CGER Reports

CGER’S SUPERCOMPUTER MONOGRAPH REPORT Vol.16 Idealized Numerical Experiments on the Space-time Structure of Cumulus Convection Using a Large-domain Two-dimensional Cumulus-Resolving Model

Cumulus convection plays an indispensable role in shaping the earth’s climate systems, for example, via the water cycle, by driving atmospheric circulation by latent heating, and by influencing the radiative energy balance. In spite of the crucial importance of cloud convection, its treatment in numerical modeling constitutes a continuous struggle, mainly because of its small scale compared to the earth’s atmosphere as a whole, and the complexity of its physical processes.
With the establishment of massively parallel supercomputers such as the Earth Simulator, the ambition of “cloud system-resolving” global modeling could be finally achieved. However, a calculation like this would produce such a tremendous amount of data that the analysis and interpretation of the results would be a very difficult task.

This volume of the CGER’s Supercomputer Monograph Report describes the results of numerical experiments performed by using a large-domain (4,096 km – 65,536 km) two-dimensional cloud resolving model with highly idealized set-up of the earth’s tropical atmosphere (Fig.1).
It is demonstrated that the distribution of cumulus clouds can change in response to the large-scale structure of boundary condition, initial condition, and externally given forcing (Figs.2-3).
However, with certain specifications, the large-scale structure of cumulus activity develops spontaneously without any external forcing or initial/boundary condition (Figs.4-5).
A close inspection of the model atmosphere (Fig.6) reveals the spontaneous emergence of variabilities having a wide range of temporal and spatial scale.

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Fig.1 Schematic picture of the idealized tropical atmosphere of the earth.

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Fig.2 Temporal evolution of the horizontal distribution of cumulus activity in the experiment where sea surface temperature, radiative cooling and initial conditions are horizontally uniform. Dark tone represents intense precipitation.

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Fig.3 Response of cumulus activity to various large-scale horizontal inhomogeneities. Experiment with inhomogeneous sea surface temperature (left), experiment with inhomogeneous radiative cooling in the troposphere (upper right), and experiments with inhomogeneous distribution of mid-tropospheric humidity (lower right).

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Fig.4 Experiments with different vertical structures of radiative heating. Strong cooling in the upper troposphere (left), and strong cooling in the lower troposphere (right).

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Fig.5 Temporal evolution of the horizontal distribution of cumulus activity in the experiment where the feedback from low level wind to the surface heat and moisture fluxes is taken into account.

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Fig.6 Multiple scale structure that develops spontaneously in a 65,536 km calculation. Magnified view of a domain extending 2,800 km horizontally.