Questions & Answers

Negative elevation angles

Negative elevation angles

by Ka Hing Yau -
Number of replies: 3

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.

In reply to Ka Hing Yau

Re: Negative elevation angles

by Michel Bisson -
Ka Hing Yau,
 
Yes, you would still apply the hodograph technique and all of the other techniques you have learned to use with radars.  The only thing is that you would have to learn the reference heights for that angle... just like the heights AGL would be different when you compare 0.5 and 3.5 degree PPI images at a particular range ring.
 
Michel Bisson
In reply to Michel Bisson

Re: Negative elevation angles

by Ka Hing Yau -

For negative elevation angle, increasing range implies decreasing height. Would it affect the interpretation of WAA vs CAA? For example, if the zero line of radial velocity is curving clockwise from centre, velocity vectors are VEERING with DECREASING height in this case. So, the velocity vectors are BACKING with INCREASING height, which implies COLD AIR ADVECTION (WAA). Is my interpretation correct?

In reply to Ka Hing Yau

Re: Negative elevation angles

by David Patrick -

Negative elevation angles only result in a decreasing beam height relative to the ground for a limited number of kilometres away from the radar.  Beyond that point the beam height increases again, relative to the ground below.  This is mainly due to the curvature of the earth.  In a vacuum, the radar beam would follow a straight line at whatever elevation angle you transmitted it at.  However due to the earth's atmosphere, it "normally" curves downward with a curvature of 4/3 the earth's radius.  But of course the earth's surface curves downward even more sharply than this, at a curvature equal to the earth's radius.  Attached is a beamheights.txt file that shows some sample negative elevation angles and expected beam heights above ground at various radiuses (in kilometres) from the radar for normal atmospheric conditions.