ARPS Version 4.0 was officially released in mid-1995. Since then, many improvements have been made to the ARPS system and as a consequence many sub-releases were made available on the CAPS anonymous FTP server (ftp://ftp.caps.ou.edu). The significant improvements include:

• A completely new real-time data ingest and analysis system (ADAS) capable of handling a variety of data sources; including raw and retrieved Doppler radar data and satellite data that are used for three-dimensional cloud analysis;
• A long and short-wave radiation package based on that of the NASA Goddard Space Flight Center;
• The Skamarock adaptive grid refinement that supports multiple level two-way interactive grid nesting;
• Kain-Fritsch cumulus parameterization;
• Improved PBL parameterization;
• Schultz simplified ice microphysics;
• High resolution (~1km) terrain, land-user and land-cover data base and an option to use subgrid scale soil types;
• The flux-corrected transport (FCT) monotonic advection scheme and an efficient positive definite advection scheme for scalars;
• A radiation top-boundary condition with a relaxed limitation on the lateral boundary condition;
• Map factor in dynamic equations;
• Support of data sets from NCEP models (e.g., RUC and ETA) as analysis background and boundary condition;
• Streamlined support for MPP platforms (Cray T3E, T3D, IBM SP-2 and network of workstations) via message-passing libraries;
• Improved code efficiency;
• The ARPS ajoint with moist physics;
• A fully automated procedure for real time analysis and prediction;
• Soil model initialization procedure using antecedent precipitation index (API);
• Significantly enhanced ARPSPLT;
• Support of several new data formats (e.g., Vis5D, GRIB and GrADS) and data visualization tools.

New capabilities are being continuously developed and incorporated into the ARPS system by the CAPS scientists, and by scientists supported by the Coastal Meteorology Research Program (CMRP) at the University of Oklahoma. A major release of the next version, ARPS Version 5.0, is scheduled for late this year or early next year. Apart from the new features listed above, this version will also include:

• A portable graphical user interface (GUI) for configuring the ARPS and supporting programs;
• Improved code organization with software version control;
• A fully integrated data analysis, retrieval and assimilation package;
• ARPS adjoint with important physics;
• Improved soil model and boundary layer physics;
• Microphysics parameterization including predicted number concentrations;
• Automated placement and removal to nested grids;
• Dynamic grid adaptation in the vertical direction;
• An updated User's Guide in html format;

To ensure maximum portability, we have so far deliberately avoid non-Fortran77 constructs in the ARPS code, except for the most common extensions. Starting from the release of ARPS version 5.0, we will start to adopt and exploit new features and constructs available in Fortran-90 so as to further improve the modularity, flexibility and efficiency of the ARPS system.

During the past years, the CAPS scientists responded to many questions from ARPS users around the world. Frequently asked questions have been compiled and posted at the ARPS weg page (http://www.caps.ou.edu/ARPS), and a search function is also provided. Users are encouraged to consult these pages when you have questions and/or problems. Further questions and comments should be addressed to arpssupport@ou.edu. We welcome comments and feedbacks from our users!