bluuebunny/crossref_metadata_2025 huggingface dataset:
Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981 to 1999
Authors: Zhou Liming , Tucker Compton J. , Kaufmann Robert K. , Slayback Daniel , Shabanov Nikolay V. , Myneni Ranga B.
Published: February 2004

The northern high latitudes have warmed by about 0.8°C since the early 1970s, but not all areas have warmed uniformly [Hansen et al., 1999]. There is warming in most of Eurasia, but the warming rate in the United States is smaller than in most of the world, and a slight cooling is observed in the eastern United States over the past 50 years. These changes beg the question, can we detect the biotic response to temperature changes? Here we present results from analyses of a recently developed satellite‐sensed normalized difference vegetation index (NDVI) data set for the period July 1981 to December 1999: (1) About 61% of the total vegetated area between 40°N and 70°N in Eurasia shows a persistent increase in growing season NDVI over a broad contiguous swath of land from central Europe through Siberia to the Aldan plateau, where almost 58% (7.3×106 km2) is forests and woodlands; North America, in comparison, shows a fragmented pattern of change in smaller areas notable only in the forests of the southeast and grasslands of the upper Midwest, (2) A larger increase in growing season NDVI magnitude (12% versus 8%) and a longer active growing season (18 versus 12 days) brought about by an early spring and delayed autumn are observed in Eurasia relative to North America, (3) NDVI decreases are observed in parts of Alaska, boreal Canada, and northeastern Asia, possibly due to temperature‐induced drought as these regions experienced pronounced warming without a concurrent increase in rainfall [Barber et al., 2000]. We argue that these changes in NDVI reflect changes in biological activity. Statistical analyses indicate that there is a statistically meaningful relation between changes in NDVI and land surface temperature for vegetated areas between 40°N and 70°N. That is, the temporal changes and continental differences in NDVI are consistent with ground‐based measurements of temperature, an important determinant of biological activity. Together, these results suggest a photosynthetically vigorous Eurasia relative to North America during the past 2 decades, possibly driven by temperature and precipitation patterns. Our results are in broad agreement with a recent comparative analysis of 1980s and 1990s boreal and temperate forest inventory data [United Nations, 2000].
Tropical teleconnection and local response to SST anomalies during the 1997–1998 El Niño
Authors: Su Hui , Neelin J. David , Chou Chia
Published: February 2004

The quasi‐equilibrium tropical circulation model (QTCM) is used to examine the response to various sea surface temperature (SST) anomalies in the tropical oceans during the 1997–1998 El Niño. Both local and remote responses are noted. The negative precipitation anomalies to the north and south of the El Niño‐Southern Oscillation (ENSO) enhanced precipitation region are largely a response to the warm SST anomalies in the central and eastern Pacific. However, in the western Pacific and maritime continent, reduction of rainfall is mainly caused by local cold SST anomalies. In the winter of the 1997–1998 El Niño, strong warm SST anomalies in the Indian Ocean contributed to the local enhanced rainfall. They affect precipitation anomalies in central, eastern, and southern Africa. The drought in northern South America is clearly a remote response to ENSO warm SST anomalies in the Pacific, while the SST anomalies in the Atlantic also impact the drought. The tropical Pacific cold SST anomalies surrounding the ENSO warm anomalies appear not to be caused by surface flux changes associated with atmospheric teleconnection (in simulations with specified SST in the ENSO warm region and a mixed‐layer ocean model elsewhere). Atmospheric circulation tends to spread the warm anomalies, but ocean dynamics appears also to be important for both cold and warm SST anomalies outside the equatorial upwelling region. In both the regional specified SST experiments and mixed‐layer experiments, relative subsidence tends to occur within convective zones, and it is generally localized. On the other hand, tropospheric temperature and wind anomalies spread much farther. The typical spatial scale of the remote response in temperature and wind fields tends to be larger than the dominant scale of the precipitation response. The remote response in precipitation anomalies does not appear to be related to temperature and wind anomalies in a simple manner.
Small‐scale gravity waves in the upper mesosphere and lower thermosphere generated by deep tropical convection
Authors: Walterscheid R. L. , Schubert G. , Brinkman D. G.
Published: February 2004

A time‐dependent, nonlinear, fully compressible, axisymmetric, ƒ‐plane, numerical model is used to simulate the generation of small‐scale gravity waves in the upper mesosphere and lower thermosphere by intense deep convection in the troposphere. The simulations show that major convective storms in the tropics excite a broad spectrum of upper mesosphere‐lower thermosphere gravity waves above the storm centers. The wave field includes a component that is guided in a thermal duct in the lower thermosphere and propagates horizontally outward from above the storm. Storms excite oscillations over the source which are initially confined to a stratospheric duct but leak into the thermospheric duct over time generating a long train of small‐scale‐ducted waves. This ringing phenomenon persists for several hours after the storm has ended. The ducted disturbances may propagate large distances from the source and explain observations of a strong summertime anisotropy favoring southward propagation of small‐scale waves observed in the airglow over Adelaide more than 2000 km to the south of the storm events.
Large‐scale cloud, precipitation, and upper level features during Fronts and Atlantic Storm Track Experiment as inferred from TIROS‐N Operational Vertical Sounder observations
Authors: Chaboureau Jean‐Pierre , Claud Chantal , Cammas Jean‐Pierre , Mascart Patrick J.
Published: February 2004

Physical parameters retrieved from the measurements of the TIROS‐N Operational Vertical Sounder aboard the National Oceanic and Atmospheric Administration satellites are used to study the large‐scale cloud, precipitation, upper level features during the Fronts and Atlantic Storm Track Experiment period: the cloud top pressure, a precipitation index, and the temperature of lower stratosphere (TLS), respectively. The three dynamically consistent (i.e., temporally and spatially collocated) retrievals are able to characterize the weather regimes through their organization in locations expected from the cyclonic activity. Moreover, high‐level cloud patterns, when accompanied by rain, can be discriminated with respect to TLS field. When no (or weak) warm TLS feature can be found upstream, such patterns are frontal systems. On the contrary, a warm TLS event present upstream of a rainy cloud feature suggests a baroclinic interaction. In this case, the high‐level cloud pattern is associated with a large‐scale baroclinic cyclone, or at least a mature low. When examining the most precipitating cases using a composite technique, the nondeveloped systems (in term of absence of high‐cloud cover) are characterized by the absence of warm TLS features, grouping mostly blocking‐regime cases. On the other hand, the cases that are well developed in terms of cloud cover are characterized by warm TLS patterns upstream the precipitation area. Moreover, the well‐developed cases could be grouped into three families depending on the orientation of the warm TLS features: zonal, anticyclonic, and cyclonic. Only the later family, characterized by a cyclonically tilted trough, clusters the large‐scale baroclinic systems that all occurred during the zonal regime. This suggests that a cyclonically tilted trough has a positive impact on the development of cyclones.
Surface stress in offshore flow and quasi‐frictional decoupling
Authors: Mahrt L. , Vickers Dean , Sun Jielun , Crawford Timothy L. , Crescenti Gennaro , Frederickson Paul
Published: February 2004

Aircraft data collected at approximately 15 m above the sea surface in the coastal zone are analyzed to examine the spatial distribution of surface stress. Advection of stronger turbulence from land dominates the near‐surface turbulence for the first few kilometers offshore. With offshore flow of warm air over cold water, strong stratification leads to very small surface stress. Because the stability restricts the momentum transfer to the waves, the aerodynamic surface roughness decreases to very small values, which in turn decreases atmospheric mixing. The redevelopment of the boundary layer farther downstream is examined. Computation of fluxes from observations for stable cases is difficult due to a variety of errors including large random flux errors, possible instrumental loss of small‐scale flux, difference between the surface flux and that at the observational level, and inadvertent capture of mesoscale motions in the computed turbulent fluctuations. Although the errors appear to be substantial, the aircraft momentum fluxes compare favorably with those from sonic anemometers on two buoys and a tower at the end of a 570‐m pier, even with near collapse of the turbulence.
A scheme for improving scatterometer surface wind fields
Authors: Patoux Jérôme , Brown Robert A.
Published: February 2004

A method is presented for improving QuikSCAT surface wind fields. The University of Washington Planetary Boundary Layer model is used to retrieve a surface pressure field from any swath of QuikSCAT surface wind vectors. An alternate set of surface wind vectors is computed from the newly calculated pressure field. The latter can be smoothed and the process can be iterated. New surface wind vectors can be calculated where ambiguity removal fails and where measurements are missing. The present methodology preserves boundary layer dynamics and is an improvement over a statistical filter.
Behavior of OH and HO2 radicals during the Observations at a Remote Island of Okinawa (ORION99) field campaign: 1. Observation using a laser‐induced fluorescence instrument
Authors: Kanaya Yugo , Sadanaga Yasuhiro , Nakamura Kenji , Akimoto Hajime
Published: February 2004

The hydroxyl (OH) and hydroperoxyl (HO2) radicals were measured by a laser‐induced fluorescence instrument at Cape Hedo, Okinawa Island, Japan, in summer 1999 during the Observations at a Remote Island of Okinawa (ORION99) field campaign. The field deployment of the instrument and its calibrations are described in detail. From the frequent calibrations during the field campaign, it was shown that the instrument, utilizing an optical fiber to transmit the laser light to the detection cell located on a tower, had a sufficiently stable sensitivity to OH and HO2 in order to trace their diurnal and day‐to‐day variations. The detection limit of the instrument was typically around 4 × 106 radicals cm−3 with an integration time of 1 min. We could not examine fast OH variations. However, hourly averaged OH concentrations during daytime were statistically significant. The HO2 concentrations were higher and the detailed variations were detected. On average, OH and HO2 showed daytime maxima of around 4 × 106 radicals cm−3 and of around 17 pptv, respectively. The median, tenth, and ninetieth percentiles of the measured daytime HO2/OH concentration ratio were 76, 32, and 143, respectively. The power law dependence of HO2 on J(O1D) was about 0.5 when the NO concentration was lower than 300 pptv and was about 1 when the NO concentration was higher than 1 ppbv, which was consistent with the known radical chemistry in the lower troposphere.
Atmospheric longwave irradiance uncertainty: Pyrgeometers compared to an absolute sky‐scanning radiometer, atmospheric emitted radiance interferometer, and radiative transfer model calculations
Authors: Philipona Rolf , Dutton Ellsworth G. , Stoffel Tom , Michalsky Joe , Reda Ibrahim , Stifter Armin , Wendung Peter , Wood Norm , Clough Shepard A. , Mlawer Eli J. , Anderson Gail , Revercomb Henry E. , Shippert Timothy R.
Published: February 2004

Because atmospheric longwave radiation is one of the most fundamental elements of an expected climate change, there has been a strong interest in improving measurements and model calculations in recent years. Important questions are how reliable and consistent are atmospheric longwave radiation measurements and calculations and what are the uncertainties? The First International Pyrgeometer and Absolute Sky‐scanning Radiometer Comparison, which was held at the Atmospheric Radiation Measurement program's Southern Great Plains site in Oklahoma, answers these questions at least for midlatitude summer conditions and reflects the state of the art for atmospheric longwave radiation measurements and calculations. The 15 participating pyrgeometers were all calibration‐traced standard instruments chosen from a broad international community. Two new chopped pyrgeometers also took part in the comparison. An absolute sky‐scanning radiometer (ASR), which includes a pyroelectric detector and a reference blackbody source, was used for the first time as a reference standard instrument to field calibrate pyrgeometers during clear‐sky nighttime measurements. Owner‐provided and uniformly determined blackbody calibration factors were compared. Remarkable improvements and higher pyrgeometer precision were achieved with field calibration factors. Results of nighttime and daytime pyrgeometer precision and absolute uncertainty are presented for eight consecutive days of measurements, during which period downward longwave irradiance varied between 260 and 420 W m−2. Comparisons between pyrgeometers and the absolute ASR, the atmospheric emitted radiance interferometer, and radiative transfer models LBLRTM and MODTRAN show a surprisingly good agreement of <2 W m−2 for nighttime atmospheric longwave irradiance measurements and calculations.
Hygroscopic growth of ultrafine sodium chloride particles
Authors: Hämeri Kaarle , Laaksonen Ari , Väkevä Minna , Suni Tanja
Published: February 2004

The hygroscopic growth properties of ultrafine NaCl particles between 8 and 50 nm in mobility diameter have been studied using an ultrafine tandem differential mobility analyzer. The data include determination of hygroscopic growth curves, deliquescence behavior, and hysteresis. The measured growth factors are clearly smaller for the nanometer‐sized particles compared with particles in the size range of 0.1 μm and larger. While this behavior of the ultrafine particles can be qualitatively predicted by accounting for the Kelvin effect, quantitative differences to the experimental data remain even after the differential mobility analyzer data have been corrected to account for the cubic shape of the particles. We conclude that the differences are related to size‐dependent density and/or shape/structure of nanometer‐sized NaCl crystals. The deliquescence relative humidities obtained in this work for nanometer‐sized particles also show clear differences from the literature value obtained for large particles. The quantitative values of hygroscopic growth factors and deliquescence relative humidities for small particles have not been available previously, and our data are important in studies of the formation and properties of atmospheric aerosols.
A study of the dynamics of the equatorial lower stratosphere by use of ultra‐long‐duration balloons: 2. Gravity waves
Authors: Hertzog A. , Vial F.
Published: February 2004

This study is the companion paper of Vial et al. [this issue]. A campaign of ultra‐long‐duration, superpressure balloons in the equatorial lower stratosphere was held in September 1998. By conception these balloons evolve on isopycnic surfaces. Pressure and position were measured every 12 min, which enable to infer the characteristics of gravity waves with periods between 1 hour and 1 day in this region of the atmosphere. The intrinsic‐frequency spectra of horizontal wind fluctuations exhibit a −2 slope, while the one associated with vertical‐wind fluctuations is flat. Significant inhomogeneity of the wave activity is observed, and the variance of the shortest frequency waves is found to be linked to the position of the balloons with respect to the Intertropical Convergence Zone. On average, the total energy associated with gravity waves in the period range studied in this paper is found to be ∼ 7 J kg−1. Calculations of momentum flux have also been undertaken. It appears that there is an approximate equipartition of flux between eastward and westward propagating gravity waves and that the absolute value of the flux is 8–12 × 10−3 m2 s−2 at 20 km. A larger flux is also observed above convective regions. These values suggest that gravity waves may carry the largest part of the Eliassen‐Palm flux required for the driving of the quasi‐biennial oscillation.
M‐component mode of charge transfer to ground in lightning discharges
Authors: Rakov Vladimir A. , Crawford David E. , Rambo Keith J. , Schnetzer George H. , Uman Martin A. , Thottappillil Rajeev
Published: February 2004

The M‐component mode of charge transfer to ground is examined using (1) multiple‐station measurements of electric and magnetic fields at distances ranging from 5 to ∼ 500 m from triggered‐lightning channels and (2) measured currents at the channel base. Data have been obtained in 1997, 1999, and 2000 at the International Center for Lightning Research and Testing at Camp Blanding, Florida, for (1) “classical” M‐components that occur during the continuing currents following return strokes and (2) impulsive processes that occur during the initial stage of rocket‐triggered lightning and are similar to the “classical” M components. All lightning events considered here effectively transported negative charge to ground. For one triggered‐lightning event the electric field 45 km from the lightning channel was measured together with the current and close fields. The shapes and magnitudes of the measured close electric and magnetic fields are generally consistent with the guided‐wave mechanism of the lightning M component. Specifically, the M‐component electric field peak exhibits logarithmic distance dependence, ln(kr−1), which is indicative of a line charge density that is zero at ground and increases with height. Such a distribution of charge is distinctly different from the more or less uniform charge density that is characteristic of the dart leaders in triggered lightning, as inferred from close electric field measurements. The M‐component magnetic field peak decreases as the inverse distance (i.e., r−1), which is generally consistent with a uniform current within the lowest kilometer or so of channel. The M‐component electric field at 45 km appeared as a bipolar, microsecond‐scale pulse that started prior to the onset of the M‐component current at the channel base. M‐component‐type processes can produce acoustic signals with peak pressure values of the same order of magnitude as those from the leader/return stroke sequences in triggered lightning.
Multiscale modeling of the atmospheric fate and transport of mercury
Authors: Seigneur Christian , Karamchandani Prakash , Lohman Kristen , Vijayaraghavan Krish , Shia Run‐Lie
Published: February 2004

A numerical simulation of the atmospheric fate and transport of mercury (Hg) was conducted using a multiscale approach. Two different spatial scales were used to simulate (1) the global cycling of atmospheric Hg and (2) the atmospheric deposition of Hg in potentially sensitive areas. The global simulation was conducted using an updated version of our global Hg chemical transport model (CTM). The imbedded continental simulation was conducted using an updated version of the regional/continental CTM, TEAM. Simulations were conducted using 1998 meteorology and 1998/1999 emission inventories. Model simulation results show improved performance compared to earlier simulations. For example, the global simulation shows background concentrations of Hg species, interhemispheric gradients, and vertical gradients that are consistent with available measurements. The comparison of simulated Hg wet deposition fluxes with data from the Mercury Deposition Network in the United States shows a coefficient of determination (r2) of 0.75, little bias (−3%), and an average gross error of 21%. The major remaining sources of uncertainties, which include speciation of Hg emissions, Hg atmospheric chemistry, and dry and wet deposition processes for Hg species, are discussed.
Introduction to special section: Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) and Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT 2)
Authors: Thompson Anne M. , Singh Hanwant B. , Schlager Hans
Published: February 2004

Emissions of atmospheric species from the engines of subsonic aircraft at cruise altitude (roughly, above 7 km) are of concern to scientists, the aviation industry, and policymakers for two reasons. First, water vapor, soot, and sulfur oxides, and related heterogeneous processes, may modify clouds and aerosols enough to perturb radiative forcing in the upper troposphere/lower stratosphere (UT/LS). A discussion of these phenomena appears in chapter 3 of the Intergovernmental Panel on Climate Change (IPCC) [1999] aviation assessment. An airborne campaign conducted to evaluate aviation effects on contrail, cirrus, and cloud formation is described in Geophysical Research Letters (25(8‐10), 1998).
Dynamics of evaporation from saline water bodies
Authors: Oroud I. M.
Published: February 2004

The sensitivity of evaporation from saline solutions resulting from a variation in their salinity/activity coefficients (β) is investigated. The calculations are based on a theoretically derived parameter (γ = ∂T/∂β), which accounts for the temperature change following a departure in the salinity/activity coefficient. Results reveal that γ is a function of saline temperature, the level of the activity coefficient, and the transfer coefficients for water vapor and sensible heat. For a transfer coefficient of 15 W m−2 kPa−1, γ ranges from ≈−6 K for a saline temperature of 5°C and activity coefficient of 0.95 to ≈−27 K for a saline temperature of 40°C and activity coefficient of 0.4. The present formulation provides a robust method that can be used to identify lower and upper theoretical limits for the variations of saline temperatures, and also to derive the departures of the activity coefficient and evaporation rates for saline solutions from routine meteorological observations only. Calculations with the Penman equation reveal that the aerodynamic term is the most sensitive parameter influencing evaporation from saline solutions following a departure in their salinity/activity coefficients, with a contribution ranging from ≈50%–65% of total annual evaporation departure. Alterations to net radiation following a variation in β accounts for a contribution of about 25% of total annual change in evaporation.
Effective diffusivity as a diagnostic of atmospheric transport: 2. Troposphere and lower stratosphere
Authors: Haynes Peter , Shuckburgh Emily
Published: February 2004

The effective diffusivity diagnostic is used to analyze the isentropic transport and mixing properties of observed winds in the upper troposphere and the lower stratosphere (300–450 K), following the approach described in part 1 [Haynes and Shuckburgh, this issue]. Local minima in effective diffusivity on isentropic surfaces in the range 330–400 K indicate transport barriers in each hemisphere associated with the extratropical tropopause. The strongest part of these “tropopause barriers” are coincident with the core of the subtropical jet at about 350 K. They are shown to have a seasonal evolution in which they are strongest in winter and considerably weakened by the monsoon circulations in summer. The barrier in the Southern Hemisphere is seen to be generally stronger than that in Northern Hemisphere during the same season. The minimum value of effective diffusivity on each isentropic surface is proposed as a new definition of the tropopause. This effective‐diffusivity definition corresponds most closely to potential vorticity (PV) values of ±2 PVU at 330 K, ±2.5 PVU at 350 K, and ±4.5 PVU at 370 K (with larger values being more appropriate during the summer monsoon period), rather than to the conventional tropopause definition of a single PV value at all levels. It is also demonstrated that the lower limit of the barrier at the stratospheric polar‐vortex edge, i.e., the “sub‐vortex” transition, varies in altitude throughout the winter. In the Antarctic the transition generally occurs at 380 K and is sometimes as low as 350 K. In the Arctic the transition is higher, rarely occurring below 400 K and frequently occurring above 450 K.
Outflow patterns of pollutants from East Asia to the North Pacific in the winter monsoon
Authors: Kaneyasu Naoki , Takeuchi Koji , Hayashi Masayasu , Fujita Shin‐ichi , Uno Itsushi , Sasaki Hidetaka
Published: February 2004

Outflow patterns of air pollutants, and mechanisms associated with a “monsoon” condition that is peculiar to the East Asian region in winter, have been studied. To capture the continental outflow, airborne and shipboard measurements of aerosols and trace gases were conducted over the northern part of the East China Sea and the western part of the Sea of Japan on January 17–26, 1991 (airborne), January 28–29, 1992 (airborne), January 22–31, 1993 (shipboard), and January 29–30, 1993 (airborne). At least two types of highly polluted air masses were found at different times of the monsoon. Type 1 polluted air mass, which contained more than 10 μg m−3 of non‐sea‐salt SO42− and more than 5 μgC m−3 of black carbon aerosols, typically appeared over the East China Sea at the beginning of the monsoon period. Chemical and microphysical characteristics suggest that the type 1 polluted air mass is chemically aged. Type 2 air mass appeared in the later half of the monsoon period and had characteristics of freshly emitted pollutants. It consisted of several high concentrations of SO2 (>10 ppbv) confined in narrow horizontal width (10 to 30 km wide). Analyses using an Eulerian chemical transport model and detailed backward trajectories indicate that the type 1 polluted air mass was subcontinental in extent and its formation and the transport were dominated by a synoptic‐scale weather cycle; it was formed over the Asian continent under anticyclonic conditions that persisted for 3–4 days, and transported intermittently to the east over the Pacific Ocean by the passage and the development of traveling cyclones. This study suggests that aerosols that flow out from East Asia are enriched with carbonaceous materials and consequently have low single‐scattering albedo, which may affect the direct radiative forcing by aerosols over the North Pacific.
A nonlinear optimal estimation inverse method for radio occultation measurements of temperature, humidity, and surface pressure
Authors: Palmer Paul I. , Barnett J. J. , Eyre J. R. , Healy S. B.
Published: February 2004

An optimal estimation inverse method is presented which can be used to retrieve simultaneously vertical profiles of temperature and specific humidity, in addition to surface pressure, from satellite‐to‐satellite radio occultation observations of the Earth's atmosphere. The method is a nonlinear, maximum a posteriori technique which can accommodate most aspects of the real radio occultation problem and is found to be stable and to converge rapidly in most cases. The optimal estimation inverse method has two distinct advantages over the analytic inverse method in that it accounts for some of the effects of horizontal gradients and is able to retrieve optimally temperature and humidity simultaneously from the observations. It is also able to account for observation noise and other sources of error. Combined, these advantages ensure a realistic retrieval of atmospheric quantities. A complete error analysis emerges naturally from the optimal estimation theory, allowing a full characterization of the solution. Using this analysis, a quality control scheme is implemented which allows anomalous retrieval conditions to be recognized and removed, thus preventing gross retrieval errors. The inverse method presented in this paper has been implemented for bending angle measurements derived from GPS/MET radio occultation observations of the Earth. Preliminary results from simulated data suggest that these observations have the potential to improve numerical weather prediction model analyses significantly throughout their vertical range.
Radiative forcing due to changes in tropospheric ozone in the period 1980 to 1996
Authors: Myhre Gunnar , Karlsdóttir Sigrun , Isaksen Ivar S. A. , Stordal Frode
Published: February 2004

Radiative forcing calculations of changes in tropospheric ozone in the period 1980 to 1996 are performed. The tropospheric ozone changes used in this study are from a chemistry transport model [Karlsdóttir et al., this issue]. For the radiative transfer calculations, schemes for thermal infrared radiation and shortwave radiation are used. Effects of reduced stratospheric ozone and emissions of tropospheric ozone precursors on the changes in tropospheric ozone are taken into account. Large positive radiative forcing is estimated over Southeast Asia, due to emissions of ozone precursors. Effects of decreased stratospheric ozone have reduced the radiative forcing due to change in tropospheric ozone from 1980 to 1996, particularly at high latitudes where the radiative forcing is negative.
On the hydrological cycle under paleoclimatic conditions as derived from AGCM simulations
Authors: Lohmann Gerrit , Lorenz Stephan
Published: February 2004

The atmospheric hydrological cycle is compared for different time slices of the late Quaternary. Simulations have been conducted with an atmospheric circulation model at T42 resolution, and we have performed a global evaluation of the atmospheric water vapor transport. The water export from the Atlantic catchment area, important for driving the large‐scale thermohaline ocean circulation, is analyzed in detail. For the Last Glacial Maximum (LGM), we examine the model's sensitivity with respect to tropical cooling relative to the CLIMAP reconstruction which is motivated by recent data. We find that the LGM experiment with tropical cooling is in better agreement with proxy data available. Our experiments indicate that the water vapor transport is strongly affected by three mechanisms: continental drying, eddy moisture transport, and changes in the tropical circulation. Except for the continental drying and the blocking effect of the Laurentide ice sheet, the hydrological cycle is substancially different for both LGM experiments. We find that the hydrological system is rather sensitive to tropical temperature change which is important to understand paleoclimate and future climate changes.
Spectral signature of column solar radiation absorption during the Atmospheric Radiation Measurement Enhanced Shortwave Experiment (ARESE)
Authors: O'Hirok William , Gautier Catherine , Ricchiazzi Paul
Published: February 2004

Spectral and broadband shortwave radiative flux data obtained from the Atmospheric Radiation Measurement Enhanced Shortwave Experiment (ARESE) are compared with 3‐D radiative transfer computations for the cloud field of October 30, 1995. Because the absorption of broadband solar radiation in the cloudy atmosphere deduced from observations differs from that modeled by 135 W m−2, we performed a consistency analysis using spectral observations and the model to integrate for wavelengths between the spectral observations. To match spectral measurements, aerosols need a reduction in both single‐scattering albedo (from 0.938 to 0.82) and asymmetry factor (from 0.67 to 0.61), and cloud droplets require a threefold increase in coalbedo. Even after modifying the model inputs and microphysics the difference in total broadband absorption is still of the order of 75 W m−2. Finally, an unexplained absorber centered near 1.06 μm appears in the comparison that is much too large to be explained by any known atmospheric process.
AVHRR estimates of surface temperature during the Southern Great Plains 1997 Experiment
Authors: Kaleita Amy L. , Kumar Praveen
Published: February 2004

In this study we aim to (1) explore the differences in the accuracy of satellite‐derived land‐surface skin temperature for day and nighttime observations, (2) assess the effects of large solar zenith angles, and (3) develop an understanding of the spatial variability of the observed temperatures. Land‐surface skin temperatures are obtained using the split‐window technique from observations of the AVHRR instrument aboard the NOAA‐12 and NOAA‐14 satellites for the SGP97 (Southern Great Plains 1997) hydrology experiment. From the study of several days of observations we find that observed biases with respect to the ground temperature, both during day and night, are small. However, except for one rainy day measurement, there consistently was a warm bias during the day and cold bias during the night. Contrary to the hypothesis that at large solar zenith angles the observed temperatures are representative of the shelter height air temperature, we find that the observed temperatures are still closer to the ground temperatures than the air temperature. The spatial correlation is nearly isotropic and has an exponential decay. The correlation lengths demonstrate tremendous spread both during the day and during the night with the mean during the day being about 50% larger than that during the night. The average correlation length of 8.63 km during the day is much smaller than the grid sizes typically used in short‐time hydrology/mesoscale forecasts. This suggests that for modeling purposes the temperatures in each grid box may be treated as uncorrelated. However, the variance captured can be significantly smaller than the true value.
Climate forcings and the initiation of low‐latitude ice sheets during the Neoproterozoic Varanger glacial interval
Authors: Chandler Mark A. , Sohl Linda E.
Published: February 2004

The GISS GCM was used to determine if a diverse set of climate forcings, alone or in combination, could have initiated the low‐latitude ice sheets of the Varanger (∼600 Ma) glacial interval. The simulations use a realistic reconstruction of the paleocontinental distribution and test the following forcings, alone and in combination: 6% solar luminosity decrease, four atmospheric CO2 scenarios (1260, 315, 140, and 40 ppm), a 50% increase and a 50% decrease in ocean heat transports, and a change in obliquity to 60°. None of the forcings, individually, produced year‐round snow accumulation on low‐latitude continents, although the solar insolation decrease and 40 ppm CO2 scenarios allowed snow and ice to accumulate at high and middle latitudes. Combining forcings further cools the climate: when solar luminosity, CO2, and ocean heat transports were all decreased, annual mean freezing and snow accumulation extended across tropical continents. No simulation would have initiated low‐latitude glaciation without contemporaneous glaciation at higher latitudes, a finding that matches the distribution of glacial deposits but which argues against high obliquity as a cause of the Varanger ice age. Low‐level clouds increased in most scenarios, as did surface albedo, while atmospheric water vapor amounts declined; all are positive feedbacks that drive temperatures lower. In the most severe scenario, global snow and ice cover increased to 68%, compared to 12% under modern conditions, and water vapor dropped by 90%. These results do not necessarily preclude a ‘snowball’ Earth climate scenario for the Varanger glacial interval. However, either more severe forcings existed or radical changes occurred in the ocean/atmosphere system which are unaccounted for by the GCM. Also, as sea ice extent increased in these experiments, snow accumulation began to decline, because of an increasingly dry atmosphere. Under snowball Earth conditions, glaciation would be impossible, since the hydrological cycle would all but cease if the atmosphere's primary moisture source were cut off.
Effects of raindrop size distribution variations on microwave brightness temperature calculation
Authors: Coppens Dorothée , Haddad Ziad S.
Published: February 2004

Many current passive‐microwave rain‐retrieval methods are based on databases built off‐line using cloud models: the models are used to simulate rain events, and radiative transfer calculations produce the associated brightness temperatures. The radiative transfer models depend on, among others, the distribution of hydrometeor sizes. The most popular raindrop size distribution (DSD) used in most models is Marshall and Palmer's [1948]. Improvements were proposed later by Sekhon and Srivastava [1971], Willis and Tattelman [1989], and Feingold and Levin [1986]. Using these DSD models, we study the dependence of forward radiative transfer calculations on the hydrometeor size distribution and quantify the uncertainty due to DSD variability.
Behavior of ultrafine particles in continental and marine air masses at a rural site in the United Kingdom
Authors: Coe Hugh , Williams Paul I. , McFiggans Gordon , Gallagher Martin W. , Beswick Karl M. , Bower Keith N. , Choularton Tom W.
Published: February 2004

Particle size distribution measurements were made at a coastal site in the United Kingdom. These are presented, and the behavior of recently formed ultrafine particles is discussed. No ultrafine particles were observed in maritime air masses; however, 3 to 7 nm particles were frequently observed at enhanced concentrations when the wind direction was from the land. Their formation was favored at lower temperatures, when 1 ppbv or more of SO2 was present and in air masses that had not been aged extensively. On days when enhanced ultrafine particle concentrations were observed, 3 nm particles increased sharply in the morning, approximately 30 to 90 min after the UV solar flux first increased. By early afternoon the ultrafine particle concentration had returned to background levels. Rapid measurements of 5 nm particles showed no correlation with turbulence parameters, although the boundary layer mixing scales were similar to growth times of freshly nucleated particles to 5 nm diameter. However, ultrafine particle concentrations do correlate with the availability of sulphuric acid vapor. A delay of approximately an hour between the increase of H2SO4 in the morning and a large increase in ultrafine particle concentrations is due to the growth of particles to observable sizes, not the nucleation process itself. An analysis of the timescales for growth showed that coagulation may be important immediately after the particles have nucleated but its effectiveness reduces as number concentration falls. Conversely, growth by condensation is initially slow due to the Kelvin effect but increases in importance as the particles reach observable sizes.
Airborne intercomparison of vacuum ultraviolet fluorescence and tunable diode laser absorption measurements of tropospheric carbon monoxide
Authors: Holloway John S. , Jakoubek Roger O. , Parrish David D. , Gerbig Christoph , Volz‐Thomas Andreas , Schmitgen Sandra , Fried Alan , Wert Brian , Henry Bruce , Drummond James R.
Published: February 2004

During the fall 1997 North Atlantic Regional Experiment (NARE 97), two separate intercomparisons of aircraft‐based carbon monoxide measurement instrumentation were conducted. On September 2, CO measurements were simultaneously made aboard the National Oceanic and Atmospheric Administration (NOAA) WP‐3 by vacuum ultraviolet (VUV) fluorescence and by tunable diode laser absorption spectroscopy (TDLAS). On September 18, an intercomparison flight was conducted between two separate instruments, both employing the VUV fluorescence method, on the NOAA WP‐3 and the U.K. Meteorological Office C‐130 Hercules. The results indicate that both of the VUV fluorescence instruments and the TDLAS system are capable of measuring ambient CO accurately and precisely with no apparent interferences in 5 s. The accuracy of the measurements, based upon three independent calibration systems, is indicated by the agreement to within 11% with systematic offsets of less than 1 ppbv. In addition, one of the groups participated in the Measurement of Air Pollution From Satellite (MAPS) intercomparison [Novelli et al., 1998] with a different measurement technique but very similar calibration system, and agreed with the accepted analysis to within 5%. The precision of the measurements is indicated by the variability of the ratio of simultaneous measurements from the separate instruments. This variability is consistent with the estimated precisions of 1.5 ppbv and 2.2 ppbv for the 5 s average results of the C‐130 and the WP‐3 instruments, respectively, and indicates a precision of approximately 3.6% for the TDLAS instrument. The excellent agreement of the instruments in both intercomparisons demonstrates that significant interferences in the measurements are absent in air masses that ranged from 7 km in the midtroposphere to boundary layer conditions including subtropical marine air and continental outflow with embedded urban plumes. The intercomparison of the two VUV instruments that differed widely in their design indicates that the VUV fluorescence technique for CO measurements is not particularly sensitive to the details of its implementation. These intercomparisons help to establish the reliability of ambient CO measurements by the VUV fluorescence technique.
Long‐term variations of UV‐B doses at three stations in northern Europe
Authors: Kaurola Jussi , Taalas Petteri , Koskela Tapani , Borkowski Janusz , Josefsson Weine
Published: February 2004

Recent analysis of the total ozone observations indicate a negative trend of about 4%/decade in the Northern Hemisphere midlatitudes during the last two decades [WMO, 1999]. The effect of this decline on surface UV levels is of interest to a variety of applications. In this work the long‐term variation of UV radiation at three stations located in northern Europe (Belsk, Norrköping, and Jokioinen) has been studied using data from (1) ground‐based observations, (2) surface UV doses determined using TOMS satellite measurements, and (3) reconstructed UV doses using observations of global radiation, total ozone, and radiative transfer modeling. For each station the estimates of daily UV doses from various sources have been intercompared, and a trend analysis has been performed to reveal long‐term changes in the UV radiation. Data sets, which start in the late 1970s or early 1980s, show a general positive trend in annual doses of UV radiation. Some of these upward trends are statistically significant. For Belsk the increases are in the range of 5–15% per decade during spring and summer. The largest increases, about 20%/decade, has been observed in Norrköping during spring. At Jokioinen there has been a slight upward trend in UV throughout the year. The analysis of reconstructed Belsk data from 1966 onward shows that the positive trend since late 1970s was preceeded by a negative trend. The reason for such changes is probably not only related to the changes in the total ozone but also to changes in aerosol content and cloudiness. The agreement of the UV series based on different data sources is good. This was studied using a subset of data in which it was required that data from all possible sources were available. The different trend estimates were in very close agreement with each other. However, there were often differences in absolute values, which is probably related to problems in calibration and limitations of the models.
Simulation of arctic low‐level clouds observed during the FIRE Arctic Clouds Experiment using a new bulk microphysics scheme
Authors: Girard Eric , Curry Judith A.
Published: February 2004

A new bulk cloud microphysics scheme that accounts for aerosol microphysical properties and size distribution is implemented into the single‐column version of the ARCSyM. This scheme is distinguished from other bulk microphysics schemes by its prognostic determination of cloud particle number concentration and saturation ratio. The new scheme is compared to a simpler bulk microphysics scheme and observations taken during the FIRE Arctic Clouds Experiment in May 1998. Qualitatively, the two microphysics schemes are generally in agreement with the observed cloud formation and evolution. Comparison with aircraft measurements at 3 times shows that the new scheme better discriminates cloud phase and reproduces reasonably well the observed liquid and ice water content for two cases. The better performance of the new scheme is attributed to its more elaborated treatment of the freezing process which is made possible by the prognostic determination of cloud particle number concentration and the assumption of a bimodal lognormal cloud size distribution. Sensitivity studies are performed to assess four aerosol microphysical properties on the evolution of cloud microphysical processes. Results show that the IFN concentration, the aerosol number concentration, the slope of the aerosol size distribution, and the aerosol solubility may impact substantially on cloud phase and total water content. The liquid water path and ice water path can vary by as much as 100 g m−2 locally as a result of the variation of these parameters related to aerosols.
Interaction of binary tropical cyclones in a coupled tropical cyclone‐ocean model
Authors: Khain Alexander , Ginis Isaac , Falkovich Aleksandr , Frumin Mitia
Published: February 2004

The motion and evolution of binary tropical cyclones was investigated using a coupled tropical cyclone‐ocean movable nested grid model. The model consists of eight‐layer atmospheric and seven‐layer ocean primitive equation models. Several regimes of binary storm interaction have been identified, depending on the initial separation distance (d) and differences in storm strengths. At d less than a few hundred kilometers, interacting storms experienced complete merger (CM) or partial merger (PM). At larger d (between about 600 km and 1000 km), three regimes of storm interaction have been found: PM, straining out (SO), characterized by complete disintegration of the weaker storm, and mutual straining out (MSO), characterized by weakening and dissipation of both storms. SO occurred when the interacting storms had substantially different intensities and strengths. MSO was observed when the interacting storms were comparable in size and intensity. In the latter case the storms were unable to approach each other at distances smaller than a certain minimum distance (of about 450–500 km) without being mutually stretched out. Moreover, initial attraction of the storms in this regime was replaced by repulsion, as frequently observed in the western Pacific. At d exceeding about 1000 km, elastic interaction (EI) was found, when the storms interact without any significant changes in their intensity and structure. In additional experiments with a conditional instability of the second kind (CISK) type parameterization of convective heating the storm interaction was very different: The storms were nearly axisymmetric and very compact, and they continued approaching each other until they merged. Thus more realistic simulations of binary storm interaction can be achieved by using a physically more reasonable convective parameterization.