Extended Abstract

Figure 1: COSMO-CLM model domains for WAM (whole area, left) and ISM (outlined in grey, right). Alsothe evaluation index analysis domains are indicated. There is one common index domain for WAM (shadedgrey) and several for the different indices for ISM3 Evaluation indices and reference dataMany indices have been used to objectively describe monsoon circulation and quantify its variability. Mostof these indices are based on precipitation, but other indices are based on the magnitude of mean summervertical wind shear, or the outgoing long-wave radiation (OLR) as an indication of convection. In recentyears, there has been a lot of discussion about which index is best (e.g. Wang 1999, Fontaine et al. 2008).For this study, we chose indices based on precipitation (the well defined All India Monsoon Rainfallindes for ISM), OLR, and zonal and meridional wind components. The wind components are used to definevertical wind shear indices describing the monsoonal dynamics and monsoon cell circulation. For WAM thewind shear in 900 hPa and 200 hPA is used to define the WAMI (Fontaine et al. 2008). A larger than averageWAMI represents an enhanced southwesterly monsoon flux and a stronger tropical easterly jet. For ISM thewind components in 850 hPa and 200 hPa are used in the zonal and meridional wind shear indices (ZWSIand MWSI, resp.) following Wang and Fan (1999) and Goswami et al. (1999).Due to disagreement in the definition of the indices’ domains in the literature we have chosen acommon index domain for WAM and different domains for the different chosen indices for ISM as shown inFig. 1. All data is interpolated to the grids of the respective observational reference data sets. In the followingonly index values for the monsoon seasons are shown.We used the Global Precipitation Climatology Centre (GPCC) data product version 4 (seehtpp://gpcc.dwd.de) as the reference dataset for precipitation. GPCC provides a dataset of globally griddedrain gauge measurements with a spatial resolution of 0.5°. For OLR we used the NASA/GEWEX SRB(Surface Radiation Budget) dataset (release 2.5, July 1983 to June 2005) on a 1° grid (Gupta et al., 2006) forWAM and data from NOAA (Liebmann and Smith 1996) for ISM. Due to the low cloud-top temperature ofhigh convective clouds, a low OLR value indicates high convective activity. The meridional and zonal windcomponents for the calculation of the wind shear reference were taken from the re-analysis product of theNational Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR).The NCEP re-analysis output variables were classified into four categories, depending on the relativeinfluence of the observational data and model on the gridded variable. The wind fields were classified as“category A files”, which are highly influenced by observations and can be considered to be the most reliableclass of NCEP output variables.-256-