The authors present a very detailed analysis of a (single) fog event in China, and its spatial propagation during the night. They hypothesize that it is a low level jet that drives the propagation, especially determines its speed (almost 10 m / s). Authors present ample evidence from satellite image analysis, ground observation, and WRF modeling. The story is very convincing. In fact, it is a very creative work, presenting novel results. The presentation style itself is very good as well. The manuscript should definitively be published.

This reviewer has only a few minor suggestions:

Abstract, lines 20/21:” The fog propagation speed would decrease notably by 6.4m/s (66%) if the BLLJ-related moisture and warm advections are turned off.” You may add “in the model” to make the concept clearer.

Suggestion not to use and bold letters/words in the main text body, including abstract.

lines 148 – 151: The directions indicated and angles seem somewhat confusing: 160° means wind from 160° (SE winds) and corresponding fog propagation into the NW direction, correct? Please be more clear.

Lines 227/228 and other places in the manuscript, including Fig. 13: It seems not quite alright to say that “fog forms at upper level ahead of forming at ground”.  “Fog” at an upper level with no fog present at the ground, isn’t fog! Fog is a cloud with ground contact. If there is no ground contact of the cloud, it is a low stratus cloud. Authors, please revise the manuscript accordingly in order to be more precise with the respective wording. The process that you describe is stratus lowering, as you correctly state in the following.

This manuscript has serious issues. First how can be warm moist advection at high levels can generate a fog layer at low levels not clear? It will be lifted with vertical motion to higher levels. A schematic fig shows totally not acceptable conditions. If warm air advection at the surface over the cold surface can generate the fog but not higher levels. What is being told in the paper as stratus formation is possible but not the fog. Warm air adv above generates stable layer, an inversion but not the fog. What will cause the low level fog related t jet stream? Did you look at the rad conditions?

In addition, where is the radiative flux in the equation? 

Why the surface wind is less than the higher level as you suggested, and said there are issues in surface obs. I dont think this is true.

Did you show any obs such as lidar or time series of w at the inversion layer that mixing going on? Say from a turb tower? or aircraft?

Did you show a time series of LWC and Nd during the model simulations? I may be missed it.

Make comments truly based on models do not prove your hypothesis.

Some conclusions are very vogue, needs to be supported. I see also no discussion section.