Critical role of vertical radiative cooling contrast in triggering episodic deluges in small-domain hothouse climates

Abstract

Seeley and Wordsworth (2021) showed that in small-domain cloud-resolving simulations the 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 absorption of solar radiation 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 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 mean precipitation can be inferred from the atmospheric column energy budget and the period can be predicted from the time for radiation and reevaporation to cool the lower atmosphere.

The Potential of Absorbing Aerosols to Enhance Extreme Precipitation

“…Figure 3 also highlights that, under the Maritime Continent vertical profile, an increase in SST correlates with a reduction in the frequency of rain events and an increase in their characteristic magnitude. A recent study has shown that the period of the episodic deluges can be predicted by considering the time for radiation and reevaporation to cool the lower atmosphere (Song et al, 2023), a factor expected to vary with SST. Furthermore, it has been demonstrated that, even under the same RHR vertical profile, higher SST facilitates a smoother transition into the episodic deluge regime (Dagan et al, 2023).…”

“…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).…”

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

“…The cloud-resolving model SAM is publicly available at M. Khairoutdinov (2004). The data in this study is publicly available at Song et al (2023).…”