1. Background

RAP has continued its work in developing operational mesoscale models in collaboration with other divisions.  For example, the NCAR team works with the Army Test and Evaluation Command (ATEC) to develop and install operational, mesogamma scale, general-purpose NWP systems. Systems have been accepted by the Army for the Dugway Proving Ground, Utah; the White Sands Missile Range, New Mexico; and the Aberdeen Test Center, Maryland.  An additional system has been installed at the Yuma Proving Ground, Arizona, and is undergoing evaluation, and another one will be installed at Fort Greely, Alaska in FY02.

Numerical weather prediction work in RAP also focuses on the development of improved modeling techniques and knowledge of atmospheric processes, with the ultimate goal of improving our ability to predict and understand various weather phenomena.  Such work has ranged from an analysis of Arabian Desert boundary layer processes, to studies of the land-surface modulation of thunderstorms using a convection-resolving mesoscale model, to an analysis of locally forced circulations in the Great Basin Desert.  Much of this work has been reported in previous Annual Scientific Reports; new developments and initiatives will be discussed here.

2. Ensemble simulations with coupled atmospheric dynamic and dispersion models: Illustrating uncertainties in dosage simulations

Ensemble simulations made using a coupled atmospheric dynamic model and a probabilistic Lagrangian puff dispersion model were employed in a forensic analysis of the transport and dispersion of a toxic gas that may have been released near Al Muthanna, Iraq during the Gulf War. The ensemble study had two objectives, the first of which was to determine the sensitivity of the calculated dosage fields to the choices that must be made about the configuration of the atmospheric dynamic model.  In this test, various choices were used for model physics representations and for the large-scale analyses that were used to construct the model initial and boundary conditions. 

The second study objective was to examine the dispersion model’s ability to use ensemble inputs to predict dosage probability distributions. Here, the dispersion model was used with the ensemble mean fields from the individual atmospheric dynamic model runs, including the variability in the individual wind fields, to generate dosage probabilities. These are compared with the explicit dosage probabilities derived from the individual runs of the coupled modeling system. An example of the dosage simulated by one of the ensemble members is shown in Figure E1. 

The results demonstrate that the specific choices made about the dynamic-model configuration and the large-scale analyses can have a large impact on the simulated dosages.  For example, the area near the source that is exposed to a selected dosage threshold varies by up to a factor of four among members of the ensemble. The agreement between the explicit and ensemble dosage probabilities is relatively good for both low and high dosage levels.  Although only one ensemble was considered in this study, the encouraging results suggest that a probabilistic dispersion model may be of value in quantifying the effects of uncertainties in a dynamic model ensemble on dispersion model predictions of atmospheric transport and dispersion.

(RAP team: T. Warner, R. Sheu, D. Rife)

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3. Mechanisms for diurnal boundary-layer circulations in the Great Basin Desert

The purpose of this observation- and model-based study of the Great Basin Desert boundary layer was to illustrate the variety of locally forced circulations that can affect such an area during a diurnal cycle.  The area of the Great Basin Desert that was studied is located to the southwest of Salt Lake City, Utah.  It is characteristic of the arid “Basin and Range” province of North America in that it contains complex terrain, varied vegetation and substrates, and high water tables associated with salt-encrusted basin flats (playas).  The study area is especially well instrumented with surface meteorological stations operated by the Army’s West Desert Test Center and a collection of cooperating mesonets in northeastern Utah. The study period was chosen based on the availability of special radiosonde data in this area. 

One of the processes that is documented here that is unique to desert environments is the salt breeze that forms around the edge of playas as a result of differential heating.  The data and model solution depict the diurnal cycle of the salt breeze, wherein there is on-playa flow at night and off-playa flow during daylight.  There is also a multiplicity of drainage flows that influence the study area at different times of the night, from both local and distant terrain.  Lastly, the lake-breeze front from the Great Salt Lake and Utah Lake progresses through the complex terrain during the day, to interact with early mountain drainage flow near sunset.

Figure E2 shows the model-simulated and observed winds for 1900 LT on a clear July day with weak synoptic-scale forcing.  The front analyzed at the wind-shear zone is associated with the lake breeze from the Great Salt Lake and Utah Lake.

(RAP team: T. Warner, F. Chen, D. Rife)