Impact of Diurnal Warm Layers on Atmospheric Convection

<title xmlns:mml="http://www.w3.org/1998/Math/MathML">Abstract</title><p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en">This manuscript presents a study of oceanic diurnal warm layers (DWLs) in kilometer‐scale global coupled simulations and their impact on atmospheric convection in the tropics. With the implementation of thin vertical levels in the ocean, DWLs are directly resolved, and sea surface temperature (SST) fluctuations of up to several Kelvin appear in regions with low wind and high solar radiation. The increase of SST during the day causes an abrupt afternoon increase of atmospheric moisture due to enhanced latent heat flux (LHF), followed by an increase in cloud cover (CC) and cloud liquid water (CLW). However, although the diurnal SST amplitude is even exaggerated in comparison to reanalysis, this effect only lasts for 5–6 hr and leads to an absolute difference of 1% for CC and 0.01 kg m⁻² for CLW. This can be explained by the fact that the low wind over the SST anomalies dampens their potential effect on the LHF and hence clouds. All in all, the impact of DWLs on convective CC is found to be negligible in the tropical mean.</p>

Plain Language Summary: The diurnal fluctuations of sea surface temperature (SST) have been extensively studied for the last decades, but the assessment of the importance of this phenomenon for atmospheric convection on the global scale has come within reach only very recently, thanks to the development of simulations with a horizontal resolution of O(1 km). In this manuscript we show that we can indeed observe an impact of SST fluctuations on moisture in the atmosphere. However, the impact on the amount of clouds in the tropics is found to be short‐lived and its magnitude negligible on average.</p>

Key Points: <list list-type="bullet"> <list-item> <p xml:lang="en">diurnal warm layers (DWLs) increase atmospheric moisture</p></list-item> <list-item> <p xml:lang="en">The increase of cloud cover (CC) following the formation of a DWL is immediate and only lasts for several hours</p></list-item> <list-item> <p xml:lang="en">The magnitude of the CC increase is small and has no discernible influence on the global mean</p></list-item> </list> </p>

Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

https://gotm.net/

https://hdl.handle.net/21.11116/0000-000C-1447-E