The impact of cloud-layer environmental variability on the shallow cloud size distribution

Authors

Xiao, Heng — Pacific Northwest National Laboratory
Varble, Adam — Pacific Northwest National Laboratory
Ovchinnikov, Mikhail — Pacific Northwest National Laboratory
Fast, Jerome D — Pacific Northwest National Laboratory

Category

Warm low clouds, including aerosol interactions

Recent debates in the literature (e.g., Brast et al. 2016; Kurowski et al. 2019; Vraciu et al. 2023) point to unclear understanding of the interactions between a shallow or shallow-to-deep transitioning cloud population and the cloud-scale variability in the environment. A clear process-level understanding of the impact of such small-scale environmental variability is essential for evaluating and improving the representation of shallow or transitioning cloud populations in climate and weather forecast models. Such small-scale variability cannot be fully resolved in even kilometer-scale atmospheric models, meaning its impact must be parameterized.

In this study, we examine the impact of small-scale dynamic and thermodynamic variability in the cloud environment on the cloud size distribution using large-eddy simulations (LESs) of shallow and shallow-to-deep transitioning convection. We use the 3-D cloud tracking algorithm of Dawe and Austin (2012) to track the simulated cloud population. We find that the shape of the lifetime-mean cloud size distribution is “flatter” than the relatively more peaked cloud size distribution at the cloud initiation time when a cloud first forms from a rising subcloud thermal in a marine shallow cumulus case. We also find that the environment (approximately 0–1.5 km from the cloud edge) of the largest clouds tends to be relatively dry in the lower cloud layer. This drier environment is in turn associated with a suppression of clouds reaching into the upper part of the cloud layer. We hypothesize that the cloud-scale circulation associated with the largest, longest-lasting clouds in the cloud population creates a drier near-cloud environment that preferentially hinders the development of medium-sized clouds, leading to a mature cloud size distribution flatter than the initial cloud size distribution. Ongoing work to perform “environment homogenization” experiments where the cloud-scale environmental thermodynamic variability is removed will further clarify the impact of such variability on the shallow and shallow-to-deep transitioning convective cloud size distribution.

(Supported by ICLASS SFA, Jerome Fast PI)

References

Brast, M., R. A. J. Neggers, and T. Heus (2016), What determines the fate of rising parcels in a heterogeneous environment?, Journal of Advances in Modeling Earth Systems, 8(4), 1674-1690, doi:10.1002/2016ms000750.

Dawe, J. T., and P. H. Austin (2012), Statistical analysis of an LES shallow cumulus cloud ensemble using a cloud tracking algorithm, Atmospheric Chemistry and Physics, 12(2), 1101-1119, doi:10.5194/acp-12-1101-2012.

Kurowski, M. J., K. Suselj, and W. W. Grabowski (2019), Is Shallow Convection Sensitive to Environmental Heterogeneities?, Geophysical Research Letters, 46(3), 1785-1793, doi:10.1029/2018gl080847.

Vraciu, C. V., I. L. Kruse, and J. O. Haerter (2023), The Role of Passive Cloud Volumes in the Transition from Shallow to Deep Atmospheric Convection, Geophysical Research Letters, 50(23), doi:10.1029/2023gl105996.

Lead PI

Fast, Jerome D — Pacific Northwest National Laboratory