Characterisitics Of Fogwater Deposition During Winter In Nanjing

Intensive field observations were conducted in the northern suburb of Nanjing during November 15 to December 29 of 2007.We used an ultrasonic anemometer-thermometer in combination with a size-resolving cloud droplet spectrometer capable of providing number counts in 20 droplet size classes. Our study focuses on the physical characteristics of fog and quantifies the fog deposition flux using direct eddy covariance flux measurements,as well as the fogwater deposition differences between radiation and radiation-advection fog.The finding are as follows:fogwater deposition concentration between 5 a.m and 10 a.m. Fog water deposition flux fluctuated intensity during mature stage of fog,the fogwater depositon during this period can be 90% of the whole fog, the visibility during this period often below 200m; Because droplet sizes above 20μm contribute mostly to the fog deposition, the contribution of gravity deposition to the total fogwater deposition is 82%.and the big droplets mostly influenced by gravity settlement mechanism.Radiation fog is the most common fog in Nanjing,when radiation fog happens the atmospheric is in a stable stat,and the underlaying surface is grass, so many reasons cause the turbulent deposition smaller than gravity deposition. Droplet sizes below 7μm and above 40μm not contribute to the overall flux,the reason is small droplets have small fall terminal velocity and they always be conveyed to the upper layer by turbulence motion, and the large droplets have small particle concentration.The longer the duration of fog,the fog deposition can be more, because the deposition is happening mainly in mature stage of fog, so the duration of mature stage of fog has a more immediate effect to fogwater deposition. The fogwater flux depended linearly on liquid water content.The visibility below 200m showing an index relationship to fogwater flux,and the period that visibility below 50m have the mostly fogwater deposition. Certain wind speed is favorable to fog generation and development of, But if wind speed is too large that can strength mixed flow, that is harm for fog development,.this study found that fogwater deposition happens mainly when the wind speed between 0.2 and 2.5 m.s-1.The different characteristics of fogwater flux between radiation fog and radiation- advection fog is in the development stage.In the development stage of radiation fog,small fog droplets been conveyed to the upper layer by the turbulent diffusion,so there are surges in the development stage of radiation fog due to intensity turbulence, the upper layer moisture advection is the main water source of radiation-advection fog,so radiation-fog hasn't this phenomenon.