George C. Reid

Solar Proton Events: Stratospheric Sources of Nitric Oxide

The production of nitric oxide (NO) in the stratosphere during each of the solar proton events of November 1960, September 1966, and August 1972 is calculated to have been comparable to or larger than the total average annual production of NO by the action of galactic cosmic rays. It is therefore very important to consider the effect of solar proton events on the temporal and spatial distribution of ozone in the stratosphere. A study of ozone distribution after such events may be particularly important for validating photochemical-diffusion models.

Climatological characteristics of the tropical tropopause as revealed by radiosondes

A temporally and spatially comprehensive depiction of the tropical tropopause is presented, based on radiosonde data from 83 stations. Climatological statistics for 1961- 1990 are computed for three levels: the conventional lapse-rate tropopause (LRT), the cold-point tropopause (CPT), and the 100 hPa level. Mean values and seasonal and interannual variations of temperature, pressure, height, potential temperature, and water vapor saturation mixing ratio at these levels are compared. The tropopause is higher, colder, and at lower pressure in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH) in NH winter. This pattern reverses in NH summer, except that the tropopause remains colder in the NH than in the SH. The climatological locations of minimum tropopause temperature differ from those of maximum height and minimum pressure: In NH winter the tropopause is coldest over the western tropical Pacific warm pool region, but it is highest and at lowest pressure over the western Atlantic. Correlations of interannual anomalies in zonal-mean characteristics reveal that the height of the tropopause reflects the temperature of the underlying troposphere. Tropopause temperature, on the other hand, shows little association with tropospheric characteristics but is significantly correlated with the temperature and pressure of the lower stratosphere. The 100 hPa level is a poor surrogate for the tropical tropopause. Changes in radiosonde instrumentation limit the potential for detecting tropopause trends. However, the following (nonmonotonic) trends in the tropopause in the deep tropics during 1978 -1997 seem robust: an increase in height of about 20 m decade 21 , a decrease in pressure of about 0.5 hPa decade 21 , a cooling of about 0.5 K decade 21 , little change in potential temperature, and a decrease in saturation volume mixing ratio of about 0.3 ppmv decade 21 .

Solar total irradiance variations and the global sea surface temperature record

The record of globally averaged sea surface temperature (SST) over the past 130 years shows a highly significant correlation with the envelope of the 11-year cycle of solar activity over the same period. This correlation could be explained by a variation in the suns total irradiance (the solar “constant”) that is in phase with the solar-cycle envelope, supporting and updating an earlier conclusion by Eddy (1976) that such variations could have played a major role in climate change over the past millennium. Measurements of the total irradiance from spacecraft, rockets, and balloons over the past 25 years have provided evidence of long-term variations and have been used to develop a simple linear relationship between irradiance and the envelope of the sunspot cycle. This relationship has been used to force a one-dimensional model of the thermal structure of the ocean (Hoffert et al., 1980), consisting of a 100-m mixed layer coupled to a deep ocean and including a thermohaline circulation. The model was started in the mid-seventeenth century, at the time of the Maunder Minimum of solar activity, and mixed-layer temperatures were calculated at 6-month intervals up to the present. The total range of irradiance values during the period was about 1%, and the total range of SST was about 1°C. Cool periods, when temperatures were about 0.5°C below present-day values, were found in the early decades of both the nineteenth and twentieth centuries. There is direct evidence for the latter period from the historical SST record and some indirect evidence for the earlier cool period. While many aspects of the study are unavoidably simplistic, the results can be taken as indicating that solar variability has been an important contributor to global climate variations in recent decades. It has probably not been the only contributor, however, and in particular, the growing atmospheric burden of greenhouse gases may well have played an important role in the immediate past. This role is likely to become even more important in the near future.

On the Annual Variation in Height of the Tropical Tropopause

Abstract The existence of an annual variation in height and temperature of the tropopause over tropical regions has long been recognized, but has not been fully explained. In this paper it is proposed that the variation is a fairly direct response to the annual variation in average tropical surface insolation. The variation in insolation causes a corresponding annual cycle in average tropical sea surface temperature with a total range of order 1 K. The consequent variation in absolute humidity in turn produces an annual variation in upper tropospheric potential temperatures, and hence in the height and temperature of the tropopause. The physical link between the surface and the tropopause is provided by convection in the cores of the giant cumulonimbus clouds (hot towers) of the tropical oceanic regions, in which air parcels can achieve the maximum possible heating by release of latent heat. The process is modeled quantitatively in a simplified way, and excellent agreement is found between the predicted a...

The production of water-cluster positive ions in the quiet daytime D region

Abstract In the quiet daytime D region, the primary positive-ion species is thought to be NO + , produced by solar Lyman-alpha ionization of NO. Below the altitude of the mesopause, however, the dominant ambient species observed are water-cluster ions of the general type H + (H 2 O) n . No satisfactory reaction scheme for producing these cluster ions from NO + has yet been proposed. Following earlier suggestions, a model calculation has been carried out in which successive hydrations of NO + take place through clustering with N 2 and CO 2 , followed by “switching” reactions with H 2 O. The third hydrate of NO + is then converted into the water-cluster species H + (H 2 O) 3 , and the other water-cluster species are produced by successive clustering and thermal breakup reactions. Many of the reactions involved have not been measured in the laboratory, but reasonable estimates of their rates can be made on the basis of existing measurements of other species. Since both temperature and water-vapor content are of major importance in the model, calculations were carried out for two temperature profiles and two water-vapor profiles. It is shown that the results are in reasonably good agreement with observations as far as the water-cluster species are concerned. Under low-temperature conditions, the model predicts relatively large concentrations of various clusters of NO + , in agreement with some observations but in disagreement with others. The importance of sampling breakup of these weakly bound clusters, and their relevance to the free electron concentrations are discussed.

Effects of electron and ion reactions on atmospheric lifetimes of fully fluorinated compounds

Atmospheric lifetimes are evaluated for the fully fluorinated compounds CF4, C2F6, c-C4F8, C6F14, and SF6 using a two-dimensional transport and chemistry model which includes removal by electrons and ions in the mesosphere and lower thermosphere. Laboratory measurements of the pertinent reaction rates were carried out at thermal energy for free electrons and for the atmospheric ions O+, O2+, O−, O2−, NO+, H3O+, NO3−, and CO3−. Atmospheric removal by electrons reduces the lifetimes of c-C4F8 and SF6 from about 3200 years to 1400 and 800 years, respectively, only if the respective product anions C4F8− and SF6− do not subsequently regenerate the parent neutral compounds. Atmospheric removal by ion reactions is minor or negligible, with the largest effect (∼5%) being removal of C6F14 by O2+. Removal of CF4 and C2F6 by O+ is probably the most important single destruction process in the atmosphere for these two compounds, but their lifetimes are governed by removal at the Earths surface in high-temperature combustors. While we show that the lifetimes of c-C4F8 and SF6 may be significantly shorter than previously estimated, these compounds remain extremely long-lived with significant global warming potentials.

Ice Clouds at the Summer Polar Mesopause

Abstract Recent satellite observations have shown the existence of a persistent layer of light-scattering particles in the vicinity of the polar mesopause during the summer. The suggestion has been made that this layer consists of ice particles, and that noctilucent clouds are a sporadic manifestation of the layer near its low-latitude edge. The consequences of this proposal in terms of the water vapor content of the mesosphere are explored in this paper through the development of a model for such a cloud, in which the water vapor mixing ratio is assumed to be 1 to 10 ppm at 60 km, and the temperature is assumed to drop to values well below 140 K at the mesopause. Water vapor is transported upward by eddy diffusion, and is photodissociated by solar ultraviolet radiation. Sublimation nuclei of radius 10A are assumed to exist in the vicinity of the mesopause, and the growth of the ice particles and their terminal speed of descent are calculated from simple kinetic theory considerations. The results indicate...

Response of the middle atmosphere to the solar proton events of August-December, 1989

Intense solar activity during the period August-December 1989 gave rise to several major energetic-particle events, which were accompanied by greatly enhanced ionization rates and NO{sub y} production in the polar regions of both hemispheres. A two-dimensional model of the chemistry and dynamics of the middle atmosphere has been used to calculate the production and subsequent fate of the NO{sub y} and its effect on ozone concentrations and temperatures. In the sunlit southern polar cap, NO increases as large as a factor of 20 are estimated near 60 km altitude, with column density enhancements of 55%. Corresponding peak ozone depletions of about 20% are calculated near 40 km in late October 1989, with predicted temperature decreases of about 3-3.5 K. Effects in the northern polar regions are considerably smaller, due to the lack of sunlight during the peak phase of the events.

Solar Forcing of Global Climate Change Since The Mid-17th Century

Spacecraft measurements of the suns total irradiance since 1980 have revealed a long-term variation that is roughly in phase with the 11-year solar cycle. Its origin is uncertain, but may be related to the overall level of solar magnetic activity as well as to the concurrent activity on the visible disk. A low-pass Gaussian filtered time series of the annual sunspot number has been developed as a suitable proxy for solar magnetic activity that contains a long-term component related to the average level of activity as well as a short-term component related to the current phase of the 11-year cycle. This time series is also assumed to be a proxy for solar total irradiance, and the irradiance is reconstructed for the period since 1617 based on the estimate from climatic evidence that global temperatures during the Maunder Minimum of solar activity, which coincided with one of the coldest periods of the Little Ice Age, were about 1 °C colder than modern temperatures. This irradiance variation is used as the variable radiative forcing function in a one-dimensional ocean–climate model, leading to a reconstruction of global temperatures over the same period, and to a suggestion that solar forcing and anthropogenic greenhouse-gas forcing made roughly equal contributions to the rise in global temperature that took place between 1900 and 1955. The importance of solar variability as a factor in climate change over the last few decades may have been underestimated in recent studies.

Ion Chemistry in the D Region

Publisher Summary This chapter summarizes the current understanding of the ion chemistry of the D region of the ionosphere. The ionosphere is defined as that region of the Earths upper atmosphere in which there exists a substantial plasma component at all times. High frequency radio waves are transmitted by reflections from the E and F regions of the ionosphere, but it was realized at an early stage in ionospheric research that some underlying ionization must exist to account for the absorption of high frequency radio waves and for the transmission of radio waves of low and very low frequencies. Naturally enough this underlying ionized region became known as the D region. In terms of altitude, the D region generally lies between an upper boundary in the neighborhood of 90 km and an ill-defined lower boundary that can be taken as about 60 km for most practical purposes. The ion chemistry and the neutral chemistry of the D region cannot be decoupled, and that, in fact, any quantitative discussion of the ion chemistry is critically dependent on the concentrations of certain minor neutral constituents of the atmosphere.