From: "Gibson, Dar (South Pole)" Subject: RE: McMurdo and South Pole Station Radiosonde Data Added Date: Mon, 15 Apr 2002 23:26:14 +1200 Below is a little background on the grid wind direction errors with our sounding system. Most of this was written by Brad Halter, one of the summer meteorologists whose extensive experience in studying atmospheric sciences here at the Pole dates back to 1976. At South Pole a grid system for defining wind direction is used, since east and west directions would change by up to 180 degrees in moving around the geographic point in its immediate vicinity if the true wind direction system were used. Winds arriving at the Station along the 0 degree, 90 degree East, 180th, and 90 degree West meridians, respectively, are taken to be arriving from grid N, E, S, and W. An error in the conversion of GPS winds from the true direction to the grid system has caused a mirror image reversal in reported wind direction, with the "mirror" being along the 90W - 90E line. That is to say that there is little difference between reported AIR sonde and GPS sonde wind direction when the winds are arriving at the Station along the 90E or 90W meridians. However, both comparisons of actual balloon sonde movement with reported GPS winds and comparisons of winds from simultaneous AIR sonde and GPS sonde flights indicate that, as the actual wind turns from easterly to become increasingly northerly and northwesterly, reported GPS sonde winds turn in the opposite direction, becoming more southerly and south- westerly, as defined in the grid wind system. This error in GPS winds caused unrealistic low level wind shear to be reported on 17 NOV 2001. At the surface, winds from grid 002 degrees at 17.3 KT were reported by the GPS calculations to have become from 197 degrees at 24.1KT within 28 meters of the surface. The actual winds, I feel confident, had turned not all the way to 197 degrees in the space of 28 meters, but had in fact backed slightly from the surface direction of 002 degrees to 343 degrees, the "mirror image" of the reported 197 degrees. Note that the surface winds (2835 meters) are placed in the sonde wind data report by the weather observer and retained in the final wind report unmodified. Winds above the surface are calculated from GPS data. Below is the correction we applied to the GPS radiosonde wind directions to get values in fair agreement with the comparison AIR radiosonde data obtained last winter. The correction is based on the assumption that the current algorithm used by the MRS to calculate grid wind direction results in wind directions from the NE and NW quadrants being assigned, respectively, to the SE and SW quadrants, and vice versa. The correction then simply puts the wind directions back into the correct quadrants by calculating the complement of a 180 degree angle as follows: IF UWD < 180, CWD = 180 - UWD IF UWD >= 180, CWD = (360 - UWD) + 180 = 540 - UWD where UWD is the uncorrected wind direction and CWD is the corrected wind direction The above correction applies to all flight files from Aug 1, 2001 0000Z through and including the Jan 4, 2002 1200Z sounding. A software upgrade then resulted in the Jan 5, 2002 0000Z sounding through and including the Jan 8, 2002, 1200Z sounding displaying a constant 180 degree error. As of the Jan 9, 2002, 0000Z sounding the grid wind directions reported by our sounding system are correct. We are trying to devise a method for correcting the 5+ months of erroneous wind data but it is yet to be determined whether or not this will be possible. Please let me know if you have any further questions. Hope this helps. -Dar- Dar Gibson Sr. Meteorologist, RPSC South Pole Station, Antarctica