I noticed that a score of Doppler PPI scans are set at negative elevation angles. For example, Lac Castor (WMB) PPI are -0.3 and -0.1 degrees. Apparently, it is needed for looking down into the valley (Lac Castor sits on a hill top near Saguenay valley). A senior forecaster in my office offered his explanation, which he learned from the seminar: the 10th US/Canada Great Lakes Workshop on Operational Meteorology, 2001. He had a summary of the workshop. I copied the related section as shown below.
My question is: Can we still apply the hodograph technique, as we learned from Phil's course, for a negative Doppler PPI?
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Negative Elevation Angles for the Montague radar
Rodger A. Brown and Vincent T. Wood (National Severe Storms Laboratory, Norman, OK) suggested that radars higher than surrounding terrain should be allowed to scan at negative elevation angle. At present, the lowest elevation angle allowed is +0.5 deg due to concern of microwave radiation impact on population. This means that phenomena like microbursts, surface divergence, and low-altitude precipitation (below 2 km) will not be detected beyond 100 km from the radar site. Furthermore, winter convection (snowqalls) is usually less than 2 km in depth. A simulation was performed to find out where events below 2 km would be missed by the radars around Lake Ontario with even +0.3 deg elevation. Trenton was found to be beyond the effective coverage of King City and Franktown radars. The speaker indicated that with the KTYX radar (Montague, 500 m above surrounding terrain) scanning at -0.3 deg elevation would detect shallow convection over most of Lake Ontario, including Trenton and the upper St. Lawrence River Valley. The amount of radiation received by someone who is radiated by the beam is only equivalent to 1 m from microwave oven, 2 orders of magnitude below that from cell phone, and 4 orders of magnitude below what is considered a biological hazard. It is worthy to add that the newly installed Doppler XAM Val d'Irene radar (Quebec) operates at -0.6 deg.