Publication Date:
2023-11-13
Description:
Low‐level clouds over the tropical and sub‐tropical oceans play a crucial role in the planetary radiative energy budget. However, they are challenging to model in climate simulations because they are affected by local processes that are still partially unknown. The control that mesoscale sea surface temperature (SST) structures have on the dynamics of the lower atmosphere on daily scales is emerging to be non‐negligible and calls for more effort to be understood. During the EUREC〈sup〉4〈/sup〉A field campaign, two of the research vessels (R/Vs) involved in the experiment sampled the edge of a cold mesoscale SST patch in the north‐western tropical Atlantic, crossing a gradient of roughly 0.75°C/100 km. The comprehensive set of instruments carried by the R/Vs allows an unprecedented characterization of the atmospheric response to the cold water forcing. The cold ocean patch weakens the vertical atmospheric mixing, reducing the boundary layer depth of roughly 200 m and the horizontal wind intensity of approximately 3 m s〈sup〉−1〈/sup〉. At the same time, the humidity content in the sub‐cloud layer increases and these conditions decrease the latent heat flux (by roughly 80 W m〈sup〉−2〈/sup〉) and reduce vertical velocity fluctuations, making it less likely that moisture exceeds the lifting condensation level. As a consequence, fewer and thinner low‐level clouds form over cold water. Independent satellite measurements are found to agree with the in situ observations. The observed link between sea temperature and low‐level clouds highlights its importance in the puzzle of modeling the sea‐air‐cloud interactions.
Description:
Plain Language Summary:
Puffy clouds typically visible at sea strongly challenge climate models that struggle to represent their interaction with the sea surface and solar radiation. And as a consequence, these climate models cannot precisely estimate how much the Earth's temperature will increase 100 years from now and its uncertainty. We went into the western Atlantic ocean for a month in January–February 2020 to measure sea surface temperature and cloud properties and observe how these changes occur. We saw clouds grow deeper over the warm water patches, holding more water and eventually raining. Weaker winds and more humid air occur on cold patches. Satellite observations seem to record the same behavior over a larger area in the same region. We detected a clear difference in cloud properties and amounts over warm and cold patches from all sensors, recording essential evidence of a feature that is hard to predict. We hope these observations will help to properly simulate the intensity and signs of low‐cloud feedback over warm and cold oceanic patches of water to improve climate models.
Description:
Key Points:
Ship‐based observations are used to characterize the lower atmospheric response to a cold patch in the north‐western subtropical Atlantic.
Signatures in dynamical and thermodynamical atmospheric properties agree with a reduced vertical mixing over the cold patch.
Such a weaker vertical mixing is linked to a reduced shallow cloud cover because less moisture reaches the level of saturation.
Description:
Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659
Description:
ESA ESRIN
Description:
MIUR 2018–2022
Description:
COST European Cooperation in Science and Technology
Description:
Helmholtz Association of German Research Centers
Description:
https://doi.org/10.5281/zenodo.6365636
Description:
https://github.com/ClauClouds/SST-impact/
Keywords:
ddc:551.5
;
mesoscale SST structures
;
low clouds
;
cloud feedbacks
;
marine boundary layer
;
air‐sea interactions
;
ship‐based remote sensing
Language:
English
Type:
doc-type:article
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