Investigating the Diurnal Evolution of the Cloud Size Distribution of Continental Cumulus Convection Using Multiday LES

Abstract

The diurnal dependence of cumulus cloud size distributions over land is investigated by means of an ensemble of large-eddy simulations (LESs). A total of 146 days of transient continental shallow cumulus are selected and simulated, reflecting a low midday maximum of total cloud cover, weak synoptic forcing, and the absence of strong surface precipitation. The LESs are semi-idealized, forced by large-scale model output but using an interactive surface. This multitude of cases covers a large parameter space of environmental conditions, which is necessary for identifying any diurnal dependencies in cloud size distributions. A power-law exponential function is found to describe the shape of the cloud size distributions for these days well, with the exponential component capturing the departure from power-law scaling at the larger cloud sizes. To assess what controls the largest cloud size in the distribution, the correlation coefficients between the maximum cloud size and various candidate variables reflecting the boundary layer state are computed. The strongest correlation is found between total cloud cover and maximum cloud size. Studying the size density of the cloud area revealed that larger clouds contribute most to a larger total cloud cover, and not the smaller ones. Besides cloud cover, cloud-base and cloud-top height are also found to weakly correlate with the maximum cloud size, suggesting that the classic idea of deeper boundary layers accommodating larger convective thermals still holds for shallow cumulus. Sensitivity tests reveal that the results are only minimally affected by the representation of microphysics and the output resolution.

Size dependence in chord characteristics from simulated and observed continental shallow cumulus

“…9), the water vapor anomalies remain slightly positive beyond the cloud at the cloud-base height. This is consistent with our general understanding of subsiding shells, which, while drier than the cloud cores, are still moister than the surrounding air (Heus and Jonker, 2008;Wang and Geerts, 2010;Katzwinkel et al, 2014;Lareau, 2020).…”

Linking Large‐Eddy Simulations to Local Cloud Observations

“…For this reason we decided to also include results from a more well established model (DALES-SI). The DALES model (Heus et al, 2010) has been applied in semi-idealized mode over a wide variety of conditions (Neggers et al, 2012;Reilly et al, 2020), including the JOYCE site (Corbetta et al, 2015;van Laar et al, 2019).…”

“…The DALES simulations discussed here were generated using the identical mixed-phase microphysics scheme as used in ICON-SI (Seifert & Beheng, 2006). Its recent implementation has been thoroughly tested against observations in the Arctic (Neggers et al, 2019).…”

On the Abundance and Common Properties of Continental, Organized Shallow (Green) Clouds

“…A number of studies have examined the size distributions of shallow Cu over land. Some of them have reported an exponential CSD shape [9], [30]- [32], and others a power-law behavior [33]- [37]. However, none of these studies performed a systematic analysis of the clouds on a global scale.…”