A total of 14 fog events are identified during the study period. The MWR temperature is in close agreement with that from radiosonde, generally within 1K. The specific humidity biases, on the other hand, are of a larger magnitude, of up to 5 gm kg-1. An analysis of the seasonality of the temperature and specific humidity biases revealed that, while the latter are very similar for the different months considered, the former shows an increasing trend being mostly negative from November to January and mostly positive in March and April.
A strong near-surface inversion with lapse rates in the lowest 250 m of up to 30 K km-1 is needed for fog to form. Subsidence warming and drying associated with the subtropical anticyclone can descend down to 500 m, aiding the formation of fog at night. Cloud base height retrievals from the infrared (IR) camera capture fog onset and dissipation and complement the MWR observations.
It is found that a strong near-surface temperature inversion of up to 30 K km-1 in the bottom 250 m is needed for fog to form (e.g. RenHe et al., 2014). However, if it is too strong, here up to 40 K km-1, it may prevent the development of fog (Zhang et al., 2018). The moistening of the atmosphere in the bottom 500 m is a necessary but not sufficient condition for the development of fog, with a water vapour mixing ratio of at least 6 gm kg-1 just above the surface present in all 14 fog events considered. A synergetic use of different instruments could allow for more accurate fog detection and nowcasting.