Relationship between optical extinction and liquid water content in fogs

Abstract

Abstract. Studies carried out in the late 1970s suggest that a simple linear relationship exists in practice between the optical extinction in the thermal IR and the liquid water content (LWC) in fogs. Such a relationship opens the possibility to monitor the vertical profile of the LWC in fogs with a rather simple backscatter lidar. Little is known on how the LWC varies as a function of height and during the fog life cycle, so the new measurement technique would help understand fog physics and provide valuable data for improving the quality of fog forecasts. In this paper, the validity of the linear relationship is revisited in the light of recent observations of fog droplet size distributions measured with a combination of sensors covering a large range of droplet radii. In particular, large droplets (radius above 15 µm) are now detected, which was not the case in the late 1970s. The results confirm that the linear relationship still holds, at least for the mostly radiative fogs observed during the campaign. The impact of the precise value of the real and imaginary parts of the refractive index on the coefficient of the linear relationship is also studied. The usual practice considers that droplets are made of pure water. This assumption is probably valid for big drops, but it may be questioned for small ones since droplets are formed from condensation nuclei of highly variable chemical composition. The study suggests that the precise nature of condensation nuclei will primarily affect rather light fogs with small droplets and light liquid water contents.

Quantifying the relationship between PM<sub>2.5</sub> concentration, visibility and planetary boundary layer height for long-lasting haze and fog–haze mixed events in Beijing

“…Under high RH conditions, a large amount of water vapour coated on water-soluble particle surface enlarges the particle size, which significantly enhances the particulate light scattering efficiency and deteriorates visibility. When haze aerosols are water vapour saturated, they can be activated to form fog droplets, which leads to a further decrease of visibility (Elias et al, 2009;Klein and Dabas, 2014;. Moreover, in the aqueous phase, the production rate of sulfate and nitrate aerosols was enhanced by aqueous-phase chemistry (i.e.…”

Multi-peak accumulation and coarse modes observed from AERONET retrieved aerosol volume size distribution in Beijing

“…Third, r 2 and r 3 of 3-peak-II and 4-peak VPSDs illustrate generally a positive correlation with RH which can be thought as similar phenomena as hygroscopic growth but occurring for particles with different size (Li et al 2014;Reid et al 1998). The radii of aerosol particles can increase from accumulation to coarse size involved into the cloud/fog process under highly RH conditions (Klein and Dabas 2014), while the decrease of RH results in the evaporation of cloud/fog droplets and the residual particles (Li et al 2014) can form the extra peak around 0.25-0.40 lm (Table 3). Fourth, r 3 of the 3-peak-I VPSD cases present a general negative correlation with RH, in contrast with those of 3-peak-II and 4-peak types.…”

An empirically derived adjustable model for particle size distributions in advection fog

“…Beside the modified Γ-distribution, that is mostly used, another common type of distribution function used to describe fog particles is the lognormal distribution [KD14]. In order to fit the observed multimodality, the sum of several lognormal distributions is used.…”