Stan Solomon

Climate forcing due to tropospheric and stratospheric ozone

Radiative forcings, preindustrial to present, are presented due to changes in both tropospheric and total ozone. Unlike previous studies, we use a combination of present-day satellite data, ozonsondes, and a chemistry model to produce a hybrid monthly mean ozone data set from 1870 to 1990. We calculate the radiative forcing due to the prescribed ozone distribution using a three-dimensional climate model that employs seasonally evolving fixed dynamical heating in the stratosphere. We find that tropospheric ozone causes a seasonal fixed dynamical radiative forcing (SEFDH) of 0.30 W m -2 over the time period 1870-1990. The spatial pattern of forcing is correlated strongly with the change in tropospheric ozone. When we allow for observed changes in stratospheric ozone we calculate that the forcing due to total ozone from 1870 to 1990 is 0.29 W m -2 , with 0.18 from shortwave and 0.1 W m -2 from longwave processes. Thus reduction of stratospheric ozone since 1970 offsets the forcing by only -0.01 W m -2 . We also consider to what extent increased tropospheric ozone offsets the direct forcing by sulfate aerosols. We find that for June-July-August the sulfate forcing is -0.76 W m -2 , while the addition of ozone forcing reduces this to -0.40 W m 2 Finally, we carry out sensitivity studies with regard to the uncertainty in preindustrial ozone levels.

Radiative forcing of the Earth's climate system due to tropical tropospheric ozone production

The radiative forcing of the Earths climate system due to tropical tropospheric ozone is estimated using ozonesonde profiles and maps of the tropospheric ozone column derived from satellite data. The forcing is estimated using several different techniques in order to place bounds on its likely magnitude and to elucidate the role of biomass burning in producing the observed forcing. The results suggest that a widespread radiative forcing of at least 0.5 to 1 W m−2 exists over large areas in the tropics for much of the year. This radiative forcing is comparable in magnitude, but opposite in sign, to estimates of the aerosol forcing from tropical biomass burning. However, the burning contribution to ozone forcing is present over a larger geographic area than the aerosol forcing. The majority of the burning and thus these radiative forcings have likely been present only in the past century. These enhancements in tropical ozone are also estimated to be responsible for a radiative forcing of between 0.1 and 0.4 W m−2 when globally averaged. Comparison of these regional and global forcings due to tropical ozone changes to the estimates of forcing due to carbon dioxide and other trace gas increases since preindustrial times (about 2.45 W m−2) suggests that the effects of tropical ozone changes could be significant for evaluation of both regional and global anthropogenic forcing of climate.

Temporal variability of atomic hydrogen from the mesopause to the upper thermosphere

We investigate atomic hydrogen (H) variability from the mesopause to the upper thermosphere, on time scales of solar cycle, seasonal, and diurnal, using measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics Dynamics satellite, and simulations by the National Center for Atmospheric Research Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X). In the mesopause region (85 to 95 km), the seasonal and solar cycle variations of H simulated by WACCM‐X are consistent with those from SABER observations: H density is higher in summer than in winter, and slightly higher at solar minimum than at solar maximum. However, mesopause region H density from the Mass‐Spectrometer‐Incoherent‐Scatter (National Research Laboratory Mass‐Spectrometer‐Incoherent‐Scatter 00 (NRLMSISE‐00)) empirical model has reversed seasonal variation compared to WACCM‐X and SABER. From the mesopause to the upper thermosphere, H density simulated by WACCM‐X switches its solar cycle variation twice, and seasonal dependence once, and these changes of solar cycle and seasonal variability occur in the lower thermosphere (~95 to 130 km), whereas H from NRLMSISE‐00 does not change solar cycle and seasonal dependence from the mesopause through the thermosphere. In the upper thermosphere (above 150 km), H density simulated by WACCM‐X is higher at solar minimum than at solar maximum, higher in winter than in summer, and also higher during nighttime than daytime. The amplitudes of these variations are on the order of factors of ~10, ~2, and ~2, respectively. This is consistent with NRLMSISE‐00.