bluuebunny/crossref_metadata_2025 huggingface dataset:
Soil‐vegetation‐atmosphere processes: Simulation and field measurement for deforested sites in northern Thailand
Authors: Giambelluca Thomas W. , Tran Liem T. , Ziegler Alan D. , Menard Trae P. , Nullet Michael A.
Published: February 2004

In recent efforts to predict the climatic impacts of tropical deforestation an extreme scenario of impoverished grassland has been used to represent the future deforested landscape. Currently, deforested areas of the tropics are composed of a mosaic of crops, bare soil, grassland, and secondary vegetation of various ages. The dominant feature of deforested land is often secondary vegetation. Parameter values for important forest replacement land covers, including secondary vegetation, have been shown to differ from those of forest much less than that assumed in general circulation model (GCM) deforestation experiments. For this study, the biosphere‐atmosphere transfer scheme (BATS) is run in uncoupled mode using measured input data in place of GCM forcing and using the same parameter settings employed in recent deforestation experiments. Model output is compared with measurements taken over seven different deforested land surfaces in northern Thailand. Comparisons reveal that the simulation of deforested land overestimates reflected shortwave radiation, the diurnal range of surface temperature for secondary vegetation, surface soil moisture loss during periods without rain, and surface soil moisture increase at the start of a rainy period and underestimates net radiation, the diurnal range of surface temperature on recently used land surfaces, and root zone soil moisture increase at the start of a rainy period at most sites. Most deforested land surfaces, especially intermediate and advanced secondary vegetation, are more similar, in terms of land surface‐atmosphere interaction, to the model simulation of forest than of deforested land as depicted in GCM experiments. These comparisons suggest that modelers aspiring to make realistic simulations of deforestation should adopt parameter settings representative of the diverse range of forest replacement land covers, instead of again using the grassland scenario.
Correlations of atmospheric dynamics with solar wind‐induced changes of air‐Earth current density into cloud tops
Authors: Tinsley Brian A.
Published: February 2004

We analyze reported correlations between solar activity and weather and climate and show that in six independent data sets there is a correlation of measured changes in atmospheric dynamics with measured or inferred changes in vertical atmospheric air‐earth current density. The current density changes are due to external modulation of the global electric circuit by the solar wind. We describe the several ways in which the solar wind modulates the global circuit, and the observations that support a simple model of the circuit, with two return paths in parallel. One return path is at low latitudes with relatively constant impedance and the other is at high latitudes and is responsive to solar wind modulation. The six independent data sets exhibiting the correlations include meteorological and air‐earth current density changes on the 10 to 12‐year solar cycle as well as on the day‐to‐day timescales of Forbush decreases of galactic cosmic ray flux and of heliospheric current sheet crossings. The geographic locations include northern and southern high latitudes as well as the tropics. In regions where these correlations are found, there exists free energy in the form of supercooled water droplets near the tops of clouds that are unstable with respect to precipitation. Laboratory data and models suggest that electrostatic charge accumulating on supercooled droplets and aerosols near cloud tops affects the probability of ice nucleation and droplet freezing, enhancing the rate of growth and sedimentation of ice crystals. This proposed mechanism is also an explanation for another longstanding meteorological problem, the discrepancy between measurements at cloud tops of initial concentrations of ice and of concentrations of ice‐forming nuclei. For light cloud cover the effect of increases in ice nucleation and sedimentation can be to reduce cloud opacity and albedo. For storm cloud systems the effect can be to enhance precipitation rates and latent heat release intensifying the storm. In several cases, measured or inferred storm intensification (or weakening) is directly related to measured or inferred increases (or decreases) of air‐earth current density. Thus electrical effects on cloud microphysics may serve as connecting links between the observed or inferred increases in air‐earth current density and the observed changes in atmospheric dynamics. In cases where thunderstorm electric fields are generated there are additional cloud microphysical effects that might contribute to the correlations. We discuss the present uncertainties regarding solar wind effects on the distribution of air‐earth current density in the global electric circuit and regarding the relevant cloud microphysics. Much work is required to quantify these effects and evaluate their importance relative to competing processes.
Optimizing an inverse method to deduce time‐varying emissions of trace gases
Authors: Haas‐Laursen Danielle E. , Hartley Dana E. , Prinn Ronald G.
Published: February 2004

In previous work, an inverse method based on the Kalman filter was used to deduce regional emissions for chlorofluorocarbons (CFC‐11) in a global chemical transport model [Hartley, 1992; Hartley and Prinn, 1993]. CFC‐11 has reasonably constant emissions over the years we addressed; however, most trace gases with poorly constrained global budgets (i.e. CO2, CH4, and N2O) have seasonally varying sources and sinks. The goal of this work is to explore various adaptations that exist in Kalman filter theory and to identify the optimum method for deducing time‐varying sources/sinks. To make this study feasible in both computer time and cost, we utilize a simplified atmospheric chemical transport model to investigate the methodology. We test many available adaptations in Kalman filter theory and conclude that the most accurate method is to use an adaptive‐iterative approach [Young, 1984; Sastri, 1985; Bellgardt et al., 1986].
Models of lightning‐produced sprites and elves
Authors: Fernsler R. F. , Rowland H. L.
Published: February 2004

Three different types of optical phenomena have been observed at high altitude above thunderstorms: an enhanced airglow (“elves”) at roughly ∼90 km; a reddish glow (“sprites”) from 50 to 90 km; and an upward moving, bluish emission (“jets”) below 40 km. A likely explanation for some or all of these phenomena is gas breakdown caused by the electromagnetic fields of lightning discharges. This paper examines the connection between these fields and breakdown at high altitude, using both analytic models and numerical simulations. Included in the calculations are the radiation fields from the lightning return stroke and the quasi‐static fields from the continuing current. The different nature of the two fields is shown to produce two distinct types of breakdown, with characteristics similar to those of elves and sprites. Also mentioned is a third breakdown mechanism which may account for blue jets.
Extracting ecological and biophysical information from AVHRR optical data: An integrated algorithm based on inverse modeling
Authors: Braswell B. H. , Schimel D. S. , Privette J. L. , Moore B. , Emery W. J. , Sulzman E. W. , Hudak A. T.
Published: February 2004

Satellite remote sensing provides the only means of directly observing the entire surface of the Earth at regular spatial and temporal intervals. Key Earth system variables can be obtained from satellite data by integrating appropriate processing, interpretation, and modeling. For example, the amount of photosynthetically active radiation absorbed by plants (APAR) and land surface albedo can be inferred from remotely sensed optical measurements. Radiative transfer model inversion exploits the dependence of reflectance on the relative source‐sensor geometry to estimate surface parameters. In contrast, geometrical effects are suppressed in most other approaches. We present an algorithm for the retrieval of fractional APAR (fAPAR), albedo, and other parameters from AVHRR (advanced very high resolution radiometer) reflectance measurements by inverting a modified version of the SAIL (scattering by arbitrarily inclined leaves) canopy radiative transfer model. The model is inverted using an effective bidirectional reflectance factor (BRF) distribution created by aggregating AVHRR data into cells of size comparable to those used in current terrestrial biosphere models (50 × 50 km). Successful inversion results over an area in central Africa are presented and compared with a vegetation index‐based analysis and other satellite data. The procedure also provides unique information on phenology derived from timing of changes in leaf optical properties and canopy structure. Our methods are unique in that they explicitly incorporate a priori ecological knowledge in the choice of model parameters and constraints. This approach can eventually be employed at pixel resolution with the EOS sensors, MODIS (moderate‐resolution imaging spectrometer) and MISR (multiangle imaging spectro‐radiometer).
Radiative transfer in multifractal clouds
Authors: Borde Régis , Isaka Harumi
Published: February 2004

We studied effects of the cloud inhomogeneity in subcloud scale on the reflectance, transmittance, and absorptance of two‐dimensional, inhomogeneous clouds. We generated the inhomogeneous clouds as multifractal clouds with a lognormal multiplicative process. For such clouds, the information codimension C1 is a measure of the cloud inhomogeneity. Radiative transfer through the multifractal clouds was computed with a discrete angle radiative transfer model. The average reflectance, transmittance, and absorptance of multifractal clouds with a given codimension were estimated as averages of 200 realizations. They were computed for different sets of the C1 parameter, cloud total optical thickness, and asymmetry factor of cloud scatterers. An effective optical thickness of inhomogeneous clouds was defined empirically in the framework of a homogeneous, plane parallel cloud model. Consequently, computation of radiative flux in an inhomogeneous cloud could be transformed into that of an equivalent homogeneous, plane parallel cloud. For a two‐dimensional, inhomogeneous, absorbing cloud we found that an inhomogeneous cloud absorbs generally less energy than its homogeneous counterpart. An exception was found for inhomogeneous clouds characterized by a small information codimension, a large cloud optical thickness, and a large single‐scattering albedo and for which absorptance is much larger than for their homogeneous counterpart. However, this increase was less than 5% of the homogeneous cloud absorptance. The use of the effective optical thickness also enabled us to treat an inhomogeneous absorbing cloud as an equivalent homogeneous absorbing cloud and to estimate the radiative flux of the equivalent homogeneous cloud as an approximation to the one in the inhomogeneous cloud. We found no immediate need to conceive of a direct effect of the cloud inhomogeneity on the single‐scattering albedo, as far as we considered this treatment as a first‐order approximation. We discussed the use of the effective optical thickness in two‐stream radiative approximations. We also compared our results with those based on the independent pixel approximation.
Effect of stratospheric ozone variations on UV radiation and on tropospheric ozone at high latitudes
Authors: Taalas Petteri , Damski Juhani , Kyrö Esko , Ginzburg Maximo , Talamoni Gustavo
Published: February 2004

A negative trend of stratospheric ozone has been observed especially at high southern and northern latitudes during the last 15 years. At the stations studied here a negative trend in total ozone was detected during 1987–1994: −10% at Marambio (64°S) and −12% at Sodankylä (67°N). The strongest negative trend was detected during spring. Mostly a negative trend in tropospheric ozone was observed at the stations during the late winter to early summer period during recent years. Because stratospheric ozone is controlling the flux of solar UV‐B radiation reaching troposphere, loss of stratospheric ozone may have a strong impact on the destruction and production reactions of tropospheric ozone. We have studied the ozone sounding records of 1988–1994 at Marambio, Antarctica (64°S), and at Sodankylä, Finland (67°N), to find out observational evidence of tropospheric ozone changes due to stratospheric ozone variations. We have found that springtime stratospheric ozone loss has a pronounced impact on the upper tropospheric ozone at both hemispheres. Average ozone deviation of ‐ 12.8% from the 1988 to 1994 means in the 6‐ to 8‐km layer has been observed in Antarctica during the months with stratospheric ozone loss and −10.0% in the Arctic, respectively. Daily total ozone records and radiative transfer calculations were used to study the UV‐B doses reaching the troposphere.
Wave breaking signatures in OH airglow and sodium densities and temperatures: 1. Airglow imaging, Na lidar, and MF radar observations
Authors: Hecht J. H. , Walterscheid R. L. , Fritts D. C. , Isler J. R. , Senft D. C. , Gardner C. S. , Franke S. J.
Published: February 2004

The Collaborative Observations Regarding the Nightglow (CORN) campaign took place at the Urbana Atmospheric Observatory during September 1992. The instrumentation included, among others, the Aerospace Corporation narrowband nightglow CCD camera, which observes the OH Meinel (6–2) band (hereafter designated OH) and the O2 atmospheric (0–1) band (hereafter designated O2) nightglow emissions; the University of Illinois Na density/temperature lidar; and the University of Illinois MF radar. Here we report on observations of small‐scale (below 10‐km horizontal wavelength) structures in the OH airglow images obtained with the CCD camera. These small‐scale structures were aligned perpendicular to the motion of 30‐ to 50‐km horizontal wavelength waves, which had observed periods of about 10–20 min. The small‐scale structures were present for about 20 min and appear to be associated with an overturned or breaking atmospheric gravity wave as observed by the lidar. The breaking wave had a horizontal wavelength of between 500 and 1500 km, a vertical wavelength of about 6 km, and an observed period of between 4 and 6 hours. The motion of this larger‐scale wave was in the same direction as the ≈30‐ to 50‐km waves. While such small‐scale structures have been observed before, and have been previously described as ripple‐type wave structures [Taylor and Hapgood, 1990], these observations are the first which can associate their occurrence with independent evidence of wave breaking. The characteristics of the observed small‐scale structures are similar to the vortices generated during wave breakdown in three dimensions in simulations described in Part 2 of this study [Fritts et al., this issue]. The results of this study support the idea that ripple type wave structures we observe are these vortices generated by convective instabilities rather than structures generated by dynamical instabilities.
Ground‐based measurements of middle atmospheric water vapor at 183 GHz
Authors: Pardo J. R. , Cernicharo J. , Lellouch E. , Paubert G.
Published: February 2004

We present here the first ground‐based observations of stratospheric/mesospheric H2O emission at the 31,3 → 22,0 resonance (183.3101 GHz of central frequency). The spectra were obtained in January 1991 and January 1994 using the Institut de Radioastronomie Millimétrique (IRAM) 30‐m radiotelescope located on Pico Veleta, Spain, (altitude 2870 m, latitude 37°N), during very dry tropospheric conditions (total precipitable water within the range 0.2–0.3 mm above the observation site). Microwave studies of stratospheric and mesospheric H2O profile have been carried out so far mostly by means of observations of the weak 61,6 → 52,3 resonance at 22.2351 GHz. Some measurements at 183 GHz were previously obtained from airborne observations. The water vapor abundance in the troposphere is, in most cases, so high that the atmosphere appears optically thick at 183 GHz. However, we show that emission from the stratosphere and mesosphere can be sometimes detected from the ground (allowing upper stratospheric and mesospheric water vapor profiles to be obtained). We use a generalized non linear radiative transfer code in order to perform a least squares fit of the observed spectra. The derived H2O abundances are consistent with most measurements at 22 GHz. We suggest the possibility of carrying out this kind of observation in some selected sites during a significant fraction of the whole winter season. The required integration times to get a given signal‐to‐noise ratio are much shorter at 183 GHz than at 22 GHz.
Methane flux in a boreal fen: Season‐long measurement by eddy correlation
Authors: Suyker Andrew E. , Verma Shashi B. , Clement Robert J. , Billesbach David P.
Published: February 2004

Eddy correlation measurements of methane flux were made at a fen in central Saskatchewan, as part of the Boreal Ecosystem Atmosphere Study (BOREAS) in 1994. Data were collected from mid‐May to early October. The water table was above the average peat surface throughout the measurement period. Detailed (near continuous) measurements allowed examination of temporal variability at hourly and daily time‐scales. As compared with the average nighttime flux, the average daytime methane flux was 25–45% higher in July and in August and 5–15% higher earlier and later in the season. Distribution of midday (1130–1430 LT) methane emission showed varying trends in different parts of the season. From mid‐May to early July, methane flux gradually increased from near zero to 4.1 mg m−2 h−1. The water table height (above an average hollow surface) varied from 0.09 to 0.18 m, but the trend in methane flux followed peat temperature (at 0.1‐m depth) more closely. The peat warmed from 3.4° to 16.3°C during this time period. Methane flux was negligible until peat temperature (at 0.1‐m depth) was above 12°C. From early July to early August there was a fivefold increase in methane flux from 4.1 to its seasonal peak of 19.5 mg m−2 h−1 on August 1. The water table ranged from 0.23 m to a brief seasonal plateau of 0.30 m on July 20–23. Sharp increases in the water table were followed by increasing trends in methane flux by approximately 12 days. Peat temperature reached its seasonal maximum (19.3°C) the same time when the methane flux peaked. After early August the methane flux declined steadily and reached a value of 2.4 mg m−2 h−1 in early October. The water table and peat temperature showed comparable trends and decreased steadily to 0.06 m and 5.7°C, respectively. The seasonally integrated methane emission (mid‐May to early October) was estimated at 16.3 g C m−2. Nonlinear regression analysis of methane flux against water table (lagged by 12 days) and peat temperature was performed for different periods of the season. Except for a brief period of very high water table (when many hummocks were inundated) the regression using water table and peat temperature explained between 68 and 94% of the variability in methane flux. The sensitivity of methane flux to water table (or the slope of the log CH4 flux/water table relationship) obtained from our daily flux values ranged from 3.4 × 10−4 to 5.0 × 10−4 m−1.
A methodology for assessing ensemble experiments
Authors: Wang X. L. , Rui H. L.
Published: February 2004

Climate simulations and forecast experiments of increasingly large ensemble size are being performed to assess the predictive skill of a dynamic model on seasonal and longer timescales. Especially in the cases of ensemble climate simulation or forecast forced by observed or predicted sea surface temperatures, the model is expected to maximize potential predictability due to boundary forcing and to minimize internal variability generated from dynamic instability. In the light of small predictive skill in extratropics from boundary forcing, one must evaluate skill of the ensemble mean quantity against intersample variability or spread of the individual ensemble member. On the other hand, certain dominant signals in climate variability, such as E1 Niño‐Southern Oscillation, have been documented. Predictability for these major signals is the hope of seasonal and climate forecasting using a dynamic model. It may be unrealistic to anticipate a model being able to simulate or forecast the full spectra of climate variability. The question is how to evaluate a model's performance in capturing the dominant climate signals in ensemble experiments with increasingly large sample size. These issues have motivated us to develop a compact methodology for assessing climate experiments with large ensemble size. This method treats the ensemble mean as signal and intersample variability as spread or noise in a common framework. Hence not only dominant signals from boundary forcing can be isolated, but also sensitivity of these signals to the forcing can be assessed. Other potential applications of the method to climate simulation and forecasting are also discussed.
A soil‐vegetation‐atmosphere transfer scheme for modeling spatially variable water and energy balance processes
Authors: Peters‐Lidard C. D. , Zion M. S. , Wood E. F.
Published: February 2004

In support of the eventual goal to integrate remotely sensed observations with coupled land‐atmosphere models, a soil‐vegetation‐atmosphere transfer scheme is presented which can represent spatially variable water and energy balance processes on timescales of minutes to months. This scheme differs from previous schemes developed to address similar objectives in that it: (1) represents horizontal heterogeneity and transport in a TOPMODEL framework, and (2) maintains computational efficiency while representing the processes most important for our applications. The model is based on the original TOPMODEL‐based land surface‐atmosphere transfer scheme [Famiglietti and Wood, 1994a] with modifications to correct for deficiencies in the representation of ground heat flux, soil column geometry, soil evaporation, transpiration, and the effect of atmospheric stability on energy fluxes. These deficiencies were found to cause errors in the model predictions in quantities such as the sensible heat flux, to which the development of the atmospheric boundary layer is particularly sensitive. Application of the model to the entire First International Satellite Land Surface Climatology Project Field Experiment 1987 experimental period, focusing on Intensive Field Campaigns 3 and 4, shows that it successfully represents the essential processes of interest.
Effect of aqueous phase cloud chemistry on tropospheric ozone
Authors: Liang Jinyou , Jacob Daniel J.
Published: February 2004

The sensitivity of tropospheric O3 to aqueous HOx (OH + HO2) phase chemistry in clouds is examined using photochemical model calculations of the O3 production efficiency per unit NOx and the chemical lifetime of O3, combined with estimates for the residence time of air in clouds. It is found that the maximum perturbation to O3 from cloud chemistry in the tropics and midlatitudes summer is less than 3%. This result is supported by calculations using a three‐dimensional, continental‐scale model for North America.
On the forcing of seasonal changes in surface pressure over Antarctica
Authors: Parish Thomas R. , Bromwich David H.
Published: February 2004

A 10‐year record (1985–1994) of output statistics from the European Centre for Medium‐Range Weather Forecasts (ECMWF) model shows that profound seasonal changes in surface pressure take place over the Antarctic continent. The most pronounced changes occur during the periods straddling the brief Antarctic summer, from September to December and again from January to April. Surface pressures atop the high Antarctic plateau often display changes in excess of 20 hPa during these periods. Temperatures in the lower troposphere also exhibit marked changes during these transitional periods; surface temperature changes during these 3‐month periods reach a maximum near 40 K over the high interior of Antarctica. Hydrostatic considerations suggest that the thermal adjustment in the lowest levels of the atmosphere alters the vertical distribution of pressure with height and hence is consistent with the dramatic seasonal surface pressure changes over the elevated Antarctic ice sheets. A strong interplay exists between the thermal forcing and the katabatic wind circulation over the continent. The large seasonal changes in solar insolation reaching the Antarctic ice surface modulate the intensity of the katabatic wind regime and thus the resulting mean meridional circulation between the continent and the subpolar latitudes. It is proposed that the diabatic adjustment in the lower levels of the atmosphere over Antarctica disrupts the mean meridional circulation creating a seasonal mass imbalance and hence surface pressure changes. It is through the meridional transports that the mass and wind fields reach a quasi‐equilibrium adjusted state. The seasonal mass movement over Antarctica requires large‐scale mass compensation over much of the southern hemisphere and shows that the diabatic influences at the Antarctic surface have far‐field impacts.
A global climatology of stratospheric aerosol surface area density deduced from Stratospheric Aerosol and Gas Experiment II measurements: 1984–1994
Authors: Thomason L. W. , Poole L. R. , Deshler T.
Published: February 2004

A global climatology of stratospheric aerosol surface area density has been developed using the multiwavelength aerosol extinction measurements of the Stratospheric Aerosol and Gas Experiment (SAGE) II for 1984–1994. The spatial and temporal variability of aerosol surface area density at 15.5, 20.5, and 25.5 km are presented as well as cumulative statistical distributions as a function of altitude and latitude. During this period, which encompassed the injection and dissipation of the aerosol associated with the June 1991 Mount Pinatubo eruption as well as the low loading period of 1989–1991, aerosol surface area density varied by more than a factor 30 at some altitudes. Aerosol surface area density derived from SAGE II and from the University of Wyoming optical particle counters are compared for 1991–1994 and are shown to be in generally good agreement though some differences are noted. An extension of the climatology using single‐wavelength measurements by the Stratospheric Aerosol Measurement II (1978–1994) and SAGE (1979–1981) instruments is also presented.
The role of gravity waves in the quasi‐biennial oscillation
Authors: Dunkerton Timothy J.
Published: February 2004

The role of gravity wave momentum transport in the quasi‐biennial oscillation (QBO) is investigated using a two‐dimensional numerical model. In order to obtain an oscillation with realistic vertical structure and period, vertical momentum transport in addition to that of large‐scale, long‐period Kelvin and Rossby‐gravity waves is necessary. The total wave flux required for the QBO is sensitive to the rate of upwelling, due to the Brewer‐Dobson circulation, which can be estimated from the observed ascent of water vapor anomalies in the tropical lower stratosphere. Although mesoscale gravity waves contributeto mean flow acceleration, it is unlikely that the momentum flux in these waves is adequate forthe QBO, especially if their spectrum is shifted toward westerly phase speeds. Short‐period Kelvin and inertia‐gravity waves at planetary and intermediate scales also transport momentum. Numerical results suggest that the flux in all vertically propagating waves (planetary‐scale equatorial modes, intermediate inertia‐gravity waves, and mesoscale gravity waves), in combination, is sufficient to obtain a QBO with realistic Brewer‐Dobson upwelling if the total wave flux is 2–4 times as large as that of the observed large‐scale, long‐period Kelvin and Rossby‐gravity waves. Lateral propagation of Rossby waves from the winter hemisphere is unnecessary in this case, although it may be important in the upper and lowermost levels of the QBO and subtropics.
Remote sensing aerosols using satellite infrared observations
Authors: Ackerman Steven A.
Published: February 2004

Aerosol detection techniques using infrared wavelengths have a distinct advantage over visible techniques by providing coverage over bright surfaces and during the night. This study investigates detection of volcanic and soil‐derived aerosols, two important aerosols in studies of the earth's climate, using infrared observations at the following approximate wavelengths 8.5, 11, and 12 μm. Detection is based on brightness temperature differences among the three channels BT11‐BT12 and BT8‐BT11. It is demonstrated that these three infrared channels are useful for detecting stratospheric volcanic aerosols over oceans. Theoretical simulations agree with observations from current satellite instruments. Detection of the stratospheric aerosol over land is complicated by spectral variation of surface emissivity. Retrieving aerosol optical depth over land requires defining the surface spectral emittance. Detecting the presence of soil‐derived aerosols can also be aided with infrared observations. Increasing the dust optical depth increases BT11‐BT12 and BT8‐BT11. The effect is opposite to that of an H2SO4 stratospheric aerosol and differs from an increase in atmospheric precipitable water, though addition of ice clouds moves the differences in the same direction. Retrievals of aerosol optical depth over the desert must account for surface emissivity and the vertical distribution of the dust. Negative differences in BT11‐BT12 are observed to occur for dust storms over the Arabian Peninsula, Africa, and the southwest United States and is useful for remote sensing source regions of dust outbreaks. These negative differences can be simulated using the theoretical model but requires a specific dust aerosol model. There are inconsistencies between theoretical simulations of the infrared properties of heavy dust loadings and the satellite observations. Negative differences in BT11‐BT12 are useful for detecting and tacking dust storms.
An assessment of the radiative effects of anthropogenic sulfate
Authors: Chuang Catherine C. , Penner Joyce E. , Taylor Karl E. , Grossman Allen S. , Walton John J.
Published: February 2004

We use a coupled climate/chemistry model with cloud nucleation processes parameterized in terms of local aerosol number, anthropogenic sulfate mass concentration, and updraft velocity to investigate both direct and indirect anthropogenic sulfate radiative forcings. We estimate that the global direct radiative forcing is about −0.4 W m−2with a maximum over Europe where the strongest anthropogenic sulfur emissions occur. With different approaches for the formation of anthropogenic sulfate and its relation to aerosol size distribution, we estimate that the indirect forcing may range from −0.6 to −1.6 W m−2. This range reduces to −0.4 to −1.1 W m−2if a prescribed marine background particle number concentration is universally applied over the ocean. Contrary to the direct effect which is more significant over continents, the calculated maximum of indirect forcing is located over the Atlantic Ocean near the coastline of North America. Our simulations indicate that anthropogenic sulfate may result in important increases in reflected solar radiation, which would mask locally the warming from increased greenhouse gases. We also compare the simulated cloud drop effective radii with those retrieved from satellite data to validate the accuracy of our cloud drop parameterization.
Comparison of BOREAS and Atmospheric Environment Service humidity sensors at Meadow Lake, Saskatchewan
Authors: Betts Alan K. , Ball John H. , Smith Brett J. , Shewchuk Stanley R.
Published: February 2004

A comparison of co‐located humidity measurements by two different sensors, a Campbell Scientific 207F (at an operational Atmospheric Environment Service autostation) and a Campbell HMP35 (used in the Boreal Ecosystem‐Atmosphere Study research mesonet) suggests that the older sensor, the 207F, has significant errors.
Actinic fluxes in broken cloud fields
Authors: Los A. , van Weele M. , Duynkerke P. G.
Published: February 2004

Photochemical processes in the atmosphere are driven by solar ultraviolet radiation. The photodissociation rate coefficients of atmospheric species are determined by the actinic flux, which is defined as 4π times the mean ultraviolet intensity. Because of the presence of clouds the actinic flux can change drastically throughout the atmosphere. Therefore clouds have large effects on photodissociation rate coefficients. At cloud top, photodissociation rate coefficients can be 300% higher than in clear sky conditions. We use Monte Carlo simulations to investigate the reflectance, the transmittance, and the actinic flux for cloud fields at various degrees of cloudiness. Scattering processes in the clouds are due to cloud particles only. We do not take absorption of radiation into account. The atmosphere outside the clouds is assumed to be completely transparent. The simulated reflectance and transmittance of plane‐parallel cloud fields and in broken cloud field conditions reproduce the results of previous model studies within statistical uncertainties. The results of actinic flux calculations for plane‐parallel cloud fields agree with the results obtained with a doubling‐adding algorithm. Horizontal and vertical actinic flux profiles in broken cloud fields are studied for various solar zenith angles and for different cloud optical thicknesses. The aim of the present model study is to obtain insight into the effect of broken cloud fields on the actinic flux.
Transport of atmospheric water vapor by volcanic eruption columns
Authors: Glaze Lori S. , Baloga Stephen M. , Wilson Lionel
Published: February 2004

Contrary to assumptions often made in the literature, explosive volcanic eruptions are capable of transporting significant amounts of water into the stratosphere. In addition to the magmatic water component, atmospheric water vapor is entrained by the column at lower levels. A theoretical model for the conservation of mass, momentum, and thermal energy of four separate components (dry air, water vapor, liquid condensates, and solid particles) is used to determine the extent of atmospheric water redistribution. We examine the effects of water vapor condensation on dynamical characteristics and ambient water vapor transport. A simple technique is presented for deriving canonical forms for the complex system of ordinary differential equations governing the column components. Solutions of this model are presented that show the influence of different volcanic boundary conditions and a range of ambient water vapor distributions on transport of the buoyant column. We show that the water component (vapor + liquid) of small eruption columns rising through a wet atmosphere is dominated by entrained water, whereas larger columns are dominated by the magmatic water. This is due, in part, to the proportionately smaller entrainment surface area in relation to the control volume for the larger columns. We also show that a maintained column with an initial mass flux of 2.7×108 kg s−1 erupted into a wet atmosphere would inject 96 Mt of water vapor into the stratosphere over 24 hours, comparable to the annual input from methane oxidation or 100 midlatitude thunderstorms. This increase may accelerate the conversion of simultaneously erupted volcanic SO2 into sulfuric acid.
Atmospheric water vapor estimate by a differential absorption technique with the polarisation and directionality of the Earth reflectances (POLDER) instrument
Authors: Bouffiès S. , Bréon F. M. , Tanré D. , Dubuisson P.
Published: February 2004

The polarization and directionality of the Earth reflectances (POLDER) instrument, to be launched in 1996 on the Japanese ADEOS (advanced Earth observing satellite) platform includes a channel which covers the 910 nm water vapor absorption band (near IR), as well as a channel centered at 865 nm. An estimate of the total atmospheric water vapor content can be derived from the ratio of the two reflectance measurements. In addition to the spectroscopic parameterization, the major uncertainties of this estimate result from (1) the surface reflectance spectral signature, (2) scattering by atmospheric aerosol, and (3) the water vapor vertical profile. A radiative transfer model has been developed in order to quantify these uncertainties. From radiative transfer simulations an uncertainty on the order of 10% is expected on the total water vapor amount. An airborne version of the POLDER instrument has been developed and flown over various targets. These targets include semiarid surfaces (Sahel), bog, coniferous and deciduous forest (Boreal forest), and the ocean in the glint direction. Water vapor measurements from radiosondes, concomitant with the POLDER measurements, are available for the method validation. Over water bodies with glint present, an overestimate is found, which is still not understood at present. For other surfaces the comparisons are, in general, in good agreement with the uncertainty analysis.
Density of rime in laboratory simulations of thunderstorm microphysics and electrification
Authors: Williams Earle R. , Zhang Renyi
Published: February 2004

Results from earlier laboratory studies are used to estimate the density of rime ice in laboratory simulations of thunderstorm microphysics and electrification. Comparisons of results in the works of Takahashi [1978], Jayaratne et al. [1983], and Saunders et al. [1991] show that kernel and opaque rime (ρ>0.33 g/cm3) are prevalent in the former study, whereas low‐density feather rime (ρ<0.33 g/cm3) is prevalent in the latter studies. It is suggested that rime density may affect the charging process by either susceptibility to surface fracture on collision or impact of surface roughness on heat and vapor transfer at the ice surface.
Distribution of charge along the lightning channel: Relation to remote electric and magnetic fields and to return‐stroke models
Authors: Thottappillil Rajeev , Rakov Vladimir A. , Uman Martin A.
Published: February 2004

We derive exact expressions for remote electric and magnetic fields as a function of the time‐ and height‐varying charge density on the lightning channel for both leader and return‐stroke processes. Further, we determine the charge density distributions for six return‐stroke models. The charge density during the return‐stroke process is expressed as the sum of two components, one component being associated with the return‐stroke charge transferred through a given channel section and the other component with the charge deposited by the return stroke on this channel section. After the return‐stroke process has been completed, the total charge density on the channel is equal to the deposited charge density component. The charge density distribution along the channel corresponding to the original transmission line (TL) model has only a transferred charge density component so that the charge density is everywhere zero after the wave has traversed the channel. For the Bruce‐Golde (BG) model there is no transferred, only a deposited, charge density component. The total charge density distribution for the version of the modified transmission line model that is characterized by an exponential current decay with height (MTLE) is unrealistically skewed toward the bottom of the channel, as evidenced by field calculations using this distribution that yield (1) a large electric field ramp at ranges of the order of some tens of meters not observed in the measured electric fields from triggered‐lightning return strokes and (2) a ratio of leader‐to‐return‐stroke electric field at far distances that is about 3 times larger than typically observed. The BG model, the traveling current source (TCS) model, the version of the modified transmission line model that is characterized by a linear current decay with height (MTLL), and the Diendorfer‐Uman (DU) model appear to be consistent with the available experimental data on very close electric fields from triggered‐lightning return strokes and predict a distant leader‐to‐return‐stroke electric field ratio not far from unity, in keeping with the observations. In the TCS and DU models the distribution of total charge density along the channel during the return‐stroke process is influenced by the inherent assumption that the current reflection coefficient at ground is equal to zero, the latter condition being invalid for the case of a lightning strike to a well‐grounded object where an appreciable reflection is expected from ground.
Air‐snow interactions and the surface energy and mass balance over the melting zone of west Greenland during the Greenland Ice Margin Experiment
Authors: Gallée Hubert , Duynkerke Peter G.
Published: February 2004

A meso‐γ‐scale atmospheric model coupled to a snow/ice model has been used to study the atmospheric circulation observed during the Greenland Ice Margin Experiment [Oerlemans and Vugts, 1993] and its impact on the ice sheet mass balance. The simulations shown here are two dimensional and cover the July 5–24, 1991, period, a typical fine weather period in that area. The synoptic scale wind, temperature, and humidity evolution are included. Simulated wind and temperature compare reasonably well with available observations. The simulated snow/ice melting is examined, and the sensitivity of the model to the ice‐sheet surface representation is discussed by replacing the snow/ice model by a classical force‐restore model. It is found that the melting‐refreezing of liquid water in the snow pack impacts significantly the surface energy and mass balances of the ice sheet. In particular, the nighttime surface temperature minimum and katabatic wind speed are less marked when the refreezing process is activated in the model.
A three‐dimensional simulation of the Antarctic ozone hole: Impact of anthropogenic chlorine on the lower stratosphere and upper troposphere
Authors: Brasseur Guy P. , Tie XueXi , Rasch Philip J. , Lefèvre Frank
Published: February 2004

We have developed a global three‐dimensional transport‐chemical model of the stratosphere (called the Study of Transport and chemical Reactions in the Stratosphere (STARS)) which includes a representation of the formation of polar stratospheric clouds (PSCs) and heterogeneous reactions on the surfaces of PSCs and sulfate aerosols. The formation of the observed springtime “Antarctic ozone hole” is well reproduced by the model. A maximum of 40% total ozone depletion occurs in October. Calculated ozone and chlorine concentrations are consistent with satellite observations. After the breakdown of the polar vortex in December, air with depleted ozone is transported to midlatitudes in the southern hemisphere, resulting in a 2–4% ozone decrease at 50°S in December and a 1% decrease in the sub tropics. Ozone‐poor air masses are also transported to the troposphere and produce a significant decrease in upper tropospheric ozone. Model calculations show that the calculated ozone depletion is not significantly modified when type I PSC particles are assumed to be liquid ternary solutions (H2SO4/HNO3/H2O) rather than solid nitric acid trihydrates. Ice particles (type II PSCs) sediment into the troposphere, producing a large decrease in the concentrations of stratospheric HNO3 and NO2. As a result, the conversion of ClO into ClONO2 is reduced, and the concentration of reactive chlorine remains high for two additional weeks. The ozone column abundance is reduced by an additional 10% during the month of October. The model results show that the ozone minimum observed in Antarctica several decades ago (preindustrial chlorine levels) is produced by (natural) dynamical processes. Under these conditions the polar ozone depletion caused by chemical processes was very small (maximum of 3%) in October. In November the ozone concentration even increased above 22 km in response to PSC processes.
NOx from lightning: 1. Global distribution based on lightning physics
Authors: Price Colin , Penner Joyce , Prather Michael
Published: February 2004

This paper begins a study on the role of lightning in maintaining the global distribution of nitrogen oxides (NOx) in the troposphere. It presents the first global and seasonal distributions of lightning‐produced NOx (LNOx) based on the observed distribution of electrical storms and the physical properties of lightning strokes. We derive a global rate for cloud‐to‐ground (CG) flashes of 20–30 flashes/s with a mean energy per flash of 6.7×109 J. Intracloud (IC) flashes are more frequent, 50–70 flashes/s but have 10% of the energy of CG strokes and, consequently, produce significantly less NOx. It appears to us that the majority of previous studies have mistakenly assumed that all lightning flashes produce the same amount of NOx, thus overestimating the NOx production by a factor of 3. On the other hand, we feel these same studies have underestimated the energy released in CG flashes, resulting in two negating assumptions. For CG energies we adopt a production rate of 10×1016 molecules NO/J based on the current literature. Using a method to simulate global lightning frequencies from satellite‐observed cloud data, we have calculated the LNOx on various spatial (regional, zonal, meridional, and global) and temporal scales (daily, monthly, seasonal, and interannual). Regionally, the production of LNOx is concentrated over tropical continental regions, predominantly in the summer hemisphere. The annual mean production rate is calculated to be 12.2 Tg N/yr, and we believe it extremely unlikely that this number is less than 5 or more than 20 Tg N/yr. Although most of LNOx, is produced in the lowest 5 km by CG lightning, convective mixing in the thunderstorms is likely to deposit large amounts of NOx, in the upper troposphere where it is important in ozone production. On an annual basis, 64% of the LNOx, is produced in the northern hemisphere, implying that the northern hemisphere should have natural ozone levels as much as 2 times greater than the southern hemisphere, even before anthropogenic influences. The amount of O3 produced from this NOx is expected to exceed the stratospheric source by a factor of 1.5, and thus the hemispheric asymmetry in LNOx would lead to a significant excess of northern hemisphere O3 even in the preindustrial troposphere. (The monthly climatologies for LNOx on a 1°×1° latitude‐longitude grid can be obtained by e‐mail to cprice@flash.tau.ac.il).
Stratospheric warming due to Agung, El Chichón, and Pinatubo taking into account the quasi‐biennial oscillation
Authors: Angell J. K.
Published: February 2004

Radiosonde data are used to estimate the impact on stratospheric temperature of Agung, El Chichon, and Pinatubo volcanic eruptions, taking into account the quasi‐biennial oscillation (QBO). This is done by comparing the temperature variations following the three eruptions with the average quasi‐biennial variation in temperature obtained by applying the superposed epoch method to the 13 QBO cycles (centered on Singapore 50 mbar east wind maxima) between 1958 and 1996 which do not include these three eruptions. Confidence estimates are based on the 2‐standard deviation value of the 13 comparisons. Shown are estimates of the volcanic impact and approximate 95% confidence intervals in 300–100 and 100–50 mbar layers of polar, temperate, subtropical, and equatorial zones (and the globe as a whole), as well as in north subtropical and equatorial zones at pressure surfaces of 50, 30, 20, and 10 mbar. Based on this procedure, and ignoring other possible influences on stratospheric temperature, all three eruptions warmed the tropical low stratosphere by nearly 2°C, with the largest warming 3°C at 50 mbar in the equatorial zone following Agung and Pinatubo. Above 50 mbar, the volcanic warming usually decreases with height in the tropics, an exception being the 2°C warming of the 10 mbar surface in the north subtropics following El Chichon. While the volcanic warming is most frequently a maximum 2 seasons after the eruption, the warming duration can be 2 years. In the polar zones there is evidence of cooling of stratospheric 300–100 and 100–50 mbar layers following the eruptions. Comparisons are made with other volcanic warming estimates.