bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of seasonal dimethyl sulphide fluxes over the North Atlantic Ocean and their contribution to European pollution levels
Authors: Tarrasón L. , Turner S. , Fløisand I.
Published: February 2004

A monthly resolved emission budget which aims to reproduce the observed seasonal variations of dimethyl sulphide (DMS) production over the oceans is derived for the North Atlantic Ocean. The monthly cycle of DMS concentrations in seawater and their distribution over the North Atlantic Ocean was determined from a collation of available field measurements. DMS fluxes are calculated on the basis of Liss and Merlivat's (1986) formulation of sea‐to‐atmosphere gas exchange, using six‐hourly meteorological data from the European Center for Medium‐Range Forecasts for the year 1988. This allows to show the influence of meteorological conditions on the DMS fluxes to the atmosphere. Results show averaged fluxes over the North Atlantic Ocean in the range 39–390 μg S m−2d−1, with a monthly summer maximum between 67 and 1260 μg S m−2d−1, about 20 times larger than the winter minimum. The contribution of natural oceanic sulphur to the acidic deposition levels over Europe is evaluated with the help of a three‐dimensional Eulerian dispersion model and is compared to the separate contributions of European and North American anthropogenic sources. The relative contribution of biogenic sulphur to total sulphur deposition over the west coast of Europe does not exceed 2% on a yearly basis although it can be close to 10% during the summer months. Considering the uncertainty ranges associated both to the estimation of DMS fluxes and to the subsequent dispersion in the atmosphere, the actual contribution of North Atlantic DMS to European pollution levels can vary by a factor of 3.
Estimation of canopy water content in Konza Prairie grasslands using synthetic aperture radar measurements during FIFE
Authors: Saatchi Sasan S. , van Zyl Jakob J. , Asrar Ghassem
Published: February 2004

This paper presents the development of an algorithm to retrieve the canopy water content of natural grasslands and pasture from synthetic aperture radar (SAR) measurements. The development of this algorithm involves three interrelated steps: (1) calibration of SAR data for ground topographic variations, (2) development and validation of backscatter model for grass canopies, and (3) estimation of canopy water content by inverting a backscattered model for cross‐polarized ratio. The polarimetric radar data acquired by the Jet Propulsion Laboratory AIRSAR system during the 1989 First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) are used for this study. The SAR data have been calibrated and corrected for the topographical effects by using the digital elevation map of the study area. The backscattering coefficients obtained from the SAR data for each pixel are related to the canopy and soil parameters by employing a discrete random media model for vegetation. The model simulations indicate that biomass variations and surface treatments (burned and unburned) of grass canopies affect the C‐band backscatter signal but does not influence the L‐band signal. This model is then validated and adjusted over training areas where ground measurements were collected. An inversion technique is proposed to estimate the canopy water content by using the cross‐polarized and copolarized ratios of the SAR data at C band. The result of the inversion algorithm shows a good agreement with the grass biomass data collected during FIFE 1989 intensive field campaign.
The evolution of the dehydration in the Antarctic stratospheric vortex
Authors: Vömel H. , Oltmans S. J. , Hofmann D. J. , Deshler T. , Rosen J. M.
Published: February 2004

In 1994 an intensive program of balloon‐borne frost point measurements was performed at McMurdo, Antarctica. During this program a total of 19 frost point soundings was obtained between February 7 and October 5, which cover a wide range of undisturbed through strongly dehydrated situations. Together with several soundings from South Pole station between 1990 and 1994, they give a comprehensive picture of the general development of the dehydration in the Antarctic stratospheric vortex. The period of dehydration typically starts around the middle of June, and a rapid formation of large particles leads to a fast dehydration of the vortex. The evaporation of falling particles leads to rehydration layers, which have significantly higher water vapor concentrations than the undisturbed stratosphere. Through the formation of these rehydration layers in the early stages of the dehydration we can estimate a particle fall speed of ⅓ km/d and thus a mean particle size of 4 μm. Ice saturation was observed over McMurdo in only two cases and only well after the onset of the dehydration. From the inspection of synoptic maps it then follows that a small cold region inside the vortex seems to be sufficient to dehydrate the entire vortex. Above 20 km the dehydration is completed by the end of July. From the descent of the upper dehydration edge we can estimate a mean descent rate inside the vortex of 1.5 km/month. In McMurdo we observed occasional penetration of the vortex edge in cases where the vortex edge was close to McMurdo, however, these cases seem to have little effect on the bulk of the vortex. A sounding from November 3, 1990, at South Pole shows that the dehydration may persist into November and indicates that there is no significant transport into the vortex throughout winter and early spring.
Space‐time integrity of improved stratospheric and mesospheric sounder and microwave limb sounder temperature fields at Kelvin wave scales
Authors: Stone E. M. , Stanford J. L. , Ziemke J. R. , Allen D. R. , Taylor F. W. , Rodgers C. D. , Lawrence B. N. , Fishbein E. F. , Elson L. S. , Waters J. W.
Published: February 2004

Space‐time analyses, which are sensitive to details of retrieval and gridding processes not seen in zonal and time means, are used to investigate the integrity of version 8 gridded retrieved temperatures from the improved stratospheric and mesospheric sounder (ISAMS) on the upper atmosphere research satellite (UARS). This note presents results of such analyses applied to ISAMS tropical data. Comparisons are made with microwave limb sounder (MLS), also on UARS, temperatures. Prominent zonal wave number 1 features are observed with characteristics similar to those expected for Kelvin waves. Time versus longitude plots reveal quasi‐regular eastward phase progression from November 1991 to mid‐January 1992. The perturbations extend throughout the upper stratosphere and lower mesosphere (altitudes of 32–64 km), exhibiting peak‐to‐peak amplitudes of up to 2°–3° K and periods from ∼ 2 weeks in midstratosphere to ∼ 1 week at higher altitudes. Faster Kelvin waves with periods of 3–5 days are also found in the lower mesosphere. Height versus time plots reveal downward phase and upward group velocities, consistent with forcing from below. Vertical wavelengths are ∼ 20 km for the slower mode and about twice this scale for the faster 3 to 5‐day mode. The features are trapped within ±10°–15° of the equator. Kelvin wave signatures in ISAMS and MLS temperatures are compared at 10 and 1 hPa. Good agreement is found, illustrating the internal consistency and ability of both ISAMS and MLS temperature grids to capture relatively small amplitude features with space‐time scales of fast, zonally asymmetric equatorial modes.
The role of a deep convective storm over the tropical Pacific Ocean in the redistribution of atmospheric chemical species
Authors: Wang Chien , Crutzen Paul J. , Ramanathan V. , Williams Steven F.
Published: February 2004

A deep convective storm observed during the Central Equatorial Pacific Experiment (CEPEX) has been simulated using the two‐dimensional version of a three‐dimensional cloud dynamics, microphysics, and chemistry model. The simulated storm is characterized by the penetration of the cloud tower into the lower stratosphere and by a widespread cloud anvil. Clear evidence of vertical transport of water vapor through the convective turret to the upper troposphere has been found. A large amount of small ice crystals formed in the cloud top and in the anvil together with graupels induced by frozen raindrops. Intense mixing of boundary layer air into the cloud caused by the deep convection can result in low ozone area inside the cloud turret and inside the anvil. However, simultaneously, stratospheric air with high ozone volume mixing ratio was brought into the cloud region in the troposphere, especially in the upper part of the anvil, which could play a significant role in the tropical tropospheric O3 budget. Due to the deep convection, a significant fraction of boundary layer dimethyl sulfide (DMS) or other ocean‐derived chemicals is also transferred to the free troposphere, but we find no evidence for significant transport to the stratosphere.
Properties of Mount Pinatubo aerosols as derived from Nimbus 7 total ozone mapping spectrometer measurements
Authors: Torres O. , Herman J. R. , Bhartia P. K. , Ahmad Z.
Published: February 2004

The perturbations to the radiances measured by the Nimbus 7 total ozone mapping spectrometer (TOMS) during the 18 months after the eruption of Mount Pinatubo are used to derive weekly zonal mean values for the stratospheric aerosol optical thickness at 312.5 nm and zonal‐mean values for the area‐weighted or effective radius. The method uses the TOMS observations, on both sides of the orbital track, of the detailed structure in the backscattering region of the aerosol‐scattering phase function. Spatial and temporal evolution of the aerosol optical thickness and effective radius is obtained for the tropical region (25°N to 25°S) during most of the period mid‐July 1991 to December 1992. The largest derived value of optical thickness was 0.22 (+36%/−10%), obtained for the latitude zone from 5° to 15°S at the end of July 1991. By the end of 1992, tropical optical depths varied from 0.02 to 0.06 over the 25°N to 25°S geographical area. The main source of uncertainty in the derived optical depth is the altitude of the aerosol layer. The inferred time evolution of the effective radius clearly shows an increase in particle size. At the end of July 1991, effective radius values of about 0.5 μm were derived, while in the fall of 1992, these values were between 0.7 and 1.4 μm. Corrections and error estimates are obtained for the measured ozone amounts. The zonal average retrieved ozone amounts corrected for the presence of aerosols are within 1% of the unconnected zonal averages. Individual scan angles can have ozone amount corrections of ±3%, with a nadir view correction of 2%.
Carbon dioxide exchange in a peatland ecosystem
Authors: Shurpali N. J. , Verma S. B. , Kim J. , Arkebauer T. J.
Published: February 2004

Micrometeorological measurements of carbon dioxide exchange were made in an open peatland in north central Minnesota during two growing seasons (1991 and 1992). The vegetation at the site was dominated by Sphagnum papillosum, Scheuchzeria palustris, and Chamaedaphne calyculata. The objective of the study was to examine the diurnal and seasonal variations in canopy photosynthesis (P) and develop information on the net ecosystem CO2 exchange. The two seasons provided contrasting microclimatic conditions: as compared with 1991, the 1992 season was significantly wetter and cooler. Canopy photosynthesis was sensitive to changes in light, temperature, and moisture stress (as indicated by water table depth and atmospheric vapor pressure deficit). Under moderate conditions (temperature 18–28°C, vapor pressure deficit 0.7–1.5 kPa, and water table near the surface) during the peak growth period, midday (averaged between 1000–1400 hours) P values ranged from 0.15 to 0.24 mg m−2 s−1. Under high‐temperature (30°–34°C) and moisture stress (water table 0.16–0.23 m below the surface and vapor pressure deficit 2.2–3.0 kPa) conditions, midday P was reduced to about 0.03–0.06 mg m−2 s−1. There was a high degree of consistency in the values of P under similar conditions in the two seasons. Seasonally integrated values of the daily net ecosystem CO2 exchange indicated that the study site was a source of atmospheric CO2, releasing about 71 g C m−2 over a 145‐day period (May‐October) in 1991. Over a similar period in 1992, however, this ecosystem was a sink for atmospheric CO2 with a net accumulation of about 32 g C m−2. These results are consistent with previous investigations on CO2 exchange in other northern wetland sites during wet and dry periods.
Estimate of diffusion parameters of aircraft exhaust plumes near the tropopause from nitric oxide and turbulence measurements
Authors: Schumann U. , Konopka P. , Baumann R. , Busen R. , Gerz T. , Schlager H. , Schulte P. , Volkert H.
Published: February 2004

Horizontal and vertical plume scales and respective diffusivities for dispersion of exhaust plumes from airliners at cruising altitudes are determined from nitric oxide (NO) and turbulence data measured with the DLR Falcon research aircraft flying through the plumes. Ten plumes of known source aircraft were encountered about 5 to 100 min after emission at about 9.4 to 11.3 km altitude near the tropopause in the North Atlantic flight corridor at 8°W on three days in October 1993. The ambient atmosphere was stably stratified with bulk Richardson numbers greater than 10. The measured NO peaks had half widths of 500 to 2000 m with maximum concentrations up to 2.4 parts per billion by volume (ppbv), clearly exceeding the background values between 0.13 and 0.5 ppbv. For analysis the measured plumes are approximated by an analytical Gaussian plume model which accounts for anisotropic diffusion in the stably stratified atmosphere and for shear. Two methods are given to obtain diffusivity parameters from either the individual plume data or the set of all plume measurements. Using estimates of the emitted mass of NO per unit length, the vertical plume width is found to be 140 m on average. This width is related to mixing in the initial trailing vortex pair of the aircraft. The range of the plume data suggests vertical diffusivity values between 0 and 0.6 m2 s−1. The turbulence data exhibit strong anisotropic air motions with practically zero turbulent dissipation and weak vertical velocity fluctuations. This implies very small vertical diffusivities. The horizontal diffusivity is estimated as between 5 and 20 m2 s−1 from the increase of horizontal plume scales with time. For constant diffusivities, shear dominates the lateral dispersion after a time of about 1 hour even for the cases with only a weak mean shear of 0.002 s−1.
Influence of cirrus cloud radiative forcing on climate and climate sensitivity in a general circulation model
Authors: Lohmann Ulrike , Roeckner Erich
Published: February 2004

Six numerical experiments have been performed with a general circulation model (GCM) to study the influence of high‐level cirrus clouds and global sea surface temperature (SST) perturbations on climate and climate sensitivity. The GCM used in this investigation is the third‐generation ECHAM3 model developed jointly by the Max Planck Institute for Meteorology and the University of Hamburg. It is shown that the model is able to reproduce many features of the observed cloud radiative forcing with considerable skill, such as the annual mean distribution, the response to seasonal forcing, and the response to observed SST variations in the equatorial Pacific. In addition to a reference experiment where the cirrus emissivity is computed as a function of the cloud water content, two sensitivity experiments have been performed in which the cirrus emissivity is either set to zero everywhere above 400 hPa (“transparent cirrus”) or set to 1 (“black cirrus”). These three experiments are repeated identically, except for prescribing a globally uniform SST warming of 4 K. Similar to earlier GCM studies, the changed cloud radiative heating within the troposphere has a profound impact on the model climate. Since the initial radiative forcing introduced by the changed cirrus emissivity is much smaller than the convective or dynamical response, we conclude that the tropical circulation, in particular, is maintained through a positive feedback loop involving cirrus radiative heating, deep cumulus convection, and moisture supply through the large‐scale dynamics. Since this interaction has been identified in at least two other GCMs employing different cumulus parameterizations, it does not crucially depend on the respective closure assumption. Moreover, the radiative‐convective‐dynamical coupling in the tropics is relevant also in the global warming experiment through the increase of cloud water and hence cirrus radiative heating in the warmer atmosphere. It is shown that the spin‐up of the Walker circulation in both the global warming and the increased cirrus emissivity experiments is a result of a selection process which enhances the diabatic heat source through asymmetries of the circulation itself, and the extra differential heating feeds back positively on the circulation. It is also shown that cirrus clouds have a significant influence on the global climate sensitivity of the model. In the climate change experiment with the standard model, the climate sensitivity is 20% higher than in a clear‐sky reference atmosphere because the increase of cirrus emissivity in the warmer atmosphere contributes substantially to the overall positive cloud feedback. In the transparent cirrus model the cloud feedback is negative, and the global sensitivity is reduced by 20% as compared to a clear‐sky reference atmosphere.
Stratospheric temperature response to improved solar CO2 and H2O parameterizations
Authors: Freidenreich S. M. , Ramaswamy V.
Published: February 2004

A fixed‐dynamical heating model is used to investigate the temperature changes in the stratosphere due to improved CO2 and H2O shortwave heating parameterizations. Besides being governed by the magnitude of the local heating, the temperature change in any layer due to the improved parameterizations is also dependent on the distribution of the solar heating in other stratospheric layers. This is a consequence of the longwave radiative exchange process, in which the temperature change in other layers, due to the imposed heating perturbations, leads to an exchange of longwave radiative energy with the layer in question, thus affecting its response. Thus the vertical profile of the heating rate becomes a significant factor in determining the stratospheric thermal profile. This investigation also confirms the sensitivity of the temperature response in the lower stratosphere to perturbations in the shortwave CO2 and H2O heating.
The effects of horizontally extended clouds on backscattered ultraviolet sunlight
Authors: Wen Guoyong , Frederick John E.
Published: February 2004

This work examines the effects of clouds on backscattered ultraviolet radiation via analysis of satellite observations and radiative transfer modeling. Measurements from the continuous scan mode of the Nimbus 7 SBUV instrument show that the enhancement in backscatter albedo associated with clouds generally increases with wavelength. However, high clouds enhance the backscattered ultraviolet radiation at short wavelengths more than do low or middle level clouds. For an overhead Sun in the tropics, the enhancements in backscattered radiance from high clouds are about 5 times those from low clouds at a wavelength of 300 nm. For a typical mid‐latitude atmosphere with a solar zenith angle of 60°, the enhancements in backscattered radiance from high clouds are approximately double those due to low level clouds.
Daily variability of lower tropospheric winds over the tropical western Pacific
Authors: Gutzler David S. , Hartten Leslie M.
Published: February 2004

Daily variability in lower tropospheric winds over the tropical western Pacific is examined using data from a network of wind‐profiling radars deployed during the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). The amplitude of the diurnal cycle in both u and v at the the westernmost sites, located just to the east of the large island of New Guinea, is of the order of 0.5 ms−1, significantly larger than the diurnal cycle observed east of the dateline in previous studies. The larger diurnal variability is attributed to the pronounced nocturnal maximum in deep convection over the island of New Guinea; the influence of this diurnal circulation extends (with smaller amplitude) to the more distant profiler sites 10°–15° E of New Guinea. The semidiurnal cycle of the zonal wind is smaller in amplitude but coherent in phase at all sites; the meridional wind exhibits a negligible semidiurnal cycle. These results are consistent with the solar semidiurnal tidal oscillation in pressure and entirely consistent with previous studies of eastern tropical Pacific winds. Two of the profilers were deployed on ships rather than islands, and we find no systematic differences between the ship data and the small island data attributable to the complete absence of land at the ship sites.
Fuel biomass and combustion factors associated with fires in savanna ecosystems of South Africa and Zambia
Authors: Shea Ronald W. , Shea Barbara W. , Kauffman J. Boone , Ward Darold E. , Haskins Craig I. , Scholes Mary C.
Published: February 2004

Fires are dominant factors in shaping the structure and composition of vegetation in African savanna ecosystems. Emissions such as CO2, NOx, CH4, and other compounds originating from these fires are suspected to contribute substantially to changes in global biogeochemical processes. Limited quantitative data exist detailing characteristics of biomass, burning conditions, and the postfire environment in African savannas. Fourteen test sites, differentiated by distinct burn frequency histories and land‐use patterns, were established and burned during August and September 1992 in savanna parklands of South Africa and savanna woodlands of Zambia. Vegetation physiognomy, available fuel loads, the levels of biomass consumed by fire, environmental conditions, and fire behavior are described. In the South African sites, total aboveground fuel loads ranged from 2218 to 5492 kg ha−1where fire return intervals were 1–4 years and exceeded 7000 kg ha−1at a site subjected to 38 years of fire exclusion. However, fireline intensity was only 1419 kW m−1at the fire exclusion site, while ranging from 480 to 6130 kW m−1among the frequent fire sites. In Zambia, total aboveground fuel loads ranged from 3164 kg ha−1in a hydromorphic grassland to 7343 kg ha−1in a fallow shifting cultivation site. Dormant grass and litter constituted 70–98% of the total fuel load among all sites. Although downed woody debris was a relatively minor fuel component at most sites, it constituted 43–57% of the total fuel load in the fire exclusion and shifting cultivation sites. Fire line intensity ranged between 1734 and 4061 kW m−1among all Zambian sites. Mean grass consumption generally exceeded 95%, while downed woody debris consumption ranged from 3 to 73% at all sites. In tropical savannas and savanna woodlands of southern Africa, differences in environmental conditions, land‐ use patterns, and fire regimes influence vegetation characteristics and thus influence fire behavior and biomass consumption.
The Regional Particulate Matter Model: 1. Model description and preliminary results
Authors: Binkowski Francis S. , Shankar Uma
Published: February 2004

The Regional Acid Deposition Model has been modified to create the Regional Particulate Model, a three‐dimensional Eulerian model that simulates the chemistry, transport, and dynamics of sulfuric acid aerosol resulting from primary emissions and the gas phase oxidation of sulfur dioxide. The new model uses a bimodal lognormal distribution to represent particles in the submicrometer size range. In addition to including the horizontal and vertical advection and vertical diffusion of the aerosol number concentration and sulfate mass concentration fields, the model now explicitly treats the response of the distribution parameters to particle coagulation within and between the modes, condensation of sulfate vapor onto existing particles, formation of new particles, evaporation and condensation of ambient water vapor in the presence of ammonia, and particle‐size‐dependent dry deposition. The model has been used to study how the degree of sulfuric acid neutralization by ambient ammonia affects the total aerosol concentrations and particle size distributions over eastern North America. Preliminary results for three representative locations, rural, near‐source, and nominal downwind of source, show that the effect is greatest for the rural and smallest for the near‐source regions, which corresponds with the largest and smallest values, respectively, of ammonium‐to‐sulfate molar ratios. The results indicate that the model could provide a tool for investigating the effects of various pollution control strategies, as well as new or alternative formulations of important aerosol processes.
Large‐scale properties of simulated soil water variability
Authors: Guetter Alexandre K. , Georgakakos Konstantine P.
Published: February 2004

Surface water balance computations were applied to historical data from the central and southeastern United States. The focus of the study was to characterize soil water variability across spatial scales in these two different parts of the United States. The regions studied in each area span the range of spatial scales from 106 km2 to 103 km2. Routinely collected precipitation, temperature, pan evaporation, and discharge data were utilized together with widely used parameterizations to reconstruct soil water (depth‐integrated and areal‐averaged soil moisture) for the period from January 1960 through December 1988. The results suggest that in the central United States, larger regions possess a greater range of temporal soil water variability than smaller embedded regions and that extreme soil water anomalies possess scales of spatial coherence that range from 105 km2 to 103 km2, with stronger anomalies characterized by smaller scales. In the Mississippi‐Missouri region, variability of aggregate soil water cannot be fully characterized by soil water variability in a few “representative” catchments. Remarkable similarity of the spatiotemporal variability of soil water across scales (106 km2 to 103 km2) was found for the southeastern United States. In this area the variability across scales was characterized by a frequency scaling ratio which decreased linearly with soil water anomaly strength and which remained substantially higher than that corresponding to random occurrence, even for extreme soil water anomalies. In both regions of the United States and for all spatial scales, extreme positive anomalies show smaller spatial coherence than negative anomalies of the same magnitude.
Large‐scale air mass characteristics observed over western Pacific during summertime
Authors: Browell E. V. , Fenn M. A. , Butler C. F. , Grant W. B. , Merrill J. T. , Newell R. E. , Bradshaw J. D. , Sandholm S. T. , Anderson B. E. , Bandy A. R. , Bachmeier A. S. , Blake D. R. , Davis D. D. , Gregory G. L. , Heikes B. G. , Kondo Y. , Liu S. C. , Rowland F. S. , Sachse G. W. , Singh H. B. , Talbot R. W. , Thornton D. C.
Published: February 2004

Remote and in situ measurements of gases and aerosols were made with airborne instrumentation to investigate the sources and sinks of tropospheric gases and aerosols over the western Pacific during the NASA Global Tropospheric Experiment (GTE)/Pacific Exploratory Mission‐West A (PEM‐West A) conducted in September–October 1991. This paper discusses the general characteristics of the air masses encountered during this experiment using an airborne lidar system for measurements of the large‐scale variations in ozone (O3) and aerosol distributions across the troposphere and airborne in situ instrumentation for comprehensive measurements of air mass composition. In low latitudes of the western Pacific the airflow was generally from the east, and under these conditions the air was observed to have low aerosol loading and low ozone levels throughout the troposphere. Ozone was found to be below 10 parts per billion volume (ppbv) near the surface to 40–50 ppbv in the middle to upper troposphere. In the middle and high latitudes the airflow was mostly westerly, and the background O3 was generally less than 55 ppbv. On 60% of the PEM‐West A flights, O3 was observed to exceed these levels in regions that were determined to be associated with stratospheric intrusions. In convective outflows from typhoons, near‐surface air with low ozone (<25 ppbv) was transported into the upper troposphere (>10 km). Several cases of continental plumes from Asia were observed over the Pacific during westerly flow conditions. These plumes were found in the lower troposphere with ozone levels in the 60–80 ppbv range and enhanced aerosol scattering. At low latitudes over the central Pacific the troposphere primarily contained air with background or low ozone levels; however, stratospherically influenced air with enhanced ozone (40–60 ppbv) was observed several times in the lower troposphere. The frequency of observation of the air masses and their average chemical composition are also discussed in this paper.
Model study of tropospheric trace species distributions during PEM‐West A
Authors: Liu S. C. , McKeen S. A. , Hsie E‐Y. , Lin X. , Kelly K. K. , Bradshaw J. D. , Sandholm S. T. , Browell E. V. , Gregory G. L. , Sachse G. W. , Bandy A. R. , Thornton D. C. , Blake D. R. , Rowland F. S. , Newell R. , Heikes B. G. , Singh H. , Talbot R. W.
Published: February 2004

A three‐dimensional mesoscale transport/photochemical model is used to study the transport and photochemical transformation of trace species over eastern Asia and western Pacific for the period from September 20 to October 6, 1991, of the Pacific Exploratory Mission‐West A experiment. The influence of emissions from the continental boundary layer that was evident in the observed trace species distributions in the lower troposphere over the ocean is well simulated by the model. In the upper troposphere, species such as O3, NOy (total reactive nitrogen species), and SO2 which have a significant source in the stratosphere are also simulated well in the model, suggesting that the upper tropospheric abundances of these species are strongly influenced by stratospheric fluxes and upper tropospheric sources. In the case of SO2 the stratospheric flux is identified to be mostly from the Mount Pinatubo eruption. Concentrations in the upper troposphere for species such as CO and hydrocarbons, which are emitted in the continental boundary layer and have a sink in the troposphere, are significantly underestimated by the model. Two factors have been identified to contribute significantly to the underestimate: one is emissions upwind of the model domain (eastern Asia and western Pacific); the other is that vertical transport is underestimated in the model. Model results are also grouped by back trajectories to study the contrast between compositions of marine and continental air masses. The model‐calculated altitude profiles of trace species in continental and marine air masses are found to be qualitatively consistent with observations. However, the difference in the median values of trace species between continental air and marine air is about twice as large for the observed values as for model results. This suggests that the model underestimates the outflow fluxes of trace species from the Asian continent and the Pacific rim countries to the ocean. Observed altitude profiles for species like CO and hydrocarbons show a negative gradient in continental air and a positive gradient in marine air. A mechanism which may be responsible for the altitude gradients is proposed.
Validation of UARS Microwave Limb Sounder ozone measurements
Authors: Froidevaux L. , Read W. G. , Lungu T. A. , Cofield R. E. , Fishbein E. F. , Flower D. A. , Jarnot R. F. , Ridenoure B. P. , Shippony Z. , Waters J. W. , Margitan J. J. , McDermid I. S. , Stachnik R. A. , Peckham G. E. , Braathen G. , Deshler T. , Fishman J. , Hofmann D. J. , Oltmans S. J.
Published: February 2004

This paper describes the validation of ozone data from the Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS). The MLS ozone retrievals are obtained from the calibrated microwave radiances (emission spectra) in two separate bands, at frequencies near 205 and 183 GHz. Analyses described here focus on the MLS Version 3 data (the first set of files made publicly available). We describe results of simulations performed to assess the quality of the retrieval algorithms, in terms of both mixing ratio and radiance closure. From actual MLS observations, the 205‐GHz ozone retrievals give better closure (smaller radiance residuals) than that from the 183‐GHz measurements and should be considered more accurate from the calibration aspects. However, the 183‐GHz data are less noise limited in the mesosphere and can provide the most useful scientific results in that region. We compare the retrieved 205‐GHz ozone profiles in the middle‐to lower stratosphere to ozonesonde measurements at a wide range of latitudes and seasons. Ground‐based lidar data from Table Mountain, California, provide a good reference for comparisons at higher altitudes. Based on these analyses, comparisons with balloon‐borne measurements and others, as well as a detailed budget of estimated uncertainties, MLS results appear to be generally of high quality, with some biases worth mentioning. Results for the lowermost stratosphere (∼50 to 100 hPa) are still in need of improvement. A set of estimated precision and accuracy values is derived for the MLS ozone data sets. We also comment on recent updates in the retrieval algorithms and their impact on ozone values.
Distribution and budget of O3 in the troposphere calculated with a chemistry general circulation model
Authors: Roelofs Geert‐Jan , Lelieveld Jos
Published: February 2004

We present results of global tropospheric chemistry simulations with the coupled chemistry/atmospheric general circulation model ECHAM. Ultimately, the model will be used to study climate changes induced by anthropogenic influences on the chemistry of the atmosphere; meteorological parameters that are important for the chemistry, such as temperature, humidity, air motions, cloud and rain characteristics, and mixing processes are calculated on‐line. The chemical part of the model describes background tropospheric CH4‐CO‐NOx‐HOx photochemistry. Emissions of NO and CO, surface concentrations of CH4, and stratospheric concentrations of O3 and NOy are prescribed as boundary conditions. Calculations of the tropospheric O3 budget indicate that seasonal variabilities of the photochemical production and of injection from the stratosphere are represented realistically, although some aspects of the model still need improvement. Comparisons of calculated O3 surface concentrations and O3 profiles with available measurements show that the model reproduces O3 distributions in remote tropical and midlatitudinal sites. Also, the model matches typical profiles connected with deep convection in the Intertropical Convergence Zone (ITCZ). However, the model tends to underestimate O3 concentrations at the poles and in relatively polluted regions. These underestimates are caused by the poor representation of tropopause foldings in midlatitudes, which form a significant source of tropospheric O3 from the stratosphere, too weak transport to the poles, and the neglect of higher hydrocarbon chemistry. Also, mixing of polluted continental boundary layer air into the free troposphere may be underestimated. We discuss how these model deficiencies will be improved in the future.
First direct kinetic study of isotopic enrichment of ozone
Authors: Sehested Jens , Nielsen Ole John , Egsgaard Helge , Larsen Niels Wessel , Pedersen Thorvald , Christensen Lene Krogh , Wiegell Mette
Published: February 2004

The formation kinetics of ozone has been studied using isotopes and pulse radiolysis combined with time‐resolved UV absorption spectroscopy. An enhancement of (9.8±2.6)% was found for the rate constant for the reaction of 18O with 18O18O relative to that for the 16O+ 16O16O reaction. The average formation rate for unsymmetric 18O16O16O and 16O18O18O from O2 and 18O18O respectively was enhanced by (14.7±2.8)%. For the formation of a mixture of symmetric and unsymmetric ozone species from O18O the enhancement was (3.6±2.4)%. This leads to the conclusion that both mass and symmetry affect the rate constant for formation of isotopic ozone. The results are compared with recent enhancement studies from the literature, and an apparent conflict is discussed.
Biogenic emissions in Europe: 1. Estimates and uncertainties
Authors: Simpson David , Guenther Alex , Hewitt C. Nicholas , Steinbrecher Rainer
Published: February 2004

Several biogenic volatile organic compound (VOC) emission algorithms have been used, together with meteorological data from the EMEP MSC‐W ozone model, to generate estimates of the emissions of isoprene from European forests and agricultural crops over several summer periods. The most up‐to‐date estimate combines the recently updated isoprene emission factors from the United States with available knowledge of European tree species and emission factors. In some cases these European emission factors are significantly different from their U.S. equivalents because of differences in the tree species represented within a forest classification, especially with regard to spruce genera and Mediterranean oak genera. The new estimates have resulted in an approximate factor of 2–3 increase in isoprene emissions from northern Europe but a factor of 2 decrease in isoprene estimates for southern Europe. Overall, European isoprene emissions are estimated to be about 4000 kt C yr−1, approximately 50–100% greater than previous estimates. Preliminary estimates are also made of the emissions of the so‐called OVOC (other VOC) from forests and of soil NOx emissions. All of these estimates of biogenic emissions are subject to considerable uncertainty, not least because of a lack of knowledge of the species coverage in most European countries and of the appropriate emission factors which should be applied. Factors of 5–10 uncertainty are not unlikely for episodic ozone calculations. The implications of these uncertainties for the results of control strategy evaluations for rural ozone in Europe are assessed in a companion paper.
Variability of the global precipitable water with a timescale of 90–150 days
Authors: Chen Tsing‐Chang , Pfaendtner James , Chen Jau‐Ming , Wikle Christopher K.
Published: February 2004

A 90–150 day signal is identified in the global precipitable water field generated by the Global Data Assimilation Systems (GDAS) of the Goddard Laboratory for Atmospheres (GLA), National Meteorological Center, and European Centre for Medium‐Range Weather Forecasts. The finding of this intraseasonal signal in global precipitable water is significant for two reasons: (1) it suggests that there is 90–150 day intraseasonal variability in the atmospheric branch of the global hydrological cycle and (2) it provides a useful parameter to test the sensitivity of the GDAS‐generated hydrological data. This newly identified intraseasonal signal in the global precipitable water was verified with Special Sensor Microwave/Imager precipitable water data over oceans and station‐mixing ratio data over the continental United States. Based upon some simple statistical analyses and global and regional composite charts, it was found that the 90–150 day low‐frequency oscillations contained in different GDAS data sets are more coherent with each other in regions with good data coverage but are poorly correlated over the data‐sparse areas. Furthermore, the GLA GDAS provides the most realistic representation of this intraseasonal global precipitable water signal.
On the derivation of kernels for kernel‐driven models of bidirectional reflectance
Authors: Wanner W. , Li X. , Strahler A. H.
Published: February 2004

A new approximation to Ross' (1981) radiative transfer theory for small values of leaf area index (LAI) and two new approximations to Li and Strahler's (1992) geometric‐optical mutual shadowing model are derived. These, together with Roujean et al.'s (1992) approximation to Ross' theory for large LAI and their geometric‐optical model of rectangular protrusions, may be used for formulating semiempirical models of the bidirectional reflectance distribution function (BRDF) of the land surface through linear combinations. Because the functions superimposed depend only on viewing and illumination geometry, the BRDF models derived may be called kernel‐driven; but Nilson and Kuusk's (1989) modified version of Walthall et al.'s (1985) model is an example of an empirical model that belongs to this same class. The linearity of kernel‐driven models is advantageous to global BRDF and albedo processing needs in several respects, most notably analytical invertibility, making possible look‐up table approaches to albedo calculation, accommodation of mixed pixel situations, and spatial scaling. The models discussed here are being proposed for BRDF/albedo processing for the moderate resolution imaging spectroradiometer (MODIS) sensor of NASA's Earth Observing System (EOS).
Studies of North American continental‐scale hydrology using Eta model forecast products
Authors: Berbery Ernesto H. , Rasmusson Eugene M. , Mitchell Kenneth E.
Published: February 2004

In this study, the forecast products of the National Meteorological Center's Eta model during the period August 1993 to March 1994 are critically examined to gain insight into the quality of the first guess fields used in the model's four‐dimensional data assimilation and the usefulness of the forecast products in diagnostic studies involving unmeasured hydrological variables. We find that the Eta model 12–36 hour forecasts produce patterns of monthly precipitation that are a reasonably good approximation of the monthly observed precipitation fields for the period under consideration. The diurnal cycle of the water vapor fluxes and their seasonal changes obtained from the model forecasts are also in good agreement with observations. During the warm months the stationary component of the forecast fluxes includes a good representation of the low‐level jet (LLJ) that is an important means for the transport of moisture from the Gulf of Mexico into the Great Plains. The LLJ decays during autumn, when it is replaced by the transient component of the fluxes as the means of transport of water vapor from the Gulf of Mexico region into the United States. Finally, evaporation estimates were derived from the convergence of the fluxes of the forecast fields and model and observed values of precipitation. All estimates agree within 0.5 mm d−1 and showed a reasonable seasonal cycle, with maximum evaporation during the warm season and a minimum evaporation during February.
A multiple‐level trajectory analysis of vortex filaments
Authors: Schoeberl Mark R. , Newman Paul A.
Published: February 2004

Simultaneous, multiple‐level contour advection and reverse domain‐filling simulations of the polar vortex are performed to examine the three‐dimensional structure of filaments. The evolution of a contour near the vortex edge over three northern hemisphere winters is examined. The results show that filaments forming at the edge have a fairly coherent vertical structure often over the whole stratosphere. The filaments linked together form vertically aligned material sheets which tend to tilt outward with height. Three specific examples of vortex edge evolution over 10‐day periods are shown; two of these examples appear to be somewhat similar to the “remote” and “local” wave‐breaking cases studied by Dritschel and Savaranan [1994]. The third example, a final warming, is a combination of the two other cases. Using the change in the vortex edge contour length to compute the Liapunov coefficient, we show the variation in that quantity, the geopotential height variance, and the eddy diffusion coefficient Kyy for the three winters. The changes in Liapunov coefficient roughly correlate with the changes in the other two quantities.
Atmospheric aerosol major ion composition and cloud condensation nuclei over the northeast Atlantic
Authors: Harrison Roy M. , Peak John D. , Kaye Andrew D.
Published: February 2004

Concentrations of the major soluble ions, non‐sea‐salt (nss) sulfate, nitrate, chloride, and ammonium have been measured in aerosol samples collected at various locations and altitudes over the North Atlantic Ocean. Concentrations of nss sulfate, nitrate, and ammonium were found to be fairly uniform within the maritime boundary layer in an air mass aged in the Azores anticyclone; average values obtained were 0.36 ± 0.11, 0.18 ± 0.03, and 0.16 ± 0.06 μg m−3, respectively. The mean cloud condensation nuclei (CCN) concentration at 1% supersaturation was 368 cm−3, corresponding to air containing the major ion concentrations above. Incursion of continental air with a travel time of about 3 days over the sea led to an increase of around fivefold in nss sulfate, nitrate, and ammonium concentrations, while CCN concentrations increased less than threefold to 1076 cm−3. Two air samples collected during the clean air period above the marine boundary layer showed much reduced concentrations of soluble ions, but a mean CCN concentration of 358 cm−3. Regression analysis of all air sample data indicates that the nss sulfate and nitrate data are correlated to the observed CCN number density. While these ions are clearly making a significant contribution to CCN activity, the degree of correlation indicates that other chemical species also play a role as CCN.
Microscale tortuosity and its variation as observed in triggered lightning channels
Authors: Idone Vincent P.
Published: February 2004

Slow‐streak photographic recordings of the lowest approximately 4–5 m of lightning channels in four triggered flashes are presented to document examples of microscale tortuosity variations (∼ 10 cm or smaller geometric features). These recordings have both high spatial resolution (centimeter scale) and modest temporal resolution (∼ 1.5 ms) such that the channel geometry is accurately rendered formost return strokes and their ensuing M‐component events. The M‐component events, verified as such from ancillary fast‐streak data records, often show a significant enhancement of the microscale tortuosity relative to the previous return stroke channel. Channel length increases (two‐dimensional) corresponding to the tortuosity enhancements are found to be as large as 55%, though the mean for 10 evaluations is 23%. In contrast, two M‐component events show a marked overall reduction of the microscale tortuosity relative to an immediately preceding sequence of several discharge pulses along the same meter scale channel. Ensuing return strokes occurring along common but previously distorted channels also show a tendency for overall tortuosity reduction or “straightening.” These tortuosity variations appear to be unrelated to the presence of the vaporized trigger wire residue. Three primary mechanisms of tortuosity enhancement are examined: Z pinch magnetohydrodynamic instability (MHD), discharge generated turbulence, and ambient planetary boundary layer turbulence. The Z pinch instability is assessed quantitatively and may be a contributor through either the high current phase or integrated over a continuing current period. Three variants of discharge generated turbulence also could be contributors, but a critical test of these mechanisms cannot be made with the present data set. Ambient planetary boundary layer turbulence appears unable to account for the observed tortuosity enhancements. An inductive mechanism of tortuosity reduction is also examined as a possible explanation for the disappearance of individual acute kinks or loops as well as overall channel tortuosity reductions. This mechanism cannot explain M‐component related tortuosity reductions, but it might be a viable explanation for return stroke straightening given the much larger current derivatives involved. A full accounting of microscale tortuosity variations in lightning discharge channels remains to be achieved.