GATE Cloud System

A Cloud Resolving Model (CRM) was run to simulate the formation and evolution of cloud systems using data from a unique 3D experiment performed during September 1-7, 1974, during Phase III of the Global atmospheric research programme Atlantic Tropical Experiment (GATE). more...
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QuickTime | Real | MPEG 1.7 MB The white isosurface represents a .5 g/kg ice water isovalue, the yellow isosurface represents a .5 g/kg liquid water isovalue, and the red isosurface represents a .5 g/kg rain water isovalue. The vertical slice at the rear of the model domain shows the East-West Wind Component (U), and the horizontal colored slice at the bottom of the domain shows contours of rain water at the surface.		QuickTime | Real | MPEG 1.7 MB With a substantial increase of the large-scale forcing (advective cooling and moistening effects) during day 2, deep convective clouds and nonsquall (slow-moving) cloud clusters develop. Although the large-scale forcing decreases during day 3, the lower-tropospheric wind shear increases as an easterly jet develops near 4km. The squall line (fast-moving) is reproduced under the moderate forcing and strong low-level wind shear (day 4). It is identified by the pronounced deep convection at the leading edge of the system, extensive anvil clouds (about 200 km wide) to the rear, and a few deep and shallow clouds. As the large-scale forcing further decreases on day 7, only scattered convection occurs and has a southwest-northeast oriented line structure.

Tropical oceanic cloud systems, which are recognized as an essential component of the global climate system, are complex phenomena in geophysical flows due to the vast range of scales of motion involved and the nonlinearity of interactions among the attendant physical processes (e.g., phase changes of water, cloud-interactive radiation, turbulence and surface fluxes) . Cloud systems directly couple dynamical and hydrological processes in the atmosphere through the release of latent heat of condensation and evaporation, through precipitation, and through the vertical redistribution of sensible heat, moisture and momentum. They have an equally important effects on the large-scale radiation budget, through the reflection, absorption and emission of radiation, and the surface energy budget, through the modification of net radiative fluxes and net heat fluxes at the ocean surface.

The cloud-resolving model (CRM) has been increasingly used to study various aspects of cloud dynamics and individual processes in the cloud systems during the past two decades. However, until recently an impediment to the effective use of CRMs for clouds-in-climate problems was the small (100's km) domains and the short (several hours) simulations. With modern computers and parallel technology, CRMs can now explicitly simulate cloud systems over large domains (1000's km) for long periods (several months for two-dimensional simulations and a month for three-dimensional). Since the CRM can span many climate model grid volumes and integrate up to intraseasonal time scales, the model-generated fields (e.g., cloud properties and cloud radiative properties) are statistically significant in terms of climate studies, which can be used to study various problems such as the parameterization of cloud systems, the cloud-radiation interaction, and the coupling of clouds and oceanic processes.

A unique 3D experiment was performed for a period of September 1 through September 7, 1974 during the Phase III of GATE (Global atmospheric research programme Atlantic Tropical Experiment). The 3D simulation produced remarkably realistic cloud system evolutions. The comparison of three cloud systems with the GATE radar composites shows overall similarities ( Wu and Moncrieff 1996; Grabowski et al. 1997). Evolution of cloud systems over the 7-day period is directly related to the evolution of the large-scale environment. Since the large--scale forcing is weak during day 1, only shallow clouds occur. With a substantial increase of the large-scale forcing (advective cooling and moistening effects) during day 2, deep convective clouds and nonsquall (slow-moving) cloud clusters develop. They are characterized by a circular type of structure with various types of clouds such as deep and shallow clouds, and small cloud bands. The maximum intensity of the convection coincides with the maximum forcing. Although the large-scale forcing decreases during day 3, the lower-tropospheric wind shear increases as an easterly jet develops near 4km. The squall line (fast-moving) is reproduced under the moderate forcing and strong low-level wind shear (day 4). It is identified by the pronounced deep convection at the leading edge of the system, extensive anvil clouds (about 200 km wide) to the rear, and a few deep and shallow clouds. With the decrease of the lower-tropospheric wind shear during days 5 and 6, organized squall line activity is replaced by less organized nonsquall clusters. As the large-scale forcing further decreases on day 7, only scattered convection occurs and has a southwest-northeast oriented line structure. The short line convection is controlled by the low-level wind field.

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