The Mass of the Atmosphere: a Constraint on Global Analyses

Kevin E. Trenberth and Lesley Smith

National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307

voice: (303) 497 1318
fax: (303) 497 1333


The total mass of the atmosphere varies mainly from changes in water vapor loading; the former is proportional to global mean surface pressure and the water vapor component is compute directly from specific humidity and precipitable water, using the ERA-40 reanalyzes from the European Centre for Medium Range Weather Forecasts (ECMWF). Their difference, the mass of the dry atmosphere is estimated to be constant for the equivalent surface pressure to within 0.01 hPa based on changes in atmospheric composition, and global reanalyzes satisfy this constraint for monthly mean for 1979-2001 with standard deviation of 0.065 hPa. New estimates of the total mass of the atmosphere its dry component, and their corresponding surface pressures, are larger than previous estimates owing to new topography of the Earth.s surface that is 5.5 m lower for the global mean. Global mean total surface pressure is 985.50 hPa, 0.9 hPa higher than previous best estimates. The total mean mass of the atmosphere is 5.1480 x 1018 with an annual range due to water vapor of 1.2 or 1.5x1015 kg depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated a 1.27x1016 kg and the dry air mass as 5.1352±0.0003x1018 kg. The water vapor contribution varies with an annual cycle of 0.29 hPa range, a maximum in July of 2.62 hPa and a minimum in December of 2.33 hPa, although the total global surface pressure has a slightly smaller range. During the 1982-83 and 1997-98 El Niño events, water vapor amounts and thus total mass increased by about 0.1 hPa in surface pressure or 0.5x1015 kg for several months. Some evidence exists for slight decreases following the Mount Pinatubo eruption in 1991 and also for upward trends associated with increasing global mean temperatures, but uncertainties due to the changing observing system compromise the evidence.

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