The Effects of Evaporation-Wind Feedback, Mid-Latitude Storms, and Stochastic Convective Processes on Tropical Intraseasonal Variability


Johnny Wei-Bing Lin
Doctor of Philosophy in Atmospheric Sciences
University of California, Los Angeles, 2000
Professor J. David Neelin, Chair

Mechanisms involved in the maintenance of intraseasonal variability are investigated using various versions of the Neelin-Zeng Quasi-Equilibrium Tropical Circulation Model (QTCM1). This model is an intermediate-level atmospheric model that includes primitive equation nonlinearity. Mechanisms investigated include evaporation-wind feedback, mid-latitude storms, and stochastic convective processes.

Experiments indicate that evaporation-wind feedback partially organizes model intraseasonal variability by reducing damping, but is not by itself sufficient to sustain this oscillation for the most realistic parameters. Excitation by extratropical variability is a major source of energy for the intraseasonal variability in this model. When mid-latitude storms are suppressed, tropical intraseasonal variability is nearly eliminated. However, the eastward propagating intraseasonal signal appears most clearly when mid-latitude excitation is aided by the evaporation-wind feedback.

Convective parameterizations used in general circulation models (GCMs) generally only simulate the mean or first-order moment of convective ensembles and do not explicitly include higher-order moments. It is proposed that the influence of including unresolved higher-order moments can be investigated using stochastic convective parameterizations. Two approaches are identified: a modeling approach and a statistical empirical approach.

As an example of the first approach, a simple stochastic convective parameterization is implemented that includes a random contribution to the convective available potential energy (CAPE). Adding convective noise noticeably affects tropical intraseasonal variability, suggesting inclusion of such noise in GCMs might be beneficial. Model response to the noise is sensitive not only to the noise amplitude, but also to such particulars of the stochastic parameterization as autocorrelation time.

As an example of the second approach, an empirically-based stochastic convective parameterization is developed that uses an assumed mixed lognormal distribution of rainfall, tuned with parameter values derived from observations, to control selected non-mean statistical properties of convection. Inclusion of the unresolved variance using this scheme is also found to have a noticeable impact upon atmospheric intraseasonal variability in the tropics. Testing of this stochastic convective parameterization reveals that large-scale model dynamics interacts heavily with the convective parameterization, in ways such that the resulting output is fundamentally different from the input. This suggests stochastic parameterizations cannot be calibrated outside of a model's dynamical framework.

© Copyright 2000 by Johnny Lin. This article is governed by the following license.