A modified microphysics scheme is implemented in the Community Atmosphere Model, version 5 (CAM5). The new scheme features prognostic precipitation. The coupling between the microphysics and the rest of the model is modified to make it more flexible. Single-column tests show the new microphysics can simulate a constrained drizzling stratocumulus case. Substepping the cloud condensation (macrophysics) within a time step improves single-column results. Simulations of mixed-phase cases are strongly sensitive to ice nucleation. The new microphysics alters process rates in both single-column and global simulations, even at low (200 km) horizontal resolution. Thus, prognostic precipitation can be important, even in low-resolution simulations where advection of precipitation is not important. Accretion dominates as liquid water path increases in agreement with cloud-resolving model simulations and estimates from observations. The new microphysics with prognostic precipitation increases the ratio of accretion over autoconversion. The change in process rates appears to significantly reduce aerosol-cloud interactions and indirect radiative effects of anthropogenic aerosols by up to 33% (depending on substepping) to below 1 W m 22 of cooling between simulations with preindustrial (1850) and present-day (2000) aerosol emissions.
For the first time a mesoscale-resolving whole atmosphere general circulation model has been developed, using the National Center for Atmospheric Research Whole Atmosphere Community Climate Model with ∼0.25 • horizontal resolution and 0.1 scale height vertical resolution above the middle stratosphere (higher resolution below). This is made possible by the high accuracy and high scalability of the spectral element dynamical core from the High-Order Method Modeling Environment. For the simulated January-February period, the latitude-height structure and the magnitudes of the temperature variance compare well with those deduced from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations. The simulation reveals the increasing dominance of gravity waves (GWs) at higher altitudes through both the height dependence of the kinetic energy spectra, which display a steeper slope (∼ −3) in the stratosphere and an increasingly shallower slope above, and the increasing spatial extent of GWs (including a planetary-scale extent of a concentric GW excited by a tropical cyclone) at higher altitudes. GW impacts on the large-scale flow are evaluated in terms of zonal mean zonal wind and tides: with no GW drag parameterized in the simulations, forcing by resolved GWs does reverse the summer mesospheric wind, albeit at an altitude higher than climatology, and only slows down the winter mesospheric wind without closing it. The hemispheric structures and magnitudes of diurnal and semidiurnal migrating tides compare favorably with observations.
Climate models rely on parameterizations of a variety of processes in the atmospheric physics, but a common concern is that the temporal resolution is too coarse to consistently resolve the behavior that individual parameterizations are designed to capture. This study examines timescales numerically derived from the Morrison-Gettelman (MG2) microphysics as implemented within the Energy Exascale Earth System Model, version 1 (E3SMv1). Numerically relevant timescales in MG2 are derived by computing the eigenspectrum of its Jacobian. These timescales are found to often be smaller than the default 5 min time step used for MG2. The fast timescales are then heuristically connected to individual microphysics processes. By substepping a few particular rain processes within MG2, the time discretization error for those processes was considerably reduced with minimal additional expense to the overall microphysics. While this improvement has a substantial effect on the target processes and on the vertical distribution of stratiform-derived rain within E3SMv1, the overall model climate is found to not be sensitive to MG2 time step. We hypothesize that this is because the surface climate does not depend strongly on certain process rates, especially MG2's rain evaporation rate. Plain Language SummaryThe atmospheric components of climate models contain a number of physics parameterizations, subcomponents that are designed to capture particular aspects of the atmospheric physics. Cloud microphysics models are parameterizations designed to represent very small scale cloud processes, including phase changes and the formation of precipitation. The accuracy of these parameterizations depends on the model time step: A shorter time step typically requires more computational resources, but also improves the model's accuracy. This paper examines a particular microphysics model, MG2, used with a time step of 5 min in the E3SMv1 climate model. By linearizing MG2, we can find characteristic timescales associated with this model, which are often much shorter than 5 min. This suggests that the usual time step is too large to fully capture the physics that MG2 represents. We also experiment with using a shorter time step for parts of the rain physics and find that in many cases the rain mass is strongly affected. However, reducing the MG2 time step does not have much effect on the overall climate of E3SMv1.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.