Regular users of weather radar products are quite familiar with the issue of non-meteorological echoes produced by ground clutter. This occurs at all hours of the day, and often is widespread under stable atmospheric conditions, such as those that typically develop during the overnight hours. Some radars, including the WSR-88D used by the National Weather Service, have built in tools that can automate the identifcation and suppression of most ground clutter. With the recent deployment of dual-polarization radar at the Chicago NWS, the automated Clutter Mitigation Decision (CMD) tool will not be active until a new radar software release becomes available sometime next summer. In the meantime, areas of clutter will have to be manually suppressed by NWS staff.
An example of extensive ground clutter (with relatively few real meteorological echoes) was present in imagery from NWS Chicago during the predawn and early morning hours of 10/31/2011:
Believe it or not, there was in fact some default clutter suppression applied to the previous image. In the map below, the red areas indicate locations where radar echoes are assumed to affected by ground clutter and thus are suppressed by default:
In contrast to the clutter suppression map shown above, which does not vary over time, the previously available CMD tool was able to interrogate various properties of the radar data and calculate a unique clutter suppression for each radar volume scan.
For example, during early morning hours when ground clutter becomes more pronounced, the dynamically determined CMD clutter suppression map might look like:
Meanwhile, during typical daytime conditions when the radar beam is not intersecting as many ground targets, the dynamically determined CMD clutter suppression map might contain much less area, such as:
Notice that this dynamically generated clutter suppression map looks very similar to the first one shown above. So during typical daytime conditions, the default (standard, fixed) clutter suppression map generally will suffice. Overnight however, until the automated CMD tool returns next summer, expect to see varying levels of clutter suppression effectiveness.