bluuebunny/crossref_metadata_2025 huggingface dataset:
Response of the seasonal carbon cycle in high latitudes to climate anomalies
Authors: Wu Wanli , Lynch Amanda H.
Published: February 2004

Many observational studies have demonstrated that the amplitude of the seasonal carbon cycle in high latitudes is increasing and advancing in phase. Several hypotheses related to natural and anthropogenic landscape disturbance and climate change have been proposed to explain these observations. This study employs a regional atmosphere‐biosphere model (the Arctic Regional Climate System Model (ARCSyM)) to investigate the impacts of the climatic effects of elevated carbon dioxide on terrestrial carbon exchange over Alaska. A perturbation experiment, driven by a “transient” global climate scenario at the point at which atmospheric carbon dioxide has doubled, is compared to a validated present‐day simulation. The focus of this paper is on the role of physical climate in seasonal carbon exchange between the atmosphere and the terrestrial biosphere at regional scales. In the present‐day simulation the ARCSyM reproduces most observed features of climate and the surface energy budget and simulates a clearly defined growing season in the seasonal cycle of carbon dioxide exchange. In the perturbation experiment, the model atmosphere shows a generally warmer and wetter climate, with increased permafrost thaw. Two dominant biomes, tundra and boreal forest, are analyzed in detail for their responses to the perturbed climate. The amplitudes of the seasonal carbon dioxide cycle for Alaskan boreal forest and tundra increase 12 and 8%, respectively, with phase advances of ∼7 and 2 days, respectively. The forcing of photosynthetic productivity by soil moisture changes is a dominant contributor to the simulated changes in net ecosystem exchange. Boreal forest ecosystem is more sensitive to moisture anomalies due to a drier soil, whereas in the tundra regions, which are often close to saturation, changes are forced primarily by temperature variations. Thus in the event of a northward migration of tree line, sensitivity of regional carbon exchange to climate perturbations could increase.
Black carbon and other species at a high‐elevation European site (Mount Sonnblick, 3106 m, Austria): Concentrations and scavenging efficiencies
Authors: Hitzenberger R. , Berner A. , Kromp R. , Kasper‐Giebl A. , Limbeck A. , Tscherwenka W. , Puxbaum H.
Published: February 2004

During a recent measurement project, several intensive campaigns were performed on Mount Sonnblick (3106 m above sea level) in the Austrian central range of the Alps. Cloud water and interstitial aerosol samples were obtained from supercooled clouds by using a specially designed cloud water sampler [Kruisz et al., 1993]. The samples were analyzed for black carbon (BC) by an optical technique (integrating sphere [Hitzenberger et al. 1996]) using liquid samples, for major inorganic ions by ion chromatography and for total carbon (TC) by a combusion method. During the fall campaign of 1996, cloud water BC concentrations ranged from 0.45 to 3.64 μg/mL (average concentration 0.85 μg/mL). During the spring 1997 campaign, cloud water BC concentrations ranged from 0.55 to 2.95 μg/mL (average concentration 1.07 μg/mL). The dominant ion in cloud water was SO42− with concentrations from 0.36 to 86.5 μg/mL (average 6.83 μg/mL) in fall 1996 and 0.31–15.4 μg/mL (average 3.06 μg/mL) in spring 1997. In the individual samples, the BC/SO42− ratio ranged from 0.036 to 1.2 (average 0.316) in fall 1996 and 0.036 to 2.04 (average 0.79) in spring 1997. The extreme values were usually confined to short periods within one cloud event. Scavenging efficiencies ε were calculated by using cloud water and interstitial aerosol concentrations from samples obtained simultaneously with the cloud water sampler for the 1997 campaign. For BC, εBC = 0.74 (±0.19) was found, while the values for SO42− and TC were εSO4 = 0.91 (±0.08) and εTC = 0.57 (±0.21), respectively. The findings of an earlier study [Kasper‐Giebl et al., 2000], where εSO4 depended on the liquid water content, were confirmed here for all the three substances.
Sulfate in air and snow at the South Pole: Implications for transport and deposition at sites with low snow accumulation
Authors: Harder Susan , Warren Stephen G. , Charlson Robert J.
Published: February 2004

Air and surface snow were sampled at Amundsen‐Scott Station at the South Pole from July through December of 1992. Four‐day averages of non‐sea‐salt sulfate (nss‐SO4=) aerosol show a strong seasonal trend, increasing by a factor of about 30 from winter to summer as oceanic biogenic sources become more active and atmospheric transport pathways change. Three‐dimensional sampling of small‐scale surface topography (sastrugi) provides evidence supporting wind pumping and filtration of aerosol by snow as a significant mechanism for dry deposition at this site. The estimated monthly flux of nss‐SO4= to the snow surface also increases from winter to spring, but by only a factor of 2, suggesting that the efficiency of deposition for this aerosol‐borne species from the near‐surface air to the snow is greater in winter. The strong surface‐based temperature inversion in winter inhibits vertical motion and may limit the rate of delivery of aerosol to the boundary layer from the free troposphere. Because the snow surface is a sink for aerosol, near‐surface measurements of aerosol in the stable inversion layer may not be representative of the free troposphere. Air and snow data in summer (when the inversion is weak) are used to estimate a tropospheric residence time of 4–20 days for nss SO4=.
Vertical distributions of cloud condensation nuclei spectra over the springtime Arctic Ocean
Authors: Yum Seong Soo , Hudson James G.
Published: February 2004

More than 23 hours of continuous condensation nuclei (CN) and cloud condensation nuclei (CCN) spectral data from the Arctic Clouds Experiment are presented and analyzed. These measurements were made at altitudes ranging from 6 km to 30 m in eight flights during May 1998 over the mostly frozen Arctic Ocean at least 500 km north of the Alaskan coast. Concentrations generally increased with altitude with a pronounced deficit in the boundary layer when low stratus clouds were present. The low‐level vertical gradient could be demonstrated to be a result of cloud scavenging. Boundary layer concentrations at 0.8% supersaturation averaged 76 cm−3 with low cloud (below the low cloud) and 250 cm−3 when no low cloud was present. The ratio of CCN to CN (total particles) was relatively high, usually exceeding 0.6. The relatively high concentrations at higher altitudes, the high CCN/CN ratio, and the lower CCN spectral slopes are characteristic of an aged aerosol probably due to long‐range transport.
Ultrafine particle size distributions measured in aircraft exhaust plumes
Authors: Brock Charles A. , Schröder Franz , Kärcher Bernd , Petzold Andreas , Busen Reinhold , Fiebig Markus
Published: February 2004

Fast‐response measurements of particle size distributions were made for the first time in the near‐field plume of a Boeing 737–300 aircraft burning fuel with fuel sulfur (S) contents (FSCs) of 56 and 2.6 ppmm, as well as in fresh and dissipating contrails from the same aircraft, using nine particle counters operating in parallel. Nonsoot particles were present in high concentrations, with number maxima at diameters ≤3nm. From these and ancillary measurements we determined the apparent emission index, EI*, or amount produced per kilogram of fuel burned, for particle nuipber, surface, and volume, and the value of η*, the apparent fraction of fuel S found in the particulate phase in the plume assuming the particles were composed of sulfuric acid and water. All of these parameters were functions of the age of the plume since emission, FSC, and presence or absence of contrail. The measurements support the use of values of η* of <10% in numerical models of the effects of the current aircraft fleet on the atmosphere, suggest that non‐S species become important contributors to particulate mass at very low FSCs, and place significant constraints on numerical models of plume microphysical processes.
A comparative study of statistical total column ozone forecasting models
Authors: Plets Hans , Vynckier Coleet
Published: February 2004

Since the beginning of the 1990s, total column ozone is being forecasted on an operational basis at several institutes around the world. It usually serves as an input parameter for the UV index forecast. The ozone forecasting procedures described in the literature are ad hoc statistical regression models. They yield sufficiently accurate predictions. So far, no detailed description, including the validity of the underlying assumptions of regression analysis, has been communicated. We studied this subject and conclude that they are violated by the models currently in use. Taking into account the causes for this violation, we come up with a more sophisticated model, which combines a stepwise multilinear regression model with an autoregressive conditional heteroskedasticity error model. This model still has shortcomings, but it yields a reduction of variance with respect to the commonly used ordinary least squares stepwise regression model of the order of 35%. Expressed in terms of R2, the improvement is 6%. The root‐mean‐square error of the model we propose is 3.2 Dobson units lower than the usually applied regression model.
Development of a global stratospheric aerosol climatology: Optical properties and applications for UV
Authors: Stevermer Amy J. , Petropavlovskikh Irina V. , Rosen James M. , DeLuisi John J.
Published: February 2004

A long‐term stratospheric aerosol climatology is constructed using SAGE II spectral extinction measurements, worldwide lidar observations, and time series of atmospheric turbidity and transmission data. The results contain important information for assessing stratospheric aerosol effects on ultraviolet (UV) radiation and are intended to provide more accurate stratospheric aerosol corrections for the Umkehr‐retrieved ozone profiles. The record, dating from 1953 to 1997, is also useful for climate studies and for estimating errors to other remote sensing methods. A significant part of this study involves examination of the relationships between aerosol size distribution and optical properties as a function of wavelength. These relationships provide empirical means for estimating aerosol extinction at various wavelengths, including UV‐B wavelengths, from a single lidar backscatter value or from an extinction value at a different wavelength. A total of 134 size distributions, reported in the literature by several investigators, were used in this analysis. The results suggest the existence of a bounded and predictable domain for aerosol optical properties and indicate distinct differences between volcanic and background aerosol regimes. This paper summarizes the data and methods used in the development of a stratospheric aerosol climatology and illustrates the resulting long‐term time series of monthly and zonally averaged stratospheric aerosol optical depth at 0.320 μm. This wavelength was selected because it is close to the midpoint of the Umkehr C‐pair wavelengths of 0.311 and 0.332 μm.
Trend analysis of O3 and CO in the period 1980–1996: A three‐dimensional model study
Authors: Karlsdóttir Sigrún , Isaksen Ivar S. A. , Myhre Gunnar , Berntsen Terje K.
Published: February 2004

During the last 20–30 years, noticeable changes have occurred in emissions related to anthropogenic activity over the globe. In this study a three‐dimensional chemical transport model for the troposphere (the 3‐D Oslo CTM1 model) is used to investigate how changes in the concentration of carbon monoxide (CO) and ozone (O3) in the period 1980–1996 can be related to changes in emission rates and patterns. The results indicate that changes in both species reflect the emission changes well, especially CO because it is to a large degree driven by CO emissions. The most pronounced effect of the changes in emissions for CO and O3 is seen in eastern Europe and Southeast Asia. The annual trend in CO and O3 concentrations in the planetary boundary layer during the period 1980–1996 in eastern Europe is −1.94±0.42 ppbv/yr and −0.01±0.02 ppbv/yr, respectively. In Southeast Asia a positive trend is seen, with 2.29±0.13 ppbv/yr in the CO concentration and 0.6 ± 0.03 ppbv/yr in the O3 concentration. The agreement between the modeled CO and O3 concentrations and observations is good. For O3 an improvement is gained in the modeled results compared with observations, by including changes in emissions from aircraft, and two effects of reduced stratospheric O3 level (altered photolysis rates and stratospheric‐tropospheric exchange) during the period of study.
The deuterium content of atmospheric molecular hydrogen: Method and initial measurements
Authors: Gerst Steve , Quay Paul
Published: February 2004

A new technique for determining the δD of atmospheric molecular hydrogen (H2) at background mixing ratios is described in detail. An air sample is compressed into a high‐pressure cylinder, after which the cylinder is cooled to −196°C to condense the air and generate a headspace greatly enriched in H2. The H2‐enriched headspace is collected and admitted into a high‐vacuum extraction line, where the H2 is separated from the headspace gases by combustion to water followed by cryogenic collection. The H2‐derived water is then reduced back to H2 for measurement of its D/H content via isotope‐ratio mass spectrometry. The primary limitation of the method is contamination during compressor collection and subsequent storage of air samples in high‐pressure cylinders. The δD of atmospheric H2 was measured on air samples collected between 46°N and 72°S during a research cruise in the Pacific Ocean during 1998 and at Cheeka Peak, Washington (48°N), during 1997–1999 and Point Barrow, Alaska (71°N), during 1996–1997. These measurements yield a global average δD of atmospheric H2 of +130±4‰ and average Northern and Southern Hemispheric δD values of +123±3‰ and +138±8‰, respectively (versus Vienna standard mean ocean water).
The seasonal cycle of water vapor and saturation vapor mixing ratio in the extratropical lowermost stratosphere
Authors: Pan Liwen L. , Hintsa Eric J. , Stone Elizabeth M. , Weinstock Elliot M. , Randel William J.
Published: February 2004

Both in situ measurements and satellite remote sensing data show that water vapor in the extratropical lowermost stratosphere has a pronounced seasonal cycle, with a maximum in summer. In this paper we show that the seasonal cycles derived from satellite measurements by the Stratospheric Aerosol and Gas Experiment (SAGE) II and the Microwave Limb Sounder (MLS) and ER‐2 aircraft measurements are in reasonable agreement when compared using potential vorticity (PV) and potential temperature (θ) binned monthly averages. The annual means and amplitudes of the water vapor seasonal cycle are derived from the SAGE II data for 320–360 K and 2–7 PV units (covering latitudes near and poleward of the tropopause). The high moisture content and the summer maxima indicate that the effect of transport from the troposphere across the extratropical tropopause can be seen in nearly the entire range examined and is more significant in the lower PV‐θ bins. We further investigate mechanisms that control the amount and seasonal cycle of water vapor in the lowermost stratosphere. One important issue is whether local temperature near the extratropical tropopause limits isentropic transport of water vapor into the lowermost stratosphere, similar to the freeze‐out of water vapor crossing the tropical tropopause. A quantitative comparison of saturation vapor mixing ratios (derived from National Centers for Environmental Prediction temperature analyses) with the water vapor measurements shows that the amount of water vapor near the extratropical tropopause is substantially lower than that given by saturation over ice. This demonstrates that local temperature effects do not set the upper limit of water vapor in this region. Analyses using ER‐2 measurements confirm that although saturation and supersaturation do occur, on average, air near the extratropical tropopause is under saturated. The seasonal cycle of the fraction of the overworld air in the lowermost stratosphere is inferred using the water vapor and saturation vapor mixing ratio climatology.
Arctic stratus cloud properties and their effect on the surface radiation budget: Selected cases from FIRE ACE
Authors: Dong Xiquan , Mace Gerald G. , Minnis Patrick , Young David F.
Published: February 2004

To study Arctic stratus cloud properties and their effect on the surface radiation balance during the spring transition season, analyses are performed using data taken during three cloud and two clear days in May 1998 as part of the First ISCCP Regional Experiment (FIRE) Arctic Cloud Experiment (ACE). Radiative transfer models are used in conjunction with surface‐ and satellite‐based measurements to retrieve the layer‐averaged microphysical and shortwave radiative properties. The surface‐retrieved cloud properties in Cases 1 and 2 agree well with the in situ and satellite retrievals. Discrepancies in Case 3 are due to spatial mismatches between the aircraft and the surface measurements in a highly variable cloud field. Also, the vertical structure in the cloud layer is not fully characterized by the aircraft measurements. Satellite data are critical for understanding some of the observed discrepancies. The satellite‐derived particle sizes agree well with the coincident surface retrievals and with the aircraft data when they were collocated. Optical depths derived from visible‐channel data over snow backgrounds were overestimated in all three cases, suggesting that methods currently used in satellite cloud climatologies derive optical depths that are too large. Use of a near‐infrared channel with a solar infrared channel to simultaneously derive optical depth and particle size appears to alleviate this overestimation problem. Further study of the optical depth retrieval is needed. The surface‐based radiometer data reveal that the Arctic stratus clouds produce a net warming of 20 W m−2 in the surface layer during the transition season suggesting that these clouds may accelerate the spring time melting of the ice pack. This surface warming contrasts with the net cooling at the top of the atmosphere (TOA) during the same period. An analysis of the complete FIRE ACE data sets will be valuable for understanding the role of clouds during the entire melting and refreezing process that occurs annually in the Arctic.
The Global Modeling Initiative assessment model: Application to high‐speed civil transport perturbation
Authors: Kinnison D. E. , Connell P. S. , Rodriguez J. M. , Rotman D. A. , Considine D. B. , Tannahill J. , Ramaroson R. , Rasch P. J. , Douglass A. R. , Baughcum S. L. , Coy L. , Waugh D. W. , Kawa S. R. , Prather M. J.
Published: February 2004

The NASA Atmospheric Effects of Aviation Project (AEAP) Global Modeling Initiative (GMI) three‐dimensional (3‐D) chemical transport model (CTM) was applied to assess the impact of a fleet of high‐speed civil transports (HSCTs) on abundances of stratospheric ozone, total inorganic nitrogen (NOy), and H2O. This model is specifically designed to incorporate a diversity of approaches to chemical and physical processes related to the stratosphere in a single computing framework, facilitating the analysis of model component differences, modeling intercomparison and comparison with data. A proposed HSCT fleet scenario was adopted, in which the aircraft cruise in the lower stratosphere, emitting nitrogen oxides (NOx) and water (H2O). The model calculated an HSCT‐induced change in Northern and Southern Hemisphere total column ozone of +0.2% and +0.05%, respectively. This change is the result of a balance between an increase in local ozone below approximately 25 km and a decrease above this altitude. When compared to available NOy observations, we find that the model consistently underestimates lower stratospheric NOy. This discrepancy is consistent with the model bias toward less negative ozone impact, when compared to results from other models. Additional analysis also indicates that for an HSCT assessment it is equally important for a model to accurately represent the lower stratospheric concentrations of ozone and H2O. The GMI model yields good agreement in comparisons to ozone data for present‐day conditions, while H2O is constrained by climatology as much as possible; thus no further biases would be expected from these comparisons. Uncertainties due to discrepancies in the calculated age of air compared to that derived from measurements, and of the impact of emissions on heterogeneous and polar chemistry, are difficult to evaluate at this point.
The effects of future supersonic aircraft on stratospheric chemistry modeled with varying meteorology
Authors: Rogers H. L. , Chipperfield M. P. , Bekki S. , Pyle J. A.
Published: February 2004

The potential impact of a future fleet of high‐speed civil transports (HSCTs) on the year 2015 atmosphere is examined using the three‐dimensional stratospheric chemical transport model, SLIMCAT. The model scenarios performed are identical to those used by the recent Intergovernmental Panel on Climate Change [1999] aircraft assessment. The importance of the background meteorological conditions in determining the potential impact of HSCTs is assessed by forcing the model with U.K. Meteorological Office analyses for the years 1992–1998. The use of annually varying meteorological analyses leads to an interannual variability in the modeled transport of aircraft‐emitted NOx and H2O from the North Atlantic flight corridor. The resulting calculated O3 perturbation exhibits an interannual variability of ∼50 ppbv in the middle stratosphere, which is of a magnitude similar to the differences between the O3 perturbations calculated by models within Intergovernmental Panel on Climate Change [1999], This result highlights both the importance of the chosen background meteorological conditions in aircraft assessments and the extent to which differences in meteorology contribute to differences between model calculations.
Lamination and polar vortex development in fall from ATMOS long‐lived trace gases observed during November 1994
Authors: Manney G. L. , Michelsen H. A. , Irion F. W. , Toon G. C. , Gunson M. R. , Roche A. E.
Published: February 2004

Long‐lived trace‐gas profiles observed by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument around the developing polar vortex (the “protovortex”) during early November 1994 show distinctive inversions (laminae). High‐resolution profiles calculated using a reverse trajectory (RT) model reproduce the majority of these laminae, demonstrating that most of these features arise from advection by the large‐scale winds. The relationship between CH4 and N2O sampled in filaments of low‐latitude air changes from the canonical midlatitude correlation to one typical of tropical air, confirming the identification of tropical laminae. N2O/CH4 correlations for laminae in vortex filaments indicate that enough confined descent and mixing has already occurred in the middle and upper stratosphere by early November to alter the tracer relationship in the protovortex from that in midlatitudes. Distributions of laminae in ATMOS observations agree with regions of greater or less filamentation seen in high‐resolution RT maps. There is a deep minimum in laminae occurrence between 30° and 90°E at all levels, extending to about 150°E above ∼35 km. Maxima in lamination in the lower stratosphere near 90°–150°E and 210°–270°E are associated with the main regions where tropical air is drawn up around the developing vortex. Continuous filamentation associated with the developing vortex and anticyclone in early November 1994 shows that transport and mixing processes previously reported during winter are already important during fall.
Cloud coverage and height during FIRE ACE derived from AVHRR data
Authors: Minnis Patrick , Chakrapani Venkatesan , Doelling David R. , Nguyen Louis , Palikonda Rabindra , Spangenberg Douglas A. , Uttal Taneil , Arduini Robert F. , Shupe Matthew
Published: February 2004

Cloud cover and height are derived from NOAA‐12 and NOAA‐14 advanced very high resolution radiometer (AVHRR) data taken over the Arctic Ocean for an 8° latitude by 30° longitude domain centered on the Surface Heat Budget of the Arctic Ocean (SHEBA) ship Des Groseilliers. Multispectral thresholds were determined subjectively and applied to each image, providing excellent temporal coverage during the May‐July 1998 First ISCCP Regional Experiment Arctic Clouds Experiment (FIRE ACE). Mean cloud amounts were near 70% for the entire period but varied regionally from 55 to 85%. On the basis of a limited climatology of ship observations, these values appear to be typical for this part of the Arctic, suggesting that most of FIRE ACE was conducted in representative cloud conditions. A diurnal cycle of mean cloud amount was found for the domain during June and July having a range of 10% with a middle‐to‐late morning maximum. The AVHRR‐derived cloud amounts are in good agreement with visual and radar measurements taken from the Des Groseilliers, except for a few subvisual and low cloud cases. Average AVHRR‐derived cloudiness differ from the mean values obtained at the surface by −1 to +3%; this represents a significant improvement over previous satellite retrievals. The satellite‐derived cloud heights are very accurate for most of the low cloud cases. Higher cloud altitudes are less certain because cloud optical depths were not available to adjust the temperature observed for the optically thin high clouds, and the radiating temperature of many of the high clouds is representative of some altitude deep in the cloud rather than the highest altitude level of condensate. The development of a more accurate automated algorithm for detecting polar clouds at AVHRR wavelengths will require inclusion of variable thresholds to account for the angular dependence of the surface reflectance as well as the seasonally changing albedos of the ice pack. The use of a 1.6‐μm channel on the AVHRR, or other complement of instruments, will greatly enhance the capabilities for detecting clouds over poles during summer.
Testing directional properties of a forest reflectance model
Authors: Kuusk Andres , Nilson Tiit
Published: February 2004

A multiangular and multispectral forest reflectance model is tested using Boreal Ecosystem‐Atmosphere Study (BOREAS) data. A series of simulations have been carried out for three boreal forest stands from the BOREAS Southern Study Area in Canada. The results of simulation are compared to the PARABOLA and POLDER data on these stands. The comparisons have confirmed the ability of the new model to adequately simulate angular distribution. However, there seems to be a systematic overestimation of the near‐ and middle‐infrared reflectance by the model in conifer stands when the leaf biophysical/biochemical parameters are either measured or determined by the inversion of the measured needle reflectance and transmittance spectra.
Seasonal observations of chlorine monoxide in the stratosphere over Antarctica during the 1996–1998 ozone holes and comparison with the SLIMCAT three‐dimensional model
Authors: Solomon Philip , Barrett James , Connor Brian , Zoonematkermani Saeid , Parrish Alan , Lee Adrian , Pyle John , Chipperfield Martyn
Published: February 2004

We report ground‐based microwave measurements of chlorine monoxide (ClO) mixing ratio profiles above Scott Base, Antarctica (latitude = 77.85°S), during the late winter and early spring of 1996, 1997, and 1998 and analyze in detail the 1996 data. The instrument is permanently installed at the site, permitting continuous measurements throughout the ozone hole period, starting before the end of the polar night. The development of the lower stratospheric ClO layer is strikingly similar in all three years, with the peak (day‐night) ClO mixing ratio at 22 km rising from 0.1 to 1.9–2.0 ppbv in the 21 days from August 13 to September 4. The altitude of the peak drifts downward from 22 to 17–18 km by the end of September. In 1997 and 1998 the chlorine remained in active species until about September 29, while in 1996 there was a rapid and sharp decline in ClO beginning on September 12. We compare the daily average ClO profiles for 1996 with the output of a three‐dimensional off‐line chemical transport model. The model output is sampled to match the data and is used as an a priori profile for retrieval of the measured profiles from the data, allowing a rigorous comparison of the consistency between the model and measurements without bias from an a priori. The model captures all of the major features of the measurements, including the rapid rise in August, the sharp drop on September 13 due to the arrival of warmer, ClOx‐poor air over Scott Base, and the deactivation of ClOx into the reservoir species, HCl and ClONO2, during the last half of September. However, the model using recommended rate coefficients consistently underestimates the measured ClO, suggesting that the ratio of the ClO dimer photolysis rate to the dimer formation rate, j/kƒ, needs to be increased by a factor of 1.8±0.3. This has important implications for model calculations of ozone loss.
Intra‐annual and interannual ensemble forcing of a regional climate model
Authors: Dutton Jan F. , Barron Eric J.
Published: February 2004

The use of ensemble modeling within the framework of dynamical downscaling of climate change scenarios derived from global climate model scenarios has not been fully explored. This study uses a six member ensemble of RegCM2 regional climate model simulations forced by the CCM3 global climate model to explore the one‐way boundary forcing of regional interannual variability of 500 mbar heights, precipitation, and surface temperature. Anomaly pattern correlations (APCs) between the CCM3 and the RegCM2 500 mbar heights, precipitation, and surface temperature show distinct annual cycles. The January ensemble averaged APCs for 500 mbar heights, precipitation, and surface temperature are 0.95, 0.65, and 0.90, respectively. The July correlations for the same variables are 0.63, 0.14, and 0.52, respectively. This indicates that the RegCM2 winter interannual variability is strongly dependent on the GCM interannual variability. The summer interannual variability of precipitation is found to contain little GCM‐supplied signal. The ensemble run variance of the CCM3 and RegCM2 is also explored. The ratio of RegCM2 to CCM3 500 mbar height normalized ensemble run variance (NERV), a measure of climate reproducibility, is near 1.0 for various regions in the simulated domain. The RegCM2 precipitation NERV is greater than CCM3 NERV, suggesting less reproducibility and therefore less predictability. Certain regions show statistically significant reduced RegCM2 surface temperature NERV, suggesting that greater reproducibility may exist in these regions. The effect of increased topographic resolution in the RegCM2 domain was not found to significantly enhance reproducibility.
Simulations of ice clouds during FIRE ACE using the CCCMA single‐column model
Authors: Lohmann U. , Humble J. , Leaitch W. R. , Isaac G. A. , Gultepe I.
Published: February 2004

The single‐column model (SCM) of the Canadian Centre for Climate Modelling and Analysis (CCCMA) solves prognostic equations for the number concentration and mass mixing ratios of ice crystals. Ice crystal formation is specified via different ice nucleation mechanisms. The CCCMA SCM was used to simulate the evolution of ice clouds for three different flights during the First ISCCP Regional Experiment Arctic Cloud Experiment (FIRE ACE) in April 1998, where measurements of cloud droplets, ice crystals, and aerosols >0.08 μm in radius were conducted. The CCCMA SCM in its original setup predicts larger ice crystal concentrations than measured with the 2DC probe. The agreement with observations of ice crystals detectable with the 2DC probe was improved if a condensation freezing parameterization depending on supersaturation with respect to ice was used. The best correlation between observed and simulated ice crystal number, which still showed a lot of scatter, was obtained if an empirical linear relationship between the number of aerosols and the number of ice crystals deduced from FIRE ACE was used.
Cloud water contents and hydrometeor sizes during the FIRE Arctic Clouds Experiment
Authors: Shupe Matthew D. , Uttal Taneil , Matrosov Sergey Y. , Frisch A. Shelby
Published: February 2004

During the year‐long Surface Heat Budget of the Arctic Experiment (1997–1998) the NOAA Environmental Technology Laboratory operated a 35‐GHz cloud radar and the DOE Atmospheric Radiation Measurement Program operated a suite of radiometers at an ice station frozen into the drifting ice pack of the Arctic Ocean. The NASA/FIRE Arctic Clouds Experiment took place during April‐July 1998, with the primary goal of investigating cloud microphysical, geometrical, and radiative properties with aircraft and surface‐based measurements. In this paper, retrieval techniques are utilized which combine the radar and radiometer measurements to compute height‐dependent water contents and hydrometeor sizes for all‐ice and all‐liquid clouds. For the spring and early summer period, all‐ice cloud retrievals showed a mean particle diameter of about 60 μm and ice water contents up to 0.1 g/m3, with the maximum sizes and water contents at approximately one fifth of the cloud depth from the cloud base. The all‐liquid cloud retrievals had a mean effective particle radius of 7.4 μm, liquid water contents up to 0.7 g/m3, and a mean droplet concentration of 54 cm−3. Maximum retrieved liquid drop sizes, water contents, and concentrations occurred at three fifths of the cloud depth from the cloud base. As a measure of how representative the FIRE‐ACE aircraft flight days were of the April‐July months in general, retrieval statistics for flight‐day clouds are compared to the mean retrieval statistics. From the retrieval perspective the ice particle sizes and water contents on flight days were ∼30% larger than the mean retrieved values for the April‐July months. Retrieved liquid cloud parameters during flight days were all about 20% smaller. All‐ice and/or all‐liquid clouds acceptable for these retrieval techniques were observed about 34% of the time clouds were present; at all other times, mixed‐phase clouds precluded the use of these single‐phase retrieval techniques.
Observations by the Lidar In‐Space Technology Experiment (LITE) of high‐altitude cirrus clouds over the equator in regions exhibiting extremely cold temperatures
Authors: Omar Ali H. , Gardner Chester S.
Published: February 2004

Data from the Lidar In‐Space Technology Experiment (LITE) were used to locate clouds near the equatorial tropopause and retrieve temperatures in cloud layers. The experiment was conducted aboard the space shuttle Discovery on September 9–20, 1994. LITE observations over the equator frequently exhibit cloud layers near the tropopause with scattering ratios as high as 2.5 at 532 nm. These clouds have been characterized as cirrus and subvisible cirrus. The clouds were observed near the tropopause with cloud top temperatures ranging from 185 to 200 K. Statistical averages of two optical properties observed by LITE in the 5°N to 15°S latitude band, the wavelength dependence of the extinction due to aerosols (Angstrom coefficient) and scattering ratios, were computed. The Angstrom coefficients show that a significant number (31%) of these high‐altitude cirrus clouds observed by LITE in the 5°N to 15°S latitude band have optical similarities to type 1a polar stratospheric cloud (PSC) particles. Sixty‐six percent of the clouds observed are thin, with scattering ratios ranging from 1.1 to 1.5. Since these clouds are most likely to form at temperatures of 185±2.5 K (a 90% chance), particles of a size and composition similar to type 1a PSCs must be considered a possibility. Since these high‐altitude tropical cirrus clouds may have significant implications for both ozone chemistry and radiative transfer, a closer examination of their composition and phase (using, for example, both in situ methods and depolarization lidar) together with simultaneous meteorological measurements are needed.
Sensitivity of multiangle imaging to natural mixtures of aerosols over ocean
Authors: Kahn Ralph , Banerjee Pranab , McDonald Duncan
Published: February 2004

Multiangle remote sensing data can discriminate among aerosol air mass types, as represented by climatologically probable, external mixtures of component particles. Retrievals are performed over a comparison space of four‐component mixtures, selected from six commonly observed components having assumed, fixed microphysical properties but with mixing ratios free to vary from 0% to 100%. We refer to this approach, which assumes climatologically probable component particles and derives aerosol mixtures from the observations, as a “climatological retrieval.” On the basis of simulated Multiangle Imaging Spectroradiometer (MISR) observations over dark water, the retrieval can distinguish mixtures containing large, spherical particles (sea salt), nonspherical particles (accumulation and coarse mode dust), and small, dark particles (black carbon) to within 20% or better of each component's true mixing ratio. This is sufficient to distinguish maritime from continental aerosol air masses. The retrievals, which use all nine MISR angles and the two wavelengths least affected by ocean surface reflectance (672 and 867 nm), are not good at distinguishing medium, spherical, nonabsorbing (sulfate) from medium, spherical, absorbing (carbonaceous) particles. However, the sum is retrieved to within 20% of the true mixing ratio or better. This is significantly more detail about the properties of particle mixtures than has previously been retrieved from satellite data, and in all cases, the derived total column aerosol optical depth remains well constrained, to at least 0.05 or 20%, whichever is larger. We expect the MISR data, with its frequent global coverage, to complement in situ and field data, which can provide greater detail about aerosol size and composition locally. This combined effort should advance our knowledge of aerosol behavior globally and our ability to model the impact of aerosols on the climatically important solar radiation budget.
Assessment of long‐term vegetation changes on potential isoprenoid emission for a Mediterranean‐type ecosystem in France
Authors: Schaab Gertrud , Steinbrecher Rainer , Lacaze Bernard , Lenz Roman
Published: February 2004

For the region St‐Martin‐de‐Londres (southern France), information on vegetation formations is available for 5 different years between 1946 and 1979. Isoprenoid emission factors are assigned according to dominant plant species that were related to the vegetation formations. Potential isoprenoid emissions are simulated with the help of the Solar Radiation Model (SORAM) considering spatial effects of the complex terrain as well as temporal effects of photosynthetically active radiation and surface temperature, the main controlling parameters for biogenic volatile organic compound (BVOC) emission. The SORAM is embedded in a geographic information system environment and considers a digital elevation model of the modeling domain. Results illustrate long‐term changes of emission fluxes due to plant succession between 1946 and today. By comparing the daily amount of isoprenoid emissions over a time period of 34 years, today's totals have increased by a factor of 4.4 as compared to the 1946 modeling results. The results presented give an example of possible impact of increasing natural plant cover on isoprenoid emissions for the Mediterranean region. These scenarios at the landscape level further emphasize the need for detailed information concerning vegetation (species, distribution, coverage, biomass) as well as the need for reliable emission factors in order to improve inputs of potential BVOC emissions to tropospheric chemistry models.
Cloud modulation of zenith sky oxygen photon path lengths over Boulder, Colorado: Measurement versus model
Authors: Portmann R. W. , Solomon S. , Sanders R. W. , Daniel J. S. , Dutton E. G.
Published: February 2004

Cloudy sky average photon path lengths were measured using the gamma band of oxygen near 628 nm on many days over Boulder, Colorado, using a zenith‐looking spectrograph with a resolution of 0.8 nm. The approach allows accurate measurement of the average photon path length. Days characterized by relatively extensive cloud cover are examined here, which exhibit very large path length variations as the cloud fields overhead evolve. The ability of a plane parallel line‐by‐line model with a single uniform cloud layer to predict the measured O2 path lengths has been tested by constraining the model to the (independently) observed surface irradiance. Overall, its performance is quite good, which demonstrates that the relationship between the diffuse transmission by clouds and the average photon path length is consistent with plane parallel radiative transfer calculations for the conditions studied. Thus large errors in the cloud radiative transfer of such models, as suggested by some recent cloud absorption measurements, are not supported by this study. Furthermore, direct observations in the blue and red spectral regions do not support even a 1% differential absorption between these spectral regions.
Prevalence of ice‐supersaturated regions in the upper troposphere: Implications for optically thin ice cloud formation
Authors: Jensen Eric J. , Toon Owen B. , Vay Stephanie A. , Ovarlez Joëlle , May Randy , Bui T. P. , Twohy Cynthia. H. , Gandrud Bruce W. , Pueschel Rudolf F. , Schumann Ulrich
Published: February 2004

In situ measurements of water vapor and temperature from recent aircraft campaigns have provided evidence that the upper troposphere is frequently supersaturated with respect to ice. The peak relative humidities with respect to ice (RHI) occasionally approached water saturation at temperatures ranging from −40°C to −70°C in each of the campaigns. The occurrence frequency of ice supersaturation ranged from about 20% to 45%. Even on flight segments when no ice crystals were detected, ice supersaturation was measured about 5–20% of the time. A numerical cloud model is used to simulate the formation of optically thin, low ice number density cirrus clouds in these supersaturated regions. The potential for scavenging of ice nuclei (IN) by these clouds is evaluated. The simulations suggest that if less than about 5×10−3 to 2×10−2 cm−3 ice nuclei are present when these supersaturations are generated, then the cirrus formed should be subvisible. These low ice number density clouds scavenge the IN from the supersaturated layer, but the crystals sediment out too rapidly to prevent buildup of high supersaturations. If higher numbers of ice nuclei are present, then the clouds that form are visible and deposition growth of the ice crystals reduces the RHI down to near 100%. Even if no ice clouds form, increasing the RHI from 100% to 150% between 10 and 10.5 km results in a decrease in outgoing longwave radiative flux at the top of the atmosphere of about 8 W m−2. If 0.02–0.1 cm−3 IN are present, the resulting cloud radiative forcing reduces the net radiative flux several watts per square meter further. Given the high frequency of supersaturated regions without optically thick clouds in the upper troposphere, there is a potential for a climatically important class of optically thin cirrus with relatively low ice crystal number densities. The optical properties of these clouds will depend very strongly on the abundance of ice nuclei in the upper troposphere.
Evaluation of a sky/cloud formula for estimating UV‐B irradiance under cloudy skies
Authors: Sabburg J. , Wong J.
Published: February 2004

A new sky/cloud formula is evaluated using results collected over 1 year from an integrated sky camera and irradiance measurement system. Sky properties were recorded every 6 min, simultaneously alongside solar radiation measurements at Toowoomba, Australia (27.6°S, 151.9°E), over the solar zenith angle range of 4.2° to 64.3°. This is the first sky formula that incorporates a complement of parameters based on sky properties around the Sun. The sky properties included cloud cover, cloud brokenness, cloud brightness variation, the angle of maximum cloud cover and aureole brightness. The sky formula was developed by correlating the ratio of the measured UV‐B irradiance to the modeled clear sky UV‐B irradiance for the same atmospheric conditions, with each of the five sky properties listed above. Variation of the UV‐B ratio for each individual sky property is examined. This sky formula, used in conjunction with a clear sky semiempirical UV‐B model, was compared to three published cloud formulas based on cloud cover alone. Using a data set restricted to cases of cloud obscuring the solar disk, the new formula was found to have the highest r2 value of 0.82. Further comparison with a fourth published formula using a subset of data containing predominately cirrus cloud type, produced an explained variance of 0.91 between the measured and the modeled data.
A model study of the dynamical connection between the Arctic Oscillation and stratospheric vacillations
Authors: Christiansen Bo
Published: February 2004

The dynamical connection between the stratosphere and the troposphere in the Northern Hemisphere winter is investigated with general circulation model (GCM) simulations under perpetual January conditions. The variability in the stratosphere is strongly dominated by vacillations on a timescale of 100 days. One‐point correlation maps of the zonal mean zonal wind reveal the characteristic downward propagation of the stratospheric disturbances. The meridional structure of the stratospheric vacillations is well described by a few empirical orthogonal functions (EOFs) of geopotential height. The leading EOF describes a standing oscillation of the stratosphere, while the two next describe the vertical and horizontal propagation. In the troposphere the leading EOF of the surface pressure shows the characteristic circumpolar structure of the Arctic Oscillation. The covariance between the leading EOF of the surface pressure and the zonal mean zonal wind reveals a vertical structure similar to the observed below 10 hPa, while the sign of the covariance changes above 10 hPa where observations are scarce. The downward propagating stratospheric modes play a prominent role in the vertical coupling. Singular value decomposition of the cross‐covariance matrix between the stratospheric zonal mean zonal wind and the surface pressure shows that a significant part of the tropospheric variability can be related to the downward propagating anomalies. The leading pattern of surface pressure anomalies found in this way closely resembles the pattern of the Arctic Oscillation and describes 20–30% of the total variance. This result is confirmed by studying the covariance between the geopotential height fields and the stratospheric principal components.