bluuebunny/crossref_metadata_2025 huggingface dataset:
Micrometer‐sized hygroscopic particles in the atmosphere: Aircraft measurement in the Arctic
Authors: Meyers Matthew B. , Hallett John
Published: February 2004

Aircraft measurements of concentrations and size distributions of nuclei between 1 and 11 μm diameter were made in cloud‐free air at altitudes below 6.2 km (approximately −30°C), using a new instrument (the cloudscope) north of Barrow, Alaska, during the First International Satellite Cloud Climatology Project Regional Experiment/Arctic Clouds Experiment (FIRE III/ACE) in May 1998. Data were obtained at various flight levels in the vicinity of a Canadian icebreaker frozen into and moving with the Arctic Ocean ice field. Ice field movements caused the frozen ocean surface to develop leads that were visible from the aircraft. Particles were collected at the stagnation point of a forward facing 3 mm diameter optical flat maintained at or above stagnation point temperatures. Images from a 425 μm × 320 μm area were video‐recorded, and concentrations were corrected for collection efficiency based on ambient airspeed, temperature, and assumed particle density. At low ambient relative humidities, crystallized particles were collected directly, and their hygroscopic character was determined by observing growth characteristics following flight into regions of higher relative humidity. Solution drops collected at high relatively humidities were crystallized with controlled heating to approximately a hemispherical shape. Particles with a dry diameter greater than 1 μm, the detection threshold, to 11 μm were found at concentrations up to 4 L−1. Concentrations were variable with altitude, and on occasion no particles were observed for periods up to 1 hour about 50 L of air. Concentrations sufficient to initiate coalescence drizzle as clouds form were observed and may be of importance in ice nucleation. Comparisons with cloud condensation nuclei (CCN) measurements (providing the collected droplets) show that on occasion the large particles (providing the collector drops) comprised a significant component of the mass of hygroscopic material. There is a gap in measurement of particles having known hygroscopic properties in that measurements of CCN have an upper limit of 0.3 μm dry diameter and the cloudscope has a lower limit of 1 μm; substantial mass of hygroscopic material could lie within this range.
Column‐integrated aerosol optical properties over the Maldives during the northeast monsoon for 1998–2000
Authors: Eck T. F. , Holben B. N. , Dubovik O. , Smirnov A. , Slutsker I. , Lobert J. M. , Ramanathan V.
Published: February 2004

Measurements made during the Indian Ocean Experiment (INDOEX) have shown the presence of large aerosol loadings over the region of the northern Indian Ocean and Arabian Sea. In recent years there has been significant interannual variability in the magnitude of this aerosol loading during the NE monsoon months of January‐April. Monitoring of the integrated atmospheric column effective aerosol optical properties was initiated in early 1998 and continued in 2000 on the island of Kaashidhoo in the Republic of Maldives. An Aerosol Robotic Network Sun‐sky radiometer at the Kaashidhoo Climate Observatory made spectral measurements of the direct Sun and directional sky radiances which were utilized to infer spectral aerosol optical depths τa, single scattering albedos, asymmetry factors, and aerosol size distributions. Monthly average aerosol optical depths at 500 nm varied by more than a factor of 2 during January through April for the 3 years that were investigated, 1998–2000. Interannual variations in the monthly mean Angstrom wavelength exponent were also observed, resulting from differences in the bimodal aerosol size distributions. Spectral variations in the Angstrom wavelength exponent were observed, especially at high aerosol optical depths when fine mode aerosols dominated over the optical influence of coarse‐mode aerosols. Some differences in spectral single scattering albedo and asymmetry factor were observed for 1999 versus 2000 in the infrared wavelengths, but with relatively little change in the visible wavelengths. The spectral variation in the retrieved single scattering albedo was large, with approximately linear wavelength dependence averaging from 0.91 at 440 nm to 0.83 at 1020 nm for January–March 1999 for observations where τa at 440 nm >0.4.
Relationships between synoptic‐scale transport and interannual variability of inorganic cations in surface snow at Summit, Greenland: 1992–1996
Authors: Slater John F. , Dibb Jack E. , Keim Barry D. , Kahl Jonathan D. W.
Published: February 2004

To fully utilize the long‐term chemical records retrieved from central Greenland ice cores, specific relationships between atmospheric circulation and the variability of chemical species in the records need to be better understood. This research examines associations between the variability of surface snow inorganic cation chemistry at Summit, Greenland (collected during 1992–1996 summer field seasons) and changes in air mass transport pathways and source regions, as well as variations in aerosol source strength. Transport patterns and source regions are determined through 10‐day isentropic backward air mass trajectories during a 1 month (late May to late June) common season over the 5 years. Changes in the extent of exposed continental surfaces in source regions are evaluated to estimate aerosol‐associated calcium and magnesium ion source strength, while forest fire activity in the circumpolar north is investigated to estimate aerosol ammonium ion source strength. During the 1995 common season, 3 times more calcium and magnesium accumulated in the snowpack than the other study years. Also, an increasing trend of ammonium concentration was noted throughout the 5 years. Anomalous transport pathways and velocities were observed during 1995, which likely contributed to the high levels of calcium and magnesium. Increased forest fire activity in North America was concurrent with increased levels of ammonium and potassium, except for 1996, when ion levels were above average and forest fire activity was below average. Because of the ubiquitous nature of soluble ions, we conclude that it is very difficult to establish a quantitative link between the ion content of snow and firn at Summit and changes in aerosol source regions and source strength.
Large‐scale air mass characteristics observed over the remote tropical Pacific Ocean during March‐April 1999: Results from PEM‐Tropics B field experiment
Authors: Browell Edward V. , Fenn Marta A. , Butler Carolyn F. , Grant William B. , Ismail Syed , Ferrare Richard A. , Kooi Susan A. , Brackett Vincent G. , Clayton Marian B. , Avery Melody A. , Barrick John D. W. , Fuelberg Henry E. , Maloney Joseph C. , Newell Reginald E. , Zhu Yong , Mahoney Michael J. , Anderson Bruce E. , Blake Donald R. , Brune William H. , Heikes Brian G. , Sachse Glen W. , Singh Hanwant B. , Talbot Robert W.
Published: February 2004

Eighteen long‐range flights over the Pacific Ocean between 38°S to 20°N and 166°E to 90°W were made by the NASA DC‐8 aircraft during the NASA Pacific Exploratory Mission (PEM) Tropics B conducted from March 6 to April 18, 1999. Two lidar systems were flown on the DC‐8 to remotely measure vertical profiles of ozone (O3), water vapor (H2O), aerosols, and clouds from near the surface to the upper troposphere along their flight track. In situ measurements of a wide range of gases and aerosols were made on the DC‐8 for comprehensive characterization of the air and for correlation with the lidar remote measurements. The transition from northeasterly flow of Northern Hemispheric (NH) air on the northern side of the Intertropical Convergence Zone (ITCZ) to generally easterly flow of Southern Hemispheric (SH) air south of the ITCZ was accompanied by a significant decrease in O3, carbon monoxide, hydrocarbons, and aerosols and an increase in H2O. Trajectory analyses indicate that air north of the ITCZ came from Asia and/or the United States, while the air south of the ITCZ had a long residence time over the Pacific, perhaps originating over South America several weeks earlier. Air south of the South Pacific Convergence Zone (SPCZ) came rapidly from the west originating over Australia or Africa. This air had enhanced O3 and aerosols and an associated decrease in H2O. Average latitudinal and longitudinal distributions of O3 and H2O were constructed from the remote and in situ O3 and H2O data, and these distributions are compared with results from PEM‐Tropics A conducted in August‐October 1996. During PEM‐Tropics B, low O3 air was found in the SH across the entire Pacific Basin at low latitudes. This was in strong contrast to the photochemically enhanced O3 levels found across the central and eastern Pacific low latitudes during PEM‐Tropics A. Nine air mass types were identified for PEM‐Tropics B based on their O3, aerosols, clouds, and potential vorticity characteristics. The data from each flight were binned by altitude according to air mass type, and these results showed the relative observational frequency of the different air masses as a function of altitude in seven regions over the Pacific. The average chemical composition of the major air mass types was determined from in situ measurements in the NH and SH, and these results provided insight into the origin, lifetime, and chemistry of the air in these regions.
Complex effects arising in smoke plume simulations due to inclusion of direct emissions of oxygenated organic species from biomass combustion
Authors: Mason Sherri A. , Field Richard J. , Yokelson Robert J. , Kochivar Michael A. , Tinsley Mark R. , Ward Darold E. , Hao Wei Min
Published: February 2004

Oxygenated volatile organic species (oxygenates), including HCOOH, H2CO, CH3OH, HOCH2CHO (hydroxyacetaldehyde), CH3COOH, and C6H5OH, have recently been identified by Fourier transform infrared measurements as a significant component of the direct emissions from biomass combustion. These oxygenates have not generally been included in the hydrocarbon‐based initial emission profiles used in previous photochemical simulations of biomass combustion smoke plumes. We explore the effects of oxygenates on this photochemistry by using an established initial emission hydrocarbon profile and comparing simulation results obtained both with and without addition of the above six oxygenates. Simulations are started at noon and carried out for 30 hours in an expanding Lagrangian plume. After an initial transient period during which [NOx] falls rapidly, conditions within the oxygenated smoke plume are found to be strongly NOx‐sensitive, and the simulated final species profile is thus strongly dependent upon the Δ[NO]/Δ[CO] initial emission profile. Oxygenate addition results in very significant and complex effects on net O3production, as well as on the relative amounts of long‐lived HOxand NOxreservoir species (H2O2, organic hydroperoxides, HNO3, and peroxyacetyl nitrate (PAN)) that are mixed into the surrounding atmosphere. Oxygenates may either increase or decrease net O3production (depending upon the initial Δ[NO]/Δ[CO]). However, they always increase H2O2and organic hydroperoxide production as a result of increased rates of radical + radical reactions. These effects spring largely from accelerated removal of NOxfrom the smoke plume due to increased radical concentrations resulting both from photolysis of oxygenates (mainly CH2O) and from their relatively high reactivity. Predicted concentrations of H2O2, Δ[O3]/Δ[CO], Δ[NH3]/Δ[CO], and Δ[HCOOH]/Δ[CO] are compared with some available measured values.
Mass balance of gaseous and particulate products analysis from α‐pinene/NOx/air in the presence of natural sunlight
Authors: Jaoui Mohammed , Kamens Richard M.
Published: February 2004

The daytime oxidation of α‐pinene in the presence of natural sunlight and oxides of nitrogen, NOx, was studied in a large outdoor smog chamber using gas chromatograph mass spectrometry as method of identification and quantification. A Scanning Mobility Particle Sizer system (3936, TSA) and a Condensation Particle Counter (3025A, TSA) were used to study the secondary organic aerosol formation, and a filter pack/denuder sampling system was used for simultaneously collecting gas phase and particle phase products for analysis. A gas chromatograph coupled to an electron impact mass spectrometry (GC‐EIMS) method is described for the identification and quantification of gas and aerosol products. The sensitivity of the method was sufficient to produce good quality mass spectra over the range of concentrations used in this study, and compares very well with classical methods based on derivatization and liquid chromatography‐ion trap mass spectroscopy methods. Mass balances for gaseous and aerosol reaction products are reported over the course of the reaction. More than 16 products were identified and quantified in this study. On average, measured gas and particle phase products accounted for ∼54% to ∼71% of the carbon reacted α‐pinene. Measurements show that 10‐hydroxypinonic acid, 10‐hydroxypinonaldehyde, 4‐oxopinonic acid, and 10‐oxopinonic acid are observed in the early stage in the aerosol phase and may play an important role in the early formation of secondary aerosols.
Tropospheric ozone layers observed during PEM‐Tropics B
Authors: Thouret Valérie , Cho John Y. N. , Evans Mathew J. , Newell Reginald E. , Avery Melody A. , Barrick John D. W. , Sachse Glen W. , Gregory Gerald L.
Published: February 2004

In this paper we analyze distinct layers seen in the tropospheric ozone and water vapor profiles taken during NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission in the Tropical Pacific (PEM‐Tropics) Phase B campaign. In summary, fewer layers were observed in this campaign than during the PEM‐Tropics A campaign. However, of those layers found, there were relatively more ozone‐rich‐water‐vapor‐poor layers (68% versus 52%). The percentage of the sampled troposphere occupied by layers during PEM‐Tropics B was less than half of that found during PEM‐Tropics A (8% versus 20%). The differences between these two campaigns suggest a seasonal variation in the occurrence of layers. This is confirmed using measurements made by the Southern Hemisphere Additional Ozonesondes (SHADOZ) network.
Atmospheric torques on land and ocean and implications for Earth's angular momentum budget
Authors: Ponte Rui M. , Rosen Richard D.
Published: February 2004

The exchange of angular momentum between the atmosphere and the oceans and solid Earth is examined using 40 years of atmospheric angular momentum (AAM) and ocean and land torque data from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis. Land torques are the dominant driving mechanism for AAM at submonthly periods. Ocean torques are as important as land torques at periods of 3 months and longer, however. With the exception of the annual and semiannual bands the ocean torque seems to mainly damp the AAM signals. The importance of the ocean torque implies a three‐way interaction among atmosphere, oceans, and solid Earth. For an ocean that simply transmits to the solid Earth the angular momentum exchanged with the atmosphere, with a delay of a few days at most, the analyzed torques imply that AAM should lead the length of day (LOD), which is contrary to the observations at monthly and longer periods. Sources of missing angular momentum variability, either from atmospheric or other origins, that can potentially explain the observed AAM and LOD phase relationship are discussed.
Stratospheric filamentation into the upper tropical troposphere
Authors: Scott R. K. , Cammas J.‐P. , Mascart P. , Stolle C.
Published: February 2004

We investigate the process of Rossby wave breaking on the subtropical tropopause and the resulting filamentary structures of stratospheric air transported into the upper tropical troposphere. We make extensive use of the Measurements of Ozone by Airbus In‐Service Aircraft (MOZAIC) flight database, both for identifying individual wave‐breaking events and for analyzing the small‐scale features of the filamentation process. Two models are used to provide a dynamically consistent description of the evolution of a particular wave‐breaking event, one a limited area, high‐resolution general circulation model, MesoNH, the other an isentropic contour advection model. The ability of both of these to represent the small‐scale evolution of the filament is examined by explicit comparison with the MOZAIC ozone data; at larger scales, comparison is made with analyses from the European Centre for Medium‐Range Weather Forecasts. Two other wave‐breaking events are presented briefly, on the one hand, verifying that the filamentary structure is ubiquitous, on the other hand, illustrating additional, distinct features of the evolution. A simple analysis of all the MOZAIC flights over the southern tropical Atlantic provides an indication of the frequency of wave‐breaking events over this region. Together with the small‐scale development suggested by the model studies and present in the MOZAIC data we infer a potentially significant contribution to the global stratosphere‐troposphere exhange from the wave‐breaking processes.
Radio occultation bending angle and impact parameter errors caused by horizontal refractive index gradients in the troposphere: A simulation study
Authors: Healy S. B.
Published: February 2004

Radio occultation (RO) bending angle and impact parameter values are derived from a Doppler shift measurement, assuming spherical symmetry. The purpose of this work is to illustrate the errors that arise when this assumption is not valid. Doppler shift values have been simulated for ray paths through a three‐dimensional refractive index field derived from a mesoscale model forecast, which has a horizontal grid of 12 km by 12 km, and includes water vapor. These have then been inverted, making the spherical symmetry assumption. It is demonstrated that refractive index gradients perpendicular to the ray path can cause errors in both the bending angle and impact parameter values, but the latter is the more significant. It is shown that the impact parameter value at the tangent point can differ by around ∼100 m from the derived value. This can cause an effective bending angle error exceeding ∼10% near the surface. A statistical analysis of the errors caused by horizontal gradients for simulations through 54 mesoscale forecasts, using fixed spacecraft trajectories and tangent point locations, is presented. In general, the bending angle errors are found to be ∼3% near the surface. A new set of analytical expressions for errors has been derived. These are based on integrating the horizontal gradients along the ray path and are found to be in good agreement with the simulation results. The implications of this work for the assimilation of RO data into numerical weather prediction models are discussed and areas of future work are outlined.
Simulations of lightning optical waveforms as seen through clouds by satellites
Authors: Light T. E. , Suszcynsky D. M. , Kirkland M. W. , Jacobson A. R.
Published: February 2004

We present three‐dimensional simulations of photon transport through clouds, specifically designed to address the characteristics and detection of optical lightning waveforms collected by satellites. The model uses a Monte Carlo approach, in which discrete photons are advanced by a standard time step through a distribution of scattering water droplets, whose size and number density distributions are variable. The model is different from previous work, in that it considers both finite and infinite cloud geometries and simulates sources of emission with arbitrary spatiotemporal properties. The model outputs are designed to be directly comparable to data obtained by the FORTE satellite photodiode detector, which records optical waveforms for lightning events with 15 μs resolution. The model treats the light propagation through clouds having a variety of shapes, sizes, and optical depths and constructs the delayed/dispersed/attenuated light curve as seen from arbitrary locations outside of the cloud. We compare the simplest case results to previous models and to data from the FORTE satellite and consider certain special cases such as the signal received from impulses occurring below the cloud. We find that the shape of the cloud and the position of the event within the cloud, rather than the motion or extent of the event itself, are the greatest determinants of the resultant distribution of photons in the sky. We also find that the position of the event within the cloud can be as large a determinant in the apparent attenuation of the signal as the cloud optical depth. We find that the class of FORTE optical waveforms with durations ≥1 ms cannot be accounted for by photon scattering alone, but rather, the intrinsic source duration must itself be quite long, which is not the case for return strokes.
Cross‐tropopause and interhemispheric transports into the tropical free troposphere over the Indian Ocean
Authors: Zachariasse M. , Smit H. G. J. , van Velthoven P. F. J. , Kelder H.
Published: February 2004

Ozone (O3) and relative humidity (RH) profiles over the Indian Ocean, obtained during the 1999 Indian Ocean Experiment, were analyzed. Upper‐tropospheric O3 mixing ratios were generally very high (varying from 100 ppbv up to 150 ppbv at the (upper) tropopause). Maxima were observed as laminae (between 14–16 km) and as O3‐ rich layers between the chemopause in O3 and the tropopause (between 15–18 km), resembling a transition zone between tropospheric and stratospheric O3 mixing ratios. Very dry, high O3 air is observed in the midtroposphere (between 5–8 km). The 1999 laminae are similar to those observed in 1998 in a pre‐INDOEX campaign, although they appear older. This complicates the attribution of an origin but there are indications, i.e., by back trajectory analyses, that the air in all these O3 maxima originates in the subtropical jet stream (STJ). This is a favored region for stratosphere‐troposphere exchange (STE). A new feature is that the transition zone is supplied with air from both the northern and the southern STJ. A pair of anticyclones located over the western Pacific/Australia directs this interhemispheric transport (IHT). IHT also occurs in the midtroposphere where a flow channel along the east coast of Africa connects the upper troposphere of both hemispheres. Although our analysis, involving the ozone profiles, trajectories, and ECMWF model data, supports the idea that all O3 maxima have sources similar to 1998, the upper‐tropospheric flow is anomalously easterly in 1999. This is probably associated with the La Nina phase of the tropospheric quasi‐biennial oscillation as opposed to the 1998 El Niño. Thus, although climatological conditions are very different, the tropospheric O3 budget over the Indian Ocean during the winter monsoon is still largely influenced by stratospheric intrusions.
Analysis of polar stratospheric clouds using temperature and aerosols measured by the Alomar R/M/R lidar
Authors: Fierli Federico , Hauchecorne Alain , Knudsen Bjoern
Published: February 2004

In this work we analyze and interpret stratospheric temperature and polar stratospheric cloud measurements made with the ALOMAR Rayleigh Mie Raman (R/M/R) lidar located at Andøya (69°N, 16°E) in northern Norway. The ALOMAR R/M/R lidar is in operation since July 1994. Vertical profiles of polar stratospheric clouds (PSCs) are measured at three different wavelengths and used to classify PSC types. Vertical temperature profiles of the stratosphere are also obtained with the lidar by using the rotational Raman technique. The backscatter ratios at three different wavelengths and the Angstrom coefficients derived from backscatter ratio are used to study the dependence of the PSC aerosol properties on the measured air temperature. We observed PSCs during the cold winters 1994/1995, 1995/1996, and 1996/1997. The clouds are classified in three types, Ia, Ib, and II. The temperature at which type Ib PSCs appear fits well with Supercooled Ternary Solution (STS) theory, while the type Ia PSCs are often measured at temperatures above TNAT. Trajectory analysis indicates that the type Ia and type Ib PSCs are characterized by different thermal histories. The last part of the paper is devoted to the interpretation of the results that are not in full agreement with the current theories of the formation of PSCs. In fact, it is found that a subtype of PSC, characterized by different values of optical parameters is measured inside clouds composite of different PSCs types. Moreover, it is observed that for a large fraction of the type Ia PSC observations the prevailing air temperature is above the thermodynamic equilibrium temperature for NAT PSCs
Simulation of the aerosol optical depth over Europe for August 1997 and a comparison with observations
Authors: Jeuken A. , Veefkind J. P. , Dentener F. , Metzger S. , Gonzalez C. Robles
Published: February 2004

A chemical transport model has been extended with an aerosol model describing processes which determine the mass distribution of sulfate, nitrate, ammonium, and aerosol associated water. A specific summer episode is simulated, and the results have been compared to surface concentration measurements and with the aerosol optical depth (AOD) retrieved from satellite measurements, with a focus on the European continent. This study is one of the first to use satellite retrievals over land for this purpose. An average difference in AOD between model and satellite measurements of 0.17–0.19 is calculated, and on average only 40–50% of the observed satellite signal can be explained by our modeled aerosol. In contrast, the observed patterns of optical thickness are well simulated by the model. Also, surface concentrations of simulated aerosol components are in close agreement with measurements. Errors in the vertical distribution of sulfate, ammonium, and nitrate, and hence in the vertical distribution of hygroscopic growth, and errors in modeled optical parameters may partly account for the observed differences in AOD. However, we argue that the most important reason for the large difference is due to the fact that organic and mineral aerosol are not taken into account in this model simulation. A sensitivity study with reduced SO2 emissions in Europe showed that reduction of the emissions of S2 in the model leads to a better agreement with surface measurements of SO2; however, calculated sulfate was less strongly influenced.
Aircraft observations of boundary layer turbulence: Intermittency and the cascade of energy and passive scalar variance
Authors: Cho John Y. N. , Anderson Bruce E. , Barrick John D. W. , Thornhill K. Lee
Published: February 2004

We analyze boundary layer velocity and temperature measurements acquired by aircraft at 22 Hz. The calculated longitudinal velocity third‐order structure function yields approximate agreement with Kolmogorov's four‐fifths law for the scale range ∼10–100 m with a downscale energy flux of ∼4×10−5 m2 s−3. For scales greater than ∼10 km the sign is reversed, implying an inverse energy cascade with an estimated flux of ∼10−5 m−2 s−3 associated with two‐dimensional stratified turbulence. The mixed structure function of longitudinal velocity and squared temperature increment follows Yaglom's four‐thirds law in the same scale range, yielding an estimated downscale temperature variance flux of ∼5×10−7 K2 s−1. Analysis of higher‐order structure functions yields anomalous scaling for both velocity and temperature. The scaling also reveals second‐order multifractal phase transitions for both velocity and temperature data. Above the transition moments, asymptotes varying with the number of realizations argue against the log‐Poisson model. The log‐Levy model is better able to explain the observed characteristics.
Areal distribution of sulphur and oxygen isotopes in sulphate of rain over eastern North America
Authors: Wadleigh Moire A. , Schwarcz Henry P. , Kramer James R.
Published: February 2004

Three summer rainfall events were sampled simultaneously across a network of 24 stations extending from the Mississippi River to the Atlantic seaboard and from the Gulf of Mexico to subarctic Canada. The chemical and isotopic compositions of the rain and its dissolved sulphate were measured on each sample. Each event was characterized by a different air mass back trajectory, suggesting a different source region for the transported sulphate. Two of the air masses apparently originated in northern Canada, one near Great Slave Lake and the other in arctic Canada. The third air mass originated in the Gulf of Mexico. In spite of this, sulphur isotopic composition was very homogeneous over most of the network, averaging +3.41±0.95‰, indistinguishable from the means of the individual events. This value is similar to that reported for long‐range transported sulphate reaching the coastal Atlantic site of Shelburne, Nova Scotia, and supports the existence of a continent‐wide, well‐mixed atmospheric reservoir of sulphur. No correlation was observed between percent sea spray and sulphur isotopic composition presumably because of the small (less than 10%) calculated sea salt contributions. Sulphate‐oxygen isotopic compositions varied very widely from −0.7 to 18‰. Two of the events apparently derived their sulphate through scavenging of aged sulphate aerosols. Sulphate‐oxygen isotopes for the other event were correlated with those of the associated rainwater, suggesting that sulphate from that event was oxidized within the transporting air mass, implying that sulphur was transported in the atmosphere in an isotopically exchangeable form.
A new neural network approach including first guess for retrieval of atmospheric water vapor, cloud liquid water path, surface temperature, and emissivities over land from satellite microwave observations
Authors: Aires F. , Prigent C. , Rossow W. B. , Rothstein M.
Published: February 2004

The analysis of microwave observations over land to determine atmospheric and surface parameters is still limited due to the complexity of the inverse problem. Neural network techniques have already proved successful as the basis of efficient retrieval methods for nonlinear cases; however, first guess estimates, which are used in variational assimilation methods to avoid problems of solution nonuniqueness or other forms of solution irregularity, have up to now not been used with neural network methods. In this study, a neural network approach is developed that uses a first guess. Conceptual bridges are established between the neural network and variational assimilation methods. The new neural method retrieves the surface skin temperature, the integrated water vapor content, the cloud liquid water path and the microwave surface emissivities between 19 and 85 GHz over land from Special Sensor Microwave Imager observations. The retrieval, in parallel, of all these quantities improves the results for consistancy reasons. A database to train the neural network is calculated with a radiative transfer model and a global collection of coincident surface and atmospheric parameters extracted from the National Center for Environmental Prediction reanalysis, from the International Satellite Cloud Climatology Project data, and from microwave emissivity atlases previously calculated. The results of the neural network inversion are very encouraging. The theoretical RMS error of the surface temperature retrieval over the globe is 1.3 K in clear‐sky conditions and 1.6 K in cloudy scenes. Water vapor is retrieved with a theoretical RMS error of 3.8 kg m−2 in clear conditions and 4.9 kg m−2 in cloudy situations. The theoretical RMS error in cloud liquid water path is 0.08 kg m−2. The surface emissivities are retrieved with an accuracy of better than 0.008 in clear conditions and 0.010 in cloudy conditions. Microwave land surface temperature retrieval presents a very attractive complement to the infrared estimates in cloudy areas: time record of land surface temperature will be produced.
Observation and analysis of ice flow in the largest Greenland ice stream
Authors: Joughin Ian , Fahnestock Mark , MacAyeal Doug , Bamber Jonathan L. , Gogineni Prasad
Published: February 2004

We have applied satellite radar interferometry methods to map the velocity field of the recently discovered Northeast Greenland Ice Stream. We have used these data in conjunction with ice thickness and accumulation data to determine that the ice stream is in balance within the measurement errors. We used control methods to invert a finite element model of ice sheet flow constrained by the data to infer the ice stream's basal shear stress distribution. Our results reveal that flow in a section of the downstream end has much in common with the streaming flow of the Ross Ice Streams of West Antarctica (e.g., a weak bed and fast flow in the presence of low driving stresses). For several hundred kilometers along the middle of the ice stream, the basal shear stress balances the driving stress. In the upstream area, where the ice stream is first visible in the velocity data, the bed appears to be weak, which may contribute to the initiation of the ice stream.
Improved ozone profile retrieval from combined nadir/limb observations of SCIAMACHY
Authors: van der A Ronald J.
Published: February 2004

The spectrometer instrument SCIAMACHY is planned to be launched in 2001 to observe trace gas concentrations in the atmosphere. This instrument will alternately observe the same air mass in limb and nadir direction in the wavelength range of 240 to 2400 nm. The combination of these nadir and limb observations is likely to improve the ozone profile observations in the troposphere and stratosphere. In this sensitivity study the method of calculating the retrieval errors and covariances of the ozone profile values will be described, and the results will be presented for a few representative scenarios. It will be shown that the accuracy of ozone profile retrievals based on combined nadir/limb observations will no longer be limited by instrument noise. Therefore the accuracy of the retrieved ozone profiles in troposphere and stratosphere has the potential to be higher than of single‐nadir observations from similar instruments.
Chemical characteristics of air from different source regions during the second Pacific Exploratory Mission in the Tropics (PEM‐Tropics B)
Authors: Maloney Joseph C. , Fuelberg Henry E. , Avery Melody A. , Crawford James H. , Blake Donald R. , Heikes Brian G. , Sachse Glen W. , Sandholm Scott T. , Singh Hanwant , Talbot Robert W.
Published: February 2004

Ten‐day backward trajectories are used to determine the origins of air parcels arriving at locations of airborne DC‐8 chemical measurements during NASA's second Pacific Exploratory Mission in the Tropics B that was conducted during February‐April 1999. Chemical data at sites where the trajectories had a common geographical origin and transport history are grouped together, and statistical measures of chemical characteristics are computed. Temporal changes in potential temperature are used to determine whether trajectories experienced a significant convective influence during the 10‐day period. Trajectories describing the aged marine Southern Hemispheric category remain over the South Pacific Ocean during the 10‐day period, and their corresponding chemical signature indicates very clean air. The category aged marine air in the Northern Hemisphere is found to be somewhat dirtier. Subdividing its trajectories based on the direction from which the air had traveled is found to be important in explaining the various chemical signatures. Similarly, long‐range northern hemispheric trajectories passing over Asia are subdivided depending on whether they had followed a mostly zonal path, had originated near the Indian Ocean, or had originated near Central or South America and subsequently experienced a stratospheric influence. Results show that the chemical signatures of these subcategories are different from each other. The chemical signature of the southern hemispheric long‐range transport category apparently exhibits the effects of pollution from Australia, southern Africa, and South America. Parcels originating over Central and northern South America are found to contain the strongest pollution signature of all categories, due to biomass burning and other sources. The convective category exhibits enhanced values of nitrogen species, probably due to emissions from lightning associated with the convection. Values of various species, including peroxides and acids, confirm that parcels were influenced by the removal of soluble gas and particle species due to precipitation. Finally, current results are compared with those from the first PEM‐Tropics mission that was conducted in the same region during the southern hemispheric dry season (August‐October 1996) when extensive biomass burning occurred. Results show that air samples during PEM‐Tropics B are considerably cleaner than those of its dry season counterpart.
Evidence for short vertical wavelength Kelvin waves in the Department of Energy‐Atmospheric Radiation Measurement Nauru99 radiosonde data
Authors: Holton James R. , Alexander M. Joan , Boehm Matthew T.
Published: February 2004

Stratospheric zonal wind and temperature oscillations observed in a 30 day time series of four times per day radiosonde data taken as part of Nauru99 show a strong negative quadrature relationship at periods of 9.5 days and 5 days. The 9.5‐day waves have vertical wavelength of about 4.5 km, and are deduced to be Kelvin waves of zonal wave number 2; the 5‐day waves have vertical wavelengths in the range of 3–4.5 km, and are deduced to be Kelvin waves of zonal wavenumber 4. Vertical momentum fluxes associated with the 9.5‐ and 5‐day modes are estimated to be 1×10−4 m2s−2 and 2×10−4 m2s−2, respectively, in the 20–25 km altitude range where the mean zonal flow is characteristic of the westerly phase of the quasi‐biennial oscillation (QBO). These are an order of magnitude smaller than momentum fluxes associated with traditional wavenumber 1 Kelvin waves analyzed in the easterly phase of the QBO.
Methane emission from Swedish mires: National and regional budgets and dependence on mire vegetation
Authors: Nilsson Mats , Mikkelä Catharina , Sundh Ingvar , Granberg Gunnar , Svensson Bo H. , Ranneby Bo
Published: February 2004

A national land use inventory program was used to estimate the flux of methane from Swedish mires. During the snow‐free season in 1994, methane fluxes were monitored at 60% (n = 619) of the total number of permanent inventory plots for the four mire vegetation classes included. The methane flux was measured only once at each inventory plot using transparent static chambers, yielding 3157 (89%) accepted flux rates. The temperature during the measurement period was close to the long‐term (30 year) average in the north and much warmer in the south. The precipitation was lower than the long‐term average. Average flux rates, specific for each mire vegetation class and geographical region, varied between 8 and 238 mg CH4 m−2 d−1. The national averages ± SE (mg CH4 m−2 d−1) were hummock communities, 24±6; transitional fens, 15±2; short sedge fens, 49±4; and tall sedge fens, 97±22. To derive the annual flux, the number of days with a daily average air temperature over 5°C was used. The annual methane fluxes for each mire vegetation class and region varied between 1.5 and 40 g CH4 m−2 yr−1. The annual methane fluxes (average ± SE in g m−2 yr−1) for each mire vegetation class for the whole of Sweden were hummock sites, 4.9±1.3; transitional fens, 2.5±0.5; low sedge fens, 8.2±0.8; and tall sedge fens, 16.5±3.6. The methane flux (±95% confidence limits) in 1994 from the monitored mire types in Sweden was 0.29±0.05 Tg CH4. The low and tall sedge mires accounted for 96% of the methane emitted.
Assessment of the Uccle ozone sounding time series quality using SAGEII data
Authors: Lemoine René , De Backer Hugo
Published: February 2004

Vertical ozone profiles measured with balloon sondes at Uccle are compared with version 5.96 ozone profiles retrieved by the SAGEII instrument to investigate instrumentally induced breaks and/or trends in the Uccle ozone sounding time series. This study covers the period 1985–1999, during which several changes took place in the ozone sounding procedure and processing method. These changes introduced breaks in the time series of ozone soundings, and a number of corrections have been developed that are not part of the standard processing method for ozone soundings. In this paper, we review the standard corrections applied to ozone soundings and describe additional corrections developed at Uccle. The improvement of the ozonesonde data set against SAGE II ozone measurements is shown. Jumps of up to 6% in the relative difference between SAGEII and ozonesondes are removed with corrections taking into account quality changes of the ozonesonde and the radiosonde. New corrections and refinements to the existing correction procedure reduce the drift between SAGEII and ozonesondes from −0.51% yr−1 to −0.07% yr−1 between 17 and 22 km with nonsignificant values at the 2σ level at all altitudes.
Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo‐Asian haze
Authors: Ramanathan V. , Crutzen P. J. , Lelieveld J. , Mitra A. P. , Althausen D. , Anderson J. , Andreae M. O. , Cantrell W. , Cass G. R. , Chung C. E. , Clarke A. D. , Coakley J. A. , Collins W. D. , Conant W. C. , Dulac F. , Heintzenberg J. , Heymsfield A. J. , Holben B. , Howell S. , Hudson J. , Jayaraman A. , Kiehl J. T. , Krishnamurti T. N. , Lubin D. , McFarquhar G. , Novakov T. , Ogren J. A. , Podgorny I. A. , Prather K. , Priestley K. , Prospero J. M. , Quinn P. K. , Rajeev K. , Rasch P. , Rupert S. , Sadourny R. , Satheesh S. K. , Shaw G. E. , Sheridan P. , Valero F. P. J.
Published: February 2004

Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo‐Asian haze at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one‐ and four‐dimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long‐range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single‐scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (±10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo‐Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (‐20±4 W m−2) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate.
Antarctic surface mass balance and systematic biases in general circulation models
Authors: Genthon Christophe , Krinner Gerhard
Published: February 2004

Atmospheric general circulation models (GCMs) simulate two of the main components of the Antarctic surface mass balance (SMB), precipitation and sublimation, which are generally assumed to dominate the SMB. Resemblances between the Antarctic SMB simulated by seven different GCMs run at high (≈100–200 km) resolution, and differences with a recently produced observation‐based map are analyzed. A number of these differences are common to all seven models. They are called systematic model biases and are summarized as a composite of all seven models. It is found unlikely that higher model resolution would significantly affect the systematic biases. All but one of the models studied here use an inaccurate prescribed topography of Antarctica, with errors as large as 1000 m. Although wrong topography does not seem to consistently explain model SMB biases, it is strongly recommended that the Antarctic topography in GCMs be updated. Wind erosion and drifting snow are not simulated in GCMs. Because the processes of wind erosion are complex and nonlinear, evaluation of its possible contribution to systematic model biases is not straightforward. Partial correspondence between regions of strongest winds and model biases suggest that wind erosion may contribute and should be formulated in GCMs. Sublimation is another significant potential negative term in the SMB of Antarctica, but it does not seem to explain any systematic model bias. Finally, it is proposed that some of the systematic differences between models and observation‐based maps actually signal errors in the latter rather than in the models. These errors occur in regions devoid of field observation. They may thus be related to the process of interpolation used to build the SMB maps.
Marine latitude/altitude OH distributions: Comparison of Pacific Ocean observations with models
Authors: Davis D. , Grodzinsky G. , Chen G. , Crawford J. , Eisele F. , Mauldin L. , Tanner D. , Cantrell C. , Brune W. , Tan D. , Faloona I. , Ridley B. , Montzka D. , Walega J. , Grahek F. , Sandholm S. , Sachse G. , Vay S. , Anderson B. , Avery M. , Heikes B. , Snow J. , O'Sullivan D. , Shetter R. , Lefer B. , Blake D. , Blake N. , Carroll M. , Wang Y.
Published: February 2004

Reported here are tropical/subtropical Pacific basin OH observational data presented in a latitude/altitude geographical grid. They cover two seasons of the year (spring and fall) that reflect the timing of NASA's PEM‐Tropics A (1996) and B (1999) field programs. Two different OH sensors were used to collect these data, and each instrument was mounted on a different aircraft platform (i.e., NASA's P‐3B and DC‐8). Collectively, these chemical snapshots of the central Pacific have revealed several interesting trends. Only modest decreases (factors of 2 to 3) were found in the levels of OH with increasing altitude (0–12 km). Similarly, only modest variations were found (factors of 1.5 to 3.5) when the data were examined as a function of latitude (30°N to 30°S). Using simultaneously recorded data for CO, O3, H2O, NO, and NMHCs, comparisons with current models were also carried out. For three out of four data subsets, the results revealed a high level of correspondence. On average, the box model results agreed with the observations within a factor of 1.5. The comparison with the three‐dimensional model results was found to be only slightly worse. Overall, these results suggest that current model mechanisms capture the major photochemical processes controlling OH quite well and thus provide a reasonably good representation of OH levels for tropical marine environments. They also indicate that the two OH sensors employed during the PEM‐Tropics B study generally saw similar OH levels when sampling a similar tropical marine environment. However, a modest altitude bias appears to exist between these instruments. More rigorous instrument intercomparison activity would therefore seem to be justified. Further comparisons of model predictions with observations are also recommended for nontropical marine environments as well as those involving highly elevated levels of reactive non‐methane hydrocarbons.