bluuebunny/crossref_metadata_2025 huggingface dataset:
Noninertial flow in NSCAT observations of Tehuantepec winds
Authors: Bourassa Mark A. , Zamudio Luis , O'Brien James J.
Published: September 2002

NASA scatterometer (NSCAT) observations provide high temporal and spatial resolution wind fields, which are used to examine gap flow through the Chivela Pass and the influence of Hurricane Marco. For approximately 1 week in November 1996, Hurricane Marco caused the flow through the gap to change from its usual flow pattern: the winds turned to the left rather than the right. Previous studies of this gap flow have used monthly averages of sparse ship observations, European Centre for Medium‐Range Weather Forecasts fields, mesoscale models, or proxies such as cloud motion or sea surface temperatures (SSTs). NSCAT provided unprecedented accuracy (rms differences less than 1.5 m s−1 and 15°) and resolution (daily and 1° × 1° in this study). The observations show that the gap flow often turns to the right (dominated by the Coriolis force); however, unusual events can cause highly noninertial flow for several days. The SSTs respond slowly to changes in the wind pattern, and they are a poor proxy for short‐term wind patterns. The NSCAT winds, gridded with a new methodology presented herein, have the spatial and temporal resolution required to show the evolution of the gap outflow in the presence of a hurricane. The wind fields are used to generate parcel pseudotrajectories, which show that Marco caused Tehuantepec winds to turn to the left and pass through the Gulf of Papagayo into the Caribbean Sea. Gridding techniques are required to fill gaps in fields of NSCAT observations. NSCAT coverage of the (ice free) global oceans is approximately 77% in 1 day and 90% in 2 days. The new gridding technique temporally averages the winds in a manner that leaves very little evidence of the pattern of satellite tracks in the wind fields and little evidence of wind field curl and divergence. Daily fields of 1° × 1° resolution are generated. The temporal sampling characteristics of the wind fields are shown to be nonhomogeneous, with the distribution of characteristic sampling times peaking at 1 day and usually within the range of 0.75 to 1.75 days.
Comparison of NSCAT, ERS 2 active microwave instrument, special sensor microwave imager, and Carbon Interface Ocean Atmosphere buoy wind speed: Consequences for the air‐sea CO2exchange coefficient
Authors: Boutin J. , Etcheto J. , Rafizadeh M. , Bakker D. C. E.
Published: September 2002

Wind speedsUobtained using three different sensor types are compared to assess which might be most useful for calculating gas fluxes across the air‐sea interface.Uremotely sensed using scatterometer (ERS 2 and NASA scatterometer (NSCAT)) and microwave radiometer (special sensor microwave imager (SSM/I)) are compared with each other and withUmeasured in situ (Carbon Interface Ocean Atmosphere (CARIOCA) drifting buoy). The effect of different gridding schemes is also evaluated for the satellite data. Global comparisons for January 1997 indicate that 1° and monthly ERS 2Uare systematically 0.3 to 2 m s−1lower than NSCATUat mid and low latitudes. The NSCAT‐SSM/I comparisons are similar for the three SSM/Is. SSM/IUappear to be higher by 1 to 2 m s−1than NSCATUin the tropics and to be lower at high latitudes. When individual satellite measurements are collocated, ERS 2Uappear to be more coherent with NSCAT (rms of the difference equal to 0.8 m s−1) than SSM/IU(rms of the difference equal to 1.3 m s−1). However, when comparing 1° one‐week interpolated fields, NSCAT‐ERS 2 differences are more scattered than NSCAT‐SSM/I differences, because of the high ERS 2 undersampling. This flaw is attenuated when monthly interpolated 1° fields are compared. The CO2exchange coefficientsKdeduced from these remotely sensedUare compared. Longitudinal profiles of monthlyKare consistent within 0 to about 25% depending on the regions and on the instruments. Comparisons with in situ measurements in a region of very lowUin the eastern equatorial Pacific show excellent agreement between NSCAT and buoyU(bias of 0.3 m s−1) whereas ERS 2Uare underestimated by 1.3 m s−1and SSM/IUare overestimated by 0.7 m s−1on average.
Sources of upper tropospheric HOX: A three‐dimensional study
Authors: Müller Jean‐François , Brasseur Guy
Published: September 2002

The sources of odd hydrogen radicals (HOx = OH + HO2) in the upper troposphere are investigated and quantified using a three‐dimensional model. While the reaction of O(1D) with water vapor constitutes the single major source of HOx in the lower and middle troposphere, the model calculations suggest that acetone photooxidation represents a large, almost ubiquitous source of HOx in the upper troposphere (around 20–40% of the total primary source in the main aircraft corridors, poleward of 40°N), while the convective injections of peroxides and aldehydes are the dominant sources in the tropics, above the oceans and the continents, respectively. The consequences for ozone photochemical production in the upper troposphere are discussed, in particular, in the perspective of the aircraft impact. The role of acetone on ozone photochemical production is estimated and discussed. It is calculated that the presence of acetone might enhance by about 20% the sensitivity of upper tropospheric ozone to the current aircraft emissions of NOx.
Short‐term variations in the 13C/12C ratio of CO as a measure of Cl activation during tropospheric ozone depletion events in the Arctic
Authors: Röckmann Thomas , Brenninkmeijer Carl A. M. , Crutzen Paul J. , Platt Ulrich
Published: September 2002

Analyses of the CO stable isotopic composition in surface air samples collected at high northern latitudes during Arctic spring reveal significant depletions in the 13C content of atmospheric CO, which coincide with episodes of tropospheric O3 loss. This isotope signal can be explained by production of small amounts of highly 13C‐depleted CO, formed in Cl‐initiated oxidation of CH4 due to the presence of enhanced Cl levels during O3 depletion events. Using the recently measured fractionation factor for the CH4 + Cl reaction, the observed 13C depletion is employed to estimate the time‐integrated amount of Cl radicals encountered by the O3 depleted air mass. The results support findings obtained from measurements of hydrocarbon mixing ratios during O3 depletion events. The 13C content of atmospheric CO can thus be used as a sensitive indicator for enhanced Cl radical levels in the atmosphere.
Fast in situ stratospheric hygrometers: A new family of balloon‐borne and airborne Lyman α photofragment fluorescence hygrometers
Authors: Zöger M. , Afchine A. , Eicke N. , Gerhards M.‐T. , Klein E. , McKenna D. S. , Mörschel U. , Schmidt U. , Tan V. , Tuitjer F. , Woyke T. , Schiller C.
Published: September 2002

A new set of hygrometers based on the Lyman α photofragment fluorescence technique has been developed for operation on aircraft and balloons in the stratosphere and upper troposphere. They combine technical details from existing fluorescence hygrometers with several improvements in order to achieve the highest data quality and to minimize maintenance and operational procedures. With these instruments, stratospheric H2O measurements can be accomplished with a precision of < 0.2 ppmv at 1 s integration time, as has been demonstrated both in the laboratory and under field deployment. The design enables a rapid exchange of the air sample of the order of 1 s for fast measurements of small‐scale variations of the H2O mixing ratio in the atmosphere. The hygrometer is calibrated using a laboratory calibration bench with approximately 4% accuracy. Measurements made by the hygrometer are compared with a frost point hygrometer during an aircraft mission at H2O mixing ratios from 280 to 8 ppmv, yielding an agreement between both techniques within the instrumental errors.
Emission of reactive terpene compounds from orange orchards and their removal by within‐canopy processes
Authors: Ciccioli Paolo , Brancaleoni Enzo , Frattoni Massimiliano , Di Palo Vincenzo , Valentini Riccardo , Tirone Giampiero , Seufert Guenther , Bertin Nadia , Hansen Ute , Csiky Olav , Lenz Roman , Sharma Meeta
Published: September 2002

VOC emission from orange orchards was determined in the framework of two field campaigns aimed at assessing the contribution of vegetation emissions to tropospheric ozone formation in the Valencia Citrus belt. Branch emission from different varieties of Citrus sinensis and Citrus Clementi was dominated by β‐caryophyllene during the summer period and by linalool during the blossoming season (April‐May). Large emission of D‐limonene from soil was also measured. Data collected with the enclosure technique were upscaled to determine canopy emission rates of terpene compounds. Values obtained were compared with fluxes measured by relaxed eddy accumulation. Substantial removal of β‐caryophyllene and linalool was detected during transport from the canopy into the atmospheric boundary layer. While within‐canopy removal of the sesquiterpene component was fully consistent with laboratory studies indicating the high reactivity of this compound with ozone, linalool losses were more difficult to explain. Although high canopy fluxes of acetone and acetaldehyde suggested linalool decomposition by gas‐phase reactivity, removal by heterogeneous chemistry seems the more likely explanation for the observed losses.
A mass‐balance/photochemical assessment of DMS sea‐to‐air flux as inferred from NASA GTE PEM‐West A and B observations
Authors: Chen G. , Davis D. , Kasibhatla P. , Bandy A. , Thornton D. , Blake D.
Published: September 2002

This study reports dimethyl sulfide (DMS) sea‐to‐air fluxes derived from a mass‐balance/photochemical‐modeling approach. The region investigated was the western North Pacific covering the latitude range of 0°–30°N. Two NASA airborne databases were used in this study: PEM‐West A in September‐October 1991 and PEM‐West B in February‐March 1994. A total of 35 boundary layer (BL) sampling runs were recorded between the two programs. However, after filtering these data for pollution impacts and DMS lifetime considerations, this total was reduced to 13. Input for each analysis consisted of atmospheric DMS measurements, the equivalent mixing depth (EMD) for DMS, and model estimated values for OH and NO3. The evaluation of the EMD took into account both DMS within the BL as well as that transported into the overlying atmospheric buffer layer (BuL). DMS fluxes ranged from 0.6 to 3.0 μmol m−2 d−1 for PEM‐West A (10 sample runs) and 1.4 to 1.9 μmol m−2 d−1 for PEM‐West B (3 sample runs). Sensitivity analyses showed that the photochemically evaluated DMS flux was most influenced by the DMS vertical profile and the diel profile for OH. A propagation of error analysis revealed that the uncertainty associated with individual flux determinations ranged from a factor of 1.3 to 1.5. Also assessed were potential systematic errors. The first of these relates to our noninclusion of large‐scale mean vertical motion as it might appear in the form of atmospheric subsidence or as a convergence. Our estimates here would place this error in the range of 0 to 30%. By far the largest systematic error is that associated with stochastic events (e.g., those involving major changes in cloud coverage). In the latter case, sensitivity tests suggested that the error could be as high as a factor of 2. With improvements in such areas as BL sampling time, direct observations of OH, improved DMS vertical profiling, direct assessment of vertical velocity in the field, and preflight (24 hours) detailed meteorological data, it appears that the uncertainty in this approach could be reduced to ±25%.
An assessment of the world ground‐based total ozone network performance from the comparison with satellite data
Authors: Fioletov V. E. , Kerr J. B. , Hare E. W. , Labow G. J. , McPeters R. D.
Published: September 2002

Total ozone data available from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) have been compared with Version 7 of the Nimbus 7 (for the 1978–1993 period) and Meteor 3 total ozone mapping spectrometer (TOMS) data sets (for the 1991–1994 period) and with Version 6 of the Nimbus 7 solar backscattered ultraviolet (SBUV) data set (for the 1978–1990 period). Comparison between the ground‐based and satellite observations resulted in parameters such as bias, scattering, relative trend, and seasonal component. About 80% of all Dobson, Brewer, and filter ozonometer stations have standard deviations of monthly mean differences with TOMS that are less than 2.5%. Typically, results of the comparisons between ground‐based stations and SBUV are similar to those for ground‐based and TOMS comparisons. For the ground‐based direct sun total ozone measurements the standard deviations between TOMS and Dobson daily mean values are about 2.4%. The standard deviations for Brewer and filter ozonometer stations are 2.2 and 3.5%, respectively. For the less accurate zenith sky measurements, standard deviations are 3.8% for Dobsons, 4% for Brewers, and 4.7% for filter ozonometer data. Comparisons of individual ground‐based measurements with satellite overpasses yield standard deviations which for middle latitudes increase approximately linearly from about 2% at coincidence to 6% for a 24 hour difference in time or a 600 km spatial difference. Typical problems with ground‐based observations and results of the comparisons for individual stations are also discussed.
Sulfur dioxide distribution over the Pacific Ocean 1991–1996
Authors: Thornton D. C. , Bandy A. R. , Blomquist B. W. , Driedger A. R. , Wade T. P.
Published: September 2002

In this study we combined the sulfur dioxide (SO2) data from the NASA Pacific Exploratory Missions (PEM) and the First Aerosol Characterization Experiment (ACE 1) to create a data set containing 4679 observations of SO2 in the troposphere of the Pacific Ocean during the period 1991–1996. These data have exceptionally high precision due to the use of isotopically labeled SO2 as an internal standard in each sample. The lower limit of detection was less than 2 pptv. The spatial extent of the data ranged from 60°N to 72°S, 110°E to 80°W, and from 50 m to 12 km above the ocean surface. A significant zonal gradient was observed between the northern and southern hemispheres. The western North Pacific was particularly well characterized during the NASA PEM‐West A and B missions that focused on that region. Our data show that anthropogenic sources in eastern Asia dominated the sulfur chemistry in the lower troposphere of the western North Pacific eastward from the Asian continent for more than 1500 km and substantially farther in the mid and upper troposphere. The impact of Asian sources far from the continent was due primarily to transported SO2 with a substantially smaller impact from transported sulfate. Dimethyl sulfide was a significant source of SO2 only in the tropical boundary layer. In the southern hemisphere, anthropogenic sources had much less impact with very little SO2 detected in biomass burning plumes. Sulfur dioxide in the middle and upper troposphere of both hemispheres was strongly influenced by volcanic sources. Sulfur dioxide from the eruption of Mount Pinatubo dominated the SO2 distribution in the upper troposphere in the northern hemisphere in the second half of 1991. A significant fraction of the SO2 in the upper free troposphere in the northern hemisphere was attributed to SO2 transported from the stratosphere to the upper troposphere. Evidence for the transport of SO2 from the stratosphere to troposphere existed as far south as 30°N, but it was most intense at high latitudes. In the absence of major volcanic activity, such as the cataclysmic eruption of Mount Pinatubo, volcanic sources in East Asia contribute significant amounts of SO2 in the mid and upper troposphere of the northern hemisphere. In the southern hemisphere where anthropogenic sources are much weaker, volcanoes may contribute most of the SO2 found in the mid and upper troposphere. Deep convection by tropical and extratropical storms appeared to be a significant process contributing to long‐range transport of volcanic SO2 for the southern hemisphere.
Field measurement of positive ion mobility and mass spectra at a Colorado site in winter
Authors: Nagato Kenkichi , Tanner David J. , Friedli Hans R. , Eisele Fred L.
Published: September 2002

Mobility and mass spectra of positive ions were measured near ground level in winter time. The mobility distribution of positive ions generated from ambient air and aged in the 0.1‐s range is strongly dependent on air temperature. At low temperature, two peaks were observed in the mobility range > 1.5 cm2 V−1 s−1 and interpreted as H3O+ and NH4+ clusters. Ions with mobilities < 1.5 cm2V−1 s−1 became dominant with increasing temperature and ion age. It is suggested that the neutral species responsible for these ions have soil and vegetation sources, the emissions from which are temperature dependent. Mass spectra were obtained via proton transfer reactions to trace gases with proton affinities > 185.6 kcal/mol (from ethanolH+) or > 194.1 kcal/mol (from acetoneH+). The mass spectra contain NH4+ clusters and numerous unidentified peaks extending to the highest mass scanned, 450 amu. Ions in the high mass range > 200 amu also show temperature dependence similar to ions with lower mobilities.
Influence of regional‐scale anthropogenic activity in northeast Asia on seasonal variations of surface ozone and carbon monoxide observed at Oki, Japan
Authors: Pochanart Pakpong , Hirokawa Jun , Kajii Yoshizumi , Akimoto Hajime , Nakao Makoto
Published: September 2002

Surface O3 and CO measurements were carried out at Oki, Japan during March 1994 to February 1996 in order to elucidate the processes determining temporal variations of O3 and CO in the northeast Asian Pacific rim region. The isentropic trajectory analysis was applied to sort out the influences of the air mass exchange under the Asian monsoon system and the regional‐scale photochemical buildup of O3. The trajectories were categorized into five groups which cover background and regionally polluted air masses. The seasonal cycles of O3 and CO in the background continental air mass revealed spring maximum‐summer minimum with averaged concentrations ranging from 32 and 120 ppb to 45 and 208 ppb, respectively. In contrast, O3 concentrations in the regionally polluted continental air mass ranged from 44 to 57 ppb and showed a winter minimum and a spring‐summer‐autumn broad maximum, which was characterized by photochemical O3 production due to anthropogenic activities in northeast Asia. CO concentrations in the same air mass showed a spring maximum of 271 ppb and a summer‐autumn minimum of 180 ppb. The photochemical buildup of O3 resulting from anthropogenic activities in this region was estimated to be 21 ppb in summer, while its production was insignificant, an average 3 ppb, in winter. A comparison between data in northeast Asia and in Europe shows many similarities, supporting the contention that photochemical buildup of O3 from large‐scale precursor emissions in both regions is very significant.
Natural emissions of chlorine‐containing gases: Reactive Chlorine Emissions Inventory
Authors: Khalil M. A. K. , Moore R. M. , Harper D. B. , Lobert J. M. , Erickson D. J. , Koropalov V. , Sturges W. T. , Keene W. C.
Published: September 2002

Although there are many chlorine‐containing trace gases in the atmosphere, only those with atmospheric lifetimes of 2 years or fewer appear to have significant natural sources. The most abundant of these gases are methyl chloride, chloroform, dichloromethane, perchloroethylene, and trichloroethylene. Methyl chloride represents about 540 parts per trillion by volume (pptv) Cl, while the others together amount to about 120 pptv Cl. For methyl chloride and chloroform, both oceanic and land‐based natural emissions have been identified. For the other gases, there is evidence of oceanic emissions, but the roles of the soils and land are not known and have not been studied. The global annual emission rates from the oceans are estimated to be 460 Gg Cl/yr for CH3Cl, 320 Gg Cl/yr for CHCl3, 160 Gg Cl/yr for CH2Cl2, and about 20 Gg Cl/yr for each of C2HCl3, and C2Cl4. Land‐based emissions are estimated to be 100 Gg Cl/yr for CH3Cl and 200 Gg Cl/yr for CHCl3. These results suggest that the oceans account for about 12% of the global annual emissions of methyl chloride, although until now oceans were thought to be the major source. For chloroform, natural emissions from the oceans and lands appear to be the major sources. For further research, the complete database compiled for this work is available from the archive, which includes a monthly emissions inventory on a 1° × 1° latitude‐longitude grid for oceanic emissions of methyl chloride.
Is aerosol production within the remote marine boundary layer sufficient to maintain observed concentrations?
Authors: Capaldo Kevin P. , Kasibhatla Prasad , Pandis Spyros N.
Published: September 2002

To evaluate the impact of aerosols on climate we must consider the aerosol dynamics of the remote marine atmosphere. Marine aerosols are subject to losses due to precipitation, dry deposition, and coagulation; yet, observed remote marine aerosol concentrations and size distributions are relatively constant. This maintenance of the aerosol distribution requires a particle source. This work focuses on the potential of H2SO4 nucleation within the marine boundary layer (MBL) to supply these particles. Spatial and temporal variability in meteorology and species concentrations are considered in a mathematical model to evaluate the effect of natural deviations from average MBL conditions on the highly nonlinear aerosol system. A dynamic, vertically dimensioned, size‐resolved aerosol model is used with parameterized heterogeneous chemical processes. The results suggest that MBL nucleation may be an important source of new particle number in the remote MBL. However, though our model shows that typical remote MBL aerosol distributions can on average be maintained by MBL nucleation and sea‐salt emissions, large oscillations in particle number concentration occur. Because such oscillations are only occasionally reported in measurements, MBL nucleation may not be the dominant source of new particles in the remote MBL. The nucleation events, which cause these oscillations, are predicted to occur at the top of the MBL after rain and/or entrainment of clean free tropospheric air. Predictions are particularly sensitive to the H2SO4 accommodation coefficient, nucleation tuner, and washout efficiency. Reduction of the accommodation coefficient is shown to increase the predicted accumulation mode concentration because the nucleation rate is enhanced. Entrainment of clean free tropospheric air is shown to increase the frequency of nucleation events within the MBL and may help to explain the observed correlation between subsidence and MBL small particles. A small constant addition of particles from the free troposphere, ocean, etc. suppresses H2SO4 nucleation and can lead to a reduction in total predicted aerosol number. This is because nucleation events require low total aerosol surface area and the constant addition of particles reduces the severity of aerosol surface area minimums. Larger external aerosol sources can maintain the observed remote MBL aerosol distribution. However, the temporal and spatial variability of such sources could have a large impact on aerosol concentrations and requires further investigation.
Use of a mixed‐layer model to estimate dimethylsulfide flux and application to other trace gas fluxes
Authors: Lenschow Donald H. , Paluch Ilga R. , Bandy Alan R. , Thornton Donald C. , Blake Donald R. , Simpson Isobel
Published: September 2002

We discuss two techniques based on mixed‐layer scaling for estimating trace gas surface emission fluxes from aircraft using instruments that do not have sufficient frequency response for direct eddy correlation measurements. The first is the mixed‐layer gradient technique, which requires accurate measurements of mean concentrations at several heights in the clear convective planetary boundary layer (CBL) to resolve gradients from even strong surface sources of short‐lived trace gases. The flux‐gradient relationship is obtained from large‐eddy numerical simulations of the CBL. We show that this technique is limited to trace gases with lifetimes of about a day or less. An example is dimethylsulfide (DMS), which is emitted from the ocean and has a lifetime of about a day. Surface DMS flux was estimated from data collected from the NASA P‐3B research aircraft during the Pacific Exploratory Mission‐Tropics (PEM‐Tropics) flight 7 (August 24, 1996), when the aircraft flew a sequence of constant altitude circles about 50 km in diameter at different heights in and above the boundary layer, following the boundary layer air trajectory. The flight took place between 0530 and 1330 local solar time, providing a good opportunity to observe diurnal changes within a Lagrangian framework under nearly clear‐sky conditions. The resulting DMS flux of 2.5±0.8 pptv m s−1 (6.1±1.9)×1013 molecules m−2 s−1, or 8.8±2.8 μmols m−2 d−1 is on the high end of previous measurements in this milieu. The second technique is the mixed‐layer variance technique, which uses measurements of the variance at several heights in the CBL to estimate the surface flux. A major problem with the variance technique is accounting for the contribution of mesoscale variability to the measured variance. Several sources of mesoscale variability were identified: clusters of small cumulus clouds rising through the top of the boundary layer, mesoscale variations in the horizontal wind leading to inaccurate tracking of the air mass and, to a lesser extent, the presence of horizontal roll vortices in some areas of the boundary layer. We show that the variance technique should be applicable to estimating surface fluxes of short‐lived trace gases in cumulus‐free boundary layers that are horizontally uniform, if sample‐collection times of about 10 s or less are used. We also show that it may be possible to utilize mesoscale variance measurements to estimate surface fluxes and lifetimes of species which have lifetimes of perhaps a week or more.
Carbonaceous aerosols and their potential role in atmospheric chemistry
Authors: Lary D. J. , Shallcross D. E. , Toumi R.
Published: September 2002

This paper considers the nature of carbonaceous surfaces, the means by which they are activated, the nature of some functional groups that they support, and some reaction mechanisms that may be involved. Because of the strong affinity of carbonaceous surfaces for organic species and because of the ease with which compounds in a high oxidation state can oxidize the carbonaceous surface, it is highly likely that carbonaceous aerosols are interacting chemically with a range of organic species in ways that have, as yet, not been fully characterized but may significantly affect the oxidizing capacity of our atmosphere. If HONO is formed on the surface of carbonaceous aerosols then this could be a significant source of HOx as HONO is readily photolyzed to give OH, and it could explain the large values of HONO often observed in the troposphere. In general, the reduction of NOy on carbonaceous aerosols is an important consideration, and it is addressed here.
An analysis of ozonesonde data for the troposphere: Recommendations for testing 3‐D models and development of a gridded climatology for tropospheric ozone
Authors: Logan Jennifer A.
Published: September 2002

I present an analysis of ozonesonde data, synthesizing what is known about the distribution of tropospheric ozone. Major features of the distribution are highlighted, and recommendations are given for testing three‐dimensional models of tropospheric chemistry and transport with these data. The data are analyzed on pressure surfaces and relative to the height of the thermal tropopause. A minimum of 20 soundings are required for 95% confidence intervals of the ozone monthly means to be less than ±30% near the extratropical tropopause. Twenty soundings also ensures means reliable to better than ±15% for 800–500 hPa for the extratropics and for 800–100 hPa in the tropics. Ozone variability is higher in the upper troposphere for subtropical locations than for tropical locations, and 35 soundings are required for 400–100 hPa for the means to be defined to better than ±15%. For northern middle and high latitudes, the broad summer maximum in ozone in the middle troposphere extends all the way up to the tropopause. Median concentrations at the tropopause are highest in June and July, typically 125–200 ppb, and are a factor of 2 smaller in winter. Highest values of ozone are in spring 2 km above the tropopause. The change in the phase of the annual cycle of ozone between the tropopause and the region immediately above it, and the steep concentration gradients across the tropopause, suggest that high vertical resolution (∼1 km) will be required in models to simulate this behavior. Mean ozone values in the middle troposphere are approximately constant from 30° to 75° in the winter in both hemispheres, while there is a maximum from 35° to 50°N in summer. In the northern subtropics, there is a summer minimum in middle tropospheric ozone over the Pacific and a summer maximum over the Atlantic which appear to be related to differences in circulation. Mean ozone values over Samoa are similar to those measured 20–30 years ago over Panama. Ozone is higher over the tropical South Atlantic (Natal) than over the western Pacific (Samoa) all year from about 800 hPa to the tropopause; ozone is most similar in May and June over the Atlantic and Pacific, the months with minimum burning in the tropics. The ozone maximum at Samoa in the middle and upper troposphere in October is caused by long‐range transport of ozone and its precursors from biomass burning, with the peak lagging that at Natal by about a month. The secondary peak in ozone in January and December at South Atlantic sites reflects transport of biomass burning effluents from the Northern Hemisphere. The sonde data were used in combination with surface and satellite data to derive a gridded climatology for tropospheric ozone.
Heterogeneous reactions on and in sulfate aerosols: Implications for the chemistry of the midlatitude tropopause region
Authors: Hendricks J. , Lippert E. , Petry H. , Ebel A.
Published: September 2002

A model is presented to simulate heterogeneous reactions of nitrogen, chlorine, and bromine compounds on and in sulfate aerosols under conditions encountered in the lowermost stratosphere and upper troposphere. Box model calculations were performed to investigate the impact of these heterogeneous reactions on the chemistry of the midlatitude tropopause region. The simulations reveal that in the lowermost stratosphere the considered heterogeneous conversions effectively dilute NOx and cause a strong enhancement of the HOx and ClOx mixing ratios. Lowermost stratospheric NOx/NOy and OH/HO2 ratios can efficiently be reduced by the heterogeneous reactions. The model calculations further suggest that also in the upper troposphere the heterogeneous transformations lead to denoxification. The modeled upper tropospheric OH abundance is reduced when the heterogeneous reactions are taken into account. The upper troposphere appears to show a large potential for heterogeneous activation of ClOx and BrOx. The simulations further indicate that the considered heterogeneous chemistry induces ozone loss in the entire tropopause region. This modeled ozone loss is governed by the hydrolysis reactions of N2O5 and BrONO2. In the simulations of the upper troposphere, also heterogeneous reactions of chlorine compounds on and in sulfate aerosols have a major impact on ozone chemistry when an enhanced aerosol loading is assumed.
Phosphorus 32, phosphorus 37, beryllium 7, and lead 210: Atmospheric fluxes and utility in tracing stratosphere/troposphere exchange
Authors: Benitez‐Nelson Claudia R. , Buesseler Ken O.
Published: September 2002

The deposition of 32P, 33P, 7Be, and 210Pb was measured in individual rain events at Woods Hole, Massachusetts, from March 1996 to February 1998 and in bimonthly integrated samples from Portsmouth, New Hampshire, from March to November 1997. Annual depositional fluxes of 32P and 33P were 0.178 and 0.165 disintegrations per minute (dpm) cm−2 yr−1, respectively, at Woods Hole and 0.212 and 0.175 dpm cm−2 yr−1 at Portsmouth. Beryllium 7 and 210Pb fluxes averaged 12.8 and 1.43 dpm cm−2 yr−1, respectively, at Woods Hole and 16.6 and 0.95 dpm cm−2 yr−1 at Portsmouth. High ratios of 33P/32P were observed in rain during severe storm events, suggesting that these isotopes trace stratosphere/troposphere exchange processes. Similar occurrences, however, were not observed in 7Be/32P, 7Be/33P, and 7Be/210Pb ratios. This indicates that there is substantial fractionation occurring among phosphorus, beryllium, and lead between time of production and atmospheric removal via precipitation.
Monte Carlo calculations of polarized microwave radiation emerging from cloud structures
Authors: Roberti Laura , Kummerow Christian
Published: September 2002

The last decade has seen tremendous growth in cloud dynamical and microphysical models that are able to simulate storms and storm systems with very high spatial resolution, typically of the order of a few kilometers. The fairly realistic distributions of cloud and hydrometeor properties that these models generate has, in turn, led to a renewed interest in the three‐dimensional microwave radiative transfer modeling needed to understand the effect of cloud and rainfall inhomogeneities upon microwave observations. Monte Carlo methods and particularly backward Monte Carlo methods have shown themselves to be very desirable because of the quick convergence of the solutions. Unfortunately, backward Monte Carlo methods are not well suited to treat polarized radiation. This study reviews existing Monte Carlo methods and presents a new polarized Monte Carlo radiative transfer code. The code is based on a forward scheme but uses biasing techniques to keep the computational requirements equivalent to the backward solution. Radiative transfer computations have been performed using a microphysical‐dynamical cloud model, and the results are presented together with the algorithm description.
Comparison of climate change scenarios generated from regional climate model experiments and statistical downscaling
Authors: Mearns L. O. , Bogardi I. , Giorgi F. , Matyasovszky I. , Palecki M.
Published: September 2002

We compare regional climate change scenarios (temperature and precipitation) over eastern Nebraska produced by a semiempirical statistical downscaling (SDS) technique and regional climate model (RegCM2) experiments, both using large scale information from the same coarse resolution general circulation model (GCM) control and 2 × CO2 simulations. The SDS method is based on the circulation pattern classification technique in combination with stochastic generation of daily time series of temperature and precipitation. It uses daily values of 700 mbar geopotential heights as the large‐scale circulation variable. The regional climate model is driven by initial and lateral boundary conditions from the GCM. The RegCM2 exhibited greater spatial variability than the SDS method for change in both temperature and precipitation. The SDS method produced a seasonal cycle of temperature change with a much larger amplitude than that of the RegCM2 or the GCM. Daily variability of temperature mainly decreased for both downscaling methods and the GCM. Changes in mean daily precipitation varied between SDS and RegCM2. The RegCM2 simulated both increases and decreases in the probability of precipitation, while the SDS method produced only increases. We explore possible dynamical and physical reasons for the differences in the scenarios produced by the two methods and the GCM.
A comparison of present and doubled CO2 climates and feedbacks simulated by three general circulation models
Authors: Watterson I. G. , Dix M. R. , Colman R. A.
Published: September 2002

The present and doubled CO2 equilibrium climates simulated by slab ocean versions of the atmospheric general circulation models from the Commonwealth Scientific and Industrial Research Organisation (CSIRO, Mark 1 and Mark 2) and from the Bureau of Meteorology Research Centre (BMRC) are examined, with the aim of explaining the large variation in mean warming (4.8°C, 4.3°C, and 2.1°C). The present climates are compared firstly with observations. A graphical display of nondimensional measures of local and mean errors is used. For 15 quantities the models produce broadly similar skill, which indicates that such an evaluation is of limited use as a validation of these models for climate change prediction. Comparison of the two climates indicates that for temperature, snow/ice cover, and water column (but not necessarily other fields) the typical magnitudes of local changes are in rough proportion to the mean warming. For tropical precipitation, however, the BMRC model shows a similar sensitivity to CO2 doubling as do the CSIRO models. A standard diagnostic feedback analysis shows that the Mark 1 model has stronger albedo, water vapor, and cloud feedbacks than the BMRC model. A novel regional net feedback analysis is then applied to all three models. Feedbacks for the snow/ice region and clear‐sky and cloud forcing components of the snow‐free region indicate similar intermodel differences to those from the diagnostic approach. The feedbacks are examined in relation to the simulated climates and model parameterizations. As the application of the regional method requires only standard climatological fields, it is proposed as a convenient analysis tool in further model comparisons.
A seasonal cycle simulation over eastern Asia and its sensitivity to radiative transfer and surface processes
Authors: Giorgi Filippo , Huang Yan , Nishizawa Keiichi , Fu Congbin
Published: September 2002

A multiseasonal simulation over eastern Asia and its sensitivity to cloud radiation and surface processes is analyzed. The model used is the NCAR Regional Climate Model (RegCM), the period of simulation is 13 months in length, and the emphasis of the analysis is on surface climatology, monsoon circulations, cloudiness, surface solar radiation, and top of the atmosphere outgoing longwave radiation. The model reproduces the basic features of the seasonal evolution of east Asia climate, but it overpredicts the intensity of the mature phase of the eastern Asia monsoon. Surface solar radiation is comparable with observations, while outgoing longwave radiation is mostly underestimated. Different formulations of large‐scale cloud‐producing mechanisms are shown to significantly affect cloud profiles, the surface and top of the atmosphere radiation budgets, and as a consequence, the simulated surface climatology. Variations in surface characteristics affect precipitation simulation in different ways during the cold/dry and warm/wet seasons. This paper emphasizes the importance of examining cloud radiation processes in regional climate models in greater detail than has been done in past studies.
Satellite remote sensing of soil moisture in Illinois, United States
Authors: Vinnikov Konstantin Y. , Robock Alan , Qiu Shuang , Entin Jared K. , Owe Manfred , Choudhury Bhaskar J. , Hollinger Steven E. , Njoku Eni G.
Published: September 2002

To examine the utility of using satellite passive microwave observations to measure soil moisture over large regions, we conducted a pilot study using the scanning multichannel microwave radiometer (SMMR) on Nimbus‐7, which operated from 1978 to 1987, and actual in situ soil moisture observations from the state of Illinois, United States, which began in 1981. We examined SMMR midnight microwave brightness temperatures on a 0.5° × 0.5° grid, and compared them with direct soil moisture measurements at 14 sites in Illinois for the period 1982–1987. The results suggest that both the polarization difference and the microwave emissivity for horizontal polarization at frequencies ≤18 GHz have real utility for use as a soil moisture information source in regions with grass or crops where the vegetation is not too dense. While SMMR observations ended in 1987, special sensor microwave/imager observations at 19 GHz start then and extend to the present, and advanced microwave scanning radiometer instruments will fly on satellites beginning soon. Together with SMMR, they have the potential to produce a soil moisture record over large regions for more than two decades and extend it into the future. Satellite observations from these low‐resolution satellite instruments measure the component of large‐scale long‐term soil moisture variability that is related to atmospheric forcing (from precipitation, evapotranspiration, and snowmelt).
Changes in measured lightning flash count and return stroke peak current after the 1994 U.S. National Lightning Detection Network upgrade: 1. Observations
Authors: Wacker Robert S. , Orville Richard E.
Published: September 2002

A total of more than 134 million cloud‐to‐ground lightning flashes (127 million negative, 7 million positive), occurring during 1989–1995 in the continental United States, have been studied on a monthly and yearly basis for variations in flash count, first stroke peak current, and polarity. The years 1989–1993 cover a period in which similar instrumentation was used throughout the United States. In 1994 the National Lightning Detection Network (NLDN) underwent a system‐wide upgrade to improve location accuracy and detection efficiency. As a result of this upgrade, we observe in the NLDN that the negative mean peak current decreased from a preupgrade (1989–1993) mean of 37.5 kA to a 1995 value of 30.2 kA, a decrease of 3.4 standard deviations. The positive mean peak current decreased from 54.4 to 31.6 kA, a 5.0 standard deviation decrease. The NLDN negative flash count increased 1.2 standard deviations, from a preupgrade mean of 16.7 million flashes yr−1 to 20.6 million flashes in 1995. The positive flash count increased 6.2 standard deviations, from an average of 696,000 flashes yr−1 before the upgrade to 2.1 million flashes in 1995. Both the negative and the positive flash count increases were predominantly at low peak currents.
The possible role of organics in the formation and evolution of ultrafine aircraft particles
Authors: Yu Fangqun , Turco Richard P. , Kärcher Bernd
Published: September 2002

We investigate the potential contribution of organic species emitted by aircraft engines to the formation of ultrafine volatile particles detected in jet wakes. The model employed here includes both acidic and organic aerosols—in their “pure” states and mixed with soot—as well as small ion clusters and electrically charged particles. The microphysical treatment accounts for the effects of charge, within the framework of a multicomponent size‐resolved kinetically controlled vapor‐aerosol system. Owing to their high proton affinities, water/acid solubility, and/or reactivities with liquid sulfuric acid and other inorganic compounds, certain organic constituents known to be generated by aircraft engines are likely to condense onto ions and charged particles and/or to be taken up by liquid sulfuric acid particles, where these particles are expected to act as efficient reaction sites. The present simulations, which are constrained by in situ measurements, indicate that in the case of very low sulfur emissions, organic species in the exhaust stream can dominate the mass of volatile particles detected at very early times. In the case of fuels with medium to high sulfur contents, the organic emissions may still contribute a significant fraction of the total aerosol mass and alter the composition of the exhaust particles.
Investigating k distribution methods for parameterizing gaseous absorption in the Hadley Centre Climate Model
Authors: Cusack S. , Edwards J. M. , Crowther J. M.
Published: September 2002

A technique for generating the coefficients of the k distribution method has been developed which uses very few exponential terms to represent absorption in a spectral region. This new technique, using k distributions derived from HITRAN 1992 spectroscopic data, predicts vertical profiles of irradiances and heating rates due to the major radiatively active gases with a better balance between speed of computation and accuracy than previous methods. Absorption coefficients were specified in a manner similar to that of the correlated k distribution method, whereas the weights of each term were determined using a fitting procedure similar in principle to the exponential‐sum fitting of transmissions method. Errors in irradiances and heating rates for various atmospheric profiles and the reference profile (with which fitting of weights was performed) were of a similar magnitude, thereby indicating the technique's suitability for use in global model studies. This parameterization is now used by the Hadley Centre climate model.
Atmospheric and geometrical effects on land surface albedo
Authors: Lyapustin A. I.
Published: September 2002

In spite of extensive experimental and theoretical studies of land surface albedo, there is a need of generalization and systematic quantification of its dependence on solar zenith angle and type of surface, and of atmospheric effects in these relationships. This work presents a systematic numerical study of surface albedo for 12 land cover types, described by realistic anisotropic reflectance, under widely varying atmospheric conditions. Our research is based on the rigorous solution of the radiative transfer equation with the spherical harmonics method. It uses a new original analytical formalism for treatment of the anisotropic boundary conditions, which avoids numeric integration. This formalism is described in the first part of the paper along with the major steps in solution of the radiative transfer equation. The second part of the paper presents the results of our study, which can be summarized as follows: for a large number of vegetation and soil surfaces, the range of relative variation of surface albedo with atmospheric opacity does not exceed 10–15% at θ0 ≤ 50° and 20–30% at θ0 ≤ 70°. Among atmospheric parameters, optical thickness is the main factor affecting albedo. The effects of the scattering function and single‐scattering albedo are small and can be neglected. For small solar zenith angles, surface albedo increases with atmospheric opacity, while for big angles θ0, it decreases. The watershed lies at θ0 ≈ 52°–57° when the albedo is almost insensitive to the atmospheric opacity. Albedo measurements at these angles can be used directly for the surface characterization. At large solar zenith angles (θ0 > 70°–80°), albedo drops off with the further increase of angle θ0.
Atmospheric transmission of North Atlantic Heinrich events
Authors: Hostetler S. W. , Clark P. U. , Bartlein P. J. , Mix A. C. , Pisias N. J.
Published: September 2002

We model the response of the climate system during Heinrich event 2 (H2) by employing an atmospheric general circulation model, using boundary conditions based on the concept of a “canonical” Heinrich event. The canonical event is initialized with a full‐height Laurentide ice sheet (LIS) and CLIMAP sea surface temperatures (SSTs), followed by lowering of the LIS, then warming of North Atlantic SSTs. Our modeled temperature and wind fields exhibit spatially variable responses over the Northern Hemisphere at each stage of the H2 event. In some regions the climatic responses are additive, whereas in other regions they cancel or are of opposite sign, suggesting that Heinrich event climatic variations may have left complex signatures in geologic records. We find variations in the tropical water balance and the mass balance of ice sheets, and implications for variations in terrestrial methane production from the contraction of northern permafrost regions and the expansion of tropical wetlands.