Air pressure and Baltic Sea loading corrections to gravity data at Metsahovi.

摘要

Variation in atmospheric mass changes surface gravity through two effects: through the direct Newtonian attraction and through deformation of the solid Earth due to surface pressure. Since air columns are approximately in hydrostatic equilibrium, both effects can be modelled using surface pressure. The range of air pressure at Metsahovi (100 hPa) corresponds to about 300 nms~(-2) in gravity. Variation in the level of the Baltic Sea changes gravity at Metsahovi mainly through the induced deformation. A uniform 1 m layer of water corresponds to 31 nms~(-2) in gravity. We investigate corrections to the record of the superconducting gravimeter T020 at Metsahovi for the years 1994-2002. A standard method for air pressure is to use a single admittance, obtained by regressing gravity on the local barometer record. We show that an analogous correction for Baltic loading using a local tide gauge is feasible and further reduces gravity residuals by 10...20%. The local barometer does not adequately describe the air pressure distribution. We test an alternative approach and calculate the correction by integrating atmospheric loading gravity Green's functions over a detailed regional surface pressure field from HIRLAM (HIgh Resolution Limited Area Model). Although the integration only extends to 5°...10° in angular distances from Metsahovi, in terms of rms gravity residuals the method performs nearly as well as the single-admittance even when Baltic loading is not modelled. Then the combination of HIRLAM and local tide gauge brings a remarkable decrease in gravity residual. The admittance obtained by regressing gravity on the local barometer partly includes Baltic loading, and thus the independent atmospheric corrections using the HIRLAM bring better out the contribution of the Baltic.