Multiscale analyses, forecasts and climatologies over complex terrains using the NCAR/ATEC real-time four-dimensional data assimilation and forecast (RTFDDA) system

High-resolution weather analyses, forecasts and climatologies over complex terrain are very valuable and, sometimes, essential for many applications. However, analyzing and forecasting weather in complex terrain areas are very challenging, because 1) flows and environmental thermodynamic properties in these regions can be very complicated and contain rich detailed structures; 2) local circulations are controlled by interactions of multi-scale systems, and strong dynamic and thermodynamic forcing due to orography and/or heterogeneous underlying properties; and 3) observations are normally too sparse to properly describe either the macro- or micro- structures of local circulations.

In the last three years, NCAR, in collaboration with the Army Test and Evaluation Command (ATEC), has developed a multiscale weather analysis and forecast (RTFDDA) system. The system employs a Four-Dimensional Data Assimilation (FDDA) method, by which various observations are dynamically combined into a full-physics mesoscale models, to generate real-time analyses and short-term forecasts on a set of multiscale domains. The grid increments of the model domains vary from a few hundred meters to tens of kilometers. By incorporating detailed terrain and land use information, and using the synoptic-scale model analyses from national weather centers, a full physics high-resolution mesoscale model has proven itself capable of simulating various physical forcing factors and producing many realistic local circulations. The NCAR/ATEC RTFDDA system is constructed to effectively combine these model advantages with all available observation information. Because of the complex forcing and evolution of local circulations over complex terrain, the spatial correlation can decrease very quickly with distance. Thus, the FDDA scheme is designed to allow the model to adjust toward observations at and near the observation time and location, allowing the model to spread the observation information to other regions according to the model dynamics. When the system is set up for a specific region, it runs continuously to produce four-dimensional dynamically and physically consistent analyses, and in many cases, high-quality short-term forecasts are also produced at a selected cycling interval of 1 - 12 hours based on application needs and availability of computer capacity.

As of February 1, 2004, five RTFDDA systems have been operating at 5 Army test ranges, with fine-grids of 1.1 - 3.3 km, for operation periods varying from 2 to 3 years. The Army test ranges are located in very different geographic and climatological zones, e.g. from the White Sand Missile Range in New Mexico, to Aberdeen Test Center in Maryland, to Cold Region Test Center in Alaska. Each of ranges possesses unique mountain range distributions and/or land use contrasts. Cases selected from the operational analyses and forecasts will be used to demonstrate the models capabilities of producing accurate real-time analysis and forecasts over complex terrain. A fine-scale 3D climatology, and model verification statistics, are calculated using the archived hourly FDDA analyses at the ranges. Applications of the RTFDDA analyses, forecasts and local-scale climatic statistics will be discussed.