Dehydration in the tropical tropopause layer: Implications from the UARS Microwave Limb Sounder

Measurements of H2O from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) are used to investigate the structure of H2O in the near tropopause region and dehydration mechanisms in the tropical tropopause layer (TTL). The new MLS data are consistent with convective input of H2O into the bottom of the TTL followed by slow ascent with a maximum relative amplitude in the seasonal cycle occurring near the tropopause nearly in phase with the tropopause temperature seasonal cycle. The relative amplitude of the seasonal cycle shows a minimum at 121 hPa in the upwelling moist phase. These features are reproduced with the “cold-trap” dehydration hypothesis. Seasonal maps show wettest tropical 100 hPa H2O colocated with continental convection

Short-term variability of solar wind number density, speed and dynamic pressure as a function of the interplanetary magnetic field components: A survey over two solar cycles

The variability of hourly values of solar wind number density, number density variation, speed, speed variation and dynamic pressure with IMF Bz and magnitude |B| has been examined for the period 1965–1986. We wish to draw attention to a strong correlation in number density and number density fluctuation with IMF Bz characterised by a symmetric increasing trend in these quantities away from Bz = 0 nT. The fluctuation level in solar wind speed is found to be relatively independent of Bz. We infer that number density and number density variability dominate in controlling solar wind dynamic pressure and dynamic pressure variability. It is also found that dynamic pressure is correlated with each component of IMF and that there is evidence of morphological differences between the variation with each component. Finally, we examine the variation of number density, speed, dynamic pressure and fluctuation level in number density and speed with IMF magnitude |B|. Again we find that number density variation dominates over solar wind speed in controlling dynamic pressure

The diurnal variation of hydrogen, nitrogen, and chlorine radicals: implications for the heterogeneous production of HNO_2

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained through sunrise and sunset in the lower stratosphere during SPADE are compared to results from a photochemical model constrained by observed concentrations of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N_(2)O_(5) on sulfate aerosols agree with measured concentrations of NO, NO_(2), and ClO throughout the day, but fail to account for high concentrations of OH and HO_(2) observed near sunrise and sunset. The morning burst of [OH] and [HO_(2)] coincides with the rise of [NO] from photolysis of NO_(2), suggesting a new source of HO_(x) that photolyzes in the near UV (350 to 400 nm) spectral region. A model that allows for the heterogeneous production of HNO_(2) results in an excellent simulation of the diurnal variations of [OH] and [HO_(2)]

The distribution of hydrogen, nitrogen, and chlorine radicals in the lower stratosphere: Implications for changes in O_3 due to emission of NO_y from supersonic aircraft

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained in the lower stratosphere during SPADE are compared to results from a photochemical model that assimilates measurements of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N_2O_5 agree well with measured concentrations of NO and ClO, but concentrations of HO_2 and OH are underestimated by 10 to 25%, concentrations of NO_2 are overestimated by 10 to 30%, and concentrations of HCl are overestimated by a factor of 2. Discrepancies for [OH] and [HO_2] are reduced if we allow for higher yields of O(^1D) from O_3 photolysis and for heterogeneous production of HNO_2. The data suggest more efficient catalytic removal of O_3 by hydrogen and halogen radicals relative to nitrogen oxide radicals than predicted by models using recommended rates and cross sections. Increases in [O_3] in the lower stratosphere may be larger in response to inputs of NO_y from supersonic aircraft than estimated by current assessment models

Trajectory model simulations of ozone (O<sub>3</sub>) and carbon monoxide (CO) in the lower stratosphere

A domain-filling, forward trajectory model originally developed for simulating stratospheric water vapor is used to simulate ozone (O3) and carbon monoxide (CO) in the lower stratosphere. Trajectories are initialized in the upper troposphere, and the circulation is based on reanalysis wind fields. In addition, chemical production and loss rates along trajectories are included using calculations from the Whole Atmosphere Community Climate Model (WACCM). The trajectory model results show good overall agreement with satellite observations from the Aura Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) in terms of spatial structure and seasonal variability. The trajectory model results also agree well with the Eulerian WACCM simulations. Analysis of the simulated tracers shows that seasonal variations in tropical upwelling exerts strong influence on O3 and CO in the tropical lower stratosphere, and the coupled seasonal cycles provide a useful test of the transport simulations. Interannual variations in the tracers are also closely coupled to changes in upwelling, and the trajectory model can accurately capture and explain observed changes during 2005–2011. This demonstrates the importance of variability in tropical upwelling in forcing chemical changes in the tropical lower stratosphere

Nighttime ozone profiles in the stratosphere and mesosphere by the Global Ozone Monitoring by Occultation of Stars on Envisat

The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European Space Agency's Envisat satellite measures ozone and a few other trace gases using the stellar occultation method. Global coverage, good vertical resolution and the self-calibrating measurement method make GOMOS observations a promising data set for building various climatologies. In this paper we present the nighttime stratospheric ozone distribution measured by GOMOS in 2003. We show monthly latitudinal distributions of the ozone number density and mixing ratio profiles, as well as the seasonal variations of profiles at several latitudes. The stratospheric profiles are compared with the Fortuin-Kelder daytime ozone climatology. Large differences are found in polar areas and they can be shown to be correlated with large increases of NO2. In the upper stratosphere, ozone values from GOMOS are systematically larger than in the Fortuin-Kelder climatology, which can be explained by the diurnal variation. In the middle and lower stratosphere, GOMOS finds a few percent less ozone than Fortuin-Kelder. In the equatorial area, at heights of around 15–22 km, GOMOS finds much less ozone than Fortuin-Kelder. For the mesosphere and lower thermosphere, there has previously been no comprehensive nighttime ozone climatology. GOMOS is one of the first new instruments able to contribute to such a climatology. We concentrate on the characterization of the ozone distribution in this region. The monthly latitudinal and seasonal distributions of ozone profiles in this altitude region are shown. The altitude of the mesospheric ozone peak and the semiannual oscillation of the number density are determined. GOMOS is also able to determine the magnitude of the ozone minimum around 80 km. The lowest seasonal mean mixing ratio values are around 0.13 ppm. The faint tertiary ozone peak at 72 km in polar regions during wintertime is observed

Origins of the Ambient Solar Wind: Implications for Space Weather

The Sun's outer atmosphere is heated to temperatures of millions of degrees, and solar plasma flows out into interplanetary space at supersonic speeds. This paper reviews our current understanding of these interrelated problems: coronal heating and the acceleration of the ambient solar wind. We also discuss where the community stands in its ability to forecast how variations in the solar wind (i.e., fast and slow wind streams) impact the Earth. Although the last few decades have seen significant progress in observations and modeling, we still do not have a complete understanding of the relevant physical processes, nor do we have a quantitatively precise census of which coronal structures contribute to specific types of solar wind. Fast streams are known to be connected to the central regions of large coronal holes. Slow streams, however, appear to come from a wide range of sources, including streamers, pseudostreamers, coronal loops, active regions, and coronal hole boundaries. Complicating our understanding even more is the fact that processes such as turbulence, stream-stream interactions, and Coulomb collisions can make it difficult to unambiguously map a parcel measured at 1 AU back down to its coronal source. We also review recent progress -- in theoretical modeling, observational data analysis, and forecasting techniques that sit at the interface between data and theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue connected with a 2016 ISSI workshop on "The Scientific Foundations of Space Weather." 44 pages, 9 figure

Sources of increase in lowermost stratospheric sulphurous and carbonaceous aerosol background concentrations during 1999–2008 derived from CARIBIC flights

This study focuses on sulphurous and carbonaceous aerosol, the major constituents of particulate matter in the lowermost stratosphere (LMS), based on in situ measurements from 1999 to 2008. Aerosol particles in the size range of 0.08–2 µm were collected monthly during intercontinental flights with the CARIBIC passenger aircraft, presenting the first long-term study on carbonaceous aerosol in the LMS. Elemental concentrations were derived via subsequent laboratory-based ion beam analysis. The stoichiometry indicates that the sulphurous fraction is sulphate, while an O/C ratio of 0.2 indicates that the carbonaceous aerosol is organic. The concentration of the carbonaceous component corresponded on average to approximately 25% of that of the sulphurous, and could not be explained by forest fires or biomass burning, since the average mass ratio of Fe to K was 16 times higher than typical ratios in effluents from biomass burning. The data reveal increasing concentrations of particulate sulphur and carbon with a doubling of particulate sulphur from 1999 to 2008 in the northern hemisphere LMS. Periods of elevated concentrations of particulate sulphur in the LMS are linked to downward transport of aerosol from higher altitudes, using ozone as a tracer for stratospheric air. Tropical volcanic eruptions penetrating the tropical tropopause are identified as the likely cause of the particulate sulphur and carbon increase in the LMS, where entrainment of lower tropospheric air into volcanic jets and plumes could be the cause of the carbon increase

Trajectory model simulations of ozone (O 3) and carbon monoxide (CO) in the lower stratosphere

A domain-filling, forward trajectory model originally developed for simulating stratospheric water vapor is used to simulate ozone (O3) and carbon monoxide (CO) in the lower stratosphere. Trajectories are initialized in the upper troposphere, and the circulation is based on reanalysis wind fields. In addition, chemical production and loss rates along trajectories are included using calculations from the Whole Atmosphere Community Climate Model (WACCM). The trajectory model results show good overall agreement with satellite observations from the Aura Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) in terms of spatial structure and seasonal variability. The trajectory model results also agree well with the Eulerian WACCM simulations. Analysis of the simulated tracers shows that seasonal variations in tropical upwelling exerts strong influence on O3 and CO in the tropical lower stratosphere, and the coupled seasonal cycles provide a useful test of the transport simulations. Interannual variations in the tracers are also closely coupled to changes in upwelling, and the trajectory model can accurately capture and explain observed changes during 2005–2011. This demonstrates the importance of variability in tropical upwelling in forcing chemical changes in the tropical lower stratosphere

Bulk Properties of Isentropic Mixing into the Tropics in the Lower Stratosphere

Timescales for mixing of midlatitude air into the tropical lower stratosphere are deduced from observations of long-lived tracers N2O and CCl3F. Bulk mixing between tropical and midlatitude regions is assumed to be isentropic and relatively slow compared with local mixing within each region. The mean value of the mixing timescale ranges from 12 to 18 months near 20 km. There is a tendency for shorter mixing times at higher and lower altitudes, although vertical profiles of mixing cannot be definitively established by the data. A more robust quantity is given by the fraction of midlatitude air entrained into the tropical upwelling region. Implied mixing fractions exceed 50% above 22 km