While not as evident a distinctive trait as the correlation coefficient, differential reflectivity sometimes shows a pattern where it is low near the fire, and becomes quite large near the edges of the plume. One of the distinguishing characteristics of smoke in dual-polarization data is the very low correlation coefficient. Smoke from large fires sometimes shows up on the radar as a plume moving downwind and spreading out from a source point. Reflectivity and velocity image from Edwards AFB, CA radar showing vehicles on multiple nearby highways Smoke There will be a few pixels of high reflectivity and generally large velocities (of either/alternating sign) right along the road (if the highway layer is on in RadarScope). Sometimes radar will pick up vehicles on interstates or other highways near the radar. This vertical gradient causes the anomalous propagation of the beam, which in turn causes the clutter echoes. Note that just above the surface temperature increases and moisture decreases sharply. The red line is temperature, and the green line is dew point. Observed sounding (skew-T diagram) near the time of the above radar image. Widespread clutter echoes (some biological, some ground-based) The morning sounding from Dodge City shows the dramatic increase in temperature and decrease in moisture just above the surface that produced the anomalous propagation conditions. Even though it looks like widespread rain near Larned and St. Below is a severe example from Dodge City, which includes some wind farms also. AP is most common overnight or early in the morning, but can also occur behind thunderstorms or strong cold fronts. Its time near the ground is then prolonged, and it can encounter biological scatterers and in some cases contact with ground-based targets such as hills, trees, etc. This type of clutter occurs when the vertical temperature and moisture profile is such that the radar beam is bent back toward the ground more than usual. Very large wind farm just east of San Francisco Bay Anomalous PropagationĪdditional ground clutter beyond what is always visible for a given radar site sometimes appears as a result of “anomalous propagation” or AP. Wind farm very close to the Dodge City radar where the rows of turbines can be seen in the reflectivity data Even though the turbines themselves don’t move, the radar will frequently display non-zero velocity in wind farms because of the spinning blades. Some radars, such as KVNX, KDDC, and KDAX, have nearby wind farms that always show up, but most only show up on radar when the beam stays closer to the ground than normal, usually early in the morning. There is also a map of wind turbines maintained by the USGS, though it may not include some of the newest wind farms. The satellite layer on Google maps can confirm the presence of a wind farm. Sometimes, it is possible to see the individual rows of turbines, as in the example from Dodge City. It has unrealistically large reflectivity gradients along the edges (goes from nothing to 60+ dBZ in one or two pixels) and noisy values in the interior (a 40 next to a 50, next to a 70, next to a 30, etc.). Wind farms are distinguished on radar data as small regions of very high reflectivity that don’t move. One of the most common ground clutter artifacts that attracts significant attention is wind farms, mostly because of the tendency for very high reflectivity that looks like a thunderstorm. Part 3 - Ground Clutter and Other Stuff Wind Farms Satellite data can also be helpful for determining if there is a fire or significant cloud cover in the area of interest. Smoke and chaff can be most readily distinguished from precipitation by the low correlation coefficient. Some ground clutter is almost constant, but in general, it tends to be worst during the night or early in the morning. When watching a loop, it tends to suddenly appear and disappear in the same area, rather than moving. The correlation coefficient is generally very low, and differential reflectivity tends to be near zero but can be variable. Sometimes there will be pixels of missing/no data/zero within a group of pixels with high values. Ground clutter is generally characterized by isolated, noisy large values of reflectivity and velocity that move very little if at all. The third part of the series will focus on various forms of ground clutter and miscellaneous things like smoke and chaff.
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