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- Author or Editor: C. G. Jones x
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Abstract
A statistical analysis has been made of individual particle transport in a homogeneous, turbulent fluid flow. Expressions for dispersion, correlation coefficients, and turbulent energy content have been obtained. In the course of the development two parameters were found to characterize particulate transport, one of which relates to inertial effects acting on the particle, while the other describes the effects of crossing trajectories. As in previous studies by others, crossing-trajectories effects are found to he of particular importance; inertial effects, however, even for heavy particles, are not insignificant. Comparison of theoretical predictions with experimental data shows good agreement.
Abstract
A statistical analysis has been made of individual particle transport in a homogeneous, turbulent fluid flow. Expressions for dispersion, correlation coefficients, and turbulent energy content have been obtained. In the course of the development two parameters were found to characterize particulate transport, one of which relates to inertial effects acting on the particle, while the other describes the effects of crossing trajectories. As in previous studies by others, crossing-trajectories effects are found to he of particular importance; inertial effects, however, even for heavy particles, are not insignificant. Comparison of theoretical predictions with experimental data shows good agreement.
Abstract
The introduction and subsequent redesign of the HO-83 thermometer have brought the homogeneity of temperature time series from many U.S. Weather Service stations into question. The nature of these inhomogeneities was investigated using data from a 1 5-month side-by-side comparison of the old and new versions of the HO-83 thermometers.
Examination of differences between the two instruments found that the original version of the HO-83 read approximately 0.6°C warmer than the redesigned instrument. Significant changes in the differences between the two instruments were noted between winter and summer. It is suggested that, for stations with climatology similar to the ones used in this study, monthly mean temperature reported by the original version of the HO-83 be adjusted by adding −0.4°C to June, July, August, and September observations and by adding −0.7°C for the remainder of the year.
The physical basis of the problem with the old HO-83 seems to he related to beating of the instrument housing by internal heat sources coupled with inadequate ventilation. The mean differences between the two instruments were negative for all hours, in contrast with a change in sign between day and night as would be expected if the housing was heated and cooled by radiation. The effects of the wind speed and ambient temperature on the observed temperature differences are consistent with this finding.
Abstract
The introduction and subsequent redesign of the HO-83 thermometer have brought the homogeneity of temperature time series from many U.S. Weather Service stations into question. The nature of these inhomogeneities was investigated using data from a 1 5-month side-by-side comparison of the old and new versions of the HO-83 thermometers.
Examination of differences between the two instruments found that the original version of the HO-83 read approximately 0.6°C warmer than the redesigned instrument. Significant changes in the differences between the two instruments were noted between winter and summer. It is suggested that, for stations with climatology similar to the ones used in this study, monthly mean temperature reported by the original version of the HO-83 be adjusted by adding −0.4°C to June, July, August, and September observations and by adding −0.7°C for the remainder of the year.
The physical basis of the problem with the old HO-83 seems to he related to beating of the instrument housing by internal heat sources coupled with inadequate ventilation. The mean differences between the two instruments were negative for all hours, in contrast with a change in sign between day and night as would be expected if the housing was heated and cooled by radiation. The effects of the wind speed and ambient temperature on the observed temperature differences are consistent with this finding.
Abstract
Cross-stream mixing and Lagrangian transport caused by chaotic advection within a baroclinic (2½ layer) meandering jet are investigated. The quasi-steady meanders arise as a result of evolution from an initial small-amplitude instability. The investigation keys on the proposition, made in earlier work, that the cross-jet mixing and transport resulting from the meandering motions are maximized at a subsurface level. It is found that the results depend largely on the size of the shear between the two active layers (which are referred to as the upper and lower layer), as measured by a parameter α. For weak vertical shear (α greater than about 0.5) the primary instability is barotropic and there is no cross-jet transport in either of the active layers. Barriers to transport are identified as plateaus in the probability density function (PDF) of potential vorticity distributions. For stronger shear (α less than about 0.4), baroclinic instability comes into play, and the lower layer experiences barrier destruction followed by cross-jet exchange and mixing. The upper-layer barrier remains intact. The barrier destruction has a dynamical effect as evidenced by the decay of total variance of potential vorticity in the lower layer. Of interest is that the value of α estimated for the Gulf Stream lies in the range 0.4–0.5.
Abstract
Cross-stream mixing and Lagrangian transport caused by chaotic advection within a baroclinic (2½ layer) meandering jet are investigated. The quasi-steady meanders arise as a result of evolution from an initial small-amplitude instability. The investigation keys on the proposition, made in earlier work, that the cross-jet mixing and transport resulting from the meandering motions are maximized at a subsurface level. It is found that the results depend largely on the size of the shear between the two active layers (which are referred to as the upper and lower layer), as measured by a parameter α. For weak vertical shear (α greater than about 0.5) the primary instability is barotropic and there is no cross-jet transport in either of the active layers. Barriers to transport are identified as plateaus in the probability density function (PDF) of potential vorticity distributions. For stronger shear (α less than about 0.4), baroclinic instability comes into play, and the lower layer experiences barrier destruction followed by cross-jet exchange and mixing. The upper-layer barrier remains intact. The barrier destruction has a dynamical effect as evidenced by the decay of total variance of potential vorticity in the lower layer. Of interest is that the value of α estimated for the Gulf Stream lies in the range 0.4–0.5.
Abstract
In summer 2010, the weather conditions in the Euro-Russian sector were affected by a long-lasting atmospheric block that led to a heat wave in Russia and floods in Pakistan. Following previous studies describing the block’s predictability, the present study aims to investigate uncertainties in the upper-level wave pattern and diabatic processes that were responsible for the block’s forecast variability during its onset, mature, and decay phases. With this aim, an ensemble sensitivity analysis (ESA) is performed for three medium-range THORPEX Interactive Grand Global Ensemble multimodel ensemble forecasts, one associated with each phase of the block’s life cycle. The ESA revealed that the block’s predictability was influenced by forecast uncertainties in the general wave pattern and in the vertically integrated water vapor transport (IVT), used here as a proxy for diabatic processes. These uncertainties are associated with spatial shifts and intensity changes of synoptic waves and IVT during the whole life cycle of the block. During the onset phase, specific features include an Atlantic precursor block and the occurrence of several cyclones. During the mature stage, the blocking ridge itself was highly predictable, while forecast uncertainties in the wave pattern and in IVT primarily were associated with uncertainties in the block’s western flank. During the decay phase, the ESA signals were less intense, but the forecast variability significantly depended on the transformation of the block into a high-over-low pattern. It can be concluded that ESA is suitable to investigate the block’s forecast variability in multimodel ensembles.
Abstract
In summer 2010, the weather conditions in the Euro-Russian sector were affected by a long-lasting atmospheric block that led to a heat wave in Russia and floods in Pakistan. Following previous studies describing the block’s predictability, the present study aims to investigate uncertainties in the upper-level wave pattern and diabatic processes that were responsible for the block’s forecast variability during its onset, mature, and decay phases. With this aim, an ensemble sensitivity analysis (ESA) is performed for three medium-range THORPEX Interactive Grand Global Ensemble multimodel ensemble forecasts, one associated with each phase of the block’s life cycle. The ESA revealed that the block’s predictability was influenced by forecast uncertainties in the general wave pattern and in the vertically integrated water vapor transport (IVT), used here as a proxy for diabatic processes. These uncertainties are associated with spatial shifts and intensity changes of synoptic waves and IVT during the whole life cycle of the block. During the onset phase, specific features include an Atlantic precursor block and the occurrence of several cyclones. During the mature stage, the blocking ridge itself was highly predictable, while forecast uncertainties in the wave pattern and in IVT primarily were associated with uncertainties in the block’s western flank. During the decay phase, the ESA signals were less intense, but the forecast variability significantly depended on the transformation of the block into a high-over-low pattern. It can be concluded that ESA is suitable to investigate the block’s forecast variability in multimodel ensembles.
Abstract
An evaluation of the ability of an integrated (slab) marine atmospheric boundary-layer (MABL) model to predict changes in the inversion and mixed-layer temperature and humidity using data from the Los Angeles-San Diego Basin is described. The model microphysics and initialization methods are evaluated separately. The Stage and Businger stratocumulus entrainment closure formulation is used. Standard radiative flux approximations (e.g., delta-Eddington) are employed with up-to-date cloud microphysical parameterizations. The assumption of well-mixed properties is relaxed to permit a constant vertical gradient that is a function of the surface flux and the entrainment rate. Initialization of the subsidence rate receives considerable attention and is analyzed using data from suitably spaced multiple stations and from a single station. Two cases, a cloud covered and a clear sky period, are examined. In both cases the island and shoreline data are from regularly reporting locations and from a research ship which moved around the region. In one case an instrumented aircraft also provided vertical temperature profiles.
Evaluation of the prediction for the cloudy sky case concentrates on the model physics. In this case, the external forcing (subsidence, surface wind and sea-surface temperature) is based on observations during the prediction period and updated every 6 h. Comparison of observations and model results illustrates the important role of both long- and shortwave radiation, and the validity of the Stage and Businger entrainment closure. The agreement is quite good for mixed-layer parameters, mixed-layer depth and the cloud base.
Evaluation of the prediction for the clear sky case emphasizes the initialization problem. Entrainment and radiative flux divergences are roughly an order of magnitude smaller in the cloud-free situation. External forcing for this case is based on data available prior to the prediction (versus updates during the prediction). Since subsidence was large during the period, the initialization was well-tested. A period when the subsidence rate was well-established from radar and acoustic remote sensing showed excellent agreement between observed and predicted values for more than 18 h of a 24 h forecast. Results during a period when subsidence was based on single-station-derived information showed reasonable agreement only during the first 12 h of a 24 h forecast. The influence of very near coastline effects is evident in the comparison of mixed-layer temperatures and humidities at the land stations.
It is concluded that existing integrated mixed-layer predictive models can, with caution, be applied to coastal prediction problems on the basis of multiple- or single-station data. Specification of the subsidence and the effects of near-coastal circulations are critical.
Abstract
An evaluation of the ability of an integrated (slab) marine atmospheric boundary-layer (MABL) model to predict changes in the inversion and mixed-layer temperature and humidity using data from the Los Angeles-San Diego Basin is described. The model microphysics and initialization methods are evaluated separately. The Stage and Businger stratocumulus entrainment closure formulation is used. Standard radiative flux approximations (e.g., delta-Eddington) are employed with up-to-date cloud microphysical parameterizations. The assumption of well-mixed properties is relaxed to permit a constant vertical gradient that is a function of the surface flux and the entrainment rate. Initialization of the subsidence rate receives considerable attention and is analyzed using data from suitably spaced multiple stations and from a single station. Two cases, a cloud covered and a clear sky period, are examined. In both cases the island and shoreline data are from regularly reporting locations and from a research ship which moved around the region. In one case an instrumented aircraft also provided vertical temperature profiles.
Evaluation of the prediction for the cloudy sky case concentrates on the model physics. In this case, the external forcing (subsidence, surface wind and sea-surface temperature) is based on observations during the prediction period and updated every 6 h. Comparison of observations and model results illustrates the important role of both long- and shortwave radiation, and the validity of the Stage and Businger entrainment closure. The agreement is quite good for mixed-layer parameters, mixed-layer depth and the cloud base.
Evaluation of the prediction for the clear sky case emphasizes the initialization problem. Entrainment and radiative flux divergences are roughly an order of magnitude smaller in the cloud-free situation. External forcing for this case is based on data available prior to the prediction (versus updates during the prediction). Since subsidence was large during the period, the initialization was well-tested. A period when the subsidence rate was well-established from radar and acoustic remote sensing showed excellent agreement between observed and predicted values for more than 18 h of a 24 h forecast. Results during a period when subsidence was based on single-station-derived information showed reasonable agreement only during the first 12 h of a 24 h forecast. The influence of very near coastline effects is evident in the comparison of mixed-layer temperatures and humidities at the land stations.
It is concluded that existing integrated mixed-layer predictive models can, with caution, be applied to coastal prediction problems on the basis of multiple- or single-station data. Specification of the subsidence and the effects of near-coastal circulations are critical.
Abstract
The atmospheric component of the new Hadley Centre Global Environmental Model (HadGEM1) is described and an assessment of its mean climatology presented. HadGEM1 includes substantially improved representations of physical processes, increased functionality, and higher resolution than its predecessor, the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). Major developments are the use of semi-Lagrangian instead of Eulerian advection for both dynamical and tracer fields; new boundary layer, gravity wave drag, microphysics, and sea ice schemes; and major changes to the convection, land surface (including tiled surface characteristics), and cloud schemes. There is better coupling between the atmosphere, land, ocean, and sea ice subcomponents and the model includes an interactive aerosol scheme, representing both the first and second indirect effects. Particular focus has been placed on improving the processes (such as clouds and aerosol) that are most uncertain in projections of climate change.
These developments lead to a significantly more realistic simulation of the processes represented, the most notable improvements being in the hydrological cycle, cloud radiative properties, the boundary layer, the tropopause structure, and the representation of tracers.
Abstract
The atmospheric component of the new Hadley Centre Global Environmental Model (HadGEM1) is described and an assessment of its mean climatology presented. HadGEM1 includes substantially improved representations of physical processes, increased functionality, and higher resolution than its predecessor, the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). Major developments are the use of semi-Lagrangian instead of Eulerian advection for both dynamical and tracer fields; new boundary layer, gravity wave drag, microphysics, and sea ice schemes; and major changes to the convection, land surface (including tiled surface characteristics), and cloud schemes. There is better coupling between the atmosphere, land, ocean, and sea ice subcomponents and the model includes an interactive aerosol scheme, representing both the first and second indirect effects. Particular focus has been placed on improving the processes (such as clouds and aerosol) that are most uncertain in projections of climate change.
These developments lead to a significantly more realistic simulation of the processes represented, the most notable improvements being in the hydrological cycle, cloud radiative properties, the boundary layer, the tropopause structure, and the representation of tracers.
Abstract
We describe annual and seasonal changes in air temperatures over high latitudes of the Northern Hemisphere during the period 1881–1980. Trends (that is, fluctuations on time scales greater than 20 years) in the average temperature of the Arctic are compared with those of the Northern Hemisphere. Seasonal and regional departures from the long-term trends in the average temperature of the Arctic are identified. Spatial patterns of variation in the Arctic temperature field are determined by principal component analysis and the major characteristics of the time series of the dominant patterns are summarized.
Trends in Arctic temperatures have been broadly similar to those for the Northern Hemisphere during the study period. The Arctic variations were, however, greater in magnitude and more rapid. The spatial pattern of change associated with the trend in Arctic temperatures is clearly identified by principal component analysis. It shows that the trends have, in general, been Arctic-wide, but that certain regions are particularly sensitive to long-term variations, most notably northwest Greenland and around the Kara Sea. There is some evidence that the pattern of Arctic cooling that occurred after 1940 was more complex than the warming that affected the whole Arctic during the 1920's and 1930's. Warming of the Arctic has occurred during the 1970's, but is not yet of sufficient duration to be considered long term, except, perhaps, in spring. The average temperature of the Arctic during the 1970's was equal to that of the 1960's, indicating a cessation of the long-term cooling trend but not, as yet, a shift to long-term warming. Short-term variations in temperature appear to be most pronounced close to major regions of sea-ice production and decay.
Abstract
We describe annual and seasonal changes in air temperatures over high latitudes of the Northern Hemisphere during the period 1881–1980. Trends (that is, fluctuations on time scales greater than 20 years) in the average temperature of the Arctic are compared with those of the Northern Hemisphere. Seasonal and regional departures from the long-term trends in the average temperature of the Arctic are identified. Spatial patterns of variation in the Arctic temperature field are determined by principal component analysis and the major characteristics of the time series of the dominant patterns are summarized.
Trends in Arctic temperatures have been broadly similar to those for the Northern Hemisphere during the study period. The Arctic variations were, however, greater in magnitude and more rapid. The spatial pattern of change associated with the trend in Arctic temperatures is clearly identified by principal component analysis. It shows that the trends have, in general, been Arctic-wide, but that certain regions are particularly sensitive to long-term variations, most notably northwest Greenland and around the Kara Sea. There is some evidence that the pattern of Arctic cooling that occurred after 1940 was more complex than the warming that affected the whole Arctic during the 1920's and 1930's. Warming of the Arctic has occurred during the 1970's, but is not yet of sufficient duration to be considered long term, except, perhaps, in spring. The average temperature of the Arctic during the 1970's was equal to that of the 1960's, indicating a cessation of the long-term cooling trend but not, as yet, a shift to long-term warming. Short-term variations in temperature appear to be most pronounced close to major regions of sea-ice production and decay.
