Cloud Base Height Determines Fog Occurrence Patterns in the Namib Desert and Can Be Estimated from Near-Surface Relative Humidity

Malik, Deepanshu; Andersen, Hendrik; Cermak, Jan; Vogt, Roland; Adler, Bianca

doi:https://doi.org/10.5194/egusphere-2025-2645

Preprints

https://doi.org/10.5194/egusphere-2025-2645

Preprints

22 Jul 2025

| 22 Jul 2025

Cloud Base Height Determines Fog Occurrence Patterns in the Namib Desert and Can Be Estimated from Near-Surface Relative Humidity

Deepanshu Malik, Hendrik Andersen, Jan Cermak, Roland Vogt, and Bianca Adler

Abstract. In the hyper-arid Namib Desert, fog serves as the only regular source of moisture, vital for sustaining local ecosystems. While fog occurrence in the region is typically associated with the advection of marine stratus clouds and their interaction with topography, its spatial distribution is strongly influenced by cloud base height, which remains poorly understood. To address this gap, this study utilizes ground-based remote sensing and in-situ observations to analyze systematic spatial and temporal patterns of cloud base height. Our results reveal clear seasonality and a diurnal cycle, with cloud base lowering moderately (10–50 m h⁻¹) during the evening and early night and lifting rapidly (30–150 m h⁻¹) after sunrise, especially inland. A strong and consistent negative correlation (r ≈ -0.75) between cloud base height and near-surface relative humidity was identified using quantile regression, enabling accurate cloud base height estimation with a mean absolute error of 46 m and a mean absolute percentage error of 19 % relative to ground-based measurements. In a case study, the potential value of the estimated cloud base height for separating fog from low clouds in satellite-based products is shown. In the future, a full integration of the estimated cloud base height with a satellite-based fog and low-cloud product can enable a spatially continuous mapping of fog in the region for the first time, which would facilitate fog ecological impact studies.

Received: 10 Jun 2025 – Discussion started: 22 Jul 2025

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Deepanshu Malik, Hendrik Andersen, Jan Cermak, Roland Vogt, and Bianca Adler

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-2645', Anonymous Referee #1, 12 Aug 2025
Review of “Cloud Base Height Determines Fog Occurrence Patterns in the Namib Desert and Can Be Estimated from Near-Surface Relative Humidity” by Malik et al.
This study presents an analysis of cloud base height (CBH) and CBH time tendency for clouds along the Namib Desert coast. The methods are reasonable. I had a hard time following all of the process-based reasoning and I diagram may be helpful in this regard. I am highly skeptical that the CBH-RH relationships represent a novel contribution to the field. Overall, I think that the paper could be publishable with revisions, but I find the scientific significance of the manuscript to not be as high as I would expect for an article published in ACP. Possibly the significance could be increased with revision.
Major Comments:
The authors find, through fitting, that the CBH rises ~22m for every 1% change in surface RH. Much is made of this result. While the authors acknowledge that this result is qualitatively consistent with expectations, I think it is important to note that it is also quantitatively consistent with expectations. Using the approximate equation LCL [km] = (T-Td)/8 and a formulation for Td (dewpoint temperature) in terms of RH and T, I likewise find that the CBH rises ~23m for every 1% change in surface RH (when T=25°C). This suggests to me that all the authors have really done is show that the boundary layer is indeed typically well-mixed during fog and low cloud events. As such, we can apply known equations about the relationship between surface conditions and cloud base height. I don’t think that this result needs to be highlighted so strongly (it’s even in the title) and instead could possibly even be presented first and then applied to all met stations so that an even more comprehensive look at cloud base climatology in the region could be conducted.

I appreciate the efforts the authors have made to infer processes driving the cloud/fog lifecycle, but I had a really hard time following the logic of their conclusions. In general, I think the manuscript could be improved by breaking up long paragraphs into shorter paragraphs with distinct themes. This is particularly true in section 3.2.2 where the paragraph runs to 40 lines. I also think that a diagram could help explain some of the authors’ ideas.
Lines 196-197: I’m not sure what is meant by this sentence. If vertical evolution is variable (which I take to mean that different parts of profile evolve differently) then why would CBH variability be sufficient. I think I am misinterpreting the sentence. Please clarify.

Lines 233-236: How exactly would CBH rate of change vary spatially as a result of this process? I’m having a hard time visualizing this idea.

Line 239: Can the authors explain more clearly why a temporal delay in lowering is indicative of an increasingly thick stratus that is advected over the locations?

Lines 243-244: Would the RH-CBH analysis suggest that decoupling is not occurring?

Lines 345-347: Why exactly does the delay in CBH lowering indicate these things?

Minor Comments:
References are needed on Lines 231 (for Central Europe conditions) and Line 256 (low-level easterly winds)

Lines 226-227: This is an unsatisfying explanation for cloud-base lowering. How can evaporation of water lead to condensation of water at a lower altitude? I typically think of cloud base lowering being driven by either cooling or moistening of the boundary layer. Evaporation of droplets cannot moisten the boundary layer since the water is already present in the layer.
Citation: https://doi.org/10.5194/egusphere-2025-2645-RC1
RC2:
'Comment on egusphere-2025-2645', Anonymous Referee #2, 26 Aug 2025
This paper investigates how cloud base height (CBH) its related with fog occurrence in the Namib Desert, a hyperarid region where fog is a vital source of moisture. Using data from ceilometers and weather stations (FogNet), the authors analyze spatial and temporal patterns of CBH and develop a statistical model to estimate CBH from relative humidity (RH) near the surface. I think the structure of the article, methods, and discussions are of a high standard.
I believe that the main value of the article does not necessarily lie in quantifying the relationship between CBH and RH (although its estimation and validation are important), since this relationship is expected both conceptually and physically. Rather, its value lies in the quantification of spatiotemporal variations (seasonal and daily cycles) and the interpretative efforts of these dynamics.
It is an interesting contribution to a long-standing question in spatial-satellite analysis, concerning whether the low-stratus interacts with the terrain and thus becomes fog. Since RH is a standard measurement within atmospheric/environmental variables, and therefore has greater spatial coverage, it opens up an important avenue for future work and analysis of low-level strata and fog based on comprehensive satellite-ground observation models.
In particular, I believe that this work should be published with some improvements incorporated:
Regarding the introduction and main messages, I would emphasize the quantification of spatial dynamics and their interpretations more than the quantification of the CBH-RH relationship itself.

I found chapter 2.2 (Ceilometer measurements) somewhat difficult to follow, as there is a lot of data, biases, %, etc. mentioned. Please simplify (It could be a table)

Chapter 2.4 (FLC Satellite producto) It is well presented as a satellite product and its validations, but it is not clear what it will be used for in particular in this research.

As I find the processes and interpretations very interesting, I suggest incorporating an interpretative figure of these processes (for different time scales), which could be very helpful in understanding what is expressed in the results (especially chapters 3.2.1 and 3.2.2).

Specific comments:
Line 45: There are some efforts in Atacama, see https://doi.org/10.1016/j.jhydrol.2021.126190

Table 1. When latitudes are indicated as °S, I understand that the negative sign would be redundant.

Line 90: indicate the temporal resolution of meteo stations (10 minutes?)

Line 200: specify which case study you are referring to (there is another case study later in the text)

Figure 9. colors and FLC symbolgy can be improve
Citation: https://doi.org/10.5194/egusphere-2025-2645-RC2

Deepanshu Malik, Hendrik Andersen, Jan Cermak, Roland Vogt, and Bianca Adler

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Short summary

We investigated cloud base height changes in the Namib Desert and developed a method to estimate it using ground-based humidity data. This improves fog monitoring by distinguishing fog from low clouds, which satellites alone cannot reliably do. Our results reveal diurnal patterns and linkages to coastal proximity in the vertical dynamics of fog and low clouds, highlighting key atmospheric processes with potential importance for future research.


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