bluuebunny/crossref_metadata_2025 huggingface dataset:
Artificial neural network estimation of rainfall intensity from radar observations
Authors: Orlandini Stefano , Morlini Isabella
Published: February 2004

Volumetric scans of radar reflectivityZand gage measurements of rainfall intensityRare used to explore the capabilities of three artificial neural networks to identify and reproduce the functional relationship betweenZandR.The three networks are a multilayer perceptron, a Bayesian network, and a radial basis function network. For each of them, numerical experiments are conducted incorporating in the network inputs different descriptions of the space‐time variability ofZ.Space variability refers to the observations ofZalong the vertical atmospheric profile, at 11 constant altitude plan position indicator levels, namelyZT= (Z1,…,Z11). Time variability refers to the observations ofZat the time intervals prior to that for which the estimate ofRis provided. Space variability is evaluated by performing a principal component analysis over standardized values ofZ, namely, and the first two principal components of(which describe 91% of the original variance) are used to synthesize the elements ofZinto fewer orthogonal inputs for the networks. Network predictions significantly improve when the models are trained with the two principal components ofwith respect to the case in which onlyZ1is used. Increasing the time horizon further improves the performances of the Bayesian network but is found to worsen the performances of the other two networks.
Detrended fluctuation analysis of rainfall and streamflow time series
Authors: Matsoukas Christos , Islam Shafiqul , Rodriguez‐Iturbe Ignacio
Published: February 2004

Several recent studies have shown that a remarkably wide variety of natural systems display fluctuations that may be characterized by long‐range power‐law correlations. Such correlations hint toward fractal geometry of the underlying dynamical system. Existence and determination of power‐law correlations would help to quantify the underlying process dynamics. Traditional time series analysis of rainfall, for example using spectral analysis, often produces spurious results due to the highly nonstationary nature of rainfall signals. Here we present some new results using detrended fluctuation analysis (DFA) to quantify the correlation properties of rainfall time series. The DFA allows the detection of long‐range correlations embedded in an apparently nonstationary time series and has been successfully applied to analyze highly nonstationary DNA sequences, heartbeat time series, and temperature fluctuations. Our analysis of rainfall data ranges from hourly to annual timescales for geographically and meteorologically distinct regions. Results show a common scaling behavior for all stations analyzed. Two distinct scaling regions are identified, one corresponding to short‐range and the other to long‐range correlations, on either side of a breakpoint. Additional synthetic experiments demonstrate that what controls the correlation structure is not the actual rainfall values but the pattern of alternating wet and dry spells. The application of DFA on streamflow reflects the fact that streamflow is characterized by smooth fluctuations compared to its driving process, rainfall. Our results highlight the dampening effect that the soil has on the rainfall correlations.
A comparison of water vapor derived from GPS occultations and global weather analyses
Authors: Kursinski E. R. , Hajj G. A.
Published: February 2004

Despite its fundamental importance in radiative transfer, atmospheric dynamics, and the hydrological cycle, atmospheric water is inadequately characterized particularly at a global scale. Occultation measurements from the Global Positioning System (GPS) should improve upon this situation. Individual occultations yield profiles of specific humidity accurate to 0.2 to 0.5 g/kg providing sensitive measurements of lower and middle tropospheric water vapor with global coverage in a unique, all‐weather, limb‐viewing geometry with several hundred meters to a kilometer vertical resolution. We have derived water vapor profiles from June 21 to July 4, 1995, using GPS occultation data combined with global temperature analyses from the European Center for Medium‐Range Weather Forecasts (ECMWF) and reanalyses from the National Centers for Environmental Prediction (NCEP). The zonal mean structure of the profiles exhibits basic climatological features of tropospheric moisture. Specific humidity biases between the GPS results and the ECMWF global humidity analyses in the middle to upper troposphere are ∼0.1 g/kg or less. Occultation results below 6 km altitude are generally drier than those of ECMWF with the bias generally increasing toward warmer temperatures. Near the height of the trade wind inversion, the ECMWF analyses are significantly moister than the occultation results due to vertical smoothing and overextension of the boundary layer top in the analyses. Overall, the occultation results are drier than the NCEP reanalyses with a marked exception near the Intertropical Convergence Zone (ITCZ) where occultation results are wetter by more than 10%. The occultation results are significantly wetter near the ITCZ and drier in the subtropics than the classical moisture climatology of Peixoto and Oort. Similarities between the NCEP and the Peixoto and Oort near‐ITCZ differences suggest that a common analysis/model problem may be responsible. The generally wetter Peixoto and Oort results in the subtropics are due in part to moist radiosonde biases. Discrepancies between these data sets are significant and limit our ability to resolve uncertainties in moisture control and feedbacks in a changing climate.
Ozone climatology using interactive chemistry: Results from the Canadian Middle Atmosphere Model
Authors: de Grandpré J. , Beagley S. R. , Fomichev V. I. , Griffioen E. , McConnell J. C. , Medvedev A. S. , Shepherd T. G.
Published: February 2004

The climatology of ozone produced by the Canadian Middle Atmosphere Model (CMAM) is presented. This three‐dimensional global model incorporates the radiative feedbacks of ozone and water vapor calculated on‐line with a photochemical module. This module includes a comprehensive gas‐phase reaction set and a limited set of heterogeneous reactions to account for processes occurring on background sulphate aerosols. While transport is global, photochemistry is solved from about 400 hPa to the top of the model at ∼95 km. This approach provides a complete and comprehensive representation of transport, emission, and photochemistry of various constituents from the surface to the mesopause region. A comparison of model results with observations indicates that the ozone distribution and variability are in agreement with observations throughout most of the model domain. Column ozone annual variation is represented to within 5–10% of the observations except in the Southern Hemisphere for springtime high latitudes. The vertical ozone distribution is generally well represented by the model up to the mesopause region. Nevertheless, in the upper stratosphere, the model generally underestimates the amount of ozone as well as the latitudinal tilting of ozone isopleths at high latitude. Ozone variability is analyzed and compared with measurements. The comparison shows that the phase and amplitude of the seasonal variation as well as shorter timescale variations are well represented by the model at various latitudes and heights. Finally, the impact of incorporating ozone radiative feedback on the model climatology is isolated. It is found that the incorporation of ozone radiative feedback results in a cooling of ∼8 K in the summer stratopause region, which corrects a warm bias that results when climatological ozone is used.
A solar reflectance method for retrieving the optical thickness and droplet size of liquid water clouds over snow and ice surfaces
Authors: Platnick S. , Li J. Y. , King M. D. , Gerber H. , Hobbs P. V.
Published: February 2004

Cloud optical thickness and droplet effective radius retrievals from solar reflectance measurements are traditionally implemented using a combination of spectral channels that are absorbing and nonabsorbing for water particles. Reflectances in nonabsorbing channels (e.g., 0.67, 0.86, 1.2 μm spectral window bands) are largely dependent on cloud optical thickness, while longer‐wavelength absorbing channels (1.6, 2.1, and 3.7 μm window bands) provide cloud particle size information. Cloud retrievals over ice and snow surfaces present serious difficulties. At the shorter wavelengths, ice is bright and highly variable, both characteristics acting to significantly increase cloud retrieval uncertainty. In contrast, reflectances at the longer wavelengths are relatively small and may be comparable to that of dark open water. A modification to the traditional cloud retrieval technique is presented. The new algorithm uses only a combination of absorbing spectral channels for which the snow/ice albedo is relatively small. Using this approach, retrievals have been made with the MODIS airborne simulator (MAS) imager flown aboard the high‐altitude NASA ER‐2 from May to June 1998 during the Arctic FIRE‐ACE field deployment. Data from several coordinated ER‐2 and in situ University of Washington Convair‐580 aircraft observations of liquid water stratus clouds are examined. MAS retrievals of optical thickness, droplet effective radius, and liquid water path are shown to be in good agreement with in situ measurements. The initial success of the technique has implications for future operational satellite cloud retrieval algorithms in polar and wintertime regions.
Derivation of split window algorithm and its sensitivity analysis for retrieving land surface temperature from NOAA‐advanced very high resolution radiometer data
Authors: Qin Zhihao , Dall'Olmo Giorgio , Karnieli Arnon , Berliner Pedro
Published: February 2004

Retrieval of land surface temperature (LST) from advanced very high resolution radiometer (AVHRR) data is an important methodology in remote sensing. Several split window algorithms have been proposed in last two decades. In this paper we intend to present a better algorithm with less parameters and high accuacry. The algorithm involves only two essential parameters (transmittance and emissivity). The principle and method for the linearization of Planck's radiance equation, the mathematical derivation process of the algorithm, and the method for determining the atmospheric transmittance are discussed with details. Sensitivity analysis of the algorithm has been performed for evaluation of probable LST estimation error due to the possible errors in transmittance and emissivity. Results from the analysis indicate that the proposed algorithm is able to provide an accurate estimation of LST from AVHRR data. Assuming an error of 0.05 in atmospheric transmittance estimate and 0.01 in ground emissivity for the two AVHRR thermal channels, the average LST error with the algorithm is 1.1°C. Two methods have been used to validate the proposed algortihm. Comparison has also been done with the existing 11 algorithms in literature. Results from validation and comparison using the standard atmospheric simulation for various situations and the ground truth data sets demonstrate the applicability of the algorithm. According to the root mean square (RMS) errors of the retrieved LSTs from the measured or assumed LSTs, the proposed algorithm is among the best three. Considering the insignificant RMS error difference among the three, the proposed algorithm is better than the other two because they require more parameters for LST retrieval. Validation with standard atmospheric simulation indicates that this algorithm can achieve the accuacry of 0.25°C in LST retrieval for the case without error in both transmittance and emissivity estimates. The accuary of this algorithm is 1.75°C for the ground truth data set without precise in situ atmospheric water vapor contents. The accuracy increases to 0.24°C for another ground truth data set with precise in situ atmospheric water vapor contents. The much higher accuracy for this data set confirms the appplicability of the proposed algorithm as an alternative for the accurate LST retrieval from AVHRR data.
Study of the interannual ozone loss and the permeability of the Antarctic polar vortex from aerosol and ozone lidar measurements in Dumont d'Urville (66.4°S, 140°E)
Authors: Godin Sophie , Bergeret Valérie , Bekki Slimane , David Christine , Mégie Gérard
Published: February 2004

Systematic ground‐based measurements of the ozone and aerosol stratospheric profile performed in the Dumont d'Urville (66.4°S, 140°E) Antarctic station are used to study the air subsidence in fall, the springtime ozone loss, and the vortex confinement on an interannual basis. The data obtained mostly from March to October in the 1992–1998 time period are analyzed as a function of equivalent latitude in order to discriminate the measurements performed inside, at the edge or outside the vortex. The subsidence rate derived from the ozone measurements in the lower stratosphere prior to the destruction period and from the volcanic aerosol measurements in 1992 is in good agreement with the estimation provided by radiative transfer models. The analysis of the ozone seasonal variation shows that the ozone destruction starts in the beginning of August and affects essentially the inner vortex or inner edge regions. The destruction lasts up to the end of September and low ozone values are found up to the end of October, confirming the isolation of the polar vortex above 400 K. Ozone destruction rates of the order of 3%/d in the 400–475 K region and 1.5%/d around 550 K are derived from the data obtained inside the vortex. The ozone measurements obtained in October allow to study the vortex confinement which shows the increase of the ozone meridional gradient through the vortex edge from 400 to 650 K. A fine view of the vortex edge is provided by a near daily series of aerosol lidar measurements performed in October 1992, a period when the vortex was deformed by planetary waves and slanted with respect to the vertical direction.
Atmospheric program on the Arctic Ocean Expedition 1996 (AOE‐96): An overview of scientific goals, experimental approach, and instruments
Authors: Leck Caroline , Nilsson E. Douglas , Bigg E. Keith , Bäcklin Leif
Published: February 2004

The broad aim of the atmospheric program on an expedition to the central Arctic Ocean during July‐September 1996 (AOE‐96) was to improve the accuracy of models predicting climatic change by reducing uncertainties associated with the indirect climate effect of aerosols. A major part of this aim was to assess what feedback processes might be involved and their implications for both regional and global radiative forcing. An immediate objective therefore was to study the biogeochemical sources that were thought to lead to aerosol formation and possible growth processes as well as the characterization of the aerosol that actually influences cloud properties in a region where natural processes prevailed. The program was designed as an integrated package to study processes from the sea‐ice/air interface to the top of the Arctic boundary layer and involved the contribution from about 25 participating scientists from 8 countries. The results from a previous expedition to the same area were used to specify the research topics from which new or improved information was needed to achieve the objectives. The instruments, techniques, and models developed and deployed to provide the answers are described. The relevant properties of the region, the route of the expedition, the conditions encountered, and the special circumstances of measurements made on an ice floe are also discussed.
Mechanisms for midlatitude ozone loss: Heterogeneous chemistry in the lowermost stratosphere?
Authors: Smith Jessica B. , Hintsa Eric J. , Allen Norton T. , Stimpfle Richard M. , Anderson James G.
Published: February 2004

The question of midlatitude ozone erosion by chlorine free radical catalysis is examined. We present and analyze simultaneous, high‐resolution observations of ClO, H2O, tropopause height, particle reactive surface area, and ice saturation occurrence frequency obtained from the NASA ER‐2 aircraft. The objective is to test the hypothesis that the existence of cirrus clouds or cold aerosols in the first few kilometers above the tropopause at midlatitudes is responsible for increasing the ratio of chlorine free radicals to total inorganic chlorine, thus amplifying the rate of catalytic ozone destruction. The observations reveal a sharp decrease in ice saturation frequency at the tropopause, a marked degree of under‐saturation just above the tropopause, a corresponding sharp gradient in the product of cold aerosol reactive surface area and reaction probability, γSa, and, finally, the consistent absence of enhanced concentrations of ClO immediately above the tropopause. These results suggest that midlatitude ozone loss is not controlled in situ by the mechanism of cirrus cloud and/or cold aerosol enhancement of chlorine radicals in the vicinity of the tropopause.
A method for assessing the quality of model‐based estimates of ground temperature and atmospheric moisture using satellite data
Authors: Wu Man Li C. , Schubert Siegfried , Lin Ching I. , Štajner Ivanka
Published: February 2004

A method is developed for validating model‐based estimates of atmospheric moisture and ground temperature using satellite data. The approach relates errors in estimates of clear‐sky longwave fluxes at the top of the Earth‐atmosphere system to errors in geophysical parameters. The fluxes include clear‐sky outgoing longwave radiation (CLR) and radiative flux in the window region between 8 and 12 μm (RadWn). The approach capitalizes on the availability of satellite estimates of CLR and RadWn and other auxiliary satellite data, and multiple global four‐dimensional data assimilation (4‐DDA) products. The basic methodology employs off‐line forward radiative transfer calculations to generate synthetic clear‐sky longwave fluxes from two different 4‐DDA data sets. Simple linear regression is used to relate the clear‐sky longwave flux discrepancies to discrepancies in ground temperature (δTg) and broad‐layer integrated atmospheric precipitable water (δpw). The slopes of the regression lines define sensitivity parameters which can be exploited to help interpret mismatches between satellite observations and model‐based estimates of clear‐sky longwave fluxes. For illustration we analyze the discrepancies in the clear‐sky longwave fluxes between an early implementation of the Goddard Earth Observing System Data Assimilation System (GEOS2) and a recent operational version of the European Centre for Medium‐Range Weather Forecasts data assimilation system. The analysis of the synthetic clear‐sky flux data shows that simple linear regression employing δTg and broad layer δpw provides a good approximation to the full radiative transfer calculations, typically explaining more than 90% of the 6 hourly variance in the flux differences. These simple regression relations can be inverted to “retrieve” the errors in the geophysical parameters. Uncertainties (normalized by standard deviation) in the monthly mean retrieved parameters range from 7% for δTg to ∼20% for the lower tropospheric moisture between 500 hPa and surface. The regression relationships developed from the synthetic flux data, together with CLR and RadWn observed with the Clouds and Earth Radiant Energy System instrument, are used to assess the quality of the GEOS2 Tg and pw. Results showed that the GEOS2 Tg is too cold over land, and pw in upper layers is too high over the tropical oceans and too low in the lower atmosphere.
Lidar estimation of tropospheric aerosol extinction, surface area and volume: Maritime and desert‐dust cases
Authors: Barnaba Francesca , Gobbi Gian Paolo
Published: February 2004

A numerical model, based on a Monte Carlo approach, is presented to determine functional relationships linking backscatter and other important properties as extinction, surface area, and volume of tropospheric aerosols. If existing, such relationships allow for a direct estimate of such properties by means of a single‐wavelength lidar measurement. To be employed in a lidar inversion procedure, the extinction to backscatter ratio is also analyzed. Maritime and desert dust aerosol particles are addressed. In the latter case, both spherical and nonspherical shape of particles are considered. Large differences (up to 200%) result from the comparison of extinction computed for spherical and nonspherical particles. On the whole, maximum errors to be associated to the model estimation of the aerosol extinction coefficient and surface area are of the order of 50%. Conversely, errors associated to volume estimates range from 15% to 100%. To validate the model, a first comparison is performed between lidar and Sun‐photometer‐derived aerosol optical thickness of both maritime aerosols and Saharan dust.
Sources of variability in mercury flux measurements
Authors: Edwards G. C. , Rasmussen P. E. , Schroeder W. H. , Kemp R. J. , Dias G. M. , Fitzgerald‐Hubble C. R. , Wong E. K. , Halfpenny‐Mitchell L. , Gustin M. S.
Published: February 2004

Chamber and micrometeorological mercury flux data collected during the Nevada STORMS intercomparison study were used to identify natural and methodological factors controlling data variability. Micrometeorological and chamber measurements revealed that flux variability at a site is closely related to the Hg concentrations in the substrate, which were found to vary with mineral composition, grain size, and sampling depth. Environmental factors also influenced flux variability. Following two rainfall events, fluxes measured by chamber and micrometeorological methods increased substantially. The micrometeorological flux was enhanced five fold following the rain event. Fluxes measured by both methods were also influenced by net radiation and temperature as evidenced by their tendency to follow the diel cycle in these variables. Daytime fluxes were 6 times greater than nighttime fluxes. Data analysis revealed that interactions between environmental and geochemical variables complicate relationships between the flux and these variables. Understanding the variability at a flux monitoring site is important to establish relationships for scaling up and for the development of consistent sampling protocols that allow comparisons from one study to another and adequately quantify mercury fluxes from natural sites to provide representative emission data that can be used for scaling up to regional and global scales.
Introduction to special section: Outstanding problems in quantifying the radiative impacts of mineral dust
Authors: Sokolik I. N. , Winker D. M. , Bergametti G. , Gillette D. A. , Carmichael G. , Kaufman Y. J. , Gomes L. , Schuetz L. , Penner J. E.
Published: February 2004

This paper provides an introduction to the special section of the Journal of Geophysical Research on mineral dust. We briefly review the current experimental and theoretical approaches used to quantify the dust radiative impacts, highlight the outstanding issues, and discuss possible strategies to overcome the emerging problems. We also introduce the contributing papers of this special section. Despite the recent notable advances in dust studies, we demonstrate that the radiative effects of dust remain poorly quantified due to both limited data and incomplete understanding of relative physical and chemical processes. The foremost needs are (1) to quantify the spatial and temporal variations of dust burden in the atmosphere and develop a predictive capability for the size‐ and composition‐resolved dust particle distribution; (2) to develop a quantitative description of the processes that control the spatial and temporal variabilities of dust physical and chemical properties and radiative effects; (3) to develop new instrumentation (especially to measure the dust particle size distribution in a wide range from about 0.01 μm to 100 μm, scattering phase function and light absorption by dust particles); and (4) to develop new techniques for interpreting and merging the diverse information from satellite remote sensing, in situ and ground‐based measurements, laboratory studies, and model simulations. Because dust distribution and effects are heterogeneous, both spatially and temporally, a promising strategy to advance our knowledge is to perform comprehensive studies at the targeted regions affected by mineral dust of both natural and anthropogenic origin.
Modeling with explicit spectral water and ice microphysics of a two‐layer cloud system of altostratus and cirrus observed during the FIRE Arctic Clouds Experiment
Authors: Khvorostyanov V. I. , Curry J. A. , Pinto J. O. , Shupe M. , Baker B. A. , Sassen K.
Published: February 2004

A one‐dimensional version of a cloud model with an explicit microphysics scheme is used to simulate a case study of middle and upper level cloud formation and evolution that was observed during the FIRE Arctic Clouds Experiment. In the simulations, the midlevel altostratus cloud is initially liquid phase, then partially freezes, and exists in mixed phase for several hours with a relative equilibrium between the rate of drop production by condensation and their depletion by freezing. The dominant mode of cirrus formation was periodic homogeneous freezing of deliquescent submicron haze particles. These crystal layers form near the tropopause and, subsequently, precipitate into the middle troposphere, causing seeding of the underlying altostratus cloud. Sensitivity tests are conducted varying the initial humidity and nucleation schemes.
Differences in early contrail evolution of two‐engine versus four‐engine aircraft: Lidar measurements and numerical simulations
Authors: Sussmann Ralf , Gierens Klaus M.
Published: February 2004

Jet‐ and vortex‐regime evolution of contrails behind cruising aircraft is investigated by focusing on the role of aircraft type. Cross‐section measurements by ground‐based lidar and observational analysis are combined with numerical simulations of fluid dynamics and microphysics in the wake of a two‐engine aircraft. Depending on ambient humidity levels, contrail evolution behind short‐/medium‐range twin‐turbofan airliners is classified into two scenarios, which is in contrast to the three scenarios observed for a wide‐body four‐turbofan aircraft. In the case of ice‐subsaturated ambient air, a short visible contrail is formed behind a two‐engine aircraft that disappears before the ice is fully entrained into the wingtip vortices (in most cases ≈4 s behind aircraft). The early evaporation of the ice is mainly due to the fast initial jet expansion, mixing the exhaust with the ambient air. Contrails behind a wide‐body four‐engine aircraft always survive at least until vortex breakdown (i.e., typically 2 min behind aircraft). This is simply due to the larger ice mass in the contrail because of the higher fuel flow rate. Generally, in the case of ice supersaturation, a diffuse secondary wake evolves above the primary vortex wake. For a two‐engine aircraft, always the whole contrail persists, while for a four‐engine aircraft, the ice inside the primary wake disappears in most cases after vortex breakdown, when the relative humidity is only slightly above ice saturation. In the more rare cases of higher ice‐supersaturation the ice in the primary wake survives vortex breakdown and becomes part of the persistent contrail.
Intercomparison of smoke aerosol optical thickness derived from GOES 8 imager and ground‐based Sun photometers
Authors: Zhang Jianglong , Christopher Sundar A. , Holben Brent N.
Published: February 2004

Using high temporal resolution GOES 8 imager data and radiative transfer calculations, smoke aerosol optical thickness (τ) is retrieved over selected sites in South America and Central America. The degradation of the signal response in the GOES 8 visible channel is estimated and the satellite‐retrieved τ values are then compared against ground‐based Sun photometer derived values. The satellite‐retrieved values are in good agreement with ground‐based τ for two sites in South America with mean linear correlation coefficients of 0.97. For Central America the mean correlation coefficient is 0.80. A single scattering albedo of 0.90 (at 0.67 μm) yields the best agreement between ground‐based and satellite retrieved values and is consistent with previous studies. However, our results show that the retrieved optical thickness results are sensitive to single scattering albedo and surface reflectance. For example, a ±3.3% change in single scattering albedo (0.90±0.03) yields an uncertainty in τ of 10% for small optical thickness (τ = 0.5) and an uncertainty of about 25% for larger optical thickness values (τ = 1.5). Although the GOES 8 visible channel has undergone significant degradation in signal response since launch, smoke aerosol optical thickness can be estimated if proper procedures are used to account for this effect.
Production of sea spray aerosol in the surf zone
Authors: de Leeuw Gerrit , Neele Filip P. , Hill Martin , Smith Michael H. , Vignati Elisabetta
Published: February 2004

A quantitative source function for sea spray aerosol produced by waves breaking in the surf zone was determined from data collected with optical particle counters at both sides of the surf zone at two locations on the Californian coast. Three optical particle counters were used to measure profiles at the base of a pier; a fourth instrument was used at the end of the pier. Careful calibration and intercomparisons of the instruments were made to avoid systematic errors. Aerosol concentrations measured downwind from the surf, in wind speeds of up to 9 m s−1, were 1–2 orders of magnitude higher than those upwind. Surf aerosol concentration gradients and plume heights vary with particle size and with wind speed. The derived surf aerosol source functions are compared with current estimates for the open ocean, taking into account the different proportions of the ocean surface covered by whitecaps. Application of a simple transport model indicates that surf‐produced sea spray contributes significantly to the aerosol concentrations at fetches up to at least 25 km. This has implications for, for example, heterogeneous chemistry and electro‐optical propagation.
Effects of lightning on reactive nitrogen and nitrogen reservoir species in the troposphere
Authors: Tie Xuexi , Zhang Renyi , Brasseur Guy , Emmons Louisa , Lei Wenfang
Published: February 2004

The impact of lightning on tropospheric reactive nitrogen NOx (NO + NO2) and nitrogen reservoir species (HNO3, peroxyacetyl nitrate (PAN), N2O5, and HNO4) has been evaluated using a global chemical/transport model. Comparison of calculations made with and without lightning show that lightning has a significant effect on the nitrogen species on a global scale, resulting in significant enhancements of NOx, HNO3, and PAN over the no lightning case. Of the nitrogen species, HNO3 is influenced the most, comprising approximately 60 to 80% of the total increase in the nitrogen species concentration. The increase in PAN accounts for approximately 20 to 30% of the nitrogen enhancement by lightning in the middle troposphere. In the lower troposphere of the tropics, NOx is rapidly converted into HNO3 due to the high OH concentration in this region. As a result, the enhancement in NOx from direct lightning emission is limited primarily to the upper troposphere. The conversion between NOx and less reactive nitrogen species (PAN and HNO3) also plays an important role in affecting NOx, especially over the oceans where lightning activity is low. The model results suggest that recycling of NOx from the lightning‐enhanced PAN and HNO3 produces 2 to 10 parts per trillion by volume (pptv) increases in NOx over the oceans in the lower troposphere of the tropics. The enhancement of NOx over the oceans can partially explain the observations of NOx (mixing ratios of typically 10–50 pptv) over the tropical oceans, which are characterized by higher concentrations than would be expected from direct transport of NOx from the continents.
Measurements of carbonyl sulfide (COS) in surface seawater and marine air, and estimates of the air‐sea flux from observations during two Atlantic cruises
Authors: Xu X. , Bingemer H. G. , Georgii H.‐W. , Schmidt U. , Bartell U.
Published: February 2004

Carbonyl sulfide (COS) was measured in surface seawater and in marine air during two Atlantic cruises of the R/V Polarstern between Bremerhaven, Germany, and Cape Town, South Africa. The cruises took place in the fall of 1997 and in the summer of 1998. The concentration of COS showed clear diurnal, seasonal, and latitudinal variations, as did its saturation ratio. The concentration of dissolved COS averaged 14.7 pmol L−1 and 18.1 pmol L−1 for the fall and summer cruises, respectively. On most days, seawater was undersaturated in COS during the late night and early morning but was supersaturated during the rest of the day, implying that the ocean can act as both a source and a sink for COS on the same day. The COS content in seawater was correlated significantly with the global radiation, the CH3SH concentration, and the seawater temperature. The air‐sea flux of COS from the open Atlantic Ocean was estimated using exchange coefficients calculated according to Erickson's stability dependent model for air‐sea gas exchange. The largest COS flux into the atmosphere occurred in productive regions (the Benguela Current, the West African upwelling area, and the northeastern Atlantic) during the warmer seasons. A small net oceanic uptake of COS was found in the Benguela Current during the southern winter. The average open ocean fluxes were 13.5 nmol COS m−2 d−1 and 28.6 nmol COS m−2 d−1 for the two cruises, respectively. A global open ocean source of 0.10 Tg COS yr−1 is extrapolated from the measured data. The atmospheric mixing ratio of COS averaged 474±33 and 502±38 pptv for the fall and summer cruises, respectively, and had no significant interhemispheric gradient.
Equatorial mesoscale wind and temperature fluctuations in the lower atmosphere
Authors: Cot Charles
Published: February 2004

We present spectrally analyzed high‐resolution balloon measurements of vertical temperature and horizontal wind in the troposphere and lower stratosphere in French Guyana (≈5°N, 52°W) made in 1996/1997. The paper is principally concerned with spectra at vertical scales from a few kilometers down to 100 m. A special balloon has been developed to solve scales below 500 m. For the first time, the equatorial region has been studied extensively at this resolution. We have found wind and temperature spectra following the canonical shape in the logarithmic representation with a slope of ≈−3 for wavenumbers ≧10−3 cycle/m with day‐to‐day significant amplitude variations but a mean spectrum constant with the seasons. We have found isotropic wave horizontal direction of propagation for the troposphere and anticorrelated with the quasi‐biennial oscillation in the stratosphere. The vertical propagation of wave has been found symmetric in the troposphere but highly asymmetric in the stratosphere with dominant upward propagation. We were not able to find any correlation between deep moist convection and wave activity. The ratio kinetic to potential energy spectra, which is the ratio of velocity spectra to temperature spectra multiplied by N2T¯2b/g2 for the troposphere and stratosphere, is constant versus the vertical wavenumber and in good agreement with a model intrinsic frequency spectrum varying as ω−p with 5/3 ≤ p ≤ 2 and without necessity of energy enhancement near the inertial frequency which is close to zero at the latitude measurements.
London methane emissions: Use of diurnal changes in concentration and δ13C to identify urban sources and verify inventories
Authors: Lowry David , Holmes Craig W. , Rata Nigel D. , O'Brien Phillip , Nisbet Euan G.
Published: February 2004

Diurnal air sampling campaigns at the Royal Holloway site on the western fringe of London, United Kingdom, have been used to: (1) test the validity of using carbon isotopes to identify local methane sources, (2) determine the isotopic signature of overnight build‐up profiles, in order to estimate regional emissions, and (3) verify statistical estimates of emissions. For CH4 an overall London δ13C source mix of −48.7±0.3‰ has been calculated from gradual overnight methane buildup in air masses moving from the east. Isotopic characterization of specific methane peaks shows them to be derived either from natural gas leaks (δ13C −33‰ to −35‰) or waste treatment emissions (δ13C −51‰ to −53‰). While landfill/waste emissions dominate, gas distribution losses represent ∼20% of the bulk local source. Various estimates of total London methane emissions for 1996 were made, using diurnal excess, isotopic data and trajectory movement across London to the sampling station. The results are in the range 240–312 kt/yr, higher than the London estimate in current U.K. greenhouse gas inventory assessments of emissions but within error of earlier statistical estimates for 1996. The results show that it is possible to use atmospheric concentration, isotopic, and meteorological data together to verify statistical estimates testing them for internal consistency and using better constrained data to calibrate more poorly known source fluxes. Importantly, atmospheric data can place constraints on poorly constrained landfill emission estimates for the region. These quasi‐independent methods for verification of greenhouse gas emissions will contribute in assessing compliance with the Kyoto agreement.
A theoretical analysis of cloud condensation nucleus (CCN) instruments
Authors: Nenes Athanasios , Chuang Patrick Y. , Flagan Richard C. , Seinfeld John H.
Published: February 2004

The behavior and performance of four cloud condensation nucleus instruments are theoretically analyzed. They include the static diffusion cloud chamber (SDCC), the Fukuta continuous flow spectrometer (FCNS), the Hudson continuous flow spectrometer (HCNS), and the California Institute of Technology continuous flow spectrometer (CCNS). A numerical model of each instrument is constructed on the basis of a general fluid dynamics code coupled to an aerosol growth/activation model. Instrument performance is explored by simulating instrument response when sampling a monodisperse ammonium sulfate aerosol. The uncertainty in the wall temperature boundary condition is estimated for all the instruments and is found to be appreciable only for the CCNS. The CCNS and HCNS models reasonably reproduced experimental data, while reported limits were also verified by the FCNS model. Regarding the performance of each instrument, simulations show that the SDCC produces droplets that are monodisperse to within 10% of the particle diameter (for particles of a constant critical supersaturation). The FCNS can potentially activate particles over a wide range of critical supersaturations, but the prevailing design exhibits low sensitivity to particles with critical supersaturations below 0.1% as a result of the short time available for droplet growth under low supersaturations. The resolution capability of both HCNS and CCNS with respect to critical supersaturation is shown to be particularly sensitive to operational parameters. This is a consequence of the strongly nonlinear nature of droplet growth; droplet size cannot always be used to distinguish particles with different critical supersaturation because of the growing droplets' trend toward monodispersity. Of the two instruments, the HCNS generally displays higher resolution capability. This is attributed to the smoother and monotonic supersaturation profiles established in the HCNS. While different design parameters or operating conditions may lead to modest shifts in the performance from that predicted here for any of the four instruments, the essential features described in this paper are inherent to their designs.
Low‐frequency temperature variations from a northern tree ring density network
Authors: Briffa Keith R. , Osborn Timothy J. , Schweingruber Fritz H. , Harris Ian C. , Jones Philip D. , Shiyatov Stepan G. , Vaganov Eugene A.
Published: February 2004

We describe new reconstructions of northern extratropical summer temperatures for nine subcontinental‐scale regions and a composite series representing quasi “Northern Hemisphere” temperature change over the last 600 years. These series are based on tree ring density data that have been processed using a novel statistical technique (age band decomposition) designed to preserve greater long‐timescale variability than in previous analyses. We provide time‐dependent and timescale‐dependent uncertainty estimates for all of the reconstructions. The new regional estimates are generally cooler in almost all precalibration periods, compared to estimates obtained using earlier processing methods, particularly during the 17th century. One exception is the reconstruction for northern Siberia, where 15th century summers are now estimated to be warmer than those observed in the 20th century. In producing a new Northern Hemisphere series we demonstrate the sensitivity of the results to the methodology used once the number of regions with data, and the reliability of each regional series, begins to decrease. We compare our new hemisphere series to other published large‐regional temperature histories, most of which lie within the 1σ confidence band of our estimates over most of the last 600 years. The 20th century is clearly shown by all of the palaeoseries composites to be the warmest during this period.
Raman lidar measurements of water vapor and cirrus clouds during the passage of Hurricane Bonnie
Authors: Whiteman D. N. , Evans K. D. , Demoz B. , Starr D. O'C. , Eloranta E. W. , Tobin D. , Feltz W. , Jedlovec G. J. , Gutman S. I. , Schwemmer G. K. , Cadirola M. , Melfi S. H. , Schmidlin F. J.
Published: February 2004

The NASA GSFC Scanning Raman Lidar (SRL) was stationed on Andros Island in the Bahamas during August‐September 1998 as a part of the third Convection and Moisture Experiment (CAMEX 3) which focused on hurricane development and tracking. During the period August 21–24, Hurricane Bonnie passed near Andros Island and influenced the water vapor and cirrus cloud measurements acquired by the SRL. Two drying signatures related to the hurricane were recorded by the SRL and other sensors. Cirrus cloud optical depths (at 351 nm) were also measured during this period. Optical depth values ranged from less than 0.01 to 1.5. The influence of multiple scattering on these optical depth measurements was studied. A correction technique is presented which minimizes the influences of multiple scattering and derives information about cirrus cloud optical and physical properties. The UV/IR cirrus cloud optical depth ratio was estimated on the basis of a comparison of lidar and GOES 8 measurements. Simple radiative transfer model calculations compared with GOES satellite brightness temperatures indicate that satellite radiances are significantly affected by the presence of cirrus clouds if IR optical depths are ∼0.005 or greater. Using the International Satellite Cloud Climatology Project (ISCCP) detection threshold for cirrus clouds on the GOES data presented here, a high bias of up to 40% was found in the GOES precipitable water retrieval.
A case study of orographic rainfall processes incorporating multiscaling characterization techniques
Authors: Purdy J. C. , Harris D. , Austin G. L. , Seed A. W. , Gray W.
Published: February 2004

The Southern Alps field experiment was designed to identify the dominant rainfall processes in intense orographic events in the South Island of New Zealand and included the deployment of a rain gauge network and meteorological radar. Multiscaling statistics, used to characterize the rainfall from a single extreme event, revealed both orographic and temporal changes in the rainfall nature, with significantly more incessant rainfall observed in the higher‐altitude regions. Central to this work was physical interpretation of the statistical parameters, which contributes toward forming links between multiscaling analysis and meteorological processes necessary for practical applications of multiscaling statistics. A further step was taken by combining the statistical results with other meteorological data to infer details of the physical processes, hence providing an example of the utility of multiscaling characterization of rainfall for improving our understanding of physical rainfall processes. Evidence is presented of lateral broadening of precipitating elements as the alpine divide is approached and is used, in conjunction with the wind profile, to explain the quasi‐incessant rainfall observed near the divide.
Interannual variations in the stratospheric circulation with a perfectly steady troposphere
Authors: Chen Mingyue , Mechoso Carlos R. , Farrara John D.
Published: February 2004

The seasonal and interannual variations of the stratospheric circulation are investigated using a quasi‐geostrophic, β‐plane channel model of the middle atmosphere. In the horizontal the model truncation retains at most two zonal and meridional wavenumbers. There are 27 layers in the vertical. All conditions at the model's lower boundary in the lower stratosphere are constant in time. No interannual variations are found if the flow is forced to a radiative equilibrium condition that is annually periodic with a radiative damping time of ∼20 days in the lower stratosphere. The seasonal evolution of the stratospheric circulation differs for different amplitudes of zonal wavenumbers 1 and 2 prescribed at the model's lower boundary (hB1 and hB2, respectively). When hB2 is small (50 m), variations are weak during winter and strong during spring, thus resembling the southern stratosphere. When hB2 is larger (150 m), variations have significant intensity during the entire winter, thus resembling the northern stratosphere. It is demonstrated that interannual variations can be obtained during winter and spring if two requirements are met. First, the radiative damping time is doubled in the lower stratosphere to a value in closer agreement with observational estimates. Second, if the model's truncation includes the antisymmetric mode about the channel center. The two requirements complement each other in the following way: Larger radiative damping times imply that the system can “remember” different conditions at the beginning of the previous summer, when otherwise the system memory is lost. Nonsymmetric conditions about the channel center imply that the critical level of the mean zonal wind in early winter does not extend across the entire domain. It is concluded that intraseasonal and interannual variations in the stratospheric circulation could exist even in the presence of planetary waves with constant amplitude in the troposphere and radiative forcing in the stratosphere, which is annually periodic.
The effect of precipitation variability on ENSO/precipitation teleconnections in the contiguous United States
Authors: Tsonis A. A. , Young‐Molling C. C.
Published: February 2004

An investigation of the spatiotemporal tendencies of the parameters of the probability density function describing precipitation in the last century in the contiguous United States reveals the existence of statistically significant local trends in both space and time. In such a case the distribution describing precipitation at a site is not constant in time, meaning that the population characteristics have not remained the same throughout the century. Here we show that when this variability is taken into account, it alters the effect of El Niño‐Southern Oscillation (ENSO) on precipitation significantly and may often lead to erroneous predictions of ENSO‐related precipitation patterns.