the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Effects of longwave radiative cooling on advection fog over the Northwest Pacific Ocean: Observations and large eddy simulations
Abstract. During the boreal summer, the prevailing southerlies traverse the sharp sea surface temperature (SST) front in the Northwest Pacific (NWP) Ocean, creating a stable air-sea interface characterized by surface air temperature (SAT) higher than SST, which promotes the frequent occurrence of advection fog. However, long-term shipborne observations reveal that during episodes of advection fog, SAT usually decreases below SST, with a peak relative frequency (~34.5 %) to all fog observations before sunrise and a minimum relative frequency (~18.8 %) before sunset. From a Lagrangian perspective, this study employs a turbulence-closure large-eddy simulation (LES) model to trace a fog column across the SST front and investigates how SAT drops below the SST during an advection fog event. The LES model, incorporating constant solar radiation, successfully simulates the evolution of advection fog and the negative SAT-SST. Simulation results show that once the near-surface air condenses, the thermal turbulence is generated by strong longwave radiation cooling (LWC) at the fog top. The influence of LWC on the fog layer surpasses the cooling effect of the near-surface mechanical turbulence ~2 hours after the fog formation, while the fog column is still positioned over the SST front. When the fog column arrives the cold flank of the SST front, the top-down developing mixed layer induced by the LWC reaches the surface, causing the SAT to drop below SST. The LES model with diurnal solar radiation well simulates the diurnal variation in SAT-SST during the fog event, suggesting that the model captures the essential processes responsible for negative SAT-SST. This study highlights the significance of fog-top cooling and its associated thermal turbulence in the evolution of advection fog. Given the challenges faced by numerical weather prediction models in forecasting sea fog, our findings suggest that observations of negative SAT-SST during advection fog episodes present an opportunity to enhance the performance of these models in simulating the thermal turbulence induced by the LWC at the fog top.
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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
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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.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2023-1494', Hermann Gerber, 13 Sep 2023
This paper deals with advection fog over the Pacific Ocean using observations and the UCLA-LES. It would be appreciated
if the authors explain the behavior of predicted radiative cooling shown on their Fig. 9. In particular, why is it so small after the initial surge?
Also, this ir cooling amount is substantially less than in stratocumulus under clear sky conditions, assuming that this cooling acts similarly
for advection fog and for stratocumulus (e.g., see Gerber et al.: 14th AMS Conf. on Atm. Radiation, 7-11 July, 2014, Boston, MA.,
paper 9.3.). Further, can you comment on the grid spacing used in your model if it is sufficient to deal with the details of the usually very narrow
fog-top interface that may need sub-grid parameterization. Another paper that applies is by Malinowski et al., ACP, doi:10.5194/
acp-13-12171-2013.
Citation: https://doi.org/10.5194/egusphere-2023-1494-CC1 -
AC2: 'Reply on CC1', Jing-Wu Liu, 21 Jan 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1494/egusphere-2023-1494-AC2-supplement.pdf
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AC2: 'Reply on CC1', Jing-Wu Liu, 21 Jan 2024
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RC1: 'Comment on egusphere-2023-1494', Anonymous Referee #1, 10 Oct 2023
Review of the article titled “Effects of longwave radiative cooling on advection fog over the Northwest Pacific Ocean: Observations and large eddy simulations” by Yang et al. for publication in atmospheric chemistry and physics.
The authors have analyzed a fog event in the Northwest Pacific region using observations and LES model. The focus is on advection fog that crosses the SST front thereby advecting over a scenario with the sea surface heating the fog layer as opposed to cooling it. The article is overall well-written, easy to follow, and the analysis support the conclusions. I recommend minor revisions before the article is published and hence mention few things below that can further improve the manuscript.
Title: Solar radiation is included in all of the analysis, so not sure why the authors have “longwave” in the title?
Figure 6c suggests that there is a lot of cloud water in the fog. Especially towards the end the cloud water content is constant at ~1 g/kg throughout the layer. I assume that at this high value, there is got to be some cloud water to rainwater conversion. Now if the rain is leaving the fog and falling into the sea surface, it will thin the fog layer and also affect the budget terms. Normally the surface rain flux is a heating and drying term in boundary layer budgets, see Caldwell and Bretherton (2005 JAS). Can you please expand on this a bit more? The cloud droplet number concentration is set constant in the simulations, but there is no mention of rain, so it is hard to tell what might cause this. Lack of rain might be the reason that the fog is persisting over a long time. I assume there are no aerosols in the model.
Section 2.2: please mention the vertical and horizontal resolution of the LES model.
Line 151-152: you mean “respectively”?
Line 169-170: How were the fog events tracked, was there a trajectory model used for this analysis?
Figure 1: Please show the scale of the wind barb and mention the contour levels. I understand that you have mentioned two SST contours in the legend shown in thick lines, but it will be good to show them in the figure.
Figure 4: A lot of work has gone into this figure. I think snapshots of visible satellite imagery will be hugely beneficial to the readers.
Section 4.2 heading: “Heat and Moisture Budgets”
Figure 10b and Figure 12 are fascinating. Compared to those in the constant solar radiation simulations, the fog layer bottom and top undulate a lot during the diurnal cycle simulations. Can you please elaborate causes of this? Thank you.
Citation: https://doi.org/10.5194/egusphere-2023-1494-RC1 -
AC3: 'Reply on RC1', Jing-Wu Liu, 21 Jan 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1494/egusphere-2023-1494-AC3-supplement.pdf
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AC3: 'Reply on RC1', Jing-Wu Liu, 21 Jan 2024
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RC2: 'Comment on egusphere-2023-1494', Mónica Zamora Zapata, 17 Oct 2023
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AC1: 'Reply on RC2', Jing-Wu Liu, 21 Jan 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1494/egusphere-2023-1494-AC1-supplement.pdf
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AC1: 'Reply on RC2', Jing-Wu Liu, 21 Jan 2024
Interactive discussion
Status: closed
-
CC1: 'Comment on egusphere-2023-1494', Hermann Gerber, 13 Sep 2023
This paper deals with advection fog over the Pacific Ocean using observations and the UCLA-LES. It would be appreciated
if the authors explain the behavior of predicted radiative cooling shown on their Fig. 9. In particular, why is it so small after the initial surge?
Also, this ir cooling amount is substantially less than in stratocumulus under clear sky conditions, assuming that this cooling acts similarly
for advection fog and for stratocumulus (e.g., see Gerber et al.: 14th AMS Conf. on Atm. Radiation, 7-11 July, 2014, Boston, MA.,
paper 9.3.). Further, can you comment on the grid spacing used in your model if it is sufficient to deal with the details of the usually very narrow
fog-top interface that may need sub-grid parameterization. Another paper that applies is by Malinowski et al., ACP, doi:10.5194/
acp-13-12171-2013.
Citation: https://doi.org/10.5194/egusphere-2023-1494-CC1 -
AC2: 'Reply on CC1', Jing-Wu Liu, 21 Jan 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1494/egusphere-2023-1494-AC2-supplement.pdf
-
AC2: 'Reply on CC1', Jing-Wu Liu, 21 Jan 2024
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RC1: 'Comment on egusphere-2023-1494', Anonymous Referee #1, 10 Oct 2023
Review of the article titled “Effects of longwave radiative cooling on advection fog over the Northwest Pacific Ocean: Observations and large eddy simulations” by Yang et al. for publication in atmospheric chemistry and physics.
The authors have analyzed a fog event in the Northwest Pacific region using observations and LES model. The focus is on advection fog that crosses the SST front thereby advecting over a scenario with the sea surface heating the fog layer as opposed to cooling it. The article is overall well-written, easy to follow, and the analysis support the conclusions. I recommend minor revisions before the article is published and hence mention few things below that can further improve the manuscript.
Title: Solar radiation is included in all of the analysis, so not sure why the authors have “longwave” in the title?
Figure 6c suggests that there is a lot of cloud water in the fog. Especially towards the end the cloud water content is constant at ~1 g/kg throughout the layer. I assume that at this high value, there is got to be some cloud water to rainwater conversion. Now if the rain is leaving the fog and falling into the sea surface, it will thin the fog layer and also affect the budget terms. Normally the surface rain flux is a heating and drying term in boundary layer budgets, see Caldwell and Bretherton (2005 JAS). Can you please expand on this a bit more? The cloud droplet number concentration is set constant in the simulations, but there is no mention of rain, so it is hard to tell what might cause this. Lack of rain might be the reason that the fog is persisting over a long time. I assume there are no aerosols in the model.
Section 2.2: please mention the vertical and horizontal resolution of the LES model.
Line 151-152: you mean “respectively”?
Line 169-170: How were the fog events tracked, was there a trajectory model used for this analysis?
Figure 1: Please show the scale of the wind barb and mention the contour levels. I understand that you have mentioned two SST contours in the legend shown in thick lines, but it will be good to show them in the figure.
Figure 4: A lot of work has gone into this figure. I think snapshots of visible satellite imagery will be hugely beneficial to the readers.
Section 4.2 heading: “Heat and Moisture Budgets”
Figure 10b and Figure 12 are fascinating. Compared to those in the constant solar radiation simulations, the fog layer bottom and top undulate a lot during the diurnal cycle simulations. Can you please elaborate causes of this? Thank you.
Citation: https://doi.org/10.5194/egusphere-2023-1494-RC1 -
AC3: 'Reply on RC1', Jing-Wu Liu, 21 Jan 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1494/egusphere-2023-1494-AC3-supplement.pdf
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AC3: 'Reply on RC1', Jing-Wu Liu, 21 Jan 2024
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RC2: 'Comment on egusphere-2023-1494', Mónica Zamora Zapata, 17 Oct 2023
-
AC1: 'Reply on RC2', Jing-Wu Liu, 21 Jan 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1494/egusphere-2023-1494-AC1-supplement.pdf
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AC1: 'Reply on RC2', Jing-Wu Liu, 21 Jan 2024
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Liu Yang
Saisai Ding
Jing-Wu Liu
Su-Ping Zhang
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2309 KB) - Metadata XML