The ARM LASSO project provides the high resolution simulations and observations needed to comprehensively evaluate a new model formulation of boundary layer mixing and shallow cloud formation.
Shallow cumulus fields have important effects on the surface and atmosphere, but are challenging to model and difficult to observe. Within weather and climate models, numerical formulations (called parameterizations) are needed to represent turbulent mixing processes that cannot be explicitly resolved in the model. A new class of parameterizations, which integrate both the boundary layer mixing processes and cloud formation, are starting to be used more widely in weather and climate models. These formulations are known as “eddy diffusivity mass flux (EDMF)” parameterizations. In this study, a new EDMF parameterization implemented in the Weather and Research Forecasting (WRF) model is evaluated against a set of observationally-forced high resolution large eddy simulations and observations produced from the Atmospheric Radiation Measurement (ARM) user facility’s LASSO project.
The new EDMF parameterization for boundary layer mixing and cloud performed very well in the single column tests conducted in this work. The 15 cases examined in this study are far more than shown in previous work evaluating new parameterizations, and allow scientists to consider how the boundary layer and shallow cloud schemes handle the variation in cloud amount from day to day and across different forcing data. This study shows the value of the LASSO dataset in moving beyond single case studies and the typical idealized test cases used during previous studies. Using multiple simulated days under the same meteorological regime of shallow convection helps identify how well EDMF captures this regime.
Representation of shallow cumulus is a challenge for mesoscale numerical weather prediction models. These cloud fields have important effects on temperature, solar irradiance, convective initiation, and pollutant transport, among other processes. Recent improvements to physics schemes available in the Weather Research and Forecasting (WRF) Model aim to improve representation of shallow cumulus, in particular over land. The DOE large-eddy simulation (LES) Atmospheric Radiation Measurement (ARM) Symbiotic Simulation and Observation Workflow (LASSO) project provides several cases that are used here to test the new physics improvements. The LASSO cases use multiple large-scale forcings to drive large-eddy simulations, and the model output is easily compared to output from WRF single-column simulations driven with the same initial conditions and forcings. The new Mellor-Yamada-Nakanishi-Niino (MYNN) eddy diffusivity mass-flux (EDMF) boundary layer and shallow cloud scheme produces clouds with timing, liquid water path, and cloud fraction that agree well with LES over a wide range of those variables. Here we examine those variables and test the scheme’s sensitivity to perturbations of a few key parameters. We also discuss the challenges and uncertainties of single-column tests. The older, simpler total energy mass-flux (TEMF) scheme is included for comparison, and its tuning is improved. This is the first published use of the LASSO cases for parameterization development, and the first published study to use such a large number of cases with varying cloud amount. This is also the first study to use a more precise combined infrared and microwave retrieval of liquid water path to evaluate modeled clouds.
Contacts (BER PM)
ARM Program Manager
Wayne M. Angevine
NOAA Earth System Research Laboratory
This work was supported by the NOAA Atmospheric Science for Renewable Energy program. Gustafson and Endo are supported by the DOE Office of Science Biological and Environmental Research via the Atmospheric Radiation Measurement User Facility. Kay Suselj’s research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Parts of his research were supported by the NASA MAP Program and the NOAA/CPO MAPP Program.
Angevine, WM, J Olson, J Kenyon, WI Gustafson, S Endo, K Suselj, and DD Turner. "Shallow Cumulus in WRF Parameterizations Evaluated against Lasso Large-Eddy Simulations." Monthly Weather Review 146(12), 4303-4322 (2018). [DOI:10.1175/mwr-d-18-0115.1]
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