bluuebunny/crossref_metadata_2025 huggingface dataset:
NSCAT high‐resolution surface wind measurements in Typhoon Violet
Authors: Jones W. Linwood , Cardone Vincent J. , Pierson Willard J. , Zec Josko , Rice Larry P. , Cox Andrew , Sylvester Winfield B.
Published: September 2002

NASA scatterometer (NSCAT) measurements of the western Pacific Supertyphoon Violet are presented for revolutions 478 and 485 that occurred in September 1996. A tropical cyclone planetary boundary layer numerical model, which uses conventional meteorological and geostationary cloud data, is used to estimate the winds at 10‐m elevation in the cyclone. These model winds are then compared with the winds inferred from the NSCAT backscatter data by means of a novel approach that allows a wind speed to be recovered from each individual backscatter cell. This spatial adaptive (wind vector) retrieval algorithm employs several unique steps. The backscatter values are first regrouped in terms of closest neighbors in sets of four. The maximum likelihood estimates of speed and direction are then used to obtain speeds and directions for each group. Since the cyclonic flow around the tropical cyclone is known, NSCAT wind direction alias selection is easily accomplished. The selected wind directions are then used to convert each individual backscatter value to a wind speed. The results are compared to the winds obtained from the tropical cyclone boundary layer model. The NSCAT project baseline geophysical model function, NSCAT 1, was found to yield wind speeds that were systematically too low, even after editing for suspected rain areas of the cyclone. A new geophysical model function was developed using conventional NSCAT data and airborne Ku band scatterometer measurements in an Atlantic hurricane. This new model uses the neural network method and yields substantially better agreement with the winds obtained from the boundary layer model according to the statistical tests that were used.
Global ERS 1 and 2 and NSCAT observations: Upwind/crosswind and upwind/downwind measurements
Authors: Quilfen Y. , Chapron B. , Bentamy A. , Gourrion J. , El Fouhaily T. , Vandemark D.
Published: September 2002

This paper presents an analysis of the wind speed dependence of upwind/downwind asymmetry (UDA) and upwind‐crosswind anisotropy (UCA) as derived from global C band VV‐polarized ERS 1 and 2 and Ku band VV‐ and HH‐polarized NASA scatterometer (NSCAT) data. Interpretation of the results relies on identifying relationships between the differing frequencies and incidence angles that are consistent with Bragg scattering theory from gravity‐capillary waves. It is shown that globally derived parameters characterizing UDA and UCA hold information on the wind dependence of short gravity and gravity‐capillary wave growth and dissipation. In particular, the UCA behavior is found quadratic for both the C and Ku band, peaking at moderate wind speeds. In addition, the dual‐frequency results appear to map out the expected, more rapid adjustment of centimeter‐scale (Ku band) waves to the wind direction at light winds. However, as wind increases, the directionality associated with these shorter waves saturates at a lower speed than for the slightly longer waves inferred at C band. It is suggested that this observed phenomenon may be related to increasing wave‐drift interactions that can potentially inhibit short‐scale surface wave growth along the wind direction. Concerning UDA properties, our present analysis reveals that the NSCAT and ERS 1 and 2 scatterometers give quite different results. Our preliminary interpretation is that C band measurements may be easier to interpret using composite Bragg scattering theory and that upwind/downwind contrasts are mainly supported by short gravity waves.
Determination of the geophysical model function of NSCAT and its corresponding variance by the use of neural networks
Authors: Mejia C. , Badran F. , Bentamy A. , Crepon M. , Thiria S. , Tran N.
Published: September 2002

We have computed two geophysical model functions (one for the vertical and one for the horizontal polarization) for the NASA scatterometer (NSCAT) by using neural networks. These neural network geophysical model functions (NNGMFs) were estimated with NSCAT scatterometer σOmeasurements collocated with European Centre for Medium‐Range Weather Forecasts analyzed wind vectors during the period January 15 to April 15, 1997. We performed a studentttest showing that the NNGMFs estimate the NSCAT σOwith a confidence level of 95%. Analysis of the results shows that the mean NSCAT signal depends on the incidence angle and the wind speed and presents the classical biharmonic modulation with respect to the wind azimuth. NSCAT σOincreases with respect to the wind speed and presents a well‐marked change at around 7 m s−1. The upwind‐downwind amplitude is higher for the horizontal polarization signal than for vertical polarization, indicating that the use of horizontal polarization can give additional information for wind retrieval. Comparison of the σOcomputed by the NNGMFs against the NSCAT‐measured σOshow a quite low rms, except at low wind speeds. We also computed two specific neural networks for estimating the variance associated to these GMFs. The variances are analyzed with respect to geophysical parameters. This led us to compute the geophysical signal‐to‐noise ratio, i.e.,Kp. TheKpvalues are quite high at low wind speed and decrease at high wind speed. At constant wind speed the highestKpare at crosswind directions, showing that the crosswind values are the most difficult to estimate. These neural networks can be expressed as analytical functions, and FORTRAN subroutines can be provided.
Roughness variability of sea ice and snow cover thickness profiles in the Ross, Amundsen, and Bellingshausen Seas
Authors: Adolphs Ute
Published: September 2002

Investigation of top and bottom surface roughness was carried out on snow and ice thickness profiles sampled on 57 floes in the Ross, Amundsen, and Bellingshausen Seas between May and September 1995. An objective classification scheme was used to group the studied sea ice floes according to the degree of deformation they had undergone. Different roughness criteria and methods were applied to the classified floes. Roughness variances and other properties such as fractal dimensions, derived from Fourier spectra, depend critically on the deformational history of the floes under consideration. The wavelet transform is presented as a new, two‐dimensional method to find dominant structure lengths and, at the same time, localize them in the studied profiles. Finally, air, water and ice surface drag coefficients are calculated from the surface roughness characteristics. Air drag coefficients ranged between 1.1 and 1.7 × 103 and increased with increasingly deformed ice.
Buffering interactions in the modeled response of stratospheric O3 to increased NOx and HOx
Authors: Nevison C. D. , Solomon S. , Gao R. S.
Published: September 2002

The response of stratospheric O3 to changes in NOx and HOx is tested in a two‐dimensional model with the goal of exploring photochemical buffering mechanisms. The model tests are performed under background aerosol conditions at both current (1996) and preindustrial levels of stratospheric halogens. The tests are prompted in part by comparisons to observations from the POLARIS (Polar Ozone Loss in the Arctic Region in Summer) high‐altitude aircraft campaign, which show that models significantly underestimate the NOx/NOy ratio in the lower stratosphere but are more successful at reproducing observed O3. Model tests with increased NOx show that increases in NOx‐catalyzed O3 loss are subject to photochemical buffering in the lower stratosphere through reduced HOx‐catalyzed O3 loss and, at current halogen levels, reduced halogen‐catalyzed O3 loss. In contrast, these tests show that O3 is not well buffered with respect to increased NOx in the middle stratosphere, especially at preindustrial halogen levels. Consequently, an NOy increase that reaches the middle stratosphere could lead to modest depletion of column O3, while an NOy increase confined to the lower stratosphere more likely would have a neutral or slightly positive effect on column O3. These results are only mildly sensitive to uncertainties in the NOx/NOy ratio. Additional model tests with increased HOx show some photochemical buffering, but generally lead to mixed effects on O3 which depend on the nature of the HOx source gas. This source gas dependence is especially strong at current halogen levels.
Ozone depletion at the edge of the Arctic polar vortex 1996/1997
Authors: Hansen Georg , Chipperfield Martyn P.
Published: September 2002

In winter 1996/1997 the Arctic polar stratospheric vortex was extremely long‐lived. During most of its lifetime the vortex was centered at the pole, and its edge was almost permanently located over Northern Scandinavia. The ozone lidar at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) was operated from mid‐December 1996 until mid‐May 1997 when the final breakup of the vortex occurred. Comparison of the measurements with three‐dimensional model calculations reveals ozone depletion of up to 40% at the 475 and 550 K level. Maximum depletion occurred by around May 5 at levels up to 550 K and by around April 20 at 675 K. Analysis of the chemical model shows that while much of the early spring ozone depletion was due to halogen chemistry, associated with chlorine activation on polar stratospheric clouds, the ongoing depletion in late April and early May was due to “summertime” NOx (= NO + NO2) chemistry. The unusual persistence of the vortex, with the isolation of high‐latitude air masses until early May, permitted the occurrence of this depletion. The observations of air masses with low O3 show a very sudden end around May 10, indicating that even in the final phase of its existence, with continuously decreasing potential vorticity, some vortex air masses were well‐confined and mixing with the surroundings was small. A possible explanation of the strong confinement of vortex air is the scarcity of disturbances due to wave activity from below the vortex. It is supported by the frequent observation of intravortex layering in spring 1997.
Polar stratospheric descent of NOy and CO and Arctic denitrification during winter 1992–1993
Authors: Rinsland C. P. , Salawitch R. J. , Gunson M. R. , Solomon S. , Zander R. , Mahieu E. , Goldman A. , Newchurch M. J. , Irion F. W. , Chang A. Y.
Published: September 2002

Observations inside the November 1994 Antarctic stratospheric vortex and inside the April 1993 remnant Arctic stratospheric vortex by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer are reported. In both instances, elevated volume mixing ratios (VMRs) of carbon monoxide (CO) were measured. A peak Antarctic CO VMR of 60 ppbv (where 1 ppbv = 10−9 per unit volume) was measured at a potential temperature (Θ) of 710 K (∼27 km), about 1 km below the altitude of a pocket of elevated NOy (total reactive nitrogen) at a deep minimum in N2O (<5 ppbv). The Arctic observations also show a region of elevated vortex CO with a peak VMR of 90 ppbv at 630–670 K (∼25 km) but no corresponding enhancement in NOy, perhaps because of stronger dynamical activity in the northern hemisphere polar winter and/or interannual variability in the production of mesospheric or lower thermospheric NO. By comparing vortex and extravortex observations of NOy obtained at the same N2O VMR, Arctic vortex denitrification of 5 ± 2 ppbv at 470 K (∼18 km) is inferred. We show that our conclusion of substantial Arctic winter 1992–1993 denitrification is robust by comparing our extravortex observations with previous polar measurements obtained over a wide range of winter conditions. Correlations of NOy with N2O measured at the same Θ by ATMOS in the Arctic vortex and at midlatitudes on board the ER‐2 aircraft several weeks later lie along the same mixing line. The result demonstrates the consistency of the two data sets and confirms that the ER‐2 sampled fragments of the denitrified Arctic vortex following its breakup. An analysis of the ATMOS Arctic measurements of total hydrogen shows no evidence for significant dehydration inside the vortex.
Nitric oxide emission from a Norway spruce forest floor
Authors: Pilegaard K. , Hummelshøj P. , Jensen N. O.
Published: September 2002

Although the available data on soil NO emission are increasing, only few measurements have been made in temperate forests. Emissions from these forests are generally low, but recent studies have shown that NO emission from N‐affected forests can be much higher. In this study the emission of NO from the floor of a moderately N‐affected Norway spruce forest in Denmark was measured using dynamic chambers during two seasons (April 1995 and September 1995). At the same time, fluxes of CO2 and NO2, soil water content inside and outside the chambers, soil temperature at 10 cm depth, soil surface temperature and air temperature were recorded. The emission of NO in April was low (< 0.3 ng N m−2 s−1), whereas it was significantly higher in September (21 ng N m−2 s−1 with a range 0–82 ng N m−2 s−1). In April the soil surface temperature was low (< 6°C) and the soil moisture was high (26% vol/vol). In September the soil surface temperature was high (16°C) and soil moisture was significantly lower (14% vol/vol) than in April. There was a high correlation between CO2 emission and NO emission. The fluxes of NO2 were very small and not related to NO emission. Generally, the variation within a single plot was small, whereas the variation between plots was large. About half of the total variation of NO emission in September could be explained by a combination of soil moisture, distance to tree trunks, and moss cover. No significant relationship could be found between NO emission and soil surface temperature or soil temperature. Other parameters such as the C/N dynamics in the soil must be taken into account to provide a better mechanistic understanding of the NO emission. An annual emission of 3.15 kg NO‐N ha−1 yr−1 was estimated by the amount of NH4+ in throughfall. Compared to most of the previous published measurements of NO emission from temperate forest soils, the values presented here are high and of the same magnitude as found in N‐affected forests in Central Europe.
Physical and chemical characteristics of aerosols at Spitsbergen in the spring of 1996
Authors: Staebler Ralf , Toom‐Sauntry Desiree , Barrie Leonard , Langendörfer Uwe , Lehrer Eckhard , Li Shao‐Meng , Dryfhout‐Clark H.
Published: September 2002

Ground level measurements of aerosol physical and chemical properties were conducted at Ny Ålesund on Spitsbergen from late March to mid‐May 1996 as part of the European Arctic Tropospheric Ozone Chemistry study (ARCTOC'96) using laser‐scattering aerosol‐spectrometer probes and a denuder‐filterpack system. When the air comes from Eurasia, the number, area, and volume distributions of aerosols at Ny Ålesund are similar to those observed in the high Arctic at Alert, Canada, while when air comes off open water areas of the North Atlantic or Greenland, they are most different. The marine air has relatively more supermicrometer particles than the polluted arctic air. In Eurasian air the inorganic ionic component is dominated by H+ and SO4= and to a lesser extent NH4+. The average molar ratio of NH4+/H+ is 1/3. The average dry aerosol mass concentration and composition at Ny Ålesund varied depending on origin of air. The period mean mass concentration was 4.1 μg m−3. It was lowest (2.2 μg m−3) in air coming from the open North Atlantic and Greenland and was highest (6.2 μg m−3) in air coming from the east. On average, aerosols in the North Atlantic and Greenland air were composed of 19% non‐sea‐salt SO4=, 22% sea salt, 25% “other inorganic ions,” and 34% of an uncharacterized residual fraction that is likely mostly carbonaceous compounds. In contrast, for the most polluted eastern air these fractions were 48% non‐sea‐salt SO4=, 7% sea salt, 8% other inorganic ions, and 37% residual. For the period the median diameters of the mean accumulation mode number, surface area, and volume distributions at ambient relative humidity (78–90%) were 0.23 μm, 0.37 μm, and 0.50 μm effective scattering diameter (ESD), respectively. Number concentrations (for dry particle diameters >0.065 μm) ranged from 10 to 922 cm−3 with an average of 270 cm−3. Mixing ratios of particulate Br− compounds were observed to be anticorrelated with that of ozone, increasing markedly when the ozone mixing ratio was <10 nmol/mol. There was no obvious dependence of the ratio of gas to particle Br− on ozone mixing ratio. Observations suggested that during ozone depletion events the number concentration of particles increased in the 1.0 to 1.4 μm ESD diameter range (ambient relative humidity).
On the budget of photooxidants in the marine boundary layer of the tropical South Atlantic
Authors: Junkermann Wolfgang , Stockwell William R.
Published: September 2002

Measurements of ozone, hydrogen peroxide, methylhydroperoxide, and formaldehyde in the marine boundary layer of the tropical southern Atlantic Ocean were performed on board the research vessel Polarstern during its cruise from Bremerhaven, Germany (54°N, 8°E), to Punta Arenas, Chile (53°S, 71°W), in October and November 1994. Model calculations were made to estimate the deposition rates of H2O2 and CH3OOH. The modeling required that long‐range transport and advection processes of photochemical aged air masses that originated in African biomass burning plumes be taken into account. For H2O2 the dry deposition rate was estimated to be 1 times; 10−5/s and the air‐sea flux was estimated to be 4.5 × 1010 molecules/cm2/s, while for CH3OOH the dry deposition and the air‐sea flux were estimated to be 0.8 × 10−5/s and 3 × 1010 molecules/cm2 s, respectively. These values compared well with other estimates. Because of the uncertainties in the rate constants for the photochemical production and loss of methyl hydrogen peroxide the accuracy of the calculated deposition rates for both CH3OOH and H2O2 is limited. These chemical rate constants are not accurate enough to provide an unambiguous estimate of the deposition rate constants through simulation of field observations.
Calculated OH A2Σ+−X2Π (0,0) band rotational emission rate factors for solar resonance fluorescence
Authors: Stevens Michael H. , Conway Robert R.
Published: September 2002

The Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) measured vertical density profiles of hydroxyl (OH) by observing A2Σ+−X2Π (0,0) solar resonance fluorescence in the mesosphere near 309 nm [Conway et al., this issue]. Interpretation of the MAHRSI results requires calculation of the OH(0,0) rotational emission rate factors (g factors) which describe the molecule's fluorescent scattering efficiency. This calculation relies heavily on a solar irradiance measurement with high enough spectral resolution to identify the amount of excitation for each rotational transition (resolving power >3.1×105). Recent high‐resolution ground‐based measurements of the solar irradiance and more accurate absolute solar irradiance observations allow for much more accurate g factors. We find that the relative intensities of calculated OH (0,0;) rotational g factors are in excellent agreement with MAHRSI spectra. The calculated OH (0,0) band g factor is found to be 3.51×10−4 s−1 at 200 K, in good agreement with the most recent work on the subject.
Automated chamber technique for gaseous flux measurements: Evaluation of a photoacoustic infrared spectrometer‐trace gas analyzer
Authors: Yamulki S. , Jarvis S. C.
Published: September 2002

Experiments were made in order to evaluate the accuracy and sensitivity of a photoacoustic infrared trace gas analyzer (TGA) in conjunction with an automatic opening and closing chamber system developed for near‐continuous (2 min intervals) soil gaseous flux measurements. Humidity interference tests on N2O, CH4, and CO2 concentrations measured by the TGA were carried out, and the results showed a linear interference, with correction factors of 3 × 10−5x, 1.9 × 10−3x and 4.4 × 10−3x(x = H2O vapor ppm), respectively. CO2 interference on N2O and CH4 signals were also linear, with average correction factors of 2.8 × 10−4x and 6 × 10−5 x (x = CO2 ppm), respectively. Laboratory intercomparisons between the TGA and GC measurements of N2O and CH4 standards showed good agreement (R2 > 0.993), indicating the accuracy of the TGA for measurement of these gases at concentrations up to 100 and 40 ppm N2O and CH4, respectively. The relatively rapid measurement time for up to five gases simultaneously in 2 min, linearity, and ease of operation of the TGA represent major advantages compared to gas chromatography (GC). The automated chamber system provides a continuous measurement of fluxes with minimum disturbance to the soil environment enclosed by the chamber and provides the means, for example, of quantifying diurnal variability. In situ measurements of N2O‐N and CH4‐C fluxes with a sensitivity <10 g ha−1 d−1 (11.6 ng m−2 s−1), as well as of CO2 and water vapor (H2O), can be measured by the TGA when used with the automated system, and fluxes at background levels (i.e., from unfertilized soils) can be determined.
Ozone profiles from GOME satellite data: Algorithm description and first validation
Authors: Hoogen Ricarda , Rozanov Vladimir V. , Burrows John P.
Published: September 2002

In April 1995 the Global Ozone Monitoring Experiment (GOME) was launched aboard the European Space Agency's second European Remote Sensing (ERS‐2) satellite. GOME is a nadir‐viewing spectrometer measuring solar radiation backscattered by the system Earth‐atmosphere in the ultraviolet and visible spectral range. For the retrieval of ozone vertical distributions from GOME measurements the Full Retrieval Method (FURM) has been developed. It is based on the optimal estimation approach and on results from information theory. The joint retrieval of the ozone profile and several additional parameters (e.g., surface albedo) enables the ozone profile retrieval to be extended below the ozone number density maximum. A first validation of the GOME profiles is performed by a comparison with approximately 200 coincident ozonesonde measurements from five European stations. The RMS difference between ozone subcolumn amounts in 10 km layers as measured by GOME and by the sondes is approximately 7–8% in the middle stratosphere and 15–20% in the lower stratosphere. Furthermore, it is demonstrated that information about the ozone content in the troposphere can be retrieved from the GOME measurements.
Atmospheric lifetime and global warming potential of HFC‐245fa
Authors: Ko Malcolm , Shia Run‐Lie , Sze Nien‐Dak , Magid Hillel , Bray Robert G.
Published: September 2002

We describe the method used to compute the global warming potential of hydrofluorocarbon (HFC) 245fa (CHF2CH2CF3). The Atmospheric and Environmental Research (AER) two‐dimensional (latitude‐height) chemistry‐transport model was used to calculate the atmospheric lifetime and atmospheric scale height of HFC‐245fa. Assuming that reaction with OH is the only removal mechanism, the recommended rate constant fromJet Propulsion Laboratory[1997] (6.1 × 10−13exp (−1330/T) cm3s−1) implies a lifetime of 7.6 years and an average atmospheric scale height of 35 km in the stratosphere. Using the IR absorption cross sections for HFC‐245fa and CFC‐11 determined in the laboratory, the AER one‐dimensional radiative‐convective model was used to calculate the radiative forcing. The value for HFC‐245fa is 1.14 times larger than that for CFC‐11 on a mass basis and 1.11 larger on a per molecule basis. The global warming potentials for HFC‐245fa (relative to carbon dioxide) are 2400, 760, and 240 (based on the values for absolute global warming potential for carbon dioxide reported byIntergovernmental Panel on Climate Change[1996]) at integration time horizons of 20, 100, and 500 years, respectively.
Measurement of free radicals OH and HO2 in Los Angeles smog
Authors: George L. A. , Hard T. M. , O'Brien R. J.
Published: September 2002

Atmospheric free radicals hydroxyl and hydroperoxyl (OH and HO2, collectively HOx) are the catalysts that cause secondary or photochemical air pollution. Chemical mechanisms for oxidant and acid formation, on which expensive air pollution control strategies are based, must accurately predict these radical concentrations. We have used the fluorescence assay with gas expansion (FAGE) technique to carry out the first simultaneous, in situ measurements of these two radicals in highly polluted air during the Los Angeles Free Radical Experiment. A complete suite of ancillary measurements was also made, including speciated hydrocarbons, carbon monoxide, aldehydes, nitric oxide, nitrogen dioxide, and ozone along with meteorological parameters. Using this suite of measurements, we tested the ability of a lumped chemical mechanism to accurately predict radical concentrations in polluted air. Comparison of model predictions with measured radical concentrations revealed generally good agreement for OH early and late in the day, including the early evening hours, when OH persisted at low concentrations after dark. During midday, however, modeled [OH] was high by about 50%. Agreement for HO2 was quite good in the early morning hours, but model‐calculated HO2 concentrations were significantly too high during midday. When we used our measured HO2 concentrations as model input, agreement between calculated and measured OH concentrations was improved. It seems likely that (1) the model's HOx sources are too large, (2) there are unaccounted HOx loss processes in Los Angeles air, and/or (3) the complex parameterization of RO2/HO2 radical chemistry in the reaction mechanism does not adequately describe the behavior of these radicals in the Los Angeles atmosphere.
Two‐day wave structure and mean flow interactions observed by radar and High Resolution Doppler Imager
Authors: Fritts David C. , Isler Joseph R. , Lieberman Ruth S. , Burrage Mark D. , Marsh Daniel R. , Nakamura Takuji , Tsuda Toshitaka , Vincent Robert A. , Reid Iain M.
Published: September 2002

Data obtained with four MF and meteor radars at equatorial and subtropical sites and with the High Resolution Doppler Imager (HRDI) instrument aboard the UARS satellite were used to examine the structure, wave‐mean flow interactions, and potential sources of the 2‐day wave in the middle atmosphere during three southern hemisphere summers. The three wave events were highly transient, having typical durations of 20 to 30 days and exhibiting modulation at shorter periods. Temporal variations were found to exhibit good correlations between radar and HRDI data. Radar and HRDI data were used to estimate those components of the Eliassen‐Palm flux that could be assessed with these data. Meridional fluxes of momentum and heat were computed using HRDI data and agree reasonably with the momentum fluxes computed from radar data at discrete locations. These fluxes were found to exhibit consistent latitudinal structures each year, suggesting systematic wave excitation and wave‐mean flow interactions. Meridional momentum flux gradients were seen to be anticorrelated with zonal wind accelerations in a manner consistent with wave forcing of the large‐scale circulation. The apparent wave‐mean flow interactions suggest that the 2‐day wave could be a transient response to baroclinic instability of the summer hemisphere mesospheric jet. A calculation of the meridional gradient of quasi‐geostrophic potential vorticity using HRDI winds and the COSPAR International Reference Atmosphere (CIRA 1986) temperatures exhibits a region of instability in the lower and middle mesosphere extending into subtropical latitudes and provides additional evidence of a possible source of this motion via baroclinic instability of the summer hemisphere jet structure.
Origin and recycling processes of precipitating water over the Eurasian continent: Experiments using an atmospheric general circulation model
Authors: Numaguti Atusi
Published: September 2002

By using an atmospheric general circulation model, origin and transport processes of water in the atmosphere‐land system are examined. The water vapor and the land‐surface water in the model are tagged according to the geographical regions of water input (evaporation) and are separately treated. The results are examined focusing on the water cycle over the Eurasian continent. In the winter season, most of the precipitating water over the Eurasian continent is supplied by evaporation from the oceans. On the other hand, the precipitating water in the summer is mostly supplied by the evaporation from the continental surface, indicating active recycling of water between the atmosphere and the land in this season. Considering that the water in the continental surface should be supplied from the oceans sometime before, the history of the water from its origin (evaporation from the oceans) is examined, by separately treating the components of the soil water and the snow according to the geographical regions of the origin. Two additional types of tracers are included in order to determine the timescale of the transport and the frequency of the recycling between the atmosphere and the land. The results show that the main origin of water in the northern part of the Eurasian continent is the Northern Atlantic Ocean and that in the southern part is the Northern Indian Ocean. In the southern part the mean age of the precipitating water since its origin is 1 month or shorter, and the mean count of recycling is less than one, indicating that the water comming directly from the oceans by atmospheric transport is dominant. In the northern inland part in summer, however, the mean age is 3 months or longer, and the mean count of recycling is above two. These results suggest that a significant portion of the precipitating water in inland Eurasia in the summer originals in the Atlantic Ocean in the previous winter and is transported eastward with a few recycling cycles between the atmosphere and the continental surface.
Vertical dispersion of an aircraft wake: Aerosol‐lidar analysis of entrainment and detrainment in the vortex regime
Authors: Sussmann Ralf
Published: September 2002

Vertical dispersion of contrails in the vortex regime is investigated by focusing on the role of entrainment and detrainment of exhaust with respect to the pair of trailing vortices. A ground‐based backscatter‐depolarization lidar with an integrated CCD camera provides information on optical and geometrical parameters of the contrail in the time span between 5.7 and 50.3 s behind a B747‐400 aircraft. This is combined with coincident airborne in situ measurements of turbulence and the vertical profiles of temperature and wind speed in a case study. The two wingtip vortices, separated by 47 m, are descending with an increasing speed (2.5–3.1 m/s for 10.8–47.8 s behind aircraft) in the weakly non‐stably‐stratified atmosphere. The turbulent vertical dissipation rate on the day of the study above southern Germany is a factor of 1000 higher than found typically above oceans at cruising altitude. At 4.2 s behind the aircraft, a diffuse secondary wake starts to evolve above the two wingtip vortices. After ≈ 50 s the secondary wake encloses a cross‐sectional area (4410 m2) comparable to that of the primary wake (4620 m2) and a relative ice surface area of 1:5. The observed early onset of the secondary wake is conjectured to be due to turbulent detrainment of fluid out of the primary wake which can be enhanced by detrainment due to baroclinic forces later in the vortex regime evolution. By exclusion of other mechanisms of secondary wake formation, detrainment of fluid from the primary wake is concluded to be the precondition for secondary wake formation. Detrainment due to baroclinic forces, shear or turbulence is, in general, unlikely to be absent for typical atmospheric conditions. It is suggested that the ambient humidity level may determine when a secondary wake is visible above a vortex pair and when it is not.
Simulation of upper tropospheric clouds with the Colorado State University general circulation model
Authors: Fowler Laura D. , Randall David A.
Published: September 2002

We have compared the climatology of upper tropospheric clouds simulated with the Colorado State University (CSU) general circulation model against cloud products retrieved by the International Satellite Cloud Climatology Project (ISCCP). Following the ISCCP cloud classification, upper tropospheric clouds are defined as clouds with cloud tops above 440 hPa. We refined our comparison by considering separately clouds with cloud tops above 180, 310, and 440 hPa in order to exhibit the optical characteristics of the highest clouds in the model and satellite cloud products. Four ranges of visible optical depths (τ) were used to distinguish cirrus (τ ≤ 3.6) from optically thicker cirrostratus (3.6 < τ ≤ 23) and deep convective clouds (τ > 23) and to further differentiate between thin (0.02 < τ ≤ 1.6) and thick (1.6 < τ ≤ 3.6) cirrus. Results show that the CSU GCM simulates satisfactorily the zonally averaged distribution of upper tropospheric clouds when all values of τ are included but systematically underpredicts the frequency of occurrence of clouds with values of τ less than 3.6 when compared against ISCCP‐D1 data. This result reveals that simulated total‐column optical depths for columns that include upper tropospheric clouds are too large relative to satellite‐derived values. The CSU GCM simulates upper tropospheric clouds in the tropics more successfully than those in the middle latitudes. In the middle latitudes the model fails to simulate upper tropospheric clouds over the continents, especially over high plateaus and mountain ranges. Discrepancies between the CSU GCM and the ISCCP cloud products can be addressed in terms of our simple formulation of the optical thickness as a function of the prognostic liquid/ice water content, the prescribed value of the effective radius, and the geometrical thickness of the upper tropospheric model layers. We investigate the impact of the vertical resolution used in the GCM on the calculation of the optical depths of single‐layer clouds using estimates of the geometrical thickness of cloudy layers from the Lidar In‐Space Technology Experiment.
Horizontal structure of marine boundary layer clouds from centimeter to kilometer scales
Authors: Davis Anthony B. , Marshak Alexander , Gerber H. , Wiscombe Warren J.
Published: September 2002

Horizontal transects of cloud liquid water content (LWC) measured at unprecedented 4‐cm resolution are statistically analyzed scale‐by‐scale. The data were collected with a Particulate Volume Monitor (PVM) probe during the winter Southern Ocean Cloud Experiment (SOCEX) on July 26, 1993, in a broken‐stratocumulus/towering‐cumulus cloud complex. Two scaling regimes are found in the sense that two distinct power laws, k−β are needed to represent the wavenumber spectrum E(k) over the full range of scales r ≈ 1/k. Detailed numerical simulations show that the scale break at 2–5 m is not traceable to the normal variability of LWC in the PVM's instantaneous sampling volume (1.25 cm3) driven by Poissonian fluctuations of droplet number and size. The two regimes therefore differ physically. The non‐Poissonian character of the small‐scale LWC variability is consistent with a similar finding by Baker [1992] for droplet number concentration obtained from Forward Scattering Spectrometer Probe (FSSP) data: at scales of a few centimeters, spatial droplet distributions do not always follow a uniform Poisson law. With β = 0.9 ± 0.1, the small‐scale (8–12 cm ≲ r ≲ 2–5 m) regime is stationary: jumps in LWC are highly variable in size and rapidly cancel each other, leading to short‐range correlations. By contrast, the large‐scale (5 m ≲ r ≲ 2 km) variability with β = 1.6 ± 0.1 is nonstationary: jumps are generally quite small, conveying a degree of pixel‐to‐pixel continuity and thus building up long‐range correlations in the low‐pass filtered signal. The large‐scale structure of the complex SOCEX cloud system proves to be multifractal, meaning that large jumps do occur on an intermittent basis, that is, on a sparse fractal subset of space. Low‐order, hence more robust, multifractal properties of the SOCEX clouds are remarkably similar to those of their First ISCCP Regional Experiment (FIRE) and Atlantic Stratocumulus Transition EXperiment (ASTEX) counterparts, and also to those of passive scalars in fully developed turbulence. This is indicative of a remarkable similarity in the micro‐physical and macrophysical processes that determine cloud structure in the marine boundary layer at very remote locales, especially since the particular SOCEX cloud system investigated here was rather atypical. Interesting differences are also found: in the scaling ranges on the one hand, and in higher‐order moments on the other hand. Finally, we discuss cloud‐radiative effects of the large‐ and small‐scale variabilities.
The importance of thermodynamics for modeling the volume of the Greenland Ice Sheet
Authors: van de Wal R. S. W.
Published: September 2002

Two different kinds of ice flow models, one two‐dimensional (2‐D) and the other three‐dimensional (3‐D), have been used to test the importance of the thermodynamic response of the Greenland ice sheet. The basic difference between the two models is that the thermodynamics are neglected in the two‐dimensional model, whereas these are incorporated in the three‐dimensional model. The 2‐D and 3‐D models are compared in terms of volume response time and total volume of the ice sheet. Results indicate that the sawtooth character of the ice volume on glacial‐interglacial timescales is, among other possible reasons, a result of the thermodynamic coupling. The changes in elevation over the last 130,000 years calculated with the three‐dimensional version are 230 m for the Summit drill site and 190 m for the N‐GRIP site. The standard deviation of the changes in elevation is 55 m for the Summit site and 43 m for the N‐GRIP site. The present‐day imbalance is merely a result of the rather constant climate over the last 10 kyr and is not determined by the thermodynamics. Consequently, Pleistocene temperatures do still exist in the ice sheet but are not important for volume calculations of the present‐day ice sheet. For short‐term perturbation experiments in the future, the pronounced sensitivity of the mass balance will determine the response of the Greenland ice sheet, whereas thermodynamics will play only a minor role.
Mapping global land surface albedo from NOAA AVHRR
Authors: Csiszar I. , Gutman G.
Published: September 2002

A set of algorithms is combined for a simple derivation of land surface albedo from measurements of reflected visible and near‐infrared radiation made by the advanced very high resolution radiometer (AVHRR) onboard the National Oceanic and Atmospheric Administration (NOAA) polar orbiting satellites. The system consists of a narrowband‐to‐broadband conversion and bidirectional correction at the top of the atmosphere and an atmospheric correction. We demonstrate the results with 1 month worth of data from the NOAA National Environmental Satellite, Data, and Information Service (NESDIS) global vegetation index (GVI) weekly data set and the NOAA/NASA Pathfinder Atmosphere (PATMOS) project daily data. Error analysis of the methodology indicates that the surface albedo can be retrieved with 10–15% relative accuracy. Monthly albedo maps derived from September 1989 GVI and PATMOS data agree well except for small discrepancies attributed mainly to different preprocessing and residual atmospheric effects. A 5‐year mean September map derived from the GVI multiannual time series is consistent with that derived from low‐resolution Earth Radiation Budget Experiment data as well as with a September map compiled from ground observations and used in many numerical weather and climate models. Instantaneous GVI‐derived albedos were found to be consistent with surface albedo measurements over various surface types. The discrepancies found can be attributed to differences in areal coverage and representativeness of the satellite and ground data. The present pilot study is a prototype for a routine real‐time production of high‐resolution global surface albedo maps from NOAA AVHRR Global Area Coverage (GAC) data.
Late 20th Century increase in South Pole snow accumulation
Authors: Mosley‐Thompson Ellen , Paskievitch J. F. , Gow A. J. , Thompson L. G.
Published: September 2002

A compilation of the 37‐year history of net accumulation at the South Pole [Mosley‐Thompson et al., 1995] suggests an increase in net annual accumulation since 1965. This record is sporadic and its quality is compromised by spatially restricted observations and nonsystematic measurement procedures. Results from a new, spatially extensive network of 236 accumulation poles document that the current 5‐year (1992–1997) average annual net accumulation at the South Pole is 84.5 ± 8.9 mm water equivalent (w.e.). This accumulation rate reflects a 30% increase since the 1960s when the best, although not optimal, records indicate that it was 65 mm w.e. Identification of two prominent beta radioactivity horizons (1954/1955 and 1964/1965) in six firn cores confirms an increase in accumulation since 1965. Viewed from a longer perspective of accumulation provided by ice cores and a snow mine study, the net accumulation of the 30‐year period, 1965–1994, is the highest 30‐year average of this millennium. Limited data suggest this recent accumulation increase extends beyond the South Pole region and may be characteristic of the high East Antarctic Plateau. Enhanced accumulation over the polar ice sheets has been identified as a potential early indicator of warmer sea surface temperatures and may offset a portion of the current rise in global sea level.
A seasonal air transport climatology for Kenya
Authors: Gatebe C. K. , Tyson P. D. , Annegarn H. , Piketh S. , Helas G.
Published: September 2002

A climatology of air transport to and from Kenya has been developed using kinematic trajectory modeling. Significant months for trajectory analysis have been determined from a classification of synoptic circulation fields. Five‐point back and forward trajectory clusters to and from Kenya reveal that the transport corridors to Kenya are clearly bounded and well defined. Air reaching the country originates mainly from the Saharan region and northwestern Indian Ocean of the Arabian Sea in the Northern Hemisphere and from the Madagascan region of the Indian Ocean in the Southern Hemisphere. Transport from each of these source regions show distinctive annual cycles related to the northeasterly Asian monsoon and the southeasterly trade wind maximum over Kenya in May. The Saharan transport in the lower troposphere is at a maximum when the subtropical high over northern Africa is strongly developed in the boreal winter. Air reaching Kenya between 700 and 500 hPa is mainly from Sahara and northwest Indian Ocean in the months of January and March, which gives way to southwest Indian Ocean flow in May and November. In contrast, air reaching Kenya at 400 hPa is mainly from southwest Indian Ocean in January and March, which is replaced by Saharan transport in May and November. Transport of air from Kenya is invariant, both spatially and temporally, in the tropical easterlies to the Congo Basin and Atlantic Ocean in comparison to the transport to the country. Recirculation of air has also been observed but on a limited and often local scale and not to the extent reported in southern Africa.
Bias correction of daily precipitation measurements for Greenland
Authors: Yang Daqing , Ishida Shig , Goodison Barry E. , Gunther Thilo
Published: September 2002

A methodology for correcting the Hellmann gauge‐measured daily precipitation for wind‐induced undercatch, wetting loss, and trace amount of precipitation was presented and applied at 12 climate stations in Greenland for years 1994–1997. The results show the following: (1) daily corrections of the biases increase the gauge‐measured annual precipitation by 35–280 mm for the 4 years (about 25–90% of the gauge‐measured yearly total) in Greenland; (2) of the biases, wind‐induced undercatch is the source of greatest error; wetting loss and trace amount of precipitation are also significant biases in the northern Greenland regions of low precipitation; (3) monthly correction factors (corrected/measured precipitation) differ by station and, at an individual station, by type of precipitation; (4) considerable intra‐annual variation of the yearly corrections has been found in Greenland due to the fluctuation of wind speed, air temperature, and frequency of snowfall. These results suggest that annual precipitation in Greenland is much higher than previously reported, particularly in the southern regions of high precipitation; and the latitudinal precipitation gradient may also be greater over Greenland. These results will have a significant impact to water budget and glacier mass balance studies in Greenland.
Impact of thermomechanical ice sheet coupling on a model of the 100 kyr ice age cycle
Authors: Tarasov L. , Peltier W. Richard
Published: September 2002

Simulations of the most recent 100 kyr glaciation‐deglaciation cycle of the late Pleistocene Ice Age are presented which feature a newly constructed thermomechanically coupled three‐dimensional ice sheet model which is itself coupled to a previously employed global energy balance climate model with realistic geography. The model incorporates both orbital insolation forcing due to the slow time evolution of orbital geometry (arising from many body effects in the solar system) as well as the forcing due to varying atmospheric concentrations of greenhouse gases. Simulations of the Greenland ice sheet are presented with which we are able to investigate the extent to which the ice flow law employed in the ice dynamics component of the model is constrained. The good agreement that we are able to achieve between the model‐generated ice sheet topography and the observed Greenland topography provides a clear demonstration of the quality of the ice dynamical model we have developed. The incorporation of full thermomechanical coupling using the standard Glen flow law in the ice dynamics component of the model is shown to increase the difficulty of achieving complete termination of the 100 kyr cycle that the model delivers. Furthermore, a 20 fold flow parameter enhancement relative to that used for Greenland is required to match the aspect ratio of the ICE‐4G reconstruction [Peltier, 1994]. Analyses presented herein suggest that basal processes are unlikely to account for this need for flow parameter retuning. However, the new thermomechanically coupled model now provides a clear separation of the Cordilleran and Hudson Bay domes at Last Glacial Maximum in contradistinction to analyses previously performed with isothermal ice sheet models. This innovation therefore leads to a major improvement of the generated ice sheet topography in relation to geological inferences. Aside from this important difference, the overall results regarding the ability of the model to fully account for the most recent 100 kyr cycle of glaciation and deglaciation without the necessity of introducing additional ad hoc feedbacks confirms the validity of this conclusion, reached previously on the basis of isothermal model integrations.