**2.1 Model**

We use the NRCM based on the WRF model that was developed at the National Center for Atmospheric Research (NCAR). This is also known as a tropical channel model (TCM), since the model's computational domain is global (periodic) in the zonal direction. Conceptually, the configuration is similar to the TCM developed at the University of Miami based on the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5, Dudhia, 1993; Grell et al., 1995), known as the Tropical MM5 (TMM5, Ray et al., 2009; Ray and Zhang, 2010). The NRCM is atmosphere only and employs Mercator projection centered at the equator with open boundaries in the North-South direction. Global reanalyses data are used to provide the initial and boundary conditions for the model (see section 2.3).

The horizontal resolution of the NRCM is 36 km, and the meridional boundaries are placed at 30°S and 45°N. The model top is at 50 hPa, and 35 vertical levels are used. Output is taken every 3 hours. Based on a series of tests, the suite of parameterizations used for this study are: Kain-Fritsch cumulus parameterization (KF, Kain, 2004), WSM6 cloud microphysics (Hong et al., 2004), CAM 3.0 radiation scheme (Collins et al., 2006), YSU boundary layer scheme (Hong et al., 2006), and Noah land surface model (Chen and Dudhia, 2001). The model was integrated for 5 years from January 1, 1996 to January 1, 2000.

### **2.2 Method**

70 Atmospheric Model Applications

wind and specific humidity, and boundary-layer moisture convergence in models are essential for them to reproduce realistic statistics of the intraseasonal variability. On the other hand, MJO events that are initiated by the extratropical influences may have less dependence on the mean state (e.g., Ray et al., 2011). A review of our present understanding

The objective of this chapter is to further explore the role of the mean state on the MJO using a high-resolution nested regional climate model (NRCM). We use the NRCM (http://www.nrcm.ucar.edu), based on the Weather Research and Forecasting Model (WRF). The domain of this NRCM is global (periodic) in the zonal direction and is bounded in the meridional direction. The main advantage of the NRCM compared to a regular regional model is that, without the east-west boundaries, it isolates the external influences arriving solely from the extratropics. The added constraint provided by the lateral boundary conditions is expected to improve the simulated MJO statistics. Also, compared to a GCM, the NRCM has higher resolution and sophisticated physics that may be helpful to better capture the multi-scale organized convection associated with the MJO (Chen et al., 1996;

The strategy of this study is to integrate the NRCM for several years and evaluate the role of the mean state on the MJO statistics. Our goal is to provide unique perspectives to the MJO

Section 2 describes the configuration of the model, method and data. Section 3 explores the atmospheric mean state and its role on the MJO with an emphasis on the roles played by the mean precipitation and zonal winds at the 850 hPa (U850). Section 4 summarizes the results

We use the NRCM based on the WRF model that was developed at the National Center for Atmospheric Research (NCAR). This is also known as a tropical channel model (TCM), since the model's computational domain is global (periodic) in the zonal direction. Conceptually, the configuration is similar to the TCM developed at the University of Miami based on the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5, Dudhia, 1993; Grell et al., 1995), known as the Tropical MM5 (TMM5, Ray et al., 2009; Ray and Zhang, 2010). The NRCM is atmosphere only and employs Mercator projection centered at the equator with open boundaries in the North-South direction. Global reanalyses data are used to provide the initial and boundary conditions for the

The horizontal resolution of the NRCM is 36 km, and the meridional boundaries are placed at 30°S and 45°N. The model top is at 50 hPa, and 35 vertical levels are used. Output is taken every 3 hours. Based on a series of tests, the suite of parameterizations used for this study are: Kain-Fritsch cumulus parameterization (KF, Kain, 2004), WSM6 cloud microphysics (Hong et al., 2004), CAM 3.0 radiation scheme (Collins et al., 2006), YSU boundary layer scheme (Hong et al., 2006), and Noah land surface model (Chen and Dudhia, 2001). The

model was integrated for 5 years from January 1, 1996 to January 1, 2000.

of the MJO can be found in Zhang (2005).

Houze, 2004; Moncrieff, 2010).

along with the implications and limitations of this study.

dynamics and mean state.

**2. Model and data** 

model (see section 2.3).

**2.1 Model** 

The NRCM simulation is used to document the mean state and the MJO statistics. The MJO is defined as a planetary scale (zonal wavenumber 1 to 5), eastward propagating, intraseasonal (20-90 day) components in the U850 coupled with precipitation (P). To extract the coupled MJO signal, a singular vector decomposition (SVD) method (Wallace et al., 1992) is applied to U850 and P. This method is similar to EOF analysis, but with one advantage: it considers the wind-precipitation coupling associated with the MJO. The leading modes are selected based on North et al. (1982) rule. Three leading modes are found for both observations and model, and they explain 41% and 31% of the covariance for the observation and the NRCM, respectively. These selected modes represent the intraseasonal coupled components between U850 and P. Time series of U850 and P reconstructed through linear regression of intraseasonal bandpass filtered U850 and P upon their selected leading SVD modes, are considered to represent the MJO. Hereafter, they are referred to as U850\* and P\*.
