Environmental Monitoring: 3-Dimensional Atmospheric Simulations

HOTMAC and RAPTAD are two simulations which allow modeling of atmospheric flows and dispersion of contaminants over any given topography. HOTMAC is a 3-Dimensional model useful for airflow forecasts over complex terrain and RAPTAD is a 3-Dimensional model for transport and diffusion forecasts of airborne materials. Together they help model how contaminants released into the atmosphere travel and disperse. Be sure to view the movie of air flow and contaminant release in the Rio Grande basin near Sandia National Laboratory's Cooperative Monitoring Center (CMC).

The model in the CMC has two components, namely HOTMAC and RAPTAD. HOTMAC forecasts winds inside a 3-D grid over an extended time interval such as 24 or 36 hours. A minimum set of meteorological conditions such as surface air temperature, surface barometric pressure, atmospheric temperature profile, surface wind speed and direction, etc. are user-input at startup time. The model then works forward in time and calculates horizontal and vertical wind vectors as well as wind turbulence parameters in each of the many cells that encompass the terrain. The model is versatile and can be used for simulations world-wide. The CMC version of the model comes equipped with terrain data for the entire continental US. Topographical data for other portions of the world can be also be accessed from other media such as CD-ROM or tape. The RAPTAD module is run following completion of the HOTMAC run. In preparation for a RAPTAD run, the user specifies source location, height, diameter, particle or gas release rate, plume vertical velocity and buoyancy etc. RAPTAD then utilizes tracer particles which are released into the wind field computed by HOTMAC. The particles are tracked in time as they move through the grid cells and are influenced by the various wind characteristics. Plume dimensions are determined by using a distribution about each of the tracer particles. The shape of the particle distribution is influenced by the turbulence parameters computed earlier in HOTMAC.

The 3-D graphics module of HOTMAC/RAPTAD enables the plume release to be visualized in an on-screen movie format. In the movie, the terrain elevation is exaggerated by a factor of four to assist in recognition of land features. An analog clock is also shown in the upper right hand portion of the screen and tracks the time of day or night during the simulation. Particle release and ensuing downwind movement with time is depicted either by colored ellipsoids or white/gray smoke, depending on which movie is being shown.