A Comparative Evaluation on Automatic and Manual Observations of Fog and Haze in Tianjin

Based on the past records by manual observation at ten meteorological stations during 1951-2014 and parallel observations by automatic and manual observations in February 2014, discrepancies of mist, fog and haze phenomena are evaluated in Tianjin, in order to meet the reform direction of surface meteorological observation business, and to improve the quality and availability of data observed by new automatic system. Results indicate that in February, the mean number of mist days in Tianjin is 10, while numbers for fog or haze days are both 2. The proportion for mist accompanying haze days occupies 7.4% in all the weather phenomena, but for fog and haze days, it is only 0.7%. There is an increasing trend in mist days, but not for fog, and since 1990s, haze days have begun to increase obviously, especially for the year of 2007 during the last 60 years. Comparative analysis in parallel period by manual indicates there are 11 more mist days but 6 less fog and haze days than those by automation. Discrepancies of mist and fog phenomena arise at 0800 BT, manual observation captures more mist but automatic observation records more fog. Discrepancies of haze phenomenon arise at 0800, 1400, 1700 BT and 2000 BT, which are observed by manual way, but none according to automatic observations, while the phenomena of haze or mist and haze are totally the opposite. However, characteristics of phenomena judged by automatic and manual observations are of the same, i.e., mist days are the most, haze days are the second, fog days are the least, and the number of mist accompanying haze days is more than that for fog accompanying haze. Comparing visibility values by manual observation with automation indicates that the average relative error between them is 25.1% in all the samples, 54.5% of automatic observation is less than manual observation, 28.5% out of these the difference exceeding 1 km, manual observation meets with automatic observation only for 6.5% of all cases. For differences of visibility in different order of magnitude, 60%-76% of automatic observation is smaller, when the visibility is less than 15.0 km, especially for 0800 BT and 2000 BT. The median of relative error within the threshold ranges of [1.0 km, 5.0 km) and [5.0 km, 10.0 km), judging the phenomenon of mist and haze, are both about 22%, and larger range of [10.0 km, 15.0 km) and [15.0 km, +∞) are only about 15%. Accordingly, observation error of visibility is an important cause leading to discrepancies of mist, fog and haze phenomena by automatic and manual observations, in the case of relative humidity to meet suitable conditions.