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Abstract
This review examines laboratory observations of the ultraviolet absorption and dissociation processes of CO2 and the reactions of the dissociation products. The following conclusions about photolytic formation of CO and O atoms are reached. In the pressure and temperature conditions found in planetary atmospheres, CO2 is dissociated at the rate implied by the absorption measurements made at higher pressure in the laboratory. Although most of the O atoms produced in the λ < 1650 Å dissociation process are initially formed in the O(1D) state, these atoms are quenched to ground state O atoms after a few collisions with CO2. There is no persuasive laboratory evidence to support the hypothesis that a metastable CO3 species is formed in the O(1D)-CO2 quenching process.
The possible fates of the CO and O atoms after they are formed in the photolysis process are examined to see if known reactions can account for the observations of low O atom concentrations in the upper atmosphere and low overall mixing ratios of O2 and CO on Mars and Venus. It is concluded that there is no known two-body process that can remove O atoms fast enough to provide, through chemistry, a concentration as low as that observed in the outer atmospheres of Mars and Venus. It is therefore probable that the O atoms are removed from the upper atmospheres by downward transport. However, a comparison of known three-body loss rates shows that at lower altitudes the O atoms will be recombined to form O2 rather than CO2 which raises the question of how CO2 is regenerated from CO and O2. Comparison of measured rate constants suggests that regeneration of CO2 on Venus can probably take place via direct reaction between O2 and CO if the atmosphere is efficiently mixed. There does not appear to be any known gas phase process capable of explaining the low mixing ratios of CO and O2 in the Martian atmosphere. The possibility of heterogeneous recombination mechanisms is briefly discussed.
Abstract
This review examines laboratory observations of the ultraviolet absorption and dissociation processes of CO2 and the reactions of the dissociation products. The following conclusions about photolytic formation of CO and O atoms are reached. In the pressure and temperature conditions found in planetary atmospheres, CO2 is dissociated at the rate implied by the absorption measurements made at higher pressure in the laboratory. Although most of the O atoms produced in the λ < 1650 Å dissociation process are initially formed in the O(1D) state, these atoms are quenched to ground state O atoms after a few collisions with CO2. There is no persuasive laboratory evidence to support the hypothesis that a metastable CO3 species is formed in the O(1D)-CO2 quenching process.
The possible fates of the CO and O atoms after they are formed in the photolysis process are examined to see if known reactions can account for the observations of low O atom concentrations in the upper atmosphere and low overall mixing ratios of O2 and CO on Mars and Venus. It is concluded that there is no known two-body process that can remove O atoms fast enough to provide, through chemistry, a concentration as low as that observed in the outer atmospheres of Mars and Venus. It is therefore probable that the O atoms are removed from the upper atmospheres by downward transport. However, a comparison of known three-body loss rates shows that at lower altitudes the O atoms will be recombined to form O2 rather than CO2 which raises the question of how CO2 is regenerated from CO and O2. Comparison of measured rate constants suggests that regeneration of CO2 on Venus can probably take place via direct reaction between O2 and CO if the atmosphere is efficiently mixed. There does not appear to be any known gas phase process capable of explaining the low mixing ratios of CO and O2 in the Martian atmosphere. The possibility of heterogeneous recombination mechanisms is briefly discussed.
Abstract
This note describes how to generate vertically stretched grids within the context of vertical nesting that are consistent with the conservative interpolation formula used by Clark and Farley. It is shown that all nested grids derive their structure directly from the parent grid, where the only flexibility allowed for nested grids is their grid ratio relative to the parent grid. Formulas are presented that can he used to analyze resulting nested grid structures, and an example showing how these formulas were used to generate relatively smooth inner meshes is described. Suggestions for further improvements in grid design are also provided.
Abstract
This note describes how to generate vertically stretched grids within the context of vertical nesting that are consistent with the conservative interpolation formula used by Clark and Farley. It is shown that all nested grids derive their structure directly from the parent grid, where the only flexibility allowed for nested grids is their grid ratio relative to the parent grid. Formulas are presented that can he used to analyze resulting nested grid structures, and an example showing how these formulas were used to generate relatively smooth inner meshes is described. Suggestions for further improvements in grid design are also provided.
Abstract
In Part I of this paper, the infrared bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) are analyzed for volcanic ash signals using principal component image analysis. Target volcanoes included Popocatepetl volcano near Mexico City and Cleveland volcano in the Aleutian Islands. The analyses were performed to determine the MODIS bands that contribute the most to detecting volcanic ash. Even though the explained variance and signal-to-noise ratio of most of these new images are generally small, several of them provide views of volcanic ash with good contrast to the image background and other image features. Both day and night examples indicate that volcanic ash can be readily detected by combinations of MODIS bands to determine not only the ash extent but also qualitative variations in the concentration and height of the ash. The 36 bands on MODIS give much more flexibility for ash detection than the 4 bands on the current Geostationary Operational Environmental Satellite (GOES) Imager. In particular, MODIS bands 28–32, in the water vapor and longwave infrared portions of the spectrum, contributed most frequently to the detection of airborne volcanic ash. These include bands 28–30 in the 7.3–9.7-μm portion of the spectrum known for volcanic signals. Several of the MODIS bands that proved useful are bands projected for inclusion in the next major upgrade to the GOES Imager (scheduled for 2012). However, band 30 (9.7 μm) is neither available on the current GOES series nor planned for future GOES Imagers. In Part II of this paper, MODIS data for the same volcanic cases examined in Part I are used for specific simulations of current and near-term GOES imagery. The purpose of the simulations is to assess the impact of changes that will occur in the spectral bands of the GOES-M Imager (launched in 2001 and renamed GOES-12) when it becomes operational.
Abstract
In Part I of this paper, the infrared bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) are analyzed for volcanic ash signals using principal component image analysis. Target volcanoes included Popocatepetl volcano near Mexico City and Cleveland volcano in the Aleutian Islands. The analyses were performed to determine the MODIS bands that contribute the most to detecting volcanic ash. Even though the explained variance and signal-to-noise ratio of most of these new images are generally small, several of them provide views of volcanic ash with good contrast to the image background and other image features. Both day and night examples indicate that volcanic ash can be readily detected by combinations of MODIS bands to determine not only the ash extent but also qualitative variations in the concentration and height of the ash. The 36 bands on MODIS give much more flexibility for ash detection than the 4 bands on the current Geostationary Operational Environmental Satellite (GOES) Imager. In particular, MODIS bands 28–32, in the water vapor and longwave infrared portions of the spectrum, contributed most frequently to the detection of airborne volcanic ash. These include bands 28–30 in the 7.3–9.7-μm portion of the spectrum known for volcanic signals. Several of the MODIS bands that proved useful are bands projected for inclusion in the next major upgrade to the GOES Imager (scheduled for 2012). However, band 30 (9.7 μm) is neither available on the current GOES series nor planned for future GOES Imagers. In Part II of this paper, MODIS data for the same volcanic cases examined in Part I are used for specific simulations of current and near-term GOES imagery. The purpose of the simulations is to assess the impact of changes that will occur in the spectral bands of the GOES-M Imager (launched in 2001 and renamed GOES-12) when it becomes operational.
Abstract
In Part I of this paper the infrared bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) were analyzed using principal component image analysis for volcanic ash signals. The analyses performed determined that several of the thermal infrared bands of MODIS contributed significantly to detecting volcanic ash in the cases examined. Most, but not all, of these bands will be included in the next major upgrade to the Geostationary Operational Environmental Satellite (GOES) Imager scheduled for 2012. In Part II, MODIS data for the same volcanic cases examined in Part I (Popocatepetl near Mexico City and Cleveland in the Aleutian Islands) are used to simulate the impact of changes that will occur in spectral bands between current and near-term GOES imagery. The change from the 12.0-μm band to a 13.3-μm band on GOES-M (launched in 2001 and renamed GOES-12) was made to improve cloud-height determinations. However, when GOES-M becomes operational, the change in bands will have a potential negative impact on image products that are heavily utilized for volcanic ash detection. Image products generated from the three GOES infrared bands with the 13.3-μm band substituted for the 12.0-μm band indicate that volcanic ash can be detected but with diminished ability, especially for diffuse ash. For both day and night cases the increased contamination by clouds leads to increased chances of false ash detection.
Abstract
In Part I of this paper the infrared bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) were analyzed using principal component image analysis for volcanic ash signals. The analyses performed determined that several of the thermal infrared bands of MODIS contributed significantly to detecting volcanic ash in the cases examined. Most, but not all, of these bands will be included in the next major upgrade to the Geostationary Operational Environmental Satellite (GOES) Imager scheduled for 2012. In Part II, MODIS data for the same volcanic cases examined in Part I (Popocatepetl near Mexico City and Cleveland in the Aleutian Islands) are used to simulate the impact of changes that will occur in spectral bands between current and near-term GOES imagery. The change from the 12.0-μm band to a 13.3-μm band on GOES-M (launched in 2001 and renamed GOES-12) was made to improve cloud-height determinations. However, when GOES-M becomes operational, the change in bands will have a potential negative impact on image products that are heavily utilized for volcanic ash detection. Image products generated from the three GOES infrared bands with the 13.3-μm band substituted for the 12.0-μm band indicate that volcanic ash can be detected but with diminished ability, especially for diffuse ash. For both day and night cases the increased contamination by clouds leads to increased chances of false ash detection.
Abstract
The Clark nonhydrostatic anelastic code is extended to allow for interactive grid nesting in both two and three spatial dimensions. Tests are presented which investigate the accuracy of three different quadratic interpolation formulae which are used to derive boundary conditions for the fine mesh model. Application of the conservation condition of Kurihara and others is shown to result in significant improvements in the treatment of interactive nesting. A significant improvement in the solutions for interactive versus parasitic nesting is also shown in the context of forced gravity wave flow. This result, for the anelastic system, is in agreement with the earlier results of Phillips and Shukla, who considered the hydrostatic shallow water system of equations.
The interactive nesting model is applied to the simulation of the severe downslope windstorm of 11 January 1972 in Boulder using both two and three spatial dimensions. The three-dimensional simulation results in a gustiness signature in the surface wind speed. The cause of this gustiness is attributed to the development of turbulent eddies in the convectively unstable region of the topographically forced wave. These eddies are transported to the surface by downdrafts formed in the leading edge of the convectively unstable region. A type of periodicity to the wind gustiness signature is then produced by a competition between the two physical processes of wave build up via forced gravity wave dynamics and wave breakdown via convective instability. The actual source/sink terms for the turbulence are still under investigation. Some preliminary comparisons between the two- and three-dimensional windstorm simulations are also presented.
Abstract
The Clark nonhydrostatic anelastic code is extended to allow for interactive grid nesting in both two and three spatial dimensions. Tests are presented which investigate the accuracy of three different quadratic interpolation formulae which are used to derive boundary conditions for the fine mesh model. Application of the conservation condition of Kurihara and others is shown to result in significant improvements in the treatment of interactive nesting. A significant improvement in the solutions for interactive versus parasitic nesting is also shown in the context of forced gravity wave flow. This result, for the anelastic system, is in agreement with the earlier results of Phillips and Shukla, who considered the hydrostatic shallow water system of equations.
The interactive nesting model is applied to the simulation of the severe downslope windstorm of 11 January 1972 in Boulder using both two and three spatial dimensions. The three-dimensional simulation results in a gustiness signature in the surface wind speed. The cause of this gustiness is attributed to the development of turbulent eddies in the convectively unstable region of the topographically forced wave. These eddies are transported to the surface by downdrafts formed in the leading edge of the convectively unstable region. A type of periodicity to the wind gustiness signature is then produced by a competition between the two physical processes of wave build up via forced gravity wave dynamics and wave breakdown via convective instability. The actual source/sink terms for the turbulence are still under investigation. Some preliminary comparisons between the two- and three-dimensional windstorm simulations are also presented.
Abstract
Numerical simulations of stochastic coalescence in a parcel framework are presented using a series of distribution functions. The equations governing the distribution parameter tendencies are derived using a variational approach with constraints. Solutions with two and three log-normal distribution functions are compared with a conventional benchmark model and the distribution model is shown to produce accurate solutions. Although only coalescence is considered within this paper, the procedures for including further physical processes is discussed. All of the simulations presented use the log-normal distribution although the method is general enough that it could be adapted to use other distributions such as the gamma distribution.
A decrease in the number of dependent variables by as much as by a factor of 10 as well as an equivalent reduction in computation time required for the treatment of the coalescence equation makes the distribution model attractive for multi-dimensional cloud model simulations. Further research in the direction of extending the distribution model for such purposes is currently in progress.
Abstract
Numerical simulations of stochastic coalescence in a parcel framework are presented using a series of distribution functions. The equations governing the distribution parameter tendencies are derived using a variational approach with constraints. Solutions with two and three log-normal distribution functions are compared with a conventional benchmark model and the distribution model is shown to produce accurate solutions. Although only coalescence is considered within this paper, the procedures for including further physical processes is discussed. All of the simulations presented use the log-normal distribution although the method is general enough that it could be adapted to use other distributions such as the gamma distribution.
A decrease in the number of dependent variables by as much as by a factor of 10 as well as an equivalent reduction in computation time required for the treatment of the coalescence equation makes the distribution model attractive for multi-dimensional cloud model simulations. Further research in the direction of extending the distribution model for such purposes is currently in progress.
Abstract
An equation is developed, to show the magnitude of pressure change required to produce a given amplitude for a given period on the galvanometric record of the Macelwane electromagnetic microbarograph. The simultaneous occurrence of certain random oscillations recorded by the Macelwane microbarograph and various weather elements is investigated. It is found that there is a close analogy between these oscillations and atmospheric turbulence. With this analogy as a working hypothesis, a mathematical framework is developed which explains the observed relationships.
Abstract
An equation is developed, to show the magnitude of pressure change required to produce a given amplitude for a given period on the galvanometric record of the Macelwane electromagnetic microbarograph. The simultaneous occurrence of certain random oscillations recorded by the Macelwane microbarograph and various weather elements is investigated. It is found that there is a close analogy between these oscillations and atmospheric turbulence. With this analogy as a working hypothesis, a mathematical framework is developed which explains the observed relationships.
Abstract
The effect of vertical differencing on equatorial inertial instability is studied and explicit results obtained for growth rates as a function of the vertical resolution. It is found that for a basic state independent of height, the form of the growing modes is the same as that without vertical discretization except that the vertical wavenumber is replaced by an effective vertical wavenumber in the differential equation for the horizontal structure. This effective vertical wavenumber is bounded above by a value that depends on the spacing of the model levels, which implies that growing modes only occur when the shear exceeds a certain value.
The upper bound is crucially dependent on the form of the difference scheme. For a scheme in which horizontal velocities and geopotential are evaluated on full levels and temperature and vertical velocity are evaluated on half levels (the Charney–Phillips scheme) the upper bound on the effective vertical wavenumber is 2/δ in the Boussinesq limit, where δ is the spacing between the model levels. For a scheme in which the horizontal velocity, geopotential, and temperature are evaluated on full levels, and only the vertical velocity on half levels (the Lorenz scheme), there is no upper bound on the effective vertical wavenumber in the Boussinesq limit so that growing modes occur for any nonzero value of the shear. This is contrary to the expectation that there is a minimum critical shear for instability because the vertical resolution limits the vertical wavenumber.
The effect of Newtonian cooling is also considered and an expression for the growth rate as a function of the cooling coefficient and the effective vertical wavenumber is found. It is found that provided the shear at the equator is nonzero, there are growing modes for all vertical wavenumbers, unlike the case without Newtonian cooling, where a mode grows only if its vertical wavenumber exceeds a critical value that depends on the shear. The consequences for numerical models with finite vertical resolution are discussed.
Abstract
The effect of vertical differencing on equatorial inertial instability is studied and explicit results obtained for growth rates as a function of the vertical resolution. It is found that for a basic state independent of height, the form of the growing modes is the same as that without vertical discretization except that the vertical wavenumber is replaced by an effective vertical wavenumber in the differential equation for the horizontal structure. This effective vertical wavenumber is bounded above by a value that depends on the spacing of the model levels, which implies that growing modes only occur when the shear exceeds a certain value.
The upper bound is crucially dependent on the form of the difference scheme. For a scheme in which horizontal velocities and geopotential are evaluated on full levels and temperature and vertical velocity are evaluated on half levels (the Charney–Phillips scheme) the upper bound on the effective vertical wavenumber is 2/δ in the Boussinesq limit, where δ is the spacing between the model levels. For a scheme in which the horizontal velocity, geopotential, and temperature are evaluated on full levels, and only the vertical velocity on half levels (the Lorenz scheme), there is no upper bound on the effective vertical wavenumber in the Boussinesq limit so that growing modes occur for any nonzero value of the shear. This is contrary to the expectation that there is a minimum critical shear for instability because the vertical resolution limits the vertical wavenumber.
The effect of Newtonian cooling is also considered and an expression for the growth rate as a function of the cooling coefficient and the effective vertical wavenumber is found. It is found that provided the shear at the equator is nonzero, there are growing modes for all vertical wavenumbers, unlike the case without Newtonian cooling, where a mode grows only if its vertical wavenumber exceeds a critical value that depends on the shear. The consequences for numerical models with finite vertical resolution are discussed.
Abstract
Micro-oscillations in the atmosphere of the order of magnitude of minutes have been known and studied for many years. In 1936 Macelwane, and in 1939 Benioff, with their respective electromagnetic microbarographs showed that the spectrum of these micro-oscillations extends down into the order of seconds. The two different types of microbarographs respond to the same types of stimuli and low-level turbulence is an important source of the micro-oscillations. It is shown that fronts, as such, are not a source of these micro-oscillations, although micro-oscillations may accompany a front. The microbarograph has produced observational evidence supporting Haurwitz's theoretically derived conclusion that there is a similarity between internal wave patterns and convective patterns. It is shown that electromagnetic microbarographs are useful in studying cumulus activity.
Abstract
Micro-oscillations in the atmosphere of the order of magnitude of minutes have been known and studied for many years. In 1936 Macelwane, and in 1939 Benioff, with their respective electromagnetic microbarographs showed that the spectrum of these micro-oscillations extends down into the order of seconds. The two different types of microbarographs respond to the same types of stimuli and low-level turbulence is an important source of the micro-oscillations. It is shown that fronts, as such, are not a source of these micro-oscillations, although micro-oscillations may accompany a front. The microbarograph has produced observational evidence supporting Haurwitz's theoretically derived conclusion that there is a similarity between internal wave patterns and convective patterns. It is shown that electromagnetic microbarographs are useful in studying cumulus activity.
Abstract
A three-dimensional numerical model is used to study the effect of small-scale supersaturation fluctuations on the evolving droplet distribution in the first 150 m above cloud base. The primary purpose of this research is to determine whether the irreversible coupling between the thermodynamics and dynamics due to finite phase relaxation time scales τs is sufficient to produce significant small-scale horizontal variations in supersaturation. Thus, the paper is concerned only with this internal source for thermodynamic variability. All other source terms, such as the downgradient flux of the variance of thermodynamic fields, have purposely been neglected.
Lagrangian particle experiments were run in parallel with the basic Eulerian model. The purpose of these experiments is to relax some of the microphysical parameterization assumptions with respect to assumed distribution shape and as a result add credibility to the results of distribution broadening.
Model results of five cases are presented, representing the cloud condensation nuclei characteristics of typical continental and maritime cumulus with mean dissipation rate of −100 cm2 s−3. The results show that for a maritime case of N≈100 cm−3 and w¯=0.5 m s−1 the standard deviation of the supersaturation is as large as its horizontal mean. The horizontal variability of all thermodynamic fields is shown to increase significantly with τs. The droplet broadening response to this irreversible coupling effect is found to be significant for the larger values of τs in the Eulerian experiments. The Lagrangian particle experiments showed a somewhat reduced but still significant effect.
Although the experiments do show a broadening effect caused by finite values of τs, in no case were we able to show a continual increase in distribution broadening with height as reported from cumulus observations.
Abstract
A three-dimensional numerical model is used to study the effect of small-scale supersaturation fluctuations on the evolving droplet distribution in the first 150 m above cloud base. The primary purpose of this research is to determine whether the irreversible coupling between the thermodynamics and dynamics due to finite phase relaxation time scales τs is sufficient to produce significant small-scale horizontal variations in supersaturation. Thus, the paper is concerned only with this internal source for thermodynamic variability. All other source terms, such as the downgradient flux of the variance of thermodynamic fields, have purposely been neglected.
Lagrangian particle experiments were run in parallel with the basic Eulerian model. The purpose of these experiments is to relax some of the microphysical parameterization assumptions with respect to assumed distribution shape and as a result add credibility to the results of distribution broadening.
Model results of five cases are presented, representing the cloud condensation nuclei characteristics of typical continental and maritime cumulus with mean dissipation rate of −100 cm2 s−3. The results show that for a maritime case of N≈100 cm−3 and w¯=0.5 m s−1 the standard deviation of the supersaturation is as large as its horizontal mean. The horizontal variability of all thermodynamic fields is shown to increase significantly with τs. The droplet broadening response to this irreversible coupling effect is found to be significant for the larger values of τs in the Eulerian experiments. The Lagrangian particle experiments showed a somewhat reduced but still significant effect.
Although the experiments do show a broadening effect caused by finite values of τs, in no case were we able to show a continual increase in distribution broadening with height as reported from cumulus observations.
