Abstract
Seven boundary‐layer cloud cases are simulated with UCLA‐LES (The University of California, Los Angeles – large eddy simulation) model with different horizontal and vertical gridspacing to investigate how the results depend on gridspacing. Some variables are more sensitive to horizontal gridspacing, while others are more sensitive to vertical gridspacing, and still others are sensitive to both horizontal and vertical gridspacings with similar or opposite trends. For cloud‐related variables having the opposite dependence on horizontal and vertical gridspacings, changing the gridspacing proportionally in both directions gives the appearance of convergence. In this study, we mainly discuss the impact of subgrid‐scale (SGS) kinetic energy (KE) on the simulations with coarsening of horizontal and vertical gridspacings. A running‐mean operator is used to separate the KE of the high‐resolution benchmark simulations into that of resolved scales of coarse‐resolution simulations and that of SGSs. The diagnosed SGS KE is compared with that parameterized by the Smagorinsky‐Lilly SGS scheme at various gridspacings. It is found that the parameterized SGS KE for the coarse‐resolution simulations is usually underestimated but the resolved KE is unrealistically large, compared to benchmark simulations. However, the sum of resolved and SGS KEs is about the same for simulations with various gridspacings. The partitioning of SGS and resolved heat and moisture transports is consistent with that of SGS and resolved KE, which means that the parameterized transports are underestimated but resolved‐scale transports are overestimated. On the whole, energy shifts to large‐scales as the horizontal gridspacing becomes coarse, hence the size of clouds and the resolved circulation increase, the clouds become more stratiform‐like with an increase in cloud fraction, cloud liquid‐water path and surface precipitation; when coarse vertical gridspacing is used, cloud sizes do not change, but clouds are produced less frequently. Cloud fraction and liquid water path decrease.
Highlights
Boundary-layer clouds are parameterized in general circulation models (GCMs) because they are too small to resolve given even the most highly resolved representation of the global atmosphere
The drizzling shallow cumulus case follows the case based on observations taken during the Rain In Cumulus over the Ocean field study (RICO, Rauber et al 2007)
In the analysis presented below, we will focus on the role that kinetic energy plays in the vertical transport of heat and moisture and on the mean states when the horizontal and vertical gridspacings are systematically coarsened
Summary
Boundary-layer clouds are parameterized in general circulation models (GCMs) because they are too small to resolve given even the most highly resolved representation of the global atmosphere. Even at the gridspacings used in the cloud resolving model (CRM) component of the Multi-scale Modeling Framework (MMF, Randall et al 2003) or in a state of the art global cloud resolving model (GCRM, Tomita and Satoh 2004), circulations associated with boundary layer clouds are wholly unresolved. Since the gridspacing helps determine the extent to which the small scale processes are resolved, it is important to understand the effects of gridspacing on the simulation of boundarylayer clouds so as to better guide the use of the MMF (or GCRM) to represent the boundary-layer cloud processes
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