Perhaps one of the fundamental challenges facing the operational meteorologist is understanding the fully three-dimensional motion of the atmosphere, that is, visualizing air parcel trajectories. This is not a trivial task and there have been few explicit methods of accomplishing this visualization. Until recently, the main tool at the forecaster's disposal for trajectory analysis has been two-dimensional streamline analysis. However, these two-dimensional streamline patterns neglect the vertical component of the flow and duplicate horizontal trajectories only if they are stationary. As a consequence of these inadequacies, forecasters can be misled as to the actual path that air parcels take to arrive in an area of concern. This can lead to errors in temperature and moisture forecasts among other problems.

Output from a trajectory model has been available to forecasters for the last several decades via both an alphanumeric product and several crude graphic products. Using the output of this model has been limited, probably due to several reasons: 1) the model uses only NGM (LFM previously) output for its computations; 2) the text product shows parcel positions in terms of latitude, longitude and pressure for only select locations and pressure levels; 3) the graphic output consists of just a few static and non-interactive depictions.

With the arrival of AWIPS, many of the aforementioned difficulties in visualizing air parcel trajectories are eliminated. The Display Three Dimensions (D3D) software that runs as part of AWIPS makes it a simple matter to calculate and display three-dimensional parcel trajectories from essentially any data set that is available in the system. This software has been running in an evaluation mode at the Denver/Boulder WFO since the spring of 2001 and has been used to compute three-dimensional air parcel trajectories from model data (eta and MM5) as well as analysis data (LAPS).

Using just a few mouse clicks, forecasters can compute either forward or backward trajectories for any point in the domain, color code them according to a wide variety of parameters (pressure, temperature, potential vorticity, lapse rate, relative humidity, etc.) and animate them in time. These trajectory displays have shown some very interesting patterns and have demonstrated the significant differences that exist between streamlines and trajectories. In some cases this has lead to increased understanding of model behaviour and also better physical understanding of actual observed flow patterns.

In this presentation, we will compare the old methods of estimating three-dimensional air parcel trajectories with these new ones and through case study examples show how visualizing trajectories has helped improve operational forecasts.