ERA-40 Adjusted Surface Pressure Anomalies
An evaluation of ERA-40 surface pressure data (Trenberth and Smith 2004), in the context of the conservation of mass of of the atmosphere, found spurious trends in both the mass of dry air and atmospheric moisture arising from changes in the observing system, especially prior to 1979 when reliable satellite data became available for global analyses. Spurious fluctuations in global mean surface pressure of order 0.6 hPa occur before 1979 and primarily arise from low quality analyses over the southern oceans. Surface pressures are generally higher around Antarctica and contribute to global mean values of order 0.3 hPa higher before 1973 in ERA-40. Locally, around Antarctica, errors were shown to be of order 5 hPa. The VTPR (Vertical Temperature Profile Radiometer) data, available from 1973-1978, improve both the mean state and cut down on spurious variability, but are not as good as the post-1979 data in this regard. It also leads to water vapor column values that are too high in the subtropics. Water vapor mean annual cycle variations are found to contribute to global mean surface pressure variations of 0.3 hPa. In general water vapor contributions to total mass anomalies are much smaller and can be neglected for current purposes.However, the spurious variations in mass prior to 1979 are regarded as a potential problem for exploring variability unless corrected. Accordingly, based on the mean errors documented in Trenberth and Smith (2004), some adjustments were made to the surface pressure anomaly time series in a region bounded by 56°S and the Antarctic coast. In this region for post 1979, and other regions as well, the mean annual cycle from 1979-2001 is used to determine the monthly anomalies. A core region from 56°S to within 2.25° latitude of the coast was established with a mask, where full adjustments are made, whereby the anomalies prior to 1979 were computed relative to the mean for 1958-1978. For two grid points immediately north and south of the core region, a merge is devised such that the weights are two thirds and one third, with the two thirds weight closest to the core (inner) region. Some minor smoothing to the mask was made in the vicinity of the Antarctic Peninsula. Accordingly decadal variability across 1978-79 is suppressed in this region.
FORMAT: netCDF
GRID: T63 Gaussian (192 x 96)
LEVELS: surface
FIELDS: Description Name units
-------------------------------------------------------------------------
Adjusted Surface Pressure Anomalies PSANOM Pa
netCDF metadata:
netcdf era40.PS.anom.1958-2001 {
dimensions:
lon = 192 ;
lat = 96 ;
time = 528 ;
variables:
float lon(lon) ;
lon:long_name = "longitude" ;
lon:short_name = "lon" ;
lon:units = "degrees_east" ;
float lat(lat) ;
lat:long_name = "latitude" ;
lat:short_name = "lat" ;
lat:units = "degrees_north" ;
float time(time) ;
time:long_name = "Year, month, YYYYMM" ;
time:short_name = "time" ;
time:units = "unitless" ;
float PSANOM(time, lat, lon) ;
PSANOM:long_name = "Adjusted Surface Pressure Anomaly" ;
PSANOM:short_name = "PSANOM" ;
PSANOM:units = "Pa" ;
PSANOM:_FillValue = 1.e+36f ;
// global attributes:
:DATA = "ERA-40 Surface Pressure Anomaly (Adjusted Data Set) " ;
}
TIMES: Monthly means
FTP Directory:ftp://ftp.cgd.ucar.edu/pub/CAS/ERA-40/PSANOM/
Filename: era40.PS.anom.1958-2001.nc
Current holdings: January 1958 - December 2001
References
Trenberth, K. E., and L. Smith, 2005: The mass of the atmosphere — A constraint on global analyses.
J. Climate, 18, 864-875.
Trenberth, K. E., D. P. Stepaniak, and L. Smith, 2005: Interannual variability of patterns of
atmospheric mass distribution. J. Climate, 18, 2812-2825.
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