Clouds and their impact on climate remain the key uncertainty in estimating the sensitivity of earth's climate to increased greenhouse gases. At GFDL this challenging problem is being addressed with a sustained research effort, led by V. Ramaswamy and Leo Donner, aimed at understanding the complex interplay of clouds, radiation, and climate in the context of global climate change.

THE CHALLENGE

Weather and climate modeling is a particularly challenging scientific problem because it involves processes covering a very wide range of space and time scales. For example, jet streams extending over thousands of kilometers and cloud droplets and aerosols of microscopic size are all important components of the climate system. The timescales of interest can range from thousands of years (e.g., ice ages) to fractions of a second (e.g., droplet collisions). Given that the current generation of global climate models represent the earth in terms of gridpoints spaced several hundred miles apart, many observed features on smaller scales, such as individual thunderstorms, are not explicitly resolved by the global models.

A NEW GENERATION OF MODELS

To address the climate sensitivity problem, as well as other applications, models have been developed at GFDL that cover a much more limited area of the globe with a much higher concentration of gridpoints (currently with one gridpoint every kilometer or so).

Three-dimensional simulation of cumulus clouds using the Frank Lipps-Rick Hemler cloud-system model, an atmospheric model developed at GFDL for resolving processes on the scale of individual cumulus clouds. The surfaces of various colors indicate regions where rain (blue), snow (white), and suspended liquid or ice cloud particles (yellow) exceed specific threshold concentrations. The model's horizontal domain shown is 96 kilometers by 64 kilometers.

This new class of models resolves atmospheric motions and other processes on a much finer scale than the current global models. Thus, they will be useful for examining the impact of some of the smaller-scale features on climate sensitivity. The models also include detailed treatment of cloud processes and atmospheric radiation.

Scientists at GFDL will use the detailed information coming from the very high resolution limited-area models to develop improved ways to represent small-scale features in global models, despite the fact that the features themselves are not explicitly resolved in the global models. As an example, GFDL scientists are working to incorporate into global scale models the net effects of unresolved thunderstorms and cumulus cloud systems.

In a unique effort at GFDL, Steve Fels, V. Ramaswamy, and Dan Schwarzkopf have also developed high-precision numerical tools for atmospheric radiation calculations. The 'exact' solutions obtained at GFDL with these tools are used as benchmarks for developing, testing, and calibrating the simplified radiation codes used in atmospheric models at major research institutions worldwide.

Only through intensive modeling, theoretical, and observational efforts can progress be expected on narrowing the uncertainty in climate models. GFDL is committed to improving scientific understanding in these challenging and important research areas.