摘要:

We examine simulations of the surface mass balance of the sector of the East Antarctic Ice Sheet between 2.4°W and 110.5°E as produced by the United Kingdom Meteorological Office Unified Climate Model. Estimates of the actual mass balance of this sector can be obtained from glaciological observations of snow accumulation and from studies of the atmospheric water vapor budget using radiosonde observations. The former technique gives an average sector accumulation of 104 mm yr−1, and the latter yields 157 mm yr−1. The modeled accumulation in this sector, 122 mm yr−1, lies between these two estimates, suggesting that the model can accurately represent the processes controlling surface mass balance. However, examination of the atmospheric water vapor budget in the model shows that only 30% of the water vapor precipitated in this sector is carried by resolved‐scale transport. Although the model produces the “correct” accumulation in present‐day climate simulations, it is not clear that this would change appropriately if the model were used to simulate future climates. By producing synthetic estimates of water vapor transport at radiosonde station locations we have used the model data to investigate the uncertainties in estimating sector accumulation from radiosonde data. The results of this study indicate that the radiosonde technique will tend to overestimate sector accumulation, thus reconciling the two observat

ISSN:0148-0227    

DOI:10.1029/95JD03034

年代:1996

数据来源: WILEY

摘要:

Natural volatile organic compound (VOC) emissions were investigated at two forested sites in the southeastern United States. A variety of VOC compounds including methanol, 2‐methyl‐3‐buten‐2‐ol, 6‐methyl‐5‐hepten‐2‐one, isoprene and 15 monoterpenes were emitted from vegetation at these sites. Diurnal variations in VOC emissions were observed and related to light and temperature. Variations in isoprene emission from individual branches are well correlated with light intensity and leaf temperature while variations in monoterpene emissions can be explained by variations in leaf temperature alone. Isoprene emission rates for individual leaves tend to be about 75% higher than branch average emission rates due to shading on the lower leaves of a branch. Average daytime mixing ratios of 13.8 and 6.6 ppbv C isoprene and 5.0 and 4.5 ppbv C monoterpenes were observed at heights between 40 m and 1 km above ground level the two sites. Isoprene and monoterpenes account for 30% to 40% of the total carbon in the ambient non‐methane VOC quantified in the mixed layer at these sites and over 90% of the VOC reactivity with OH. Ambient mixing ratios were used to estimate isoprene and monoterpene fluxes by applying box model and mixed‐layer gradient techniques. Although the two techniques estimate fluxes averaged over different spatial scales, the average fluxes calculated by the two techniques agree within a factor of two. The ambient mixing ratios were used to evaluate a biogenic VOC emission model that uses field measurements of plant species composition, remotely sensed vegetation distributions, leaf level emission potentials determined from vegetation enclosures, and light and temperature dependent emission activity factors. Emissions estimated for a temperature of 30°C and above canopy photosynthetically active radiation flux of 1000 μmol m−2s−1are around 4 mg C m−2h−1of isoprene and 0.7 mg C m−2h−1of monoterpenes at the ROSE site in western Alabama and 3 mg C m−2h−1of isoprene and 0.5 mg C m−2h−1of monoterpenes at the SOS‐M site in eastern Georgia. Isoprene and monoterpene emissions based on land characteristics data and emission enclosure measurements are within a factor of two of estimates based on ambient measurements in most cases. This represents reasonable agreement due to the large uncertainties associated with these models and because the observed differences are at least partially due to differences in the size and location of the source region (“fl

ISSN:0148-0227    

DOI:10.1029/95JD03006

年代:1996

数据来源: WILEY

摘要:

We report on an ecosystem modeling approach that integrates global satellite, climate, vegetation, and soil data sets to (1) examine conceptual controls on nitrogen trace gas (NO, N2O, and N2) emissions from soils and (2) identify weaknesses in our bases of knowledge and data for these fluxes. Nitrous and nitric oxide emissions from well‐drained soils were estimated by using an expanded version of the Carnegie‐Ames‐Stanford (CASA) Biosphere model, a coupled ecosystem production and soil carbon‐nitrogen model on a 1° global grid. We estimate monthly production of NO, N2O, and N2based on predicted rates of gross N mineralization, together with an index of transient water‐filled pore space in soils. Analyses of model performance along selected climate gradients support the hypothesis that low temperature restricts predicted N mineralization and trace gas emission rates in moist northern temperate and boreal forest ecosystems, whereas in tropical zones, seasonal patterns in N mineralization result in emission peaks for N2O that coincide with wetting and high soil moisture content. The model predicts the annual N2O:NO flux ratio at a mean value of 1.2 in wet tropical forests, decreasing to around 0.6 in the seasonally dry savannas. Global emission estimates at the soil surface are 6.1 Tg N and 9.7 Tg N yr−1for N2O and NO, respectively. Tropical dry forests and savannas are identified by using this formulation as important source areas for nitrogen trace gas emissions. Because humans continue to alter these ecosystems extensively for agricultural uses, our results suggest that more study is needed in seasonally dry ecosystems of the tropics in order to understand the global impacts of land use change on soil sources fo

ISSN:0148-0227    

DOI:10.1029/95JD02028

年代:1996

数据来源: WILEY

摘要:

Simultaneous measurements are reported of the nitrate radical (NO3), nitrogen dioxide (NO2), ozone (O3), and dimethylsulfide (DMS) in the nighttime marine boundary layer over Biscayne Bay in South Florida. These field observations are analyzed and used to initialize a boundary layer box model which examines the relative importance of the various sinks for NOxin the marine boundary layer. The results show that the observed lifetime of NO3(≤6 min.) is probably controlled both by the loss of nitrogen pentoxide (N2O5) to reaction with water vapor and aerosols and by the reaction between NO3and DMS. The model is then extended to investigate the loss of nitrogen oxides from an air parcel that remains in the boundary layer with a constant sea‐to‐air DMS flux for several days. The principal conclusions are (1) that DMS is a much more important sink for NO3at lower NO2levels and (2) that the reaction between NO3and DMS is an important sink for DMS in the marine boundary layer and could exceed that of the daytime removal

ISSN:0148-0227    

DOI:10.1029/95JD02905

年代:1996

数据来源: WILEY

摘要:

The vertical ozone distribution over the Atlantic Ocean has been measured in situ by shipborne ozone soundings during three RVPolarsternmeridional transects in January/February 1993, October/November 1993, and May/June 1994. The low ozone column densities measured by Nimbus 7 and Meteor 3 satellites in 1993 could be confirmed by our investigations. We observed distinct differences in the vertical distribution pattern of tropospheric ozone between the northern and the southern hemisphere: The ozone mixing ratio gradients were flat in the northern hemisphere and ozone mixing ratios in the free troposphere never did exceed 80 parts per billion by volume (ppbv) up to the tropopause, while the southern hemisphere exhibited a pronounced vertical gradient. Extremely dry air masses with enhanced ozone amounts up to 120 ppbv have been found in the tropical free troposphere of the southern hemisphere between 0° and 20°S. The vertical ozone stratification in the troposphere of the southern hemisphere was dominated by this large‐scale feature. Photochemical ozone production as a consequence of the emissions of natural fires or intrusions of stratospheric air masses are the most probable sources for these ozone‐rich layers. On the basis of our results, a stringent differentiation between both alternatives could not be

ISSN:0148-0227    

DOI:10.1029/95JD02838

年代:1996

数据来源: WILEY

摘要:

A mesoscale chemistry transport model is coupled to a numerical weather prediction model (NWP) for Europe and the North Atlantic. It is applied to show that air traffic emissions significantly increase the concentrations of NOxas well as the net chemical formation rate of ozone over both the Atlantic Ocean and over central Europe. The time period studied was July 1–10, 1991, a period characterized by a high‐pressure ridge over North Europe. In convection the vertical gradient of NOxis strongly reduced because the midtropospheric concentrations increase. In the updraft region of convective plumes, chemical ozone formation up to 1.8 ppbv/h is calculated in the upper troposphere, and this is about a factor 50 higher than the average for 10.5 km over continental Europe during the 10‐day period. The high production rates are not sustained for more than a few hours, and it would be difficult to identify the effect of the process in a local measurement of the ozone concentration. The calculations indicate that 10–25 molecules of O3(0.03–0.05 ppb/h) are generated on the average per NOxmolecule emitted from aircraft (2–3 ppt/h). On convective days over regions with significant surface sources of NOx, ozone formation in the upper troposphere due to NOxbrought there by convection, outweighs the role of aircraft NOxemissions. The net chemical formation rate of ozone across the model domain depends on the boundary concentrations of NOxand O3, but the change in the upper troposphere in the net chemical formation rate of ozone induced by aircraft NOxemissions is quite independent of the concentrations

ISSN:0148-0227    

DOI:10.1029/95JD03070

年代:1996

数据来源: WILEY

摘要:

Three‐dimensional model calculations suggest that the world's fleet of subsonic aircraft has enhanced the abundance of nitrogen oxides in the upper troposphere by up to 20–35% and has produced a significant increase in the ozone concentration in this region of the atmosphere (4% in summer and 1% in winter). In year 2050, on the basis of current scenarios for growth in aviation, the concentration of NOxat 10 km could increase by 30–60% at midlatitudes, and the concentration of ozone could be enhanced by 7% and 2% in summer and winter, respectively (relative to a situation without aircraft effects). The perturbation is not limited to the flight corridors but affects the entire northern hemisphere. The magnitude (and even the sign) of the ozone change depends on the level of background atmospheric NOxand hence on NOxsources (lightning, intrusion from the stratosphere, and convective transport from the polluted boundary layer) and sinks which are poorly quantified in this region of the atmosphere. On the basis of our model estimates, 20% of the NOxfound at 10 km (midlatitudes) is produced by aircraft engines, 25% originates from the surface (combustion and soils), and approximately 50% is produced by lightning. For a lightning source enhanced in the model by a factor of 2, the increase in NOxand ozone at 10 km due to aircraft emissions, is reduced by a factor of 2. The magnitude of aircraft perturbations in NOxis considerably smaller than the uncertainties in other NOxso

ISSN:0148-0227    

DOI:10.1029/95JD02363

年代:1996

数据来源: WILEY

摘要:

The vertical transport between the stratosphere and the troposphere is studied by evaluating a matrix that gives the amount of air transferred between the different model layers of a three‐dimensional off‐line transport model coupled to European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses. The latitudinal structure of vertical exchange reflects the Hadley circulation with upward transport in the tropics and with downward transport at mid‐latitudes. Maxima in downward transport occur near the subtropical and polar jet streams. The seasonal cycle of downward transport and the asymmetry between the two hemispheres are at least qualitatively represented by the model. However, it is found that the evaluated total amount of air transferred from the stratosphere to the troposphere per month is sensitive to the chosen horizontal model resolution. Refinement of this resolution from 8° × 10° to 4° × 5° leads to almost a doubling of the computed downward transport. Also, the vertical transport is found to depend strongly on changes in the formulation of the parent model (ECMWF). It is therefore important to use internally consistent meteorological input data when performing multiyear simulations with off‐line chemistry

ISSN:0148-0227    

DOI:10.1029/95JD03407

年代:1996

数据来源: WILEY

摘要:

Evidence is presented pointing to the existence of rich and coherent subsynoptic and mesoscale flow features at tropopause levels. These features are related to, and evolve from, the classical V‐shaped intrusions of stratospheric air down to tropospheric elevations. It is shown that intrusions can develop into elongated (∼2000–3000 km) and slender (∼200 km) streamers, and that thereafter such a streamer can roll up to form a train of stalactite‐shaped vortex subentities with an accompanying substantial thinning of the intervening filament. In addition there are indications that the vortices themselves can develop a spirallike interior structure of interleaved stratospheric and tropospheric air. These inferences are based upon two independent but complementary sources: analysis of the potential vorticity distribution on tropopause transcending isentropic surfaces derived from the analysis fields of the European Centre for Medium‐Range Weather Forecasts either directly, or indirectly using a contour advection technique; and imagery from the water vapor channel of the European Space Agency Meteosat 4 satellite. Streamers were observed to occur with a frequency of approximately one per week over central and southern Europe during the winter of 1991–1992. The fragmentation is linked to the instability or self‐development of a filament of enhanced potential vorticity and it can modify or instigate surface weather systems. Moreover, by inducing a substantial and rapid enlargement of the intrusion's surface area it greatly enhances the potential for local irreversible mixing of stratospheric and t

ISSN:0148-0227    

DOI:10.1029/95JD02674

年代:1996

数据来源: WILEY

摘要:

Rarely does the atmosphere allow direct observation of the photochemical evolution of ozone. In most of the troposphere and lower stratosphere this slow chemistry cannot be understood without including much larger changes caused by the circulation. Yet in the tropical stratosphere, where ozone‐poor air of tropospheric origin enters and rises slowly in near isolation, it can be demonstrated that O3is created by dissociation of O2at a rate consistent with current theory. The parallel photolytic destruction of the unreactive source gases (for example, N2O and CFCl3) and the consequent evolution of chemically active odd‐nitrogen (NOy) and chlorine (Cly) species, however, indicate a small amount of mixing of much older, photochemically aged air from the midlatitude stratosphere into this tropical pl

ISSN:0148-0227    

DOI:10.1029/95JD03010

年代:1996

数据来源: WILEY