bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupled general circulation modeling of the tropical Pacific
Authors: Delecluse Pascale , Davey Michael K. , Kitamura Yoshiteru , Philander S. G. H. , Suarez Max , Bengtsson Lennart
Published: February 2004

During the Tropical Ocean‐Global Atmosphere (TOGA) program substantial progress was made in the development of coupled general circulation models with regard to representation of the tropical mean state and climate variability. This paper provides a review of the main developments, focusing on the tropical Pacific region. Early coupled general circulation models were relatively crude; with coarse resolution and limited physical parameterizations and poor surface fluxes, the model drift from the observed mean state was often substantial, and their use for investigating climate variability such as El Niño‐Southern Oscillation (ENSO) was limited. Improvements in resolution and parameterizations led to rapid progress. Through the TOGA program it has been possible to assess coupled model mean states and variability against high‐quality observations, particularly for the tropical Pacific. Both components of the coupled system (ocean and atmosphere) have benefited from an improved understanding of the physics. Coupled experiments have revealed deficiencies in each model component that were concealed in separate forced runs. Several general systematic errors have yet to be eliminated, especially in the east Pacific. The variety of behavior obtained with coupled models provides evidence that more than one mechanism is active in the generation and evolution of ENSO events. There are also indications that interannual variability is linked to the mean structure of the equatorial Pacific.
Upstream separation in a rotating channel flow
Authors: Borenäs K. M. , Whitehead J. A.
Published: February 2004

The question of upstream separation of a current of dense rotating fluid, below a stagnant layer of lower density, that is critically controlled by a weir is addressed in laboratory experiments that follow from theoretical calculations. For a constant potential vorticity flow in a gradually varying channel, it is shown that a range of parameters lead to upstream separation. In all cases the flow reattaches before reaching the sill crest, so that a closed gyre is produced. In laboratory experiments separation and reattachment was observed in numerous cases where the theory predicts closed gyres. Although the shape and size of the gyre are in accordance with theory, the circulation within the gyre differs markedly from theoretical predictions. The mechanism that feeds energy into the gyre appears to be an important unsolved problem. The results indicate that upstream separation is a fact and that the dynamics of the gyre is significant, so the gyre dynamics may modify the volume flux of fluid over the weir.
Water masses and properties at 165°E in the western Pacific
Authors: Kawabe Masaki , Taira Keisuke
Published: February 2004

Full‐depth conductivity‐temperature‐depth‐oxygen profiler (CTDO2) data at 165°E in 5°S–40°N in summer 1991 (World Ocean Circulation Experiment (WOCE) Hydrographic Program P13C) and 27°–48°N in spring 1993 (P13J) are analyzed. Distributions of salinity and temperature are characterized by the fronts associated with ocean currents, such as the Kuroshio Extension front at 34°–35°N, the Kuroshio Bifurcation front at 38°–40°N, the subarctic front at 40°–42°N, other salinity fronts in the tropical waters, and some oxygen fronts. Dissolved oxygen increases upward sharply, called oxycline, just above the intermediate oxygen minimum layer. The processes of water‐mass extension are clarified mainly from oxygen and potential vorticity (PV). The North Pacific Tropical Water (NPTW) forms at the sea surface in 18°–31°N from winter to summer, and the winter‐formed NPTW of 23.0–25.0σθ (where σθ is potential density) extends to 12°N by the subtropical ocean gyre, so that it occupies a layer of 23.0–25.0σθ in 12°–31°N. The Subtropical Mode Water (STMW) forms at the surface in 29°–34°N in winter and extends to 25°N by the ocean circulation; it occupies a layer around 25.4σθ between the main and seasonal pycnoclines in 25°–34°N. The North Pacific Intermediate Water (NPIW) extends to 34°–38°N on 26.83σθ by the Kuroshio Extension and its northern branch and extends to 27°–33°N on 26.77σθ and to 15°–22°N on 26.61σθ by westward currents; it occupies 15°–38°N in the intermediate subtropical gyre. The NPIW is located in the oxycline with similar PV to the STMW. The boundary of the subtropical and tropical gyres is concluded to be around 12°N (14°–16°N) in a surface (intermediate) layer <25.0σθ (>26.0σθ). Bottom waters are also examined.
A consideration of tracer advection schemes in a primitive equation ocean model
Authors: Hecht M. W. , Bryan F. O. , Holland W. R.
Published: February 2004

We consider several tracer advection schemes in the context of a simple sector configuration of a Bryan‐Cox‐Semtner type ocean model. The schemes are centered‐in‐time centered‐in‐space, donor cell upwind differencing, flux corrected transport (FCT), QUICKEST and MPDATA. While we discuss the transport of temperature and salinity, and apply alternative schemes to this transport within our model, we concentrate primarily on a problem which involves passive tracer advection in which vertical gradients are high and concentrations asymptote to zero near the sea surface, as is the case in biogeochemical ocean modeling. We find that flux correction (available optionally with the MPDATA scheme, as well as the FCT scheme) is required in order to produce acceptable results on this challenging problem. A method of supercycling of passive tracers relative to dynamic variables, whereby the time step for passive tracers is larger that that used for the other prognostic variables of the model, is introduced in order to apply these more expensive flux limited schemes efficiently. We also present a general method for the second‐order accurate application of one‐dimensional forward‐in‐time advection schemes in three dimensions.
An optimized design for a moored instrument array in the tropical Atlantic Ocean
Authors: Hackert Eric C. , Miller Robert N. , Busalacchi Antonio J.
Published: February 2004

This paper presents a series of observing system simulation experiments (OSSEs) which are intended as a design study for a proposed array of instrumented moorings in the tropical Atlantic Ocean. Fields of TOPEX/Poseidon sea surface height anomalies are subsampled with the goal being reconstruction of the original fields through the use of reduced‐space Kaiman filter data assimilation at a restricted number of locations. Our approach differs from typical identical and fraternal twin experiments in that real observed data (i.e., TOPEX/Poseidon data) are subsampled and used in place of synthetic data in all phases of the OSSEs. In this way the question of how closely a particular model‐generated data set resembles nature is avoided. Several data assimilation runs are performed in order to optimize the location of a limited number of moorings for the proposed Pilot Research Moored Array in the Tropical Atlantic (PIRATA). Results of experiments in which data are assimilated at 2°N, 2°S and the equator and the longitude is systematically varied by 5° show that the greatest impact of the assimilated data occurs when the observations are taken between 15°W and 30°W. Next, a more systematic technique is presented which allows us to determine optimal points in an objective fashion by applying a least squares regression approach to reconstruct the errors on a dense array of points from the data misfits at any three selected points. The forecast error structure from the Kaiman filter is used in a novel way to assess the optimality of mooring locations. From a large sample of triads of points, the optimal mooring locations are found to be along the equator at 35°W, 20°W, and 10°W. Additional experiments are performed to demonstrate the efficacy of the initial and final PIRATA configurations and the added value that can be expected from PIRATA observations beyond existing expendable bathythermograph observations.
Recent observations and modeling in the Arabian Sea Laccadive High region
Authors: Bruce J. G. , Kindle J. C. , Kantha L. H. , Kerling J. L. , Bailey J. F.
Published: February 2004

Recent observations that include an airborne expendable bathythermograph (AXBT) survey and coincident satellite altimeter measurements together with a high‐resolution numerical simulation indicate that the Laccadive High, an anticyclonic circulating feature that forms off the SW coast of India during the NE monsoon, is comprised of multiple eddies. It is hypothesized that in addition to local and remote seasonal forcing, the Laccadive High region is influenced by an intraseasonal signal that originates in the Bay of Bengal.
Characterization of the principal tidal current constituents on the Texas‐Louisiana shelf
Authors: DiMarco Steven F. , Reid Robert O.
Published: February 2004

We analyzed 81 current meter records of varying lengths (3 to 30 months) to describe the principal diurnal (O1, K1, P1, and Q1) and semidiurnal (S2, M2, K2, and N2) tidal current constituents on the Texas‐Louisiana continental shelf. The Louisiana‐Texas Shelf Physical Oceanography Program (LATEX) had 81 current meters at 31 sites and varying depths from April 1992 to December 1994. The local inertial period range across the shelf (24.4 hours to 26.2 hours) and thermal diurnal cycling during the summer season make it difficult to estimate the diurnal tidal constituents. Dominant tidal modes on the shelf are K1, O1, and M2. Absolute and relative energy contained in each tidal constituent varies with shelf location. The northeast corner of the shelf, near Atchafalaya Bay, has the largest tidal currents with maximum surface current amplitudes (at 3 m depth) of about 9 cm s−1, while typical maximum tidal surface currents near the shelf break are between 1 and 2 cm s−1 for each of the K1, O1, and M2 components. In general, the surface tidal currents decrease in magnitude as water depth increases toward the shelf break, although the semidiurnal components are amplified more at midshelf locations than the diurnal components. Examination of tidal ellipses at different depths suggests that the M2 tide has less vertical structure, while the diurnal tides exhibit more shear, particularly at the more shallow locations. Sea surface height constituents estimated at five locations along the Texas‐Louisiana coast are in agreement with historical values.
A review of the predictability and prediction of ENSO
Authors: Latif M. , Anderson D. , Barnett T. , Cane M. , Kleeman R. , Leetmaa A. , O'Brien J. , Rosati A. , Schneider E.
Published: February 2004

A hierarchy of El Niño‐Southern Oscillation (ENSO) prediction schemes has been developed during the Tropical Ocean‐Global Atmosphere (TOGA) program which includes statistical schemes and physical models. The statistical models are, in general, based on linear statistical techniques and can be classified into models which use atmospheric (sea level pressure or surface wind) or oceanic (sea surface temperature or a measure of upper ocean heat content) quantities or a combination of oceanic and atmospheric quantities as predictors. The physical models consist of coupled ocean‐atmosphere models of varying degrees of complexity, ranging from simplified coupled models of the “shallow water” type to coupled general circulation models. All models, statistical and physical, perform considerably better than the persistence forecast in predicting typical indices of ENSO on lead times of 6 to 12 months. The TOGA program can be regarded as a success from this perspective. However, despite the demonstrated predictability, little is known about ENSO predictability limits and the predictability of phenomena outside the tropical Pacific. Furthermore, the predictability of anomalous features known to be associated with ENSO (e.g., Indian monsoon and Sahel rainfall, southern African drought, and off‐equatorial sea surface temperature) needs to be addressed in more detail. As well, the relative importance of different physical mechanisms (in the ocean or atmosphere) has yet to be established. A seasonal dependence in predictability is seen in many models, but the processes responsible for it are not fully understood, and its meaning is still a matter of scientific discussion. Likewise, a marked decadal variation in skill is observed, and the reasons for this are still under investigation. Finally, the different prediction models yield similar skills, although they are initialized quite differently. The reasons for these differences are also unclear.
A comparison of TOVS ocean skin and surface air temperatures with other data sets
Authors: Anyamba Ebby , Susskind Joel
Published: February 2004

A data set of geophysical fields has been processed at the Goddard Laboratory for Atmospheres from Tiros operational vertical sounder (TOVS) high‐resolution infrared sounder 2 (HIRS2) and microwave sounding unit (MSU) radiance observations. The retrieval system is the physically based Pathfinder Path A methodology. The data set currently covers the period 1985–1995. We compare monthly means of TOVS skin temperature Ts and surface air temperature Ta to the Tropical Ocean Global Atmosphere (TOGA) buoy observations, National Center for Environmental Prediction (NCEP) Ts and the Comprehensive Ocean Atmosphere Data Set (COADS) Ts and Ta for the time period 1987–1989. We also compare the Ta data to the European Center for Medium‐Range Weather Forecasts (ECMWF) 2 m temperature. The TOVS Ts has a mean cold bias of ≤0.5 K relative to NCEP Ts, while the Ta has a mean cold bias of ≤1.0 K in the tropics relative to both ECMWF and COADS Ta. The bias in the TOVS Ta is largely due to the diurnal sampling of NOAA 10 observations. The bias in the Ts has additional contributions from a possible cool skin effect as well as retrieval errors associated with cloud clearing and accounting for water vapor attenuation and reflected solar radiation. Because of the holistic retrieval of TOVS Ts and Ta there is a near cancellation of biases due to diurnal sampling and cloud clearing, so that the derived Ts‐Ta have higher accuracy than the individual Ts and Ta and give a more realistic representation of the evolution of known features of the ocean surface circulation than does the COADS. These results indicate that TOVS Ts and Ta may be suitable for climate variability studies involving air‐sea interactions. The data set is available on a daily, 5‐daily, or monthly mean basis with a spatial resolution of 1° by 1°.
Fifteen years of global wave hindcasts using winds from the European Centre for Medium‐Range Weather Forecasts reanalysis: Validating the reanalyzed winds and assessing the wave climate
Authors: Sterl A. , Komen G. J. , Cotton P. D.
Published: February 2004

The ERA (European Centre for Medium‐Range Weather Forecasts Reanalysis) project resulted in a homogeneous data set describing the atmosphere over a time span of 15 years, from 1979 to 1993. To validate (part of) these data against independent observations we use the ERA surface winds to drive the WAM wave model. The modeled significant wave heights are then compared with observations. From this comparison the quality of the forcing winds is assessed. The patterns of computed wave heights agree well with observed patterns, and they are of the right magnitude. This confirms the realistic nature of the ERA winds. If one looks in detail, it appears that the significant wave heights resulting from the model are systematically lower than the observed ones in areas of high winds and waves and higher in areas of low winds and waves. It is argued that underestimation at high winds speeds is most likely a resolution effect, as wind and thus wave peaks are missed by finite resolution in space and time, while overestimation at low wind speeds most likely results from internal WAM errors. It is concluded that the monthly mean ERA winds are slightly (less than 5%) too low in areas of high winds, while from this study it is not possible to draw a decisive conclusion on the quality of ERA winds at low wind speeds. At the same time, the hindcast data form a 15‐year climatology of global waves. This climatology is analyzed in terms of annual cycle and trends. The largest trends in significant wave height occur in the North Atlantic with an increase of more than 12 cm/yr in January, and south of Africa where the increasing trend exceeds 7 cm/yr in July. These trends, however, are only marginally significant. Furthermore, they exhibit a large month‐to‐month variability, so that on a seasonal basis the trends are significant only in small parts of the ocean. In conclusion, we are unable to confirm a significant change in wave height during the ERA period.
Relation between underwater irradiance and quantum irradiance in dependence on water transparency at different depths in the water bodies
Authors: Reinart Anu , Arst Helgi , Blanco‐Sequeiros A. , Herlevi A.
Published: February 2004

The ratio between underwater quantum irradiance (q) and irradiance (E) for Jerlov's [1976] oceanic and coastal water types and for 11 Estonian and Finnish lakes was studied. This ratio was found to depend on the depth in the water body and the transparency of the water. The ratio q/E for the photosynthetically active radiation (PAR) region of the spectrum may differ from its value in air by up to 24%. The results of the present paper can be used to convert the underwater radiation data from units of W m−2 to μmol s−1 m−2 (or vice versa) and to estimate the errors caused by applying the “air” value of q/E instead of its real value in the water.
Freezing behavior of single sulfuric acid aerosols suspended in a quadrupole trap
Authors: Carleton K. L. , Sonnenfroh D. M. , Rawlins W. T. , Wyslouzil B. E. , Arnold S.
Published: February 2004

The freezing properties of sulfuric acid droplets were studied by suspending single 20‐ to 30‐μm‐diameter particles in a quadrupole trap and cooling them to stratospheric temperatures (≥191.5 K). Each particle's dc balance voltage was measured to determine the particle composition as a function of temperature and map out the particle's trajectory relative to the sulfuric acid phase diagram. Angularly resolved optical scattering patterns were monitored to detect freezing events. Particles cooled through the sulfuric acid tetrahydrate region (35–70 wt % H2SO4) did not freeze and remained spherical liquid droplets for several hours. Only particles cooled through the ice‐liquid equilibrium region (<35 wt% H2SO4) showed evidence of freezing. This supports previous experimental and field observations that stratospheric sulfuric acid aerosols are likely to remain liquid to within a few degrees of the ice frost point.
On the possible role of aircraft‐generated soot in the middle latitude ozone depletion
Authors: Bekki S.
Published: February 2004

A large range of observations have established that there is a significant downward trend in column ozone at middle latitudes in the northern hemisphere (NH) with most of the ozone depletion occurring in the lower stratosphere. Models largely underestimate this lower stratospheric ozone depletion. Recent one‐dimensional model calculations [Lary et al, 1997] suggested that heterogeneous chemistry on black carbon could play a major role in stratospheric chemistry. Here, a two‐dimensional aerosol/soot photochemical model is used to calculate the stratospheric global distribution of black carbon soot produced by aircraft emissions. The model‐calculated distribution is found to be broadly consistent with observations [Blake and Kato, 1995]. By taking into account a catalytic heterogeneous reduction of O3 on aircraft‐generated soot, the model is now able to reproduce a large part of the lower stratospheric ozone trend observed at NH middle latitudes. However, the results are sensitive to the assumed O3 uptake rate on soot which is still uncertain. Renoxification by the heterogeneous conversion of HNO3 into NOx on soot cannot account for most of the possible discrepancies between modeled and observed NOy partitioning because, similarly to the ozone uptake, its impact is mainly confined to the aircraft emissions region.
Uncertainties in modeled UV irradiances due to limited accuracy and availability of input data
Authors: Schwander Harry , Koepke Peter , Ruggaber Ansgar
Published: February 2004

Uncertainties in modeled spectral UV irradiances under cloud‐free conditions are analyzed with respect to limited measurement accuracy of actual atmospheric input parameters or their nonavailability under the assumption that no uncertainty results from the used model or from the spectral values of the extraterrestrial solar irradiance and the gaseous absorption coefficients. The resulting mean uncertainty of spectral UV irradiance is calculated using a root‐mean‐square (rms) procedure for various scenarios, defined by differing qualities of the used sets of input values. The results are discussed with respect to the possibility of reducing the uncertainty in modeled UV irradiances by additional measurements of input parameters and, on the other hand, assessing which of such measurements may be redundant since greater measurement expense leads to no significant improvement in accuracy of modeled irradiances. The uncertainties in modeled UV irradiances are mainly produced by the uncertainties of the measured ozone amount, by the aerosol optical depth if it is not directly measured, and by the soot concentration of the aerosol in the haze layer. Additional uncertainties can arise where snow cover is present. If O3 and SO2 contents, spectral aerosol optical depth, and aerosol soot concentration near the ground are measured under actual conditions, the uncertainties in input parameters result in a mean uncertainty of about 5% for spectral integrals of UV irradiance. These results cannot be improved significantly, even when measured values of vertical profiles of all atmospheric constituents are used. Using only the observed visibility without the measurement of aerosol optical properties, the mean uncertainty for modeled UV integrals is about 10–15%.
Modeling cloud fraction and horizontal variability in marine boundary layer clouds
Authors: Considine Geoffrey , Curry Judith A. , Wielicki Bruce
Published: February 2004

Boundary layer clouds play an important role in the global energy budget. The radiative properties of these clouds depend upon the cloud fraction and the horizontal variability of liquid water path within patches of unbroken cloud. A simple statistical model for describing the probability distributions of liquid water path and cloud fraction in low clouds has been developed. The model is shown to reproduce realistically the diurnal cycle in the cloud fraction and horizontal inhomogeneity of marine stratocumulus. Further, the model produces realistic statistics for the horizontal variability of liquid water path within boundary layer clouds. The modeled distributions of cloud liquid water path depend upon the cloud fraction, similar to Landsat retrievals. The results are discussed in the context of improving the representation of boundary layer clouds in large‐scale models.
Observations of the 1995 ozone hole over Punta Arenas, Chile
Authors: Kirchhoff V. W. J. H. , Sahai Y. , Casiccia C. A. R. S. , Zamorano B. F. , Valderrama V. V.
Published: February 2004

We examine the appearance of the ozone hole over a populated area with more than 100,000 inhabitants. The largest population concentrations on the South American continent nearest the ozone hole region are Punta Arenas, Chile (53.0°S, 70.9°W) and Ushuaia, Argentina (54.5°S, 68.0°W), located close to the strait of Magallanes, opposite the Antarctic Peninsula. A special field mission was held in Punta Arenas, in September‐October 1995 to investigate the vertical distribution of ozone during the appearance of the Antarctic ozone hole. Previous work has shown that the city of Punta Arenas is located at the edge of the hole area and is affected every year during a few days in the October period. The ozone trend near these locations is −0.5% per year using the yearly averages and −1.2% per year using the October means. This trend is 2 to 5 times larger than the global average. Several ozonesondes of the electrochemical concentration cell type were launched from Punta Arenas to determine the vertical distribution of ozone during “normal” and “perturbed” conditions. The ozone hole passed over Punta Arenas on October 12, 13 and 14, 1995. In addition to the sondes, which were launched once a day, ozone column amounts and UVB radiation were measured with a ground‐based ozone Brewer spectrophotometer. The strongest ozone depletion over Punta Arenas in 1995 occurred on October 13, when the ozone column decreased from a “normal” value of about 325 Dobson Units (DU) to 200 DU; the vertical distribution of ozone on October 13 compared with October 6 shows depleted ozone roughly 50% less during hole conditions in the stratosphere. The UVB intensities have increased accordingly. The spectral ratio for October 13 to October 4 is 13 times larger at 297 nm.
A synthesis of warm air advection to the South Polar Plateau
Authors: Hogan Austin
Published: February 2004

Surface temperature and wind chronologies of varying length and 10 m snow temperatures have been used to produce a synthesis of airflow into Antarctica. This synthesis shows relatively warm air over Ellsworth Land, which appears to enter the South Polar Plateau through a slightly lower‐lying trough between the summit of the Transantarctic Mountains and the Polar Plateau. Comparison of recent automatic weather station (AWS) records from Mount Howe (88.5°S, 150°W) with four AWS surrounding South Pole at 89°S show that this upslope inflow is a frequent and persistent phenomenon.
Numerical simulation of cloud plumes emanating from Arctic leads
Authors: Burk Stephen D. , Fett Robert W. , Englebretson Ronald E.
Published: February 2004

A two‐dimensional, steady state boundary layer model is used to investigate the formation of cloud plumes over open Arctic leads. Satellite observations from the period of an intense storm in the Beaufort Sea during April 1992 are used to document lead plume formation [Fett et al., 1997]. These observations show a marked variability of open leads with and without cloud plumes dependent on synoptic weather conditions. Model simulations are conducted (using soundings before and after the storm's passage) which show extensive plumes forming in the postfrontal environment but not in the prefrontal atmosphere. To better isolate important factors contributing to plume development and plume dynamics, a large set of idealized model experiments is performed. This matrix of model simulations permits exploration of the parameter space of plume formation. The model is found capable of forecasting the onset of steam fog over a lead in a manner which agrees with empirical relationships. Such steam fog, however, dissipates quickly upon advecting over the surrounding ice and does not therefore appear generally responsible for extensive plumes observed to emanate from leads. Instead, extended cloud plumes form at the top of the convective thermal internal boundary layer (TIBL) that develops over a lead. These elevated plumes are quite sensitive to the relative humidity of the environmental air. Unlike steam fog, the elevated plumes also are found to be sensitive to the lapse rate of the environmental air. Changes in wind speed have little impact on the integrated heat and moisture amounts transferred to the atmosphere from a lead (provided advection balances flux divergence) but do impact the cloud plumes by altering shear‐driven entrainment. For wide leads, indications are that lead refreezing must be in an advanced stage before having a major impact on the plume formation process. Additionally, we investigate the conditions under which a cloud plume that is detached from its parent lead may form downwind of a lead. Often, the plumes are predominantly water clouds near the lead, with the ice crystal fraction increasing as the plume cools downstream. A simple formula for estimating plume penetration depth is found to agree reasonably well with the model simulations for 1 km wide leads but deviates considerably for wider leads.
Retrieval of land surface parameters from airborne POLDER bidirectional reflectance distribution function during HAPEX‐Sahel
Authors: Roujean Jean‐Louis , Tanré Didier , Bréon François‐Marie , Deuzé Jean‐Luc
Published: February 2004

Spaceborne instruments such as POLDER (polarization and directionality of Earth's reflectances) provide multidirectional estimates of the surface reflectance (after atmospheric correction). A better description of the surface cover is needed for an improvement in climate modeling. This paper describes how the inversion of a three‐parameter bidirectional model from the measurements yields some variables of interest such as the surface albedo, the leaf area index, the fraction of vegetation, and the daily integrated photosynthetically active radiation absorption. Airborne measurements of POLDER were made during the HAPEX‐Sahel experiment over various surface types and for different atmospheric conditions. These measurements together with some “ground truth” allow a validation of the proposed method. The reflectance bidirectional signature of the Sahelian landscape shows some classical features such as a general increase toward backscattering and the hot spot phenomenon. The bidirectional model allows an accurate representation of the observed signatures. However, a large range of angular measurements is necessary to constrain the model inversion and to decrease the uncertainty in the three‐parameter retrieval. The surface variables derived from these three parameters are found to be in agreement with the in situ measurements. Obviously, a validation on other landscapes with a higher vegetation coverage is now needed. The technique described in this paper will be applied to the spaceborne POLDER data sets.
Variability of soil moisture near flux towers in the BOREAS southern study area
Authors: Peck Eugene L. , Carroll Thomas R. , Maxson Robert , Goodison Barry , Metcalfe John
Published: February 2004

Soil moisture measurements, using an airborne gamma radiation system, were collected over the southern BOREAS study area in Saskatchewan, Canada, during 1993 and 1994. Results of microscale analyses of the soil moisture measurements near four flux towers established in different boreal forest areas (old aspen, old black spruce, old jack pine, and young aspen) are presented. Included is information on the spatial and temporal variability of soil moisture of the mineral soil in the vicinity of each tower site and estimates of the water content in overlaying moss/humus layers. For the old black spruce site, estimates are presented of the change in the amount of standing water from September 1993, when there was considerable standing water, to September 1994, when the area had completely dried out and no standing water was observed.
Five years of NO2 vertical column measurements at Faraday (65°S): Evidence for the hydrolysis of BrONO2 on Pinatubo aerosols
Authors: Slusser J. R. , Fish D. J. , Strong E. K. , Jones R. L. , Roscoe H. K. , Sarkissian A.
Published: February 2004

Summertime measurements Of NO2 vertical column amounts over a 5 year period from May 1990 until February 1995 from Faraday Base, Antarctica, show a marked reduction following the arrival of the Mount Pinatubo volcanic aerosol in December 1991. Model calculations show that this reduction can be explained by BrONO2 and N2O5 hydrolysis on the volcanically enhanced aerosol, with the former dominating. Given the measurement and model uncertainties and lack of any treatment the effects of the quasi‐biennial oscillation, the reduction in NO2 is consistent with a BrONO2 sticking coefficient gamma of 0.4. However, the best agreement between the model and the measurements occurs using a gamma of 0.2. Over the time span of the measurements the known increases in chlorine and bromine loadings have an effect of less than 2% on midsummer NO2 columns. With background aerosols, summertime ozone catalytic losses are dominated by the HOx cycle between 12 and 18 km and by the NOx cycle at greater altitudes. With heavy aerosol loading, HOx is the primary loss cycle from 12 to 22 km. The total ozone loss increases by 38% at 16 km as a result of heavy aerosol loading.
Biogenic emissions of volatile organic compounds from gorse (Ulex europaeus): Diurnal emission fluxes at Kelling Heath, England
Authors: Cao X.‐L. , Boissard C. , Juan A. J. , Hewitt C. N. , Gallagher M.
Published: February 2004

Volatile organic compound (VOC) emission fluxes from Gorse (Ulex europaeus) were measured during May 30–31, 1995 at Kelling Heath in eastern England by using bag enclosure and gradient methods simultaneously. The enclosure measurements were made from branches at different stages of physiological development (flowering, after flowering, and mixed). Isoprene was found to represent 90% of the total VOC emissions, and its emission rates fluctuated from 6 ng (g dwt)−1 h−1 in the early morning to about 9700 ng(g dwt)−1 h−1 at midday. Averaged emission rates standardized to 20°C were 1625, 2120, and 3700 ng (g dwt)−1 h−1 for the new grown, “mixed,” and flowering branch, respectively. Trans‐ocimene and α‐pinene were the main monoterpenes emitted and represented, on average, 47.6% and 36.9% of the total monoterpenes. Other monoterpenes, camphene, sabinene, β‐pinene, myrcene, limonene and γ‐terpinene, were positively identified but together represented less than 1.5% of the total VOC emissions from gorse. Maximum isoprene concentrations in air at the site were measured around midday at 2 m (174 parts per trillion by volume, or pptv) and 6 m (149 pptv), and minimum concentrations were measured during the night (8 pptv at both heights). Mean daytime α‐pinene air concentrations of 141 and 60 pptv at 2 and 6 m height were determined, but trans‐ocimene concentrations were less than the analytical detection limit (4 pptv), suggesting rapid chemical removal of this compound from air. The isoprene fluxes calculated by the micrometeorological gradient method showed a pattern similar to that of those calculated by the enclosure method, with isoprene emission rates maximum at midday (100 μg m−2 h−1) and not detectable during the nighttime. Assessment of the fraction of the site covered by gorse plants enabled an extrapolation of emission fluxes from the enclosure measurements. When averaged over the 2 day experiment, isoprene fluxes of 29.8 and 27.8 μg m−2 h−1 were obtained from the gradient and the enclosure extrapolation respectively. These isoprene fluxes to the atmosphere represented between 0.12% and 0.35% of the net assimilated carbon (as CO2) uptake rate for gorse.
Modeling the impact of averaging on aggregation of surface fluxes over BOREAS
Authors: Cooper Harry J. , Smith Eric A. , Gu Jiujing , Shewchuk Stanley
Published: February 2004

Surface meteorological observations, soil and vegetation parameters, and satellite‐retrieved surface radiation fluxes are used as input to a biosphere‐atmosphere flux exchange model to estimate the fine‐scale surface fluxes over a 75,000 km2 region within the large‐scale BOREAS study area under cloud‐free conditions. The size of the study area chosen for the analysis is synonymous with the characteristic grid size of a modern global climate model. The variables used to force the model are area‐averaged and applied in such a fashion to assess the errors in area‐wide fluxes that would arise in flux aggregation schemes based on the area‐averaged inputs. In the absence of large spatial variance of downwelling radiative fluxes, characteristic of the clear sky case under study, progressive area averaging of the input parameters indicates that the most important quantities influencing the aggregated fluxes are root layer depth, soil moisture, and soil type, all of which are interrelated in the formulation of the transpiration capacity. Meteorological variance over the site and variability in a number of other parameters relating to vegetation characteristics and initial soil temperatures are not critical to the aggregation process. Notably, major errors in aggregated surface fluxes arise due to area averaging soil moisture alone, even when all other parameters are not area‐averaged and allowed to maintain their spatial variability. The errors materialize as significant phase shifts in the frequency distributions of the fine‐scale surface fluxes calculated after input averaging. Application of a soil mosaic averaging technique, based on soil type and the soil moisture associated with each soil type, restores the characteristic mean values of the baseline area‐averaged fluxes. This procedure also simplifies the surface flux distributions to dependence on the percentages of grid area covered by each soil type but without altering the basic multimodal character of the baseline distributions. It is demonstrated and explained why such a soil‐type designation over the area forms the kernels for the surface flux distribution spectrums, around which the other input parameters act to disperse the frequency distribution properties of the aggregated surface fluxes.
Mineralogical composition of African dust delivered by red rains over northeastern Spain
Authors: Avila Anna , Queralt‐Mitjans Ignasi , Alarcón Marta
Published: February 2004

The African dust deposition, mineralogy, source regions, and influence on rain chemistry were analyzed for a site in the Montseny mountains (Catalonia, northeastern Spain) for the period August 1, 1983, through August 1, 1994. Dust deposition was highly variable: two events (collected on November 9–11, 1984, and March 22–25, 1991) accounted for 62% of the total dust input in the 38 red rain events recorded in the period. The average annual dust deposition was 5.3 g m−2 (SE 2.6). Three source regions in the African continent were identified with back trajectory analysis: (1) Western Sahara, (2) Moroccan Atlas, and (3) Central Algeria. Events from the Moroccan Atlas predominated in terms of number of events, amount of rainfall, and dust deposition. The meteorological situations during red rains were identified: Western Sahara events occurred principally with a depression at high latitudes (50°N) forming a trough in the North Atlantic, Moroccan Atlas events occurred with a depression in front of Portugal, and events from central Algeria were associated with a depression over Spain or North Africa. By order of abundance (median of n=13) the minerals identified in the dust by X ray diffraction were as follows: illite > quartz > smectite > palygorskite > kaolinite > calcite > dolomite > feldspars. Differences in mineralogy between source regions were significant for smectite, kaolinite, quartz, and dolomite. Smectite and kaolinite content was highest in the Algerian events, while quartz and dolomite content was lowest. Events from the Moroccan Atlas had the lowest smectite and kaolinite. The red rain soluble chemistry was clearly influenced by the dissolution of calcite (high alkalinity, calcium concentration and basic pH) and by marine components (high Na+, Cl− arid Mg+2 concentrations). Because of the very alkaline character of red rains they had a strong weight on the annual mean pH of rainwater. This was reflected by the significant relationship (r=0.71, p<0.01) between the percentage of annual precipitation as red rain and annual mean precipitation pH. Red rains provided 25–45% of the annual dissolved input of the major ions in precipitation and 34% of the calcium required for holm oak (Quercus ilex L.) forest growth at the study site.
ClONO2 vertical profile and estimated mixing ratios of ClO and HOCl in winter Arctic stratosphere from Michelson interferometer for passive atmospheric sounding limb emission spectra
Authors: von Clarmann T. , Wetzel G. , Oelhaf H. , Friedl‐Vallon F. , Linden A. , Maucher G. , Seefeldner M. , Trieschmann O. , Lefèvre F.
Published: February 2004

Nighttime limb emission spectra recorded by the balloon‐borne Michelson interferometer for passive atmospheric sounding (MIPAS) on February 11, 1995, near Kiruna were used to infer a vertical profile of ClONO2 as well as estimates of ClO and HOCl volume mixing ratios. The highest ClONO2 mixing ratio (2.6 parts per billion by volume (ppbv)) was found at 22.69 km altitude and is explained by an early recovery of this chlorine reservoir in the upper part of the formerly chlorine‐activated height range. Inferred nighttime ClO mixing ratios appear to be rather high for the lower stratosphere (0.38 ppbv at 16.43 km altitude) and indicate chlorine activation at this altitude region. The HOCl mixing ratio is estimated as 0.03 ppbv at 28.04 km altitude, while for lower altitudes the HOCl concentrations are clearly below the detection limit of MIPAS. The measurements are compared with three‐dimensional chemical transport model calculations. Results agree reasonably well but show differences in detail.
Radiative forcing calculations for CH3Cl and CH3Br
Authors: Grossman Allen S. , Grant Keith E. , Blass William E. , Wuebbles Donald J.
Published: February 2004

Methyl chloride, CH3Cl, and methyl bromide, CH3Br, are particularly important in the global atmosphere as major natural sources of chlorine and bromine to the stratosphere. The production of these gases is dominated by natural sources, but smaller, important anthropogenic sources, such as agricultural fumigation and/or biomass burning, also exist. As absorbers of infrared radiation these gases are of interest for their potential effect on the tropospheric energy balance as well as for chemical interactions. In this study we estimate the radiative forcing and Global Warming Potentials (GWPs) of CH3Cl and CH3Br. Our calculations use an infrared radiative transfer model based on the correlated k‐distribution algorithm for band absorption. Radiative forcing values of 0.0047 W/m2 per part per billion by volume (ppbv) for CH3Cl in the troposphere and 0.0049 W/m2 per ppbv for CH3Br in the troposphere were obtained. On a per molecule basis the radiative forcing values are about 2% of the forcing of CFC‐11 and about 270 times the forcing of CO2. GWPs for these gases are about 8 for CH3Cl and about 4 for CH3Br (100 year time integration, CO2=1). These results indicate that while CH3Cl and CH3Br have direct GWPs similar to that of CH4, the current emission rates are too low to contribute meaningfully to atmospheric greenhouse heating effects.
Impact of sulfate aerosols on albedo and lifetime of clouds: A sensitivity study with the ECHAM4 GCM
Authors: Lohmann Ulrike , Feichter Johann
Published: February 2004

A coupled sulfur chemistry‐cloud microphysics scheme (COUPL) is used to study the impact of sulfate aerosols on cloud lifetime and albedo. The cloud microphysics scheme includes precipitation formation, which depends on the cloud droplet number concentration (CDNC) and on the liquid water content. On the basis of different observational data sets, CDNC is proportional to the sulfate aerosol mass, which is calculated by the model. Cloud cover is a function of relative humidity only. Additional sensitivity experiments with another cloud cover parameterization (COUPL‐CC), which also depends on cloud water, and with a different autoconversion rate of cloud droplets (COUPL‐CC‐Aut) are conducted to investigate the range of the indirect effect due to uncertainties in cloud physics. For each experiment, two simulations, one using present‐day and one using preindustrial sulfur emissions are carried out. The increase in liquid water path, cloud cover, and shortwave cloud forcing due to anthropogenic sulfur emissions depends crucially upon the parameterization of cloud cover and autoconversion of cloud droplets. In COUPL the liquid water path increases by 17% and cloud cover increases by 1% because of anthropogenic sulfur emissions, yielding an increase in shortwave cloud forcing of −1.4 W m−2. In COUPL‐CC the liquid water path increases by 32%, cloud cover increases by 3% and thus shortwave cloud forcing increases by −4.8 W m−2. This large effect is caused by the strong dependence of cloud cover on cloud water and of the autoconversion rate on CDNC, cloud water, and cloud cover. Choosing a different autoconversion rate (COUPL‐CC‐Aut) with a reduced dependence on CDNC and cloud water results in an increase of liquid water path by only 11% and of cloud cover by 1%, and the increase in shortwave cloud forcing amounts to −2.2 W m−2. These results clearly show that the uncertainties linked to the indirect aerosol effect are higher than was previously suggested.
Relationships between ozone photolysis rates and peroxy radical concentrations in clean marine air over the Southern Ocean
Authors: Penkett S. A. , Monks P. S. , Carpenter L. J. , Clemitshaw K. C. , Ayers G. P. , Gillett R. W. , Galbally I. E. , Meyer C. P.
Published: February 2004

Measurements of the sum of inorganic and organic peroxy radicals (RO2) and photolysis rate coefficients J(NO2) and J(O1D) have been made at Cape Grim, Tasmania in the course of a comprehensive experiment which studied photochemistry in the unpolluted marine boundary layer. The SOAPEX (Southern Ocean Atmospheric Photochemistry Experiment) campaign included measurements of ozone, peroxides, nitrogen oxides, water vapor, and many other parameters. This first full length paper concerned with the experiment focuses on the types of relationships observed between peroxy radicals and J(NO2), J(O1D) and √[J(O1D)] in different air masses in which ozone is either produced or destroyed by photochemistry. It was found that in baseline air with ozone loss, RO2 was proportional to √[J(O1D)], whereas in more polluted air RO2 was proportional to J(O1D). Simple algorithms were derived to explain these relationships and also to calculate the concentrations of OH radicals in baseline air from the instantaneous RO2 concentrations. The signal to noise ratio of the peroxy radical measurements was up to 10 for 1‐min values and much higher than in other previous deployments of the instrument in the northern hemisphere, leading to the confident determination of the relationships between RO2 and J(O1D) in different conditions. The absolute concentration Of RO2 determined in these experiments is in some doubt, but this does not affect our conclusions concerned either with the behavior of peroxy radicals with changing light levels or with the concentrations of OH calculated from RO2. The results provide confidence that the level of understanding of the photochemistry of ozone leading to the production of peroxide via recombination of peroxy radicals in clean air environments is well advanced.