the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Aug 2023
Effect of boundary layer low-level jet on fog fast spatial propagation
Shuqi Yan
Xiaohui Liu
Fan Zu
Abstract. The spatiotemporal variation of fog reflects the complex interactions among fog, boundary layer ther-modynamics and synoptic systems. Previous studies revealed that fog can present fast spatial propagation feature and attribute it to boundary layer low-level jet (BLLJ), but the effect of BLLJ on fog propagation is not quantitatively understood. Here we analyze a large-scale fog event in Jiangsu, China from 20 to 21 January 2020. Satellite retrievals show that fog propagates from southeast coastal area to northwest inland with the speed of 9.6 m/s, which is three times larger than the ground wind speeds. The ground meteorologies are insufficient to explain the fog fast propagation, which is further investigated by WRF simulations. The fog fast propagation could be attributed to the BLLJ occurring between 50 and 500 m, because the wind speeds (10 m/s) and directions (southeast) of BLLJ core are consistent with fog propagation. Through sensitive experiments and process analysis, three possible mechanisms of BLLJ are revealed: 1) The abundant oceanic moisture is transported inland, increasing the humidity of boundary layer and promoting condensation; 2) The oceanic warm air is transported inland, enhancing the inversion layer and favouring moisture accumulation; 3) The moisture advection probably promotes upper-level fog formation, and later it subsides to ground by turbulent mixing of fog droplets. The fog propagation speed would decrease notably by 6.4 m/s (66 %) if the BLLJ-related moisture and warm advections are turned off.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(2737 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2737 KB) - Metadata XML
- BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Shuqi Yan et al.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1299', Anonymous Referee #1, 30 Aug 2023
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.
Citation: https://doi.org/10.5194/egusphere-2023-1299-RC1 - AC2: 'Reply on RC1', Shuqi Yan, 07 Sep 2023
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CC1: 'Comment on egusphere-2023-1299', Binbin Zhou, 30 Aug 2023
Fog is high impact weather but its prediction skill is pretty low in current weather centers. This article investigated the impact of high jet streams on formation and development of an area fog event over eastern China by using WRF model simulations. The article is generally good but there are some issues that must be clarified before it can be accepted for publication:
1) The authors claimed that the WRF model well simulated the atmospheric status of the fog event. But this conclusion was based only on comparison of simulated variables to the surface observations. Just surface environment well simulated is not enough. The high level atmospheric fields should also be well simulated since this study focuses on the influence of high level wind jets on the fog event. Please add such a comparison of the model with high level fields. This can gives readers more confidence to the results and conclusions.
2) In this study, simulated LWC was used to investigate the fog development during the fog event. The LWC equation included several important items such as condensation part, sedimentation part, vertical mixing part, etc. Then how to identify these items in the study? I don't remember there are items in WRF's output files. These items were calculated or diagnosed from the WRF outputs?
3) This study only focused on the fog event before 8:00AM due to satellite data restriction. As we know that fog may develop further after sunrise . Ignoring the evolution time after 8:00AM may loss many more important features of the fog event, or even the conclusion could be different from this study. Please clarify this concern.
4) In the introduction section line 48: Pleas also add the article by Zhou and Ferrier (2008) about the role of turbulence in fog. Yes, Ye's article (2015) investigated the role of turbulence, but Ye's article is not the first but just confirmed the findings of Zhou and Ferrier in 2008.
Citation: https://doi.org/10.5194/egusphere-2023-1299-CC1 -
AC1: 'Reply on CC1', Shuqi Yan, 07 Sep 2023
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CC2: 'Reply on AC1', Binbin Zhou, 07 Sep 2023
Thanks for addressing all of the questions and making necessary modifications accordingly. The authors' explanation have clarified all of my concerns.
I have no further questions and suggest "accepted" for the revision.
Citation: https://doi.org/10.5194/egusphere-2023-1299-CC2
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CC2: 'Reply on AC1', Binbin Zhou, 07 Sep 2023
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AC1: 'Reply on CC1', Shuqi Yan, 07 Sep 2023
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RC2: 'Comment on egusphere-2023-1299', Anonymous Referee #2, 11 Sep 2023
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.
Citation: https://doi.org/10.5194/egusphere-2023-1299-RC2 - AC3: 'Reply on RC2', Shuqi Yan, 28 Sep 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1299', Anonymous Referee #1, 30 Aug 2023
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.
Citation: https://doi.org/10.5194/egusphere-2023-1299-RC1 - AC2: 'Reply on RC1', Shuqi Yan, 07 Sep 2023
-
CC1: 'Comment on egusphere-2023-1299', Binbin Zhou, 30 Aug 2023
Fog is high impact weather but its prediction skill is pretty low in current weather centers. This article investigated the impact of high jet streams on formation and development of an area fog event over eastern China by using WRF model simulations. The article is generally good but there are some issues that must be clarified before it can be accepted for publication:
1) The authors claimed that the WRF model well simulated the atmospheric status of the fog event. But this conclusion was based only on comparison of simulated variables to the surface observations. Just surface environment well simulated is not enough. The high level atmospheric fields should also be well simulated since this study focuses on the influence of high level wind jets on the fog event. Please add such a comparison of the model with high level fields. This can gives readers more confidence to the results and conclusions.
2) In this study, simulated LWC was used to investigate the fog development during the fog event. The LWC equation included several important items such as condensation part, sedimentation part, vertical mixing part, etc. Then how to identify these items in the study? I don't remember there are items in WRF's output files. These items were calculated or diagnosed from the WRF outputs?
3) This study only focused on the fog event before 8:00AM due to satellite data restriction. As we know that fog may develop further after sunrise . Ignoring the evolution time after 8:00AM may loss many more important features of the fog event, or even the conclusion could be different from this study. Please clarify this concern.
4) In the introduction section line 48: Pleas also add the article by Zhou and Ferrier (2008) about the role of turbulence in fog. Yes, Ye's article (2015) investigated the role of turbulence, but Ye's article is not the first but just confirmed the findings of Zhou and Ferrier in 2008.
Citation: https://doi.org/10.5194/egusphere-2023-1299-CC1 -
AC1: 'Reply on CC1', Shuqi Yan, 07 Sep 2023
-
CC2: 'Reply on AC1', Binbin Zhou, 07 Sep 2023
Thanks for addressing all of the questions and making necessary modifications accordingly. The authors' explanation have clarified all of my concerns.
I have no further questions and suggest "accepted" for the revision.
Citation: https://doi.org/10.5194/egusphere-2023-1299-CC2
-
CC2: 'Reply on AC1', Binbin Zhou, 07 Sep 2023
-
AC1: 'Reply on CC1', Shuqi Yan, 07 Sep 2023
-
RC2: 'Comment on egusphere-2023-1299', Anonymous Referee #2, 11 Sep 2023
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.
Citation: https://doi.org/10.5194/egusphere-2023-1299-RC2 - AC3: 'Reply on RC2', Shuqi Yan, 28 Sep 2023
Peer review completion
Journal article(s) based on this preprint
Shuqi Yan et al.
Shuqi Yan et al.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2737 KB) - Metadata XML