Mesoscale (20-2000 km) meteorological processes differ from synoptic circulations in that mesoscale weather changes rapidly in space and time, and physics processes that are parameterized in NWP models play a great role. Complex interactions of synoptic circulations, regional and local terrain, land-surface heterogeneity, and associated physical properties, and the physical processes of radiative transfer, cloud and precipitation and boundary layer mixing, are crucial in shaping the regional weather and climate. The imperfectness of representations of these factors in NWP systems can frequently impair the ability of single-model deterministic forecasts.

In this paper, we describe an innovative mesoscale Ensemble Real-Time Four Dimensional Data Assimilation (E-RTFDDA) and forecasting system. E-RTFDDA extends the single deterministic mesoscale RTFDDA, a continuous data assimilation and forecasting model system, developed jointly by NCAR and ATEC (Army Test Evaluation Commands) in the last 10 years (Liu et al. 2008 a,b, JAMC), to ensemble analysis and forecasting. E-RTFDDA samples the uncertainties in various aspects of the RTFDDA modeling system. The core models of the system include the Weather Research and Forecasting model (WRF) and Penn-State NCAR Mesoscale Model version 5 (MM5) that are furnished with a continuous four-dimensional data assimilation (FDDA) capabilities. The FDDA, applied to each member, allows effective uses of all observation and produces dynamically-balanced and physically-consistent 4D analyses and "spun-up" initial conditions to initialize each forecast cycles. The ensemble realizations include perturbations in model initial conditions (IC), lateral boundary conditions (LBC), model physical parameterizations (PP), and the underlying land-surface (LS) properties. A 30-member E-RTFDDA system has been implemented on an Army HPC platform and started forecast operation since September 2007. The system has three nested domains with grid sizes of 30, 10 and 3.33 km respectively with a fine-mesh domain covering a 230 by 230 km region in the middle Northwest Utah. The system cycles at 6-hr intervals, producing 6h final analyses and 48h forecasts in each cycle. The performance of the system and the indication of mesoscale predictability over the complex terrain region will be presented.