A New Resource for Studying Processes in Shallow Cumulus Clouds

The Science

The daily cycle and timing of shallow cumulus clouds is difficult to simulate accurately in weather and Earth system models because many of the processes controlling their development occur at scales smaller than a model grid cell. Detailed high-resolution models, known as large-eddy simulation (LES) models, are often used to determine how to parameterize these complicated processes for larger-scale models.  LES studies are often based on observations from a single “golden” day that is assumed to be representative of a particular meteorological regime.  In this study, scientists supported by the Atmospheric System Research (ASR) program develop a case study for active surface-forced shallow cumulus that is based on 60 selected days of observations from the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site.

The Impact

The new modeling case study is developed using long-term observational data rather than a single day.  This new modeling case study consists both of observationally-based forcing data to drive the model and observed cloud statistics to evaluate the model. The authors identify features that are statistically significant across all of the surface-forced shallow cumulus cases in their database, leading to a case study that is more representative of this meteorological regime. This new dataset is a resource to the wider scientific community for the development of convection and cloud parameterizations in large-scale models.

Summary

The new modeling case study, “Continental Active Surface-forced Shallow cumulus (CASS)”, represents a typical day-time non-precipitating shallow cumulus cloud field composited from 60 observed days where cloud formation and dissipation are found to be driven by the local atmospheric conditions and land-surface forcing, and are not influenced by synoptic weather events.

The case includes: early-morning initial profiles of temperature and moisture with a residual layer; diurnally-varying sensible and latent heat fluxes which represent a domain average over different land-surface types; simplified large-scale horizontal advective tendencies and subsidence; and horizontal winds with prevailing direction and average speed. Observed composite cloud statistics are provided for model evaluation.  CASS data is available to the public via http://portal.nersc.gov/project/capt/CASS/

The study shows that the observed diurnal cycle is well-reproduced by LES. However the cloud amount, liquid water path, and shortwave radiative effect are generally underestimated. LES are compared between simulations with an all-or-nothing bulk microphysics and a spectral bin microphysics. The latter shows improved agreement with observations in the total cloud cover and the amount of clouds penetrating deeper than 300 meters. When compared with ARM radar retrievals of in-cloud air motion, LES produce comparable downdraft vertical velocities, but a larger updraft area, velocity and updraft mass flux. Both observation and LES show a significantly larger in-cloud downdraft fraction and down-draft mass flux than marine shallow cumulus in previous studies.

Principal Investigator(s)

Yunyan Zhang
Lawrence Livermore National Laboratory
[email protected]

Funding

Data from the U.S. Department of Energy (DOE) as part of the Atmospheric Radiation Measurement (ARM) Climate Research Facility Southern Great Plains site were used. This work was supported by the Atmospheric Systems Research (ASR) program and ARM Facility in the Office of Biological and Environmental Research, Office of Science.

References

Zhang, Y., S.A. Klein, J. Fan, A.S. Chandra, P. Kollias, S. Xie, and S. Tang. 2017. “Large-Eddy Simulation of Shallow Cumulus Over Land: A Composite Case Based on ARM Long-Term Observations at its Southern Great Plains Site. J. Atmos. Sci., 74, 3229-3251. DOI:10.1175/JAS-D-16-0317.1