The Effect of the Atmospheric and Cloud Properties on the Retrieval of Fog Microphysics at Night for MSG/SEVIRI: A Sensitivity Test Using LibRadtran

The simplest way of building a look-up table (LUT) for the retrieval of cloud microphysical properties is to use a standard atmospheric profile and vertically uniform cloud microphysics. Such an assumption has been demonstrated to be incoherent with in-cloud observations. This paper aims to show the effect of some atmospheric conditions associated with fog as well as its macro-and microstructure on brightness temperature (BT) for the MSG/ SEVIRI satellite using libRadtran. The sensitivity tests were performed by gradually changing some features from the initial data, such as cloud cover, total water vapor column, thermal inversion intensity, fog depth, fog microstructure, and others. The results revealed that some variables can cause significant variations on BT and, consequently, discrepancies in the retrieval of fog microphysical properties. Also, a variation as high as 0.5°C was found on BT just by switching uniform to the non-uniform profile of fog microphysics depending on the channel, the droplet size, and optical thickness.

Analysis of the Microphysical Structure of Heavy Fog Using a Droplet Spectrometer:A Case Study

The microphysical properties of a long-lasting heavy fog event are examined based on the results from a comprehensive field campaign conducted during the winter of 2006 at Pancheng （32.2°N, 118.7°E）, Jiangsu Province, China. It is demonstrated that the key microphysical properties （liquid water content, fog droplet concentration, mean radius and standard deviation） exhibited positive correlations with one another in general, and that the 5-min-average maximum value of fog liquid water content was sometimes greater than 0.5 g m-3. Further analysis shows that the unique combination of positive correlations likely arose from the simultaneous supply of moist air and fog condensation nuclei associated with the advection of warm air, which further led to high liquid water content. High values of liquid water content and droplet concentration conspired to cause low visibility （〈50 m） for a prolonged period of about 40 h. Examination of the microphysical relationships conditioned by the corresponding autoconversion threshold functions shows that the collision-coalescence process was sometimes likely to occur, weakening the positive correlations induced by droplet activation and condensational growth. Statistical analysis shows that the observed droplet size distribution can be described well by the Gamma distribution.

The Influences of Macro- and Microphysical Characteristics of Sea-Fog on Fog-Water Chemical Composition

ABSTRACT During a sea-fog field observation campaign on Donghai Island in the spring of 2011, fog-water, visibility, meteorological elements, and fog droplet spectra were measured. The main cations and anions in 191 fog-water samples were Na＋, NH2, H＋, NO3, C1- and SO] , and the average concentrations of cations and anions were 2630 and 2970 p-eq L 1, respectively. The concentrations of Na＋ and C1- originated from the ocean were high. The enhancement of anthropogenic pollution might have contributed to the high concentration of NH＋, H＋, and NO^-3. The average values ofpH and electrical conductivity （EC） were 3.34 and 505 uS cm-1, respectively, with a negative correlation between them. Cold fronts associated with cyclonic circulations promoted the decline of ion loadings. Air masses from coastal areas had the highest ion loadings, contrary to those from the sea. The ranges of wind speed, wind direction and temperature corresponding to the maximum total ion concentration （TIC） were 3.5-4 m s-1, 79°-90° and 21°C-22°C, respectively. In view of the low correlation coefficients, a new parameter Lr was proposed as a predictive parameter for TIC and the correlation coefficient increased to 0.74. Based on aerosol concentrations during the sea-fog cases in 2010, we confirmed that fog-water chemical composition also depended on the species and sizes of aerosol particles. When a dust storm passed through Donghai Island, the number concentration of large aerosol particles （with diameter 〉 1 p-m） increased. This caused the ratio of CaZ＋/Na＋ in fog-water to increase significantly.

ANALYSIS OF THE MICROPHYSICAL STRUCTURE OF RADIATION FOG IN XUANEN MOUNTAINOUS REGION OF HUBEI,CHINA

Based on data of radiation fog events in Xuanen, Hubei province, 2010, this paper analyzes the microphysical process and evolution characteristics of radiation fogs with complicated substrate in the upper and middle reaches of the Yangtze River, and compares them with findings in other areas. Results are as follows: radiation fog in Xuanen is evidently weaker in droplet number concentration and liquid water content than land fogs in other areas. Its liquid water content fluctuates obviously, 0.01g/m3 with visibility of 1,000 meters, which is quite different from that in urban areas, but similar to the Nanling Mountains. Bi-modal droplet distribution is likely to occur in Xuanen mountain radiation fog(MRF) events. Statistical analysis shows that the observed droplet size distribution can be piecewise described well by the Gamma distribution. There is a positive correlation between liquid water content, fog droplet concentration and mean radius, especially in the development and dissipation stage. Condensation growth and droplet evaporation are major processes of Xuanen MRF. The dissipation time coincided with the time when the grass temperature reached the peak value, which indicated that dew evaporation is a key role in maintaining Xuanen MRF. In the early stage of dense fog's growth, droplets with diameter of over 20 micrometers can be observed with visibility of800-1,000 m, which might be caused by the transportation of low cloud droplets to earth's surface by turbulence. Big droplets in the initial stage correspond to higher water content, leading to the higher observed value of water content of Xuanen MRF.

Examination of Microphysical Relationships and Corresponding Microphysical Processes in Warm Fogs

In this paper, the microphysical relationships of 8 dense fog events collected from a comprehensive fog observation campaign carried out at Pancheng（32.2 N, 118.7 E） in the Nanjing area, China in the winter of 2007 are investigated. Positive correlations are found among key microphysical properties（cloud droplet number concentration, droplet size, spectral standard deviation, and liquid water content） in each case, suggesting that the dominant processes in these fog events are likely droplet nucleation with subsequent condensational growth and/or droplet deactivation via complete evaporation of some droplets. The abrupt broadening of the fog droplet spectra indicates the occurrence of the collision-coalescence processes as well, although not dominating. The combined efects of the dominant processes and collision-coalescence on microphysical relationships are further analyzed by dividing the dataset according to visibility or autoconversion threshold in each case. The result shows that the specific relationships of number concentration to volume-mean radius and spectral standard deviation depend on the competition between the compensation of small droplets due to nucleation-condensation and the loss of small droplets due to collision-coalescence. Generally, positive correlations are found for diferent visibility or autoconversion threshold ranges in most cases, although negative correlations sometimes appear with lower visibility or larger autoconversion threshold. Therefore, the compensation of small droplets is generally stronger than the loss, which is likely related to the sufcient fog condensation nuclei in this polluted area.