bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the snow cover in climate studies: 1. Long‐term integrations under different climatic conditions using a multilayered snow‐cover model
Authors: Loth Bettina , Graf Hans‐F.
Published: February 2004

A physically based model of a snow cover, which has been designed for climate studies, was tested under different conditions (Arctic, midlatitudes, alpine regions). This multilayered model can be used for stand‐alone experiments as well as for implemention into sophisticated coupled models. One version of the model, which considers a partial snow coverage of the grid cells and an albedo masking effect by vegetation, is formulated for the Hamburg climate model ECHAM [Roeckner et al., 1992]. The key parameters of the snow‐cover model are snow albedo, description of liquid water (storage capacity, transmission rate), turbulent fluxes at the snow surface, heat conduction (internal heat conduction and heat exchange with ground), new snow density and density changes due to aging, and the choice of snow layers. Wind drifting processes and an additional form of metamorphism, the wind compaction, have to be considered in areas with high wind speeds. In order to obtain the simulation quality which is necessary for climate studies when using observed forcing data, the precipitation measurements should be corrected and information concerning the precipitation type should be given in the input data sets. A further improvement of the model could be reached by the implementation of a more sophisticated transmission scheme for liquid water, the use of a wind dependent new snow density, and the modification of snow albedo due to different rates of pollution.
Modeling the snow cover in climate studies: 2. The sensitivity to internal snow parameters and interface processes
Authors: Loth Bettina , Graf Hans‐F.
Published: February 2004

In order to find an optimal complexity for snow‐cover models in climate studies, the influence of single snow processes on both the snow mass balance and the energy fluxes between snow surface and atmosphere has been investigated. Using a sophisticated model, experiments were performed under several different atmospheric and regional conditions (Arctic, midlatitudes, alpine regions). A high simulation quality can be achieved with a multilayered snow‐cover model resolving the internal snow processes (cf. part 1,[Loth and Graf, this issue]). Otherwise, large errors can occur, mostly in zones which are of paramount importance for the entire climate dynamics. Owing to simplifications of such a model, the mean energy balance of the snow cover, the turbulent heat fluxes, and the long‐wave radiation at the snow surface may alter by between 1 W/m2 and 8 W/m2. The snow‐surface temperatures can be systematically changed by about 10 K.
Sulfate dry deposition measured with a water surface sampler: A comparison to modeled results
Authors: Yi Seung‐Muk , Holsen Thomas M. , Zhu Xiang , Noll Kenneth E.
Published: February 2004

Sulfate can be deposited to water from the atmosphere as both a particulate (SO42−) and a gas (SO2) (SO2 is quickly hydrolyzed and oxidized to SO42− in water). To determine the relative importance of these two types of deposition, a circular water surface sampler (WSS) was used to measure the dry deposition flux of atmospheric SO2 and sulfate containing particles in Chicago, Illinois, during the spring, summer, and fall of 1994 and 1995. A sharp‐edged, greased surrogate surface was used to simultaneously measure the dry deposition flux of atmospheric sulfate containing particles. The total sulfate flux to the WSS was consistently higher than the flux to the greased surface due to gas phase deposition. When the dry deposition of SO2 gas to the WSS was calculated by subtracting the greased surface flux (particulate sulfate flux) from the WSS flux (particulate sulfate plus SO2 flux), it was found to contribute significantly more (70–96%) than the particulate phase to overall dry deposition of sulfate to the water surface. For each sample the SO2 flux was also calculated as the product of the individual SO2 gas phase mass transfer coefficient calculated using two different models and the measured ambient SO2 concentration. The predicted SO2 fluxes calculated from both models were highly correlated with the measured SO2 flux obtained with the WSS. The resistance model predictions were statistically the same as the measured values; however the evaporation model significantly underpredicted the deposition of compounds that exist in both the gas and particulate phases.
Forcings and chaos in interannual to decadal climate change
Authors: Hansen J. , Sato M. , Ruedy R. , Lacis A. , Asamoah K. , Beckford K. , Borenstein S. , Brown E. , Cairns B. , Carlson B. , Curran B. , de Castro S. , Druyan L. , Etwarrow P. , Ferede T. , Fox M. , Gaffen D. , Glascoe J. , Gordon H. , Hollandsworth S. , Jiang X. , Johnson C. , Lawrence N. , Lean J. , Lerner J. , Lo K. , Logan J. , Luckett A. , McCormick M. P. , McPeters R. , Miller R. , Minnis P. , Ramberran I. , Russell G. , Russell P. , Stone P. , Tegen I. , Thomas S. , Thomason L. , Thompson A. , Wilder J. , Willson R. , Zawodny J.
Published: February 2004

We investigate the roles of climate forcings and chaos (unforced variability) in climate change via ensembles of climate simulations in which we add forcings one by one. The experiments suggest that most interannual climate variability in the period 1979–1996 at middle and high latitudes is chaotic. But observed SST anomalies, which themselves are partly forced and partly chaotic, account for much of the climate variability at low latitudes and a small portion of the variability at high latitudes. Both a natural radiative forcing (volcanic aerosols) and an anthropogenic forcing (ozone depletion) leave clear signatures in the simulated climate change that are identified in observations. Pinatubo aerosols warm the stratosphere and cool the surface globally, causing a tendency for regional surface cooling. Ozone depletion cools the lower stratosphere, troposphere and surface, steepening the temperature lapse rate in the troposphere. Solar irradiance effects are small, but our model is inadequate to fully explore this forcing. Well‐mixed anthropogenic greenhouse gases cause a large surface wanning that, over the 17 years, approximately offsets cooling by the other three mechanisms. Thus the net calculated effect of all measured radiative forcings is approximately zero surface temperature trend and zero heat storage in the ocean for the period 1979–1996. Finally, in addition to the four measured radiative forcings, we add an initial (1979) disequilibrium forcing of +0.65 W/m2. This forcing yields a global surface warming of about 0.2°C over 1979–1996, close to observations, and measurable heat storage in the ocean. We argue that the results represent evidence of a planetary radiative imbalance of at least 0.5° W/m2; this disequilibrium presumably represents unrealized wanning due to changes of atmospheric composition prior to 1979. One implication of the disequilibrium forcing is an expectation of new record global temperatures in the next few years. The best opportunity for observational confirmation of the disequilibrium is measurement of ocean temperatures adequate to define heat storage.
On the significance of regional trace gas distributions as derived from aircraft campaigns in PEM‐West A and B
Authors: Ehhalt D. H. , Rohrer F. , Kraus A. B. , Prather M. J. , Blake D. R. , Rowland F. S.
Published: February 2004

With the help of a chemical transport model we calculate the global distributions of three synthetic tracers with uniform emission rates over the continents, zero emissions over the ocean, and constant lifetimes of 5, 20, and 100 days. These distributions serve as a first approximation to those of the lower alkanes and are analyzed in detail for the region between 115°E and 155°E longitude, 0°N and 56°N latitude, within which the Pacific Exploratory Mission‐West (PEM‐West) A and B campaigns took place. We show that the longitudinally averaged latitude by altitude distributions of the tracers over that domain obtained from sampling the model data along the flight tracks closely represent those obtained from complete averages over the full data sets for the months March and September when the campaigns took place. We further show that for each campaign the modeled distributions closely resemble those measured for ethane and propane, although these distributions change significantly between the two seasons. Taken together, this demonstrates that the average distributions of ethane and propane measured during PEM‐West A and B describe reasonably well the true pattern of these gases between 0°N and 50°N latitude and 115°E and 155°E longitude in both seasons, despite incomplete and irregular sampling of that domain. In turn, the measured distributions can be used to test the model, mainly for correct parameterization of transport.
Observing Earth's atmosphere with radio occultation measurements using the Global Positioning System
Authors: Kursinski E. R. , Hajj G. A. , Schofield J. T. , Linfield R. P. , Hardy K. R.
Published: February 2004

The implementation of the Global Positioning System (GPS) network of satellites and the development of small, high‐performance instrumentation to receive GPS signals have created an opportunity for active remote sounding of the Earth's atmosphere by radio occultation at comparatively low cost. A prototype demonstration of this capability has now been provided by the GPS/MET investigation. Despite using relatively immature technology, GPS/MET has been extremely successful [Ware et al., 1996;Kursinski et al., 1996], although there is still room for improvement. The aim of this paper is to develop a theoretical estimate of the spatial coverage, resolution, and accuracy that can be expected for atmospheric profiles derived from GPS occultations. We consider observational geometry, attenuation, and diffraction in defining the vertical range of the observations and their resolution. We present the first systematic, extensive error analysis of the spacecraft radio occultation technique using a combination of analytical and simulation methods to establish a baseline accuracy for retrieved profiles of refractivity, geopotential, and temperature. Typically, the vertical resolution of the observations ranges from 0.5 km in the lower troposphere to 1.4 km in the middle atmosphere. Results indicate that useful profiles of refractivity can be derived from ∼60 km altitude to the surface with the exception of regions less than 250 m in vertical extent associated with high vertical humidity gradients. Above the 250 K altitude level in the troposphere, where the effects of water are negligible, sub‐Kelvin temperature accuracy is predicted up to ∼40 km depending on the phase of the solar cycle. Geopotential heights of constant pressure levels are expected to be accurate to ∼10 m or better between 10 and 20 km altitudes. Below the 250 K level, the ambiguity between water and dry atmosphere refractivity becomes significant, and temperature accuracy is degraded. Deep in the warm troposphere the contribution of water to refractivity becomes sufficiently large for the accurate retrieval of water vapor given independent temperatures from weather analyses [Kursinski et al., 1995]. The radio occultation technique possesses a unique combination of global coverage, high precision, high vertical resolution, insensitivity to atmospheric particulates, and long‐term stability. We show here how these properties are well suited for several applications including numerical weather prediction and long‐term monitoring of the Earth's climate.
Uncertainty analysis of direct radiative forcing by anthropogenic sulfate aerosols
Authors: Pan Wenwei , Tatang Menner A. , McRae Gregory J. , Prinn Ronald G.
Published: February 2004

Uncertainty in the direct radiative forcing by anthropogenic sulfate aerosols is analyzed for four different aerosol model structures with 13 uncertain parameters using a second‐order probabilistic collocation method. The resulting probability density functions agree well with those determined here by a computationally much more expensive 10,000‐point Monte Carlo method. The structural difference, measured by the range of the mean responses in models with different approximations, is −0.28 to −1.3 W m−2, and the parametric uncertainty, induced by the uncertainties in the model parameters, is −0.1 to −4.2 W m−2, both with 95% confidence. This implies that refining uncertain input parameters may be more important than improving models in order to minimize the overall uncertainty in the direct radiative forcing by anthropogenic sulfate aerosols in these four models. The variance analysis indicates that the parametric uncertainty comes mainly from sulfate yield, sulfate lifetime, and ambient relative humidity. Variance contributions from aerosol size parameters are much smaller, and this finding agrees with the sensitivity analysis by Boucher and Anderson [1995]. Note that the conclusions reached here are dependent on the chosen model structures and parameter distributions. This study presents a computationally efficient framework for assessing uncertainty in aerosol radiative forcing which could be used to address an even wider range of structures and parameters in the future.
A comparative study of POAMII and electrochemical concentration cell ozonesonde measurements obtained over northern Europe
Authors: Deniel Carole , Dalaudier F. , Chassefiére E. , Bevilacqua R. M. , Shettle E. P. , Hoppel K. W. , Hornstein J. S. , Lumpe J. D. , Rusch D. W. , Randall C. E.
Published: February 2004

Ozone profiles measured by the Polar Ozone and Aerosol Measurement (POAM II) instrument (version 5 retrievals) flown on the SPOT 3 spacecraft are compared with coincident electrochemical concentration cell (ECC) ozonesonde profiles at 11 stations over polar northern regions. The comparison study extends from December 1993 to March 1996 and contains about 500 coincidences. The satellite and ozonesonde profile coincidences are averaged separately for winter and nonwinter periods and for winter coincidences occurring inside and outside of the polar vortex. The mean differences versus altitude show a dependence on the latitude separation between the sondes and the POAM II measurements. The best estimate of the biases between the POAM II and the ECC ozone measurements is obtained after removal of the latitudinal gradient effect on the relative mean differences for the dynamically quiet nonwinter periods. The biases are negative at all altitudes indicating that the POAM version 5 retrievals are low relative to the ECC sondes. The magnitude of this bias ranges from 2–3% near the lowest and highest altitudes of profile overlap (17 and 30 km) to 7.6% from 21 to 24 km, which is comparable to the evaluated errors of both instruments.
The influence of the quasi‐biennial oscillation on global constituent distributions
Authors: O'Sullivan Donal , Dunkerton Timothy J.
Published: February 2004

Long‐lived stratospheric constituents observed by the Upper Atmosphere Research Satellite are compared between late northern winters 1991–1992 and 1992–1993. The phase of the quasi‐biennial oscillation (QBO) was such that the equatorial zonal wind was easterly (westerly) in the lower stratosphere during the first (second) winter. In the tropics, winter‐to‐winter differences in the latitudeheight distribution of zonally averaged N2O and H2O are associated with differences in large‐scale upwelling, the QBO's mean meridional circulations, and lateral mixing by incident extratropical Rossby waves. In the extratropics, significant differences in zonally averaged N2O and H2O are also found which are believed to represent the extratropical constituent QBO. The transport processes creating the extratropical constituent QBO are tentatively identified: In the winter extratropics, QBO modulation of the residual circulation appears to dominate in the upper stratosphere, while QBO‐modulated Rossby‐wave isentropic mixing dominates below about 10 mbar. Analysis of Rossby waves at low latitudes, as deduced from isobaric constituent distribution maps, suggests that the meridional distribution of Rossby wave breaking depends on the tropical zonal winds. Easterly QBO winds prevent Rossby‐wave radiation into the tropics, so that a strong constituent concentration gradient is generated in the winter subtropics. Westerly QBO winds allow some wave radiation into the tropics, accompanied by isentropic mixing in the winter subtropics, cross‐equatorial propagation, and occasional mixing in the summer subtropics. These effects depend on altitude and appear to vary in accord with the descending phases of the QBO.
Ultraviolet optical model of volcanic clouds for remote sensing of ash and sulfur dioxide
Authors: Krotkov N. A. , Krueger A. J. , Bhartia P. K.
Published: February 2004

The total ozone mapping spectrometer (TOMS) instruments have detected every significant volcanic eruption from November 1978 to December 1994 on the Nimbus 7 and Meteor 3 satellites and since July 1996 on the new satellites, TOMS‐Earth Probe and ADEOS. We apply a radiative transfer model to simulate the albedos of these fresh eruption clouds to study the limitations of the present algorithm which assumes an absorbing cloud above a scattering atmosphere. The conditions are found to be approximated when the total absorption optical depth is less than 2 (i.e., 100 Dobson units (DU) SO2at 312 nm or 300 DU SO2at 317 nm). The spectral dependence of the albedo of a nonabsorbing Rayleigh atmosphere can be specified by only two parameters which are uniquely different when ash or sulfate aerosols are present in the stratosphere. However, the interaction between ash scattering and SO2absorption within a volcanic cloud produces a nonlinear effect at strongly absorbing wavelengths that accounts for overestimation of sulfur dioxide in ash‐laden volcanic clouds by theKrueger et al. [1995] algorithm. Correction of this error requires knowledge of the ash properties. A method for determining two of the ash parameters from the longer TOMS wavelengths is described. Given the altitude of the cloud, surface reflectivity, and an estimate of effective variance of the ash size distribution, the optical thickness and either the effective radius or the index of refraction can be deduced. The ash retrievals are also needed to evaluate the tephra/gas ratio of eruptions and to compare the ash properties of different volcanoes.
Sea‐salt aerosol in coastal Antarctic regions
Authors: Wagenbach D. , Ducroz F. , Mulvaney R. , Keck L. , Minikin A. , Legrand M. , Hall J. S. , Wolff E. W.
Published: February 2004

Continuous year round records of atmospheric sea‐salt concentrations have been recovered at three coastal Antarctic stations (Halley, Dumont D'Urville, and Neumayer) at temporal resolutions typically between 1 day and 2 weeks. The records were evaluated in terms of their spatial and seasonal variability as well as with respect to changes in the relative ion composition of airborn sea‐salt particles. Annual mean sea‐salt concentrations vary between 1400 ng m−3 at Dumont D'Urville, 850 ng m−3 at Neumayer, and 200 ng m−3 at Halley, respectively. They are thus considerably lower than the mean levels previously observed at the north tip of the Antarctic Peninsula but are, at their lower end, comparable to the level previously reported from Mawson. The representativeness of the atmospheric sea‐salt data appears to be weak due to their high temporal variability, strong impacts of site specific aspects (such as site topography) but also due to the nonuniform sampling techniques applied so far. In accordance with the ice core evidence, the seasonal change in the atmospheric sea‐salt load is found to be clearly out of phase with the seasonal cycle of the open water fraction offshore from the station as (with the exception of Dumont D'Urville) the lowest concentrations are generally observed during the local summer months. Major ion analyses of bulk aerosol and concurrently sampled fresh snow show a strong, systematic depletion of the SO42− to Na+ (Cl−) ratios with respect to bulk sea water, which appeared to be confined to the local winter half year. During that time, sea‐salt SO42− was found to be depleted typically by 60–80% along with a concurrent Na+ deficit, which is in accordance with the precipitation of mirabilite. No significant fractionation of Mg2+, K+, and Ca2+ between seawater and sea‐salt particles is observed. Laboratory experiments failed to simulate the SO42− fractionation in airborne seawater droplets or in the skin of seawater bubbles at low air temperatures. They gave, however, SO42− depletion factors, similar to the field observation in air and snow, in the remaining brine of seawater which was partly frozen below −8°C to an artificial sea ice surface. It is suggested therefore that the mobilization of brine from the sea ice surface constitutes an important sea‐salt source in winter which may dominate the atmospheric sea‐salt load at high latitudes of coastal Antarctica.
Physical properties of Amazonian soils: A modeling study using the Anglo‐Brazilian Amazonian Climate Observation Study data
Authors: Delire C. , Calvet J.‐C. , Noilhan J. , Wright I. , Manzi A. , Nobre C.
Published: February 2004

The hydraulic properties of some Amazonian soils differ significantly from the properties of the temperate soils. Most of the soil water release functions implemented in the atmospheric models used in deforestation studies were developed for temperate soils. It is necessary to check the validity of these soil water models with in situ data. In this study, the Anglo‐Brazilian Amazonian Climate Observation Study (ABRACOS) soil data have been used to modify the parameter values of Clapp and Hornberger's water release model. Different relations between hydraulic parameters and texture are proposed. These relations are included in the ISBA land surface scheme which is used to simulate the longterm evolution of the soil water content and the surface energy balance of three contrasting ABRACOS sites: two pasture sites with distinct soil properties and a forest site. The sensitivity of the simulations to the use of either the original Clapp and Hornberger water release model or the ABRACOS‐derived one is shown.
The terdiurnal tide in the mesosphere and lower thermosphere over London, Canada (43°N, 81°W)
Authors: Thayaparan T.
Published: February 2004

Radars have been used successfully for many years to measure atmospheric diurnal and semidiurnal tides, but studies of the atmospheric terdiurnal tide have only rarely been reported to date in the literature. In this paper we investigate the amplitude, phase, and vertical wavelength for the terdiurnal tide using the University of Western Ontario (UWO) 2 MHz radar near London, Canada. Emphasis is placed on short‐term, monthly, seasonal, and annual variations of the terdiurnal tide. The results of the present study bring out certain new aspects of the terdiurnal tide at midlatitudes in the northern hemisphere. We show that terdiurnal tides have significant amplitudes in the mésosphère and lower thermosphère; indeed, the terdiurnal amplitudes are often comparable to the diurnal ones. We also draw attention to the annual variability and especially highlight the occurrence of the terdiurnal tide in non winter months. For example, the occurrence of the terdiurnal tide in spring months is not uncommon. Thus it seems that the 8‐hour component observed over London, Canada, cannot be explained by the terdiurnal tide being due to solar heating alone. Particular cases of the terdiurnal tide are examined, and the observed amplitude and phase profiles of the terdiurnal tide at certain times of the year are discussed in terms of nonlinear interaction between the diurnal and semidiurnal tides. We suspect that this effect may also explain the fact that diurnal and semidiurnal tides at our station often seem suppressed relative to other sites [Thayaparan et al., 1995a]. On the basis of our previous and present observations, it is also possible that large terdiurnal amplitudes observed at certain times of the year may be partly explained in terms of tidal/gravity wave interactions. However, this possibility needs to be further investigated in the future.
Tropospheric mixing ratios of NO obtained during TROPOZ II in the latitude region 67°N–56°S
Authors: Rohrer Franz , Brüning Dirk , Ehhalt Dieter H.
Published: February 2004

Vertical profiles of NO were measured during the TROPOZ II aircraft campaign in January 1991. Altogether, 30 flights were made along the coastlines of western Europe, eastern North America, South America, and western Africa, covering latitudes between 67°N and 56°S and altitudes between 0 and 11 km. As a result of the general wind pattern, westerly winds at all northern and southern midlatitudes, the NO data show large‐scale outflow of NO‐polluted air along the east coasts of the North and South American continents and inflow of clean maritime air over the west coasts of South America, North Africa, and Europe. The NO concentrations in the free tropospheric air flowing out from the (winter) North American continent lie between 100 and 300 parts per trillion by volume (pptv) with a weakly C‐shaped vertical profile. The air coming off the (summer) South American continent has a factor of 3 lower NO concentrations, between 40 and 100 pptv, but a more pronounced C‐shaped vertical profile with NO mixing ratios of up to 200 pptv in the upper troposphere of the tropics. The salient features in the NO distribution are discussed with the help of 3 day back trajectories.
Contribution of different aerosol species to the global aerosol extinction optical thickness: Estimates from model results
Authors: Tegen Ina , Hollrig Peter , Chin Mian , Fung Inez , Jacob Daniel , Penner Joyce
Published: February 2004

We combine global distributions of aerosol loading resulting from transport models for soil dust, sulfate, sea salt, and carbonaceous aerosol. From the aerosol distributions we estimate optical thicknesses and compare them with Sun photometer measurements and satellite retrievals, thereby revealing problems with both model results and comparisons with such measurements. Globally, sulfate, dust, and carbonaceous particles appear to contribute equally to the total aerosol optical thickness. Owing to the different optical properties of different aerosol types, aerosol composition should be taken into consideration for estimating the aerosol climate effect as well as for aerosol retrievals from satellite measurements.
Observations of high concentrations of total reactive nitrogen (NOy) and nitric acid (HNO3) in the lower Arctic stratosphere during the Stratosphere‐Troposphere Experiment by Aircraft Measurements (STREAM) II campaign in February 1995
Authors: Fischer H. , Waibel A. E. , Welling M. , Wienhold F. G. , Zenker T. , Crutzen P. J. , Arnold F. , Bürger V. , Schneider J. , Bregman A. , Lelieveld J. , Siegmund P. C.
Published: February 2004

Simultaneous in situ measurements of NOy, HNO3, O3, N2O, and CO have been performed in the lower stratosphere during the Stratosphere‐Troposphere Experiment by Aircraft Measurements (STREAM) II intensive winter campaign in February 1995 from Kiruna airport (northern Sweden) with a Cessna Citation II twinjet aircraft up to a maximum altitude of 12.8 km. The flights were coordinated with the Arctic Second European Stratospheric Arctic and Midlatitude Experiment (SESAME) winter campaign. Strongly elevated levels of total reactive nitrogen (NOy) and its most abundant contributing species, nitric acid (HNO3), with mixing ratios up to 9 parts per billion by volume (ppbv), were observed during all flights at altitudes near 12 km. On average, the measured NOy concentrations exceed the expected levels by a factor of 2–3. Normal background NOy has been calculated from observed N2O mixing ratios using the NOy‐N2O correlation reported for the undisturbed northern hemisphere. This indicates that subsidence of air in the vortex alone cannot explain these findings. We propose that the elevated NOy concentrations were caused by nitrification of the lower stratosphere associated with sedimentation and evaporation of polar stratospheric cloud particles that carry down HNO3 from higher altitudes, that is, from altitudes up to about 25 km.
Representations of transport, convection, and the hydrologic cycle in chemical transport models: Implications for the modeling of short‐lived and soluble species
Authors: Rasch P. J. , Mahowald N. M. , Eaton B. E.
Published: February 2004

We compare chemical transport simulations performed in a model using archived meteorological data (an off‐line transport model) to those performed in a model in which the meteorological data are predicted every time step (an on‐line model). We identify the errors associated with using data sampled at timescales much longer than those operating in the atmosphere or in the on‐line model, and strategies for ameleorating those errors. The evaluation is performed in the context of a global off‐line chemical transport model called the Model of Atmospheric Transport and Chemistry (MATCH) for three test problems: (1) the passive advection of blobs initially concentrated in the lower and upper troposphere; (2) the surface emission of radon and its decay to lead; and (3) the removal of lead from the atmosphere by wet and dry deposition processes. These problems exercise the important processes of transport by resolved scale winds, rapid transport by smaller scale convection processes, and wet removal (which depends on the representation of the hydrologic cycle). We show that the errors in off‐line model simulations (compared to the on‐line simulations) can be made small when the sampling interval is order 6 hours or less. We also show that one can accurately reproduce the subgrid‐scale processes within the off‐line model, rather than needing to archive the results of those processes as input to the off‐line model. This suggests that for the spatial and temporal scales treated in global models it is possible to treat many problems nearly as accurately in an off‐line mode as one can with an on‐line treatment.
New insights into lightning processes gained from triggered‐lightning experiments in Florida and Alabama
Authors: Rakov V. A. , Uman M. A. , Rambo K. J. , Fernandez M. I. , Fisher R. J. , Schnetzer G. H. , Thottappillil R. , Eybert‐Berard A. , Berlandis J. P. , Lalande P. , Bonamy A. , Laroche P. , Bondiou‐Clergerie A.
Published: February 2004

Analyses of electric and magnetic fields measured at distances from tens to hundreds of meters from the ground strike point of triggered lightning at Camp Blanding, Florida, and at 10 and 20 m at Fort McClellan, Alabama, in conjunction with currents measured at the lightning channel base and with optical observations, allow us to make new inferences on several aspects of the lightning discharge and additionally to verify the recently published “two‐wave” mechanism of the lightningMcomponent. At very close ranges (a few tens of meters or less) the time rate of change of the final portion of the dart leader electric field can be comparable to that of the return stroke. The variation of the close dart leader electric field change with distance is somewhat slower than the inverse proportionality predicted by the uniformly charged leader model, perhaps because of a decrease of leader charge density with decreasing height associated with an incomplete development of the corona sheath at the bottom of the channel. There is a positive linear correlation between the leader electric field change at close range and the succeeding return stroke current peak at the channel base. The formation of each step of a dart‐stepped leader is associated with a charge of a few millicoulombs and a current of a few kiloamperes. In an altitude‐triggered lightning the downward negative leader of the bidirectional leader system and the resulting return stroke serve to provide a relatively low‐impedance connection between the upward moving positive leader tip and the ground, the processes that follow likely being similar to those in classical triggered lightning. Lightning appears to be able to reduce, via breakdown processes in the soil and on the ground surface, the grounding impedance which it initially encounters at the strike point, so at the time of channel‐base current peak the reduced grounding impedance is always much lower than the equivalent impedance of the channel. At close ranges the measuredM‐component magnetic fields have waveshapes that are similar to those of the channel‐base currents, whereas the measuredM‐component electric fields have waveforms that appear to be the time derivatives of the channel‐base current waveforms, in further confirmation of the “two‐wave”M‐component mechanism.
Saturated‐cascade similitude theory of gravity wave spectra
Authors: Dewan Edmond
Published: February 2004

A theory is presented, which is based mainly on dimensional analysis (but also on gravity wave theory), that attempts to explain all the types of gravity wave power spectral densities (PSDs) now being measured. This theory is based on two concepts, namely, wave saturation and wave cascade. The immediate result of the simultaneous presence of these two processes is that there should exist a unique relation between the vertical (or horizontal) wavelength of a gravity wave and its period (provided the Brunt Period and dissipation rate are given and Doppler effects are omitted). This relation provides a way to derive all of the intrinsic spectra from the fundamental one which is the vertical wavenumber PSD of the horizontal winds. The most important suggestion to emerge from this theory is that ϵ, the dissipation rate, is the main controlling independent variable for the amplitude of all but 3 of the 12 spectra predicted. It would also control the wavelength‐period relations. Comparisons are made between observations and theory, and important experimental tests are proposed. This model presently appears to be useful in the analysis of gravity wave data obtained by means of lidars, radars, interferometers, and imagers. In addition, it raises a number of new scientific issues for future research.
Near‐field measurements on contrail properties from fuels with different sulfur content
Authors: Petzold A. , Busen R. , Schröder F. P. , Baumann R. , Kuhn M. , Ström J. , Hagen D. E. , Whitefield P. D. , Baumgardner D. , Arnold F. , Borrmann S. , Schumann U.
Published: February 2004

Microphysical properties of jet exhaust aerosol and contrails were studied in the near field of the emitting aircraft for different fuel sulfur contents. Measurements were performed behind two different aircraft (ATTAS test aircraft of type VFW 614 and Airbus A310‐300) using fuels with sulfur contents of 6 ppm and 2700 ppm, respectively. At closest approach (plume age < 1 s), the total number concentrations exceeded the measuring range of the condensation particle counter, i.e., N>105 cm−3. The concentration of the dry accumulation mode aerosol, i.e., predominantly soot particles, was not affected by the fuel sulfur content. At a plume age of 10 s, an increase in total number concentration (Dp>0.01 µm) by a factor of 3.5 in the high sulfur case compared to the low sulfur case was observed. The ultrafine condensation nuclei fraction (0.007 µm <Dp<0.018 µm) contributed at maximum 70% to the total aerosol in the plume while this fraction was much less outside the plume. The high fuel sulfur content also caused an increase in the typical number concentrations of contrail particles by about one third with respect to low sulfur fuel, while the effective diameter of the size distribution was lowered at a fuel sulfur independent ice water content. The major differences in accumulation mode aerosol and microphysical contrail properties between the used aircraft were an increased number concentration of both the accumulation mode aerosol and the contrail particles in the Airbus A310‐300 plume relative to the ATTAS plume. Part of the difference in contrail particles may be caused by different ambient conditions, but the major differences are assumed to be caused by different engine and wake properties.
A three‐dimensional chemistry/general circulation model simulation of anthropogenically derived ozone in the troposphere and its radiative climate forcing
Authors: Roelofs Geert‐Jan , Lelieveld Jos , van Dorland Rob
Published: February 2004

We present results from the tropospheric chemistry/climate European Center Hamburg Model by comparing two simulations that consider a preindustrial and a contemporary emission scenario. Photochemical O3 production from anthropogenically emitted precursors contributes about 30% to the present‐day tropospheric O3 content, which is roughly equal to the natural photochemical production. Transports of stratospheric O3 into the troposphere contribute about 40%. As a result of anthropogenic emissions, the O3 maximum over remote northern hemisphere (NH) areas has shifted from winter to spring, when photochemical production of O3 is relatively efficient. Over NH continents the preindustrial seasonal variability is relatively weak whereas a distinct surface O3 summer maximum appears in the contemporary simulation. In the (sub)tropical southern hemisphere (SH), anthropogenic biomass burning emissions cause an increase of O3 mixing ratios in the dry season (September–November). We calculate a relative increase in O3 mixing ratios due to anthropogenic emissions of about 30% in the pristine SH middle and high latitudes to about 100% in the polluted NH boundary layer. The model simulations suggest that the absolute increase of tropospheric O3 maximizes in the middle troposphere. Through convection, upper tropospheric O3 mixing ratios are significantly affected in the tropical regions and, during summer, in the middle and high NH latitudes. Under these conditions the radiative forcing of climate by increasing O3 is relatively large. We calculate a global and annual average radiative forcing by tropospheric O3 perturbations of 0.42 W m−2, i.e., 0.51 W m−2 in the NH and 0.33 W m−2 in the SH.
Fast time response measurements of HNO3 in air with a chemical ionization mass spectrometer
Authors: Huey L. Gregory , Dunlea Edward J. , Lovejoy Edward R. , Hanson David R. , Norton Richard B. , Fehsenfeld Fred C. , Howard Carleton J.
Published: February 2004

A chemical ionization mass spectrometer has been developed for fast time response measurements of HNO3 in ambient air. The apparatus was characterized in an informal intercomparison campaign at Green Mountain Mesa in Boulder, Colorado. Nitric acid was measured over a wide range of meteorological conditions. Detection limits of less than 15 pptv for a 1 s integration period were routinely obtained. The apparatus was sufficiently sensitive to measure ambient levels of HNO3 for all conditions except for periods of fog or heavy precipitation when HNO3 mixing ratios were less than 10 pptv.
Effects of anthropogenic emissions on tropospheric ozone and its radiative forcing
Authors: Berntsen T. K. , Isaksen I. S. A. , Myhre G. , Fuglestvedt J. S. , Stordal F. , Larsen T. Alsvik , Freckleton R. S. , Shine K. P.
Published: February 2004

Tropospheric ozone changes since preindustrial times due to changes in emissions have been calculated by the University of Oslo global three‐dimensional photochemical model. The radiative forcing caused by the increase in ozone has been calculated by two independent radiative transfer models; the University of Reading model (Reading), and the University of Oslo/Norwegian Institute for Air Research model (OsloRad). Significant increases in upper tropospheric ozone concentrations are found at northern midlatitudes (15–40 ppbv, depending on season) at about 10 km altitude. In the tropical regions the largest increase (about 20 ppbv for all seasons) is found at about 15 km altitude. The increase is found to be caused mainly by enhanced in situ production due to transport of precursors from the boundary layer, with a smaller contribution from increased transport of ozone produced in the boundary layer. The lifetime of ozone in the troposphere decreased by about 35% as a result of enhanced concentrations of HO2. The calculated increase in surface ozone in Europe is found to be in good agreement with observations. The calculations of radiative forcing include the effect of clouds and allow for thermal adjustment in the stratosphere. The global and annual averaged radiative forcing at the tropopause from both models (0.28 W m−2 and 0.31 W m−2, for the Reading and OsloRad models, respectively) are in the lower part of the Intergovernmental Panel on Climate Change [1995] estimated range. The calculated radiative forcing is similar in magnitude to the negative radiative forcing by sulfate aerosols, but displaced southward in source regions at northern midlatitudes. The increase in tropospheric ozone is calculated to have cooled the lower stratosphere by up to 0.9 K, with possibly half of this cooling occurring in the past 2 to 3 decades.
Accumulation and turnover of carbon in organic and mineral soils of the BOREAS northern study area
Authors: Trumbore S. E. , Harden J. W.
Published: February 2004

Rates of input, accumulation, and turnover of C differ markedly within soil profiles and in soils with different drainage in the BOREAS northern study area. Soil C storage increases from ∼3 kg C m−2 in well‐drained, sandy soils to greater than 100 kg C m−2 in wetlands. Two modes of C accumulation were observed in upland soil profiles. Large annual C inputs (0.06–0.1 kg C m−2 yr−1) and slow decomposition (turnover times of 6–250 years) lead to rapid C accumulation in regrowing surface moss and detrital layers following fire. Deep organic layers that have accumulated over the millennia since the initiation of soil development, and are located below the most recent charred horizon, show slower rates of input (0.015–0.03 kg C m−2 yr−1) and turnover (100–1600 years) and accumulate C about 10 times slower than surface detrital layers. Rates of C input to soils derived from C and 14C data were in accord with net primary production estimates, with highest rates of input (0.14–0.6 kg C m−2 yr−1) in wetlands. Turnover times for C in surface detrital layers were 6–15 years for well‐drained sand soils that showed highest soil temperatures in summer, 30–40 years for wetlands, and 36–250 years for uplands with thick moss cover and black spruce trees. Long (>100 years) turnover times in upland black spruce/clay soils most likely reflect the influence of woody debris incorporated into detrital layers. Turnover times for deep organic and mineral layer C were controlled by drainage, with fastest turnover (80–130 years) in well‐drained sand soils and slowest turnover (>3000 years) in wetlands. Total C accumulation rates, which account for C losses from both deep organic and surface detrital layers, are close to zero for sand/jack pine soils, 0.003–0.01 kg C m−2 yr−1 for moderately to poorly drained sites in mature forest stands, and 0.03 kg C m−2 yr−1 for a productive fen. Decomposition of organic matter more than several decades old accounts for 9–22% of total heterotrophic respiration at these sites. The rates of C accumulation derived here are decadal averages for specific stands and will vary as stands age or undergo disturbance. Extrapolation to larger regions and longer timescales, where burning offsets C gains in moss layers, will yield smaller rates of C storage.
BIOME‐BGC simulations of stand hydrologic processes for BOREAS
Authors: Kimball John S. , White Michael A. , Running Steve W.
Published: February 2004

BIOME‐BGC is a general ecosystem model designed to simulate hydrologic and biogeochemical processes across multiple scales. The objectives of this investigation were to compare BIOME‐BGC estimates of hydrologic processes with observed data for different boreal forest stands and investigate factors that control simulated water fluxes. Model results explained 62 and 98% of the respective variances in observed daily evapotranspiration and soil water; simulations of the onset of spring thaw and the dates of snowpack disappearance and accumulation also generally tracked observations. Differences between observed and simulated evapotranspiration were attributed to model assumptions of constant, growing season, overstory leaf areas that did not account for phenological changes and understory effects on stand daily water fluxes. Vapor pressure deficit and solar radiation accounted for 58–74% of the variances in simulated daily evapotranspiration during the growing season, though low air temperature and photosynthetic light levels were found to be the major limiting factors regulating simulated canopy conductances to water vapor. Humidity and soil moisture were generally not low enough to induce physiological water stress in black spruce stands, though low soil water potentials resulted in approximate 34% reductions in simulated mean daily canopy conductances for aspen and jack pine stands. The sensitivity of evapotranspiration simulations to leaf area (LAI) was less than expected because of opposing responses of transpiration and evaporation to LAI. The results of this investigation identify several components within boreal forest stands that are sensitive to climate change.
Chemical characteristics of tropospheric air over the Pacific Ocean as measured during PEM‐West B: Relationship to Asian outflow and trajectory history
Authors: Gregory G. L. , Merrill J. T. , Shipham M. C. , Blake D. R. , Sachse G. W. , Singh H. B.
Published: February 2004

The Pacific Exploratory Missions (PEM) were designed to study the chemistry of tropospheric air within the Pacific Rim region extending from the equator to about 50°N. Missions emphasized the importance of Asian outflow to the chemistry of tropospheric air. PEM‐West A was conducted in September and October 1991, and PEM‐West B was conducted in February and March 1994. The PEM‐West B winter mission coincides with the time of maximum impact of Asian outflow on the Pacific Rim region. This paper examines the chemical composition of air measured during PEM‐West B aircraft ascents/descents. Chemical composition of tropospheric air is related to its history as determined from 5‐ to 10‐day back trajectory calculations at multiple altitudes of the vertical profiles. Locations and the altitudes for trajectory calculations are selected to elucidate relationships between Asian source regions, transport within the region, and the chemical characteristics of tropospheric air. Data are over‐ocean measurements at locations ranging from hundreds of kilometers from the Asian coast to remote ocean sites thousands of kilometers east of Asia. Seasonal differences are illustrated by comparing PEM‐West A and B results. In general, the chemical composition of tropospheric air throughout the Pacific Rim region is influenced by Asian outflow, and transported continental emissions are an important source of pollution to the region during both seasons.
Chemical apportionment of aerosol column optical depth off the mid‐Atlantic coast of the United States
Authors: Hegg Dean A. , Livingston John , Hobbs Peter V. , Novakov T. , Russell Philip
Published: February 2004

Aerosol column optical depths derived from airborne Sun photometer and in situ measurements of aerosol properties in 14 vertical profiles off the mid‐Atlantic coast of the United States in June show excellent agreement. Simultaneous measurements of the chemical compositions of the aerosol allows an assessment of the chemical apportionment of the aerosol column optical depths. The optical depths had essentially three chemical components, which, in order of descending average contributions, were condensed water, carbonaceous species, and sulfate. These results do not support the common assumption that sulfate dominates aerosol optical depths in polluted regions.