BOX 8.1

Low-altitude coverage

A fundamental limitation of any network comprising widely-spaced radars is the inability to comprehensively cover the lowest regions of the troposphere, owing to both the curvature of the earth and terrain blockage. The solid curves in Figure 8.2 show the percentage of the volume in a thin layer at various heights above ground level covered versus radar spacing, assuming a smooth earth. At 230 km separation, which is the approximate spacing of the NEXRAD radars in the eastern half of the United States, coverage is nearly complete at a height of 3000 m but decreases to less than 10 percent at 300 m above ground level.

Denser radar placement can overcome this limitation, but larger numbers of radars would be needed in the network, as shown in the dashed line in the figure. Achieving comprehensive coverage down to 300 m, for example, would require a network of several thousand radars spaced tens of kilometers apart. Realizing such a network cost effectively would require substantial reductions in radar acquisition, siting and recurring costs compared to today’s radars. The Center for Collaborative Adaptive Sensing of the Atmosphere (CASA), an Engineering Research Center chartered by the National Science Foundation, is investigating the feasibility of small low-cost radars and the associated software architecture and data handling issues that would enable future deployment of such networks (McLaughlin et al., 2007).

FIGURE 8.2. Volume coverage at different heights (solid lines) and number of radars needed for CONUS coverage versus radar spacing (calculations based on smooth earth).

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