The Effects of Explicit versus Parameterized Convection on the MJO in a Large-Domain High-Resolution Tropical Case Study. Part I: Characterization of Large-Scale Organization and Propagation*

Abstract

High-resolution simulations over a large tropical domain (;208S-208N, 428E-1808) using both explicit and parameterized convection are analyzed and compared to observations during a 10-day case study of an active Madden-Julian oscillation (MJO) event. The parameterized convection model simulations at both 40-and 12-km grid spacing have a very weak MJO signal and little eastward propagation. A 4-km explicit convection simulation using Smagorinsky subgrid mixing in the vertical and horizontal dimensions exhibits the best MJO strength and propagation speed. Explicit convection simulations at 12 km also perform much better than the 12-km parameterized convection run, suggesting that the convection scheme, rather than horizontal resolution, is key for these MJO simulations. Interestingly, a 4-km explicit convection simulation using the conventional boundary layer scheme for vertical subgrid mixing (but still using Smagorinsky horizontal mixing) completely loses the large-scale MJO organization, showing that relatively high resolution with explicit convection does not guarantee a good MJO simulation. Models with a good MJO representation have a more realistic relationship between lower-free-tropospheric moisture and precipitation, supporting the idea that the moisture-convection feedback is a key process for MJO propagation. There is also increased generation of available potential energy and conversion of that energy into kinetic energy in models with a more realistic MJO, which is related to larger zonal variance in convective heating and vertical velocity, larger zonal temperature variance around 200 hPa, and larger correlations between temperature and ascent (and between temperature and diabatic heating) between 500 and 400 hPa.

Improving MJO simulation by enhancing the interaction between boundary layer convergence and lower tropospheric heating

“…Hsu and Li (2012) found that the BL convergence-induced moistening to the east of MJO convection is a primary cause for the MJO eastward propagation, while the BL convergence is related to the heating-induced free atmosphere wave dynamics. The second mechanism is stratiform cloud-wave interaction (Mapes 2000;Khouider and Majda 2006;Kuang 2008;Fu and Wang 2009;Seo and Wang 2010;Holloway et al 2013). Fu and Wang (2009) and Seo and Wang (2010) showed that the latent heating released in stratiform clouds produces MJO eddy available potential energy (EAPE), which allows direct interaction between heating and largescale circulation and sustains the MJO.…”

The parametric sensitivity of CAM5's MJO

“…Given moisture's central role, it is not surprising that some improvements can also be found by altering the moistening tendencies of convection, for example, by increasing the amount of evaporation of convective rain [Maloney, 2009;Del Genio et al, 2012]. While model resolution does not consistently impact a simulated MJO [Crueger et al, 2012;Holloway et al, 2013], an interactive ocean usually improves the MJO simulation [Waliser et al, 1999;Sperber et al, 2005;Woolnough et al, 2007;X. Jiang et al, submitted manuscript, 2014].…”

A general theoretical framework for understanding essential dynamics of Madden–Julian oscillation

“…In addition to theoretical modeling, various processes and mechanisms have been identified or speculated to play significant roles in MJO dynamics based on numerical model experiences and observations, including shallow convection-BL circulation interaction (Johnson et al 1999;Kikuchi and Takayabu 2004;; stratiform cloud-wave interaction (Mapes 2000;Khouider and Majda 2006;Kuang 2008;Fu and Wang 2009;Seo and Wang 2010;Holloway et al 2013); cloud-radiation interaction (Lee et al 2001;Raymond 2001;Bony and Emanuel 2005;Andersen and Kuang 2012); moisture-convection feedback (Woolnough et al 2001;Grabowski and Moncrieff 2004;Benedict et al 2014), and moisture transport (Maloney 2009;Maloney et al 2010;Andersen and Kuang 2012;Hsu and Li 2012;Kim et al 2014;Pritchard and Bretherton 2014). More details were reviewed by Zhang (2005) and Wang (2005Wang ( , 2012.…”