bluuebunny/crossref_metadata_2025 huggingface dataset:
A model study on internally generated variability in subtropical mode water formation
Authors: Hazeleger W. , Drijfhout S. S.
Published: September 2002

Internally generated variability in the subtropical gyre is studied as a possible mechanism for the observed interannual to decadal variability in subtropical mode water formation. An isopycnic ocean model with idealized geometry and forcing which mimics the North Atlantic subtropical gyre is used for this purpose. The horizontal resolution is sufficiently high and the friction and diffusion sufficiently low for the flow to become barotropically and baroclinically unstable. Two modes of low‐frequency variability are found. Both modes consist of westward propagating thickness anomalies. The anomalies have a first baroclinic modal structure with a maximum amplitude at the thermocline. One mode has a timescale of 8 years and a basin wide spatial scale; the other has a timescale of 4.5 years and a smaller spatial scale. The modes appear to be damped when the diffusion is high. In that case, the 8‐year mode can be excited by a spatially coherent stochastic wind stress. The evolution of the modes is determined by an interaction between the mean flow and the low‐frequency variability itself. The modes are instabilities of the mean flow determined by the basic stratification. It appears that coupling to the atmosphere and a parameterization of surface mixing are necessary for the low‐frequency variability to appear in the mixed layer. The coupling and surface mixing do not play a role in generating the modes. It is concluded that these internally generated modes may play a role in the observed variability in mode water formation.
Quasi‐biennial variability in the Japan Sea
Authors: Hirose Naoki , Ostrovskii Alexander G.
Published: September 2002

The TOPEX/Poseidon (T/P) altimetry reveals quasi‐biennial (QB) variability in the southern Japan Sea. Sea surface height anomalies of a biennial nature are most energetic in the Yamato Basin, the southeastern most part of the Japan Sea, where they can be as large as 20 cm and extend for 100–200 km. On the basis of the in situ measurements of Maizuru Marine Observatory, the 2–3 year variations are associated with thermohaline anomalies in the upper 300 m layer. The local QB oscillation is studied with a reduced gravity model of the Japan Sea. The model is forced by European Centre for Medium‐Range Weather Forecasting daily wind and seasonal inflows/outflows through the three major straits of the sea. The model sensitivity experiments suggest that the QB variability can be associated with the sea's response to wind forcing of particular years, more specifically, 1992 and 1996, and to some extent, 1994. An approximate Kaiman filter is employed for assimilation of the T/P altimeter data into the reduced gravity model. It filters out observational noise and intraseasonal sea level variability and allows the model to dynamically interpolate T/P observations. The results of the assimilation indicate that the QB anomalies are strongest at 37.5°N, 134.5°E and propagate west‐northwest with a speed of ∼0.01 m s‐1, contributing to variations of the Tsushima Warm Current.
A computer model study of multiphase chemistry in the Arctic boundary layer during polar sunrise
Authors: Michalowski Brian A. , Francisco Joseph S. , Li Shao‐Meng , Barrie Leonard A. , Bottenheim Jan W. , Shepson Paul B.
Published: September 2002

A multiphase chemical box model of Arctic halogen chemistry has been developed using a PC‐based modeling program developed by Environment Canada called the Chemical Reactions Modeling System (CREAMS). The multiphase model contains 125 gas phase reactions, 19 photolysis reactions, and 16 aqueous reactions occurring in suspended aerosol particles and the quasi‐liquid component of snow. The model simulates mass transfer of species between the gas phase and particles, and between the gas phase and the snowpack. Model simulations were conducted for the Arctic for the period April 16 to April 24 at 245 K within a 400 m boundary layer. The complete model simulates halogen‐catalyzed ozone depletion within 5 days from the start of the model run, via known gas and heterogeneous phase activation mechanisms. A critically important model reaction is BrO + HCHO → HOBr + CHO, which has a substantial impact on gas phase HOBr, and subsequent condensed phase chemistry. When coupled with a necessary snowpack efflux of aldehydes, required to maintain the aldehyde concentrations at observed levels, the new BrO chemistry has a significant impact on the concentrations of gas phase bromine species, particle bromide, and chlorine atoms, through chemistry occurring in the snowpack. We also find that O3 depletion cannot be simulated without the presence of heterogeneous halogen chemistry occurring in the snowpack and that the rate of O3 depletion is limited by the mass transfer rate of HOBr to the snowpack.
Migration of methane sulphonate in Antarctic firn and ice
Authors: Pasteur Elizabeth C. , Mulvaney Robert
Published: September 2002

We investigate the seasonal relationship of the sulphur‐bearing anions methane sulphonate (MSA−) and non‐sea‐salt sulphate (nssSO42−) in sections of firn and ice cores from the Antarctic Peninsula and Weddell Sea region of Antarctica. In cores from Dolleman Island and Berkner Island, MSA− has clearly migrated from the summer snow layer, where it is initially deposited, to become concentrated in the winter layer. A similar behavior is evident in a core from the Dyer Plateau, though in deeper layers. Cores from Gomez Nunatak and Beethoven Peninsula show little evidence of relocation of MSA−, though migration at greater depth in the ice sheet cannot be ruled out. In contrast, in all these cores, non‐sea‐salt sulphate remains in the summer layer. From comparisons between the ice core characteristics and the migration behavior at these sites, we conclude that the movement of MSA− does not occur via percolation and refreezing of meltwater. Simple concentration‐driven diffusion is also not a factor, as the MSA− peaks are sharp in the winter layer. The data presented indicate that the movement of MSA− in firn is likely to be linked to the concentration of other ionic species in the snowpack and to the snow accumulation rate. A possible mechanism for the migration of MSA− in the snowpack is via an initial diffusion in the liquid or vapor phase, which is halted by trapping in the winter layer when the MSA− forms a salt with a cation.
Cumulus transport of chemical tracers: 1. Cloud‐resolving model simulations
Authors: Lu Rong , Lin Chichung , Turco Richard , Arakawa Akio
Published: September 2002

Convective transport in the tropics and middle latitudes is a critical mechanism leading to the vertical and, eventually, the horizontal redistribution of chemical tracers throughout the lower atmosphere. Accordingly, a better understanding and accurate representation of convective transport are essential in simulating tracer distributions and long‐range dispersion in chemical transport models. We employ the UCLA cloud‐resolving model (CRM) to simulate a 5‐day period during the GATE phase III experiment, during which several mesoscale convective systems were observed. The dynamical structures and thermodynamic balances of the simulated versions of these events compare favorably with observations. The distributions of six tagged tracers originating at different levels in the atmosphere are calculated and analyzed. This represents the first in‐depth analysis of regional dispersion over a multi‐day period utilizing multiple tracers in a high‐resolution model. It is found that the dominant process affecting tracer transport by individual clouds involves convective updrafts and related downdrafts. Entrainment, anvil cloud outflow, and detrainment at various levels all have important effects in different regions of the troposphere. In addition to the convective‐scale transport, mesoscale subsidence induced by deep convection is capable of moving chemical tracers downward as much as several hundred hPa during a single deep convective event, which can substantially modify tracer distributions in the middle and lower troposphere. On the basis of our analysis we conclude that deep convection profoundly affects the distribution of tropospheric chemical tracers in at least three fundamental ways: (1) by quickly and directly transporting tracers from the boundary layer to the upper troposphere, (2) by accelerating downward transport of chemical tracers from the upper troposphere to the surface, both locally in downdrafts and over broader areas via subsidence, and (3) by inducing the exchange of air between the upper troposphere and the lower stratosphere.
Boundary layer vertical exchange processes and the mass budget of ozone: Observations and model results
Authors: Berkowitz Carl M. , Fast Jerome D. , Easter Richard C.
Published: September 2002

An Eulerian chemical model is used to assess the relative importance of a variety of processes associated with producing high surface ozone episodes during selected periods of the North American Research Strategy for Tropospheric Ozone (NARSTO) 1995 field campaign over the northeastern United States. A comparison of the observed and predicted hourly surface ozone mixing ratios showed that the model qualitatively reproduced the observed ozone trends over the northeastern United States. The model had a mean hourly bias of ∼15 ppb with a wide range of skill compared to individual observing stations clustered around one node. The simulated mass budget tendency terms are compared for days with low ozone values immediately followed by days with high values. The later days showed observed and simulated ozone mixing ratios aloft to be of order twice that found on preceding days, although the associated chemical mix appeared to have relatively little potential for the subsequent generation of “new” ozone. Under conditions of shallow mixing over urban regions, simulated surface ozone production rates were negative (a net loss) throughout much of the day with convective mixing bringing newly produced ozone from aloft to the surface. It is noted that surface ozone levels appeared to be relatively insensitive to mixing layer growth rates.
A cloud‐scale model study of lightning‐generated NOxin an individual thunderstorm during STERAO‐A
Authors: DeCaria Alex J. , Pickering Kenneth E. , Stenchikov Georgiy L. , Scala John R. , Stith Jeffrey L. , Dye James E. , Ridley Brian A. , Laroche Pierre
Published: September 2002

Understanding lightning NOx(NO+NO2) production on the cloud scale is key for developing better parameterizations of lightning NOxfor use in regional and global chemical transport models. This paper attempts to further the understanding of lightning NOxproduction on the cloud scale using a cloud model simulation of an observed thunderstorm. Objectives are (1) to infer from the model simulations and in situ measurements the relative production rates of NOxby cloud‐to‐ground (CG) and intracloud (IC) lightning for the storm; (2) to assess the relative contributions in the storm anvil of convective transport of NOxfrom the boundary layer and NOxproduction by lightning; and (3) to simulate the effects of the lightning‐generated NOxon subsequent photochemical ozone production. We use a two‐dimensional cloud model that includes a parameterized source of lightning‐generated NOxto study the production and advection of NOxassociated with a developing northeast Colorado thunderstorm observed on July 12, 1996, during the Stratosphere‐Troposphere Experiment—Radiation, Aerosols, Ozone (STERAO‐A) field campaign. Model results are compared with the sum of NO measurements taken by aircraft and photostationary state estimates of NO2in and around the anvil of the thunderstorm. The results show that IC lightning was the dominant source of NOxin this thunderstorm. We estimate from our simulations that the NOxproduction per CG flash (PCG) was of the order of 200 to 500 mol flash−1. NOxproduction per IC flash (PIC) appeared to be half or more of that for a CG flash, a higher ratio ofPIC/PCGthan is commonly assumed. The results also indicate that the majority of NOx(greater than 80%) in the anvil region of this storm resulted from lightning as opposed to transport from the boundary layer. The effect of the lightning NOxon subsequent photochemical ozone production was assessed using a column chemical model initialized with values of NOx, O3, and hydrocarbons taken from a horizontally averaged vertical profile through the anvil of the simulated storm. The lightning NOxincreased simulated ozone production rates by a maximum of over 7 ppbv d−1in the upper troposphere downwind of this storm.
Estimating the effect of clouds on the arctic surface energy budget
Authors: Beesley John A.
Published: September 2002

The relationship between cloudiness and sea ice thickness over the Arctic Ocean is examined using an idealized equilibrium energy budget model and a single‐column model (SCM) forced by incoming solar radiation, atmospheric poleward energy flux, oceanic heat flux, and sea ice divergence. The sensitivity of ice thickness to cloud perturbations in the SCM is qualitatively different from previous modeling studies, which did not allow the atmospheric temperature profile to respond to perturbations in cloudiness. It is shown that this restriction implicitly changes the poleward energy flux in model experiments and obscures the intrinsic effect of clouds on the arctic climate. In the present model, equilibrium ice thickness has a strong negative correlation with cloud radiative forcing at the top of the atmosphere and no correlation with cloud radiative forcing at the surface. The results demonstrate how local processes, such as the surface albedo feedback, affect the sensitivity of ice thickness to cloud perturbations and reduce its predictability. Exploratory model experiments suggest that interactions between the Arctic and the lower latitudes (via the poleward energy flux) could strongly influence the dependence of ice thickness on cloudiness. The impact of these interactions in the model was predicted qualitatively by considering how the radiative effect of clouds is partitioned between the atmosphere and the surface. The results demonstrate the importance of using cloud radiative forcing at both the surface and the top of the atmosphere when estimating the climatic effect of perturbation in cloudiness.
Tropospheric temperature variations adjusted for El Niño, 1958–1998
Authors: Angell James K.
Published: September 2002

The El Niño‐Southern Oscillation (ENSO) is not an extrinsic forcing mechanism on the atmosphere, so its impact thereon cannot be cleanly removed. Nevertheless, it is of interest to estimate what atmospheric temperatures would have been if ENSO had not been operating. This is done here by using a 63‐station global radiosonde network to find the relation, for the period 1958–1998, between sea‐surface temperature (SST) in the El Niño region of the eastern equatorial Pacific (Nino3 SST) and tropospheric (850–300 mbar) temperature. Seasonal lag correlations for the 41‐year interval show maximum (and significant) correlations of 0.67, 0.57, and 0.34 between Nino3 SST and the 850 to 300‐mbar temperature in tropics, globe, and north temperate zone, respectively, two seasons later. Accordingly, the annual 850 to 300‐mbar temperatures for those regions are adjusted (effect of Nino3 SST on them minimized) by means of the linear least squares relation between the annual values of 850 to 300‐mbar temperature and annual values of Nino3 SST shifted two seasons earlier, omitting the years 1964, 1983, and 1992 following Agung, El Chichon, and Pinatubo eruptions, as well as the anomalously warm year of 1998. In 1998 the global 850 to 300‐mbar temperature is a record 0.7 K above the 1961–1990 average but, after adjustment in the above manner for the strong 1997–1998 El Niño, only 0.2 K above this average and the twelfth warmest year of record. During 1958–1998 the global 850 to 300‐mbar layer warms by 0.10 K/decade, but after adjustment for ENSO events the warming is 0.07 K/decade, so that about one third of the global tropospheric warming is related to ENSO. The adjustment provides a different perspective on the significance of anomalously warm years, as well as warming trends, to the issue of climate change.
Rainfall controls on evaporation at the regional scale: An example from the Sahel
Authors: Taylor Christopher M. , Blyth Eleanor M.
Published: September 2002

The validation of general circulation model (GCM) land surface schemes requires observations of energy and water fluxes averaged over an area of a typical GCM grid cell (100 km×100 km) for a time period long enough to include the typical climatic conditions in that area. To create such a consolidated data set, soil moisture and surface evaporation are estimated with a detailed model of the surface energy and water balance. Observations from the Hydrological Atmospheric Pilot Experiment in the Sahel (HAPEX‐Sahel) in Niger, West Africa, have shown that rainfall is a key source of spatial variability in the water and energy balance in semi arid regions. The model is therefore run for two wet seasons, forced by observations from a dense rain gauge network over an area of 10,000 km2. Analysis of the estimated surface fluxes indicates that bare soil evaporation has a strong influence on the spatial variability of evaporation during the wet season. On the other hand, leaf‐level transpiration is relatively uniform and unstressed by soil moisture for most of the period. Modest spatial variability in transpiration develops through rainfall‐induced contrasts in growth rates. The representation in GCMs of these moisture controls on evaporation can be problematic. In this case, where leaf area index is < 1, transpiration is well‐represented by areal mean surface properties. However, an interactive growth model may be a useful component of the surface scheme when considering the large interannual rainfall variability typical of the Sahel. It is also found that the effect of spatial variability due to subgrid rainfall can not be ignored when modeling interception and bare soil evaporation. Methods of calculating mean fluxes based on mean moisture variables are therefore proposed. Using such a subgrid scheme, total evaporation errors are reduced by 50%.
Radiative effects of absorbing aerosols and the impact of water vapor
Authors: Kay Merlinde J. , Box Michael
Published: September 2002

The radiative effects (both forcing at the tropopause and absorption in the lower troposphere) of standard aerosol models are examined as a function of relative humidity. Several of the models are also modified by the inclusion of additional soot, in line with recent observations. Increasing relative humidity causes many aerosol types to expand and also increases their single‐scattering albedo. We have examined the impacts of these changes and also the interaction between aerosol growth and absorption of solar radiation by water vapor. These effects are seen not to be additive, especially in the case of flux divergence due to absorbing aerosols.
Isotopic evidence of trace metal sources and transport in eastern Canada as recorded from wet deposition
Authors: Simonetti Antonio , Gariépy Clément , Carignan Jean , Poissant Laurier
Published: September 2002

Abundances of heavy metals and Pb and Sr isotope ratios have been determined in precipitation collected between December 1997 and June 1999 at St. Anicet and L'Assomption, near Montréal, Canada. Median enrichment factors for the heavy metals, relative to upper crustal abundances, are all »10 indicative of an anthropogenic origin. The 206Pb/207Pb values exhibit a large variation at both stations, with values of 1.170–1.190 and 1.130–1.150 recorded during the winter and summer seasons, respectively. The Pb isotope systematics defined by most of the samples from both stations may be explained by mixing of anthropogenic emissions from U.S. and Canadian sources. Comparison of Pb isotope ratios and heavy metal abundances between the same, individual precipitation events from both stations, however, do not systematically yield similar values and may be attributable to significant, local anthropogenic sources. The 87Sr/86Sr values for precipitations also exhibit a seasonal variation at both stations with the least radiogenic values being recorded, in general, during the winter months (<0.7085). These are possibly related to increased atmospheric emissions during the winter season from coal‐fired power plants located in North America. For the remaining months the 87Sr/86Sr values are either variable (0.7085–0.7100) at St. Anicet or constant (0.7085–0.7088) at L'Assomption. The latter possibly reflect Sr anthropogenic emissions from urban activity associated with Montreal.
Auroral effects on the gas phase chemistry of meteoric sodium
Authors: Heinselman Craig J.
Published: September 2002

The effects of auroral ionization on the gas phase chemistry of meteoric sodium in the upper atmosphere are considered via computer simulation and with a case study. We find that under some circumstances, auroral ionization can significantly affect the chemical state of sodium. In particular, at auroral altitudes, where recently deposited material or sudden atom layers are typically found, neutral Na can be ionized via charge transfer reactions with aurorally enhanced ionization products. The time constants for these reactions are of the order of 15 min. Measurements collected at the Sondrestrom Facility, in Greenland, are analyzed, and a case study is presented for which this gas phase chemistry is sufficient to explain the results. In addition to supporting the predicted influence of auroral ionization on neutral sodium, this case study is also consistent with a chemical mechanism for the formation of a sporadic atom layer.
Stratospheric sudden warmings and slowly propagating zonal‐mean zonal wind anomalies
Authors: Kodera Kunihiko , Kuroda Yuhji , Pawson Steven
Published: September 2002

The relationship between stratospheric sudden warming (SSWs) and slowly propagating zonal wind anomalies during the northern hemisphere cold season (November to March) is investigated using two leading empirical orthogonal functions (EOFs) of 15‐day mean zonal‐mean zonal wind. Poleward and downward propagation of the zonal‐mean zonal wind anomalies can be translated as rotation of state vector in a plane spanned by the two EOFs. Major SSWs occur in phase with the slowly propagating zonal wind anomalies, which implies a “conditioning” of the atmosphere by the slowly varying state. Movement of the state vector suggests quasi‐periodicity of the order of 4 months, although generally only one cycle is achieved during a major warming winter. This characteristic feature can be compared to the stratospheric vacillation cycle in a mechanistic model. It is also shown that changes in the tropospheric planetary waves associated with the slowly propagating zonal wind anomalies coincide with changes in the Arctic Oscillation.
Microwave Limb Sounder observations of gravity waves in the stratosphere: A climatology and interpretation
Authors: McLandress Charles , Alexander M. Joan , Wu Dong L.
Published: September 2002

High‐horizontal‐resolution temperature data from the Microwave Limb Sounder (MLS) are analyzed to obtain information about high intrinsic frequency gravity waves in the stratosphere. Global climatologies of temperature variance at solstice are computed using six years of data. A linear gravity wave model is used to interpret the satellite measurements and to infer information about tropospheric wave sources. Globally uniform sources having several different spectral shapes are examined and the computed variances are filtered in three‐dimensional space in a manner that simulates the MLS weighting functions. The model is able to reproduce the observed zonal mean structure, thus indicating that the observations reflect changes in background wind speeds and provide little information about the latitudinal variation of wave sources. Longitudinal variations in the summer hemisphere do reflect source variations since the modeled variances exhibit much less variation in this direction as a consequence of the zonal symmetry of the background winds. A close correspondence between the MLS variances and satellite observations of outgoing‐longwave radiation suggests that deep convection is the probable source for these waves. The large variances observed over the tip of South America in winter are most certainly linked to orographic forcing but inferences about wave sources in Northern Hemisphere winter are difficult to make as a result of the high degree of longitudinal and temporal variability in the stratospheric winds. Comparisons of model results using different source spectra suggest that the tropospheric sources in the subtropics in summer have a broader phase speed spectrum than do sources at middle latitudes in winter.
Wind contributions to the Earth's angular momentum budgets in seasonal variation
Authors: Aoyama Yuichi , Naito Isao
Published: September 2002

The atmospheric contributions to seasonal variations in length of day (LOD) and wobble are discussed for the period 1988–1997. The data sets used here are SPACE97 and EOP97C04 for LOD and wobble, respectively. Two atmospheric angular momentum (AAM) functions are calculated from the operational objective analysis data of the Japan Meteorological Agency (JMA) and the reanalysis data of the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR). Both the axial AAM functions agree well in annual variation and roughly in semiannual variation, with the function inferred from the observed LOD. The two equatorial AAM functions show the different wind contributions to the function inferred from the observed wobble, in both annual and semiannual variations, which are attributable in part to the different vertical wind integration methods between NCEP/NCAR and JMA. The equatorial tropospheric wind contributions, calculated by the same method, are found to show discrepancies arising from the different tropospheric regional winds associated with the Asian monsoon.
Charge and mass transfer in ice‐ice collisions: Experimental observations of a mechanism in thunderstorm electrification
Authors: Mason B. L. , Dash J. G.
Published: September 2002

A microphysical mechanism that may be important in the electrification of thunderstorms is probed in an experimental study of collisional charging of ice surfaces. The study examined mass and charge transfer in single collisions between two ice samples and their dependence on growth, temperature, impact strength, and time delay between impacts. The polarity is consistent with earlier results in that the more rapidly growing particle charges positively. We find that the magnitudes of charge and mass transfer increase with growth rate over an appreciable range, but the charge tends to saturate at high rates. The systematic dependence indicates that the charge transfer is more closely related to the difference in growth rate between the contacting surfaces rather than to their temperature difference. This result provides quantitative confirmation of earlier researchers' work. An appreciable time delay is required for the magnitude of the charge to recover from a previous collision. The magnitude of charge transfer cannot be explained by the thermoelectric effect or by any other mechanism proposed to date. It appears that the charge is carried in the mass that is transferred between the colliding surfaces, as predicted by Baker and Dash, [1989, 1994] but the magnitude of the mass transfer is much greater than can be due to ordinary surface melting, as proposed.
Observations and models of irradiance profiles, column transmittance, and column reflectance during the Atmospheric Radiation Measurements Enhanced Shortwave Experiment
Authors: Pope Shelly K. , Valero Francisco P. J.
Published: September 2002

In the fall of 1995 the Atmospheric Radiation Measurement Enhanced Shortwave Experiment (ARESE) was conducted to address fundamental questions about the amount of absorption of solar radiation occurring in clear and cloudy skies. Irradiances measured from instruments flown on stacked aircraft as part of this experiment are compared to modeled irradiances for several representative days. As a basic check of the data and the model, the downwelling irradiances at 13 km are examined and found to be in good agreement, with both model and observations showing day‐to‐day variations as well as shorter timescale variations due to changing atmospheric conditions. Evidence of cloud‐induced enhanced absorption is seen from an analysis of column reflectance and absorptance as determined from simultaneous measurements made from the stacked aircraft. When compared to model results, profiles of irradiance versus altitude as measured by one aircraft indicate cloud‐induced enhanced absorption as well.
Comparison of model and observed regional temperature changes during the past 40 years
Authors: Russell Gary L. , Miller James R. , Rind David , Ruedy Reto A. , Schmidt Gavin A. , Sheth Sukeshi
Published: September 2002

Results are presented for six simulations of the Goddard Institute for Space Studies (GISS) global atmosphere‐ocean model for the years 1950–2099. There are two control simulations with constant 1950 atmospheric composition from different initial states, two greenhouse gas (GHG) experiments with observed greenhouse gases up to 1990 and compounded 0.5% CO2 annual increases thereafter, and two greenhouse gas plus sulfate (GHG+SO4) experiments with the same varying greenhouse gases plus varying tropospheric sulfate aerosols. Surface air temperature trends in the two GHG experiments (with the control simulations climate drift subtracted out) are compared between themselves and with changes in the observed temperature record between 1960 and 1998. All comparisons show high positive spatial correlation in the Northern Hemisphere except in summer when the greenhouse signal is weakest. The GHG+SO4 experiments show weaker correlations. In the Southern Hemisphere, correlations between any experiments or observations are either weak or negative, in part because the model's interannual variability of southern sea ice cover is unrealistic. The model results imply that temperature changes due to forcing by increased greenhouse gases have risen above the level of regional interannual temperature variability in the Northern Hemisphere over the past 40 years. This period is thus an important test of the reliability of coupled climate models.
Retrieval of aerosol size distributions and integral properties from simulated extinction measurements at SAGE III wavelengths by the linear minimizing error method
Authors: Yue Glenn K.
Published: September 2002

The retrieval of aerosol size distribution and integral properties from simulated aerosol extinction measurements of the new satellite instrument, the Stratospheric Aerosol and Gas Experiment (SAGE) III, is investigated. The retrieval method used in this study is the newly proposed linear minimizing error (LME) method. This approach assumes that the aerosol size distribution can be approximated by a histogram of number density as a function of particle size. The retrieved number density in each bin and the integral properties, including surface area and volume densities, are expressed as a linear combination of the SAGE III aerosol extinction coefficients at eight or fewer wavelengths. The coefficients in the weighted linear combination are obtained by minimizing the average of retrieval errors over a set of testing size distributions. The choice of aerosol channels for retrieval was first determined by comparing the average composite errors of the integral properties retrieved from different combinations of aerosol extinctions. The accuracy of the proposed method was studied by comparing the actual and retrieved properties for six aerosol size distributions. Results of this investigation indicate that unimodal, as well as bimodal, lognormal size distributions can be retrieved with reasonable accuracy. When all eight aerosol extinctions are available for retrieval, the average composite errors, which include systematic errors and errors due to measurement uncertainties for the retrieval of surface area and volume densities, are 9.4±4.3 and 4.7±2.0%, respectively. The retrieved aerosol integral properties are more accurate than those retrieved from SAGE II. However, it is not advantageous to use all eight SAGE III wavelengths for retrieval. The proper choice of aerosol extinctions for retrieval is elucidated. The derived formulas listed in this paper can be easily used by investigators to retrieve aerosol integral properties and estimate the retrieval errors from SAGE III aerosol extinctions at different altitudes for a variety of purposes. The application of the LME method to instrumental design is discussed.
Intercalibration of the Meteosat‐7 water vapor channel with SSM/T‐2
Authors: Sohn B. J. , Schmetz J. , Tjemkes S. , Koenig M. , Lutz H. , Arriaga A. , Chung E. S.
Published: September 2002

A new method has been developed to intercalibrate the Meteosat‐7 water vapor (WV) channel (5.7–7.1 μm) with collocated and calibrated satellite radiances at 183.3±1 GHz from the microwave instrument SSM/T‐2. With radiative transfer calculations it is shown that this microwave channel is sensitive to radiation from the upper troposphere, very similar to the Meteosat WV channel. Radiative transfer simulations with a set of representative climatological profiles for both channels provide a basis for the relation between Meteosat WV and SSM/T‐2 183.3±1 GHz brightness temperatures. The intercalibration method is used in a case study to assess a systematic error of the Meteosat‐7 WV channel which rather relies on vicarious calibration technique. Results for the limited period under investigation suggest that the operational calibration coefficient for the Meteosat‐7 WV channel is positively biased by about 12%, causing a bias of about 3°K in brightness temperature. It is shown that such a bias would have a significant impact on the retrieval of upper tropospheric humidity (UTH) with relative errors up to 30%.
Middle atmosphere age of air in a globally balanced two‐dimensional model
Authors: Zhu Xun , Yee Jeng‐Hwa , Strobel Darrell F.
Published: September 2002

We extend the concepts of the mean age derived from the age spectrum and the age of air derived from the time lag in the middle atmosphere by specifying the tracer source gases uniformly in the lower troposphere. The concepts are illustrated by use of both a time‐independent one‐dimensional diffusive model and a globally balanced two‐dimensional middle atmosphere model. We quantitatively examine several factors that may cause the difference between the mean age and the age of air. It is found that the mean age and the age of air can differ by any amount in general for a time‐dependent transport operator. For a time‐independent transport operator the age of air derived by integrating a model for a finite time is less than the mean age. The age of air at the stratopause is about 5 to 6 years in our globally balanced two‐dimensional middle atmosphere model. In the mesosphere the age of air changes with season significantly due to the reversal of the mesospheric meridional circulation between the summer and the winter. With a chemical lifetime of SF6 above 30 km being ∼38 years our two‐dimensional model shows no signs of overestimate of age of air for SF6 at 30 km. It is also demonstrated that stratospheric mean age is sensitive to the horizontal mixing in the troposphere. Weak horizontal mixing in the troposphere will reduce the mean age in the upper stratosphere but increase the mean age in the high‐latitude lower stratosphere. The former is caused by diminishing the recycling of air parcels between the troposphere and stratosphere, and the latter is caused by minimizing the portion of tracers directly transported upward from the high‐latitude troposphere into the stratosphere.
Exhumation during a continental collision inferred from the tectonometamorphic evolution of the Alpujarride Complex in the central Betics (Alboran Domain, SE Spain)
Authors: Azañón José‐Miguel , Crespo‐Blanc Ana
Published: September 2002

The tectonometamorphic evolution of the Alpujarride Complex within the internal zones of the Betics reveals a complex structural and metamorphic succession of continental collision, synmetamorphic exhumation, nappe forming in the final stage of exhumation, and rifting. A complex tectonic evolution is deduced from the superposition of structures observed in several Alpujarride units of the central Betics together with their metamorphic record. The following sequence of events is suggested: (1)A first stacking event (D1) is deduced from the presence of pre‐S2 high‐pressure‐low‐temperature metamorphic assemblages, (2) The synmetamorphic exhumation is inferred from the almost isothermal decompression pressure‐temperature paths during which the S2 main foliation developed; the condensation of the mineral zones perpendicularly to the S2 foliation suggests large‐scale vertical shortening during event D2, (3) A new stacking event (D3) is inferred from post‐metamorphic thrusts and nappes, which are probably associated with kilometer‐scale recumbent folds affecting the S2 main foliation, (4) A thinning event (D4), revealed by extensional fault systems, is associated with crustal spreading on the concave side of the Gibraltar Arc, which resulted in the opening of the Alboran Basin from the early Miocene (Burdigalian), (5) From late Tortonian to Pliocene a continuous N‐S to NW‐SE compression took place (D5). Stratigraphic, paleontological, and geochronological data suggest that event D3 could be lower Miocene in age (Aquitanian); consequently, event D1 and D2 would be pre‐Miocene.
Using meta‐analysis for benefit transfer: In‐sample convergent validity tests of an outdoor recreation database
Authors: Rosenberger Randall S. , Loomis John B.
Published: September 2002

The application of metaregression analysis models for the purpose of benefit transfer is investigated using in‐sample convergent validity tests on average value transfers. The database on which the metaregression analysis models are developed is composed of empirical outdoor recreation use value studies conducted from 1967 through 1998. Results of the convergent validity tests suggest that the national model is slightly more robust to changes in application than the Census Region models. The results suggest that the application of meta‐analysis for benefit transfers is promising considering limitations imposed by inconsistent data reporting of original studies.
Force‐restore technique for ground surface temperature and moisture content in a dry desert system
Authors: Jacobs Adrie F. G. , Heusinkveld Bert G. , Berkowicz Simon M.
Published: September 2002

The level of the surface temperature as well as surface moisture content is important for the turbulent transports of sensible and latent heat, respectively, but this level is also crucial for the survival of shrubs, plants, insects, and small animals in a desert environment. To estimate the surface conditions, the force‐restore technique for surface temperature as well as surface moisture has been applied under extreme dry desert condition. A simple physical model was constructed in which the force‐restore technique for surface moisture has been included. This model simulates the accumulated dew amounts as well as the early morning drying process. A measurement program carried out in the northwestern Negev desert of Israel (Nizzana) gathered data on daily surface temperature, daily dew deposition amounts, and micrometeorological conditions during the dew formation and drying process. Actual dew amounts were assessed using microlysimeters. Model simulations agreed well with the observations. It appeared also that the reduction of the vapor pressure in the soil's pores under extremely dry soil conditions is essential for the dew process.
Transport of polydisperse colloids in a saturated fracture with spatially variable aperture
Authors: James Scott C. , Chrysikopoulos Constantinos V.
Published: September 2002

A particle tracking model is developed to simulate the transport of variably sized colloids in a fracture with a spatially variable aperture. The aperture of the fracture is treated as a lognormally distributed random variable. The spatial fluctuations of the aperture are described by an exponential autocovariance function. It is assumed that colloids can sorb onto the fracture walls but may not penetrate the rock matrix. Particle advection is governed by the local fracture velocity and diffusion by the Stokes‐Einstein equation. Model simulations for various realizations of aperture fluctuations indicate that lognormal colloid size distributions exhibit greater spreading than monodisperse suspensions. Both sorption and spreading of the polydisperse colloids increase with increasing variance in the particle diameter. It is shown that the largest particles are preferentially transported through the fracture leading to early breakthrough while the smallest particles are preferentially sorbed. Increasing the variance of the aperture fluctuations leads to increased tailing for both monodisperse and variably sized colloid suspensions, while increasing the correlation length of the aperture fluctuations leads to increased spreading.
Thermal‐infrared remote sensing and Kirchhoff's law: 2. Field measurements
Authors: Korb Andrew R. , Salisbury John W. , D'Aria Dana M.
Published: September 2002

Laboratory and field measurements of the spectral, directional emissivity of surfaces were compared to emissivities calculated from directional hemispherical reflectances to establish the apparent validity of Kirchhoff's law under terrestrial field conditions. The laboratory measurements confirmed results of an earlier experiment that showed that rocks and coarse particulate surfaces follow Kirchhoff's law in the 8 to 14 μm atmospheric window, whereas very fine particulates may not. However, laboratory measurements of samples heated from below are not an adequate simulation of field conditions. Nonetheless, field measurements demonstrated similar results for materials heated by the Sun. Apparent deviations from Kirchhoff's law of more than 0.04 were measured for fine‐particulate (quartz) surfaces, dependent on the intensity of downwelling radiance from atmospheric water vapor and clouds. Examples of natural surfaces that may deviate from Kirchhoff's law include wind‐blown dust deposits, fresh snow, frosts, and fine volcanic ash in the vicinity of explosive volcanism. Apparent deviations from Kirchhoff's law are caused by steep thermal gradients in the infrared skin depth. A new technique probed this vertical temperature structure, with a sensitivity of 0.2°C, revealing details of a 2°C temperature difference within the infrared‐emitting region (the top ∼150 μm) of a fine‐particulate surface.
Radar backscatter statistics from the sea surface: Implications of SIR‐C/X‐SAR observations from the NE Atlantic
Authors: Macklin J. T. , Stapleton N. R.
Published: September 2002

Multichannel synthetic aperture radar (SAR) observations from the spaceborne imaging radar‐C/X‐band SAR (SIR‐C/X‐SAR) experiment in the NE Atlantic (April 1994) are analyzed to test models of both the mean and the distribution of radar backscatter from the sea surface. The data cover incidence angles from about 20° to 40° and wind speeds from about 5 to 10 m s−1. Empirical models of the mean fit the data well at C band to an accuracy of within 1–2 dB. Discrepancies at L band are a function of incidence angle, and we cannot rule out the possibility that they arise from systematic calibration errors. Single‐look SIR‐C/X‐SAR data (spatial resolution ∼7–10 m) fit well to a K distribution, but multilook data (spatial resolution ∼25 m) fit better to a lognormal distribution. The observed second moments of image intensity can be explained by the modulations of resolved ocean surface waves but only if relatively large hydrodynamic modulations, which are generally consistent with those inferred from tower radar data, are assumed.
Interaction of the Jovian magnetosphere with Europa: Constraints on the neutral atmosphere
Authors: Saur J. , Strobel D. F. , Neubauer F. M.
Published: September 2002

A three‐dimensional plasma model was developed to understand the sources and sinks that maintain Europa's neutral atmosphere and to study the interaction of the Jovian magnetosphere with this atmosphere and the formation of an ionosphere. The model includes self‐consistently the feedback of the plasma action on the atmosphere through mass balance. Suprathermal torus ions with a contribution from thermal ions sputter O2 from the water ice surface, and thermal torus ions remove the O2 atmosphere by sputtering. For an oxygen column density of 5×1018 m−2 the calculated intensities of the oxygen lines OI 130.4 nm and 135.6 nm produced by electron impact dissociation agree with observations by the Hubble Space Telescope [Hall et al., 1995]. Mass balance is also consistent with this column density, with a net atmospheric mass loss of 50 kg s−1. For a given neutral atmosphere and magnetospheric conditions, the electrodynamic model computes self‐consistently plasma density, plasma velocity, electron temperature of the thermal and the suprathermal population, electric current and electric field in the vicinity of Europa, with the assumption of a constant homogeneous Jovian magnetic field. Europa's ionosphere is created by electron impact ionization where the coupling of the ionosphere with the energy reservoir of the plasma torus by electron heat conduction supplies the energy to maintain ionization. The calculated distribution of electron densities with a maximum value of nearly 104 cm−3 is in general agreement with densities derived by Kliore et al. [1997] from the Galileo spacecraft radio occultations. The Alfvénic current system closed by the ionospheric Hall and Pedersen conductivities carries a total current of 7×105 A in each Alfvén wing.
Acetone, methanol, and other partially oxidized volatile organic emissions from dead plant matter by abiological processes: Significance for atmospheric HOx chemistry
Authors: Warneke Carsten , Karl Thomas , Judmaier Helmut , Hansel Armin , Jordan Alfons , Lindinger Werner , Crutzen Paul J.
Published: September 2002

In this paper, attention is called to the significance of abiological production of partially oxidized volatile organic carbons (POVOCs) from the decay of dead plant material. Measured relative emission of acetone and methanol can be at least 10−4 and 3 ‐ 5 × 10−4 g g−1of decaying dry plant matter, respectively. If these results may be extrapolated, global annual emissions of 6–8 Tg of acetone and 18 – 40 Tg of methanol would result, adding strongly to the estimated total emissions of these compounds to the atmosphere. Because acetone and methanol, through OH and HO2 formation, play significant roles in the chemistry of the atmosphere, further research is strongly needed to quantify the emissions of acetone, methanol, and other POVOCs
Pyroclastic flows generated by gravitational instability of the 1996–97 Lava Dome of Soufriere Hills Volcano, Montserrat
Authors: Cole P. D. , Calder E. S. , Druitt T. H. , Hoblitt R. , Robertson R. , Sparks R. S. J. , Young S. R.
Published: September 2002

Numerous pyroclastic flows were produced during 1996–97 by collapse of the growing andesitic lava dome at Soufriere Hills Volcano, Montserrat. Measured deposit volumes from these flows range from 0.2 to 9 × 106 m³. Flows range from discrete, single pulse events to sustained large scale dome collapse events. Flows entered the sea on the eastern and southern coasts, depositing large fans of material at the coast. Small runout distance (<1 km) flows had average flow front velocities in the order of 3–10 m/s while flow fronts of the larger runout distance flows (up to 6.5 km) advanced in the order of 15–30 m/s. Many flows were locally highly erosive. Field relations show that development of the fine grained ash cloud surge component was enhanced during the larger sustained events. Periods of elevated pyroclastic flow productivity and sustained dome collapse events are linked to pulses of high magma extrusion rates.