bluuebunny/crossref_metadata_2025 huggingface dataset:
Vortex stretching and bottom torques in the Bryan‐Cox ocean circulation model
Authors: Bell Michael J.
Published: September 2002

After 3 months of integration the barotropic stream functions in the 1° global versions of the Bryan‐Cox ocean models [Bryan, 1969; Cox, 1984] run at the U.K. Meteorological Office have weaker recirculations to the north and south of the Gulf Stream than are believed to exist and than are obtained in some diagnostic models of comparable resolution. Diagnostics of the contributions to the rate of change of the vorticity of the vertically meaned and vertically integrated currents in the third month of a model integration are presented. Those for the vertically integrated current show that the main patterns of northward advection of planetary vorticity are accounted for by the bottom pressure torque and viscous torque; the contributions from the curl of the wind stress and nonlinear advection are comparatively small. Using this result, a new relationship is derived between the bottom pressure torque in the model and the vortex stretching at the top of the steps in the bathymetry. Diagnostic calculations of the contributions to the stream function driven by each of the torques suggest two alternative interpretations of the weakness of the northern recirculation gyre and the poor separation of the Gulf Stream at Cape Hatteras in the model. One is based on the difference between the vortex stretching and bottom pressure torque in the model, the other on the flow driven by the viscous torque on the fluid interior (i.e., on grid cells that are not adjacent to bathymetric steps). The main changes to the patterns of these diagnostics after 10 years of integration are summarized.
A forcing mechanism for the poleward flow off the southern California coast
Authors: Oey Lie‐Yauw
Published: September 2002

It is shown that when the wind distribution along a coast is anisotropic, such that its cross‐shore scale is smaller than its alongshore scale, the coastal sea level (or the upper layer anomoly) “A” of an ocean forced by both wind and wind curl is governed by a modified (nondimensionalized) Kelvin wave equation: ∂A/∂t* + ∂A/∂y* =k0(0,y*,t*) + ∫ ∂k1/∂y*dtLwherek0andk1are wind stress and wind stress curl at the coast, respectively,yis the alongshore distance, andtis the time. Numerical experiments, from a simple reduced‐gravity type with idealized forcing and coastline to a three‐dimensional primitive equation model with a realistic coastline, bottom topography of the Southern California Bight and the Santa Barbara Channel, and observed wind stresses, were carried out to show that the observed near‐coast near‐surface poleward flow in the region is primarily forced by the equatorward weakening of the wind curl, (∂k1/∂y* >0), in the bight. Beta provides natural damping by weakening and widening the current through westward propagating Rossby waves and causes the current to lead the coastal pressure field by 1–2 months, which improves the agreement with observations of the phasing of the modeled currents but is otherwise not required in forcing the poleward flow.
Warm water formation and escape in the upper tropical Atlantic Ocean: 2. A numerical model study
Authors: Lee Sang‐Ki , Csanady G. T.
Published: September 2002

We seek the simplest mass and heat balance scenario within the upper tropical Atlantic Ocean using a 2.5‐layer numerical model, with the help of a newly suggested entrainment formula. The model ocean is driven by the seasonal wind and heat flux associated with the movement of the Intertropical Convergence Zone (ITCZ). The entrainment rate is scaled by the product of wind stress and shear at the bottom of the mixed layer. On an annual average, the northward transport of the tropical warm water is about 11 Sv, with roughly 10 Sv associated with entrainment of upper thermocline water. Out of the total 10 Sv of the needed upper thermocline water, 9 Sv enters the equatorial cell from the South Atlantic. The seasonal response to the ITCZ movement is most striking in the entrainment rate and the northward warm water escape rate. The entrainment is significant during May/December and ceases between January and March. The locally forced equilibrium response between the interface shear and the zonal wind stress east of 30°W appears to be responsible for this cycle. The warm water escape toward the North Atlantic takes place mainly from October to June and nearly stops during July/September. As a result of seasonal variations of the two key processes, namely, the entrainment and the northward warm water escape, the tropical warm water pool stores heat during May/October and lets heat escape in November/April.
Gas transfer experiment on a lake (Kerguelen Islands) using 3He and SF6
Authors: Jean‐Baptiste Philippe , Poisson Alain
Published: September 2002

Gas transfer velocities of SF6 and 3He were determined in a Kerguelen Islands lake at wind speeds in the range 0–10 m/s by injecting the two tracers into the water and measuring their concentrations over 40 days. Two methods are investigated for the determination of the relationship linking the gas transfer velocity K to the wind speed W. The first method postulates a power law relationship K = βWα. This leads to the same exponent α = 1.5±0.2 for both gases. The second method is the classic determination of the gas transfer velocity Kij between two tracer measurements at times ti and tj using the well‐mixed reservoir assumption. This method proves to be less favorable owing to its nonlinearity bias and also because it induces much scatter in the gas transfer coefficient/wind speed relationship. This dispersion is shown to arise from the experimental scatter of the data and, above all, from the high sensitivity of the method to even small heterogeneities in the tracer vertical distribution. In the present experiment, the Liss and Merlivat correlation [Liss and Merlivat, 1986] is shown to underestimate the actual mean gas exchange rate by about 40%. Our results agree with the recent dual‐tracer experiment by Wanninkhof et al. [1993] and are also consistent with CO2 transfer coefficient data derived from the study of 14C oceanic inventories. As expected from gas transfer theories and various experimental observations, the Schmidt number exponent in the comparison of 3He and SF6 transfer velocities is found to vary substantially with the transfer regime. However, its variation is found to be greater than that forecast by current gas transfer models, with values as high as n = −0.2 for intermediate to strong winds. This again raises the question of the validity of the normalization method for KCO2 calculation from gas transfer experiments, especially in high‐wind regimes.
Toward the elimination of bias in satellite retrievals of sea surface temperature: 2. Comparison with in situ measurements
Authors: Merchant C. J. , Harris A. R.
Published: September 2002

We test a new scheme for retrieving skin sea surface temperatures (SSTs) from brightness temperatures observed by the satellite‐borne along‐track scanning radiometer (ATSR). The main improvement is in the accuracy of SST retrievals using dual‐view observations at 11 and 12 μm under conditions of high optical depth of stratospheric aerosol and high tropospheric water vapor loading. This is demonstrated by comparing 10 arc min average ATSR SSTs with in situ measurements of the Tropical Atmosphere Ocean (TAO) array of buoys between September 1991 and May 1992 (when volcanic aerosol was present in the stratosphere after the eruption of Mount Pinatubo in June 1991). The 620 coincidences of ATSR and TAO SSTs are matched within 1 hour and ∼20 km. The mean and standard deviation of the difference between satellite and in situ SSTs are −0.16 K and 0.37 K, respectively, compared to −1.39 K and 0.59 K, respectively, using the prelaunch scheme. Meteorological measurements recorded with most TAO SSTs allow calculation of skin SSTs, more suitable for comparison with the “radiometric” SSTs derived from ATSR. The bias and standard deviation compared to skin SST are +0.07 K and 0.27 K, respectively. About 5% of ATSR retrievals have errors which we attribute to residual cloud contamination. The robust standard deviation describing the uncontaminated data is 0.19 K. The aerosol‐robustness of the new scheme is evident in the absence of any trend in SST error corresponding to the decay of the stratospheric aerosol loading (0.00±0.16 K yr−1). The new scheme is shown to be unbiased by water vapor loadings between 20 and 60 kg m−2. The effects of matchup and buoy error and either skin‐bulk variability or skin‐bulk adjustment error contribute to the standard deviations given above, suggesting that the random retrieval errors intrinsic to use of the new coefficients are smaller still.
Variations of ƒCO2 and air‐sea flux of CO2 in the Greenland Sea gyre using high‐frequency time series data from CARIOCA drift buoys
Authors: Hood E. M. , Merlivat L. , Johannessen T.
Published: September 2002

A 6 month, high‐frequency (hourly) time series of ƒCO2 and sea surface temperature measured by CARIOCA drift buoys in the Greenland Sea gyre is presented. The ƒCO2 shows the effects of photosynthetic activity in the summer, with ƒCO2 values as low as 260 μatm, followed by an increase to approximately 310 μatm in the late fall and winter due to mixing with CO2‐rich deep water from below, remineralization, and gas exchange. The time series shows that ƒCO2 in the Greenland Sea gyre is undersaturated with respect to the atmosphere year‐round, having an average ΔƒCO2 of −71 μatm. Linear correlations between the measured ƒCO2 normalized to a constant temperature of −1°C and sea surface temperature (SST) are used to construct high‐resolution ƒCO2 and air‐sea CO2 flux maps for the Greenland Sea gyre area using SST and wind speed data from the European Centre for Medium‐Range Weather Forecasting and ice information from the Special Sensor Microwave / Imager on a 0.5° × 0.5° grid. The CO2 flux for the Greenland Sea gyre calculated for 1996–1997 considering the effects of blockage of gas exchange by sea ice is estimated to be −2.4 to −4.2 × 1012 g C yr−1 depending on the gas exchange parameterization used.
Trapped motion around the Pribilof Islands in the Bering Sea
Authors: Kowalik Zygmunt , Stabeno Phyllis
Published: September 2002

Observations in the region of the Pribilof Islands and Canyon (PIC) reveal a clockwise circulation around the group of islands and around each of the two largest islands, St. Paul and St. George. Six current meters deployed around St. Paul Island revealed a steady clockwise flow around the island, which was strongest south of the island and weakest to the east. We use a high‐resolution tidal model in the PIC region to show that this flow pattern results from tidal rectification over the shallow topography tides. Tidal residual currents of 10–15 cm s−1 were predicted by the model, compared to mean currents of 5–20 cm s−1 observed at the mooring sites. Both diurnal and semidiurnal tidal oscillations contribute to the clockwise circulation around the islands. In the diurnal band the enhanced currents occur also at the shelf slope where a tidal wave generates a shelf wave due to resonance with topography. In the PIC region the main shelf wave occurs in the Pribilof Canyon where both observations and measurements show enhancement of the diurnal tidal currents.
A modeling study of upper ocean winter processes in the Labrador Sea
Authors: Tang C. L. , Gui Q. , DeTracey B. M.
Published: September 2002

A coupled ice‐mixed layer model is developed to study winter processes in the Labrador Sea. The seasonal change of mixed‐layer properties, air‐sea fluxes, and ice coverage is examined. The effects of severe atmospheric conditions and salinity change on the upper ocean are investigated. The model is integrated from November to June to cover the entire winter period. Objectively analyzed temperature and salinity fields for November are used as the initial conditions of the model ocean. Meteorological parameters derived from the NCEP reanalysis are used as forcings. The results show that heat loss of the ocean reaches a maximum at the end of January. High heat loss, 250 to 400 W m−2, occurs in the northern Labrador Sea between 56°N and 63°N in ice‐free waters near the ice edge. Pack ice significantly reduces the heat loss with a typical value of 50 W m−2. In the northern Labrador Sea, surface cooling causes the mixed layer to deepen continuously through winter and reaches a maximum of 150–450 m in late March. A decrease of surface salinity by 1% (for example, from 34.35 to 34) results in a decrease of mixed‐layer depth by 17–55%. The high stratification associated with the low surface salinity has little effect on ice coverage, since the reduced level of vertical mixing cannot lower the surface temperature sufficiently to increase the ice area. This suggests that freshening of the Labrador Sea during the Great Salinity Anomaly in the late 1960s to early 1970s did not promote ice production. An increase of wind speed by 40% and a decrease of air temperature up to 4°C, representing the conditions of cold winters, doubles the mixed‐layer depth to a maximum of 900 m and extends the southern ice limit by 200 km on the northern Grand Banks. This suggests that interannual variation of convection and ice extent are mainly controlled by local meteorological conditions.
Diapycnal mixing in Gulf Stream meanders
Authors: Rodríguez‐Santana A. , Pelegrí J. L. , Sangrà P. , Marrero‐Díaz A.
Published: September 2002

We use historical data [Bane et al., 1981], interpolated to isopycnic coordinates, to examine the possibility of significant diapycnal mixing within the upper thermocline layers of the Gulf Stream. The data consist of 28 air‐dropped expendable bathythermograph (AXBT) sections in the northern Blake Plateau distributed in five different surveys done within 8 days. From the data we obtain a separation index between isopycnals j = ρ ∂z/∂ρ, where z is the depth of an isopycnal and ρ is the density; the diapycnal shear ∂υ/∂ρ, where υ is the geostrophic velocity; and the gradient Richardson number Ri. Following Pelegrí and Csanady [1994], we postulate that the material derivative of the density, or density tendency wρ = Dρ/Dt, is the result of small‐scale instabilities related to near‐critical Ri values. We present the distribution of these quantities (j, ∂υ/∂ρ, Ri, and wρ) and the diapycnal convergence/divergence (∂wρ/∂ρ) over isopycnals and on sections normal to the coast. The results show the passage of steep meanders being related to anomalously low j values (strong density gradients) within the upper thermocline layers and the cyclonic filaments of the stream. The statically stable upper thermocline layers, however, are concurrent with large diapycnal shear and turn out to be dynamically unstable, characterized by low Ri and high‐density tendencies and diapycnal convergence/divergence. The errors involved in calculating the dependent variables from AXBT data are assessed using both an error propagation approach and a Monte Carlo error simulation. These errors, although significant, are not large enough to modify the observed patterns substantially.
Nitrous oxide and methane during the 1994 SW monsoon in the Arabian Sea/northwestern Indian Ocean
Authors: Upstill‐Goddard Robert. C. , Barnes Jonathan , Owens Nicholas J. P.
Published: September 2002

Partial pressures of dissolved and atmospheric nitrous oxide, N2O, and methane, CH4, were measured during the latter stages of the southwest (SW) monsoon and subsequent intermonsoon transition of 1994 in the Gulf of Oman and the northern and central Arabian Sea, during Discovery cruises D210 and D212 of the United Kingdom Joint Global Ocean Flux Study of the northwestern Indian Ocean (NWIO). Mean observed atmospheric mixing ratios, 310±3 ppbv N2O, 1706±17 ppbv CH4 (SW monsoon), and 311±3 ppbv N2O, 1784±20 ppbv CH4 (intermonsoon), were analytically indistinguishable from contemporary global baseline data. Mean surface mixed layer saturations were spatially and temporally heterogeneous. Largest variation was observed for N2O in an upwelling region adjacent to the Oman coast; mean N2O saturations were 140±40% (SW monsoon) and 119±17% (intermonsoon), with corresponding CH4 saturations of 170±55% and 179±15%. These apparent differences were largely a consequence of less detailed station coverage during D212, reflecting large variability on a relatively small spatial‐scale rather than true seasonal variation; for individual stations in the coastal upwelling, temporal changes in mean mixed layer saturations were not significant. This suggests that within this region the processes of gas exchange, net production, and supply/removal by advection and vertical mixing were more or less in balance during the period studied. Open ocean saturations were lower and less variable: 106±7% N2O, 130±5% CH4 (SW monsoon) and 104±6% N2O, 115±2% CH4 (intermonsoon). Large supersaturation maxima for N2O (saturations ∼400–800%) and CH4 (saturations ∼200–400%) just below the base of the mixed layer were ubiquitous and followed a trend of progressive deepening toward the south. All deep N2O profiles were characterized by a second, more vertically extensive maximum (saturations ∼400–600%) between 500 and 1000 m. For these, plots of ΔN2O versus AOU were consistent with their formation by coupled nitrification‐denitrification, with denitrification becoming progressively more important with distance toward the core of the oxygen depleted zone. Sea‐to‐air fluxes for a 6‐month period represented by the study were determined from measured air‐sea partial pressure differences and gas transfer velocities derived from in situ wind speeds. Estimated semiannual emissions, ∼0.41–0.75 × 1012 g N2O, ∼0.1–0.18 × 1012 g CH4, were within most previously reported ranges for the NWIO. The data indicate that seasonal changes in wind speed rather than seasonal changes in air‐sea partial pressure differences due to monsoon‐driven mixing and upwelling are the dominant control on air‐sea gas exchange in the NWIO.
Area balance of the Arctic Ocean perennial ice zone: October 1996 to April 1997
Authors: Kwok R. , Cunningham G. F. , Yueh S.
Published: September 2002

We use National Aeronautics and Space Administration scatterometer (NSCAT), RADARSAT, and ice motion data to examine the perennial ice zone (PIZ) of the Arctic Ocean between October 1996 and April 1997. The PIZ is identified by a simple backscatter‐based classification of the gridded NSCAT backscatter fields. The area of the PIZ at the beginning of October occupies an area of 5.32 × 106 km2, ∼76% of the Arctic Ocean. By the first of May, only 4.54 × 106 km2 of that area remains, a decrease of 0.78 × 106 km2 over the 7‐month period. This area loss can be explained almost entirely by ice export. Over this period the total area flux of sea ice through Fram Strait, estimated using satellite passive microwave ice motion, is 0.80 × 106 km2 or 12% of the Arctic Ocean. Approximately 0.70 × 106 km2 or 88% of the exported area is from the PIZ. Nares Strait outflow is small at 34,000 km2 and is estimated by summing the high backscatter areas exported into northern Baffin Bay. After accounting for the outflow through the Fram and Nares Straits an unexplained residual of 46,000 km2 remains. We attribute this residual to errors in our estimation process, the unaccounted for ice flux through the Canadian Archipelago, and the net divergence and convergence of the PIZ over the period. Our interpretation of the radiometry seems to be sound on the basis of the area balance arguments provided here. This study shows that (1) the PIZ converage of the Arctic Ocean can be derived from NSCAT backscatter fields and (2) the decrease in PIZ area over the winter is a good estimate of the PIZ net ice area exported through Fram Strait.
Construction of the adjoint MIT ocean general circulation model and application to Atlantic heat transport sensitivity
Authors: Marotzke Jochem , Giering Ralf , Zhang Kate Q. , Stammer Detlef , Hill Chris , Lee Tong
Published: September 2002

We first describe the principles and practical considerations behind the computer generation of the adjoint to the Massachusetts Institute of Technology ocean general circulation model (GCM) using R. Giering's software tool Tangent‐Linear and Adjoint Model Compiler (TAMC). The TAMC's recipe for (FORTRAN‐) line‐by‐line generation of adjoint code is explained by interpreting an adjoint model strictly as the operator that gives the sensitivity of the output of a model to its input. Then, the sensitivity of 1993 annual mean heat transport across 29°N in the Atlantic, to the hydrography on January 1, 1993, is calculated from a global solution of the GCM. The “kinematic sensitivity” to initial temperature variations is isolated, showing how the latter would influence heat transport if they did not affect the density and hence the flow. Over 1 year the heat transport at 29°N is influenced kinematically from regions up to 20° upstream in the western boundary current and up to 5° upstream in the interior. In contrast, the dynamical influences of initial temperature (and salinity) perturbations spread from as far as the rim of the Labrador Sea to the 29°N section along the western boundary. The sensitivities calculated with the adjoint compare excellently to those from a perturbation calculation with the dynamical model. Perturbations in initial interior salinity influence meridional overturning and heat transport when they have propagated to the western boundary and can thus influence the integrated east‐west density difference. Our results support the notion that boundary monitoring of meridional mass and heat transports is feasible.
Modeling the seasonal variation of sea ice in the Labrador Sea with a coupled multicategory ice model and the Princeton ocean model
Authors: Yao T. , Tang C. L. , Peterson I. K.
Published: September 2002

We use a multicategory sea ice model coupled to the Princeton ocean model, which is driven by monthly climatological atmospheric forcing, to study the seasonal variation of ice cover in the Labrador Sea. Initial ocean conditions are derived from a gridded, objectively analyzed temperature‐salinity data set that provides improved resolution of gradients in the vicinity of the shelf break. The model produces a realistic seasonal variation of sea ice. There is ice growth over the inner shelf and ice melt over the outer shelf and slope. Over the inner shelf, advection and diffusion decrease the ice mass; over the outer shelf, advection and diffusion increase the ice mass, which maintains the location of the ice edge. Near the offshore ice edge the melt rate exceeds 1 m per month, and the heat to melt ice together with the heat lost to the atmosphere exceeds 500 W m−2. The heat lost at the ocean surface is compensated for by advection of heat from an offshore convective region. The dominant heat source for the spring retreat of ice in the south is shortwave radiation over the open water fraction.
Reconstructed histories of the annual mean atmospheric mole fractions for the halocarbons CFC‐11 CFC‐12, CFC‐113, and carbon tetrachloride
Authors: Walker S. J. , Weiss R. F. , Salameh P. K.
Published: September 2002

Annual mean mixing ratios for the halocarbons CFC‐11 (CCl3F), CFC‐12 (CCl2F2), CFC‐113 (CClF2CCl2F), and carbon tetrachloride (CCl4) have been determined from their first year of industrial production through 1998. From the late 1970s (in the case of CFC‐11 and CFC‐12) or early 1980s (in the case of CFC‐113 and carbon tetrachloride) the reported mixing ratios have been determined from experimental observations made by the Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment/Advanced Global Atmospheric Gases Experiment program. For years prior to these times we have used estimates of industrial emissions and atmospheric lifetimes to calculate historic concentrations. The likely error bounds of the annual mean values are also reported here. Errors in the annual mean mixing ratio may primarily be a result of incorrect industrial emissions data, an incorrect atmospheric lifetime, or uncertainty in the ALE/GAGE/AGAGE observations. Each of these possible sources of error has been considered separately. These results show that atmospheric concentrations for each of these compounds have experienced a rapid rise in the early part of their production. It is only within the past decade that rise rates have decreased sharply and (except in the case of CFC‐12) in the past few years that atmospheric concentrations have begun to decrease. The uncertainties in the reconstructed histories are a similar proportion for each of the chlorofluorocarbons (<4% for most of the history). However, uncertainty in the history of carbon tetrachloride is much greater (up to 12%), and this is mainly the result of poor knowledge of CCl4 emissions.
Winter monsoon circulation of the northern Arabian Sea and Somali Current
Authors: Schott Friedrich A. , Fischer Jürgen
Published: September 2002

The winter monsoon circulation in the northern inflow region of the Somali Current is discussed on the basis of an array of moored acoustic Doppler current profiler and current meter stations deployed during 1995–1996 and a ship survey carried out in January 1998. It is found that the westward inflow into the Somali Current regime occurs essentially south of 11°N and that this inflow bifurcates at the Somali coast, with the southward branch supplying the equatorward Somali Current and the northward one returning into the northwestern Arabian Sea. This northward branch partially supplies a shallow outflow through the Socotra Passage between the African continent and the banks of Socotra and partially feeds into eastward recirculation directly along the southern slopes of Socotra. Underneath this shallow surface flow, southwestward undercurrent flows are observed. Undercurrent inflow from the Gulf of Aden through the Socotra Passage occurs between 100 and 1000 m, with its current core at 700–800 m, and is clearly marked by the Red Sea Water (RSW) salinity maximum. The observations suggest that the maximum RSW inflow out of the Gulf of Aden occurs during the winter monsoon season and uses the Socotra Passage as its main route into the Indian Ocean. Westward undercurrent inflow into the Somali Current regime is also observed south of Socotra, but this flow lacks the RSW salinity maximum. Off the Arabian peninsula, eastward boundary flow is observed in the upper 800 m with a compensating westward flow to the south. The observed circulation pattern is qualitatively compared with recent high‐resolution numerical model studies and is found to be in basic agreement.
Deep current structure above the Izu‐Ogasawara Trench
Authors: Fujio Shinzou , Yanagimoto Daigo , Taira Keisuke
Published: September 2002

Above the Izu‐Ogasawara Trench south of Japan, direct current measurements were made at 34°N from 1987 to 1996, and hydrographic observations were carried out at 34° and 30°N in 1995. The geostrophic shears calculated from the conductivity‐temperature‐depth data were consistent with the shears calculated from the current data. It is found that there are opposing currents that flow southward on the western flank and northward on the eastern flank along the isobaths. In the cross‐trench direction the magnitude of the mean velocity tended to increase with the distance from the deepest point of the trench and exceeded 10 cm s−1 on the eastern flank. In the vertical direction the mean velocity increased with depth, but it decreased just above the bottom, probably because of friction. The southward transport above the western flank was estimated to be 5–8 Sv both at 34° and 30°N. However, the northward transport above the eastern flank increased from 5 Sv at 30°N to 22 Sv at 34°N, suggesting a large inflow from the west.
Modification of the relaxed eddy accumulation technique to maximize measured scalar mixing ratio differences in updrafts and downdrafts
Authors: Bowling D. R. , Delany A. C. , Turnipseed A. A. , Baldocchi D. D. , Monson R. K.
Published: September 2002

A modification to the relaxed eddy accumulation (REA) flux measurement technique is proposed which maximizes the scalar mixing ratio difference in updrafts and downdrafts. This technique was developed with the goal of measuring the stable isotope (13C/12C and 18O/16O) ratios of updraft and downdraft air and thus the net fluxes of 13C16O2 and 12C18O16O. Current mass spectrometer precision is small relative to measured isotopic gradients in CO2 in the Earth's boundary layer, and the conventional REA approach is likely to be ineffective. The new technique, which we refer to as hyperbolic relaxed eddy accumulation (HREA), uses the conditional sampling concept of hyperbolic hole analysis to control sampling of air during only those turbulent events which contribute most strongly to the flux. Instead of basing updraft/downdraft sampling decisions strictly on vertical wind velocity, CO2 mixing ratio ([CO2]) fluctuations or those of another scalar are also used. Simulations using 10‐Hz data show that a wind‐based/scalar‐based sampling threshold can achieve a factor of 2.7 increase in scalar updraft/downdraft [CO2] differences over simple REA. During midday periods with strong photosynthetic fluxes, up/down [CO2] differences with HREA of 8–10 ppm are possible, compared with 3–5 ppm for the best conventional REA case. Corresponding isotopic differences can likely be resolved with current mass spectrometers using this approach.
Soil‐atmosphere exchange of methane in adjacent cultivated and floodplain forest soils
Authors: Burke Roger A. , Meyer Judith L. , Cruse Jennifer M. , Birkhead Karen M. , Paul Michael J.
Published: September 2002

The soil‐atmosphere exchange of methane was measured in adjacent cultivated (corn) and forest (upper floodplain, mixed hardwood) habitats of the southeastern U.S. piedmont for a period of 3 years using closed chambers. We have evaluated the effect of the following factors on soil‐atmosphere methane exchange: (1) interannual variability of climatic conditions, (2) landscape position (i.e., river levee versus terrace), and (3) disturbance ranging from intense (cultivation) through moderate (approximately annual flooding events that last from weeks to months) to subtle (approximately annual flooding of a few days duration). We found that mean methane consumption in the cultivated and forested terrace sites was <0.3 mg CH4m−2d−1, whereas the mean consumption rate in forested levee sites was about 1.4 mg CH4m−2d−1over the course of the 3 years. Moisture levels in the upper soil (0–5 cm) appear to exert little control of methane exchange in any of the habitats. We observed little seasonal variation in methane flux in the levee sites, in contrast to results observed by others in higher‐latitude and tropical forests. Our results suggest that very subtle differences in landscape position and disturbance impact the strength of the soil methane sink. We cannot conclude that agricultural development destroyed the methane sink capacity of these floodplain terrace soils because it was probably already quite low due to periodic disturbance by flooding. Limited measurements of nitrogen cycling suggest that methane flux differences observed among the different habitats are not obviously related to differences in N mineralization or nitrification as in other ecosystems.
A model for identifying the aerosol‐only mode of SAGE II 1.02‐μm extinction coefficient data at altitudes below 6.5 km
Authors: Wang Pi‐Huan , Kent Geoffrey S. , Veiga Robert E. , Yue Glenn K. , Poole Lamont R. , Fishman Jack , McCormick M. Patrick
Published: September 2002

A model is proposed for identifying the aerosol mode of the second Stratospheric Aerosol and Gas Experiment (SAGE II) 1.02‐μm extinction coefficient measurements at altitudes below 6.5 km, which also contain cloud samples. This development allows one to extend the SAGE II satellite data analysis from the lower limit at 6.5 km of the SAGE II two‐wavelength method into the lower troposphere. Thus the proposed model provides opportunities for fully utilizing the SAGE II tropospheric measurements important to the understanding of the global behavior of tropospheric aerosols, clouds, and ozone and related transports. The effectiveness of this model is examined by using the SAGE II two‐wavelength technique at 6.5 km. Sample applications of the proposed model reveal encouraging results. To assess the quality of the aerosol 1.02‐μm data, it is recommended that a comprehensive data comparison analysis be conducted by using tropospheric measurements from different instruments.
An interactive cirrus cloud radiative parameterization for global climate models
Authors: Joseph Everette , Wang Wei‐Chyung
Published: September 2002

An interactive cirrus cloud radiative parameterization is developed for global climate models from recent observations and analytical results that more accurately characterize cirrus cloud optical and microphysical properties. The radiative properties are based on the assumption that cirrus clouds are composed of hexagonal crystals. For the infrared component, a new mass absorption coefficient is parameterized to calculate emissivity, and for the solar, single‐scattering properties from an existing parameterization are modified and employed. The solar and infrared optical properties are given as a function of ice water content and effective particle size. Aircraft observations are used to parameterize the microphysical properties in terms of temperature, thus allowing the radiative properties to interact with the local model climate. The interactive cirrus radiative parameterization is evaluated in model‐to‐observation comparisons with a comprehensive set of cloud and radiation measurements obtained during the spring 1994 and fall 1995 Intensive Observation Periods of the Atmospheric Radiation Measurement program. It is shown that the model with the new parameterization calculates realistic infrared radiation and improved solar radiation incident at the surface. Specifically, biases in calculated solar direct and diffuse fluxes are reduced by 60 and 40%, respectively. Further, the shortwave flux is shown to be more sensitive than the longwave flux to variability in the ice water content and in the base and top heights of observed clouds replicated in model calculations. The potential effect of the new parameterization on climate simulations is investigated in the context of initial radiative forcing. The new parameterization calculates a significantly different ice water path distribution from an existing parameterization that has been used for global climate change studies. For example, in the high latitudes of the summer hemisphere the new ice water path is larger by more than 7.7 g m−2 (>100%), and in the tropics it can be smaller by as much as −3.5 g m−2 (∼80%). These differences lead to an increased solar albedo effect in the high latitudes of the summer hemisphere and a decreased greenhouse effect in the tropics, both of which contribute to a smaller, 2.26 W m−2, global‐ and annual‐mean forcing of the surface‐troposphere system.
RF radiation observations of positive cloud‐to‐ground flashes
Authors: Shao X. M. , Rhodes C. T. , Holden D. N.
Published: September 2002

During the summers of 1995 and 1996 we conducted broadband HF‐UHF and narrowband VHF radio frequency (RF) observations of positive cloud‐to‐ground (+CG) flashes at Langmuir and Los Alamos laboratories, New Mexico. These observations indicate that positive leaders to ground produce no or very weak radiation from HF to UHF. The broadband system was able to record 2 ms data each time it was triggered. For a +CG the system was usually triggered by the return stroke, and a 1 ms pretrigger period was coincident with the positive leader process. It was commonly observed that no or little radiation was associated with the leader process in the 1 ms pretrigger period. The narrowband VHF system employed a logarithmic power amplifier and recorded one 1 s data each time it was triggered. The narrowband observations show that strong and often continuous radiation occurs at the beginning of the +CGs, but the radiation usually becomes intermittent and impulsive during the last few tens of milliseconds preceding the return strokes. The observations for most of the +CGs also show complete lack of radiation a few ms before the beginning of the return strokes, suggesting that the ongoing downward positive leaders were quiet at VHF, at least during the final few ms. The results of this study for natural positive leaders are in agreement with the results obtained from laboratory gap discharges and rocket‐triggered lightning.
Radiosonde observations of partially trapped lee waves over Tasmania, Australia
Authors: Reeder Michael J. , Adams Neil , Lane Todd P.
Published: September 2002

A well‐defined family of lee waves over southern Tasmania, Australia, is analyzed using a routine radiosonde sounding released from Hobart at 1100 UTC June 18, 1991. Pronounced fluctuations in the ascent rate of the radiosonde are analyzed using a new technique called the horizontal projection method. The dominant horizontal wavelength is found to be 8249 m. An advanced very high resolution radiometer (AVHRR) infrared satellite image at 1554 UTC is used to check this calculation and gives a horizontal wavelength of 8.9±1.4 km. The horizontal projection method takes account of the vertical variation of background flow and the trajectory of the radiosonde. When the Scorer parameter is a strong function of height, as is the case with the 1100 UTC sounding, it is inappropriate to assume that the vertical wavenumber is constant with height. In particular, the vertical wavelength at low levels is about 4.5 km, the waves become evanescent between the heights of 2.5 km and 9 km, and above that the vertical wavelength is typically 10–15 km. As a consistency check, linear wave theory is used to calculate the resonant wave modes supported by the background profiles of wind and potential temperature. These are the wave modes that are most likely responsible for the lee‐wave clouds seen in the AVHRR satellite image. The horizontal wavelength for the resonant wave mode is 8188 m, in agreement with the estimates based on the AVHRR satellite image and from the horizontal projection method. The paper highlights the dangers of interpreting radiosonde soundings as vertical “snapshots” of the atmosphere and assuming that the vertical wavenumber can be characterized by a single value. It is shown that treating the radiosonde sounding as a vertical profile and analysing wave‐like structures in the ascent rate using standard spectral methods gives an incorrect vertical wavelength of 1906±238 m. The paper concludes that operational radiosonde soundings can be used to make accurate, quantitative measurements of orographically generated waves, provided that the vertical variation in the background flow and the motion of the radiosonde through the wave field are taken into account.
Role of vegetation and soil in the Holocene megathermal climate over China
Authors: Wang Hui‐Jun
Published: September 2002

There was significant difference between vegetation cover in mid‐Holocene and that at present over China based on various evidences as concluded by Shi et al [1992]. These changes were introduced to the climate model to study the role of the changing vegetation and associated soil to the simulated climate of mid‐Holocene at 6000 years before present (BP) with bigger seasonal cycle of insolation than present (caused by the change of orbital parameters). Results show that the changes in vegetation and soil could further strengthen the monsoon rainfall over China. We found that replacing the today's seasonal cycle of insolation by that of mid‐Holocene increases the summer (June‐August) precipitation by 20% in eastern China (100°–120°E, 18°–42°N); while considering both the effect of changing orbital forcing and the vegetation and soil, the total summer precipitation increase is 29% in the same region. Compared with the reconstructed precipitation over China, the result of that including changing vegetation and soil is quantitatively better.
Alignment of mountain wave patterns above Wales: A VHF radar study during 1990–1998
Authors: Worthington Richard M.
Published: September 2002

Mountain waves must be parameterized in numerical models of the atmosphere, but long‐term obervations of their characteristics are sparse. In this study, a large data set from the VHF radar at Aberystwyth (52.42°N, 4.00°W), during the years 1990–1998, is used to investigate the relation between background wind direction, local topography, and the mountain wave alignment. The horizontal wave vector of the mountain waves is, on average, found to be biased 10° anticlockwise from the wind direction in the lower troposphere near 2 km altitude, and biased 20° clockwise from the wind direction near the ground. This can be explained if the “wave‐launching height” (where the background wind speed and direction values can be assumed responsible for generating the waves), lies within the boundary layer, where the wind has only partly rotated between its direction near ground level and its direction in the free troposphere above the boundary layer. On average, and for most background wind directions at Aberystwyth, the wind direction within the boundary layer is a more important influence on wave alignment than the orientation of upstream mountain ridges. The relative height of the mountain peaks and the boundary layer may affect the average alignment of mountain waves and therefore the wave stress by as much as 30°.
An evaluation of the quality of Halogen Occultation Experiment ozone profiles in the lower stratosphere
Authors: Bhatt Praful P. , Remsberg Ellis E. , Gordley Larry L. , McInerney M. Joseph. , Brackett Vince G. , Russell James M.
Published: September 2002

The archived ozone profiles from the Halogen Occultation Experiment (HALOE) have already been corrected for the effects of the spectrally varying, interfering absorption due to aerosols composed of aqueous sulfuric acid, and agreement with correlative measurements in the stratosphere is generally excellent. However, comparisons of sets of coincident HALOE and ozonesonde profiles indicate occasional large differences at the lowest levels of the stratosphere. Most of those instances occur at altitudes just below a well‐defined minimum in the 5.26 μm channel aerosol extinction profile, whose wavelength dependence is not represented by a sulfuric acid aerosol model. Further, when the aerosol extinction exceeds about 10−3 km−1, the aerosol correction to the ozone channel transmittances is both large and uncertain. After screening out the HALOE ozone profile segments whose corresponding aerosol/cirrus corrections are likely uncertain and after averaging the ozonesonde profiles into 2.5 km thick layers, we find that the HALOE ozone agrees, on average, to within 10% of their coincident ozonesonde measurements down to 100 hPa at tropical/subtropical latitudes and to 200 hPa at extratropical latitudes. A tightening of the coincidence criteria for the comparisons does not improve the mean differences for the sets nearly as much. Part of the variance of the paired differences was also accounted for when the ozonesonde profile values were integrated into those 2.5 km layers, prior to taking differences. This improvement is due mainly to the vertical averaging of the local, higher‐resolution ozonesonde data, matching the lower resolution for HALOE ozone in the lower stratosphere. It is concluded that HALOE is providing accurate ozone profiles throughout the lower stratosphere, when its correction for interfering aerosols has been well characterized and when cirrus layers are not indicated.
Stable isotope composition of water vapor in the atmospheric boundary layer above the forests of New England
Authors: He Hui , Smith Ronald B.
Published: September 2002

Water vapor at multiple levels was sampled from a light aircraft in the summertime atmospheric boundary layer (ABL) over forested terrain at altitudes up to 3 km. Three sampling flights were carried out under similar weather conditions during the summer to earlier fall period of 1996. The deuterium and oxygen 18 isotope ratios, δD and δ18O, of 24 water vapor and 30 surface water samples were analyzed on a mass spectrometer. The water vapor mixing ratio, Q, at each sampling level was estimated from the amount of the sample collected. The results show constant δD and δ18O in the mixed region of the ABL and sharply decreasing values near the top. Measurement of the ratio of the vertical atmospheric flux of HD16O or H218O to the flux of H216O is of particular interest in hydrology and paleoclimatology studies. This quantity, however, cannot be measured directly at the surface due to the instantaneous turbulent mixing of the evaporating water vapor with the overlying ABL. Using a δ − 1/Q mixing line method, we are able to determine such isotope flux ratios characterizing the two‐way exchange between the Earth's surface and the free atmosphere. The varying isotope flux ratios are lighter than expected, but fall into the ranges of the isotope ratios of the winter precipitation in the area. Based on the hypothesis of no fractionation between the soil water and the water vapor transpired by plants, our measurements suggest that late summer transpiration releases soil water which comes from the precipitation that fell during the previous winter. Other explanations are also discussed.
Lead in rainwaters from intertropical Africa: Natural or anthropogenic origin?
Authors: Freydier R. , Dupré B. , Delmas R.
Published: September 2002

As part of an investigation of lead in precipitation, we sampled rainwater over a 2 year period at four locations in intertropical Africa. We clearly identified crustal dust and anthropogenic sources for lead in rain using measured concentrations, correlation matrices, enrichment factors, and the distribution of lead between dissolved and particulate fractions. In Niger, mean and most individual sample crustal enrichment factors are less than 10, suggesting the dominance of the crustal dust source for lead in rainwater at that site. Nevertheless, the few enrichment factors greater than 10 and an estimate of the concentration of noncrustal lead in rainwaters from this site suggest anthropogenic sources may be important as well. In the Central African Republic, high individual sample (600) and mean (10.5) crustal enrichment factors indicate a noncrustal source for lead in rain. Very weak correlations between lead and other elements (e.g., Rb) for both total and dissolved fractions argue against a biogenic source and suggest an anthropogenic source. In Cameroon, individual sample and mean crustal enrichment factors are greater than 10, indicating a noncrustal source for lead in rainwater. We suggest the possibility of a biogenic lead source that would partly explain its correlation to Rb in dissolved and total rain fractions. For all stations, lead crustal enrichment factors decrease with increases in aluminium concentrations (i.e., mineral dust) and with increases in the fraction of soluble lead in rainwater. Thus lead in rain appears to be made up primarily of two types of particles; mineral dust particles with low solubility and highly soluble anthropogenic and/or biogenic particles.