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P. Grossi, J-M. Giovannoni, and A. G. Russell

Abstract

In this study, four different meteorological models, one diagnostic and three prognostic, are used to develop meteorological inputs for a photochemical model, as applied to the peninsula of Athens, Greece. The comparison of meteorological models results pointed out significant differences in the calculated wind fields, mainly during the night period. These differences are linked to specific aspects of the models, such as model vertical resolution, hydrostatic versus nonhydrostatic formulation, and numerical diffusion. During the day hours, models produce quite similar wind fields, which agree correctly with the available observations related to the Athens center area. Using the different wind fields as input to a photochemical air quality model led to similar urban ozone levels in the Athens area. Outside of the city, the different wind fields transport the urban plume in different directions in a range of 50°. The more primary pollutants, for example CO and NO2 concentrations, varied significantly due to the different wind velocities predicted by meteorological models. The effect of the atmospheric deposition can be near zero or can go up to 25% for ozone and to 45% for NO2. The determination of the most appropriate wind field to be used for the photochemical modeling would have required a more comprehensive set of observed data. Therefore, when data are scarce, it may be recommended to use different wind field modeling techniques to assess the sensitivity and the robustness of the predicted concentrations.

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Petr Chýlek, G. W. Grams, G. A. Smith, and P. B. Russell

Abstract

Hemispherical backscattering cross sections σb of spherical particles are calculated using a recently derived analytic expression. Results are compared with σb values obtained by numerical integration. It is found that the analytic formula gives exact values of the hemispherical backscattering cross sections and also saves computer time. The behavior of σb in the limits of very small and very large spheres is discussed. As an aid in utilizing simple models of climate change due to aerosols, the percentage of incident solar radiation scattered into the backward hemisphere is calculated for a range of particle sizes and complex refractive indices. Similar results are also presented for the ratio of absorption to hemispheric backscattering, a critical parameter in many aerosol climate models.

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David A. R. Kristovich, Neil F. Laird, Mark R. Hjelmfelt, Russell G. Derickson, and Kevin A. Cooper

Abstract

Boundary layer rolls over Lake Michigan have been observed in wintertime conditions predicted by many past studies to favor nonroll convective structures (such as disorganized convection or cellular convection). This study examines mechanisms that gave rise to transitions between boundary layer rolls and more cellular convective structures observed during a lake-effect snow event over Lake Michigan on 17 December 1983. The purposes of this study are to better understand roll formation in marine boundary layers strongly heated from below and examine the evolution of snowfall rate and mass overturning rate within the boundary layer during periods of convective transition. A method of quantifying the uniformity of convection along the roll axes, based on dual-Doppler radar-derived vertical motions, was developed to quantify changes in boundary layer convective structure. Roll formation was found to occur after (within 1 h) increases in low-level wind speeds and speed shear primarily below about 0.3z i, with little change in directional shear within the convective boundary layer. Roll convective patterns appeared to initiate upstream of the sample region, rather than form locally near the downwind shore of Lake Michigan. These findings suggest that either rolls developed over the upwind half of Lake Michigan or that the convection had a delayed response to changes in the atmospheric surface and wind forcing. Mass overturning rates at midlevels in the boundary layer peaked when rolls were dominant and gradually decreased when cellular convection became more prevalent. Radar-estimated aerial-mean snowfall rates showed little relationship with changes in convective structure. However, when rolls were dominant, the heaviest snow was more concentrated in updraft regions than during more cellular time periods.

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Kevin A. Cooper, Mark R. Hjelmfelt, Russell G. Derickson, David A. R. Kristovich, and Neil F. Laird

Abstract

Numerical simulations are used to study transitions between boundary layer rolls and more cellular convective structures observed during a lake-effect snow event over Lake Michigan on 17 December 1983. Weak lake-effect nonroll convection was observed near the eastern (downwind) shore preceding passage of a secondary cold front. After frontal passage horizontal wind speeds in the convective boundary layer increased, with subsequent development of linear convective patterns. Thereafter the convective pattern became more three-dimensional as low-level wind speeds decreased. Little directional shear was observed in any of the wind profiles. Numerical simulations with the Advanced Regional Prediction System model were initialized with an upwind sounding and radar-derived wind profiles corresponding to each of the three convective structure regimes. Model-derived reflectivity fields were in good agreement with the observed regimes. These simulations differed primarily in the initial wind speed profiles, and suggest that wind speed and shear in the lower boundary layer are critical in determining the linearity of convection. Simulation with an upwind-overlake wind profile, with strong low-level winds, produced the most linear model reflectivity structure. Fluxes and measures of shear-to-buoyancy ratio for this case were comparable to observations.

Model sensitivity tests were conducted to determine the importance of low-level wind speed and speed shear in determining the linearity of convection. Results are consistent with trends expected from ratios of buoyancy to shear (but not proposed numerical threshold values). Eliminating all directional shear from the initial wind profile for the most linear case did not reduce the degree of linearity, thus showing that directional shear is not a requirement for rolls in lake-effect convection. Elimination of clouds (principally latent heating) reduced the vertical velocities by about 50%. It was found that variations in wind speed shear below 200-m height played a major role in determining the degree of linearity of the convection.

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J. Hansen, G. Russell, D. Rind, P. Stone, A. Lacis, S. Lebedeff, R. Ruedy, and L. Travis

Abstract

A global atmospheric model is developed with a computational efficiency which allows long-range climate experiments. The model solves the simultaneous equations for conservation of mass, energy and momentum, and the equation of state on a grid. Differencing schemes for the dynamics are based on work of Arakawa; the schemes do not need any viscosity for numerical stability, and can thus yield good results with coarse resolution. Radiation is computed with a semi-implicit spectral integration, including all significant atmospheric gases, aerosols and cloud particles. Cloud cover and vertical distribution are computed. Convection mixes moisture, heat and momentum, with buoyant air allowed to penetrate to a height determined by its buoyancy. Ground temperature calculations include diurnal variation and seasonal heat storage. Ground hydrology incorporates a water-holding capacity appropriate for the root zone of local vegetation. Snow depth is computed. Snow albedo includes effects of snow age and masking by vegetation. Surface fluxes are obtained from a drag-law formulation and parameterization of the Monin-Obukhov similarity relations.

The initial Model I is used for 60 climate sensitivity experiments with integration times from 3 months to 5 years. These experiments determine the dependence of model simulation on various physical assumptions and model parameters. Several modifications are incorporated to produce Model II, the greatest changes arising from more realistic parameterization of the effect of boundary layer stratification on surface fluxes and the addition of friction in the top stratospheric layer to minimize effects of wave reflection from the rigid model top. The model's climate simulations are compared to observations and a brief study is made of effects of horizontal resolution. It is verified that the major features of global climate can be realistically simulated with a resolution as coarse as 1000 km, which requires an order of magnitude less computation time than used by most general circulation models.

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D. Rind, R. Suozzo, N. K. Balachandran, A. Lacis, and G. Russell

Abstract

The GISS global climate model (Hansen et al.) has been extended to include the middle atmosphere up to an altitude of approximately 85 km. The model has the full array of processes used for climate research, i.e., numerical solutions of the primitive equations, calculation of radiative and surface fluxes, a complete hydrologic cycle with convective and cloud cover parameterizations, etc. In addition, a parameterized gravity wave drag formulation has been incorporated, in which gravity-wave momentum fluxes due to flow over topography, wind shear and convection are calculated at each grid box, using theoretical relationships between the grid-scale variables and expected source strengths. The parameterized waves then propagate vertically upward depending on the instantaneous wind and temperature profiles, with waves breaking at levels in which their momentum flux exceed the background saturation value. Radiative damping is also calculated, and the total momentum convergence in each layer is used to alter the local wind, while the kinetic energy dissipation warms the temperature. Thus the generation, propagation, breaking and drag are all a function of the calculated variables at each grid box for the various vertical levels.

The model has been run for five years, and the results compared with observations. The model produces generally realistic fields of temperature and wind throughout the atmosphere up to approximately 75 km. Important aspects of the current simulation include a proper break between the tropospheric and stratospheric jets, realistic closing off of the wintertime jet in the mesosphere, the observed warm winter/cold summer mesosphere, and a semiannual wind oscillation near the stratopause. The most obvious deficiencies are that the long-wave energy itself is somewhat too small in the low and midstratosphere, temperatures are too cold near the model top and are too warm in the polar Southern Hemisphere lower stratosphere during winter. Also, the model generates an inertial oscillation near the equatorial stratopause which may be excessive. Experiments are run without the various gravity wave drag mechanisms to quantify their effects. It is shown that a coarse-grid general circulation model with parameterized gravity-wave drag can produce a reasonable simulation of the middle atmosphere, which makes possible relatively long-term integrations.

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David G. Vaughan, Jonathan L. Bamber, Mario Giovinetto, Jonathan Russell, and A. Paul R. Cooper

Abstract

Recent in situ measurements of surface mass balance and improved calculation techniques are used to produce an updated assessment of net surface mass balance over Antarctica. A new elevation model of Antarctica derived from ERS-1 satellite altimetry supplemented with conventional data was used to delineate the ice flow drainage basins across Antarctica. The areas of these basins were calculated using the recent digital descriptions of coastlines and grounding lines. The delineation of drainage basins was achieved using an automatic procedure, which gave similar results to earlier hand-drawn catchment basins. More than 1800 published and unpublished in situ measurements of net surface mass balance from Antarctica were collated and then interpolated. A net surface mass balance map was derived from passive microwave satellite data, being employed as a forcing field to control the interpolation of the sparse in situ observations. Basinwide integrals of net surface mass balance were calculated using tools available within a geographic information system. It is found that the integrated net surface mass balance over the conterminous grounded ice sheet is 1811 Gton yr−1 (149 kg m−2 yr−1), and over the entire continent (including ice shelves and their embedded ice rises) it is 2288 Gton yr−1 (166 kg m−2 yr−1). These values are around 18% and 7% higher than the estimates widely adopted at present. The uncertainty in these values is hard to estimate from the methodology alone, but the progression of estimates from early studies to the present suggests that around ±5% uncertainty remains in the overall values. The results serve to confirm the great uncertainty in the overall contribution of the Antarctic Ice Sheet to recent and future global sea level rise even without a substantial collapse of the West Antarctic Ice Sheet.

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G. W. Grams, I. H. Blifford Jr., D. A. Gillette, and P. B. Russell

Abstract

The angular variation of the intensity of light scattered from a collimated beam by airborne soil particles and the size distribution of the particles were measured simultaneously 1.5 m above the ground. These measurements gave an estimate of the complex index of refraction m=n REn IM i of airborne soil particles, where n RE is the real part and n IM the imaginary part of the refractive index.

Standard microscopic analysis procedures were employed to determine n RE. Although a wide range of values was observed, the value 1.525 was taken as representative. By applying Mie scattering theory to each of the observed distributions of particle size, the expected angular variation of the intensity of the scattered light was calculated for a fixed value of n RE and a wide range of values of n IM. For each set of simultaneous measurements, the value of n IM was taken to be that value which provided the best fit to the experimental data. The upper limit of the value of n IM for the airborne particles studied in the experiment was determined to be 0.005 with an uncertainty factor of about 2. The estimate of n IM was found to be fairly insensitive to the assumed value of n RE.

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A. Gettelman, W. J. Randel, S. Massie, F. Wu, W. G. Read, and J. M. Russell III

Abstract

The interannual variability of the tropical tropopause region between 14 and 18 km is examined using observations of convection, winds, and tropopause temperatures from reanalyses and water vapor from satellites. This variability is compared to a simulation using the Community Climate Model version 3 (CCM3) general circulation model forced by observed sea surface temperatures. A coherent picture of the effect of the El Niño–Southern Oscillation (ENSO) on the tropopause region is presented in the NCEP–NCAR reanalyses and CCM3. ENSO modifies convection in the Tropics, and the temperature and circulation of the tropical tropopause region, in agreement with idealized models of tropical heating. CCM3 reproduces most details of these changes, but not the zonal mean temperature variations present in the analysis fields, which are not related to ENSO. ENSO also forces significant changes in observed and simulated water vapor fields. In the upper troposphere water vapor is at maximum near convection, while in the tropopause region water vapor is at minimum in the regions of convection and surrounding it. Convection, cirrus clouds, temperatures, and transport are all linked to describe the water vapor distribution and highlight the role of transport in the tropopause region.

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Brett F. Taubman, Lackson T. Marufu, Charles A. Piety, Bruce G. Doddridge, Jeffrey W. Stehr, and Russell R. Dickerson

Abstract

Airborne observations of trace gases, particle size distributions, and particle optical properties were made during a constant altitude transect from New Hampshire to Maryland on 14 August 2002, the final day of a multiday haze and ozone (O3) episode over the Mid-Atlantic and northeastern United States. These observations, together with chemical, meteorological, and dynamical analyses, suggest that a simple two-reservoir model, composed of the lower free troposphere (LFT), where photochemical processes are accelerated and removal via deposition does not occur, and the planetary boundary layer (PBL), where most precursor species are injected, may realistically represent the physics and chemistry of severe, multiday haze and O3 episodes over the Mid-Atlantic and Northeast. Correlations among O3, potential temperature (θ), the scattering Ångström exponent (α), and relative humidity (RH) suggest that high concentrations of O3 and relatively large, internally mixed sulfate and black carbon (BC) particles were produced in the LFT. Conversely, the PBL contained less O3 and more externally mixed, primary sulfate and BC particles than the LFT. Backward trajectories indicate source regions in the Midwest and Mid-Atlantic urban corridor, with southerly transport up the urban corridor augmented by the Appalachian lee trough and nocturnal low-level jet.

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