Convection and Convective‐Organization in Hothouse Climates

Abstract

In a “hothouse” climate, warm temperatures lead to a high tropospheric water vapor concentration. Sufficiently high water vapor levels lead to the closing of the water vapor infrared window, which prevents radiative cooling of the lower troposphere. Because water vapor also weakly absorbs solar radiation, hothouse climates feature radiative heating of the lower troposphere. In recent work, this radiative heating was shown to trigger a shift into a novel “episodic deluge” precipitation regime, where rainfall occurs in short, intense outbursts separated by multi‐day dry spells. Here, we further examine the role of the lower tropospheric radiative heating (LTRH) in the transition into the “episodic deluge” regime. We demonstrate that under high sea‐surface temperature the “episodic deluge” regime could be formed even before the LTRH turns positive. In addition, we examine whether these oscillations operate on larger scales and how these oscillations, which represent “temporal” convective self‐organization, would manifest in the presence of traditional “spatial” self‐ or forced‐aggregation in large‐domain convection‐permitting simulations. We find that the temporal oscillations become much less synchronized throughout a large domain ( 1,000 km) because gravity waves cannot propagate fast enough to synchronize convection. We also show that temporal oscillations still dominate the rainfall distribution even when there is tropical convective self‐aggregation or a large‐scale overturning circulation. These results could have important implications for extreme precipitation events under a warming climate.

Hydrologic cycle weakening in hothouse climates

“…( C ) ExoCAM experiments with increasing CO 2 concentration for an aqua-planet with modern solar constant ( 49 ). ( D ) Small-domain fixed-SST (sea surface temperature) cloud-resolving experiments using DAM ( 22 ) and SAM ( 45 ). Blue and red lines show results with the solar spectrum (blue) and an M star spectrum (red) in ( 22 ), and the purple line depicts results from ( 45 ).…”

“…Besides mean precipitation, precipitation variability should also change in hot climates, as suggested in (22,(44)(45)(46)(47). The temporal pattern of precipitation in hothouse climates may transform from quasi-steady to organized episodic deluges, with outbursts of short and heavy rain alternating with several-day dry spells.…”

The Potential of Absorbing Aerosols to Enhance Extreme Precipitation

“…Additionally, the fact that we use small domain simulations, the absence of largescale circulation effects and the use of a fixed SST in our simulations should be considered as potential limitations in the scope of our results. These factors warrant further investigation and may refine our understanding of the relationship between absorbing aerosols and extreme precipitation (e.g., see Dagan et al (2023) for large domain simulations of hothouse climate episodic deluges regime). Furthermore, previous studies using global climate simulations (Dagan et al, 2021;Persad, 2023;Williams et al, 2023) demonstrated that the location of the absorbing aerosol perturbation matters for the local-and global-mean precipitation response.…”

“…In a seemingly unrelated context to absorbing aerosols, recent research has demonstrated that under hothouse climate conditions-warmer than our current climate, believed to have existed in the distant history of our planet -precipitation shifts from the quasi-equilibrium condition observed today to an "episodic deluge" regime. This regime is characterized by short and intense outbursts of rainfall separated by multi-day dry spells (Dagan et al, 2023;Seeley & Wordsworth, 2021;Song et al, 2023;Spaulding-Astudillo & Mitchell, 2023).…”

“…Using radiative-convective-equilibrium (RCE) simulations with idealized and prescribed radiative heating rate vertical profiles, it was shown that LTRH under current tropical sea surface temperature (SST) could also generate a shift into episodic deluge regime (Dagan et al, 2023;Seeley & Wordsworth, 2021;Song et al, 2023). Hence, it was speculated that LTRH due to an absorbing aerosol perturbation in the tropics could affect extreme precipitation under our current climate conditions (Dagan et al, 2023). This paper aims to examine this speculation and provide a comprehensive examination of the impact of absorbing aerosols on tropical precipitation in RCE simulations (which exclude diurnal cycle), with a particular focus on extreme precipitation events.…”