bluuebunny/crossref_metadata_2025 huggingface dataset:
Independent component analysis of multivariate time series: Application to the tropical SST variability
Authors: Aires Filipe , Chédin Alain , Nadal Jean‐Pierre
Published: February 2004

With the aim of identifying the physical causes of variability of a given dynamical system, the geophysical community has made an extensive use of classical component extraction techniques such as principal component analysis (PCA) or rotational techniques (RT). We introduce a recently developed algorithm based on information theory: independent component analysis (ICA). This new technique presents two major advantages over classical methods. First, it aims at extracting statistically independent components where classical techniques search for decorrelated components (i.e., a weaker constraint). Second, the linear hypothesis for the mixture of components is not required. In this paper, after having briefly summarized the essentials of classical techniques, we present the new method in the context of geophysical time series analysis. We then illustrate the ICA algorithm by applying it to the study of the variability of the tropical sea surface temperature (SST), with a particular emphasis on the analysis of the links between El Niño Southern Oscillation (ENSO) and Atlantic SST variability. The new algorithm appears to be particularly efficient in describing the complexity of the phenomena and their various sources of variability in space and time.
A continental outbreak of air during the Second Aerosol Characterization Experiment (ACE 2): A Lagrangian experiment
Authors: Suhre Karsten , Johnson Doug W. , Mari Céline , Rosset Robert , Osborne Simon , Wood Robert , Bates Timothy S. , Raes Frank
Published: February 2004

A mesoscale meteorological model is used to simulate the continental outbreak of air that occured over North Atlantic Ocean during the Second Aerosol Characterization Experiment (ACE 2). Comparison with Meteosat 6 visible images shows that the model is able to reproduce most of the observed cloud features in the entire modeling domain, including a frontal system, orographic clouds over the Pyrenees and the Iberian Peninsula, boundary layer clouds in the ACE 2 region, and cloud‐free areas in the dry air outflow off the Portuguese coast. The model reproduces correctly the temperature, humidity, wind speed and direction, cloud water content, and light precipitation events observed by the aircraft during the Lagrangian experiment, while following a tagged boundary layer air parcel over a 28‐hour long period. A passive tracer, introduced in the model at the time corresponding to the balloon launch, followed the balloon track with an error in position that is smaller than 1° after 24‐hours with about 20% higher speed. An estimate of the boundary layer entrainment velocity is obtained, based on observed boundary layer growth rates and modeled vertical wind speed. The average entrainment velocity of 1.28 cm/s for the 28‐hour Lagrangian period agrees with experimentally derived entrainment velocities.
Ozone deposition to a ponderosa pine plantation in the Sierra Nevada Mountains (CA): A comparison of two different climatic years
Authors: Bauer M. R. , Hultman N. E. , Panek J. A. , Goldstein A. H.
Published: February 2004

Ponderosa pine is one of the most sensitive species to ozone and a dominant species in the Sierra Nevada Mountains. To assess the factors controlling ozone deposition to the forest and to study the forest response, we established a research site in a ponderosa pine plantation ∼75 km northeast of Sacramento. Ozone concentration and ozone flux, along with relevant environmental variables, were measured from June to September in 1997 and from May to November in 1998. Summer of 1997 had very low soil moisture and an early budbreak, while summer of 1998 had very high soil moisture and later budbreak. Soil moisture and vapor pressure deficit exerted a strong control on ozone deposition in the dry year (1997), but the relationship was less clear in the wet year (1998). During the dry year ozone concentration and flux became decoupled owing to stomatal closure, but this did not occur explicitly in 1998. Phenology also proved to be important in controlling ozone deposition. Early in summer 1997 cumulative ozone flux was 50% higher than that of 1998: the difference can be attributed to the late budbreak in 1998. Further, the highest ozone deposition velocity in both years occurred 3–4 weeks after budbreak. Total cumulative ozone flux during the summer was 6% lower in 1998 due mainly to later budbreak even though drought stress reduced ozone flux late in summer 1997. Our results show that interannual climate variability impacts temporal patterns, physiological controls, and magnitude of ozone deposition to sensitive Mediterranean‐type ecosystems.
Gravity waves in the tropical lower stratosphere: An observational study of seasonal and interannual variability
Authors: Vincent Robert A. , Joan Alexander M.
Published: February 2004

Radiosonde observations made at Cocos Islands (12°S, 97°E) in the Indian Ocean between September 1992 and June 1998 are used to study seasonal and interannual variations in gravity wave activity in the lower stratosphere (18–25 km). The islands are located in a region of generally strong convection that occurs at all times of the year, with the period of strongest convective activity between December and July (wet season). The prevailing zonal winds during the observational period and height range are westward with a quasi‐biennial oscillation (QBO) superimposed. Time series of wave energy show that largest wave amplitudes occur during the wet season when convection is strongest, but a QBO‐like variation is also apparent. Maximum energy densities of about 25 J kg−1 occur early 1993, 1995, and 1997 at the times when the westward shears are largest. Wave energy is found to be propagating upward, and in the horizontal there is considerable azimuthal anisotropy, with predominate eastward propagation against the prevailing wind. Upward fluxes of zonal momentum flux
are estimated by combining the temperature and wind information. Fluxes show a similar temporal behavior to the energy. The motion and temperature fields are dominated by waves with vertical wavelengths ∼2 km. Using a Stokes parameter analysis, it is found that the intrinsic frequencies are, on average, 2–3 times the inertial frequency, corresponding to intrinsic periods of 20–25 hours. Horizontal wavelengths between 200 and 2000 km are inferred, with a mean value of about 1000 km. The mean intrinsic phase speeds are about 10 ms−1, but ground‐based phase speeds are centered on 0 ms−1.
A statistical study of gravity waves in the polar regions based on operational radiosonde data
Authors: Yoshiki Motoyoshi , Sato Kaoru
Published: February 2004

Gravity waves in the lower polar stratosphere are examined using operational radiosonde observations gathered from 33 stations over a period of 10 years. Both the potential and kinetic energies of the gravity waves vary annually and reach maxima in winter in the Arctic and in spring in the Antarctic. In the Antarctic spring a region of large gravity wave energy propagates downward following the movement of a zone of high static stability associated with Southern Hemispheric warming. Moreover, the enhanced energy region and the high stability zone coincide in the horizontal plane and move gradually from 135°E, 50°S to 45°W, 70°S over the South Pole. The vertical and horizontal directions of wave propagation are examined using hodograph analysis in the vertical. Most gravity waves transfer energy upward in the Arctic, while the percentage of downward energy propagation is relatively high in winter and spring in the Antarctic. Horizontally, gravity waves propagate westward relative to the mean wind in the Arctic, while in the Antarctic the dominant direction varies from station to station. The correlation between gravity wave energy in the lower stratosphere and the mean wind is also examined. In the Arctic, gravity wave energy is highly correlated with the surface wind, though in the Antarctic it correlates with the stratospheric wind. These results suggest that gravity waves observed in the Arctic are forced by topography, whereas in Antarctica some sources may exist in the stratosphere. One such source candidate is likely to be the polar night jet.
Propagation of small‐scale gravity waves through large‐scale internal wave fields: Eikonal effects at low‐frequency approximation critical levels
Authors: Walterscheid R. L.
Published: February 2004

The propagation of small‐scale internal gravity wave packets through a background of large‐scale internal gravity waves is investigated using Eikonal theory. When background variations are sufficiently slow, it is possible to ignore both the time dependency and the vertical velocity of the background while retaining the Doppler effects of the horizontal winds. This approximation, here referred to as the low‐frequency approximation (LFA), is used as a reference approximation to elucidate the peculiar aspects of wave propagation through time‐dependent wave backgrounds. The greatest difference between stationary and time‐dependent wave background occurs near levels where the frequency measured in the Earth‐fixed frame is canceled by the Doppler term due to the horizontal background wind (LFA critical level). In stationary flow, propagation is blocked, and wave amplitude can become very large. The focus of this paper is non‐LFA effects near LFA critical levels (LFA/CLs). Major findings are as follows: the LFA overpredicts variations in the intrinsic frequency in a time‐dependent background wave field because the variation of observed frequency is exactly out of phase with the variation of the Doppler term. Non‐LFA effects at an LFA/CL are strong because as a packet approaches an LFA/CL, its intrinsic frequency decreases, its ascent slows, and its wavenumber vector rotates toward the vertical and increases in magnitude; thus the packet increasingly senses both the time dependency and the vertical component of the background wind. The maximum cancelation between wave frequency and the Doppler term (i.e., minimum intrinsic frequency) will usually occur above LFA/CL. The minimum intrinsic frequency is significantly nonzero and is not a strong function of vertical wavelength. The background vertical velocity allows freer propagation through intrinsic frequency minima. Counterintuitively, the time variation of wave action near such levels appears less pronounced for the slower waves (the ones that should feel Doppler effects most strongly). Eikonal simulations suggest that intrinsic frequency minima associated with LFA/CL and strong Doppler effects are favored locations for wave breakdown, but wave breakdown in a time‐dependent background depends on the initial wave‐packet amplitude; thus Doppler effects and time dependency should both play a role in shaping the vertical wavenumber spectrum [Hines, 1991a, b; Broutman et al., 1997]. This work supports findings of Eckermann [1997] that the effects of time dependency can drastically reduce the response of a wave packet to a wave background comprising downward propagating waveforms (upward energy transfer).
Analysis of residual mean transport in the stratosphere: 2. Distributions of CO2 and mean age
Authors: Schneider Hans R. , Jones Dylan B. A. , Wofsy Steve C. , McElroy Michael B.
Published: February 2004

Distributions of CO2 and the mean age of stratospheric air are examined using the interactive two‐dimensional model described in the companion paper. It is shown that the model can explain the distribution of age in the lower stratosphere and reproduce the correlations between CO2 and N2O observed at midlatitudes in the lower stratosphere. Seasonal characteristics of the measured correlations are captured by the model. The model uses externally specified coefficients Kyy to account for large‐scale isentropic mixing in the stratosphere. The sensitivity of calculated distributions of CO2 to Kyy is be examined by comparing model runs using three different distributions of Kyy. It is shown that the CO2 measurements require Kyy to exceed a threshold value of approximately 105 m2 s−1 for most of the year. Once this threshold is exceeded, the slope of the CO2/N2 correlations depends only weakly on the magnitude and the structure of the distribution of Kyy.
A study of the importance of initial conditions for photochemical oxidant modeling
Authors: Berge Erik , Huang Ho‐Chun , Chang Julius , Liu Tsun‐Hsien
Published: February 2004

We present a study of the impact of initial concentrations on the modeling of photochemical oxidants. A simple impact model is employed, and an impact factor is defined which describes the ratio between the initial concentration and the time‐dependent model concentration. The calculations have been carried out with data from a box model and a comprehensive three‐dimensional (3‐D) model. Results are for three different sites in the California San Joaquin Valley. By using data from the chemical box model a very active chemistry and rather high concentrations are obtained since no transport processes are included. The impact of the initial concentrations is small already after 24 hours of model integration under such conditions. Results from the more realistic 3‐D data show that the impact factor is reduced to less than 10% within the planetary boundary layer after 48 hours for nearly all chemical components studied. An exception is the sum of selected grouped species of Ox + NO2 + NO3 + N2O5 + HNO3 + PAN (sum of reservoir species for O3) which is not below 10% before approximately after 3 days at the two least polluted sites. In the free troposphere the impact factors of the initial concentrations are large even after 3 days for paraffin, ethene, and isoprene. For selected grouped species of Ox + NO2 and Ox + NO2 + NO3 + N2O5 + HNO3 + PAN, large impact factors are still found after 3 days. The large impact of the initial concentrations in the free troposphere strongly complicates any model evaluation by use of measurements.
Simulations of tropical upper tropospheric humidity
Authors: Dessler A. E. , Sherwood S. C.
Published: February 2004

We present simulations of humidity in the tropical upper troposphere at 215 and 146 hPa made by a model that includes only advection and, whenever relative humidity exceeds 100%, condensation. Despite the simplicity of the model, the simulations agree well with measurements in both convective and nonconvective regions. We see no evidence to suggest that accurate predictions of the humidity in this region are dependent on accurate simulations of microphysical processes or on transport of ice or liquid water. Our results instead suggest that accurate predictions of the humidity primarily require realistic three‐dimensional large‐scale (greater than a few hundred kilometers) wind fields.
Interannual variability of atmospheric dimethylsulfide in the southern Indian Ocean
Authors: Sciare J. , Mihalopoulos N. , Dentener F. J.
Published: February 2004

Continuous measurements of atmospheric dimethylsulfide (DMS) have been performed over a 10‐year period (1990–1999) at Amsterdam Island in the southern Indian Ocean. Atmospheric DMS ranges from 5 to 1930 parts per trillion by volume (pptv) and shows a clear seasonal variation with a factor of 20 in amplitude between its maximum in January (austral summer) and minimum in July‐August (austral winter). Important deviations from the 10‐year monthly mean as high as 100% have been detected, which could not be explained by changes in meteorology and/or oxidation capacity of the atmosphere. Comparison with a three dimensional (3‐D) chemistry/transport model revealed that changes in the source strength of DMS as high as a factor of 2 were required to account for such DMS interannual variations. In addition, DMS variability was found to be closely related to sea surface temperature anomalies, clearly indicating a link between DMS and climate changes.
Tracing sources of precipitation sulfate in eastern Canada using stable isotopes and trace metals
Authors: Jamieson R. E. , Wadleigh M. A.
Published: February 2004

Stable isotopic and chemical evidence has been used to trace sources of sulfate in precipitation in eastern Canada and to apportion this sulfate between natural and anthropogenic origins. A site in Nepean, Ontario was chosen to characterize continental inputs from long‐range transport, while two coastal sites in Newfoundland (St. John's and Seal Cove) were chosen to represent the boundary between the marine and continental environments. Precipitation sulfate at the coastal sites should reflect mixing of these two source areas, modified by contributions from local anthropogenic sources. The results of this study show that a three end‐member isotopic mixing model using δ18O and δ34S describes the composition of sulfate at the Newfoundland sites. The three end‐members are (1) sea spray (δ34S = +21‰; δ18O = +9.5‰), (2) long‐range transported secondary (anthropogenic) sulfate (δ34S ∼ +4‰; δ18O ∼ 15‰), and (3) local primary (anthropogenic) sulfate (δ34S ∼ +4‰; δ18O = +42‰). Mass balance calculations show that as much as 63% of the sulfate deposited at the St. John's site and 18% at Seal Cove is primary sulfate emitted from local pollution sources. Up to 30% of the sulfur deposited in rain at the Newfoundland sites is of continental origin.
Implications of methyl bromide supersaturations in the temperate North Atlantic Ocean
Authors: King Daniel B. , Butler James H. , Montzka Stephen A. , Yvon‐Lewis Shari A. , Elkins James W.
Published: February 2004

Methyl bromide saturation anomalies measured in the springtime North Atlantic and summertime North Pacific Oceans during 1998 revealed persistent supersaturations in the temperate waters of the northeastern Atlantic but undersaturations in tropical waters of both oceans. A comparison of data from this study with those from a previous cruise to the northeastern Atlantic suggests that methyl bromide is cycled seasonally in these waters and perhaps in all temperate open‐ocean waters. This means that the calculated net flux of methyl bromide into the oceans is slightly less negative than previously reported. With these new insights we estimate that the global air‐sea flux of methyl bromide ranges from −11 to −20 Gg yr−1. Data combined from this and three previous cruises support a flux dependence upon sea surface temperature, as reported recently by Groszko and Moore [1998]. Whereas sea surface temperature can account for 40–70% of the observed variability in methyl bromide globally, it is able to reproduce only a small fraction of the observed seasonal cycle in the temperate northeastern Atlantic. The development of reliable predictions of air‐sea fluxes of methyl bromide will require information on additional variables as well.
NO3 at Helgoland during the NORDEX campaign in October 1996
Authors: Martinez Monica , Perner Dieter , Hackenthal Eva‐Maria , Külzer Stefan , Schütz Lothar
Published: February 2004

Nighttime observations of NO3, O3, NO2, and aerosol distributions were made at Helgoland in October 1996. Frequently polluted air was observed. Air transport from the mainland over the sea takes several hours, and lifetimes for NO3 and N2O5 can be determined. Those lifetimes are compared with aerosol size distributions measured simultaneously. The lifetimes of N2O5 show clear correlations with total aerosol surface and mean aerosol size, indicating that the most likely sink of NO3 is hydrolysis of N2O5 on wet aerosols of varying composition. The range of possible uptake coefficients is calculated from the measurements and compared with literature data.
Intra‐annual and interannual variability of ecosystem processes in shortgrass steppe
Authors: Kelly R. H. , Parton W. J. , Hartman M. D. , Stretch L. K. , Ojima D. S. , Schimel D. S.
Published: February 2004

We used a daily time step ecosystem model (DAYCENT) to simulate ecosystem processes at a daily, biweekly, monthly, and annual time step. The model effectively represented variability of ecosystem processes at each of these timescales. Evolution of CO2 and N2O, NPP, and net N mineralization were more responsive to variation in precipitation than temperature, while a combined temperature‐moisture decomposition factor (DEFAC) was a better predictor than either component alone. Having established the efficacy of CENTURY at representing ecosystem processes at multiple timescales, we used the model to explore interannual variability over the period 1949–1996 using actual daily climate data. Precipitation was more variable than temperature over this period, and our most variable responses were in CO2 flux and NEP. Net ecosystem production averaged 6 g C m−2 yr and varied by 100% over the simulation period. We found no reliable predictors of NEP when compared directly, but when we considered NEP to be lagged by 1 year, predictive power improved. It is clear from our study that NEP is highly variable and difficult to predict. The emerging availability of system‐level C balance data from a network of flux towers will not only be an invaluable source of information for assessments of global carbon balance but also a rigorous test for ecosystem models.
Impact of soil moisture anomalies on seasonal, summertime circulation over North America in a Regional Climate Model
Authors: Hong Song‐You , Pan Hua‐Lu
Published: February 2004

The impact of initial soil moisture on seasonal, summertime atmospheric circulation over North America is investigated using the National Centers for Environmental Prediction Regional Spectral Model (NCEP RSM). Three‐month‐long simulations are conducted for the two summers, May‐June‐July (MJJ) 1988 (drought in the Great Plains) and MJJ 1993 (a flooding event). Overall, the climate responses due to soil moisture anomalies in the RSM is in line with the previous findings from global models and observational studies, with strong positive feedback between initial soil moisture anomalies and simulated seasonal precipitation. The impact of the initial soil moisture anomalies persists for the 3‐month integration period, but effects are mostly confined to the near‐surface climate with little impact on the dynamic circulation in the upper troposphere. Influence of soil moisture anomaly on the low‐level structure and their interaction with precipitation mechanism are highlighted. The regional climate responses in the RSM were not significantly dependent on the size of the domain of integration.
Emissions of volatile organic compounds from Quercus ilex L. measured by Proton Transfer Reaction Mass Spectrometry under different environmental conditions
Authors: Holzinger R. , Sandoval‐Soto L. , Rottenberger S. , Crutzen P. J. , Kesselmeier J.
Published: February 2004

Volatile organic compound (VOC) emissions of the Mediterranean holm oak (Quercus ilex L.) were investigated using a fast Proton Transfer Reaction Mass Spectrometry (PTR‐MS) instrument for analysis. This technique is able to measure compounds with a proton affinity higher than water with a high time resolution of 1 s per compound. Hence nearly all VOCs can be detected on‐line. We could clearly identify the emission of methanol, acetaldehyde, ethanol, acetone, acetic acid, isoprene, monoterpenes, toluene, and C10‐benzenes. Some other species could be tentatively denominated. Among these are the masses 67 (cyclo pentadiene), mass 71 (tentatively attributed to methyl vinyl ketone (MVK) and metacrolein (MACR)), 73 (attributed to methyl ethyl ketone (MEK)), 85 (C6H12 or hexanol), and 95 (vinylfuran or phenol). The emissions of all these compounds (identified as well as nonidentified) together represent 99% of all masses detected and account for a carbon loss of 0.7–2.9% of the net photosynthesis. Of special interest was a change in the emission behavior under changing environmental conditions such as flooding or fast light/dark changes. Flooding of the root system caused an increase of several VOCs between 60 and 2000%, dominated by the emission of ethanol and acetaldehyde, which can be explained by the well described production of ethanol under anoxic conditions of the root system and the recently described subsequent transport and partial oxidation to acetaldehyde within the green leaves. However, ethanol emissions were dominant. Additionally, bursts of acetaldehyde with lower ethanol emission were also found under fast light/dark changes. These bursts are not understood.
The nitrate radical in the remote marine boundary layer
Authors: Allan Beverley J. , McFiggans Gordon , Plane John M. C. , Coe Hugh , McFadyen Gordon G.
Published: February 2004

The technique of differential optical absorption spectroscopy has been used to determine the nitrate radical (NO3) concentration in the remote marine boundary layer. The instrument was deployed in campaigns at Mace Head on the west coast of Ireland and on the north coast of Tenerife. A comprehensive set of NO3 measurements under a wide variety of conditions was obtained. For instance, nighttime NO3 levels at Mace Head ranged from 1 to 5 ppt in the clean marine atmosphere and from 1 to 40 ppt in semipolluted continental air masses. The nightly averaged NO3 lifetime varied from less than 2 min to 4 hours. At Tenerife, where there was less variability in conditions, nighttime NO3 ranged from 1 to 20 ppt, with nightly averaged lifetimes between 4 and 34 min. A photochemical box model, fully constrained by measurements of species that control the formation and removal of NO3, was then employed to determine the major loss mechanisms of the radical. This shows that NO3 in the clean marine air masses is very sensitive to small increases in the concentrations of dimethyl sulphide (DMS) and nonmethane hydrocarbons and that the radical is rarely in chemical steady state. At Tenerife, 80–90% of NO3 was removed by reaction with DMS. However, in continental air masses with little marine influence, indirect losses of NO3 via dinitrogen pentoxide (N2O5) usually dominate. It appears that in much of the North Atlantic, NO3 is a more efficient sink for DMS compared to the hydroxyl radical (OH) during the day.
A catchment‐based approach to modeling land surface processes in a general circulation model: 2. Parameter estimation and model demonstration
Authors: Ducharne Agnès , Koster Randal D. , Suarez Max J. , Stieglitz Marc , Kumar Praveen
Published: February 2004

The viability of a new catchment‐based land surface model (LSM) developed for use with general circulation models is demonstrated. First, simple empirical functions, tractable enough for operational use in the LSM, are established that faithfully capture the control of topography on the subgrid variability of soil moisture and the surface water budget, as predicted by theory. Next, the full LSM is evaluated off‐line. Using forcing and validation data sets developed for the Project for Intercomparison of Land‐Surface Parameterization Schemes Phase 2c, the minimally calibrated model is shown to reproduce observed evaporation and runoff fluxes successfully in the Red‐Arkansas River Basin. A complementary idealized study that employs the range of topographic variability seen over North America demonstrates that the simulated surface water budget does vary strongly with topography, which can, by itself, induce variations in annual runoff as high as 20%.
Effects of the stratospheric quasi‐biennial oscillation on long‐lived greenhouse gases in the troposphere
Authors: Hamilton Kevin , Fan Song‐Miao
Published: February 2004

An analysis is presented of results of an extended integration with a global general circulation model that includes treatment of a long‐lived tropospheric trace constituent as well as a momentum source that forces a realistic stratospheric quasibiennial oscillation (QBO). It is shown that the dynamical QBO may modulate stratosphere‐troposphere exchange in such a manner as to produce a QBO in global‐mean tropospheric tracer mixing ratio. For the growth rate of tropospheric methane this transport‐induced QBO is expected to have a peak‐to‐peak amplitude of 1–2 ppbv yr−1, which, while modest compared with the full range of observed variability in growth rate, is still significant. The observed methane growth rate time series during 1983–1999 is shown to be consistent with the predicted QBO effect, although the record is definitely dominated by other interannual variations.
A modified extraction technique for liberating occluded gases from ice cores
Authors: Sowers Todd , Jubenville Jennifer
Published: February 2004

We have developed a new dry extraction technique to extract air from large pieces of glacial ice. The primary reason for developing this technique was to be able to perform a single extraction and measure a single sample of air from an ice core sample for as many atmospheric constituents as possible. The procedure is modeled after the dry extraction ‐“cheese grater” design of Etheridge et al. [1988]. Extracted air samples are analyzed for the elemental and isotopic composition of O2 and N2 as well as the CH4 concentration. Extensive experimental work to determine the integrity of the extraction procedure yielded blank values and external precision which are comparable with exiting extraction procedures. Overall external precision for δO2/N2, δ18O of O2, and δ15of N2 analyses is ±2.1‰, ±0.074‰, and ±0.045‰, respectively. Variable δO2/N2 results from ice which is either completely bubbly or clathrated are in good agreement with previous measurements. Variable δO2/N2 results from Greenland Ice Sheet Project (GISP) II ice samples spanning the clathrate formation region (1000–1500 m) are markedly different from previous results obtained with a “wet” extraction procedure. We attribute the differences to variable δO2/N2 ratios in bubbles and clathrates in the clathrate formation region combined with a 21% difference in our extraction efficiency for bubbly versus clathrated ice. The overall uncertainty and blank value for CH4 measurements are ±19 ppb and 16 ppb, respectively. CH4 concentrations for ice between 115 and 140 meters below the surface (mbs) from the GISPII ice core appear to be 4.3% higher than the average value measured by five other laboratories. We attribute our elevated values to uncertainties in the actual concentration of our working standard and small differences in the CH4 concentration of the liberated air relative to the total air trapped in ice. Our corrected CH4 data spanning the last 25 kyr are indistinguishable from the Brook et al. (1996) CH4 record from the same period.
Large‐scale forcing of summertime monsoon surges over the Gulf of California and the southwestern United States
Authors: Anderson Bruce T. , Roads John O. , Chen Shyh‐Chin
Published: February 2004

Synoptic forcing of the Gulf of California summertime low‐level wind field is described using a nested regional modeling system. Under appropriate synoptic conditions, strong surge events develop that are characterized by persistent, vertically extensive (1–2 km) southerly flow extending along the entire Gulf of California and into Arizona, California, and even southern Nevada. These surge periods are initiated either by tropical‐cyclone activity to the south of Baja California or by the westward propagation of lower tropospheric troughs from over the Sierra Madre Occidental to the eastern Pacific. The flow over the Gulf is primarily geostrophic and is associated with the presence of these low‐pressure centers to the south and west of Baja California.
Influence of inhomogeneous surface albedo on UV irradiance: Effect of a stratus cloud
Authors: Degünther Markus , Meerkötter Ralf
Published: February 2004

The influence of inhomogeneous surface albedo on UV irradiance is investigated assuming the sky to be overcast by a horizontally homogeneous stratus cloud. From calculations with a three‐dimensional (3‐D) radiative transfer model the snow‐induced relative UV irradiance enhancement as well as the 1‐D albedo is derived for two different snow bidirectional reflectance distribution functions (Lambertian and anisotropic) and snow coverages varying between 0 and 100%. The 1‐D albedo is a quantity defined to consider effects of inhomogeneous surface albedo even in 1‐D radiative transfer codes. It turned out that anisotropy of snow reflectance is of minor importance for quantification of inhomogeneous surface albedo influence on UV irradiance. This holds for a cloudy as well as for a cloud‐free atmosphere. Furthermore, the comparison of the new results with corresponding data obtained for cloudless sky indicates that a stratus cloud, on the one hand, enhances the surface albedo effect by about a factor 2–3, depending on wavelength. On the other hand, the area significantly influencing UV irradiance via its surface albedo is distinctly smaller. Looking, e.g., at wavelength 330 nm and clear sky, the maximum albedo effect of the surface surrounding a quadratic area of 6400 km2 on UV irradiance at area center is 3%. In the case of a stratus cloud the same effect is found for an area of only 1500 km2.
The response of atmospheric circulation to weak solar forcing
Authors: Lawrence J. K. , Cadavid A. C. , Ruzmaikin A.
Published: February 2004

We study the effect of variable external forcing on a dynamical system proposed by Lorenz as “perhaps the simplest possible ‘general circulation’ model” of the atmosphere. When a strong annual cycle is included, numerical exploration reveals the existence of a variety of multiyear climate states, which fall into two basic types. In the space of external forcing parameters, the different kinds of climate state are interleaved in an intricate pattern at scales ≤0.01%. Since this is below the ∼0.1% level of observed solar cycle irradiance variability, then the model climate state can be modulated by the solar variability. If the solar cycle is accompanied by a steady drift in forcing, it can produce periodic modulation that appears, disappears, and even reverses its phase. A parametric drift by itself produces intervals of steady, but sometimes differing, climates punctuated by intermittent bursts of variability. Different forcing parameters for the Northern and Southern Hemispheres produce different responses to variable forcing. We test the stability of the results under the addition of noise and changes of system parameters.
Does cloud processing of aerosol enhance droplet concentrations?
Authors: Feingold Graham , Kreidenweis Sonia
Published: February 2004

In this paper we explore phase space for cloud processing of cloud condensation nuclei (CCN) via heterogeneous chemistry. A range of input CCN size spectra, parameterized as lognormal distributions, are used as input to a parcel model driven along trajectories derived from a large‐eddy simulation of the stratocumulus‐capped marine boundary layer. A simple sulfate chemistry model is coupled to the microphysical model. Gas phase concentrations of SO2, O3, H2O2, and NH3 are varied so as to generate one case for which SO2 processing is dominated by oxidation via O3 and another case for which processing is dominated by oxidation via H2O2. The processed aerosol spectra are then used as input to an adiabatic parcel model that predicts the drop concentration for a given updraft velocity. Comparisons are made between predictions of drop concentration based on input of aerosol spectra that have experienced processing and an equivalent set that has not experienced processing. It is shown that for both of the chemical processing scenarios, heterogeneous processing can either enhance or suppress the number of drops activated in the subsequent cloud cycle, depending on the input CCN distribution and the magnitude of the updraft. Enhancement of drop concentration occurs in cases where the subsequent cloud cycle has low vertical velocity. A reduction in drop number occurs when updraft velocities in the subsequent cloud cycle are high. The size of the smallest CCN size category activated in the subsequent cloud cycle, relative to that experienced in the original cloud cycle, is important in determining the change in number of activated drops. By applying probability distribution functions of the vertical velocity, we calculate that, on average, drop concentrations are likely to be enhanced by between 10% and 20% for the cases examined. Nevertheless, the potential for both positive and negative changes in drop concentration due to cloud processing may complicate predictions of the indirect effect of aerosols on climate.
Differences in the volatility of organic aerosols in unpolluted tropical and polluted continental atmospheres
Authors: Kirchstetter Thomas W. , Novakov T. , Morales Ricardo , Rosario O.
Published: February 2004

In this paper we describe experimental results that demonstrate that chemical and physical properties of the organic aerosol material in tropical trade winds are drastically different from the organic aerosol component typically found in fossil fuel combustion influenced atmospheres. These experiments were performed at a Caribbean location (Cape San Juan, Puerto Rico) and at a semiurban site influenced by common anthropogenic pollution (Berkeley, California). The results show that the volatility of the polluted and the (largely natural) tropical organic aerosols is vastly different. Specifically, we find that the tropical organic aerosol material is highly volatile. The condensed phase organic carbon (OC) appears to exist in equilibrium with its gas‐phase counterpart. Its mass concentration drastically decreases if the concentration of the gas‐phase is reduced. The condensed phase OC adjusts quickly to the new equilibrium when the equilibrium at the sampler inlet is perturbed. Consequently, the mass concentration of condensed phase OC in tropical aerosols may be governed by both natural and sampling induced changes in temperature and pressure that alter the gas‐particle partitioning. As a result, analyses based on conventional filter techniques may result in OC mass concentrations that are not representative of actual ambient concentrations. In contrast, the anthropogenic aerosol OC is much more stable and not appreciably influenced by changes in gas‐phase organic compound concentrations.
A laboratory study of the heterogeneous reaction of nitric acid on calcium carbonate particles
Authors: Goodman A. L. , Underwood G. M. , Grassian V. H.
Published: February 2004

It has been postulated that the reaction of nitric acid with calcium carbonate, namely, CaCO3(s) + 2HNO3(g) → Ca(NO3)2(s) + CO2(g) + H2O(g), plays an important role in the atmosphere. In this study, transmission FTIR spectroscopy, diffuse reflectance UV‐visible spectroscopy, transmission electron microscopy and a Knudsen cell reactor coupled to a quadrupole mass spectrometer have been used to investigate the heterogeneous reactivity of HNO3 on CaCO3 at 295 K as a function of relative humidity. Transmission FTIR spectroscopy was used to probe both gas‐phase and adsorbed products and showed that the reaction of HNO3 and CaCO3 is limited to the surface of the CaCO3 particle in the absence of adsorbed water. However, in the presence of water vapor, the reaction is greatly enhanced and is not limited to the surface of the particle producing both solid calcium nitrate and gaseous carbon dioxide. The enhanced reactivity of the particles is attributed to the presence of a layer of adsorbed water on the particle surface. The amount of adsorbed water on the particle surface is strongly dependent on the extent of the reaction. This can be understood in terms of the increased hydrophilicity of calcium nitrate as compared to calcium carbonate. Data from experiments using a mass‐calibrated Knudsen cell reactor showed the stoichiometry for the reaction determined from gas‐phase species deviated from that expected from the balanced equation. Water adsorption on the particle surface and gases dissolved into the water layer appear to be the cause of this discrepancy. The measured uptake coefficient accounting for the BET area of the sample is determined to be 2.5±0.1×10−4 for HNO3 on CaCO3 under dry conditions and is found to increase in the presence of water vapor. Atmospheric implications of the results presented here are discussed.
Consistency tests applied to the measurement of total, direct, and diffuse shortwave radiation at the surface
Authors: Cess Robert D. , Qian Taotao , Sun Moguo
Published: February 2004

Motivated by recent studies suggesting that the clear‐sky atmosphere absorbs more shortwave (solar) radiation than the theoretical models, we have performed two consistency tests on the data used in several of these studies. These data consist of broadband measurements of shortwave irradiance to the surface (total, direct, and diffuse) taken in Oklahoma. In the absence of aerosols, Rayleigh scattering is the sole source of diffuse radiation and thus without any unknown source of atmospheric SW absorption, the measured diffuse irradiance should not be less than that produced by a model incorporating both Rayleigh scattering and conventional atmospheric absorption. The measurements of broadband diffuse irradiance, however, exhibit considerable sub‐Rayleigh behavior. On the other hand, measurements of the diffuse irradiance in narrow spectral bands, centered at 415, 500, and 608 nm, indicate no sub‐Rayleigh behavior, suggesting that exhibited by the broadband measurements is probably unrealistic. Related to this is the finding that the total surface SW irradiance, when evaluated as the sum of the direct‐beam irradiance (pyrheliometer) and the diffuse irradiance (shaded pyranometer), differs considerably and diurnally from the single measurement of the total irradiance by the unshaded pyranometer under conditions in which possible cosine response errors of the unshaded pyranometer have been minimized. This indicates that the pyranometer daytime offsets differ from each other, suggesting their daytime offsets likewise differ from their nighttime offsets, which are nearly identical. We emphasize that these conclusions apply solely to the data for Oklahoma, and they are focused upon obtaining a better understanding of the clear‐sky absorption problem that analyses of these data have raised.
Stratosphere‐troposphere exchange: Chemical sensitivity to mixing
Authors: Esler J. Gavin , Tan David G. H. , Haynes Peter H. , Evans Mathew J. , Law Kathy S. , Plantevin Paul‐Henri , Pyle John A.
Published: February 2004

We present examples of how chemical evolution can exhibit sensitivity to mixing arising from stratosphere‐troposphere exchange. A chemical transport model is used to survey the chemical contrasts on isentropic surfaces that intersect the tropopause. Significant cross‐tropopause gradients in both ozone and water vapor are shown to exist between 300 and 340 K. Back trajectories are used to confirm that air parcels with widely varying chemical properties are rapidly brought together in a typical quasi‐isentropic stratosphere‐troposphere exchange event. A two‐box model is used to investigate the chemical evolution of stratospheric and tropospheric air parcels and to determine the effect of mixing between them. Mixing of stratospheric ozone and tropospheric water vapor is shown to lead to enhanced hydroxyl (OH) radical concentrations compared with background tropospheric and stratospheric values. The oxidation of CO, methane, and higher hydrocarbons is correspondingly increased, and NOx is also lost more rapidly with faster mixing. Also, in low NOx conditions, the rate of O3 loss is found to increase with faster mixing. The consequences of this anomalous chemistry for chemical transport in stratosphere‐troposphere exchange events are discussed. It is also noted that if NOx levels in either parcel are very high initially, mixing of NOx can become more important than O3 or H2O in determining OH levels.
N2O emission from tropical forest soils of Australia
Authors: Breuer Lutz , Papen Hans , Butterbach‐Bahl Klaus
Published: February 2004

Three different tropical rain forest sites (Kauri Creek, Lake Eacham, and Massey Creek) on the Atherton Tablelands, Queensland, Australia, were investigated for the magnitude of N2O emission from soils during different seasons, that is, wet season, dry season, and transition periods. Highest mean N2O emission rates were observed for soils derived from granite at the Kauri Creek site with 74.5±25.2 μg N2O‐N m−2 h−1, whereas for soils derived from Metamorphics (Lake Eacham site) mean N2O emission rates were much lower (13.1±1.1 μg N2O‐N m−2 h−1). For the Massey Creek site, with soils derived from Rhyolite, a mean annual N2O emission rate of 46. ±1.1 μg N2O‐N m−2 h−1 was calculated. The mean annual N2O emission rate calculated for all three sites over the entire observation period was 39.0 μg N2O‐N m−2 h−1 and thus at the high end of reports from tropical rain forest soils. N2O emission rates showed at all sites pronounced temporal as well as spatial variability. The magnitude of N2O emissions was strongly linked to rainfall events; that is, N2O emissions strongly increased approximately 6–8 hours after precipitation. Correlation analysis confirmed the strong dependency of N2O emissions on changes in soil moisture, whereas changes in soil temperature did not mediate considerable changes in N2O fluxes. Spatial variability of N2O fluxes on a site scale could be explained best by differences in water‐filled pore space, CO2 emission, and C/N ratio of the soil. On the basis of all published N2O flux rates from tropical rain forest soils we recalculated the contribution of such forests to the global atmospheric N2O budget and come up with a figure of 3.55 Tg N2O‐N yr−1, which is approximately 50% higher than reported by others.
Effects of quasi‐stationary large‐scale anomalies on some mesoscale features associated with the 1993 flood: A regional model simulation
Authors: Pan Zaitao , Arritt Raymond W. , Segal Moti , Chen Tsing‐Chang , Weng Shu‐Ping
Published: February 2004

We investigate the effect of quasi‐stationary large‐scale anomalies on mesoscale factors contributing to the 1993 U.S. Midwest flood by performing 30‐day simulations with a mesoscale model. The contributions of anomalous mesoscale features to the flood were quantified by contrasting model integrations forced by initial and lateral boundary conditions with and without large‐scale anomalous stationary waves that are believed to have been responsible for the flood. Supplementing previous investigations, the present study finds the following: (1) The large‐scale anomalies produced environmental conditions that favored a generation of MCC‐like systems rather than individual convective cells or squall lines that would have produced less precipitation over large areas. Ambient conditions favoring mesoscale convective complex (MCC) include an intensified east‐west orientation of quasistationary fronts, weaker upper tropospheric inertial stability, and a strong low‐level jet (LLJ). (2) The anomalous ageostrophic wind component to the far south of the flood region (Texas area) and geostrophic wind component over the flood region bridged together to form an extended moisture conveyer that sustained the long‐lasting 1993 flood.
Uncertainties in satellite‐derived estimates of surface UV doses
Authors: Martin T. J. , Gardiner B. G. , Seckmeyer G.
Published: February 2004

Satellite‐derived maps of surface UV irradiance are currently limited by their poorly assessed accuracy. Here we use an extensive data set of ground‐based spectral measurements from two Alpine sites to examine the level of uncertainty expected in model results due to the time‐limited nature of some satellite data. When the ozone column, cloud optical depth, and aerosol optical depth supplied to a radiative transfer model are fixed to a single noontime value, the root‐mean‐square difference between calculations and measurements of the erythemal daily dose is about 20%. The corresponding uncertainty in the monthly dose is less than 5%. The modeled results also show a systematic error that depends on cloud optical depth. The results suggest that satellite‐derived maps of UV irradiance cannot be expected to produce accurate values of the daily dose if they rely on a single estimate of the cloud conditions but may be able to provide reasonable estimates of the monthly dose.