Observational Study On The Winter Fong Macro-Micro-Physical Structures In Nanjing

Comprehensive fog field observations were conducted during the winters of 2006-2009 at the Nanjing University of Information Science & Technology to study macro-and micro-physical structures and physical-chemical processes of dense fogs in the area.The observations include features of fog boundary layer, characteristics of fog water, particle spectrum and chemical composition of atmospheric aerosols, radiation and heat components, turbulence, meteorological elements (air temperature, pressure, wind speed, wind direction), and environmental monitoring.A brief introduction of the field observation/instruments and data is given in section 2. Based on the primary factor that influences fog formation, maturation, and dissipation, fog episodes observed during this project are divided into four types:radiation fog, advection-radiation fog, advection fog, and precipitation fog.Macro-structure of different types of fogs, fog-layer features, fog temperature and wind fields are detailed analyses. The radiation fog had a remarkable diurnal variation. It formed mostly at sunset or midnight, lifted after sunrise or at noon, and its duration was mostly between 9 and 18 hr. Advection-radiation fog and advection fog had very long durations. Extremely dense fogs occurred only in radiation-related cases, namely, radiation fog or advection-radiation fog, and were mostly between 1 and 6 hr. The advection fog and advection-radiation fog had the thickest layers. The inversion in radiation fog was short-lived. The low-level wind was always light prior to fog formation.n radiation fog or advection-radiation fog, the maximum surface wind speed was no more than 0.6 m/s, and the wind directions were mostly northwesterly or southeasterly. The surface wind directions changed greatly before extremely dense fog, especially in the cases that extremely fogs persisted for more than 2 hr. Advection fog formed when wind directions were southerly or easterly, without leading to extremely dense fog.Microphysical characteristics and droplet spectrum distributions of different types of fogs, microphysical relationships (among fog droplet concentration, liquid water content, and mean diameter) are discussed. There were large differences of microphysical parameters among radiation fog, precipitation fog, advection fog, and advection-radiation fog. Advection-radiation fog had the largest values in N, LWC, and AveD. There were two types of radiation-fog drop spectrum distributions. One is called the broad spectrum radiation fog. And the other type is called the narrow spectrum radiation fog. The broad case spectrum lines are all above the narrow ones. The fog had different droplet spectrum broadening characteristics. The droplet spectrum "widening" has the characteristic of a ladder-type and other maximum diameter could enlarge continuously.Based on the obtained fog data and conventional meteorological observation data during 2006 to 2008 at Nanjing Meteorological Observatory, synoptic situations and macro-microstructure characteristics of precipitation fog were analyzed. Results show that:The weather situation when precipitation fog happened can be divided into 3 kinds:front, bottom of high pressure, low pressure and reverse trough; Invasion of weak cold air is an important condition for the formation of precipitation fog; Another important condition is that a strong inversion layer exists, namely the temperature is higher in upper of the boundary layer than it near the ground; Rain enhancement or increasing surface wind speed, intrusion of the durative dry cold air are likely to result in dissipation of precipitation fog, the nature of which is evaporation fog.The evolution and structure of a radiation fog event was studied, the findings are that: Strong radiative cooling occurred first, followed by air immediately above the surface losing heat to the cooled area above, causing cooling there. Meanwhile, soil-evaporated moisture increased the relative humidity in the air, resulting in fog droplets. This fog event was marked by rapid evolution that occurred after sunrise, when enhanced surface evaporation and cold air intrusion. The fog structure was two-layered, with the top of both the surface-layer and upper-layer components characterized by strong temperature and humidity inversions, and low-level jets existed in the boundary layer above each fog layer.By the analysis of the advection fog influenced by cold & warm advection, found that:It's an advection fog influenced by cold and warm advection, which Northeast cold advection in the ground layer and Southeast warm-moist advection in the upper layer. The fog maintenance due to double inversions influenced by cold & warm advection and steady supply of water. The double inversions were the thermal condition for the deep fog layer maintenance. Warm-moist advection makes not only the deep fog layer but the prolonged maintenance. Low-level jet lies near the low-level inversion also promotes its maintenance, which makes the exchange of heat, momentum, and water vapor.