Study of a unique fog event in Israel during January 2021: from measured microphysics, ground-remote sensing and satellite imagery to mesoscale forecasts and synoptic analysis

This study describes a regional-scale fog event during 4 consecutive nights in January 2021, in Israel. During the event, we performed a first-of-its kind measurements campaign in Israel, with a  variety of instruments, including both, in-situ measurements of droplets-size distribution, visibility range and meteorological parameters; and ground-based remote sensing with a Doppler Lidar and a thermal IR Whole Sky Imager. In addition, we analyzed the event with IR and visible EUMETSAT imagery, mesoscale WRF-RTFDDA  forecasts and NCEP/NCAR reanalyses.

Reanalyses showed that the event developed under dry Red Sea Trough synoptic conditions at the surface, without cyclonic upper-air circulation, suitable for radiation fog development.

The measurements of droplets-size distribution and visibility range enabled the calculation of liquid-water content and fog-droplets effective radius. The fog event was characterized by a visibility range as low as 90 m. The droplets-diameter main mode was 1-2 micrometers, followed by another around 6 micrometers. Typical liquid-water content  values were 0.01-0.025 g/m3. These parameters are typical of small-droplets fog. A similar identification as small droplets fog was provided by EUMETSAT imagery. 

Analysis of the fog event with the thermal IR Whole Sky Imager, as monitored by the sensor’s field of view, revealed three distinctive properties that make it possible to identify it. First, it exhibits an azimuthal symmetrical shape during the buildup phase. Second, the zenith brightness temperature is very close to the ground-level air temperature. Lastly, the rate of increase in cloud cover up to a completely overcast sky is very fast. Additionally, we validated the use of a Doppler Lidar as a tool for monitoring fog by proving that the measured backscatter-attenuation vertical profile agrees with the calculation of the Lidar equation fed with data measured by in-situ instruments. It was shown that fog can be monitored by those two off-the-shelf stand-off-sensing technologies not originally designed for fog purposes. 

The EUMETSAT imagery was useful to analyze the two-dimensional temporal evolution of the regional fog. The fog evolved from the southern to the central coastal area following the temperature differences from south to north.  Clear patches were observed over coastal-urban areas due to their urban heat-island effect, as previously reported in studies around the world.

High resolution WRF-RTFDDA forecasts (1.1-km grid size), together with an algorithm for fog detection based on simultaneous thresholds of wind speed, dew-point temperature and relative humidity, succeeded in predicting the temporal and spatial development of the dense fog and its disipation. Moreover, they proved useful in distinguishing between near-surface fog and elevated fog/low clouds, a distinction not possible from satellite imagery only. Clear patches at coastal areas, partially covered by urban landuse, were observed in model forecasts, too. WRF-RTFDDA forecasts proved useful in forecasting this massive fog and low-clouds event and in providing warnings to users.