bluuebunny/crossref_metadata_2025 huggingface dataset:
Drought length properties for periodic‐stochastic hydrologic data
Authors: Cancelliere Antonino , Salas Jose D.
Published: February 2004

Extreme droughts may be characterized by their duration, severity (magnitude or intensity), spatial extent, and frequency or return period. Comparing the time series of water supply and water demand and analyzing droughts based on the theory of runs may determine these characteristics. This study is focused on drought analysis where the underlying water supply process is periodic stochastic, such as for monthly streamflows. The probability mass function (pmf) of drought length and associated low‐order moments are derived assuming a periodic simple Markov chain. The derived pmf allows estimating the occurrence probability of droughts of a given length and its return period. The applicability of the drought formulations has been illustrated using a variety of water supply series such as monthly and weekly precipitation, monthly streamflows, the Palmer hydrologic drought index, and the standardized precipitation index. The results obtained confirm the validity of the analytical derivations for drought lengths and associated return periods. The overall conclusion of the study is that simple definitions of droughts enables one characterizing droughts using stochastic approaches and analytical derivations. They are particularly useful for drought analysis because the limited hydrologic records that are generally available do not allow observing many drought events of a particular duration and, in fact, extremely long droughts may not even be observable from the historical sample. This hinders the applicability of an inferential approach for finding the probability distributions of drought lengths and their associated return periods because it is either impractical or not feasible.
Thermal models of oceanic crustal accretion: Linking geophysical, geological and petrological observations
Authors: Maclennan J. , Hulme T. , Singh S. C.
Published: February 2004

Models of oceanic crustal accretion should be able to reproduce both petrological and geological observations from ophiolites and geophysical observations from active spreading ridges. We have developed thermal models of crustal accretion at fast spreading ridges that incorporate petrological variation and allow for a variable vertical distribution of crystallization. Including petrological variation has a major effect on the distribution of melt within the crust and the consequent interpretation of model results in terms of geophysical observables. In particular, the narrow melt lens imaged by seismic reflection surveys can be produced by the presence of large‐scale compositional stratification within the crust, such as that observed in the Oman ophiolite. Controlling the vertical distribution of crystallization allows a range of crustal accretion mechanisms to be tested, including the many sills and gabbro glacier models. Both gabbro glacier and many sills models can broadly match the geophysical observations, but the preferred model is a hybrid, where the uppermost 25% or more of the lower crust forms in a gabbro glacier and the lowermost 25% or more of the lower crust forms by crystallization in situ. The success of all models is dependent on the strength and distribution of hydrothermal cooling.
Eulerian and Lagrangian properties of biophysical intermittency in the ocean
Authors: Seuront Laurent , Schmitt François G.
Published: February 2004

Eulerian and Lagrangian characteristics of simultaneously recorded temperature, salinity and phytoplankton biomass time series are investigated over a seasonal cycle. Time series are analyzed in the spectral and multifractal scaling frameworks. We show that passive scalar time series (temperature and salinity) are characterized by scaling laws representative of Eulerian and Lagrangian turbulence. The two different regimes identified are separated by a transition linked to the characteristic scale of turbulent structures of the same size as the boat. While the scaling behavior exhibited by phytoplankton biomass fluctuations is close to the expected passive scalar behavior over Eulerian scales, it is quite far from it over Lagrangian scales. For these scales, the observed scaling behavior exhibits a significant density‐dependence, indicative of biological activity.
Observations of the anomalous oxygen isotopic composition of carbon dioxide in the lower stratosphere and the flux of the anomaly to the troposphere
Authors: Boering K. A. , Jackson T. , Hoag K. J. , Cole A. S. , Perri M. J. , Thiemens M. , Atlas E.
Published: February 2004

Measurements of the triple oxygen isotopic composition of stratospheric CO2 in whole air samples from the NASA ER‐2 aircraft show anomalous enrichments in 17O and 18O. The compact correlation of the isotope anomaly (defined as Δ17O = δ17O − 0.516 × δ18O) with simultaneous N2O measurements demonstrates that Δ17OCO2 is a long‐lived tracer with a stratospheric source. These characteristics, and an isotopic link to O3 production, make Δ17OCO2 potentially useful as a tracer of integrated stratospheric chemistry and transport. The Δ17OCO2:N2O correlation is also used to estimate a net Δ17OCO2 flux to the troposphere of 3.6 ± 0.9 × 1015‰ mol CO2 yr−1. This flux is required to predict and understand the CO2 and O2 isotope anomalies in the troposphere and their use as tracers of gross carbon exchanges between the atmosphere and biosphere on interannual to glacial‐interglacial time scales.
Phase space structures generated by an absorbing obstacle in a streaming plasma
Authors: Guio P. , Pécseli H. L.
Published: February 2004

The dynamic behavior of an ion flow around an obstacle in a collisionless plasma is investigated. The obstacle consists here of an absorbing cylinder, and a 2 dimensional electrostatic particle‐in‐cell simulation is used to study the flow characteristics. The formation of irregular filamented density depletions, oblique to the flow, is observed. The dynamics of these structures depend on the physical parameters of the plasma. The structures form at the edges of the wake behind the obstacle, in a region with a strong velocity shear, and are found to be associated with phase‐space vortices, observed specially in the velocity direction perpendicular to the flow. The results can be of interest in the interpretation of structures in space plasmas as observed by instrumented space crafts.
Simulation of extratropical Hurricane Gustav using a coupled atmosphere‐ocean‐sea spray model
Authors: Perrie William , Ren Xuejuan , Zhang Weiqing , Long Zhenxia
Published: February 2004

Numerical simulations of extratropical Hurricane Gustav (2002) are performed using the MC2 (Mesoscale Compressible Community) atmospheric model, coupled to the Princeton Ocean Model (POM), and a sea spray parameterization. On one hand, the impact of coupling POM to MC2 generates sea surface temperature (SST) cooling, through entrainment mixing at the bottom of the mixed layer, with the passage of the storm. SST cooling reduces the sea surface heat fluxes compared to uncoupled MC2 simulations, which have time‐invariant SST. Reduced heat fluxes lead to reduced storm intensity. On the other hand, simulation of the heat and mass flux contributions of sea spray enhances sea surface heat fluxes and slightly increases maximum storm intensity compared to coupled MC2‐POM simulations without spray.
Interactions between sea surface temperature over the South Atlantic Ocean and the South Atlantic Convergence Zone
Authors: Chaves Rosane Rodrigues , Nobre Paulo
Published: February 2004

Interactions between the sea surface temperature (SST) over the South Atlantic Ocean (40°S‐Equador) and the South Atlantic Convergence Zone (SACZ) were studied through numerical experiments with an atmospheric general circulation model (AGCM) and an ocean general circulation model (OGCM). The AGCM experiments showed that warm SST anomalies over the South Atlantic tend to intensify the SACZ and shift it northward, while cool SST anomalies over the South Atlantic tend to weaken the SACZ. The OGCM experiments, on the other hand, showed that the intensification of the SACZ contributes to cool the underlying ocean through the reduction of incident shortwave solar radiation, causing the appearance of cold SST anomalies or the weakening of pre‐existing warm SST anomalies. The most important finding in this work was the predominance of the cloud/shortwave ‐ SST negative thermodynamic feedback between the atmosphere and the ocean over the southwest tropical Atlantic, this is one order of magnitude larger than the dynamic feedback associated with Ekman pumping. The latter was verified only during strong SACZ events. The results suggest that negative SST anomalies often observed underlying the SACZ represent an ocean response to atmospheric forcing.
Coincident equatorial bubble detection by TIMED/GUVI and ROCSAT‐1
Authors: Kil Hyosub , Su S.‐Y. , Paxton Larry J. , Wolven Brian C. , Zhang Yongliang , Morrison Daniel , Yeh H. C.
Published: February 2004

The GUVI on board the NASA's TIMED satellite provides a new capability for monitoring equatorial electron density irregularities. The nighttime GUVI disk scan images of OI 135.6‐nm show numerous emission‐depleted dark features in the equatorial region. The dark features show typical equatorial bubble characteristics ‐ elongation in the north‐south direction, westward tilt, and season‐longitudinal variations in their distribution. Coincident in‐situ observations by ROCSAT‐1 satellite identify plasma depletions co‐located with the emission depletions. The individual bubble images are hardly distinguishable in the equatorial region owing to an occurrence of multiple bubbles and owing to the low background radiance at the ionization trough. Occasionally, a reversed “C”‐shaped bubble images appear on the GUVI swath of a single orbit. Those global bubble images corroborate the occurrence of plasma depletion in the whole magnetic flux tube and the presence of vertical shear of zonal bulk plasma flow in the topside ionosphere.
Theoretical relationship between maximum value of the post‐sunset drift velocity and peak‐to‐valley ratio of anomaly TEC
Authors: Basu B. , Retterer J. M. , de La Beaujardière O. , Valladares C. E. , Kudeki E.
Published: February 2004

Theoretical study of electron density distribution in the nighttime equatorial ionosphere shows that linear relationships with statistically significant correlation coefficients exist between the maximum value of the post‐sunset plasma drift velocity and the peak‐to‐valley ratio of anomaly TEC. The study is based on the low‐latitude density model of Air Force Research Laboratory (AFRL) and the obtained relationships are valid for the longitudinal sector of Jicamarca incoherent scatter radar whose drift velocity measurements are used. The significance of this finding lies in the fact that the maximum value of the post‐sunset vertical plasma drift velocity is an important parameter for determining both the intensity and the latitudinal distribution of equatorial scintillation. When the parameter is not available from any direct measurement, the linear relationships may be used to estimate it from the measured peak‐to‐valley ratio of anomaly TEC.
A new method for diagnosing radiative forcing and climate sensitivity
Authors: Gregory J. M. , Ingram W. J. , Palmer M. A. , Jones G. S. , Stott P. A. , Thorpe R. B. , Lowe J. A. , Johns T. C. , Williams K. D.
Published: February 2004

We describe a new method for evaluating the radiative forcing, the climate feedback parameter (W m−2 K−1) and hence the effective climate sensitivity from any GCM experiment in which the climate is responding to a constant forcing. The method is simply to regress the top of atmosphere radiative flux against the global average surface air temperature change. This method does not require special integrations or off‐line estimates, such as for stratospheric adjustment, to obtain the forcing, and eliminates the need for double radiation calculations and tropopause radiative fluxes. We show that for CO2 and solar forcing in a slab model and an AOGCM the method gives results consistent with those obtained by conventional methods. For a single integration it is less precise but since it does not require a steady state to be reached its precision could be improved by running an ensemble of short integrations.
Comment on ‘Kinetic effects in the acceleration of auroral electrons in small scale Alfvén waves: A FAST case study’ by Chaston et al.
Authors: Shukla P. K. , Stenflo L.
Published: February 2004

We reconsider the underlying physics and the dispersion properties of inertial and kinetic Alfvén waves in plasmas, accounting for the inertial electron and kinetic ion responses. In that way, we clarify the ambiguities in a recently published paper by Chaston et al. [Geophys. Res. Lett. 30(6), 1289, doi:10.1029/2002GL015777, 2003].
Critical thickness for two‐dimensional tearing instability
Authors: Tanaka K. G. , Shinohara I. , Fujimoto M.
Published: February 2004

Dependence of tearing mode saturation state on the current sheet thickness is investigated by two‐dimensional (2‐D) full particle simulations. When the system length Lx is taken to be the wavelength of the maximum growth mode Lx = λmax = 12D (D: half‐thickness of the current sheet), the instability is found to saturate without producing significant reconnection if D > Dcr1st = 3.5 λe, where λe is the electron inertial length. When the system length is doubled Lx = 2λmax, only insignificant effects are available for D > Dcr2nd = 2.7λh, where λh is the ion‐electron hybrid inertial length. Comparing these 2‐D results with a recent 3‐D result, it is shown clearly that a three‐dimensional effect reduces the current sheet thickness and thus leads to quick production of substantial reconnection even if D > Dcr1st.
Incorporating remotely‐sensed snow albedo into a spatially‐distributed snowmelt model
Authors: Molotch Noah P. , Painter Thomas H. , Bales Roger C. , Dozier Jeff
Published: February 2004

Basin‐average albedo estimated from remotely‐sensed Airborne Visible/Infrared Imaging Spectroradiometer (AVIRIS) data specific to the catchment typically differed by 20% from albedo estimated using a common snow‐age‐based empirical relation. In some parts of the basin, differences were as large as 0.31. Using the AVIRIS albedo estimates in a distributed snowmelt model that explicitly includes net solar radiation resulted in a much more accurate estimate of the timing and magnitude of snowmelt as compared to the same model with the empirical albedo (R2 of 0.73 versus 0.59 and magnitude error of 2% versus 36%). Model improvement was most significant in areas and at times where incident solar radiation was relatively high and temperatures low.
An updated analysis of long‐term sea level change in New Zealand
Authors: Hannah J.
Published: February 2004

The original analysis of long‐term sea level change in New Zealand is updated with a new and extended analysis. In this new analysis the original hourly sea level data have been re‐examined to remove obvious errors that were still present, new data covering the period 1989–2000 has been added, and the sea level record for Wellington extended by the inclusion of recently discovered data covering the years 1891–1893. These new results indicate that relative sea levels in New Zealand have been rising at an average rate of 1.6 mm/yr over the last 100 years ‐ a figure that is not only within the error bounds of the original determination, but when corrected for glacial‐isostatic effects has a high level of coherency with other regional and global sea level rise determinations. There continues to be no evidence of any acceleration in relative sea levels over the record period.
Tropical Atlantic surface current variability from 10 years of TOPEX/Poséïdon altimetry
Authors: Arnault Sabine , Kestenare Elodie
Published: February 2004

10 years of surface geostrophic currents from TOPEX/Poséïdon altimetric data are used to describe the low frequency variability of the tropical Atlantic circulation through Empirical Orthogonal Function analysis. The seasonal variability clearly agrees with previous studies based on climatological data. It shows the tropical Atlantic response to seasonal fluctuations of the overlying wind system. More interesting is the capability, using altimetry, to reach for the first time on a basin scale the year‐to‐year variability from measurements. Abnormal events occur in 1996–1997 and in 2001 with different spatial scales regarding both large scale zonal distribution and regional variability located in the north‐western basin. A first attempt to link these events to climatic indexes (El Niño‐Southern Oscillation, North Atlantic Oscillation) is also evocated.
Seismic constraints on mechanisms of deep earthquake rupture
Authors: Estabrook Charles H.
Published: February 2004

Observations of the depth distribution of seismicity suggest that several mechanisms cause deep earthquakes. Cold subducting slabs exhibit a broad peak in seismicity between 300 and 530 km depth that is best explained by transformational faulting of metastable olivine. Cumulative seismic moment release increases sharply at ∼530 km depth in both cold and warm slabs, implying that events deeper than this are controlled by an equilibrium rather than a metastable phase change, which would be temperature‐dependent. This, together with the observation that deep seismicity predominates near the garnet‐rich upper boundary of the subducting plate (former oceanic crust), suggests that the transformation of garnet to perovskite, which may release considerable water, is integral in the cause of earthquakes deeper than 530 km, where most deep earthquakes occur.
Osmium isotopic compositions of Os‐rich platinum group element alloys from the Klamath and Siskiyou Mountains
Authors: Meibom Anders , Frei Robert , Sleep Norman H.
Published: February 2004

We present new measurements of 186Os/188Os and 187Os/188Os in 10 Os‐rich platinum group element (PGE) alloys from placer deposits formed by the mechanical erosion of peridotite‐bearing ophiolites in the Klamath and Siskiyou Mountains in northern California and southwestern Oregon. These data nearly double our database of high‐precision 186Os/188Os measurements on such samples. Together with previously published data, our new results reinforce the conclusion that the radiogenic 186Os/188Os compositions of these PGE alloys are very difficult to reconcile with a derivation of their Os from the outer core. Such a model requires extremely early growth of the inner core to its present size, within several hundred million years after accretion of the Earth, which is geophysically implausible. Collectively, our data suggest instead that partial melting or metasomatic processes in the upper mantle play a primary role in controlling the Os isotopic systematics of these Os‐rich PGE alloys and suggest the existence of upper mantle components characterized by radiogenic 186Os/188Os ratios. Pyroxene‐rich lithologies are possible candidates.
Late Cretaceous tectonic history of the Sierra‐Salinia‐Mojave arc as recorded in conglomerates of the Upper Cretaceous and Paleocene Gualala Formation, northern California
Authors: Schott Ronald C. , Johnson Clark M. , O'Neil James R.
Published: February 2004

Upper Cretaceous and Paleogene conglomerates in the Gualala basin of northern California likely preserve the westernmost record of sedimentation in California during the interval between cessation of Sierran batholith magmatism and inception of Neogene transform activity on the San Andreas fault system. Detailed chemical and H, O, Sr, Nd, and Pb isotope analyses of conglomerate clasts, as well as U/Pb zircon ages, identify two source types: (1) evolved granite, like those found in the Salinian block or eastern Sierra Nevada batholith, and (2) primitive oceanic arcs that were restricted to the western portions of the Mesozoic arc system. The provenance of the Gualala clasts can be entirely explained through derivation from the Sierra‐Salinian‐Mojave portion of the Mesozoic Cordilleran arc, and do not support models for extensive large‐scale translation of the basin from low latitudes along a pre‐Neogene transform fault system. Contemporaneous deposition of the contrasting lithologies in the Gualala basin is interpreted to reflect tectonic juxtaposition of the source terranes approximately 80 m.y. ago. Tectonic processes responsible for juxtaposition of the two source regions in the Late Cretaceous most probably involved zones of lithospheric weakness that later localized the San Andreas fault system in the Neogene. Chemical, isotopic, and age data tie the Gualala basin to North American continental basement in the Mojave‐Salinian segment of the Mesozoic Cordilleran arc and imply minimal pre‐Neogene translation of the basin contrary to the low latitude origin for the basin inferred from previous paleomagnetic and paleontologic studies.
Low‐frequency waves in the Santa Barbara Channel observed by high‐frequency radar
Authors: Beckenbach Edwin , Washburn Libe
Published: February 2004

Three years of high‐frequency radar observations in the Santa Barbara Channel reveal sequences of alternating cyclonic and anticyclonic vortices propagating westward with a period of approximately 2 weeks. The sequences last up to a few months and occur intermittently throughout the year. The velocity distributions of cyclones and anticyclones are antisymmetric with a mean propagation speed of ∼5 km d−1 and relative vorticity of order 0.1 f. Complex empirical orthogonal function decomposition in the 10–20 day passband partitions variance into two dominant modes. The first mode represents the vortices with an average period of 14.4 days. The second mode represents alongshore fluctuations with an average period of 13.3 days. Amplitude functions of the two modes correlate with in situ current time series at 5 and 45 m depths obtained from moorings at the east and west channel entrances, suggesting coupling with the larger‐scale circulation of the northern Southern California Bight. The observations support interpretation of mode 2 as a coastal trapped wave since the period and alongshore flow are consistent with previous observations and predictions. We hypothesize that mode 1 is a resonant response of the Santa Barbara Basin in the form of a trapped topographic Rossby mode. The spatial mode resembles the predicted pattern based on a simplified analytical model of the fundamental mode. The amplitude of mode 1 is consistently large when the frequencies of modes 1 and 2 converge, suggesting that the vortices are a basin‐scale resonant response to coastal trapped waves.
Tidal and subtidal currents in the Strait of Sicily
Authors: Gasparini G. P. , Smeed D. A. , Alderson S. , Sparnocchia S. , Vetrano A. , Mazzola S.
Published: February 2004

Current measurements, from shipboard and moored instruments, from the Strait of Sicily are examined to evaluate the importance of tidal and subtidal currents during summer 2000. The results show that the principal tidal components (M2, K1, S2, and O1) are particularly important in the western shelf and sill region but are significantly less important over the eastern shelf and almost negligible in the central part of the strait. There is a significant internal tide in the narrowest passage through the western sill. The residual (nontidal) currents describe the main pathways and local structures of Atlantic Water as it crosses the strait. The residual currents are compared with the geostrophic velocity calculated from simultaneous hydrographic measurements.
Seasonal variability of secondary organic aerosol: A global modeling study
Authors: Lack Daniel A. , Tie Xuexi X. , Bofinger Neville D. , Wiegand Aaron N. , Madronich Sasha
Published: February 2004

Two secondary organic aerosol (SOA) formation modules have been implemented into a global chemical transport model (Model for Ozone And Related chemical Tracers, Version 2) to estimate the global distribution of SOA and to compare between methods. The first SOA model is based on bulk smog chamber yields while the second uses the gas to particle partitioning theory to predict SOA concentrations. Maximum SOA concentrations using the bulk yield method were found to vary up to 10 μg/m3 with concentrations peaking over the southeast United States, Europe, South America, central Africa, and southern Asia. Maximum SOA concentrations were found over the Northern Hemisphere (NH) and tropics in June while SOA concentrations in the tropics remained high through December. SOA production was found to be dependant on oxidant availability rather than volatile organic compound emissions in South America and Asia. Using the partitioning model, SOA concentrations peaked at 10 μg/m3 over South America in September related to organic carbon aerosol availability. The partitioning model produced significantly less SOA during the NH summer, and SOA production was found to be dependant on organic carbon primary aerosol and oxidant availability for South America and Asia. The total annual global production of SOA was calculated to be 24.6 Tg/yr using the bulk yield method and 15.3 Tg/yr using the partitioning method.
Long‐term variations in the occurrence and effective solar transmission of clouds as determined from surface‐based total irradiance observations
Authors: Dutton Ellsworth G. , Farhadi Alessio , Stone Robert S. , Long Charles N. , Nelson Donald W.
Published: February 2004

Time series of cloud solar transmission and cloud occurrence frequency are developed for the past 27 years at four globally remote and climatically diverse surface locations. A new methodology is developed that objectively segregates times of cloud‐free conditions from those times when clouds are detected in high‐time‐resolution total solar irradiance observations that are obtained from pyranometers. The methodology for cloud detection depends on the magnitude and short‐term variability of observed departures from clear‐sky conditions. Expected clear‐sky irradiances are based on interpolated clear‐sky observations. Results of the new cloud detection methodology are compared to four independent cloud detection methods. An effective cloud transmission is determined as the ratio of observed irradiance in the presence of clouds to that expected in the absence of clouds. Selective forward scattering by clouds toward the observation site results in computed effective transmissions frequently being >1.0. It is shown that conditional temporal averaging of effective cloud transmission over periods of three days or more virtually eliminates the unrealistic cloud transmissions exceeding 1.0. Such temporal averaging of the surface measurements is advantageous for comparing against other area‐wide cloud transmission estimates, such as those determined from satellite or by numerical climate models. The cloud occurrence frequency and the effective solar transmission for long‐term observational records are summarized into monthly and annual averages, and their long‐term variability is investigated. Temporal variations in frequency distributions of transmission are used to determine which clouds are responsible for changes in mean cloudiness. A statistically significant upward trend in cloud occurrence frequency, from 76 to 82% between 1976 and 2001, is detected at Barrow, Alaska, where clouds having solar transmission near 0.2 exhibit the largest increase. At the South Pole, decadal timescale oscillations in both cloud characteristics are detected, but no particular cloud category is identified as the source of that oscillation.
Spatial and monthly trends in speciated fine particle concentration in the United States
Authors: Malm William C. , Schichtel Bret A. , Pitchford Marc L. , Ashbaugh Lowell L. , Eldred Robert A.
Published: February 2004

In the spring of 1985 an interagency consortium of federal land management agencies and the Environmental Protection Agency established the Interagency Monitoring of Protected Visual Environments (IMPROVE) network to assess visibility and aerosol monitoring for the purpose of tracking spatial and temporal trends of visibility and visibility‐impairing particles in rural areas. The program was initiated with 20 monitoring sites and was expanded to 165 sites between 2000 and 2003. This paper reports on fine aerosol data collected in the year 2001 at 143 sites. The major fine (dp < 2.5 μm) particle aerosol species, sulfates, nitrates, organics, light‐absorbing carbon, and wind‐blown dust, and coarse gravimetric mass are monitored, and at some sites, light scattering and/or extinction are measured. Sulfates, carbon, and crustal material are responsible for most of the fine mass at the majority of locations throughout the United States, while at sites in southern California and the midwestern United States, nitrates can contribute significantly. In the eastern United States, sulfates contribute between 50 and 60% of the fine mass. Sulfate concentrations tend to be highest in the summer months while organic concentrations can be high in the spring, summer, or fall seasons, depending upon fire‐related emissions. However, at the two urban sites, Phoenix, Arizona, and Puget Sound, Washington, organics peak during the winter months. Nitrate concentrations also tend to be highest during the winter months. During the spring months in many areas of the western United States, fine soil can contribute as much as 40% of fine mass. The temporal changes in soil concentration that occur simultaneously over much of the western United States including the Rocky Mountain region suggest a large source region, possibly long‐range transport of Asian dust.
Using a geographic information system to improve Special Sensor Microwave Imager precipitation estimates over the Tibetan Plateau
Authors: Yin Zhi‐Yong , Liu Xiaodong , Zhang Xueqin , Chung Chih‐Fang
Published: February 2004

This study examines the potential of spatial modeling using geographic information systems (GIS) to improve the precipitation estimates based on Special Sensor Microwave/Imager (SSM/I) over the Tibetan Plateau. The monthly SSM/I precipitation estimates are based on the algorithm developed at the National Environmental Satellite, Data and Information Service (NESDIS) of the National Oceanic and Atmospheric Administration (NOAA). When using SSM/I estimates to predict station precipitation directly, the R2 values ranged from 0.005 to 0.624, with a mean of 0.334 for all months. When terrain and location variables obtained from the GIS were added to the models, the R2 values improved to range from 0.217 to 0.739 with a mean of 0.590 for all months. These variables represent effects of orographic forcing of topography, rain barrier/rain shadow, direction of moisture‐bearing winds, and distance to the sources of moisture over the Tibetan Plateau. Results from this study suggest that region‐specific algorithms are necessary for the SSM/I precipitation estimates over the Tibetan Plateau and that topographic analysis based on GIS can contribute significantly in improving the performance of SSM/I estimates.
Direct radiative effect of aerosols as determined from a combination of MODIS retrievals and GOCART simulations
Authors: Yu Hongbin , Dickinson R. E. , Chin M. , Kaufman Y. J. , Zhou M. , Zhou L. , Tian Y. , Dubovik O. , Holben B. N.
Published: February 2004

The aerosol direct solar effect under clear sky is assessed by (1) combining multiple aerosol characterizations and (2) using the satellite‐retrieved land surface albedo. The aerosol characterization is made through an integration of the MODerate resolution Imaging Spectroradiometer (MODIS) retrievals and the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model simulations. The spectral and bidirectional albedo of land surface is derived from MODIS. On a global average, the solar forcing at the top of atmosphere (TOA) DFTOA is −4.5 Wm−2, of which about 1/3 is contributed by a sum of natural and anthropogenic sulfate and carbonaceous aerosols. Though the optical depth is about 50% higher over land than over ocean, no significant land‐ocean contrast in this TOA forcing is observed. It is reduced by larger aerosol absorption and higher surface albedo over land. As a result of absorption by soot and dust, a much larger surface cooling and substantial atmospheric absorption coexist over land and adjacent oceans. Globally, the surface cooling DFSFC is about −9.9 Wm−2, and the atmospheric absorption DFAIR is about 5.4 Wm−2, suggesting that more than half of the surface cooling results from the atmospheric absorption. Sensitivity tests show that an inclusion of MODIS‐derived anisotropy of land surface reflection reduces the diurnal variation of TOA solar forcing, because of aerosol‐induced changes in the fraction of direct beam and hence in the effective reflection from the surface. Constraining the GOCART dust absorption with recent measurements reduces DFAIR and DFSFC by 1.3 Wm−2 and 0.9 Wm−2, respectively, and increases the TOA cooling by 0.4 Wm−2.
A search for mountain waves in MLS stratospheric limb radiances from the winter Northern Hemisphere: Data analysis and global mountain wave modeling
Authors: Jiang Jonathan H. , Eckermann Stephen D. , Wu Dong L. , Ma Jun
Published: February 2004

Despite evidence from ground‐based data that flow over mountains is a dominant source of gravity waves (GWs) for the Northern Hemisphere winter middle atmosphere, GW‐related signals in global limb radiances from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) have shown little direct evidence of mountain waves. We address this issue by combining a renewed analysis of MLS limb‐track and limb‐scan radiances with global mountain wave modeling using the Naval Research Laboratory Mountain Wave Forecast Model (MWFM). MLS radiance variances show characteristics consistent with mountain waves, such as enhanced variance over specific mountain ranges and annual variations that peak strongly in winter. However, direct comparisons of MLS variance maps with MWFM‐simulated mountain wave climatologies reveal limited agreement. We further develop a detailed “MLS GW visibility function” that accurately specifies the three‐dimensional in‐orbit sensitivity of the MLS limb‐track radiance measurement to a spectrum of GWs with different wavelengths and horizontal propagation directions. On postprocessing MWFM‐generated mountain wave fields through these MLS visibility filters, we generate MWFM variance maps that agree substantially better with MLS radiance variances. This combined data analysis and MLS‐filtered MWFM modeling leads us to conclude that many MLS variance enhancements can be associated with mountain waves forced by flow over specific mountainous terrain. These include mountain ranges in Europe (e.g., Scandinavia; Alps; Scotland; Ural, Putoran, Altai, Hangay and Sayan Mountains; Yablonovyy, Stanovoy, Khingan, Verkhoyansk and Central Ranges), North America (e.g., Brooks Range, MacKenzie Mountains, Colorado Rockies), southeastern Greenland, and Iceland. Our results show that given careful consideration of the in‐orbit sensitivity of the instrument to GWs, middle atmospheric limb radiances measured from UARS MLS, as well as from the new MLS instrument on the Earth Observing System (EOS) satellite, can provide important global information on mountain waves in the extratropical Northern Hemisphere stratosphere and mesosphere.
Parameterization of the influence of organic surfactants on aerosol activation
Authors: Abdul‐Razzak Hayder , Ghan Steven J.
Published: February 2004

Surface‐active organic compounds, or surfactants, can affect aerosol activation by two mechanisms: lowering surface tension and altering the bulk hygroscopicity of the particles. A numerical model has been developed to predict the activation of aerosol particles consisting of an internally uniform chemical mixture of organic surfactants and inorganic salts in a parcel of air rising adiabatically at constant speed. Equations reflecting water balance of the air parcel were used together with a modified form of Köhler theory to model droplet nucleation while considering surface effects. We also extend a parametric representation of aerosol activation to the case of a mixture of inorganic salts and organic surfactants by modifying the Raoult term in Köhler theory (assuming additive behavior) and using a simplified relationship between surface tension and surfactant molar concentration to account for surface effects at the critical radius for activation. The close agreement (to within 10% for most and 20% for almost all conditions) between numerical and parametric results validates our modifications. Moreover, the form of the relationship is identical to an empirical relationship between surface tension and organic carbon concentration. Thus the modified form of the parameterization provides a framework that can account for the influence of observed organics on the activation of other salts. The modified form of the parameterization is tested successfully with the Po Valley model both for single aerosol size distribution and three‐mode size distributions for marine, rural, and urban aerosols. Further measurements are required to extend the parameterization to other organic surfactants.
Tropical troposphere‐to‐stratosphere transport inferred from trajectory calculations
Authors: Fueglistaler S. , Wernli H. , Peter T.
Published: February 2004

We present an analysis of trajectory calculations in the tropical tropopause layer (TTL) based on European Centre for Medium‐Range Weather Forecasts (ECMWF) analysis wind and temperature fields. Over 500,000 forward and backward trajectories were calculated for January/February and July/August 2001. We analyze the pathways between 340 K and 400 K potential temperature (θ) of those trajectories involved in troposphere‐to‐stratosphere transport (TST). Even though trajectory calculations in this region may suffer from deficiencies in the underlying vertical wind field, they incorporate not only slow radiative ascent but also effects of deep convection, zonal and meridional transport, and their regional variability. From the trajectory calculations we derive a mean residence time of air parcels in the TTL, which shows a maximum at θ ≈360 K of ∼13 days for a change in potential temperature of ±10 K. The analysis of trajectory pathways reveals that approximately 80% of the trajectories ascending into the stratosphere enter the TTL over the western Pacific. Upon further ascent, they typically travel ∼5000–10,000 km before they arrive at the location where they assume minimum water mixing ratios. These pathways show regional and seasonal patterns and are largely controlled by the upper level circulation of the Asian‐Australian monsoon, the northern hemispherical subtropical jet and the equatorial easterly jet from South Asia to Africa. As a consequence of the interplay of these meteorological systems, about 70% of TST trajectories assume their minimum water mixing ratio over the western Pacific, which shows also a global minimum in tropopause temperatures. Average water mixing ratios of air after TST are χH2O = 1.6 ppmv for January/February and χH2O = 3.6 ppmv for July/August 2001. Mixing of stratospherically young air, which just underwent TST, with older air masses entering the lower tropical stratosphere sideways yields an estimate of χH2O = 2.3 ppmv for January/February and χH2O = 3.7 ppmv for July/August for air at θ = 400 K, which compares favorably with satellite observations. Our analysis emphasizes the importance of particular pathways for tropical TST, with the western Pacific being the dominant source of stratospheric air in general and being the place, in particular, where ∼70% of tropical TST assumes its final water mixing ratio.
Contribution of mass‐dependent fractionation to the oxygen isotope anomaly of atmospheric nitrous oxide
Authors: Kaiser Jan , Röckmann Thomas , Brenninkmeijer Carl A. M.
Published: February 2004

Similar to other oxygen‐bearing atmospheric compounds, lower‐stratospheric and tropospheric nitrous oxide (N2O) show an oxygen isotope anomaly. This anomaly can be explained by in situ atmospheric chemical sources that transfer the well‐known oxygen isotope anomaly of ozone to N2O. The isotope anomaly of ozone, in turn, is caused by non‐mass‐dependent fractionation during its formation. Nevertheless, recent work claimed that photodissociation of stratospheric N2O could account for up to half of the observed anomaly in N2O without having to invoke chemical N2O sources. It is shown that this prediction is due to the choice of inadequate parameters in the specific underlying physicochemical model of isotopic fractionation in N2O photolysis. Budget calculations based on experimentally measured fractionation factors at stratospherically relevant wavelengths show only negligible contributions of N2O photolysis to the observed oxygen isotope anomaly. However, biological sources at the Earth's surface, which are usually considered to produce mass‐dependently fractionated N2O, may actually be responsible for part of the observed anomaly. This is as a consequence of slight variations in the mass‐dependent relationships between 17O and 18O isotope effects and the relationship assumed in the definition of the oxygen isotope anomaly. Up to 44% of the observed anomaly might be explained by this “numerical source” that was not recognized previously. As a prerequisite to understand this possibly surprising result, the existing definitions of isotope anomalies and their practical consequences are analyzed. An accurate terminology will also benefit future generations of researchers in the rapidly growing fields of atmospheric isotope chemistry and physics.
Simulation of N2O emissions from a urine‐affected pasture in New Zealand with the ecosystem model DayCent
Authors: Stehfest Elke , Müller Christoph
Published: February 2004

We used the trace gas model DayCent to simulate emissions of nitrous oxide (N2O) from a urine‐affected pasture in New Zealand. The data set for this site contained year‐round daily emissions of nitrification‐N2O (N2Onit) and denitrification‐N2O (N2Oden), meteorological data, soil moisture, and at least weekly data on soil ammonium (NH4+) and nitrate (NO3−) content. Evapotranspiration, soil temperature, and most of the soil moisture data were reasonably well represented. Observed and simulated soil NH4+ concentrations agreed well, but DayCent underestimated the NO3− concentrations, due possibly to an insufficient nitrification rate. Modeled N2O emissions (18.4 kg N2O‐N ha−1 yr−1) showed a similar pattern but exceeded observed emissions (4.4 kg N2O‐N ha−1 yr−1) by more than 3 times. Modeled and observed N2O emissions were dominated by peaks following N‐application and heavy rainfall events and were favored under high soil temperatures. The contribution of N2Oden was simulated well except for a 4‐week period when water‐filled pore space was overestimated and caused high N2O emissions which accounted for one third of the simulated annual N2O emissions. N2Onit fluxes were overestimated with DayCent because they are calculated as a fixed proportion of NH4+ converted to NO3−, while the data suggest that significant rates of nitrification can occur without inducing significant N2O emissions. The comprehensive data set made it possible to explain discrepancies between modeled and observed values. In‐depth model validations with detailed data sets are essential for a better understanding of the internal model behavior and for deriving possible model improvements.
On estimating length fluctuations of glaciers caused by changes in climatic forcing
Authors: Leysinger Vieli G. J.‐M. C. , Gudmundsson G. H.
Published: February 2004

The reaction of alpine glaciers to shifts in the equilibrium line altitude (ELA) is calculated by using a two‐dimensional numerical model to solve the full equations for the velocity and stress fields (full‐system model) in the absence of basal motion. Rates of advance and retreat of the snout of typically sized alpine glaciers are found to be insensitive to the details of the flow at the snout, even when the glaciers are far from steady state. A comparison of results obtained with a full‐system model and a shallow ice approximation (SIA) model yields no significant differences in advance or retreat rates. This assumption has been implicitly made in numerous previous climatic studies and is here shown to be well justified. Using a realistic mass balance altitude feedback, only slight model‐dependent changes in steady state lengths are found. The relative importance of mass balance and glacier dynamics for the transient response of alpine glaciers to changes in the ELA is given a precise meaning by determining the model‐dependent additional shifts in ELA needed for the SIA and the full‐system models to produce identical changes in length. For alpine glaciers, these additional shifts in ELA are on the order of 10 m, which is within the error range of ELA estimates. It follows that at least in the absence of significant basal motion, there is no need to include the effects of horizontal stresses when calculating the reaction of alpine glaciers to climatic changes. Attention should focus on accurate determination of the mass balance distribution and model tuning to give realistic ice thickness distributions.
South China Sea surface water evolution over the last 12 Myr: A south‐north comparison from Ocean Drilling Program Sites 1143 and 1146
Authors: Li Baohua , Wang Jiliang , Huang Baoqi , Li Qianyu , Jian Zhimin , Zhao Quanhong , Su Xin , Wang Pinxian
Published: February 2004

Planktonic foraminifera (PF) from Ocean Drilling Program (ODP) Sites 1143 and 1146 in the southern and northern South China Sea (SCS), respectively, were quantitatively analyzed in order to reconstruct the sea‐surface environment over the last 12 Myr. The observed decrease in deep‐dwelling PF species after ∼10 Ma at both sites is interpreted to reflect a depression of the upper water thermocline, corresponding to the closure of the Indonesian Seaway around 11–9 Ma. This upper water column structure implies the intensification of equatorial Pacific warm currents and the initial formation of the western Pacific “warm pool” (WPWP) during the early Late Miocene. The consistent pattern of south‐north thermocline evolution and the synchronous disappearance of Globoquadrina dehiscens (9.8 Ma) at both Sites 1143 and 1146 together imply that the entire SCS was likely under the influence of the newly developed WPWP at ∼10 Ma. After ∼8 Ma, sea‐surface temperatures and thermocline variations evolved differently between the southern and northern SCS. The total deep‐dwelling PF fauna at Site 1143 decreased gradually in abundance from 6.6 to 2 Ma, indicating a deepening of the thermocline in the southern SCS. In contrast, deep‐dwelling PF species increased in abundance from 3.1 to 2 Ma at Site 1146, reflecting a shoaling of the thermocline in the northern SCS. This south‐north contrast reflects two major environmental regimes: (1) the southern SCS, which has mainly been under the influence of the WPWP since the late Late Miocene, and (2) the northern SCS, where effects of the east Asian winter monsoon have prevailed, especially since the Late Pliocene. Estimate of past sea‐surface temperatures (SSTs) at Site 1143 suggests a relatively stable and warm environment in the southern SCS since about 2.5 Ma, with an increased influence of warm subsurface waters after the mid‐Pleistocene transition (1.2–0.9 Ma). In the northern SCS, however, a gradual decrease in winter SST recorded at Site 1146 over the last 4 Myr records east Asian monsoon evolution, especially the enhancement of the east Asian winter monsoon between 3.1 and 2 Ma.