ATMOSPHERIC-PRESSURE LOADING PARAMETERS FROM VERY LONG-BASE-LINE INTERFEROMETRY OBSERVATIONS

Atmospheric mass loading produces a primarily vertical displacement of the Earth's crust. This displacement is correlated with surface press ure and is large enough to be detected by very long baseline interfero metry (VLBI) measurements. Using the measured surface pressure at VLBI stations, we have estimated the atmospheric loading term for each sta tion location directly from VLBI data acquired from 1979 to 1992. Our estimates of the vertical sensitivity to change in pressure range from 0 to -0.6 mm/mbar depending on the station. These estimates agree wit h inverted barometer model calculations (Manabe et al., 1991; vanDam a nd Herring, 1994) of the vertical displacement sensitivity computed by convolving actual pressure distributions with loading Green's functio ns. The pressure sensitivity tends to be smaller for stations near the coast, which is consistent with the inverted barometer hypothesis. Ap plying this estimated pressure loading correction in standard VLBI geo detic analysis improves the repeatability of estimated lengths of 25 o ut of 37 baselines that were measured at least 50 times. In a root-sum -square (rss) sense, the improvement generally increases with baseline length at a late of about 0.3 to 0.6 ppb depending on whether the bas eline stations are close to the coast. For the 5998-km baseline from W estford, Massachusetts, to Wettzell, Germany, the rss improvement is a bout 3.6 mm our of 11.0 mm. The average rss reduction of the vertical scatter for inland stations ranges from 2.7 to 5.4 mm.