Parameterization of Radiation Fog-Top Height and Methods Evaluation in Tianjin

Abstract

Different methods have been developed to estimate the fog-top height of radiation fog and evaluated using the measurements obtained from a 255-m meteorological tower located in Tianjin in 2016. Different indicators of turbulence intensity, friction velocity (u*), turbulence kinetic energy (TKE), and variance of vertical velocity (σw2) were used to estimate the fog-top height, respectively. Positive correlations between the fog-top height and u*, TKE, and σw2 were observed, with empirical parameterization schemes H = 583.35 × u * 1.12 , H = 205.4   ×   ( T K E ) 0.68 , and H = 420.10 × ( σ w 2 ) 0.51 being obtained. Among them, σw2 is the most appropriate indicators of turbulence intensity to estimate the fog-top height. Compared with sensible flux and condensation rate, the new form of convective velocity scale (w*) was the most appropriate indicator of buoyancy induced by radiative cooling, and the relationship H = 328.33 × w * 1.34 was obtained. σw2 and with w*, which represents the intensity of turbulence and buoyancy, were used to estimate the fog-top height. The relationship H = 396.26 ×   (σw + 0.1 ×   w*) − 16 was obtained, which can be used to accurately estimate the fog-top height. Moreover, the temperature convergence (TC) method was used to estimate the fog-top height; however, the results strongly rely on the threshold value.