NMT Tropical Meteorology

Evaluating Lagrangian Model Simulations of the Madden-Julian Oscillation with Metrics for Balanced Dynamics


Balanced Dynamics in the tropical atmosphere refers to the interplay between a large scale horizontal rotation (the primary circulation), and a convective scale secondary circulation working in tandem to organize convection. The primary circulation acts to modify the thermodynamic environment, which in turn contributes to the development and maintenance of the secondary circulation. Convection that is a response to balanced dynamics is potentially predictable. Data from the 2011 DYNAMO field campaign suggest that the Madden-Julian Oscillation (MJO) exhibits characteristics consistent with balanced dynamics. Namely, the mid-tropospheric potential vorticity is strongly correlated with moist atmospheric instability, which is correlated with parameters related to the secondary circulation, including a mass flux index and mid- and low-tropospheric convergence. These are also well correlated with saturation fraction (column integrated relative humidity) and precipitation. These robust correlations can be used to evaluate models which simulate the MJO. Here, we apply this strategy to two simulations using a Lagrangian model. The first simulation has a robust MJO signature, while the second (with a reduced entrainment parameter) poorly reproduces the MJO. Comparing correlations of parameters relevant to the balanced dynamics can help identify where the correlations break down in the model, and thus identify the role that entrainment plays in convective organization on the MJO scale.

References
Sessions, Sharon, K. Ryder Fox, Stipo Sentic, and Pat Haertel, 2018: Evaluating Lagrangian Model Simulations of the Madden-Julian Oscillation with Metrics for Balanced Dynamics. 33rd Conference on Hurricanes and Tropical Meteorology, Amer. Meteor. Soc., Ponte Vedra, FL.

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