MODELS & OBSERVATIONS
Models
Cumulus Clouds: A Pair of Simulations
To study the effect of the precipitation process on cumulus cloud field structure, one simulation does not allow its clouds to form rain. Precipitation causes cloud organization to change from cloud streets to arcs surrounding regions absent of clouds. This indicates the important effect of precipitation in the development of the cumulus cloud field.
Effects of precipitation on cumulus cloud organization
In the images above, clouds are denoted by white iso-surfaces. Colors contours are humidity at a height of 200 meters. The leftmost panel shows clouds that are not allowed to form rain, whereas in the panel on the right, clouds are allowed to precipitate. These simulations agree with the observed behavior of precipitating cumulus clouds. Moreover, the simulations reproduce fine details in the structure of humidity and clouds.
Matheou, G., D. Chung, L. Nuijens, B. Stevens, and J. Teixeira, 2011: On the fidelity of large-eddy simulation of shallow precipitating cumulus convection. Mon. Wea. Rev., 139, 2918–2939
Process Scale Modeling at JPL
High-resolution modeling of boundary layer clouds
Research at the Jet Propulsion Laboratory includes high-resolution simulations that aim to advance our understanding of cloud physics for the purpose of improving their representation in climate and weather models. These simulations are used to investigate the fundamental physics of the boundary layer, including clouds and precipitation, and to develop and evaluate numerical models used in weather and climate predictions.
Simulating the atmospheric boundary layer
The atmospheric boundary layer is the lowermost layer (1-4 km) of the troposphere that is in contact with Earth's surface. It is host to a plethora of physical processes, such as cloud formation, that strongly affect the radiative balance of the planet, and consequently the climate. The main driving forces of convection and clouds in the boundary layer are sensible and latent heat exchange and radiative cooling. Their combination with the large-scale atmospheric conditions--such as winds--generates vigorous turbulence which augments the vertical transport of water, pollutants, and other important gases.
Because of limitations in computing power, the smallest atmospheric motions (about 1 mm) cannot be resolved in these computations, even when using the most powerful present-day supercomputers.
A turbulence model, or a turbulence closure as it is often called, represents the effects of motions that cannot be resolved. A type of high-fidelity turbulence closure implemented at JPL, called a large-eddy simulation (LES), is used in atmospheric boundary layer simulations.
A multiscale approach
To accurately model the effects of unresolved small-scale motions, knowledge of their fundamental dynamics is required. Accordingly, simulations of the smallest scales of the atmosphere are carried out.
Analysis of these simulations are used to validate and improve the turbulence closures used in the large-eddy simulation of boundary layer clouds. In particular, these simulations aim at understanding the effects of stable density stratification that are often encountered in the atmosphere and ocean.