The MSE Budget in Hindcast Experiments During DYNAMO

The Dynamics of the MJO (DYNAMO) field campaign took place in the Indian Ocean during boreal fall and winter of 2011-2012 to collect observations of the initiation of the Madden-Julian Oscillation (MJO). Hindcast experiments are conducted with an atmospheric general circulation model with varying values of a dilute CAPE entrainment rate parameter for the first two MJO events of DYNAMO from 01 October - 15 December 2011. Higher entrainment rates better reproduce the gross features of precipitation and zonal wind, with MJO hindcast skill up to 20 days. Simulations with lower entrainment rapidly diverge from observations such that no coherent MJO convective signal is present after five days, and the model has no MJO skill beyond 12 days. Analysis of the tropical Indian Ocean column moist static energy (MSE) budget reveals that the simulations with superior MJO performance are characterized by strong positive vertical MSE advection, indicating that convection and associated divergent circulations act to moisten the column. This is inconsistent with ERA-I reanalysis in which vertical MSE advection contributes a drying tendency on average. All simulations have weaker MSE tendency due to MSE sources such as radiation and surface fluxes compared to reanalysis. The concept of gross moist stability (GMS) is invoked to interpret these MSE budget results in a normalized framework relevant to moisture mode theory. A larger entrainment rate is found to produce negative effective GMS (which includes vertical advection and all MSE sources), in agreement with observations, indicating a favorable environment for moisture mode amplification. However, the simulations with higher entrainment appear to get the right answer for the wrong reason because unrealistically strong positive vertical MSE advective tendencies per unit convective activity appear to compensate for weak radiation and surface flux feedbacks.