bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of the low to high NOxtransition in a simplified tropospheric photochemical model
Authors: Stewart Richard W.
Published: February 2004

A simplified photochemical model designed to capture the basic properties of the tropospheric Ox, HOx, NOx, CO, CH4system is used to study the transition between low and high NOxconditions. These regimes are characterized by substantially different chemical properties and by the existence of a single, stable steady state for each parameter set consistent with low or high NOxconditions. In the parameter space of the system, low and high NOxconditions are separated by a transition zone which corresponds to multiple and/or unstable solutions in the state space of the system. This paper discusses a number of different transitions between low and high NOxconditions which occur as NO and CO source strengths are varied. Emphasis is on the consequences of the existence of a region in parameter space which corresponds to unstable steady states and stable limit cycles in state space. These limit cycles are oscillatory solutions with periods ranging from hundreds to thousands of years. They consist of low and high NOxphases of relatively slowly varying composition separated by transitions in which rapid changes in composition occur. The characteristics of these phases are described in terms of an ozone production cycle based on the chemical reactions assumed to take place. Depending on the phase of the limit cycle, ozone may accelerate or inhibit its own production. The former case represents an autocatalytic cycle with possible “runaway” growth of oxidants. The transition between low and high NOxconditions is described with the aid of an “autocatalytic ratio” defined in the text.
The tropospheric distribution and budget of ethane
Authors: Rudolph J.
Published: February 2004

From about 1500 measurements of ethane in the remote troposphere the longitudinally and vertically averaged latitudinal and seasonal variability of ethane was derived. To improve the data coverage, several data sets from literature were included. There are only very few data sets available for the southern hemisphere. Nevertheless, the uncertainty of the average seasonal/latitudinal ethane profile is estimated to less than 30%. The global annually averaged ethane mixing ratio is 860 ppt. There is a strong interhemispheric gradient with an average north/south ratio of 3.5. Within the northern hemisphere there is an average gradient from the highest annual mean value of 2500 ppt around 65°N to about 600 ppt at the equator. In the southern hemisphere there is only a small gradient at low latitudes and at middle and high southern latitudes no significant gradient can be seen. In both hemispheres a significant seasonal cycle with highest mixing ratios in late winter is observed. The ethane source strength needed to balance the atmospheric budget of ethane is estimated to 15.5 Tg/yr, with most of the emissions in the northern hemisphere. An independent estimate of the sources indicate that most of the emissions are due to natural gas losses (6 Tg/yr) and biomass burning (6.4 Tg/yr). This is also compatible with the latitudinal and seasonal variation of the atmospheric ethane removal rates. However, these estimates have substantial uncertainties and it should be noted that the role of the biosphere for the atmospheric budget of ethane is presently not understood.
Satellite‐derived surface temperatures with boundary layer temperatures and geostrophic winds to estimate surface energy fluxes
Authors: Crago Richard , Sugita Michiaki , Brutsaert Wilfried
Published: February 2004

The surface sensible heat flux, H, and the latent heat flux, LE, were determined under unstable conditions for the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE) site in eastern Kansas. NOAA 9 and 11 AVHRR radiative surface temperatures, corrected for atmospheric effects using the split window technique, were used for the surface temperature in a bulk atmospheric boundary layer similarity formulation for H. Mean vertically integrated mixed layer temperatures, determined from radiosonde data, were also required in this technique. However, unlike previous applications, the present approach avoids the need for direct wind speed measurements by using instead the geostrophic wind speed, VG, as derived from the routine measurements of the surface pressure field. LE was then determined as the residual term of the surface energy balance, using H from the bulk similarity formulation. Under the unstable conditions considered, results for both H and LE were comparable to those found in previous work by means of measured wind speeds with the same FIFE data. It is shown that for slight to moderate instabilities a 1% error in the friction velocity generally produces less than 1% error in H.
Classification of clouds over the western equatorial Pacific Ocean using combined infrared and microwave satellite data
Authors: Liu Guosheng , Curry Judith A. , Sheu Rong‐Shyang
Published: February 2004

A new cloud classification scheme is presented that combines infrared and microwave satellite data. Because microwave radiation can penetrate deep into the cloud layer, this scheme is able to determine characteristics for both thin and deep clouds. Additionally, the new scheme can provide information on precipitation, which traditional infrared‐visible cloud classification schemes have been unable to. The proposed cloud classification scheme utilizes the cloud top temperature obtained from infrared measurements and a microwave index that includes both emission and scattering signals. The following eight cloud classes are defined: warm nonprecipitating cloud, warm precipitating cloud, midtop nonprecipitating cloud, midtop precipitating cloud, thin high‐top nonprecipitating cloud, deep high‐top nonprecipitating cloud, anvil with stratiform precipitating cloud, and deep convective precipitating cloud. The classification scheme is validated by aircraft radar data obtained from Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment. The new cloud classification scheme is used to investigate the clouds in the western equatorial Pacific Ocean warm pool region for the period from November 1992 to February 1993, allowing us to obtain for the first time the spectra of cloud coverage and precipitation distribution as a function of cloud types. The analysis shows that most of the cloudiness in this region is associated with warm nonprecipitating clouds. Precipitating pixels comprise only about 15% of total cloud pixels. Of the precipitating pixels, about 50% have cloud top temperatures warmer than −40°C. Using rainfall rates obtained from microwave satellite data, the relative contribution by each precipitating cloud type is investigated. It is found that the precipitating clouds with cloud top temperature warmer than −40°C contribute at least 23% of the total rainfall amount, while the remaining 77% of total precipitation is divided almost equally by anvil with stratiform precipitating cloud and the deep convective precipitating cloud.
Ice cores as an index of global volcanism from 1850 to the present
Authors: Robock Alan , Free Melissa P.
Published: February 2004

To evaluate an important cause of past climate change, climate researchers need reliable estimates of volcanic aerosol loading in the atmosphere. Previous indices, the dust veil index, volcanic explosivity index, and those by Mitchell (1970), Sato et al. (1993) and Khmelevtsov et al. (unpublished manuscript, 1993), all have drawbacks. Ice core acidity and sulfate records, because they contain physical evidence of atmospheric loading, are a promising source of information on past volcanic aerosols, but these records contain large nonvolcanic signals as well. We have compared northern hemisphere (NH) and southern hemisphere (SH) annual‐average versions of five indices with eight NH and six SH ice core records for the period 1850 to the present in an attempt to identify the volcanic signal common to all records. The indices are all highly correlated with each other. For the NH, although the individual ice core records are, in general, not well correlated with each other or with any of the indices, a composite derived from averaging the cores, the ice core‐volcano index (IVI), shows promise as a new index of volcanic aerosol loading. This new index correlates well with the existing non ice core volcanic indices and with high‐frequency temperature records. For the SH the individual ice cores and indices are better correlated. The SHIVI is again highly correlated with all indices and individual ice cores but not with high‐frequency temperature records. For both hemispheres the Southern Oscillation index shows no significant correlation with the volcanic indices, ice cores or the IVI, thus providing no evidence for the impact of volcanic eruptions on El Niño/Southern Oscillation events.
Horizontal and vertical transport of air over southern Africa
Authors: Garstang M. , Tyson P. D. , Swap R. , Edwards M. , Kållberg P. , Lindesay J. A.
Published: February 2004

Tropospheric air trajectories that occurred during the Southern African Fire‐Atmosphere Research Initiative (SAFARI) in August–October 1992 are described in terms of a circulation classification scheme and the vertical stability of the atmosphere. Three major and frequently occurring stable discontinuities are found to control vertical transport of aerosols in the subtropical atmosphere at the end of the dry season. Of these, the main subsidence‐induced feature is a spatially ubiquitous and temporally persistent absolutely stable layer at an altitude of about 5 km (3.5 km above the interior plateau elevation). This effective obstacle to vertical mixing is observed to persist without break for up to 40 days. Below this feature an absolutely stable layer at 3 km (1.5 km above the surface) prevails on and off at the top of the surface mixing layer for up to 7 days at a time, being broken by the passage of regularly occurring westerly wave disturbances. Above the middle‐level discontinuity a further absolutely stable layer is frequently discerned at an altitude of about 8 km. It is shown that five basic modes can be used to describe horizontal aerosol transportation fields over southern Africa. Dominating these is the anticyclone mode which results in frequent recirculation at spatial scales varying from hundreds to thousands of kilometers. In exiting the anticyclonic circulation, transport on the northern periphery of the system is to the west over the Atlantic Ocean via a semistationary easterly wave over the western part of the subcontinent. On the southern periphery, wave perturbations in the westerly enhance transports which exit the subcontinent to the east into the Indian Ocean. Independently derived data suggest that during SAFARI only 4% of the total transport of air from three locations south of 18°S was into the Atlantic Ocean. Over 90% of the transport was into the Indian Ocean across 35°E. This result reflects circulation fields typical of the extremely dry conditions prevailing in 1992. The integrated effect of the control exerted by atmospheric stability on vertical mixing, on the one hand, and the nature of the horizontal circulation fields, on the other, is to produce a distinctive suite of transport patterns that go a long way to explain the observed high concentrations of tropospheric aerosols and trace gases observed over the subcontinent in winter and spring, as well as over the tropical South Atlantic and southwestern Indian Oceans.
Stratospheric aerosol effective radius, surface area and volume estimated from infrared measurements
Authors: Grainger R. G. , Lambert A. , Rodgers C. D. , Taylor F. W. , Deshler T.
Published: February 2004

A technique is presented for estimating the effective radius, surface area density, and volume density of stratospheric aerosols from infrared emission measurements. These parameters are required to assess the perturbation of the climate and chemical balance of the stratosphere following the largest volcanic eruption so far this century: that of Mount Pinatubo in the Philippines. The method uses a relationship between the surface area density and the volume density derived from balloon‐borne measurements of the Mount Pinatubo aerosol cloud made at Laramie, Wyoming. It is shown that the aerosol emission value is well approximated by a linear function of effective radius and aerosol volume density. The technique relies on knowing the refractive index of the aerosol cloud, which is assumed to be composed of liquid spheres of sulphuric acid and water. It is shown that the uncertainties in the current knowledge of the refractive index of sulphuric acid solutions limit the accuracy of the inversion technique. As a case study, the aerosol effective radius, surface area density, and volume density are determined from emission measurements at 12.1 μm of the Mount Pinatubo aerosol cloud made by the improved stratospheric and mesospheric sounder (ISAMS) carried on the Upper Atmospheric Research Satellite (UARS). From these measurements it is shown that five months after the eruption the core of the Mount Pinatubo cloud had a size distribution with an effective radius of 0.5 μm, a surface area density of 35 μm2 cm−3, and a volume density of 6 μm3 cm−3.
Post‐Industrial Revolution changes in large‐scale atmospheric pollution of the northern hemisphere by heavy metals as documented in central Greenland snow and ice
Authors: Candelone Jean‐Pierre , Hong Sungmin , Pellone Christian , Boutron Claude F.
Published: February 2004

Pb, Zn, Cd and Cu have been measured using ultraclean procedures in various sections of a 70.3‐m snow/ice core covering the past 220 years (including the Industrial Revolution) drilled at Summit, central Greenland. These time series are the first reliable ones ever published for Zn, Cd, and Cu; for Pb they are the first verification of the pioneering data published more than two decades ago by C. Patterson and his coworkers [Murozumi et al., 1969]. For all four heavy metals, concentrations are found to have markedly increased up until the 1960s and 1970s before decreasing significantly during the following few decades. The timing and the amplitude of the observed changes differ significantly however from one metal to another. Comparison with concentration values obtained by analyzing ancient Holocene ice dated 7760 years B.P., that is, before humans started to impact on the atmosphere, show that no detectable increase occurred for Zn, Cd, and Cu before the Industrial Revolution. On the other hand, Pb concentrations were already one order of magnitude above natural values in late 18th century ice. Cumulative deposition of heavy metals to the whole Greenland ice cap since the Industrial Revolution ranges from 3200 t for Pb to 60 t for Cd.
Three dimensional atmospheric transport simulation of the radioactive tracers 210Pb, 7Be, 10Be, and 90Sr
Authors: Rehfeld Stefan , Heimann Martin
Published: February 2004

A global three‐dimensional transport model of the atmosphere, having a grid resolution of 7.83° (latitude) × 10° (longitude) × 19 layers in the vertical and driven by European Centre for Medium Range Weather Forecasts (ECMWF) wind field analyses of 1990, is used to simulate the distribution of natural (210Pb, 7Be, 10Be) and artificial (90Sr), radioactive, water‐soluble, aerosol‐borne tracers. Because of their well‐known source‐sink distribution these tracers build an ideal tool to depict transport processes in the whole atmosphere and to test the models ability of reproducing these. In particular, this paper focuses on mass exchange between stratosphere and troposphere, using the concentration ratio 10Be/7Be as an indicator. In general, the agreement between observations and model results is quite good, except in polar regions. Modeled arctic 210Pb and 7Be concentrations are overestimated and the annual cycle is underestimated. The modeled antarctic annual cycle of 10Be/7Be shows maxima in winter and spring, whereas the observations exhibit a summer maximum. These discrepancies are attributed to deficiencies in ECMWF wind fields, to differences between observed and model used precipitation, and to the employed parameterization schemes of dry and wet deposition which may be inadequate in the cold polar regions.
Low ozone in the marine boundary layer of the tropical Pacific Ocean: Photochemical loss, chlorine atoms, and entrainment
Authors: Singh H. B. , Gregory G. L. , Anderson B. , Browell E. , Sachse G. W. , Davis D. D. , Crawford J. , Bradshaw J. D. , Talbot R. , Blake D. R. , Thornton D. , Newell R. , Merrill J.
Published: February 2004

Aircraft measurements of ozone, its key precursors, and a variety of chemical tracers were made in the troposphere of the western and central Pacific in October 1991. These data are presented and analyzed to examine the occurrence of low ozone concentrations in the remote marine boundary layer of the tropical and equatorial Pacific Ocean. The data from these flights out of Guam, covering an area extending from the equator to 20°N and from south of the Philippines to Hawaii, show average O3 concentrations as low as 8–9 ppb (ppb = 10−9 v/v) at altitudes of 0.3–0.5 km in the boundary layer. Individual measurements as low as 2–5 ppb were recorded. Low O3 concentrations do not always persist in space and time. High O3, generally associated with the transport of upper tropospheric air, was also encountered in the boundary layer. In practically all cases, O3 increased to values as large as 25–30 ppb within 2 km above the boundary layer top. Steady state model computations are used to suggest that these low O3 concentrations are a result of net photochemical O3 destruction in a low NO environment, sea surface deposition, and low net entrainment rates (3.6 ± 1.7 mm s−1) from the free troposphere. Day/night measurements of select organic species (e.g., ethane, propane, C2Cl4) suggest that Cl atom concentrations in the vicinity of 105 molecules cm−3 may be present in the marine boundary layer in the early morning hours. This Cl atom abundance can only be rationalized if sea‐salt aerosols release active chlorine (Cl2 or HOCl) to the gas phase when exposed to sunlight. These Cl atom concentrations, however, are still insufficient, and halogen chemistry is not likely to be an important contributor to the observed low O3.
Estimating the fraction of absorbed photosynthetically active radiation (ƒAPAR) at FIFE with airborne bidirectional spectral reflectance data
Authors: de Colstoun E. C. Brown , Walthall C. L. , Russell C. A. , Irons J. R.
Published: February 2004

The relationship between the fraction of absorbed photosynthetically active radiation (ƒAPAR) and spectral vegetation indices (SVIs) is affected by various factors, including background effects and the view zenith angle of the sensor. This study investigated the effects of estimating ƒAPARfrom multiple off‐nadir airborne measurements acquired by the advanced solid‐state array spectroradiometer (ASAS) during the First International Satellite Land Surface Climatology Project Field Experiment. Using atmospherically corrected data, ƒAPARwas estimated with both directional SVIs (normalized difference vegetation index (NDVI) and simple ratio (SR)) and PAR hemispherical reflectance (RPAR) calculated from PAR bidirectional reflectance factors for the seven viewing angles of ASAS. Very weak linear relationships were found between the ground‐measured total ƒAPARvalues and ASAS measurements, assumed to be due to the presence of varying amounts of senescent plant materials in the canopy. Exclusion of data collected in October when the canopy was fully senescent somewhat improved these relationships. However, correction of total ƒAPARvalues by the fraction of green vegetation present (green ƒAPAR) provided substantial improvement. For both SVIs the highest coefficients of determination (r2= 0.839 for NDVI;r2= 0.890 for SR) were found at 15° in the backscattering direction decreasing to minimum values at ±45°, a trend thought to be associated with the effect of canopy structure on the SVIs. AlthoughRPARwas not strongly correlated with either the total or the green ƒAPAR, it was related to the independently measured PAR flux reflected from the canopy indicating a potential for deriving PAR hemispherical reflectance from directional measurements for natural cover types such as grasslands.
Measurements of 3‐methyl furan, methyl vinyl ketone, and methacrolein at a rural forested site in the southeastern United States
Authors: Montzka Stephen A. , Trainer Michael , Angevine Wayne M. , Fehsenfeld Fred C.
Published: February 2004

Three oxygenated hydrocarbons were measured in ambient air above a rural forested site in western Alabama. Mixing ratios of methyl vinyl ketone (MVK), methacrolein (MACR), and 3‐methyl furan were determined during a 3‐week period in the summer of 1992. While the mean mixing ratio for methyl vinyl ketone was determined to be 740 parts per 1012 (ppt), the mean mixing ratio for methacrolein was 480 ppt. The results for methyl vinyl ketone and methacrolein are compared to previously reported measurements from this same location during the summer of 1990. Although isoprene levels were lower by ∼50% in 1992, mixing ratios of the carbonyls were reduced by only ∼25%. These differences are discussed in light of the changes that had occurred to the forest canopy in the time elapsed between the two measurement campaigns. Despite the differences observed during the 2 years, a consistent diurnal variability is observed for methyl vinyl ketone, relative to methacrolein, during both years. In addition to a weak correlation observed between the carbonyls and temperature, levels of MVK and MACR in air sampled from just above the forest canopy are influenced by the depth of the mixed layer. A third oxidation product of isoprene, 3‐methyl furan, was identified and measured in ambient air. Mean daytime mixing ratios were estimated at 60 ppt. Nighttime levels were lower, averaging 40 ppt. Mixing ratios of 3‐methyl furan were highly correlated with isoprene (R2 = 0.82), and during the day, were typically ∼2% of levels measured for isoprene.
On the origin and nature of the interannual variability of the winter stratospheric circulation in the northern hemisphere
Authors: Kodera Kunihiko
Published: February 2004

During the northern hemisphere winter, the stratospheric circulation exhibits quite large interannual variability. In the present study, the nature and origin of the interannual variation in the winter stratosphere is studied with two different approaches: one is composite analysis assuming a priori four possible causes: solar activity, equatorial quasi‐biennial oscillation (QBO), volcanic aerosols, and unknown cause producing trends, while the other is an extended empirical orthogonal function analysis (E‐EOF). The results of composite analysis show that in November, anomalous zonal‐mean zonal winds and meridional temperature gradients are found in accordance with the assumed forcings in different regions in the stratosphere. As winter progresses, however, anomalous patterns in zonal wind become similar by forming a dipole‐type pattern and extend down into the troposphere. The common features of the evolution of the anomalies during the winter can be well explained by the first mode of the E‐EOF. The results of the present study suggest that interannual variability in the winter stratosphere in the northern hemisphere can be considered an internal mode of variability triggered by changes in forcing, such as solar activity, equatorial QBO, and volcanic aerosols. Although overall features of the evolution of zonal‐mean zonal wind associated with trends are similar, the cause of the trends is still undetermined.
UV/visible and IR absorption cross sections of BrONO2
Authors: Burkholder James B. , Ravishankara A. R. , Solomon Susan
Published: February 2004

The absorption cross sections of BrONO2 between 200 and 500 nm were measured over the temperature range 298 to 220 K using a diode array spectrometer. The BrONO2 absorption cross sections are weakly dependent on temperature at wavelengths < 450 nm (less than 10% change between 298 and 220 K) but decrease rapidly with decreasing temperature at wavelengths > 450 nm; at 480 nm the cross section decreased by ∼35% in going from 298 K to 220 K. Our room temperature absorption cross sections are in good agreement with the measurements of Spencer and Rowland (1978) over the common wavelength range of the measurements, 200 to 390 nm. We show that wavelengths longer than 390 nm must be included in the calculation of the BrONO2 atmospheric photolysis rate. We also show that photolysis of BrONO2 could be a significant atmospheric loss process for odd oxygen (due to halogen chemistry) below about 25 km. The infrared absorption cross sections for the BrONO2 band centered at 803.3 cm−1 were also measured. The integrated band strength was (2.7±0.5) × 10−17 cm2 molecule−1 cm−1.
Isentropic cross‐tropopause mass exchange in the extratropics
Authors: Chen Ping
Published: February 2004

The extratropical stratosphere‐troposphere S/T exchange is investigated on isentropic surfaces that intersect the tropopause, using a semi‐Lagrangian transport model and analyzed winds from the European Center for Medium‐Range Weather Forecasts (ECMWF). It is found that the characteristics of extratropical S/T exchange exhibit two distinctive regimes. On the 330 K isentropic surface and below, S/T exchange occurs vigorously in all seasons, mainly caused by the irreversible mixing and transport by breaking synoptic‐scale baroclinic disturbances. However, on and above the 340 K isentropic surface, S/T exchange exhibits a strong annual cycle where very little S/T exchange takes place in the winter hemisphere, but significant S/T exchange occurs in the summer hemisphere, particularly in the northern summer. It is argued that the weak S/T exchange in the winter hemisphere is mainly due to the barrier effect of the strong potential vorticity (PV) gradient at the tropopause and the active S/T exchange in the summer hemisphere is mainly associated with the summer Asian and Mexican monsoons in the northern summer and the Australia and South American monsoons in the southern summer.
Precipitation charge and size measurements in the stratiform region of two mesoscale convective systems
Authors: Bateman Monte G. , Rust W. David , Smull Bradley F. , Marshall Thomas C.
Published: February 2004

We measured the charge and size of precipitation particles with instrumented free balloons in the trailing stratiform regions of two different mesoscale convective systems (MCSs) in Oklahoma. Each of these stratiform regions had a different type of vertical electric field structure, one of each of the two types commonly found in MCS stratiform regions. Using an instrument that measured precipitation charge from 4 to 400 pC and equivalent diameters from 0.8 to 8.0 mm, we found that (1) the particles measured were not the major charge carriers in these storms, implying that smaller particles were responsible for the measured field, (2) there was no obvious relation between precipitation charge and size or polarity and size, (3) the precipitation size distribution in and near the transition zone consisted of two independent populations, one centered at 2.6 mm and one at 4.9 mm, (4) the two lower charge layers in these storms were likely created by in situ charging processes, and (5) net precipitation charge density was small because nearly equal amounts of positive and negative precipitation charge existed at all altitudes measured.
Improving the large‐scale modeling of the saltation flux of soil particles in presence of nonerodible elements
Authors: Alfaro S. C. , Gomes L.
Published: February 2004

Stress partition between nonerodible obstacles and the underlying particle bed greatly influence erosion by wind in dry areas. Several studies have already been dedicated to this problem but their results cannot be used in large‐scale modeling of the saltation flux because they always involve microscale parameters. Experiments are carried out in a wind tunnel to find a simplified expression of the stress partition coefficient, ƒ, defined by Arya. Marbles of three different diameters are used to constitute nonerodible elements arrays of various densities. They are successively laid on three substrates of varying roughness. Results deduced from wind profile analysis then show that ƒ can be expressed simply as a function of the roughness lengths of the overall terrain and of the underlying substrate, which can be determined relatively simply. The determination of ƒ allows us to propose a more accurate calculation of the saltation flux for large‐scale models of dust production.
Hypercanes: A possible link in global extinction scenarios
Authors: Emanuel Kerry A. , Speer Kevin , Rotunno Richard , Srivastava Ramesh , Molina Mario
Published: February 2004

Bolide impacts and large‐scale volcanic eruptions have been proposed as possible causes of the massive extinction of life that has occurred episodically in Earth's history. In spite of the catastrophic disruption of the local environment that accompanies bolide impacts and volcanic eruptions, it has been difficult to explain why these events sometimes lead to global extinction of species. We propose that, in some cases, the missing link may be provided by hypercanes, runaway hurricanes that are capable of injecting massive amounts of water and aerosols into the middle and upper stratosphere, where they may have profound effects on atmospheric chemistry and radiative transfer. Hypercanes are theorized to occur when the sea surface temperature exceeds a critical threshold, which may occur when sea water is locally heated by bolide impact, shallow‐sea volcanism, or possibly, by overturning of superheated brine pools formed by underwater volcanic activity. Simulations using a convection‐resolving nonhydrostatic, axisymmetric numerical model show that hypercanes can indeed develop when the sea surface temperature is high, and that they inject large amounts of mass into the stratosphere.
Precipitation and radiation modeling in a general circulation model: Introduction of cloud microphysical processes
Authors: Boucher Olivier , Le Treut Hervé , Baker Marcia B.
Published: February 2004

Cloud microphysical processes are introduced in the precipitation parameterization of a general circulation model (GCM). Three microphysical processes are included in this representation of warm cloud precipitation: autoconversion of droplets, collection of droplets by falling raindrops, and evaporation of raindrops falling in clear sky. The mean droplet radius, r, is calculated from the cloud water mixing ratio, which is computed in the model, and the cloud droplet number concentration, N, which is prescribed. The autoconversion rate is set to zero for r < r0, a prescribed threshold mean droplet radius. We investigate the model sensitivity to r0 and to N, the cloud droplet concentration, which is linked to the concentration of cloud condensation nuclei and is likely to vary. We find that an increase in N leads to an increase in the amount of cloud water stored in the atmosphere. In further experiments the mean droplet radius used in the parameterization of cloud optical properties is calculated in the same way as in the precipitation parameterization in order to bring more consistency between the different schemes. We again investigate the model sensitivity to r0 and to N and we find that an increase in N significantly enhances cloud albedo.
Changes in Antarctic stratospheric aerosol characteristics due to volcanic eruptions as monitored by the Stratospheric Aerosol and Gas Experiment II satellite
Authors: Saxena V. K. , Anderson John , Lin N.‐H.
Published: February 2004

An estimated 20–30 megatons of SO2 and crustal material was injected into the stratosphere during June 12–16, 1991, by the eruption of Mount Pinatubo (15.1°N, 120.4°E). The impact on Antarctic aerosol characteristics is of utmost concern owing to the seasonality in the observed ozone depletion and climate implications. This study focuses on Antarctic stratospheric aerosol characteristics during three temporal periods: September 23–30, September 30 to October 13, and November 13–27, 1991, at latitudes poleward of 60°S for vertically averaged characteristics, and at latitudes poleward of 50°S for temporal and spatial characteristics. Stratospheric aerosol characteristics are inferred from the Stratospheric Aerosol and Gas Experiment (SAGE) II measurements using a modified randomized minimization search technique (RMST). Aerosol characteristics such as size distribution, number concentration, mass loading, surface area concentration, and radial characteristics are derived between 15 and 30 km for particles having radii between 0.1 and 0.8 μm. Results indicate that aerosol size distributions between 15 and 30 km are bimodal in several instances for all three time periods and can be fitted with the sum of two lognormal distributions. Larger concentrations are observed for particles of all sizes between 18 and 30 km during November 1991, signaling the arrival of the Mount Pinatubo plume. An order of magnitude increase in concentration is observed for particles with radii between 0.1 and 0.2 μm and between 0.7 and 0.8 μm. Vertical aerosol profiles show that the peak in aerosol concentration shifted to a higher altitude between 21 and 26 km as compared to the preplume peak between 15 and 18 km. Using the displacement as a function of time for a mass loading of 1.7 μg m−3 isopleth, we estimated meridional velocity ≈0.9 m s−1, zonal velocity ≈16 m s−1, and downward vertical velocity of 0.5 cm s−1 during September to mid‐October, 1991, and 0.3 cm s−1 during mid to late November 1991. The mass deposition flux was estimated to be 9×10−13 g cm−2 s−1. The peeling of segments of the Pinatubo aerosol was found near the outer edge of the circumpolar vortex wall centered around 56°S. Meridional wave activity was also observed poleward of 56°S. Two meridional separation regimes were found: between 56° and 62°S and between 62° and 66°S. These two regions represent different segments of the vortex edge. In the first region we found wavelike isopleths with rising motions between 55° and 58°S and sinking motions thereafter. In the second region, rising motions were located between 60° and 63°S and sinking motions at latitudes poleward of 63°S. In contrast to previous reports, we found an indication of entrainment of volcanic aerosol into the vortex edge. The Mount Pinatubo aerosol is an effective tracer of south polar transport.
On the atmospheric impact of launching the Ariane 5 rocket
Authors: Jones A. E. , Bekki S. , Pyle J. A.
Published: February 2004

Calculations have been performed in a two‐dimensional chemical‐radiative‐transport model of the atmosphere aimed at investigating the atmospheric response to launching the Ariane 5 rocket. Various pollutants emitted from the rocket were studied: inorganic chlorine, water vapour, and aluminium oxide. The calculations also considered whether the atmospheric response differs according to the form in which inorganic chlorine is released. Both long‐term global and short‐term local responses were investigated. After 20‐model years of inorganic chlorine and water vapour emissions, with 10 launches per year, only small losses of ozone, of the order of less than 0.1%, were calculated. These losses were independent of the form of inorganic chlorine. Recovery of the atmosphere after the cessation of launches was rapid, and all but complete within 5 years. Local ozone loss after a single rocket launch was also limited, but the magnitude was dependent upon the form in which chlorine was released. The effect of releasing Al2O3 from Ariane 5 was a modest increase in the sulphate aerosol mass mixing ratio throughout most of the stratosphere.
Analysis of two balloon experiments in coincidence with SAGE II in case of large stratospheric aerosol amount: Post‐Pinatubo period
Authors: Brogniez C. , Lenoble J. , Herman M. , Lecomte P. , Verwaerde C.
Published: February 2004

The stratospheric aerosol layer is regularly monitored since 1979 by satellite experiments as Stratospheric Aerosol Measurement (SAM II) and Stratospheric Aerosol and Gas Experiment (SAGE I and II). The satellite spectral measurements of transmission permit one to obtain the spectral variations of extinction from which the aerosol size distribution may be derived. Several correlative measurements with a balloon‐borne polarimeter, Radiomètre Ballon (Radibal), were conducted during the post‐El Chichon period to validate the satellite measurements. The radiance and polarization diagrams of the sunlight scattered by the aerosols, measured from the balloon, were analyzed within the primary scattering approximation, and the retrieved aerosol size distribution was compared with the satellite results. Following the eruption of Mount Pinatubo (June 15, 1991), two flights of Radibal were planned to allow a validation in case of large aerosol amount. Because of the importance of the multiple scatterings the usual inversion of the Radibal measurements was no more valid. We propose here an improved scheme to evaluate the coherence of both SAGE II and Radibal that gives excellent results.
Ozone in the middle atmosphere as measured by the improved stratospheric and mesospheric sounder
Authors: Connor Brian J. , Scheuer C. J. , Chu D. A. , Remedios J. J. , Grainger R. G. , Rodgers C. D. , Taylor F. W.
Published: February 2004

The improved stratospheric and mesospheric sounder (ISAMS) made ozone measurements in the stratosphere and mesosphere, with dense coverage over a broad range of latitudes, on about 180 days between September 1991 and July 1992. In this paper we are concerned with version 10 of the ISAMS data, which is recommended for use in the pressure range 10–0.1 mbar at night and 10–1 mbar during daytime. We describe the measurements and the retrieval algorithm, present an error analysis, and provide comparisons of the results to both climatological and coincident data. It is demonstrated that biases with respect to other well‐validated data sets are less than 10%, which is well within the internal assessment of systematic errors.
Methyl halide emissions from savanna fires in southern Africa
Authors: Andreae M. O. , Atlas E. , Harris G. W. , Helas G. , de Kock A. , Koppmann R. , Maenhaut W. , Manø S. , Pollock W. H. , Rudolph J. , Scharffe D. , Schebeske G. , Welling M.
Published: February 2004

The methyl halides, methyl chloride (CH3Cl), methyl bromide (CH3Br), and methyl iodide (CH3I), were measured in regional air samples and smoke from savanna fires in southern Africa during the Southern Africa Fire‐Atmosphere Research Initiative‐92 (SAFARI‐92) experiment (August–October 1992). All three species were significantly enhanced in the smoke plumes relative to the regional background. Good correlations were found between the methyl halides and carbon monoxide, suggesting that emission was predominantly associated with the smoldering phase of the fires. About 90% of the halogen content of the fuel burned was released to the atmosphere, mostly as halide species, but a significant fraction (3–38%) was emitted in methylated form. On the basis of comparison with the composition of the regional background atmosphere, emission ratios to carbon dioxide and carbon monoxide were determined for the methyl halide species. The emission ratios decreased in the sequence CH3Cl > CH3Br > CH3I. Extrapolation of these results in combination with data from other types of biomass burning, e.g. forest fires, suggests that vegetation fires make a significant contribution to the atmospheric budget of CH3Cl and CH3Br. For tropospheric CH3I, on the other hand, fires appear to be a minor source. Our results suggest that pyrogenic emissions of CH3Cl and CH3Br need to be considered as significant contributors to stratospheric ozone destruction.
Principal component analysis of summertime organic aerosols at Niwot Ridge, Colorado
Authors: Veltkamp P. R. , Hansen K. J. , Barkley R. M. , Sievers R. E.
Published: February 2004

The organic constituents of atmospheric aerosols collected at Niwot Ridge, Colorado, along with various physical and meteorological data, were measured during a collaborative field study. Temporal changes in the composition of samples collected during summertime at Niwot Ridge, a rural site in the Rocky Mountains west of Denver, were studied. Volatile organic compounds were thermally desorbed from aerosol particles, separated by gas chromatography, and identified by mass spectrometry. For each of 48 2‐ to 5‐hour samples, 18 organic compounds in aerosol particles, 11 organic and inorganic compounds in the vapor phase, wind direction, and time of day were measured. Relationships among the variables were analyzed by principal component analysis in order to examine the covariations within the data set. The 31 variables are grouped into seven factors, and individual compounds, as well as the factors, serve as molecular markers for biologic and anthropogenic emission sources. Elucidation of several distinct associations within both rural and urban air masses was possible because of the high time resolution achieved in the collection and analysis of aerosols. In addition, factor scores were used to illustrate how several organic compounds vary with respect to local atmospheric dynamics.
A comparison of anticyclone and trough influences on the vertical distribution of ozone and meteorological conditions during SAFARI‐92
Authors: Diab R. D. , Jury M. R. , Combrink J. , Sokolic F.
Published: February 2004

The vertical distribution of ozone recorded over northern Namibia during SAFARI‐92 is examined as a function of prevailing atmospheric circulation. Two dominant synoptic weather types, namely, anticyclonic flow and flow associated with a westerly trough, are identified and composite ozone profiles produced for each weather type. In both cases an elevated, enriched ozone layer (9–12 km), where ozone concentrations exceed 80 ppbv on average, and 100–120 ppbv on individual days, is present. Under anticyclonic conditions, ozone increases steadily from surface values of around 40 ppbv to about 60–80 ppbv throughout the troposphere. In contrast, the westerly trough is characterized by a midtropospheric (4–8 km) ozone minimum, where values are 20 ppbv lower. This gives rise to a double ozone maximum in the troposphere. The elevated, enriched layer is thought to be a pervasive layer equatorward of 20°S and to result from biomass burning.
Mechanism of the lightning M component
Authors: Rakov Vladimir A. , Thottappillil Rajeev , Uman Martin A. , Barker Philip P.
Published: February 2004

Analysis of simultaneous measurements of the channel‐base current and the vertical electric field 30 m from triggered lightning reveals that the fields associated with M components, although essentially electrostatic, appear to be proportional to the time derivatives of the associated M currents. Based on this finding, coupled with other observations and modeling, a mechanism for the lightning M component is proposed. According to this mechanism an M component involves a downward progressing incident wave (the analog of a leader) followed by an upward progressing reflected wave (the analog of a return stroke). However, as opposed to a leader‐return stroke sequence in which the latter removes the charge deposited by the former, both the upward and the downward processes contribute about equally to the total charge flowing from the bottom of the channel at any instant of time. Such a mode of charge transfer to ground, distinctly different from a leader‐return stroke sequence, is possible because of the presence of a path capable of supporting the propagation of a traveling wave (facilitated by a continuing current flowing to ground) and the fact that the ground is essentially a short circuit for the downward incident wave, so that the magnitude of the current reflection coefficient at ground is virtually equal to unity. We show that some observed properties of M components can be explained if the lightning channel traversed by an M‐current wave is represented as a linear R‐C transmission line. In this view, the preferential attenuation of the higher‐frequency components on an R‐C line is responsible for the lack of frequencies above several kilohertz in both the M‐current pulses measured at the channel base and the M‐light pulses observed in the bottom 1 km or so of the channel. Further, the relatively high characteristic impedance of the channel, of the order of tens to hundreds of kilohms for frequencies below some kilohertz, inferred from the linear R‐C line approximation, is consistent with the observation that even a relatively poor ground is sensed by an incident M wave as essentially a short circuit. However, on a linear R‐C transmission line the higher‐frequency components travel faster than lower‐frequency components (this velocity dispersion implying that the original pulse would spread while propagating along the line), whereas the shape of the M‐light pulses does not change much within the bottom 1 km or so, as if the channel were a distortionless transmission line. We speculate, on physical grounds, that the front of the traveling M‐current pulse heats the channel so that the pulse tail encounters a lowered resistance and, as a result, accelerates. By virtue of these two opposing effects, velocity dispersion and channel nonlinearity, an M pulse is formed whose more‐or‐less symmetrical shape is preserved over a relatively large distance, as in the case of a soliton.
Variations of the total ozone trend over the northern hemisphere midlatitudes in winter‐spring seasons 1963–1964 through 1992–1993
Authors: Krzyścin Janusz W.
Published: February 2004

Monthly means of total ozone for winter‐spring seasons (December through the next March) since 1963–1964 through 1992–1993 from a network of 19 Dobson stations are averaged to form a composite time series of the winter‐spring midlatitudinal total ozone. A multiple regression statistical model, comprising overall mean level, linear trend, quasi‐biennial, El Niño, solar (including both the 11‐year and its double length “geomagnetic” oscillations), and dynamics of the winter polar vortex terms, is applied to estimate interannual variations of total ozone over the northern hemisphere midlatitudes in winter‐spring seasons. The model used explains about 88% of total variance of the winter‐spring total ozone and gives the value of the anthropogenic trend in the northern hemisphere midlatitudinal total ozone of the order of −3.5%±0.4% per decade since winter‐spring of 1969–1970. The model's results support the hypothesis that the increase of the ozone loss during the 1980s and 1990s over the northern hemispheric midlatitudes may be partially caused by a superposition of “natural” long‐term oscillations in total ozone related to the climatic changes in the atmospheric dynamics. The initial evidences for a strong manifestation of the interannual changes of the polar vortex in the winter‐spring midlatitudinal total ozone data are found. The 22‐year oscillation in the total ozone data is disclosed; however, because of the limited length of the ozone data, one should be cautious to relate this fluctuation uniquely to the solar input.