摘要:

Observations of methane, CFC‐11, and ozone losses are used along with insights from models and observations regarding interrelationships between tracers to develop a semi‐empirical framework for evaluating global ozone depletion potentials. Direct measurements of some hydrochlorofluorocarbons including HCFC‐22 in the Arctic lower stratosphere are also used to evaluate the local ozone depletion potentials there. This approach assumes that all of the observed ozone destruction in the contemporary atmosphere is due to chlorine and that the depletion is proportional to the local relative chlorine release. It is shown that the global ozone depletion potentials for compounds with relatively long stratospheric lifetimes such as HCFC‐22 and HCFC‐142b are likely to be larger than those generally predicted by gas phase chemical models, due largely to the importance of lower stratospheric ozone losses that are not simulated in gas phase studies. The analysis presented suggests that the globally averaged efficiency for ozone depletion by HCFC‐22 is as much as a factor of 2 larger than some gas phase model estimates. For compounds with short stratospheric lifetimes such as (CCl4). and (CH3CCl3), on the other hand, gas phase models likely overestimate the ozone depletion potentials for the present‐day stratosphere. Observations of polar ozone loss and reactive halogen radical abundances also imply that the globally averaged ozone depletion potentials for brominated species for the contemporary stratosphere could be as much as 1.5–3 times greater than some gas phase model predictions, depending upon lower stratospheric

ISSN:0148-0227    

DOI:10.1029/91JD02613

年代:1992

数据来源: WILEY

摘要:

This paper describes a stratospheric density and temperature retrieval experiment based on the solar occultation measurement of the Stratospheric Aerosol and Gas Experiment (SAGE II). The entire retrieval analysis involves two inversion steps: the vertical structure inversion, which derives the profile of local atmospheric extinction from SAGE II limb optical depth data, and the species inversion, which inverts the concentration of air molecules, aerosols, ozone, and nitrogen dioxide from the derived atmospheric extinction at five SAGE II short wavelengths (0.385, 0.448, 0.453, 0,525, and 0.600 μm). The inversion schemes for these steps are Twomey's (1975) modification of Chahine's nonlinear inversion algorithm and the Levenberg‐Marquardt nonlinear least squares method, respectively. The derived density profile is then used to infer the temperature distribution, assuming that the atmosphere is in hydrostatic equilibrium and obeys the ideal gas law. The retrieval analysis is illustrated in detail by using a SAGE II measurement event that occurred on September 15, 1987 (43.6°N, 4.1°W). An estimate of the error associated with the retrieved molecular density and temperature profiles based on the propagation of the instrument measurement error, the reference altitude error, and an assigned 10% error to the pressure at the top measurement level is also provided. The temperature profiles retrieved from the SAGE II observations are compared with near‐coincident, in both time and space, French Rayleigh lidar and NASA Wallops Flight Facility rocket datasonde soundings as well as the National Meteorological Center (NMC) data analyses. The results indicate that the mean SAGE II temperature agrees with the mean lidar measurement to within 2°C at altitudes from 30.5 to 52.5 km. The SAGE II and datasonde observations agree to within about 4°C in approximately the same altitude region. The mean temperature difference between the SAGE II and NMC analyses are, on average, about 1°, 4°, 2°, and 3°C at the standard pressure levels of 10, 5, 2, and 1 mbar,

ISSN:0148-0227    

DOI:10.1029/91JD02601

年代:1992

数据来源: WILEY

摘要:

The presence of a stratospheric haze layer may produce increases in both the actinic flux and the irradiance below this layer. Such haze layers result from the injection of aerosol‐forming material into the stratosphere by volcanic eruptions. Simple heuristic arguments show that the increase in flux below the haze layer, relative to a clear sky case, is a consequence of “photon trapping.” We explore the magnitude of these flux perturbations, as a function of aerosol properties and illumination conditions, with a new radiative transfer model that can accurately compute fluxes in an inhomogeneous atmosphere with nonconservative scatterers having arbitrary phase function. One calculated consequence of the El Chichon volcanic eruption is an increase in the midday surface actinic flux at 20°N latitude, summer, by as much as 45% at 2900 Å. This increase in flux in the UV‐B wavelength range was caused entirely by aerosol scattering, without any reduction in the overhead ozo

ISSN:0148-0227    

DOI:10.1029/91JD01308

年代:1992

数据来源: WILEY

摘要:

Submicron aerosols, as evidenced by the occurrence of polar mesospheric and noctilucent clouds, exist at heights from which polar mesosphere summer echoes (PMSE) are observed. We investigate the role of positively and negatively charged aerosols in the scattering processes proposed in the literature. These aerosols, if charged substantially, can account for the remarkably high radar reflectivity at both VHF and UHF by raising the electron Schmidt number through the ambipolar effect. A positively charged component may be responsible for enhanced UHF radar scatter by increasing the incoherent scatter power through a dressed dust effect, although such a process is not realistic as an explanation for VHF scatter during PMSE. Such an enhanced UHF scatter will be associated with extremely narrow backscatter spectra. We propose a model in which both negatively and positively charged aerosols are present to explain both the radar properties and the rocket probe observations of charged particle depletions. Finally, we point out that the Poker Flat 50‐MHz long‐term data, which contrary to accepted dynamical theory show average downward velocities in the summertime upper mesosphere, can be attributed to the fall speed of the aerosols responsible for P

ISSN:0148-0227    

DOI:10.1029/91JD02836

年代:1992

数据来源: WILEY

摘要:

The Stokes drift argument put forth by Coy et al. (1986) to explain the monthly mean downward vertical velocities observed in the summer polar mesosphere by radars is critically re‐examined. The size of the effect is quite sensitive to the choice of gravity wave wavenumber and phase speed. We reproduce the Coy et al. (1986) result for a monochromatic gravity wave and then generalize to a Garrett‐Munk type spectrum. This allows us to easily incorporate experimentally determined wave field parameters in order to predict a Stokes drift magnitude. The Stokes drift we calculate for mesospheric spectra reported in the literature is less than 4 cm/s. This is nearly a factor of 10 smaller than the mean June and July downward vertical velocities we have verified from 4‐year averaging of the Poker Flat, Alaska, radar data base. In this extended analysis we also find that the upward winter mean vertical velocity is smaller than previously reported but still in apparent conflict with the mesospheric winter circulation theory. We suggest, as an alternative to the Stokes drift idea, that the observed summer velocity might be the terminal velocity of charged aerosols. Such an explanation would have the advantage of relating the mean velocity observations to the large VHF radar cross section exhibited by the polar summer Mesosphere. This possible connection is discussed further in a companion

ISSN:0148-0227    

DOI:10.1029/91JD02835

年代:1992

数据来源: WILEY

摘要:

The NASA/GSFC Crustal Dynamics Project microwave water vapor radiometer (J03) is evaluated in terms of measurements of the integrated precipitable water vapor content of a particular column of the troposphere. The measurements were taken during the Atmospheric Moisture Intercomparison Study (ATMIS) held at Wallops Island, Virginia, during April 1989. Various water vapor sensing instruments were used during ATMIS, including radiometers, radiosondes, and the NASA/GSFC Raman lidar. Comparisons between water vapor measurements by the radiometer and the lidar yielded a correlation coefficient of 0.998 and rms differences for three nights of −0.2±0.2mm (April 11–12, 1989), −0.8±0.5 mm (April 16–17, 1989), and −0.4±0.3 mm (April 17–18, 1989). The integrated precipitable water vapor measurements for these three nights were approximately 5, 10, and 21 mm, respectively. The first two periods had clear meteorological conditions, while clouds were present during the third period. The lidar results during the third period are augmented with radiosonde measurements above the cloud base. This study shows that the radiometer provides accurate, continuous measurements of the water vapor integrated through the depth of

ISSN:0148-0227    

DOI:10.1029/91JD02384

年代:1992

数据来源: WILEY

摘要:

The annual net atmospheric transports of water vapor and latent heat poleward across 70°S are estimated using the latest compilation of surface mass balance for the Antarctic ice sheet and new estimates of precipitation and evaporation in sectors of the southern oceans and of seaward drifting snow transport in particular sectors of the ice sheet. The mass and energy exchange rates at the ice sheet‐atmosphere and ocean‐atmosphere interfaces are integrated strictly for areas within that latitude. The estimates of net southward water vapor transport (6.6 ± 1.3 kg m−1s−1) and latent heat transport (18.9 ± 3.6 MJ m−1s−1) are larger than reported in all preceding studies, based on atmospheric advection and moisture data collected at stations located between 66°S and 80°S, and are generally in agreement with those based on surface mass balance data and seaward drifting snow transport across the ice terminus which extends betwe

ISSN:0148-0227    

DOI:10.1029/91JD02485

年代:1992

数据来源: WILEY

摘要:

Lightning and thunderstorm observations have been made in Kathmandu, Nepal (27.4°N, 85.2°E) over the 21‐month period March 1987 to November 1988 inclusive, using direct observations of thunder and lightning, and the registrations of a CGR3 lightning flash counter that provided records of the approximate numbers of cloud flashes, positive ground flashes, and negative ground flashes with an effective range of about 14 km. The observation period effectively covered two thunderstorm seasons as almost no lightning occurred during the winter months. During 1987 and 1988 the mean thunderdays per year was 42. The principal use of the CGR3 registrations was to estimate the long‐term ratio of cloud flashes to ground flashes, denotedz; it was found thatzwas 2.1 in 1987 and 3.0 in 1988, with a two‐season mean of 2.7. The uncertainty in ratio values is about 30%. The registrations also enabled lightning flash densities to be estimated with an uncertainty of about 40%. The two‐season mean ground flash, cloud flash, and total flash densities were 2.4, 6.6, and 9.0 km−2yr−1, respectively. There were well‐marked seasonal variations in occurrence, with about 82% of all lightning occurring between mid‐February and the end of May. During this premonsoon period there were steady rises to maximum values in May of the thunderdays per month (to 10), the monthly total flash density (to 2.9 km−2month−1), and the monthly value ofz(to 3.6). The range of values ofzcalculated from the records for individual thunderstorm days was 0.17 to infinity, but very little lightning was associated with the extreme values. A distribution of values of the ratio with respect to cumulative lightning occurrence shows that 80% of all lightning was associated with values ofz(calculated on a thunderstorm day basis) between 1.3 and 6.7. There is reasonably good agreement between the value ofzfor Kathmandu and values ofzfor other places at similar latitudes. However, major differences have been found for low latitudes between recently measured values ofzand earlier predictions of the lati

ISSN:0148-0227    

DOI:10.1029/91JD02039

年代:1992

数据来源: WILEY

摘要:

A linear reference relationship between O3and N2O has been used to estimate polar winter O3loss from aircraft data taken in the lower stratosphere. Here we evaluate this relationship at high latitudes by comparing it with a two‐dimensional (2‐D) model simulation and with NIMBUS 7 satellite measurements. Although comparisons with satellite measurements are limited to January through May, the model simulations are compared during other seasons. The model simulations and the satellite data are found to be consistent with the winter O3loss analysis. It is shown that such analyses are likely to be inappropriate during other seas

ISSN:0148-0227    

DOI:10.1029/91JD02756

年代:1992

数据来源: WILEY