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

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 (σ_w²) were used to estimate the fog-top height, respectively. Positive correlations between the fog-top height and u_*, TKE, and σ_w² were observed, with empirical parameterization schemes H = 583.35 × $u_{*}^{1.12}$ , H = 205.4 $\times {\left(TKE\right)}^{0.68}$ , and H = 420.10 $\times {\left({\sigma }_w^2\right)}^{0.51}$ being obtained. Among them, σ_w² 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. σ_w² and with w_*, which represents the intensity of turbulence and buoyancy, were used to estimate the fog-top height. The relationship H = 396.26 $\times$ (σ_w + 0.1 $\times$ 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.