Search Results
Abstract
Selected portions of microtemperature data obtained continuously and with near simultaneity at several levels up to six feet over a desert surface are plotted on expanded height-time coordinates. The resulting isotherm patterns are shown to be strikingly consistent at all levels and are qualitatively analyzed in relation to the turbulence field present. Correlation coefficients between temperature fluctuations simultaneously at two levels and at a point for various time intervals are evaluated and their variation with separation, time, wind speed, and thermal stability is discussed. Tentative intensity and scale measures of turbulence derived from iemperature data are presented.
Abstract
Selected portions of microtemperature data obtained continuously and with near simultaneity at several levels up to six feet over a desert surface are plotted on expanded height-time coordinates. The resulting isotherm patterns are shown to be strikingly consistent at all levels and are qualitatively analyzed in relation to the turbulence field present. Correlation coefficients between temperature fluctuations simultaneously at two levels and at a point for various time intervals are evaluated and their variation with separation, time, wind speed, and thermal stability is discussed. Tentative intensity and scale measures of turbulence derived from iemperature data are presented.
Abstract
Airbone Doppler lidar wind measurements were obtained in the lee of Mount Shasta in northern California on 28 August 1984. These data consist of line of sight wind vectors at flight level (3000 m) and along planes tilted at 1, 2 and 3 degree below the 3000 m level. The observed field is confined to a rectangular box, encompassing the mountain, that extends about 40 km downwind and about 20 km crosswind. The spatial resolution of the measured wind field is approximately 330 m.
The upstream southwesterly flow tended to circumvent the mountain although some air did rise over the peak (at 4317 m) to initiate three-dimensional internal gravity waves in the lee. These waves are delineated in the two-dimensional divergence field D, determined from the downwind velocity components on each of the tilted planes with line of sight wind vector measurements. The observed field of D exhibits a peak in its power spectrum, determined along the downstream direction, at a wavelength of about 8 km with a secondary peaks at about 17 km. Data from upper air soundings at Medford, Oregon and from onboard sensors establish that the 8 km wavelength represents the free wave response, which is determined by the airstream characteristics. Comparison with the power spectrum of the mountain slope indicates that the longer wavelength is a forced response.
Qualitative aspect of the lee-wave pattern are reproduce in a linear model with uniform airstream characteristics. However, the amplitude of the free wave response is underestimated by a factor of two, and the forced wave amplitude is about three time that of the free wave. In addition, the wave disturbance produced by the linear model decays more rapidly in the downstream direction than the observed wave. These discrepancies are interpreted in relation to physical features that are contained in the linear model.
Abstract
Airbone Doppler lidar wind measurements were obtained in the lee of Mount Shasta in northern California on 28 August 1984. These data consist of line of sight wind vectors at flight level (3000 m) and along planes tilted at 1, 2 and 3 degree below the 3000 m level. The observed field is confined to a rectangular box, encompassing the mountain, that extends about 40 km downwind and about 20 km crosswind. The spatial resolution of the measured wind field is approximately 330 m.
The upstream southwesterly flow tended to circumvent the mountain although some air did rise over the peak (at 4317 m) to initiate three-dimensional internal gravity waves in the lee. These waves are delineated in the two-dimensional divergence field D, determined from the downwind velocity components on each of the tilted planes with line of sight wind vector measurements. The observed field of D exhibits a peak in its power spectrum, determined along the downstream direction, at a wavelength of about 8 km with a secondary peaks at about 17 km. Data from upper air soundings at Medford, Oregon and from onboard sensors establish that the 8 km wavelength represents the free wave response, which is determined by the airstream characteristics. Comparison with the power spectrum of the mountain slope indicates that the longer wavelength is a forced response.
Qualitative aspect of the lee-wave pattern are reproduce in a linear model with uniform airstream characteristics. However, the amplitude of the free wave response is underestimated by a factor of two, and the forced wave amplitude is about three time that of the free wave. In addition, the wave disturbance produced by the linear model decays more rapidly in the downstream direction than the observed wave. These discrepancies are interpreted in relation to physical features that are contained in the linear model.
Abstract
Large-amplitude planetary waves in the southern winter stratosphere are observed to occur episodically, the result of episodic tropospheric forcing. This work is an observational study of the dynamics of the planetary waves, focusing on the evolution through a typical life cycle. Time lag correlations of wave amplitude with the Eliassen-Palm flux vector reveal the characteristic heat and momentum flux patterns associated with wave evolution. Energetic studies clearly show that the stratospheric waves can be understood in terms of a life cycle of vertical propagation from the troposphere, followed by decay from barotropic interactions with the zonal mean flow. Although usually of secondary importance baroclinic decay of stratospheric wave energy is also observed, resulting from equatorward heat flux in the lower stratosphere. Good agreement in the energy balances discounts in situ instability in the stratosphere as a source of wave activity. An average or composite over several clearly propagating cases reveals the wave structure and evolution, and suggests a source of planetary wave activity in the upper tropospheric.
Abstract
Large-amplitude planetary waves in the southern winter stratosphere are observed to occur episodically, the result of episodic tropospheric forcing. This work is an observational study of the dynamics of the planetary waves, focusing on the evolution through a typical life cycle. Time lag correlations of wave amplitude with the Eliassen-Palm flux vector reveal the characteristic heat and momentum flux patterns associated with wave evolution. Energetic studies clearly show that the stratospheric waves can be understood in terms of a life cycle of vertical propagation from the troposphere, followed by decay from barotropic interactions with the zonal mean flow. Although usually of secondary importance baroclinic decay of stratospheric wave energy is also observed, resulting from equatorward heat flux in the lower stratosphere. Good agreement in the energy balances discounts in situ instability in the stratosphere as a source of wave activity. An average or composite over several clearly propagating cases reveals the wave structure and evolution, and suggests a source of planetary wave activity in the upper tropospheric.
Abstract
A case of fog formation along the California coast is examined with the aid of a one-dimensional, higher-order, turbulence-closure model in conjunction with a set of myriad observations. The event is characterized by persistent along-coast winds in the marine layer, and this pattern justifies a Lagrangian approach to the study. A slab of marine layer air is tracked from the waters near the California–Oregon border to the California bight over a 2-day period. Observations indicate that the marine layer is covered by stratus cloud and comes under the influence of large-scale subsidence and progressively increasing sea surface temperature along the southbound trajectory.
It is hypothesized that cloud-top cooling and large-scale subsidence are paramount to the fog formation process. The one-dimensional model, evaluated with various observations along the Lagrangian path, is used to test the hypothesis. The principal findings of the study are 1) fog forms in response to relatively long preconditioning of the marine layer, 2) radiative cooling at the cloud top is the primary mechanism for cooling and mixing the cloud-topped marine layer, and 3) subsidence acts to strengthen the inversion above the cloud top and forces lowering of the cloud. Although the positive fluxes of sensible and latent heat at the air–sea interface are the factors that govern the onset of fog, sensitivity studies with the one-dimensional model indicate that these sensible and latent heat fluxes are of secondary importance as compared to subsidence and cloud-top cooling. Sensitivity tests also suggest that there is an optimal inversion strength favorable to fog formation and that the moisture conditions above the inversion influence fog evolution.
Abstract
A case of fog formation along the California coast is examined with the aid of a one-dimensional, higher-order, turbulence-closure model in conjunction with a set of myriad observations. The event is characterized by persistent along-coast winds in the marine layer, and this pattern justifies a Lagrangian approach to the study. A slab of marine layer air is tracked from the waters near the California–Oregon border to the California bight over a 2-day period. Observations indicate that the marine layer is covered by stratus cloud and comes under the influence of large-scale subsidence and progressively increasing sea surface temperature along the southbound trajectory.
It is hypothesized that cloud-top cooling and large-scale subsidence are paramount to the fog formation process. The one-dimensional model, evaluated with various observations along the Lagrangian path, is used to test the hypothesis. The principal findings of the study are 1) fog forms in response to relatively long preconditioning of the marine layer, 2) radiative cooling at the cloud top is the primary mechanism for cooling and mixing the cloud-topped marine layer, and 3) subsidence acts to strengthen the inversion above the cloud top and forces lowering of the cloud. Although the positive fluxes of sensible and latent heat at the air–sea interface are the factors that govern the onset of fog, sensitivity studies with the one-dimensional model indicate that these sensible and latent heat fluxes are of secondary importance as compared to subsidence and cloud-top cooling. Sensitivity tests also suggest that there is an optimal inversion strength favorable to fog formation and that the moisture conditions above the inversion influence fog evolution.
Abstract
Global variability and budgets of stratospheric nitrous oxide (N2O) are studied using output from a stratospheric version of the NCAR Community Climate Model. The model extends over 0–80 km, incorporating an N2O-like tracer with tropospheric source and upper-stratospheric photochemical sink, the latter parameterized using linear damping rates obtained from detailed two-dimensional model calculations. Results from the model over several seasonal cycles are compared with observations of N2O from the Cryogenic Limb Array Etalon Spectrometer instrument on the Upper Atmosphere Research Satellite. The model produces N2O structure and variability that is in reasonable agreement with the observations. Global budgets of stratospheric N2O are furthermore analyzed using model output, based on the transformed Eulerian-mean, zonal-mean framework. These budgets are used to quantify the importance of planetary wave constituent transport in the stratosphere, for both slow seasonal variations and fast planetary wave events. These results demonstrate that such wave fluxes act to form and sharpen the strong subtropical N2O gradients observed in satellite measurements.
Abstract
Global variability and budgets of stratospheric nitrous oxide (N2O) are studied using output from a stratospheric version of the NCAR Community Climate Model. The model extends over 0–80 km, incorporating an N2O-like tracer with tropospheric source and upper-stratospheric photochemical sink, the latter parameterized using linear damping rates obtained from detailed two-dimensional model calculations. Results from the model over several seasonal cycles are compared with observations of N2O from the Cryogenic Limb Array Etalon Spectrometer instrument on the Upper Atmosphere Research Satellite. The model produces N2O structure and variability that is in reasonable agreement with the observations. Global budgets of stratospheric N2O are furthermore analyzed using model output, based on the transformed Eulerian-mean, zonal-mean framework. These budgets are used to quantify the importance of planetary wave constituent transport in the stratosphere, for both slow seasonal variations and fast planetary wave events. These results demonstrate that such wave fluxes act to form and sharpen the strong subtropical N2O gradients observed in satellite measurements.
Abstract
The deuterium, crystal and air bubble structures of 11 large hailstones from three severe storms have been examined. It is emphasized that there are a number of assumptions underlying the interpretation of such data and these are discussed. In seven of the hailstones the ambient temperatures at which they grew were inferred from the crystal size. The deuterium concentrations and ambient temperatures generally show similar variations and the crystal data thereby provide a useful way of placing an absolute temperature scale against the deuterium values. Throughout most of their growth, the hailstones grew in the updraft between about the ambient temperature levels of –17 to –30°C. The air bubble analyses showed that the hailstones grew near the wet growth limit or slightly wet and heat balance considerations give values of 2–3 g m−3 for the effective liquid water concentrations. On the assumption that the median volume radius of the cloud droplets is 10 µm, the actual liquid water concentrations are then about 4 to 5.5 g m−3.
Abstract
The deuterium, crystal and air bubble structures of 11 large hailstones from three severe storms have been examined. It is emphasized that there are a number of assumptions underlying the interpretation of such data and these are discussed. In seven of the hailstones the ambient temperatures at which they grew were inferred from the crystal size. The deuterium concentrations and ambient temperatures generally show similar variations and the crystal data thereby provide a useful way of placing an absolute temperature scale against the deuterium values. Throughout most of their growth, the hailstones grew in the updraft between about the ambient temperature levels of –17 to –30°C. The air bubble analyses showed that the hailstones grew near the wet growth limit or slightly wet and heat balance considerations give values of 2–3 g m−3 for the effective liquid water concentrations. On the assumption that the median volume radius of the cloud droplets is 10 µm, the actual liquid water concentrations are then about 4 to 5.5 g m−3.
Abstract
Precipitation composition was characterized at 14 remote sites between 65°N and 51°S. Anthropogenic sources contributed to non-sea-salt (nss) SO4 2−, NO3 −, and NH4 + in North Atlantic precipitation. Biogenic sources accounted for 0.4–3.3 μeq L−1 of volume-weighted-average (VWA) nss SO4 2− in marine precipitation. SO4 2− at the continental sites (2.9–7.7 μeq L−1) was generally higher. VWA NO3 − (0.5–1.3 μeq L−1) and NH4 + (0.5–2.6 μeq L−1) at marine-influenced, Southern Hemispheric sites were generally less than those at continental sites (1.4–4.8 μeq L−1 and 2.3–4.2 μeq L−1, respectively). VWA pH ranged from 4.69 to 5.25. Excluding the North Atlantic, nss SO4 2−, NO3 −, and NH4 + wet depositions were factors of 4–47, 5–61, and 3–39, respectively, less than those in the eastern United States during 2002–04. HCOOH t (HCOOHaq + HCOO−) and CH3COOH t (CH3COOHaq + CH3COO−) concentrations and depositions at marine sites overlapped, implying spatially similar source strengths from marine-derived precursors. Greater variability at continental sites suggests heterogeneity in terrestrial source strengths. Seasonality in deposition was driven by variability in precipitation amount, wind velocity, transport, and emissions. Between 1980 and 2009, nss SO4 2− at Bermuda decreased by 85% in response to decreasing U.S. SO2 emissions; trends in NO3 − and NH4 + were inconsequential. Corresponding decreases in acidity, as reflected in the significant 30% decline in VWA H+, impacted pH-dependent chemical processes. Comparisons between measurements and models indicate that current predictive capabilities are uncertain by factors of 2 or more.
Abstract
Precipitation composition was characterized at 14 remote sites between 65°N and 51°S. Anthropogenic sources contributed to non-sea-salt (nss) SO4 2−, NO3 −, and NH4 + in North Atlantic precipitation. Biogenic sources accounted for 0.4–3.3 μeq L−1 of volume-weighted-average (VWA) nss SO4 2− in marine precipitation. SO4 2− at the continental sites (2.9–7.7 μeq L−1) was generally higher. VWA NO3 − (0.5–1.3 μeq L−1) and NH4 + (0.5–2.6 μeq L−1) at marine-influenced, Southern Hemispheric sites were generally less than those at continental sites (1.4–4.8 μeq L−1 and 2.3–4.2 μeq L−1, respectively). VWA pH ranged from 4.69 to 5.25. Excluding the North Atlantic, nss SO4 2−, NO3 −, and NH4 + wet depositions were factors of 4–47, 5–61, and 3–39, respectively, less than those in the eastern United States during 2002–04. HCOOH t (HCOOHaq + HCOO−) and CH3COOH t (CH3COOHaq + CH3COO−) concentrations and depositions at marine sites overlapped, implying spatially similar source strengths from marine-derived precursors. Greater variability at continental sites suggests heterogeneity in terrestrial source strengths. Seasonality in deposition was driven by variability in precipitation amount, wind velocity, transport, and emissions. Between 1980 and 2009, nss SO4 2− at Bermuda decreased by 85% in response to decreasing U.S. SO2 emissions; trends in NO3 − and NH4 + were inconsequential. Corresponding decreases in acidity, as reflected in the significant 30% decline in VWA H+, impacted pH-dependent chemical processes. Comparisons between measurements and models indicate that current predictive capabilities are uncertain by factors of 2 or more.
