sonic tilts

The attached plots were made with our Splus function, plot.tilt, that does a linear least squares fit to find the plane of mean flow, and plots the wind vector elevation angle vs azimuth. The planar fit becomes a sine wave on the tilt plot.

From a long term plot of the sonic "diag" value, I chose two periods where the the values were consistently very small. plot.tilt discards 5 minute wind averages when "diag" is above 0.01, or more than 1% of the data has a non-zero CSAT3 diagnostic value.

For the upper sonics at 16, 30 and 43 meters, the minimum wind speed used for the fit was 1.0 m/s. For the lower sonics at 2 and 7 meters, the minimum wind speed was set to 0.5 m/s.

Tom says that typical sag of sonic booms results in 1 to 2 degree tilts, which would be 180 degees different in azimuth from the above values.

These tilts appear to be due to the slope of the terrain, which is downward in the -u direction, in the direction that the sonic boom points. If the terrain was not sloping, these would be a "backwards" boom tilt, i.e. the booms not drooping from the tower but angling upward. They generally agree on an approximate 5 degree slope of the terrain relative to the sonics, except the 9.9 degree tilt for the 7m sonic in Aug 2011.

dpar(start="2011 2 21 00:25",end="2011 4 4 07:26",coords="instrument")
dpar(hts=2)
plot.tilt(flag="diag",ellim=10,spdmin=0.5)