IMPLICATIONS OF SMALL-SCALE FLOW FEATURES TO MODELING DISPERSION OVERCOMPLEX TERRAIN

Small-scale, topographically forced wind systems often have a strong i nfluence on flow over complex terrain. A problem is that these systems are very difficult to measure, because of their limited spatial and t emporal extent. They can be important however, in the atmospheric tran sport of hazardous materials. For example, a nocturnal exit jet-a narr ow stream of cold air-which flowed from Eldorado Canyon at the interfa ce between the Rocky Mountains and the Colorado plains near the Rocky Flats Plant (RFP), swept over RFP for about 3 h in the middle of the n ight of 4-5 February 1991. It extended in depth from a few tens of met ers to approximately 800 m above the ground. Because the jet was so na rrow (2 km wide), it was poorly sampled by the meteorological surface mesonet, but it did prove to have an effect on the dispersion of trace r material released from RFP, producing a secondary peak in measured c oncentration to the southeast of RFP. The existence and behavior of th e jet was documented by Environmental Technology Laboratory's Doppler lidar system, a scanning, active remote-sensing system that provides f ine resolution wind measurements. The lidar was deployed as a part of a wintertime study of flow and dispersion in the RFP vicinity during F ebruary 1993. The MATHEW-ADPIC atmospheric dispersion model was run us ing the case study data from this night. It consists of three major mo dules: an interpolation scheme; MATHEW, a diagnostic wind-flow algorit hm that calculates a mass-consistent interpolated flow; and ADPIC, a d iffusion algorithm. The model did an adequate job of representing the main lobe of the tracer transport, but the secondary lobe resulting fr om the Eldorado Canyon exit jet was absent from the model result. Beca use the jet was not adequately represented in the input data, it did n ot appear in the modeled wind field. Thus, the effects of the jet on t he transport of tracer material were not properly simulated by the dia gnostic model.