bluuebunny/crossref_metadata_2025 huggingface dataset:
A climatology of snowfall‐temperature relationships in Canada
Authors: Davis Robert E. , Lowit Michael B. , Knappenberger Paul C. , Legates David R.
Published: September 2002

A better understanding of potential climate change impacts on the global hydrological cycle requires knowledge of the interaction between air temperature and water in its various forms. One important example is the effect of air temperature on snowfall. Proper parameterization of the snowfall‐temperature relationship in climate models is essential for accurate prediction of future snowfall changes that might arise from high‐latitude warming. On a climatological basis, at any location, air temperature and snowfall can be correlated positively (higher temperatures increase atmospheric moisture and snowfall through the Clausius‐Clapeyron relationship) or negatively (precipitation falls as rain instead of snow). Examination of 50 years of monthly snowfall water equivalent and mean temperature data indicates that the snowfall‐temperature relationship is positive in the high latitudes and negative in southern Canada, along both coasts, and east of the Rockies. The “zero line” (the transition zone north of which warmer months receive more snowfall than colder months) migrates southward from autumn to winter so that by January most of eastern and northwestern Canada has a positive snowfall‐temperature slope. The primary exception to a straightforward relationship between slope and latitude occurs east of the Rockies, where anomalous negative slopes extend far to the north. In this region, dry, adiabatically warmed air from downslope or circulation effects may reduce the number of snow events and modify the slope of the snowfall‐temperature curve. Based on first principles and Canadian observations, we develop a function relating temperature to snowfall that attempts to account for the complex spatial and seasonal nature of the snowfall‐temperature relationship. Given the importance of snowfall in the global radiation balance, this analysis could be used to improve climate change predictions.
Influence of snow and clouds on erythemal UV radiation: Analysis of Swiss measurements and comparison with models
Authors: Renaud Anne , Staehelin Johannes , Fröhlich Claus , Philipona Rolf , Heimo Alain
Published: September 2002

Global, direct and diffuse measurements of erythemal UV radiation with UV‐Biometer and radiative transfer calculations are used to estimate the influences of (1) a snow‐covered surface, (2) an overcast sky, and (3) a joined snow‐covered surface and overcast sky at Davos (1610 m asl, Swiss Alps). The influence of total ozone, zenith angle, and aerosol are first investigated. The influence of surface albedo and clouds are then estimated as a function of the zenith angle after normalization of the data to a fixed ozone amount. If the ground is covered with snow on clear‐sky days at Davos, erythemal irradiance increases by 15 to 25% due to multiple reflections between the surface and the atmosphere. This relative increase may reach 80% on overcast days. The highly significant dependence of the albedo on solar elevation is most probably due to inhomogeneities in the surrounding terrain leading to a strong non‐Lambertian behavior. The impact of clouds on radiation is highly variable: on overcast days with a snow free surface, erythemal UV radiation at Davos is reduced to a level ranging between 8% (very thick cloud cover) and 70% (thin cloud layer) relative to the radiation under clear‐sky conditions.
Nitric oxide fluxes from an agricultural soil using a flux‐gradient method
Authors: Taylor N. M. , Wagner‐Riddle C. , Thurtell G. W. , Beauchamp E. G.
Published: September 2002

Soil emission of nitric oxide may be a significant source of NOx in rural areas. Agricultural practices may enhance these emissions by addition of nitrogen fertilizers. A system that enables continuous measurement of NO fluxes from agricultural surfaces using the flux‐gradient method was developed. Hourly differences in NO concentrations in air sampled at two intake heights (0.6 and 1 m) were determined using a chemiluminescence analyzer. Eddy diffusivities were determined using wind profiles (cup anemometers), and stability corrections calculated using a 5 cm path sonic anemometer. Fast switching of sampling between air intake heights (every 30 s) and determination of concentration values at a frequency of 2 Hz minimized the errors due to fluctuations in background concentration. Low travel times for air samples in the tubing (∼8 s) were estimated to result in small errors in flux values (<0.5 ng N m−2 s−1) due to chemical reactions. The overall resolution of the system was estimated as ∼1 ng N m−2s−1. NO fluxes from a bare soil were measured quasi‐continuously from January to June 1995 at Elora, Canada, comprising a total of 1833 hourly values. Daily NO fluxes before nitrogen fertilization were small, increasing after nitrogen fertilizer was added (>10 ng N m−2 s−1). Monthly NO fluxes estimated were similar to those observed in previous studies. The designed system could be easily modified to measure NOx and NO fluxes by using an additional chemiluminescence analyzer. The system also could be adapted to measure fluxes sequentially from various plots, enabling testing of agricultural practices on NO emissions.
Variations of precipitation and evaporation over the North Atlantic Ocean, 1958–1997
Authors: Walsh John E. , Portis Diane H.
Published: September 2002

The output from recent atmospheric reanalysis projects at the National Centers for Environmental Prediction and the European Centre for Medium‐Range Weather Forecasts is used to address regional and temporal variability of precipitation P and evaporation E over the North Atlantic. The difference between P and E is the net surface freshwater flux, which directly affects the salinity and hence the stratification of the ocean surface layer. Over North Atlantic subregions, spanning 10°–15° latitude and 30°–40° longitude, the area‐averaged P from the two reanalyses are highly correlated with each other outside of the tropics and are consistent with climatologies compiled from ship data. In the annual mean, P exceeds E in the North Atlantic poleward of 50°N; E exceeds P between 20°N and 50°N and also in the eastern tropical North Atlantic (10°N–35°N). Seasonality varies considerably among the subregions: P and E both peak during late autumn/early winter in the subpolar regions, and P has an autumn peak in the subtropics, where E shows little seasonality. The year‐to‐year variations of monthly and annual regional values are larger for P than for E, implying that the interannual variations of P minus E are driven primarily by P. These interannual variations are sufficiently large and persistent that they can account for substantial portions (0.1–1.0 salinity unit (SU) per year) of observed salinity variations in the North Atlantic surface waters. Atmospheric forcing of regional P shows consistent teleconnections throughout the year, although the patterns are strongest in winter and weakest in summer. The North Atlantic Oscillation (NAO) is the dominant pattern controlling P in the eastern and northern North Atlantic; P in the western North Atlantic shows associations with the atmospheric circulation as far away as the North Pacific. P anomalies of >3 cm month−1 are associated with multiyear periods of positive and negative NAO. The high‐NAO regime of the late 1980s and early 1990s has coincided with increased P over the northeastern North Atlantic and decreased P over the southwestern North Atlantic although the seasonality of the NAO impact varies widely among the regions.
An analysis of ozonesonde data for the lower stratosphere: Recommendations for testing models
Authors: Logan Jennifer A.
Published: September 2002

I present an analysis of ozonesonde data with a focus on using these data to evaluate stratospheric models of transport and chemistry. Ozonesondes are the only instruments that provide year‐round profiles of ozone throughout the lowermost stratosphere for middle and high latitudes of the Northern Hemisphere. I show vertical profiles and the annual cycle of ozone at selected pressure levels and stations and use TOMS data to evaluate the spatial bias of the sonde stations with respect to zonal mean values of column ozone. Between 10 and 30% of the ozone column is located between 100 hPa and the tropopause at middle and high latitudes, and this region drives much of the seasonal variation of the ozone column. The sonde data allow quantification of the buildup of ozone in the lowermost stratosphere of the Northern Hemisphere in winter and of its loss in late spring and summer. The amount of ozone between the tropopause and 100 hPA in the Northern Hemisphere decreases from 175 to 75 Tg from March to September, with the maximum rate of decrease in May to June; about half of the decrease is caused by the increase in the height of the tropopause. There is a lag of about a month in the time when ozone starts to accumulate in the lowermost stratosphere, from September near 40°N to October or November near 80°N, and a similar lag in the time when ozone starts to decrease, from February or March near 40°N to March or April near 80°N. I propose that the sonde data be used to provide a test of transport in two‐ and three‐dimensional models in addition to those provided by long‐lived tracers, such as N2O, CFCs, CO2, CH4, and their correlation patterns, and age of air. The annual variation of the ozone content of the lowermost stratosphere in the Northern Hemisphere is suggested as a particularly useful constraint on transport parameterizations. I make recommendations for the use of particular stations and provide summary statistics for ozone concentrations and for the mean tropopause height for each station.
Retrievals of vertical profiles of ice cloud microphysics from radar and IR measurements using tuned regressions between reflectivity and cloud parameters
Authors: Matrosov Sergey Y.
Published: September 2002

A new approach is suggested for retrieving vertical profiles of radiatively important microphysical parameters of ice clouds from measurements of radar reflectivity and estimates of cloud optical thickness. This approach is applicable to ground‐based, vertical‐mode measurements of pure ice phase clouds observed without blockage from low‐liquid water clouds. It involves “tuning” the coefficient of the power law reflectivity‐ice water content (Ze‐IWC) relations for each individual profile of radar measurements. This tuning is done based on the independent estimates of cloud ice water path obtained from layer‐mean values of radar reflectivity and cloud optical thickness. After a vertical profile of IWC is retrieved using this approach, a corresponding vertical profile of cloud particle median sizes, Dm, can be easily calculated for an assumed type of the particle size distribution. By assuming that the exponent in the tuned Ze‐IWC relations changes linearly within a small dynamic range, better results are obtained compared with the case when it is assumed to be constant. The accuracy of the tuned regression approach is assessed by comparing the retrieval results for several high‐quality observational cases with data from the “reference” method, which uses additional measurements of Doppler velocities. This method was verified by earlier comparisons with simultaneous in situ data. The specifics of using Doppler information, however, limit the applicability of this method and prevent its real‐time applications. It is shown that the relative standard deviations of IWC and Dm retrievals using the tuned regression approach compared with the reference method are about 35% and 20%, respectively. These accuracies are much better than those that can be attained by any a priori chosen fixed regression between reflectivity and cloud microphysical parameters. At the same time, these accuracies are comparable to uncertainties of estimates of cloud parameters from direct in situ sampling. The suggested tuned regression approach is easy to implement and can be used for routine processing of ground‐based, and, potentially, satellite‐aircraft‐based radar measurements of ice clouds.
The budget and partitioning of stratospheric chlorine during the 1997 Arctic summer
Authors: Sen B. , Osterman G. B. , Salawitch R. J. , Toon G. C. , Margitan J. J. , Blavier J.‐F. , Chang A. Y. , May R. D. , Webster C. R. , Stimpfle R. M. , Bonne G. P. , Voss P. B. , Perkins K. K. , Anderson J. G. , Cohen R. C. , Elkins J. W. , Dutton G. S. , Hurst D. F. , Romashkin P. A. , Atlas E. L. , Schauffler S. M. , Loewenstein M.
Published: September 2002

Volume mixing ratio profiles of HCl, HOCl, ClNO3, CH3Cl, CFC‐12, CFC‐11, CCl4, HCFC‐22, and CFC‐113 were measured simultaneously from 9 to 38 km by the Jet Propulsion Laboratory MkIV Fourier Transform Infrared solar absorption spectrometer during two balloon flights from Fairbanks, Alaska (64.8°N), on May 8 and July 8, 1997. The altitude variation of total organic chlorine (CCly), total inorganic chlorine (Cly), and the nearly constant value (3.7±0.2 ppbv) of their sum (ClTOT) demonstrates that the stratospheric chlorine species available to react with O3 are supplied by the decomposition of organic chlorinated compounds whose abundances are well quantified. Measured profiles of HCl and ClNO3 agree well with profiles found by photochemical model (differences <10% for altitudes below 35 km) constrained by various other constituents measured by MkIV. The production of HCl by ClO + OH plays a relatively small role in the partitioning of HCl and ClNO3 for the sampled air masses. However, better agreement with the measured profiles of HCl and ClNO3 is obtained when this source of HCl is included in the model. Both the measured and calculated [ClNO3]/[HCl] ratios exhibit the expected near linear variation with [O3]2/[CH4] over a broad range of altitudes. MkIV measurements of HCl, ClNO3, and CCly agree well with ER‐2 in situ observations of these quantities for directly comparable air masses. These results demonstrate good understanding of the budget of stratospheric chlorine and that the partitioning of inorganic chlorine is accurately described by photochemical models that employ JPL97 reaction rates and production of HCl from ClO + OH for the environmental conditions encountered: relatively warm temperatures, long periods of solar illumination, and relatively low aerosol surface areas.
On the use of real‐time radar rainfall estimates for flood prediction in mountainous basins
Authors: Borga Marco , Anagnostou Emmanouil N. , Frank Enrico
Published: September 2002

This paper investigates the effect of systematic mean‐field and range‐dependent radar rainfall errors on the accuracy of runoff simulation in mountainous basins. Statistical analysis of radar rainfall and runoff simulation error is performed on six flood events for two medium size watersheds in northern Italy, located at 38 and 60 km basin‐to‐radar distances, respectively. We show significant range‐related rainfall biases, which are due to the high elevation radar scans used to minimize the interception of the radar beam with the topography. These biases are corrected by converting radar reflectivity measurements at a given altitude into their equivalent surface values, using real‐time identification of the mean vertical reflectivity profile. The mean‐field bias is adjusted using a multiplicative factor determined based on real‐time radar‐rain gauge comparisons. The impact of the above radar rainfall biases, and the improvements obtained from the proposed corrections, on areal‐rainfall estimation and runoff simulation are evaluated. It is shown that radar rainfall biases magnify through the rainfall‐runoff transformation and preclude the accurate simulation of runoff, particularly for the distant basin. The combined correction procedure results in significant reduction of the hydrologic prediction error, especially at the distant basin.
Trends in the vertical distribution of ozone: A comparison of two analyses of ozonesonde data
Authors: Logan J. A. , Megretskaia I. A. , Miller A. J. , Tiao G. C. , Choi D. , Zhang L. , Stolarski R. S. , Labow G. J. , Hollandsworth S. M. , Bodeker G. E. , Claude H. , De Muer D. , Kerr J. B. , Tarasick D. W. , Oltmans S. J. , Johnson B. , Schmidlin F. , Staehelin J. , Viatte P. , Uchino O.
Published: September 2002

We present the results of two independent analyses of trends in the vertical distribution of ozone. For most of the ozonesonde stations we use data that were recently reevaluated and reprocessed to improve their quality and internal consistency. The two analyses give similar results for trends in ozone. We attribute differences in results primarily to differences in data selection criteria, rather than in statistical trend models. We find significant decreases in stratospheric ozone at all stations in middle and high latitudes of the Northern Hemisphere from 1970 to 1996, with the largest decreases located between 12 and 21 km, and trends of −3 to −10%/decade near 17 km. The decreases are largest at the Canadian and the most northerly Japanese station and are smallest at the European stations and at Wallops Island. The mean midlatitude trend is largest, −7%/decade, from 12 to 18 km for 1970–1996, and the decrease in ozone is significant from 10.5 to 25 km. For 1980–1996, the decrease is more negative by 1–2%/decade, with a maximum trend of −9%/decade in the lowermost stratosphere. The trends vary seasonally from about 10 to 17 km, with largest ozone decreases in winter and spring. Trends in tropospheric ozone are highly variable and depend on the region. There are decreases or zero trends at the Canadian stations for 1970–1996, and decreases of −2 to −8%/decade for the mid‐troposphere for 1980–1996; the three European stations show increases for 1970–1996, but trends are close to zero for two stations for 1980–1996 and positive for one; there are increases in ozone for the three Japanese stations for 1970–1996, but trends are either positive or zero for 1980–1996. The United States stations show zero or slightly negative trends in tropospheric ozone after 1980. It is not possible to define reliably a mean tropospheric ozone trend for northern midlatitudes, given the small number of stations and the large variability in trends. The integrated column trends derived from the sonde data are consistent with trends derived from both surface‐based and satellite measurements of the ozone column.
Solar UV‐B irradiance and total ozone in Italy: Fluctuations and trends
Authors: Casale G. R. , Meloni D. , Miano S. , Palmieri S. , Siani A. M. , Cappellani F.
Published: September 2002

Solar UV irradiance spectra (290–325 nm) together with daily total ozone column observations have been collected since 1992 by means of Brewer spectrophotometers at two Italian stations (Rome and Ispra). The available Brewer irradiance data, recorded around noon and at fixed solar zenith angles, together with the output of a radiative transfer model (the STAR model) are presented and analyzed. The Brewer irradiance measurements and total ozone fluctuations and anomalies are investigated, pointing out the correlation between the high‐frequency O3 components and irradiance at 305 nm. In addition, the total ozone long time series of Arosa (170 km apart from Ispra) and Vigna di Valle (very close to Rome) are analyzed to illustrate evidence of temporal variations and a possible trend.
In‐flight comparison of MOZAIC and POLINAT water vapor measurements
Authors: Helten M. , Smit H. G. J. , Kley D. , Ovarlez J. , Schlager H. , Baumann R. , Schumann U. , Nedelec P. , Marenco A.
Published: September 2002

An intercomparison of airborne in situ water vapor measurements by two European research projects Measurement of Ozone and Water Vapor by Airbus In‐Service Aircraft (MOZAIC) and Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT) was performed from aboard the Airbus (MOZAIC) and Falcon (POLINAT) aircraft, respectively. The intercomparison took place southwest of Ireland on September 24, 1997, at 239 hPa flight level. MOZAIC uses individually calibrated capacitive humidity sensors for the humidity measurement. POLINAT employs a cryogenic frost‐point hygrometer developed for such measurements. For conversion between humidity and mixing ratio, ambient temperature and pressure measurements on board the respective aircraft are used. The Falcon followed the Airbus at a distance of 7–35 km with a time lag increasing from 30 to 160 s. The water vapor volume mixing ratio measurements in the range of 80–120 ppmv of both instruments are in excellent agreement, differing by <±5%, where the trajectories of both aircraft are very close. However, the relative humidity (RH) calculated from POLINAT frost‐point measurements and the Falcon PT500 temperature sensor is up to 15% higher relative to the RH of MOZAIC. The agreement improved to within 5% when using the temperature measurement of the PT100 sensor instead of the temperature measurement of the PT500 sensor for RH determination of POLINAT.
Nitric oxide emission from intensively managed agricultural soil in North Carolina
Authors: Li Yongxian , Aneja Viney P. , Arya S. P. , Rickman J. , Brittig J. , Roelle P. , Kim D. S.
Published: September 2002

Emissions of nitric oxide (NO) were determined from an intensively managed agricultural soil near Plymouth, in the coastal plain of North Carolina, using the dynamic chamber technique. The measurements were made over a soybean field from July 15 to August 15, 1996, as part of the project Natural Emissions of Oxidant Precursors: Validation of Techniques and Assessment (NOVA). A N‐containing fertilizer was applied at the middle of the experiment in order to investigate the effect of N‐fertilizers on NO emissions and to test the response of instruments. Soil water content was high during the experimental period, with water‐filled pore space ranging from 49% to 67%. NO emission during this period ranged between 0.28 and 18.45 ng N m−2 s−1, with an overall average of 5.01±3.03 ng N m−2 s−1. A normal diurnal pattern with low values at nighttime and high values during the day was observed during the prefertilization period, but a reverse diurnal pattern (high at nighttime, low in daytime) of NO emission variation was found during the postfertilization, closed‐canopy period, implying that interaction among canopy development, application of fertilization, and soil parameters may affect the diurnal variation of NO emission from soils. The emissions of NO were related to soil temperature, water‐filled pore space, and extractable nitrogen. Application of fertilizer at the middle of the experiment was found to disrupt the normal relations between NO emission and soil temperature and water content seen during the prefertilization period but to enhance the positive relation between NO emission and extractable N. An intercomparison of the dynamic chamber technique with the eddy‐correlation technique in this experiment indicates that in spite of large differences in the magnitudes of soil NO emission and the NO flux at 5 m, the two fluxes show similar variations with time and are strongly correlated.
Tropospheric sulfur cycle in the Canadian general circulation model
Authors: Lohmann Ulrike , von Salzen Knut , McFarlane Norman , Leighton Henry G. , Feichter Johann
Published: September 2002

Emission, transport, chemistry, and scavenging of the gaseous sulfur species dimethyl sulfide and sulfur dioxide (SO2) and sulfate aerosols are calculated on‐line with the meteorology in the general circulation model (GCM) of the Canadian Centre for Climate Modelling and Analysis (CCCMA). Additionally, prognostic equations for cloud water and cloud ice have been introduced. The sensitivity of this sulfur cycle to differences in GCM physics and dynamics has been studied by comparing the results to those obtained with the ECHAM GCM which has a very similar sulfur cycle and cloud scheme, but a different turbulent diffusion and convection scheme. The differences in the global mean burdens of SO2 and sulfate are less than 2%. Simulated surface SO2 concentrations with CCCMA in winter as well as the seasonal cycle are in better agreement with observations at several sites than those simulated with ECHAM because of stronger boundary layer mixing in CCCMA. The simulated surface SO42− with CCCMA, however, is often higher than observed and in ECHAM. Additionally, sensitivity experiments showed that the global sulfur budgets are most sensitive to changes in the cloud cover parameterization and less sensitive to changes in pH calculation and oxidation of SO2 in convective clouds. The results of the sensitivity experiments give evidence for the importance of all of these effects on the geographical and vertical distribution of sulfur and cloud liquid water.
Partitioning of the organic aerosol component between fog droplets and interstitial air
Authors: Facchini Maria Cristina , Fuzzi Sandro , Zappoli Sergio , Andracchio Antonella , Gelencsér András , Kiss Gyula , Krivácsy Zoltán , Mészáros Ernô , Hansson Hans‐Christen , Alsberg Tomas , Zebühr Yngve
Published: September 2002

Limited information is available on the nature of organic compounds in the tropospheric aerosol and their effect on aerosol hygroscopic properties and cloud condensation nuclei (CCN) ability. Here we analyze samples of liquid droplets and interstitial aerosol, concurrently collected during fog episodes, to determine how the organic compounds are partitioned between the two reservoirs. By comparing the nature and concentration of different organic carbon classes found in the two reservoirs, we find that fog acts as an efficient separator for carbon (C) species on the basis of their chemical properties, with polar water soluble species representing the greater part of total C within fog droplets, and water insoluble C species preferentially found in the interstitial reservoir. Water‐soluble organic species are scavenged by fog droplets to a comparable extent to major inorganic ions and are therefore expected to play an important role in the droplet nucleation process. The main classes of water soluble organic carbon (WSOC) identified in fog water and interstitial aerosol by the techniques traditionally used in aerosol analysis are aliphatic dicarboxylic acids, sugars, aliphatic alcohols, and aliphatic carboxylic acids. However, such species, ∼120 individual compounds, only account for a few percent (<5% on average) of total WSOC. A new class of water soluble macromolecular compounds (MMC), detected in aerosol samples from different areas of the globe, are found to constitute a large fraction (∼40% on average) of WSOC in the fog system (fog droplets plus interstitial aerosol) and represent the main class of water soluble species identified. More than 50% of WSOC still remains undetermined.
Analysis of UV‐B irradiances measured simultaneously at two stations in the Czech Republic
Authors: Dubrovský Martin
Published: September 2002

The 10 min sums of erythemal UV‐B irradiances measured during approximately 10 months at two locations with different altitudes were analyzed. The magnitude of the altitude effect averaged over the whole period varies between 4 and 8%/1000 m. The value of radiation amplification factor (RAF) derived from the relationship between UV‐B irradiance and total ozone is about 1.1. The value of RAF is slightly higher if derived from the relationship between the ratio of UV‐B to global solar irradiance and total ozone. A statistical model relating clear‐sky UV‐B irradiance with solar zenith angle, total ozone, and Julian day was developed. Unfortunately, knowledge of the daily average total column ozone does not adequately improve the forecast of UV‐B irradiance.
Implementation of a convective atmospheric boundary layer scheme in a tropospheric chemistry transport model
Authors: Wang K.‐Y. , Pyle J. A. , Sanderson M. G. , Bridgeman C.
Published: September 2002

A convective atmospheric boundary layer (ABL) scheme for the transport of trace gases in the lower troposphere has been implemented from the Community Climate Model, Version 2 [Hack et al., 1993] into a tropospheric chemistry transport model [Wang, 1998]. The atmospheric boundary layer scheme includes the calculation of atmospheric radiative transfer, surface energy balance, and land surface temperature and has a specified annual variation of sea surface temperature. The calculated diurnal variation of the height of the boundary layer is similar to the results of Troen and Mahrt [1986] and is in a good agreement with Holtslag and Boville [1993]. The modeled height of the boundary layer shows a seasonal shift between land and sea in the Northern Hemisphere. In summer (June‐July‐August), the height of the boundary layer is deeper over land (850–2250 m) and shallower over sea (50–850 m); while in winter (December‐January‐February), it is shallower over land (50–850 m) and deeper over sea (850–2850 m). The coupled ABL‐chemical transport model is verified against measurements of radon 222 and methane. Comparison of the coupled model with a non‐ABL model indicates significant differences between these model simulations and a better agreement between the coupled model and measurements. There is a significant effect on the trace gas distribution when the ABL model is compared with the non‐ABL schemes. For example, the ABL scheme shows more O3 transported from the middle troposphere down to the surface, while more CO is pumped up from the surface into the middle troposphere. The seasonal cycle of modeled CH4 is significantly improved with the inclusion of the new ABL scheme, especially in regions which are not remote from methane sources.
Simulation of carbon monoxide redistribution over central Africa during biomass burning events (Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO))
Authors: Cautenet S. , Poulet D. , Delon C. , Delmas R. , Grégoire J.‐M. , Pereira J. M. , Cherchali S. , Amram O. , Flouzat G.
Published: September 2002

The Regional Atmospheric Modeling System (RAMS) is used to assess a realistic estimate of emissions by savanna fires from NOAA advanced very high resolution radiometer (AVHRR) satellite imagery during the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) campaign. The daily amounts of gases released by biomass burning depend on burnt biomaterial, the mass of which is estimated from two independent methods: classification of vegetation formations (CV) or net primary productivity (NPP). The area of the burnt zone is derived from satellite imagery. The percent of burnt biomass or combustion factor (α) is determined by an adjustment procedure using two model runs and assumed to be 40%, a value consistent with the biomass wetness. The simulated CO redistribution is compared with aircraft measurements (tracks and profiles). Two events of 4 and 5 days are examined, where flights above forest and savanna zones have been performed. In this study we show that the main mechanisms of the chemical species spatial redistribution originates from mesoscale features. The RAMS results are in rather good agreement with the aircraft measurements (vertical profiles or horizontal legs) over forest and savanna. The difference between the CO simulated fields obtained from CV or NPP is weak (about 20% or 30%), and it is difficult to discriminate the best method because this difference is generally inferior to the gap between model and observations. One may consider this study as a preliminary approach to improve the estimate of released gases by biomass burning with the help of mesoscale modeling and the knowledge of the real sources.
Validation of an UV inversion algorithm using satellite and surface measurements
Authors: Wang Pucai , Li Zhanqing , Cihlar Josef , Wardle David I. , Kerr J.
Published: September 2002

Ultraviolet radiation in the spectral region between 280 and 315 nm (often referred to as UV‐B) is harmful to living organisms. Satellite‐based estimation of surface UV‐B supplements the sparsely distributed ground‐based UV‐B monitoring networks. This study is concerned with validation of an inversion algorithm [Li et al., this issue] for retrieving spectrally integrated UV‐B (no spectral weighting) and erythemal UV (EUV) (with spectral weighting) fluxes at the surface from satellite. The physical inversion algorithm contains a few analytical expressions and input parameters: the solar zenith angle, ozone amount, albedo at the top of the atmosphere (TOA), and aerosol variables. The algorithm is applied to satellite measurements of total ozone amount and 360 nm reflectance from Meteor 3/TOMS and visible reflectance from NOAA/AVHRR. The retrieved UV‐B and EUV fluxes are compared with ground UV observations made at six Canadian UV observation stations with Brewer instruments from 1992 to 1994. Under all‐sky conditions the comparisons showed very small mean differences and relatively large standard deviations (s.d.): 0.033 W/m2 (mean) and 0.287 W/m2 (s.d.) for total UV‐B and 3.02 mW/m2 (mean) and 12.0 mW/m2 (s.d.) for EUV radiation. The large standard deviations are attributed to the inhomogeneity in sky condition and mobility of cloudy scenes, which renders an inaccurate match between satellite and surface measurements. The comparisons under clear‐sky conditions showed very small mean and standard differences. By means of a running average over a period of time, satellite inversion can track the variation of surface‐observed UV‐B and EUV very well.
Microphysics and chemistry of sulphate aerosols at warm stratospheric temperatures
Authors: Drdla K. , Pueschel R. F. , Strawa A. W. , Cohen R. C. , Hanisco T. F.
Published: September 2002

Observations of high NOx/NOy ratios (overall 40% larger than modelled values) during the Polar Ozone Loss in the Arctic Region in Summer campaign have led us to re‐examine the heterogeneous chemistry of stratospheric aerosol particles during the polar summer period, using the Integrated MicroPhysics and Aerosol Chemistry on Trajectories model. The warm summer temperatures (up to 235 K) imply very concentrated sulphuric acid solutions (80 wt %). On the one hand, these solutions are more likely to freeze, into sulphuric acid monohydrate (SAM), reducing the efficiency of the N2O5 hydrolysis reaction. Including this freezing process increases NOx/NOy ratios but does not improve model/measurement agreement: in polar spring, SAM formation causes the NOx/NOy ratio to be overpredicted whereas freezing has a much smaller effect on nitrogen chemistry during the continuous solar exposure of polar summer. On the other hand, if sulphate aerosols remain liquid, the high acidity may promote acid‐catalysed reactions. The most important reaction is CH2O + HNO3, which effectively increases NOx/NOy ratios across a wide range of conditions, improving agreement with measurements. Furthermore, the production of HONO can either enhance gas‐phase OH concentrations or promote secondary liquid reactions, including HONO + HNO3 and HONO + HCl. Primary uncertainties include the uptake coefficient of CH2O relevant to reaction with HNO3, the amount of HONO available for secondary reaction, and the relative rates of HONO reaction with HNO3 and HCl. The fate of the formic acid product, whose presence in the stratosphere may be an indicator for the CH2O reaction, and the impact on the stratospheric hydrogen budget are also discussed.
A global model tool for three‐dimensional multiyear stratospheric chemistry simulations: Model description and first results
Authors: Rummukainen Markku , Isaksen Ivar S. A. , Rognerud Bjørg , Stordal Frode
Published: September 2002

The paper presents a new global modeling tool, Stratospheric Chemical Transport Model 2. It has been developed for effective three‐dimensional multiyear stratospheric chemistry studies, featuring an extensive chemistry scheme, heterogeneous processing on sulfate aerosols, and some polar stratospheric cloud processes. The transport algorithm maintains sub‐grid‐scale distributions and connects vertically the stratospheric layers, even in a coarse vertical grid. The model has been integrated for 49 months, recycling 1 year of precalculated transport from a middle atmosphere general circulation model. One year of daily National Centers for Environmental Prediction global analyses are used as temperatures. Diurnal cycles of photolysis rates are recalculated every 7 days to give interaction with ozone changes. The model is able to describe most of the geographical and seasonal ozone variability and the meridional distributions of ozone, reactive nitrogen, chlorine, and bromine. Stratospheric diurnal cycles for nitrogen, hydrogen, chlorine, and bromine species are captured in detail. The upper stratosphere ozone deficiency, typical to models, is large. Its sensitivity to different ways of tuning are explored. Midlatitude, rather than polar, wintertime processes have so far been the focus in this model tool. The present transport and grid resolution are not suited for realistic simulations at high latitudes. As there is only a limited inclusion of polar stratospheric cloud (PSC) microphysics, chemical processing in the cold polar lower stratosphere also cannot be well simulated. For example, the Antarctic ozone hole is not simulated, but the modeled chemistry should be suitable for warm Arctic winters when type II PSCs and particle sedimentation do not occur.
Inverse modeling of methane sources and sinks using the adjoint of a global transport model
Authors: Houweling Sander , Kaminski Thomas , Dentener Frank , Lelieveld Jos , Heimann Martin
Published: September 2002

An inverse modeling method is presented to evaluate the sources and sinks of atmospheric methane. An adjoint version of a global transport model has been used to estimate these fluxes at a relatively high spatial and temporal resolution. Measurements from 34 monitoring stations and 11 locations along two ship cruises by the National Oceanographic and Atmospheric Administration have been used as input. Recent estimates of methane sources, including a number of minor ones, have been used as a priori constraints. For the target period 1993–1995 our inversion reduces the a priori assumed global methane emissions of 528 to 505 Tg(CH4) yr−1 a posteriori. Further, the relative contribution of the Northern Hemispheric sources decreases from 77% a priori to 67% a posteriori. In addition to making the emission estimate more consistent with the measurements, the inversion helps to reduce the uncertainties in the sources. Uncertainty reductions vary from 75% on the global scale to ∼1% on the grid‐scale (8° × 10°), indicating that the grid scale variability is not resolved by the measurements. Large scale features such as the interhemispheric methane concentration gradient are relatively well resolved and therefore impose strong constraints on the estimated fluxes. The capability of the model to reproduce this gradient is critically dependent on the accuracy at which the interhemispheric tracer exchange and the large‐scale hydroxyl radical distribution are represented. As a consequence, the inversion‐derived emission estimates are sensitive to errors in the transport model and the calculated hydroxyl radical distribution. In fact, a considerable contribution of these model errors cannot be ignored. This underscores that source quantification by inverse modeling is limited by the extent to which the rate of interhemispheric transport and the hydroxyl radical distribution can be validated. We show that the use of temporal and spatial correlations of emissions may significantly improve our results; however, at present the experimental support for such correlations is lacking. Our results further indicate that uncertainty reductions reported in previous inverse studies of methane have been overestimated.
NOy and Cly partitioning in the middle stratosphere: A box model investigation using HALOE data
Authors: Randeniya L. K. , Plumb I. C. , Ryan K. R.
Published: September 2002

The partitioning of the active nitrogen and chlorine families (NOy, Cly) between 24 and 32 km and 30° and 75°N in summer and autumn is studied using a photochemical box model. Data obtained from the Halogen Occultation Experiment (HALOE) are used to initialize the model and to obtain measured values for NO, NO2 and HCl. When the recommended kinetic and photochemical parameters are used, the model underestimates the NOx/NOy ratios by 15–35%. As in analyses of aircraft, balloon, and laboratory data, it is found that the gas‐phase processes which link NOx and HNO3 are inadequately represented by the model. The inclusion of rate expressions derived recently for the reactions of OH with NO2 and OH with HNO3 in the model results in 8–20% increases in NOx, where the larger increases are obtained at lower altitudes. Despite better agreement resulting from the use of new rate expressions, differences in excess of 20% remain between NOx/NOy values obtained from measurements and calculations. The remaining differences are particularly noticeable above about 27 km. This implies that further studies of the reactions controlling partitioning in the NOy family are necessary. The use of branching ratios determined recently for the production of HCl from the reaction between ClO and OH leads to better agreement for the HCl/Cly ratios obtained from measurements and calculations for altitudes above 27 km. However, the agreement below 27 km becomes poorer as a result of calculated additional production of HCl. Overall, within the recommended uncertainties for the relevant rate coefficients, the model predictions for HCl/Cly agree with the values obtained from measurements between 24 and 32 km.
Shipboard determinations of the distribution of 13C in atmospheric methane in the Pacific
Authors: Lowe David C. , Allan W. , Manning Martin R. , Bromley Tony , Brailsford Gordon , Ferretti Dominic , Gomez Antony , Knobben Rob , Martin Ross , Mei Zhu , Moss Rowena , Koshy K. , Maata M.
Published: September 2002

Measurements of the mixing ratio and δ13C in methane (δ13CH4) are reported from large, clean air samples collected every 2.5° to 5° of latitude on four voyages across the Pacific between New Zealand and the West Coast of the United States in 1996 and 1997. The data show that the interhemispheric gradient for δ13CH4 was highly dependent on season and varied from <0.1‰ in June 1996 to >0.5‰ in November 1996 with an estimated annual mean of 0.2–0.3‰. The seasonal cycles in δ13CH4 reveal three distinct latitude bands differentiated by phase. Maxima occur in January‐February for the extratropical Southern Hemisphere, in September‐October for the tropics, and in June‐July for the extratropical Northern Hemisphere. The data are compared with results from a three‐dimensional transport and atmospheric chemistry model that simulates the observed latitudinal structure of either δ13CH4 or the methane mixing ratio well, but not both simultaneously. The requirement that a methane source‐sink budget be consistent with both types of data clearly imposes stricter constraints than arise from either mixing ratio or isotopic data alone. The seasonal δ13CH4 data in the extratropical Southern Hemisphere are used to estimate a value for the net fractionation in the CH4 sink of 12–15‰, which is larger than can be explained by current laboratory measurements of a kinetic isotope effect for the OH + CH4 reaction and soil sink processes. The hypothesis that the discrepancy is caused by competitive reaction of active chlorine with methane in the marine boundary layer is discussed.
Influence of synoptic waves on column ozone during Arctic summer 1997
Authors: Hitchman Matthew H. , Buker Marcus L. , Tripoli Gregory J.
Published: September 2002

The influence of synoptic waves on column ozone over Alaska during the summer of 1997 is explored using observations of ozone and meteorological quantities, a nonhydrostatic three‐dimensional model, and theory. From early May to early August, column ozone over Fairbanks declined from 450 to 250 Dobson Units, while 300 hPa heights rose from 8500 to 9500 m. Time series of daily column ozone and 300 hPa heights are correlated at −0.78. Week‐to‐week variations contribute strongly to this anticorrelation, with column ozone increasing as a trough approaches. Case studies of synoptic scale waves show that they disturb the lower stratosphere, stretching and folding tropospheric and stratospheric air during their life cycles. Mesoscale resolution simulations with the University of Wisconsin Nonhydrostatic Modeling System (UW‐NMS) were carried out in support of NASA's Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Layered ozone structures in an ER2 stairstep flight are represented fairly well by the UW‐NMS. Two ozone fields were transported in the model, with one subject to a sink below the dynamical tropopause. UW‐NMS ozone distributions, a simple box model, and a Stokes drift argument support the idea that synoptic waves cause a poleward and downward flow of ozone through the high latitude tropopause throughout the year. Planetary wave transport of ozone from the tropics is much reduced in the summer. Net transport out of the high latitude stratosphere likely contributes to column ozone decline during summer.
Numerical modeling of chemical‐dynamical coupling in the upper stratosphere and mesosphere
Authors: Zhu Xun , Yee Jeng‐Hwa , Lloyd Steve A. , Strobel Darrell F.
Published: September 2002

A two‐timescale chemical algorithm has been developed to solve photochemistry coupled with transport in the middle atmosphere. It is suggested that two continuity equations for each species be solved when transport processes prevent instantaneous chemical equilibrium. The simultaneous solutions of the two sets of equations correspond to quasi‐equilibrium and transient or forced states of all the modeled species. The chemical solver is incorporated in a two‐dimensional model to study the chemical‐dynamical coupling in the upper stratosphere and the mesosphere for different timescales in a consistent manner. New parameterizations for calculating photolysis rates in the Schumann‐Runge bands and Schumann‐Runge continuum are presented on the basis of an optimal k distribution method. Several distinct features of measured tracer distributions in the mesosphere can be simulated by the model. These include (1) the model daytime mean OH distribution with a secondary maximum in number density of ∼6.5 × 106 cm−3 around 70 km, (2) a semiannual oscillation in O3 mixing ratio around 85 km that characterizes the coupling effect between the OH‐O3 photochemistry and O transport, and (3) diurnal variations of O3 in the mesosphere controlled by both fast varying local photochemistry and slowly varying HOx transported from below. There is no systematic underprediction of mesospheric O3 in our model comparison with the measurements. Our model also predicts the morphology of chemical heating rate around mesopause by exothermic reactions. From 80 to 95 km the dynamically controlled atomic oxygen distribution generates a latitudinal chemical heating rate that counters the radiative heating rate gradient.
Analysis of near‐global trends and variability in Halogen Occultation Experiment HF and HCl data in the middle atmosphere
Authors: Considine Geoffrey D. , Deaver Lance E. , Remsberg Ellis E. , Russell James M.
Published: September 2002

We present an analysis of trends and variability in daily‐averaged near‐global Halogen Occultation Experiment (HALOE) retrievals of HF and HCl at 55 km. The goal of decomposing HALOE measurements into a series of components is to determine the temporal scales associated with variability and to estimate the trends in HF and HCl. To determine the significance of each component in the statistical model, partial correlations are calculated between the data and each model element using the rank‐order correlation coefficient. We also account for the effects of serial correlation in our data (i.e., the lack of independence between consecutive points) in the determination of statistical significance. The results provide evidence of highly statistically significant slowdowns in the rates of accumulation of HF and HCl at 55 km. HF shows evidence of interannual and annual variability at 55 km that is not evident in HCl due to the short chemical lifetime of HCl.