Showing papers by "Sean C. Solomon published in 1999"

The Global Topography of Mars and Implications for Surface Evolution

Abstract: Elevations measured by the Mars Orbiter Laser Altimeter have yielded a high-accuracy global map of the topography of Mars. Dominant features include the low northern hemisphere, the Tharsis province, and the Hellas impact basin. The northern hemisphere depression is primarily a long-wavelength effect that has been shaped by an internal mechanism. The topography of Tharsis consists of two broad rises. Material excavated from Hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary. The present topography has three major drainage centers, with the northern lowlands being the largest. The two polar cap volumes yield an upper limit of the present surface water inventory of 3.2 to 4.7 million cubic kilometers.

On the role of nitrogen dioxide in the absorption of solar radiation

Abstract: Direct measurements of the absorption of downwelling visible radiation by nitrogen dioxide are presented. The data show that this gas can contribute significantly to local radiative forcing under certain conditions. The observed enhancements in nitrogen dioxide absorption are likely to be due both to pollution and to production by lightning in convective clouds. Case studies of several days of observations in Colorado reveal peak absorption of downwelling radiation by NO2 of up to 5–12%, corresponding to an estimated local radiative forcing that is likely to be in the range of 5–30 W/m2. The amount of local forcing associated with thunderstorm activity depends strongly upon the cloud optical depth and on where the NO2 resides within the clouds. These case studies suggest that NO2 can play a significant role in the absorption of radiation (including but not limited to anomalous cloud absorption) either under polluted conditions or when electrically active storms are considered.

Climate forcing due to tropospheric and stratospheric ozone

Abstract: 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.

Climate change as a regulator of tectonics on Venus.

Abstract: Tectonics, volcanism, and climate on Venus may be strongly coupled. Large excursions in surface temperature predicted to follow a global or near-global volcanic event diffuse into the interior and introduce thermal stresses of a magnitude sufficient to influence widespread tectonic deformation. This sequence of events accounts for the timing and many of the characteristics of deformation in the ridged plains of Venus, the most widely preserved volcanic terrain on the planet.

Implications for water monomer and dimer solar absorption from observations at Boulder, Colorado

Abstract: We present ground-based spectral measurements of downward direct and diffuse solar intensities between 624 and 686 nm. Using the rising Sun as a light source, it is shown that the water dimer does not absorb significantly in this wavelength region. Over Boulder, Colorado, on July 10, 1998, the upper limit to the dimer vertical optical depth at these wavelengths is approximately 0.001, with an implied upper limit for instantaneous clear-sky absorption of solar radiation of about 0.8 W/m 2 for an overhead Sun. The direct Sun measurements also show that in the observed wavelength region no other unknown structured absorber can account for more than 0.6 W/m 2 absorption of the instantaneous downward direct solar flux for an overhead Sun. Observations of light scattered through a thick cloud reveal that when the atmosphere is saturated with respect to water vapor, no observable dimer absorption occurs and there is no significant spectral shape change in water monomer absorption at our spectral resolution.

Constraints on N2O sinks inferred from observed tracer correlations in the lower stratosphere

Abstract: Recent isotopic studies have suggested that the trace gas N 2 O has a missing stratospheric sink of potentially major significance. While these studies have raised interesting questions, the constraints on N 2 O photochemistry imposed by correlations between N 2 O, total reactive nitrogen, and other tracers measured in situ in the lower stratosphere also should be considered. Measured tracer correlations, when compared to the results of models using standard photochemistry, provide evidence in support of conventional N 2 O sinks. Stratospheric tracer correlations, however, cannot be used to preclude a new atmospheric source of N 2 O in the troposphere or to argue against an undiscovered stratospheric sink that contributes less than ∼20% of the total sink.