| 31 Aug 2026
The Marine Stratocumulus-topped Boundary Layer across the Pacific-Atacama Desert transition
Abstract. A semi-permanent stratocumulus-topped boundary layer (STBL) is advected daily from the Southeast Pacific toward the Atacama Desert, producing coastal fog that represents a potential water input for ecosystems and communities in this hyper-arid region (∼1 mm yr−1). The STBL is maintained by the balance between synoptic-scale subsidence and entrainment of dry, warm air, driven by cloud-top radiative and evaporative cooling, and surface fluxes over sea and land. These processes are well studied over the open ocean, but their evolution across the ocean-to-desert transition remains poorly understood. Here, we quantify cloud cover fraction (CCF) and STBL height tendency across the Pacific-Atacama Desert transition (from ∼500 km offshore to ∼50 km inland) using three years (2022–2024) of GOES satellite and ERA5 reanalysis, with a zero-order discontinuity approach and dry-cloud formulation. Our results show that CCF decreases from offshore (0.71) to inland (0.12), with higher values in winter (0.12–0.75) than in summer (0.09–0.57). Oceanic CCF variability spans days to weeks, whereas inland CCF variability is dominated by a strong diurnal cycle. The STBL height tendency shifts from near-equilibrium between processes offshore, with a weak positive tendency (∼0.5 cm s−1) driven mainly by cloud-top radiative cooling (∼58 %), to a strongly perturbed inland regime (∼3 cm s−1), where surface fluxes contribute mainly to entrainment in summer (∼43 %) and to subsidence in the other seasons (∼47 %). These results show how subsidence and entrainment govern STBL persistence and breakdown across the coastal desert, with implications for ecosystems and regional climate.
This manuscript discusses 4 regions defined by distance from the coast of Chile, using satellite-derived cloud fraction and ERA5 properties to understand the dominant influences on the ERA5-derived boundary layer depths across diurnal and seasonal time scales. It’s certainly an interesting topic, the meteorology of coastal regions is unique of the inland and offshore regions, and not all land-sea breezes are the same. That said, I had trouble understanding what the new contribution and findings were of this work, and at times, wasn’t even sure of the focus. Several recent papers have examined this region, what is this additional manuscript adding? Portions of the PAD were observationally characterized during VOCALS-REx in October 2008. A new aspect of this study is incorporation of July measurements from StratoFog. Are there interesting differences? The VOCALS-REx observations are well documented and are publicly available at https://data.eol.ucar.edu/. e.g., data from both ships and planes characterized the 20S line out to 85W. This study is also able to provide more context to those studies through its broader domain, extending down to 16S. Implied in this manuscript is that despite variations in the height of the Andes mountains, the diurnal and seasonal cycles are fairly consistent across approximately 700 km in latitude, with only Fig. 6 highlighting latitudinal differences. Is this consistency matched by station observational data and radiosondes from other locations than the StraToFog campaign and Morro Merono airport? See, e.g., fig.3 of the cited Falvey and Garreaud 2009 paper, for other observational datasets that could be used. Mostly I wish for more granularity in what new information this work is providing, I felt a little frustrated in trying to glean this for myself from the presentation. Some portions of the manuscript felt pedantic and could be placed in an Appendix. Overall I had trouble following the plot line and that explains my low evaluation so far.
Somewhere a brief description of the StraToFog campaign is useful as well
More detailed comments are provided below.
Abstract: the abstract would benefit from a clearer statement on what are the new findings. The height budget offshore and inland as discussed seems straightforward. For a research strategy that seems focused on entrainment (sec 3.2), the reader could be forgiven for not gleaning this focus from the abstract.
Introduction:
-define the domain of PAD when first introduced.
-On line 27, mention ‘austral’ before ‘winter’ and mention you will use southern hemisphere seasons throughout, as many of the readers may be more used to the northern hemisphere seasons.
-I was also surprised to read that the cloud top inversion weakens towards the coast. This is not evident to me in the radiosondes shown from along 20S in DeSzoeke et al 2012 (fig. 3) or Zuidema et al 2012 (fig. 6) or Bretherton et al 2010 (fig. 6).
Line 37: the term ‘sea-land breeze circulation’ could use a bit more description and nuance. Do the authors expect this to be consistent over the 700 km long coastline they are characterizing? And does this include the diurnal gravity that extends ~2000 km offshore, generated by diurnal heating of the Andes? See, e.g., Wood et al 2009; Allen et al 2013. I might think there are 2 sea-land breezes. There’s a reference made to Lobos-Roco et al 2021, and a quick summary of might be of use here.
Section 2, theory: mixed-layer theory has been applied before in this region but offshore e.g., Caldwell et al 2005,assessed the diurnal cycle using mixed-layer theory as 20S, 85W. Wood 2007 used mixed-layer theory to understand cloud thinning in this region. Zuidema et al 2009 further did an observational assessment of cloud top height changes using satellite-derived cloud top heights (their eqn 3). There’s nothing wrong in using the same framework of course, but a clearer explanation of what is new would be helpful. Has it not been applied over land before in this region?
I might suggest that portions of this section, detailing mixed-layer theory, can go into an appendix, to allow more focus on new findings, and/or reduced in length. For example, section 2.3.1 seems pedantic, could be summarized, along with section 2.3.2.
Section 2.4, advection: would suggest splitting out meridional and zonal advection, given a study area that is 600 km long. The writeup seems to imply that the latitudinal variation doesn’t matter so much. It’s certainly true that cross-shore variability is much larger, but can latitudinal variations really be ignored? There should be local upwelling zones in the ocean, and the height of the Andes vary as well.
Section 3.1
Section 3.2 Research Strategy.
This section would suggest the focus of the manuscript is on entrainment. If that’s the case, perhaps the introduction needs to be revisited to make this more clear to the reader.
that said, it does seem fair to summarize what was learned from the Caldwell et al 2005 study at least.
P. 10: is the Antofagasta Airport the same as the Morro Mereno Airport? If yes clarify in either the text or table 1
Section 3.3.2 Data Processing
How well does ERA5 capture the thermodynamic profiles within this this complex topography? You can assess using the radiosondes and StraToFog campaign.
Fig. 2: are these CCF values averaged across the full latitudinal stretch of the PAD - does the shading then indicate the spatial homogeneity of the cloud field, given that is it fairly small in the GOES dataset? De Szoeke et al 2016 suggests the full coast line is not entirely homogeneous in the cloud cover, although the large domain size might be exaggerating the spatial variability.
Line 365: might be good to spell out the anticipated sea-land breeze circulation for this location at some point.
Fig 5: figure 9 in Zuidema et al 2009 indicates more variability in the coastal cloud top heights than this figure suggests, while also showing higher cloud top heights near the coast than depicted in the Spring panel. I wonder if GOES is placing the cloud top heights too low, possible when the clouds are thin.
Fig. 6: it’s nice to see the latitudinal granularity in this plot. It does look like there is more near the coast than is implied by figs 2 and 5. I have interpreted fig. 2 and 5 to be averaged over the entire latitudinal range, is that perhaps not correct?
Fig. 10: I seem to miss this, where are the observations coming from? Is a brief description of StraToFog included somewhere? I’m also confused by what the observed radiative fluxes are in the figure given the caption says only ERA5 radiative fluxes were used.
Fig 11: How do these measurements compare to VOCALS-Rex values? It’s nice that the measurements are from July, representing a different time of year from VOCALS-Rex.
References mentioned:
Caldwell, P., C. S. Bretherton, and R. Wood, 2005: Mixed-Layer Budget Analysis of the Diurnal Cycle of Entrainment in Southeast Pacific Stratocumulus. J. Atmos. Sci., 62, 3775–3791, https://doi.org/10.1175/JAS3561.1
Allen, G., Vaughan, G., Toniazzo, T., Coe, H., Connolly, P., Yuter, S.E., Burleyson, C.D., Minnis, P. and Ayers, J.K. (2013), Gravity-wave-induced perturbations in marine stratocumulus. Q.J.R. Meteorol. Soc., 139: 32-45. https://doi.org/10.1002/qj.1952
Bretherton, C. S., Wood, R., George, R. C., Leon, D., Allen, G., and Zheng, X.: Southeast Pacific stratocumulus clouds, precipitation and boundary layer structure sampled along 20° S during VOCALS-REx, Atmos. Chem. Phys., 10, 10639–10654, https://doi.org/10.5194/acp-10-10639-2010, 2010.
Wood, R., Köhler, M., Bennartz, R. and O'Dell, C. (2009), The diurnal cycle of surface divergence over the global oceans. Q.J.R. Meteorol. Soc., 135: 1484-1493. https://doi.org/10.1002/qj.451
S. de Szoeke, S. Yuter, D. Mechem, C. W. Fairall, C. Burleyson and P. Zuidema, 2012: Observations of stratocumulus clouds and their effect on the eastern Pacific surface heat budget along 20S. J. Climate, 25, pp. 8542-8567. doi:10.1175/JCLI-D-11-00681.1]
de Szoeke, S. P., K. L. Verlinden, S. E. Yuter, and D. B. Mechem, 2016: The Time Scales of Variability of Marine Low Clouds. J. Climate, 29, 6463–6481, https://doi.org/10.1175/JCLI-D-15-0460.1.
Zuidema, P., D. Leon, A. Pazmany and M. Cadeddu, 2012: Aircraft millimeter-wave
passive sensing of cloud liquid water and water vapor during VOCALS-REx. Atmos. Chem. Phys., 12, pp. 355-369, doi:10.5194/acp-12-355-2012]
Zuidema, P., D. Painemal, S. deSzoeke and C. Fairall, 2009: Stratocumulus cloud top height estimates and their climatic implications. J. Clim., 22, pp. 4652-4666. doi:10.1175/2009JCLI2708.1