Movie: Flow visualization with the CAPITA Monte Carlo model

Description:

The animation shows a dynamic flow simulation of airmasses over North America during 7/1/1992 - 7/14/1992. The left hand side of the movie contains the spatial location of all particles emitted at a constant rate from a spatially uniform grid at the surface layer of the atmosphere. The particles have been colored based upon their height. The blue particles are at the surface, the red particles are above 3 km, and the yellow and green particles are between these heights. The right hand side displays the vertical position of the particles for four longitudinal slices across the map view. The vertical slices are evenly spaced over the map view and are 150km wide.

Purpose:

The flow visualization is created to illustrate the transport phenomena in the lower atmosphere, such as air stagnation, convergence of airmasses, fronts, ventilation, and general 3 dimensional motion.

Method:

The transport simulation was created using the CAPITA Monte Carlo Model. Particles were released from 504 sources uniformly distributed over the Continental US. Three particles were released from each source every two hours, and were tracked for seven days. The calculation of the transport of the particles was done using the wind fields generated by the National Meteorological Center's Nested Grid Model with a grid resolution of 180 km.

Interpretation:

This movie displays several of the characteristics of regional scale flow in the lower atmosphere. First, the daily "breathing" of the atmospheric due to the diurnal cycling of the mixing height is clearly seen. The nocturnal mixing layer is low, 100 - 300 m, and the particles released into this layer remain at the surface. During the day, the mixing height increases, and the particles are mixed throughout the layer diluting the high concentrations of particle that accumulated during the night.

Several instances are also apparent where two regional airmasses converge. In one instance extending from 7/2 to 7/4 an airmass heading southwest from Quebec, Canada and another heading northeast from the Midwest converge over New York state and Canada. At the convergence zone, the flow is stagnant and there is a high density of particles. The converging airmasses cause upward vertical motion as seen by the large number of particles transported to heights above 4 km. The simulation also illustrates the ventilating effects of high wind speeds. During 7/11 meandering winds in the Northwest and Canada cause a buildup of particles. Around 7/12, strong westerly winds transport these particles to the east leaving a low particle density.