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Clouds and Convection

Contacts, for more information:

Clouds play a central role in both the hydrological and energy cycles of our planet. Water evaporates directly at the surface or is transpired from the soil through vegetation. Once in the atmosphere, moisture is transported and redistributed by winds. Moisture condenses to form small cloud droplets or ice crystals when the temperature becomes cold enough. If clouds contain a sufficient amount of condensed water or ice, small particles undergo a transformation into larger particles that produce precipitation. Precipitation returns the water back to the surface in the form of rain or snow.

Clouds also help regulate the temperature of the Earth though their impact on the energy cycle. Globally, clouds cool the Earth by -17.1 Wm-2. This cooling results from a partial cancellation between two opposing contributions: cooling from the reflection of incoming solar radiation (-46.6 Wm-2; shading effect) and warming from the trapping of infrared radiation emitted by the Earth (+29.5 Wm-2; thermal blanket effect).

Illustration of water cycle.

GFDL Research

Modeling Clouds

GFDL builds computer models to simulate the climate of the Earth. The impact of clouds on the hydrological and energy cycles must be captured by these models in order to accurately simulate the climate. Clouds take on a variety of shapes and sizes in the atmosphere. Unfortunately, air motions driving individual clouds occur at small scales (typically tens of meters) that cannot be resolved by current climate models. Instead, we develop cloud parameterizations that attempt to capture the essence of clouds without having to resolve the full complexity of cloud motions and processes.

Clouds and Climate Change

Anthropogenic activities since the industrial revolution have had a considerable impact on the composition of the atmosphere. Concentration of greenhouse gases (CO2, CH4, N2O, …) have increased, resulting in more trapping of infrared radiation emitted by the Earth (approximately +2.6 Wm-2) and a warmer climate. In addition to greenhouse gases, anthropogenic activities have also led to an increase in emissions of aerosols. Aerosols can alter cloud properties by making clouds more reflective as well as increase their lifetime by slowing the conversion of cloud particles to precipitation. These effects are known collectively as “aerosol indirect effects”. They tend to cool the planet, potentially offsetting a portion of the warming from greenhouse gases. The exact magnitude of this cooling is, however, uncertain.

How clouds will change in a warmer climate is another source of uncertainty. Will the net cloud cooling of -17.1 Wm-2 stay the same, increase, or decrease as the climate warms? An increase in the net cloud cooling would offset a portion of the greenhouse gases warming (negative cloud feedback), whereas a decrease would accelerate the warming (positive cloud feedback). The sign and magnitude of the cloud feedback remains an open question that we are trying to address by improving the representation of clouds in our climate models.

Illustration of key cloud regimes ranging from stratocumulus clouds in coastal subtropical oceans (cold waters) to deep convection clouds in tropical oceans (warm waters).

High resolution explicit simulation of a stratocumulus cloud using a large-eddy simulation model (Golaz et al. 2005, doi:10.1007/s10546-004-7300-5). Details of turbulent air motions can be seen on the vertical cross section (warm colors: rising air; cool colors: sinking air). The extent of the entire horizontal domain (9 km) is much smaller than the size of a single grid box in a global climate model (50-200 km). This illustrates the need for cloud parameterizations in global models.

Research Highlights