| The tendency of heavy fog and haze events has increased rapidly in recent years in the North China region. The research about the type, vertical structure and physical formation mechanism of the heavy fog events in North China is few. Under supporting of the Low-Visibility Weather Monitoring and Forecasting in the Beijing-Tianjin region of the National Meteorology Public Welfare Industry Research Project, we collected the field experimental data of fog and haze events during 2009～2013 and analysed the evolution of fog and haze events, and the characteristics of transition of haze into fog and its mechanism. Firstly, the temperature and humidity profiling data of Microwave Radiometer Profiles (MWRP) with 35 channels produced by Radiometrics Corporation were examined by the routine radiosonde data, tethered balloon sounding and CloudSat cloud radar data. Then we investigated the type, vertical structure and physical formation mechanism of 13 fog events in autumn and winter in Beijing based on the atmospheric profiling data of MWRP, routine meteorological and satellite data. Finally, we studied the microphysical properties such as aerosol, cloud condensation nuclei (CCN), fog droplet spectrum and liquid water content (LWC), as well as horizontal visibility and boundary layer properties for the unusual persistent fog and haze during 1-7 December 2011, and discussed the characteristics and mechanism of formation, evolution, and transition of the fog and haze event.Based on the reliability test of microwave radiometer by contrasting the brightness temperature simulated by MonoRTM with that observed by the instrument, the temperature (T), water vapor density (ρ) and relative humidity (RH) data retrieved from MWRP were examined by various data. The comparison of MWRP’s profiles with routine radiosonde soundings shows that correlation coefficient between two observational methods for temperature data was higher than 0.98, and that for water vapor density data reached more than 0.95. The correlation for relative humidity data was much lower, only about 0.67. In the error analysis of the overall samples, T retrieved from MWRP was lower about 3℃ than that from radiosonde sounding, the mean root mean square error (E) of p was less than 1 g m-3, and RH from MWRP was larger than that from radiosonde sounding between the height of 1 and 7 km. The mean E of RH was 18% for overall samples and 23% for foggy samples. The comparison for profiles between the tethered balloon soundings and MWRP shows that the retrieved T from MWRP was also lower than that from tethered balloon, but both were in good agreement in indicating the development and evolution of fog events. The high RH at upper-levels retrieved from MWRP was relevant to the clouds based on the cloud radar data from CloudSat.The 13 fog events are mainly classified into advection fog and evaporation fog from their basic formation processes. The advection fog and evaporation fog accounted for 69% and 31% respectively. The advection fog was mainly caused by cooling process of warm wet airflow advection from southwest and southeast moving onto cold underlying surface in the Northern China region. The formed strong inversion was favorable to the sustainable development of the fog events. The evaporation fog was caused by the cooling process of water vapor evaporated from falling raindrops, or the cooling process due to cold air advection. The average top height of advection fog was less than 1.0 km, and that of precipitation evaporation fog is between 0.5 km and 1.5 km. In addition, we also categorized the 13 fog events into the single fog structure, and fog and cloud coexisted structure based on the vertical structure observed by MWRP. The proportion of single-structure fog was only about 15%, and that of co-existed structure of low-level fog and high-level cloud reached about 85%. The severe fog events lasted for more than three days were almost characterized by the co-existed structure of fog and cloud. Some typical cases were further verified by satellite and surface meteorological observation data. The heavy long-lasting fog events in Beijing were mainly caused by advection and precipitation evaporation processes, which were closely related to the abnormal weather process.The case research of fog and haze event lasted for one week took place during 1-7 December 2011 indicated that the long-lasting fog and haze event occurred in a high pressure weather system and calm wind condition. The stable boundary-layer structure resulted from temperature inversions that were built by warm advection and radiation cooling provided a favorable condition for the accumulation of polluted aerosols and the formation and development of the fog and haze event. In particular, the continuous southerly wet flow advection made the process a persistent and long-lasting event. The horizontal visibility was almost below 2 km in the whole process, and the lowest visibility was only 56 m. The average LWC was about 10-3g m-3, and the maximum LWC reached 0.16 g m-3. The aerosol number concentration was more than 10000 cm-3, and its mass concentration ranged from 50 to 160 μ,g m-3. The further study shows that the fog and haze event experienced three main processes in different intensities during the whole period, each process could be divided into three main stages:aerosol accumulation, transition and mixture of aerosol and fog, and dissipation. Each stage had different physical features:the aerosol accumulation stage was characterized by the increase of aerosol number concentration in Aitken nuclei and accumulation mode sequentially. In the transition and mixing stage of fog and haze, the latent heating produced by fog droplet condensation process and high aerosol number concentration condition intensified the Brownian coagulation process, which induced the small size of aerosols to become larger ones and enhanced the CCN activation process, thereby promoting the explosive development of the fog event. The ratio of aerosol activated to CCN reached 17%, and the ratio of CCN converted to fog droplet exceeded 100%, showing an explosively broadening of fog droplet spectrum. The decrease and dissipation of the fog was caused by an increased solar radiation heating or the passage of cold frontal system. |
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