bluuebunny/crossref_metadata_2025 huggingface dataset:
Air‐ice drag coefficients in the western Weddell Sea: 2. A model based on form drag and drifting snow
Authors: Andreas Edgar L.
Published: February 2004

In part 1 (Andreas and Claffey, this issue) we observed some characteristics of the neutral stability air‐ice drag coefficient at a reference height of 10 m (CDN10) that had not been documented before. Our main conclusion was that wind‐driven snow continually alters the sea ice surface; the resulting snowdrifts determine how largeCDN10is. In particular, part 1 reported three observations that I would like to explain. (1)CDN10is near 1.5×10−3when the wind is well aligned with the drifted snow. (2)CDN10is near 2.5×10−3when the wind makes a large angle with the dominant orientation of the snowdrifts. (3)CDN10can increase by 20% if, after being well aligned with the drift patterns, the mean wind direction shifts by as little as 20°. To investigate this behavior ofCDN10here I adapt a model developed by Raupach (1992) that partitions the total surface stress into contributions from form drag and skin friction. An essential part of this development was extending Raupach's model to the more complex geometry of sastrugi‐like roughness elements. Assuming that 10‐cm high sastrugi cover 15% of the surface, this physically based model reproduces the three main observations listed above. Thus the model seems to include the basic physics of air‐ice momentum exchange. The main conclusion from this modeling is that 10‐cm, sastrugilike snowdrifts, rather than pressure ridges, sustain most of the form drag over compact sea ice in the western Weddell Sea. Secondly, the modeling suggests that skin friction accounts for about 60% of the surface stress when the wind is well aligned with the sastrugi; but when the wind is not well aligned, form drag accounts for about 80% of the stress. The sastrugi are thus quite effective in streamlining the surface.
Determination of ocean tides from the first year of TOPEX/POSEIDON altimeter measurements
Authors: Ma X. C. , Shum C. K. , Eanes R. J. , Tapley B. D.
Published: February 2004

An improved geocentric global ocean tide model has been determined using 1 year of TOPEX/POSEIDON altimeter measurements to provide corrections to the Cartwright and Ray (1991) model (CR91). The corrections were determined on a 3°×3° grid using both the harmonic analysis method and the response method. The two approaches produce similar solutions. The effect on the tide solution of simultaneously adjusting radial orbit correction parameters using altimeter measurements was examined. Four semidiurnal (N2, M2, S2, and K2), four diurnal (Q1, O1, P1, and K1), and three long‐period (Ssa, Mm, and Mƒ) constituents, along with the variation at the annual frequency, were included in the harmonic analysis solution. The observed annual variation represents the first global measurement describing accurate seasonal changes of the ocean during an El Niño year. The corrections to the M2constituent have an RMS of 3.6 cm and display a clear banding pattern with regional highs and lows reaching 8 cm. The improved tide model reduces the weighted altimeter crossover residual from 9.8 cm RMS, when the CR91 tide model is used, to 8.2 cm RMS. Comparison of the improved model to pelagic tidal constants determined from 80 tide gauges gives RMS differences of 2.7 cm for M2and 1.7 cm for K1. Comparable values when the CR91 model is used are 3.9 cm and 2.0 cm, respectively. Examination of TOPEX/POSEIDON sea level anomaly variations using the new tide model further confirms that the tide model has been improved.
A radiative transfer model for sea surface temperature retrieval for the along‐track scanning radiometer
Authors: Závody A. M. , Mutlow C. T. , Llewellyn‐Jones D. T.
Published: February 2004

The measurements made by the along‐track scanning radiometer are now converted routinely into sea surface temperature (SST). The details of the atmospheric model which had been used for deriving the SST algorithms are given, together with tables of the coefficients in the algorithms for the different SST products. The accuracy of the retrieval under normal conditions and the effect of errors in the model on the retrieved SST are briefly discussed.
Observed directional characteristics of the wind, wind stress, and surface waves on the open ocean
Authors: Rieder Karl F. , Smith Jerome A. , Weller Robert A.
Published: February 2004

Sonic anemometer data were taken during the Surface Waves Processes Program (SWAPP) in March 1990 in the North Pacific. Measurements of the wind stress vector span several strong wind events. Significant angles between the wind stress vector and the mean wind vector are seen. Simultaneous measurements of the directional wave field were made with a surface scanning Doppler sonar. The data suggest that the wind stress direction is influenced by the direction of the surface waves, especially for stronger winds. Overall, the stress vector lies between the mean wind and the mean wave directions. At the higher wind speeds (over 8.6 m/s), there is a nonzero correlation between the variations in wave directions and stress directions as well. Finally, the stress and wave component directions have similar frequency dependence over the frequency band where wave energy is nonnegligible, suggesting a dynamic link.
Shortwave feedbacks and El Niño‐Southern Oscillation: Forced ocean and coupled ocean‐atmosphere experiments
Authors: Waliser Duane E. , Blanke Bruno , Neelin J. David , Gautier Catherine
Published: February 2004

Changes in tropical sea surface temperature (SST) can produce changes in cloudiness that modify incoming solar shortwave (SW) radiation, which in turn affects SST. The effects of this negative feedback on Pacific interannual variability are examined in forced ocean model and hybrid coupled ocean‐atmosphere model simulations. Two empirical schemes are used to model the large‐scale, low‐frequency response of surface SW to SST anomalies. The first scheme attempts to account for the nonlocal nature of the atmospheric response to SST based on patterns of covariability analyzed through singular value decomposition. In the observations the primary coupled mode of variability is composed of a SW anomaly in the central Pacific that covaries with anomalous SST in the eastern Pacific. This is applied in the model as a nonlocal SW feedback. The second scheme examines the effects of a purely local feedback with a spatially varying coefficient of magnitude chosen similar to the first scheme. In almost all cases the second scheme behaved similarly to the first, presumably because the correlation scale of SST is large enough for El Niño‐Southern Oscillation (ENSO) dynamics that there is little sensitivity to the local approximation in the SW feedback. In simulations forced by time series of observed wind stress the SW feedback induced very minor SST damping. Results from a simplified heat budget analysis showed that while the SW feedback increased the local heat flux damping on SST, it also induced a mean shallowing of the mixed layer. The resulting changes in both the local mean vertical temperature gradient and the zonal velocity response to the wind stress acted to oppose the local heat flux damping effects. When observed SW anomalies were applied to forced simulations, the simulated SST anomalies were modified as expected, and agreement with observed SST improved. In coupled simulations the SW feedbacks had greater impact than in the case of specified stress. The main effects were to decrease the magnitude of the warm and cold SST anomalies in the central Pacific, while leaving the pattern and evolution of ENSO anomalies essentially unchanged elsewhere. The SW feedbacks thus produce a modest improvement of the model ENSO SST pattern compared with observations, although they tended to shorten the period of the model ENSO cycle. Overall the results suggest that large‐scale SW feedbacks are of quantitative importance to simulating some aspects of the ENSO cycle but are not critical to the overall occurrence of the phenomenon.
Passage of a shallow front across a Beaufort Sea polynya
Authors: Burk Stephen D. , Thompson William T.
Published: February 2004

A mesoscale numerical weather prediction model is used to investigate the passage of a shallow front across a polynya in the Beaufort Sea near Point Barrow, Alaska. In order to properly resolve the phenomena with the model's grid, the polynya is specified larger than actually observed, but otherwise this represents a real data case. Prior to frontal passage, a well‐mixed boundary layer exists over the polynya to a depth of about 1.5 km, while following frontal passage, the frontal inversion acts to confine the strong fluxes to a depth of only a few hundred meters. The temporal behavior of surface stresses, heat fluxes, and two separate low‐level jets are examined as the front passes over this large polynya.
Arctic sea ice leads from advanced very high resolution radiometer images
Authors: Lindsay R. W. , Rothrock D. A.
Published: February 2004

A large number of advanced very high resolution radiometer (AVHRR) images from throughout 1989 are analyzed to determine lead characteristics. The units of analysis are square 200‐km cells, and there are 270 such cells in the data set. Clouds are masked manually. Leads are determined from images of the potential open water δ, a scaled version of the surface temperature or albedo that weights thin ice by its thermal or brightness impact. The lead fraction is determined as the mean δ; the monthly mean lead fraction ranges from 0.02 in winter to 0.06 in summer in the central Arctic and is near 0.08 in the winter in the peripheral seas. A method of accounting for lead width sampling errors due to the finite sample areas is introduced. In the central Arctic the observed mean lead width for a threshold of δ = 0.1 ranges from 2 or 3 km (near the resolution of the instrument) in the winter to 6 km in the summer. In the peripheral seas it is about 5 km in the winter. Width distributions are often more heavily weighted in the tail than exponential distributions and are well approximated by a power law. The along‐track, number density power law N = aw−b has a mean exponent of b = 1.60 (standard deviation 0.18) and shows some seasonal variability. Mean floe widths in the central Arctic are 40 to 50 km in the winter, dropping to about 10 km in the summer. For floes the power law has a mean exponent of 0.93 and exhibits a clearer annual cycle. Lead orientation is determined with a method based on the direction of maximum extent.
Ice pack and lead surface energy budgets during LEADEX 1992
Authors: Ruffieux Dominique , Persson P. Ola G. , Fairall C. W. , Wolfe Daniel E.
Published: February 2004

During a 1‐month deployment for the Arctic Leads Experiment (LEADEX) in March and April 1992 on the Arctic ice cap roughly 200 km north of Prudhoe Bay, Alaska, surface‐based mean meteorological and flux instruments plus a variety of remote sensors were operated at the main base camp. Identical systems were also deployed by helicopter on the upwind and downwind edges of several Arctic leads, two of which we describe in this paper. At the base camp the diurnal amplitude for sensible heat flux was ±10 W m−2 about a mean of −3 W m−2, net radiation was ±30 W m−2 about a mean of −15 W m−2, and net surface energy flux was ±20 W m−2 about a mean of −12 W m−2. The mean latent heat flux was +1 W m−2 with a diurnal variation of about ±1.5 W m−2. Mean values for the momentum and sensible heat transfer coefficients were CD = (1.20±0.20) × 10−3 and CH = (0.75±0.25) × 10−3 at a 10‐m reference height with only modest diurnal variations. Two lead deployments were examined. Lead 3 was approximately 1 km across. Only limited meteorological data were obtained for about 6 hours at the end of April 7 and beginning of April 8 when the lead was covered with about 10 cm of ice. Downwind of the lead, the sensible heat flux increased to about 170 W m−2 and the stress doubled, suggesting an ice‐covered lead 10‐m drag coefficient of 2.2×10−3. More than 36 hours of data were obtained upwind and downwind of lead 4, which varied in width from 80 to 120 m. Doppler minisodars upwind and downwind of the lead indicated a doubling in the depth (5 to 10 m) of the shear‐driven turbulent surface layer downwind of the lead and an intensification of intermittent wave interactions exceeding 60 m (sodar range maximum). Three prominent waves with strong downward motion were observed in this period, apparently causing increases in the downwind stress magnitude. Various sources of data were used to compute estimates over a 36‐hour period of the net surface heat flux Qg over the lead, the adjacent pack ice, and any open water that might have occurred in the lead. The results indicate that once significant ice forms, the sun is increasingly more effective in reducing the surface freezing rate and in shutting off convective mixing in the ocean under the lead. Over the period of observations the average net surface heat flux was −75 W m−2 over the pack ice, −130 W m−2 over the lead, and −250 W m−2 over the open water.
Longwave radiation budget in the Mediterranean Sea
Authors: Bignami F. , Marullo S. , Santoleri R. , Schiano M. E.
Published: February 2004

Direct measurements of infrared budget and meteorological parameters at sea were carried out in the western Mediterranean Sea during different seasons in the years 1989–1992. The spatial and time distribution of this data set allows us to perform an exhaustive test on the most widely used infrared budget bulk formulas. An underestimation of about 30 W/m2 is systematically observed, confirming previous results obtained by a limited data set. This discrepancy is independent of the time of the year and location of measurements, indicating an inability of the bulk formulas to reproduce the infrared Mediterranean budget. The completeness of the data set enables us to determine a new relation between infrared budget and meteorological parameters. Alternative expressions, to be used accordingly with the available data sets, are presented here.
Tide‐induced mixing in the Amazon Frontal Zone
Authors: Geyer W. Rockwell
Published: February 2004

The freshwater discharge from the Amazon River produces a bottom salinity front on the Amazon shelf that extends many hundreds of kilometers along the shelf, between the 10‐ and 20‐m isobaths. Strong cross‐shelf tidal currents cause advective excursions of the front of more than 20 km and supply the major source of kinetic energy for mixing on the shelf. This mixing is manifested in spring‐neap variations in the frontal structure and position, with weaker stratification and seaward migration of the front during periods of strong tidal currents, and stronger stratification and landward migration during neap tides. Mixing appears to be driven by shear instability, based on local estimates of gradient Richardson number and bulk estimates of the water column stability. Maximum mixing occurs during ebb, when vertical shear is greatest. Suspended sediment reaches such high concentrations in the frontal zone that it makes a significant contribution to the stratification, thus increasing the stability of the flow and reducing the effectiveness of tide‐induced mixing.
A comparison of satellite‐derived and aircraft‐measured regional surface sensible heat fluxes over the Beaufort Sea
Authors: Walter Bernard A. , Overland James E. , Turet Philip
Published: February 2004

Motivated by the importance of quantifying the regional surface heat balance over Arctic sea ice in studying climate processes, Lindsay and Rothrock (1994) developed a methodology for computing regional surface sensible heat fluxes using readily available advanced very high resolution (AVHRR) IR satellite imagery. Their technique is based upon the determination of the pixel‐by‐pixel sea ice surface temperature from which estimates of sensible heat fluxes are then made. We compare the sensible heat fluxes over the Beaufort Sea computed using their methodology with those measured by a gust probe system on the National Oceanic and Atmospheric Administration P‐3 aircraft on April 18, 1992, during the Leads Experiment. We use an AVHRR image recorded during the P‐3 flight at 2303 UTC. We show that individual lead heat fluxes can be large, 115 W m−2 for 1‐km average fluxes obtained from flight legs that included a 300‐m‐wide lead, but that regional values of sensible flux over 50–200 km of sea ice were small and positive, ∼8 W m−2. The sensible heat flux computed from the P‐3 showed that a value of CS = 1.1×10−3, where CS is the heat transfer coefficient relative to 10 m, is appropriate for the spring Beaufort Sea. We suggest that more realistic winds derived, for example, from the National Meteorological Center sea level pressure analyses be used instead of the constant value of 5 m s−1 now employed by Lindsay and Rothrock. We also found that the maximum value of ΔT, the difference between air and surface temperature, used in the calculation of sensible heat flux using the Lindsay and Rothrock technique was underestimated by a factor of 1.9 when compared with direct measurements. Use of this ΔT correction factor, synoptic scale winds, and the calculated value of CS gave a good comparison between the AVHRR approach and aircraft fluxes measured over the region. The effective regional momentum drag coefficient CD relative to 10 m was 2.1×10−3, typical of Arctic pack ice.
Hydrographic conditions in the Brazil‐Malvinas Confluence during austral summer 1990
Authors: Provost Christine , Gana Slim , Garçon Véronique , Maamaatuaiahutapu Keitapu , England Matthew
Published: February 2004

The Confluence 3 cruise during February 1990 provides the first hydrographic survey of the Brazil‐Malvinas Confluence region ever obtained with such a refined spatial resolution over the continental slope. The closely spaced stations on the slope permit the observation of the branching of the Malvinas Current near 40°S. The eastern branch returns south, while the eastern one is squeezed along the continental slope, managing to continue northward as far north as 36.3°S. The Brazil‐Malvinas (B‐M) front has a north‐south orientation on the slope and an east‐west direction offshore. The B‐M front is marked by very strong gradients in temperature and salinity. Fine scale vertical structures and intrusions are numerous in the vicinity of the front and cover a wide range of scales. The largest intrusion observed is a 50‐km‐wide, 350‐m‐thick subsurface lense of Thermocline Water present on the Malvinas side of the front. The fine scale vertical structure results in very high temperature and salinity fluxes across the front of the order of 2×10−2°C m−1 s−1 and 10−3 psu m−1 s−1. Two types of mode waters are identified as follows: Subtropical Mode Waters north of the front and Subantarctic Mode Waters on both sides of the front. The Subantarctic mode waters (SAMW) appear to recirculate near the western boundary. A young variety of SAMW (lighter, very oxygenated) propagates northward along the continental slope with the Malvinas Current. It detaches from the coast near 37°S and is strongly altered in the frontal region. It is then entrained southward with the Malvinas return flow and Brazil Current extension. The Confluence 3 hydrographic survey provides a good description of the North Atlantic Deep Water (NADW) and Circumpolar Deep Water (CDW) convergence near the western boundary. Strong, fine scale vertical structures are observed in the vicinity of the NADW/CDW front, and the associated temperature and salinity fluxes induced are strong, 5×10−3°cm/s and 2×10−3 psu.m/s, respectively.
On bubble clouds produced by breaking waves: An event analysis of ocean acoustic measurements
Authors: Dahl Peter H. , Jessup Andrew T.
Published: February 2004

A field experiment was carried out to measure the time‐varying properties of bubble clouds as they evolve from ocean breaking waves. The experiment was conducted from the floating instrument platform Flip and consisted of simultaneous, colocated acoustic measurements of subsurface bubbles and video recordings of the sea surface. Details of two breaking wave events are presented. The results of the acoustic measurements of bubble density and their entrainment depth are reported for the time t/Tc ≳ 3, where t is the time from breaking and Tc is a characteristic timescale based on the speed of the breaking wave crest as estimated from the video recordings. Order of magnitude values for the air‐void fraction β, vertical eddy diffusivity kν, and kinetic energy dissipation ε associated with breaking waves are presented. The wind speed dependence of average acoustic backscattering properties over the course of the experiment is also given.
Inversion for heat anomaly transport from sea surface temperature time series in the northwest Pacific
Authors: Ostrovskii Alexander , Piterbarg Leonid
Published: February 2004

We describe a heat anomaly transport in the upper ocean mixed layer in the Kuroshio extension region and the subtropical gyre of the northwest Pacific. Emphasis is on behavior in the cool season (December‐March) during the Asian Winter Monsoon. The heat anomaly transport is estimated by applying an inversion technique to the stochastic partial differential equation for the heat anomaly balance of advection, diffusion, stabilizing feedback, and atmospheric forcing. The inversion consists of (1) derivation of statistical parametric model from the heat anomaly balance equation; (2) fitting the derived statistical model to the sea surface temperature (SST) anomaly covariances; and (3) calculation of the heat anomaly net advection velocity, horizontal diffusion coefficient, feedback factor and atmospheric forcing correlation from the parameters of the evaluated statistical model. The inversion was applied to the Comprehensive Ocean‐Atmosphere Data Set Compressed Marine Reports SST data averaged at 1° latitude × 2° longitude boxes on a 10‐day mean basis from 1965 to 1990. The estimates of the net advection velocity are consistent in magnitude and direction with the general circulation in the surface layer of the Northwest Pacific in winter. SST anomalies are transported to the west at ∼0.15 m s−1 in the northern part of the North Equatorial Current. Between 21° and 29°N in the recirculating region, SST anomalies propagate westward with the mean velocity less than 0.1 m s−1. South and east of Honshu the observed pattern of the SST anomaly transport agrees broadly with the circulations of the Kuroshio current and its extension and the Oyashio current. South of Honshu, the eastward transport is about 200–300 km wide; its absolute velocity is up to 0.2 m s−1. One branch of the transport separates from the coast near the large meander path of the Kuroshio current and follows the east‐southeast direction. The second separation from the coast occurs south of Hokkaido. Over the analysis domain the estimates of the diffusion coefficient are in the range of 3×103 to 6×103 m2 s−1. The higher values of the diffusion coefficient confirm the enhancement of the mesoscale eddy processes near the subtropical convergence zone. The analysis supports Hasselmann's (1976) theory in which generation of midlatitude SST anomalies lasting the dominant timescale of atmospheric processes is primarily attributed to the short period stochastic weather forcing. However, the analysis indicates that the inertia of SST anomalies to their “memory” of earlier winds can not be neglected in the vicinity of the western boundary and in the tropics.
Time and space variability in the eastern Alboran Sea from March to May 1990
Authors: Viúdez Álvaro , Tintoré Joaquín
Published: February 2004

An intensive field experiment was carried out from March 6 to April 30, 1990, in the eastern Alboran Sea to understand the relationship between the large‐scale circulation and the location of the transition zone between new and longer‐resident Modified Atlantic Water (MAW). The detection of a strong jet (80 cm s−1) of new MAW with anticyclonic curvature near the Morocco coast suggests that the Eastern Alboran Gyre (EAG) was not fully developed. A new state of the circulation in the eastern Alboran basin, intermediate between the EAG (Viúdez et al., 1995) and the Lanoix (1974) state, is presented. Also important is that the Almería‐Oran Front was not present in the upper layer, that small‐scale instabilities were detected in the northern region, and that significant nongeostrophic flows were observed. All these features are clearly indicative of the high spatial and temporal variability of a region where complex adjustments among density, velocity, and topography occur.
Quantitative analysis of sea ice draft: 2. Application of stochastic modeling to intersecting topographic profiles
Authors: Goff John A. , Stewart W. Kenneth , Singh Hanumant , Tang Xiaoou
Published: February 2004

A recent upward looking sonar profile survey of a ∼10‐km by 10‐km area beneath the Arctic ice serves as a basis for an exploratory analysis of the quantitative characteristics of sea ice draft. In a companion paper (Goff, this issue) a method was developed for estimating profile statistical parameters and their uncertainties. These include mean draft, rms variation, characteristic length, fractal dimension, and normalized skewness. Here this methodology is applied to the intersecting profile data set, yielding 160 separate estimates for each parameter. Although not completely two‐dimensional, the data nevertheless allow an opportunity to investigate some aspects of topographic anisotropy. Our most significant observations are (1) a strong positive correlation exists between fractal dimension and characteristic length, while normalized skewness is weakly negatively correlated with both parameters; (2) high and low rms/mean ice are morphologically distinct in the rms versus characteristic length and fractal dimension versus characteristic length parameter spaces; (3) the overall correlation between rms and characteristic length can be explained entirely by the difference between high and low rms/mean type morphology; and (4) anisotropy appears to exist on a local scale but is highly variable over the entire survey. These observations could be explained by a systematic variation in morphology with age, perhaps including a progressive superposition of deformation events, and/or by regionally variable anisotropy.
Modeling the effects of frazil ice crystals on the dynamics and thermodynamics of Ice Shelf Water plumes
Authors: Jenkins Adrian , Bombosch Andreas
Published: February 2004

The seawater that comes into contact with the base of a floating ice shelf is modified as a result of the phase changes that occur. Melting is prevalent in the deepest parts of the subice cavity, and this drives a buoyant flow of Ice Shelf Water (ISW) along the sloping ice shelf base. The ascent of the ISW toward the surface of the ocean causes supercooling, because the freezing point rises with the falling pressure, and this induces a change from melting to freezing. Assuming that seed crystals exist, the ISW now fulfills the condition for the rapid growth of disc‐shaped frazil ice crystals, which may subsequently settle (upward) out of suspension under the action of gravity. A simple numerical model of these processes has been developed, based on the theory of inclined plumes. The ISW is treated as a turbulent, particle‐laden gravity current ascending a reactive boundary and containing a suspended crystal load which evolves in response to the supercooling of the water and the inverted sedimentation of the crystals. The frazil ice has two important effects on the behavior of the ISW plume. Because the generation of crystals through‐out the plume provides such a large area over which phase changes can occur, the conversion of supercooling into ice happens much more readily than is possible through turbulent transfer of heat and mass at the ice shelf base. A suspended crystal load reduces the bulk density of the ISW, so that a growing suspension causes the plume to accelerate, while deposition of crystals onto the ice shelf effects a deceleration. There are positive feedbacks in that an acceleration of the plume induces more rapid crystal growth, while a deceleration allows suspended crystals to settle out more easily.
Radiative‐convective model with an explicit hydrologic cycle: 1. Formulation and sensitivity to model parameters
Authors: Rennó Nilton O. , Emanuel Kerry A. , Stone Peter H.
Published: February 2004

A hydrological cycle is explicityl included in a one‐dimensional radiative‐convective equilibrium model which is coupled to a “swamp” surface and tested with various cumulus convection schemes: the hard and soft convective adjustment schemes, the Kuo scheme, the Goddard Institute for Space Studies (GISS) (1974) model 1 scheme, the GISS (1983) model 2 scheme, and the Emanuel scheme. The essential difference between our model and other radiative‐convective models is that in our model the moisture profile (but not cloudiness) is interactively computed by the cumulus convection scheme. This has a crucial influence on the computation of the radiative fluxes throughout the atmosphere and therefore on the model's sensitivities. Using the Emanuel scheme, we show that the climate equilibrium is very sensitive to cloud microphysical processes. Clouds with high precipitation efficiency produce cold and dry climates. Clouds with low precipitation efficiency lead to moist and warm climates. Since climate equilibrium can be very sensitive to the cloud microphysical processes, any cumulus convection scheme adequate for use in general circulation models (GCMs) should be strongly based on them. The cumulus convection schemes currently in use in GCMs bypass the microphysical processes by making arbitrary moistening assumptions. We suggest that they are inadequate for climate change studies.
SO2 flux measurements at Mount Etna (Sicily)
Authors: Caltabiano Tommaso , Romano Romolo , Budetta Gennaro
Published: February 2004

Since 1987, over 220 measurements of the SO2 flux at Mount Etna have been carried out using a correlation spectrometer (COSPEC) with different measuring techniques (mainly with COSPEC mounted on a ground‐based vehicle). This paper reports and analyzes the data obtained between October 1987 and December 1991. During this period, three distinct time intervals characterized by particular SO2 emission patterns were identified. The first interval (A) showed a mean SO2 flux of 5500 t/d associated with relatively quiet summit crater eruptive activity. The second interval (B) included two eruptive periods, September–October 1989 and January–February 1990, associated with high fluxes reaching 10,000–25,000 t/d. The third interval (C) started in concert with a regional earthquake (December 13, 1990) and showed first a decrease and then an increase of SO2 emissions before the onset of the major 1991–1993 flank eruption. Analysis of the data reveals a cyclic pattern to the SO2 emissions over prolonged periods; a nearly constant supply of SO2 from the volcano's main feeder system, especially evident in the long term; a two‐ to fivefold increase above mean flux values (from 10,000 to 25,000 t/d) when occurring with paroxysmal eruptive activity; and minimal flux values (∼1000 t/d) about 1 month prior to important eruptive events.
Reevaluation of the NOAA/CMDL carbon monoxide reference scale and comparisons with CO reference gases at NASA‐Langley and the Fraunhofer Institut
Authors: Novelli P. C. , Collins J. E. , Myers R. C. , Sachse G. W. , Scheel H. E.
Published: February 2004

The carbon monoxide (CO) reference scale created by the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) is used to quantify measurements of CO in the atmosphere, calibrate standards of other laboratories and to otherwise provide reference gases to the community measuring atmospheric CO. This reference scale was created based upon a set of primary standards prepared by gravimetric methods at CMDL and has been propagated to a set of working standards. In this paper we compare CO mixing ratios assigned to the working standards by three approaches: (1) calibration against the original gravimetric standards, (2) calibration using only working standards as the reference gas, and (3) calibration against three new gravimetric standards prepared at CMDL. The agreement between these values was typically better than 1%. The calibration histories of CMDL working standards are reviewed with respect to expected rates of CO change in the atmosphere. Using a Monte Carlo approach to simulate the effect of drifting standards on calculated mixing ratios, we conclude that the error solely associated with the maintenance of standards will limit the ability to detect small CO changes in the atmosphere. We also report results of intercalibration experiments conducted between CMDL and the Diode Laser Sensor Group (DACOM) at the NASA Langley Research Center (Hampton, Virginia), and CMDL and the Fraunhofer‐Institut (Garmisch‐Partenkirchen, Germany). Each laboratory calibrated several working standards for CO using their reference gases, and these results were compared to calibrations conducted by CMDL. The intercomparison of eight standards (CO concentrations between ∼100 and ∼165 ppb) by CMDL and NASA agreed to better than ±2%. The calibration of six standards (CO concentrations between ∼50 and ∼210 ppb) by CMDL and the Fraunhofer‐Institut agreed to within ±2% for four standards, and to within ±5% for all six standards.
Disequilibrium of radon and its short‐lived daughters near the ground with atmospheric stability
Authors: Dueñas C. , Pérez M. , Fernández M. C. , Carretero J.
Published: February 2004

We have studied the possibility of using the nonequilibrium of radon daughters as an atmospheric tracer. Over a 7‐month period, two daily measurements were taken, one in the early morning and another in the early afternoon at a sampling point with a continental climate. The Rn in the atmospheric air is determined by electroprecipitation of Ra A (218Po) ions, and the Ra B (214Pb) and Ra C (214Bi) daughter concentrations are evaluated by gamma spectrometry. The results show a great variation of the activity ratios Ra B/Rn and Ra C/Rn during the sampling period, depending on the time of measurements and weather conditions. The relations between the activity ratios Ra B/Rn and Ra B/Rn with the stability of the atmosphere has been studied. The stability of the atmosphere was established by two methods: Pasquill's and Turner's stability categories. In the second case, the relationship between the activity ratios Ra B/Rn and Ra C/Rn with stability was higher.
Land surface temperatures derived from the advanced very high resolution radiometer and the along‐track scanning radiometer: 2. Experimental results and validation of AVHRR algorithms
Authors: Prata A. J.
Published: February 2004

This is the second paper in a series of three papers which discuss the problem of deriving accurate land surface temperatures (LSTs) from infrared radiometers on board satellites, such as the NOAA advanced very high resolution radiometer (AVHRR) and the ERS 1 along‐track scanning radiometer (ATSR). In the first part a detailed description of a theoretical model for deriving LSTs based on linearization of the radiative transfer equation was given. This paper focuses on an experimental investigation aimed at establishing the validity and accuracy of the so‐called split‐window technique applied to the land surface. The paper concentrates on the use of AVHRR data for LST estimation; the third part of this series will discuss ATSR data. Because of the lack of good ground‐truth data, an appropriate method has been devised to create an LST validation data set suitable for comparison with the AVHRR or ATSR infrared measurements. Multiple regression relations between satellite temperature measurements and areal averages of in situ temperatures are presented for several surface types from two field sites. Root‐mean‐square differences of about ±1.5°C are obtained for all of the surface types and the correlations are extremely good (r2 ≥ 0.95). These results are considered to represent the limiting accuracies that can be expected from the split‐window formulation applied to high‐resolution AVHRR measurements. Results are also presented for dual‐channel and triple‐window algorithms. The effects of spatial variability and viewing geometry on surface temperature retrieval are discussed. It is found that for the vegetated surfaces studied, emissivity effects are minimized by cavity effects and angular effects are only important for structured vegetation (e.g., row crops) and certain viewing geometries. The spatial temperature variability is large at scales below 1 km2 (>3°C), is small at a scale of 1–10 km2 (<1°C), and increases only slowly with spatial scale beyond scales of 10 km2 or so. Comparisons between the LST algorithms given in part 1, published algorithms by other authors and the in situ data indicate that for these when used with climatological atmospheres and laboratory measured emissivities.
A simple hydrologically based model of land surface water and energy fluxes for general circulation models
Authors: Liang Xu , Lettenmaier Dennis P. , Wood Eric F. , Burges Stephen J.
Published: February 2004

A generalization of the single soil layer variable infiltration capacity (VIC) land surface hydrological model previously implemented in the Geophysical Fluid Dynamics Laboratory general circulation model (GCM) is described. The new model is comprised of a two‐layer characterization of the soil column, and uses an aerodynamic representation of the latent and sensible heat fluxes at the land surface. The infiltration algorithm for the upper layer is essentially the same as for the single layer VIC model, while the lower layer drainage formulation is of the form previously implemented in the Max‐Planck‐Institut GCM. The model partitions the area of interest (e.g., grid cell) into multiple land surface cover types; for each land cover type the fraction of roots in the upper and lower zone is specified. Evapotranspiration consists of three components: canopy evaporation, evaporation from bare soils, and transpiration, which is represented using a canopy and architectural resistance formulation. Once the latent heat flux has been computed, the surface energy balance is iterated to solve for the land surface temperature at each time step. The model was tested using long‐term hydrologic and climatological data for Kings Creek, Kansas to estimate and validate the hydrological parameters, and surface flux data from three First International Satellite Land Surface Climatology Project Field Experiment intensive field campaigns in the summer‐fall of 1987 to validate the surface energy fluxes.
Electrolyte effects on aqueous atmospheric oxidation of sulphur dioxide by ozone
Authors: Lagrange J. , Pallares C. , Lagrange P.
Published: February 2004

The kinetics of sulphur(IV) oxidation by ozone in an aqueous solution were studied in various supporting electrolytes (NaClO4, NaCl, NH4ClO4, Na2SO4), using the stopped flow method. The rate data in perchlorate medium (chosen as reference electrolyte) are empirically correlated by the following expression: r = −d[O3]/dt = k [O3] [S(IV)] [H+]−1/2. For each supporting electrolyte studied, the rate constant varies linearly with the ionic strength. The value of the rate constant is extrapolated to zero ionic strength (1.65 × 104 mol−0.5 L0.5 s−1). The rate constants are shown to be higher mainly when ammonium chloride and sodium sulphate are added to the solution. The effect of temperature is studied between 13 ° and 28 °C. The reaction rate is unaffected by the trace presence of metal ion (Mn2+, Fe2+, Cu2+, Fe3+, Cr3+). The rate of oxidation of dissolved SO2 by O3, in water droplets under atmospheric conditions (presence of [H2O2] = 4.31 × 10−5 mol L−1 and [O3] = 1.17 × 10−9 mol L−1), is calculated as a function of [H+] and compared to the oxidation by H2O2. At zero ionic strength the ozone reaction becomes faster than hydrogen peroxide reaction above −log [H+] = 4.70. This effect appears at higher H+ concentration when the ionic strength increases (−log [H+] = 3 in 4 mol L−1 NaCl or 2 mol L−1 Na2SO4).
Intercomparison of tropospheric OH and ancillary trace gas measurements at Fritz Peak Observatory, Colorado
Authors: Eisele Fred. L. , Mount George H. , Fehsenfeld Fred C. , Harder Jerald , Marovich Edward , Parrish David D. , Roberts James , Trainer Michael , Tanner David
Published: February 2004

The determination of the concentration of OH in the Earth's troposphere is of fundamental importance to an understanding of the chemistry of the lower atmosphere. Although many experiments to measure OH concentration have been performed in recent years, very few operate at sensitivities necesssary to measure the extremely low amount of OH in the clean troposphere (0.1–0.2 parts per trillion by volume at summertime local noon). This paper describes an informal intercomparison campaign held at Fritz Peak, Colorado, in summer 1991 to intercompare the OH concentrations determined from a spectroscopic instrument and an in situ chemical conversion instrument, both with sensitivities at or below 5×105 molecules cm−3. Ancillary measurements including those of O3, CO, NO, NO2, NOy, H2O, SO2, aerosols, solar flux, and meteorological parameters were also performed to test photochemical theories of OH formation. These measurements also provided a means for comparing air masses at the long path and in situ sites. The intercomparison was very successful with measured values of OH concentration in agreement within one standard error much of the time. OH concentrations were typically low, rarely above 4×106 cm−3, with only slow growth during the morning hours, indicating the possible presence of scavenger species. Model results suggest higher than measured OH concentrations or the presence of scavenger species.
On testing the significance of atmospheric response to smoke from the Kuwaiti oil fires using the Los Alamos general circulation model
Authors: Kao Chih‐Yue Jim , Glatzmaier Gary A. , Malone Robert C.
Published: February 2004

The response of the Los Alamos atmospheric general circulation model to the smoke from the Kuwaiti oil fires set in 1991 is examined. The model has an interactive soot transport module that uses a Lagrangian tracer particle scheme. The statistical significance of the results is evaluated using a methodology based on the classic Student's t test. Among various estimated smoke emission rates and associated visible absorption coefficients, the worst‐ and best‐case scenarios are selected. In each of the scenarios, an ensemble of 10 30‐day June simulations are conducted with the smoke and are compared to the same 10 June simulations without the smoke. The results of the worst‐case scenario show that a statistically significant wave train pattern propagates eastward‐poleward downstream from the source. The signals favorably compare with the observed climate anomalies in summer 1991, albeit some possible El Niño‐Southern Oscillation effects were involved in the actual climate. The results of the best‐case (i.e., least‐impact) scenario show that the significance is rather small but that its general pattern is quite similar to that in the worst‐case scenario.
Evaluation of a GCM cirrus parameterization using satellite observations
Authors: Soden B. J. , Donner L. J.
Published: February 2004

This study applies a simple yet effective methodology to validate a general circulation model parameterization of cirrus ice water path. The methodology combines large‐scale dynamic and thermodynamic fields from operational analyses with prescribed occurrence of cirrus clouds from satellite observations to simulate a global distribution of ice water path. The predicted cloud properties are then compared with the corresponding satellite measurements of visible optical depth and infrared cloud emissivity to evaluate the reliability of the parameterization. This methodology enables the validation to focus strictly on the water loading side of the parameterization by eliminating uncertainties involved in predicting the occurrence of cirrus internally within the parameterization. Overall the parameterization performs remarkably well in capturing the observed spatial patterns of cirrus optical properties. Spatial correlations between the observed and the predicted optical depths are typically greater than 0.7 for the tropics and northern hemisphere midlatitudes. The good spatial agreement largely stems from the strong dependence of the ice water path upon the temperature of the environment in which the clouds form. Poorer correlations (r ∼ 0.3) are noted over the southern hemisphere midlatitudes, suggesting that additional processes not accounted for by the parameterization may be important there. Quantitative evaluation of the parameterization is hindered by the present uncertainty in the size distribution of cirrus ice particles. Consequently, it is difficult to determine if discrepancies between the observed and the predicted optical properties are attributable to errors in the parameterized ice water path or to geographic variations in effective radii.
Water activities, densities, and refractive indices of aqueous sulfates and sodium nitrate droplets of atmospheric importance
Authors: Tang I. N. , Munkelwitz H. R.
Published: February 2004

Water activities, densities, and refractive indices over extended concentration ranges at 25°C are reported for solution droplets containing a single salt of either (NH4)2SO4, NH4HSO4, (NH4)3H(SO4)2, Na2SO4, NaHSO4, or NaNO3, which are common constituents of atmospheric aerosols. The extensive data reported here are obtained from experiments using the single‐particle levitation technique recently developed for measuring the thermodynamic and optical properties of microdroplets. These data should find application in mathematical models predicting the dynamic behavior, visibility reduction, and radiative effects of atmospheric sulfate and nitrate aerosols.
Northern middle‐latitude ozone profile features and trends observed by SBUV and Umkehr, 1979–1990
Authors: DeLuisi J. J. , Mateer C. L. , Theisen D. , Bhartia P. K. , Longenecker D. , Chu B.
Published: February 2004

A comparison of Umkehr ozone profile data with the reprocessed solar backscatter ultraviolet (SBUV) ozone profile data in the northern middle‐latitude region, 30° to 50°N, is reported. Although significant biases exist between the two types of observations, the long‐term variations and least squares linear regression trends agree remarkably well over the comparison period of 1979 to 1990. The ozone trend in the upper stratosphere is of the order of −0.9% yr−1. Near 25 km, little if any trend appears, but a larger negative trend is seen in the lower stratosphere near 15 km. Comparisons show that the average annual ozone cycles in the profiles also agree well. The upper stratospheric ozone results are consistent with photochemical model predictions of ozone depletion near 40 km that are due to the release of anthropogenically produced chlorofluorocarbons. The lower stratospheric ozone trend results are in reasonable agreement with published ozonesonde data trends. It is shown that the ozone trends in the lower stratospheric layers impact significantly on the total ozone trend of the order of −0.47% yr−1. The good agreement now seen between the two types of observations suggests that the combined ground‐based and satellite approach could provide a valuable database for long‐term monitoring of stratospheric ozone for trends and extraordinary variations.