Comparisons of Boundary Mixing Layer Depths Determined by the Empirical Calculation and Radiosonde Profiles

Mixing layer is one typical type of atmosphere boundary layer, and it is named after strong vertical mixing which leads to the nearly constant variables, such as potential temperature and water vapor in this layer. The depth of mixing layer is an important parameter to identify features of thermodynamics and atmospheric dynamics in the boundary layer, and also a key to monitor the air quality. Mixing layer has very distinct daily variation as different meteorological conditions and synoptic processes largely influence the structure of boundary layer. Mixing layer becomes thicker under clear sky conditions, while remains physically stable and almost invariant during a single day under cloudy or raining weather conditions. Therefore, measurements and calculation of mixing layer depth are worth studying.The depths of mixing layer at 14:00 of Beijing, Longfengshan, Lin'an, Aletai, Sanya, Xining and Tengchong are compared using two kinds of datasets: The Nozaki empirical method and the radiosonde observational data reduced by vertical profiles of potential temperature and refractivity. It shows that the two observational depths are in good agreement, and the radiosonde measurements of mixing layer can be seen as criteria in the comparison with the Nozaki empirical method. Few bad linear correlation points of mixing layer depth from potential temperature profiles and refractivity profiles indicate that depth of mixing layer determined by refractivity profiles sometimes cannot find out the actual mixing layer, possibly due to dramatic variation of refractivity profiles under stable atmosphere vertical structure conditions.The comparisons illustrate that the Nozaki method may reflect the daily variations of mixing layer as those shown in observational dataset. However, the Nozaki method underestimates mixing layer depth when the mixing layer is above 2000 m. On the contrary, it overestimates mixing layer depth when the mixing layer is lower than 1000 m. Nozaki method also overestimates mixing layer depth at the sites (Beijing, Longfengshan, Aleitai, Xining) located at higher latitudes, but underestimates mixing layer depth in the sites (Sanya, Lin'an, Tengchong) at lower latitudes. Errors of Nozaki method are smaller under cloudy (total cloud amount is about 3—7) weather conditions, while larger in clear days. The lack of considering terrain effect and the simplifying of physical process maybe sources of comparative error of Nozaki method. These results suggest that the empirical determination of mixing layer depth need more subtle consideration before extensive use.