Spatial organisation affects the pathway to precipitation in simulated trade-wind convection

Abstract

We investigate whether and how spatial organisation affects the pathway to precipitation in realistic large-domain large-eddy simulations of the North Atlantic trades. We decompose the formation of surface precipitation (P) into a production phase, where cloud condensate is converted into rain, and a sedimentation phase, where rain falls towards the ground while some of it evaporates. With strengthened organisation, rain forms in weaker updrafts from smaller mean cloud droplets so that cloud condensate is less efficiently converted into rain. At the same time, organisation creates a locally moister environment and modulates the microphysical conversion processes shaping rain properties. This acts to reduce evaporation so that more of the produced rain reaches the ground. Organisation thus affects how the two phases contribute to P. It only weakly affects the total precipitation efficiency. We conclude that the pathway to precipitation differs with spatial organisation and suggest that organisation buffers rain development.

Mesoscale cloud patterns in the trade-wind boundary layer

“…Next to the self-reinforcement of mesoscale circulations through fluctuations in water vapour, at least seven processes have recently been hypothesised to drive and feed back on SMOCs through convective heating. First, transport of aerosols may influence cloudiness and rain formation (Seifert et al, 2015;Yamaguchi et al, 2019;Manshausen et al, 2022), which may again drive the convective heating and SMOCs through the patterning of rain and its interaction with the thermodynamic environment (Vogel et al, 2021;Radtke et al, 2022Radtke et al, , 2023. Second, moist and dry intrusions may give rise to subcloud-layer water vapour variability (Villiger et al, 2022;Albright et al, 2022), which could result in convective instabilities that resolve themselves as SMOCs.…”

“…5.6, and the water vapour anomalies observed during earlier campaigns (Nuijens et al, 2009;Stevens et al, 2018). It could in turn potentially influence the transition to deeper, precipitating convection (Nuijens et al, 2009;Radtke et al, 2023) and explain our congestus-like upper lobe of 5.6 Sketching a framework for the cloud-circulation coupling in the trades153 heating 4 . This offers another analogy to the convection-circulation coupling in the deeper tropics (Bretherton et al, 2004).…”

“…Nevertheless, the simulations realistically capture the co-variability between cloudiness and precipitation and their synoptic environment (Schulz & Stevens, 2023), and the lack of co-variability between surface precipitation rates and the degree of clustering of precipitating clouds (Radtke et al, 2023). Coarser simulations overpredict the cloud-base cloud fraction; this overprediction is somewhat remedied by reducing the grid spacing.…”

“…In this relation, it is assumed that only the small-scale flow transports rain water, while S au and S ac are the autoconversion and accretion rates, which we reconstruct from q c , q r and fields of effective droplet radius following Radtke et al (2023). Rain evaporation S ev also cannot be computed without N r and is therefore (erroneously) absorbed in our definition for the divergence of P ′ m .…”

“…Increased CCN concentrations are known to delay, but invigorate precipitation formation in idealised large-domain LES, leading to more aggregated cloud structures (Seifert et al, 2015;Yamaguchi et al, 2019); we may be witnessing the result of a similar difference between our simulation setups. If nothing else, the difference emphasises that also other processes that can generate deeper, precipitating convection, such as aerosol concentrations, or dynamics associated with rain evaporation (Radtke et al, 2023), deserve consideration in explanations for SMOCs. More broadly, while we have here found that rain can help to create SMOCs, we have not studied rain evaporation dynamics, which might break them (Narenpitak et al, 2023), or even reverse their direction (Savazzi et al, 2023).…”