05/19/2015
ARM Measurements Provide Support for Conceptual Theories of Tropical Variability
Summary
A large-scale weather feature known as the Madden-Julian Oscillation (MJO) is the largest contributor to variability in tropical clouds and rainfall on weekly to monthly timescales. Global climate models (GCMs) have trouble accurately simulating the initiation, strength, and evolution of the MJO, indicating that there are still gaps in conceptual theories of the MJO or their implementation in numerical models. Scientists, funded in part by the Atmospheric System Research program, used data from the Atmospheric Radiation Measurement (ARM) MJO Investigation Experiment, along with satellite data, to evaluate the sensitivity of a GCM’s MJO simulation to physical factors including entrainment, rain evaporation, downdrafts, and cold pools. This study found that differences among model versions occur primarily at intermediate values of column water vapor, where the transition from shallow to deeper convection occurs. Simulations that have too rapid a transition from shallow to deep convection, due to weak entrainment or lack of convective organization, have poor MJO simulations. Shallow convection is important for MJO initiation because it allows sources such as surface evaporation and large-scale transport to slowly import moist static energy into the middle levels of the atmosphere, eventually triggering the MJO propagation. Premature deep convection exports the moist static energy too quickly. These results suggest that both cloud/moisture-radiative interactions and convection-moisture sensitivity are required to produce a successful MJO simulation and strongly support the “moisture mode” conceptual theory of the MJO.
References
Del Genio, A. D., J. Wu, A. B. Wolf, Y. Chen, M.-S. Yao, and D. Kim. 2015. “Constraints on Cumulus Parameterization from Simulations of Observed MJO Events,” Journal of Climate, DOI: 10.1175/jcli-d-14-00832.1.