How Moisture Shapes Low‐Level Radiative Cooling in Subsidence Regimes

Abstract

Radiative cooling of the lowest atmospheric levels is of strong importance for modulating atmospheric circulations and organizing convection, but detailed observations and a robust theoretical understanding are lacking. Here we use unprecedented observational constraints from subsidence regimes in the tropical Atlantic to develop a theory for the shape and magnitude of low‐level longwave radiative cooling in clear‐sky, showing peaks larger than 5–10 K/day at the top of the boundary layer. A suite of novel scaling approximations is first developed from simplified spectral theory, in close agreement with the measurements. The radiative cooling peak height is set by the maximum lapse rate in water vapor path, and its magnitude is mainly controlled by the ratio of column relative humidity above and below the peak. We emphasize how elevated intrusions of moist air can reduce low‐level cooling, by sporadically shading the spectral range which effectively cools to space. The efficiency of this spectral shading depends both on water content and altitude of moist intrusions; its height dependence cannot be explained by the temperature difference between the emitting and absorbing layers, but by the decrease of water vapor extinction with altitude. This analytical work can help to narrow the search for low‐level cloud patterns sensitive to radiative‐convective feedbacks: the most organized patterns with largest cloud fractions occur in atmospheres below 10% relative humidity and feel the strongest low‐level cooling. This motivates further assessment of favorable conditions for radiative‐convective feedbacks and a robust quantification of corresponding shallow cloud dynamics in current and warmer climates.

Shallow convective heating in weak temperature gradient balance explains mesoscale vertical motions in the trades

“…Furthermore, the ICON simulations lack the elevated moist layers sensed by JOANNE (fig. 3 c), which may play an important role in creating larger radiative cooling contrasts (Prange et al, 2023;Fildier et al, 2023).…”

Mesoscale cloud patterns in the trade-wind boundary layer

“…Indeed, we have performed idealised LES experiments (not shown in this thesis), where such small SST gradients powered long-lived, self-reinforcing cloudy circulation structures. Sixth, large-scale moisture modes may force convection and circulations through radiative cooling (Fildier et al, 2023); the diurnal cycle has a similar effect (Vial et al, 2021, ch. 7).…”

“…Many have responded by analysing cloud fields in the SGFF paradigm, and these studies have substantially advanced our understanding of the mesoscale variability in trade-wind cloudiness (e.g. Schulz et al, 2021;Vial et al, 2021;Vogel et al, 2021;Narenpitak et al, 2021;Beucher et al, 2022;Dauhut et al, 2023;Fildier et al, 2023).…”

“…Furthermore, the ICON simulations miss the elevated moist layers sensed by JOANNE (figs. 5.6 and 5.7), which are thought to play an important role in creating larger radiative cooling contrasts (Prange et al, 2023;Fildier et al, 2023). Finally, larger moisture variability is usually found in larger scales than the mesoscale (e.g.…”

“…Many such forcings disturb the calm long associated with the region (Nitta & Esbensen, 1974), e.g. sea-surface temperature fluctuations (Park et al, 2006;Acquistapace et al, 2022), extratropical disturbances (Aemisegger et al, 2021), dry and moist intrusions and layers (Villiger et al, 2022;Prange et al, 2023) and their radiative cooling anomalies (Fildier et al, 2023), or gravity waves off remote, deep convection (Stephan & Mariaccia, 2021). In order to understand what sets the coupling between shallow convection and shallow circulations, we first require a framework that can fit all the puzzle pieces.…”