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DOE Progress Reports

Self-Regulation Strikes a Balance Between Hydrological Cycle, Radiation Processes, and Intraseasonal Dynamic Variations

Submitter: Stephens, G. L., Colorado State University

Area of Research: Atmospheric Thermodynamics and Vertical Structures

Working Group(s): Cloud Properties

Journal Reference: Stephens, Graeme L., Webster, Peter J., Johnson, Richard H., Engelen, Richard, L'Ecuyer, Tristan. 2004: Observational Evidence for the Mutual Regulation of the Tropical Hydrological Cycle and Tropical Sea Surface Temperatures. Journal of Climate: Vol. 17, No. 11, pp. 2213-2224.

One of the major obstacles in simulating climate variability and climate change is the lack of scientific understanding of cloud related feedbacks. In one way or another, clouds play a role in the processes that affect the hydrological cycle and all relevant feedback mechanisms that alter its response to external climate forcing. This is especially true in the tropics, where the convergence of moist air along the equator results in convection that creates the deepest clouds, the heaviest rainfall, and the largest release of latent heat on the planet. Because of the vigorous feedbacks that contribute to these conditions, tropical intraseasonal variability has proven difficult to simulate, and even more difficult to predict. As described in the Journal of Climate (Stephens et al., June 2004), research supported by the Department of Energy's Atmospheric Radiation Measurement Program suggests that for tropical regions, interactions between the ocean, moist atmospheric thermodynamics, and radiation establish a phased, self-regulating feedback mechanism.

The researchers demonstrate a mutually regulating feedback mechanism—termed a "humidistat"—that evolves in three phases: destabilization, convection, and restoring. They analyzed changes in atmospheric structure during several Madden-Julian oscillations (a 30-60 day intraseasonal variation in tropical convection) and over longer periods to demonstrate their hypothesis. In the destabilization phase, the sea surface temperature (SST) warms, winds are calm, evaporation is minimal, and skies are generally clear. These conditions allow strong solar heating at the ocean surface due to a lack of cloudiness, which increases the SST, and significant cooling in the upper troposphere via radiative processes. This combination produces a vertical thermodynamic structure that is conducive to the formation of deep convection. In the convective phase, large-scale convective cloud clusters that reduce incoming solar radiation reaching the ocean surface are observed and low-level winds strengthen, which causes increased evaporation from the ocean surface and enhanced mixing in the near-surface layers of the ocean. These processes cool the ocean surface. The convective phase is associated with heavy precipitation, drying in the lower layers of the atmosphere, and significant moistening of the upper troposphere. High-level clouds maintained by residual moisture characterize the restoring phase. These clouds act like a solar "blanket" allowing the ocean surface to continue to cool. They also warm the upper atmosphere by absorbing infrared radiation, thereby returning the atmosphere to a condition that will enable a new destabilization phase to be onset.

Viewing the humidistat hypothesis in a broader sense, radiative cooling in the atmosphere is largely balanced by latent heating associated with precipitation. Recent studies suggest that increased water vapor and carbon dioxide in the atmosphere will increase the net radiative cooling. Such an increase would likely lead to an increase in the latent heating associated with precipitation, which suggests an acceleration of the global hydrologic cycle. However, model-to-model uncertainties related to the treatment of increased carbon dioxide are directly related to model-to-model differences in the effects of clouds on atmospheric radiative cooling. A global observing system designed to measure not only precipitation, but also the related hydrological parameters, such as vertical distributions of clouds and the water and ice contents of these clouds, should reduce the uncertainties in these models and increase scientific understanding of these parameters that alter the radiation balance of the atmosphere.