Modification and Performance Tests of Visibility Parameterizations for Haze Days

: In order to select a visibility parameterization scheme that can be applied well to the Beijing-Tianjin-Hebei (BTH) region and provide better forecasting, a modified parameterization of visibility based on the aerosol volume concentration and RH is developed in this study. This upgraded parameterization scheme (S1) and other schemes based on PM2.5 and RH (S2) and Mie theory (S3) are evaluated using forecast data from Rapid refresh Multi-scale Analysis & Prediction System-CHEM (RMAPS-CHEM v1.0). A performance test using data from February 2017 showed that:① The concentration of PM2.5 is forecast well in the BTH region. The correlation coefficients of the observed and forecast daily average PM2.5 in most areas are higher than 0.8, and the forecasted values are close to those observed. The mean errors (ME) are -7.54, -0.46, and -11.0μg·m-3 for the forecast domain, south and north of Hebei province, and 12.04, 2.02, and -13.31 μg·m-3 for the cities of Beijing, Tianjin, and Shijiazhuang, respectively. The correlation coefficients for the forecast and observed hourly relative humidity in the three typical cities are above 0.78, and the mean errors are lower than 3.91%. ② All three parameterization schemes predict the time evaluation of visibility in the BTH region during February 2017 well. In general, the visibility predicted with S1 is the lowest, while that of S3 is the highest; the predictions of S2 are intermediate. In most areas of the BTH region, S1 has the minimum root mean squared error (RMSE) and normalized mean error (NME) between the observed and forecast visibility, while S3 has the maximum RMSE and NME. The error of S2 is between that of S1 and S3, but it shows the best performance in the Beijing area. ③ When the observed visibility is higher than 10 km, the predicted visibilities of the three schemes are all lower than the observed visibility, and S3 has the lowest mean error (ME) and RMSE. S1 has the lowest MB, RMSE, and NME when the visibility is lower than 10 km, especially for visibilities of 1 km to 5 km, which occurred more frequently during heavy haze episodes. The comparison of the results indicated that S1 is best for application to haze forecasting in the BTH region.