[1] Results are presented from the multi-institution partnership to develop a real-time and retrospective North American Land Data Assimilation System (NLDAS). NLDAS consists of (1) four land models executing in parallel in uncoupled mode, (2) common hourly surface forcing, and (3) common streamflow routing: all using a 1/8°grid over the continental United States. The initiative is largely sponsored by the Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project (GCIP). As the overview for nine NLDAS papers, this paper describes and evaluates the 3-year NLDAS execution of 1 October 1996 to 30 September 1999, a period rich in observations for validation. The validation emphasizes (1) the land states, fluxes, and input forcing of four land models, (2) the application of new GCIP-sponsored products, and (3) a multiscale approach. The validation includes (1) mesoscale observing networks of land surface forcing, fluxes, and states, (2) regional snowpack measurements, (3) daily streamflow measurements, and (4) satellite-based retrievals of snow cover, land surface skin temperature (LST), and surface insolation. The results show substantial intermodel differences in surface evaporation and runoff (especially over nonsparse vegetation), soil moisture storage, snowpack, and LST. Owing to surprisingly large intermodel differences in aerodynamic conductance, intermodel differences in midday summer LST were unlike those expected from the intermodel differences in Bowen ratio. Last, anticipating future assimilation of LST, an NLDAS effort unique to this overview paper assesses geostationary-satellite-derived LST, determines the latter to be of good quality, and applies the latter to validate modeled LST.

Comparisons between satellite-derived winds and collocated rawinsonde observations often show a pronounced slow speed bias at mid-and upper levels of the atmosphere. A leading cause of the slow speed bias is the improper assignment of the tracer to a height that is too high in the atmosphere. Height errors alone cannot fully explain the slow bias, however. Another factor influencing the speed bias is the size of the target window used in the tracking step. Tracking with a large target window can cause excessive averaging to occur and a smoothing of the instantaneous wind field. Conversely, if too small a window is specified, there is an increased risk of finding a false match. The authors have developed a new ''nested tracking'' approach that isolates the dominant local motion within a cloud scene and minimizes the smoothing of the motion estimate. A major advantage of the new approach is the ability to identify which pixels within the cloud scene are contributing to the tracking solution. Knowing which pixels contribute to the dominant motion allows for a more representative height to be derived, thereby directly linking the height assignment to the tracking process, which is an important goal for producers of global atmospheric motion vector (AMV) data. When compared with equivalent rawinsondes, the AMVs derived with the new approach show a considerable improvement in the speed bias and root-mean-square error over a control set of AMVs derived with moreconventional methods.

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A satellite product that displays regions of aircraft icing potential, along with corresponding cloud-top heights, has been developed using data from the Geostationary Operational Environmental Satellite (GOES) imager and sounder. The icing product, referred to as the Icing Enhanced Cloud-top Altitude Product (ICECAP), is created hourly for the continental United States and southern Canada, and is color coded to show cloud-top altitudes in 1.9-km (6000 ft) intervals. Experimental ICECAP images became routinely available on the Internet during the spring of 2004. Verification of separate ICECAP components (imager icing potential and sounder cloud-top heights) using aircraft pilot reports (PIREPs) indicates that the product provides useful guidance on the spatial coverage and maximum altitude of current icing conditions, but not icing intensity, stratification, or minimum altitude. The imager icing potential component of ICECAP was compared with the operational 40-km resolution National Weather Service (NWS) current icing potential and NWS Airman’s Meteorological Advisories via the NOAA Real-Time Verification System, while GOES cloud-top heights were compared with altitudes of moderate or greater icing from PIREPs. Benefits and deficiencies of the GOES icing product are discussed.