condition is found to be around 6.5, which is high compared with the values reported from other field experiments. The drag coefficient showed a strong dependence on wind velocity at low wind conditions and it is found to obey a power law of wind speed irrespective of the various stability conditions.

Variability of surface roughness and turbulence intensities at a coastal site in India

Surface-layer features with different prevailing wind directions for two distinct seasons (Southwest Monsoon and Northeast Monsoon) on the west coast of India are studied using data obtained from tower-based sensors at a site located about 500 m from the coast. Only daytime runs have been used for the present analysis. The surface boundary-layer fluxes have been estimated using the eddy correlation method. The surface roughnessz0 obtained using the stability-corrected wind profiles (Paulson, 1970) has been found to be low for the Southwest monsson season. For the other season,z0 is relatively high. The drag coefficientCD varies with height in the NE monsoon season but not in the season with lowz0. This aspect is reflected in the wind profiles for the two seasons and is discussed in detail. The scaling behaviour of friction velocityu* and the turbulence intensity of longitudinal, lateral and vertical winds σu, σv and σw, respectively) are further examined to study their dependence on fetch. Our study shows that for the non-dimensional case, σu/u* and σv/u* do not show any surface roughness dependence in either season. On the other hand, for σw/u* for the season with lowz0, the values are seen to agree well with that of Panofskyet al. (1977) for homogeneous terrain whereas for the other season with highz0, the results seem to conform more to the values observed by Smedman and Högström (1983) for coastal terrain. The results are discussed in the light of observations by other investigators.

Estimating surface sensible heat flux using sodar and surface temperature measurements in the evolving convective boundary layer

Abstract A simple method is described for estimating the sensible heat flux by using a Doppler sodar system and a thermal probe. This method, which can be applied to a convective boundary layer in morning hours, is based on knowing the zero heat flux level from the reflectivity and the vertical wind speed.

Temperature turbulence probe for atmospheric boundary layer studies

In the conventional application of temperature turbulence probes to atmospheric boundary layer studies, not much attention is paid to the exact evaluation of frequency response. The need for this evaluation and a new technique for its achievement are described. A sample result obtained using one of the probes developed at Thumba, Trivandrum is also presented.