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.