Empirical and Process Studies

CDC conducts diverse research activities that span time scales from intraseasonal to decadal. Whereas other chapters focus on specific time bands, this chapter considers basic physical processes important in climate variability that occur across time scales. Moist atmospheric convection, clouds and climate, atmospheric angular momentum, and air-sea interaction are topics of active research. CDC scientists integrate modeling and observations to better understand physical processes, and to overcome limitations in either observational or modeling analyses alone. Coherent phenomena such as ENSO, the Madden-Julian oscillation (MJO) and the seasonal cycle provide specific case studies and important examples of scale interactions.

Moist atmospheric convection is central to all climate phenomena studied at CDC and epitomizes the complexity of the climate system. At CDC, we use observations and hierarchical modeling to conduct fundamental studies of moist convection in various environments. A key objective is to evaluate the assumptions at the heart of present day cumulus parameterization schemes. Cloud-resolving models are used to develop, test and refine parameterizations in a simplified and tractable context.

Clouds also profoundly impact climate variability. CDC research shows that stratocumulus decks over the east Pacific influence the seasonal cycle of sea surface temperature (SST). The three-dimensional structure of clouds helps determine the heating that drives such ocean-atmosphere interactions, and must be properly represented in climate models. CDC scientists have developed a simple cloud overlap scheme for GCMs, and are working on a higher order statistical cloud scheme to better represent the subgrid scale distribution of cloud properties.

A key feature of tropical variability is the MJO, which produces strong air-sea interactions over the warm pool and may influence the ENSO cycle, particularly the onset and decay phases. CDC scientists determined that SST anomalies within the MJO are driven mainly through surface energy fluxes, especially shortwave and latent heat fluxes. MJOs were also shown to play a prominent role at the onset of the 1997-98 El Niño event. MJO events initiated cooling of SSTs in the west Pacific via surface fluxes and warming in the central Pacific via Kelvin wave currents. In the first link identified with interannual SSTs, CDC scientists found that strong MJO activity during the northern winter was correlated with positive SSTA in the western Pacific during the preceding northern fall.

Most prediction models systematically lose atmospheric angular momentum (AAM) during the forecast cycle, a serious concern for such a fundamental quantity. CDC scientists used studies of the AAM budget to demonstrate the negative impact of gravity wave drag, including its role in local and global systematic forecast errors. CDC scientists also constructed a linear model of global AAM anomalies that replicated much of the observed intraseasonal variability and clarified the role of the mountain and friction torque in AAM variations. Composite studies were used to illustrate the regional patterns that produce the torques and helped identify a mid-latitude mode with a time scale similar to the MJO.

The diverse range of activities described in this chapter illustrates the breadth of CDC diagnostics research, provides insight and understanding of different physical processes, and provides a foundation for assessment of climate models.

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