The Moist Static Energy Budget in CAM5 and SP-CAM Hindcasts during Dynamo

Abstract:

The Dynamics of the MJO (DYNAMO) field campaign took place in the Indian Ocean during boreal winter of 2011-2012 to collect observations of the initiation of Madden-Julian Oscillation (MJO) events. Hindcast experiments are conducted for the first two MJO events of DYNAMO from 01 October – 15 December 2011 with two atmospheric models, one that uses conventional parameterization (CAM) and another that employs super-parameterization (SP-CAM). Increased convective entrainment in CAM improves the representation of MJO precipitation and zonal wind, with RMM predictive skill up to 20 days, whereas the control hindcasts rapidly diverge from observations with no coherent MJO convective signal after 5 days. SP-CAM exhibits superior hindcast performance, with a much more coherent convective envelope. However, a systematic drift in the large-scale fields is found to have a detrimental effect on the interpretation of RMM skill scores.

Analysis of the column moist static energy (MSE) budget reveals that the simulations with superior MJO performance exhibit a positive MSE tendency by vertical advection on average, inconsistent with reanalysis that indicates a weak negative tendency. This discrepancy in CAM hindcasts is compensates for overly weak cloud-radiative feedbacks, suggesting that increased entrainment improves the MJO in CAM for the wrong reason. Cloud-radiative feedbacks in SP-CAM show better agreement with observations.

The gross moist stability (GMS) is used to interpret these MSE budget results in a normalized framework relevant to moisture mode theory. Both CAM and SPCAM hindcasts are characterized by negative effective GMS, indicating that convection and associated divergent circulations have a net moistening effect on the column, which is a favorable condition to destabilize a moisture mode. However, this result is not consistent with observations.