Critical Role of Vertical Radiative Cooling Contrast in Triggering Episodic Deluges in Small‐Domain Hothouse Climates

• Published in: Journal of advances in modeling earth systems 2024-06, Vol.16 (6), p.n/a
• Main Authors: Song, Xinyi, Abbot, Dorian S., Yang, Jun
• Format: Article
• Language: English
• Subjects: Boundary layers; Charged particles; CIN; Climate; cloud‐resolving model; Convection; Cooling; episodic deluge; Experiments; General circulation models; Greenhouse effect; Heating; Heavy rainfall; High temperature; Hot climates; hothouse climate; Lower atmosphere; Moist static energy; Precipitation; Radiation; Radiation absorption; Radiative cooling; Radiative heating; Rainfall; Simulation; Solar radiation; Solar radiation absorption; Troposphere; Upper troposphere; Water vapor; Water vapour
• ISSN: 1942-2466; 1942-2466
• DOI: 10.1029/2023MS003912

Seeley and Wordsworth (2021, https://doi.org/10.1038/s41586‐021‐03919‐z) showed that in small‐domain cloud‐resolving simulations the temporal pattern of precipitation transforms in extremely hot climates (≥320 K) from quasi‐steady to organized episodic deluges, with outbursts of heavy rain alternating with several dry days. They proposed a mechanism for this transition involving increased water vapor greenhouse effect and solar radiation absorption leading to net lower‐tropospheric radiative heating. This heating inhibits lower‐tropospheric convection and decouples the boundary layer from the upper troposphere during the dry phase, allowing lower‐tropospheric moist static energy to build until it discharges, resulting in a deluge. We perform cloud‐resolving simulations in polar night and show that the same transition occurs, implying that some revision of their mechanism is necessary. We perform further tests to show that episodic deluges can occur even if the lower‐tropospheric radiative heating rate is negative, as long as the magnitude of the upper‐tropospheric radiative cooling is about twice as large. We find that in the episodic deluge regime the period can be predicted from the time for radiation and reevaporation to cool the lower atmosphere. Plain Language Summary Precipitation plays an important role in Earth's climate and habitability, and also influences important weathering processes such as the carbonate‐silicate cycle. In the distant future, Earth may experience a very hot and wet “hothouse” climate, just like it may have in the Archean. Modeling results show that in a hothouse climate, precipitation transforms into an “episodic deluge” pattern, with outbursts of heavy rain alternating with several dry days. In this study, we find that positive lower‐tropospheric heating is not the necessary cause for episodic deluges. Instead, vertical radiative cooling contrast is critical in triggering the episodic deluges in small‐domain hothouse climates. We also try to understand the underlying mechanism of episodic deluges through CIN and CAPE analyses. Key Points Episodic deluges can occur during polar night Lower‐tropospheric radiative heating is not necessary for the occurrence of episodic deluges A strong vertical gradient of radiative cooling is a key factor in triggering episodic deluges