Warwick Norton - Academia.edu (original) (raw)
Papers by Warwick Norton
Climate models are the only viable tool for the prediction of future climates. Similarly, prospec... more Climate models are the only viable tool for the prediction of future climates. Similarly, prospects for the development of seasonal forecasts are thought to reside in the development of such models. Substantial biases
Journal of the Atmospheric Sciences, 2006
A quantitative examination of the annual cycle in the tropical tropopause temperatures, tropical ... more A quantitative examination of the annual cycle in the tropical tropopause temperatures, tropical ascent, momentum balance, and wave driving is performed using ECMWF analyses to determine how the annual cycle in tropical tropopause temperatures arises. Results show that the annual cycle in tropical tropopause temperatures is driven by the annual variation in ascent and consequent dynamical (adiabatic) cooling at the tropical tropopause. Mass divergence local to the tropical tropopause has the dominant contribution to ascent near the tropical tropopause. The annual cycle in mass divergence, and the associated meridional flow, near the tropical tropopause is driven by Eliassen–Palm (EP) flux divergence, that is, wave dissipation. The EP flux divergence near the tropical tropopause is dominated by stationary waves with both the horizontal and vertical components of the EP flux contributing. However, the largest annual cycle is in the divergence of the vertical EP flux and in particular ...
Deterministic parametrizations in global climate models rest on the assumption (or hope) that a s... more Deterministic parametrizations in global climate models rest on the assumption (or hope) that a statistical-dynamical equilibrium exists between the resolved and subgrid-scale unresolved flows. A particular choice of parameter values then takes into account only the mean of the unresolved quantities while intrinsically neglecting all higher order moments of the distribution. In the past some studies have suggested, that representing higher order moments in parameterizations of unresolved flows may improve GCMs. Convection is thought to be the most important source of gravity waves in the tropics. Unfortunately convectively triggered gravity waves are episodic in nature and highly variable in strength. Furthermore circulation of the tropical stratosphere is strongly affected by wave-mean flow interactions. It therefore seems reasonable to study the effects on the tropical stratospheric climate of a stochastic gravity wave scheme. A version of the Hines gravity wave scheme with a stochastic source strength is developed. The scheme is implemented in a 55 level version the Unified Model. The effects of different stochastic source strengths are examined. In some cases the source is assumed to represent the intensity distribution of equatorial convection. The effects of including higher order moments of the distribution are also examined.
Gravity Wave Processes, 1997
Journal of the Atmospheric Sciences, 2005
The stratospheric sudden warming in the Southern Hemisphere (SH) in September 2002 was unexpected... more The stratospheric sudden warming in the Southern Hemisphere (SH) in September 2002 was unexpected for two reasons. First, planetary wave activity in the Southern Hemisphere is very weak, and midwinter warmings have never been observed, at least not since observations of the upper stratosphere became regularly available. Second, the warming occurred in a west phase of the quasi-biennial oscillation (QBO)
Journal of Geophysical Research, 2004
1] The sensitivity of the equatorial stratospheric climate to variations in the gravity wave para... more 1] The sensitivity of the equatorial stratospheric climate to variations in the gravity wave parameterization is examined in the Unified Model. Particular emphasis is placed on the response of the annual cycle and quasi-biennial oscillation of zonal winds. Two classes of variations are considered. The first consists of variations in the source strength of the classic deterministic formulation of a Doppler-spread Doppler-shifted gravity wave scheme. The second consists of variations in the probability density function of the source strength parameter in a stochastic formulation of the same scheme. The effects on stratospheric climate of variations in the source strength of a deterministic scheme can be predicted from theory. In such a scheme, when choosing the best possible value for the source strength, a trade-off between a proper representation of the quasi-biennial oscillation and the annual cycle is inevitable. Implementation of a stochastic source strength in the resident gravity wave scheme is justified by the need to better represent the intermittent nature of the convective generation of gravity waves. The most striking effect of the stochastic parameterization is the stabilization of the quasi-biennial oscillation in multidecadal simulations. We suggest that this is due to the ability of the stochastic gravity wave parameterization to deposit a larger portion of gravity wave drag in the middle stratosphere.
Journal of Geophysical Research, 1996
Evidence is presented pointing to the existence of rich and coherent subsynoptic and mesoscale fl... more Evidence is presented pointing to the existence of rich and coherent subsynoptic and mesoscale flow features at tropopause levels. These features are related to, and evolve from, the classical V-shaped intrusions of stratospheric air down to tropospheric elevations. It is shown that ...
Journal of Geophysical Research, 1999
A global circulation model which extends from the surface to 125 km is used to study how the stre... more A global circulation model which extends from the surface to 125 km is used to study how the strength of gravity wave drag affects the dynamics of the mesosphere. The strength of gravity wave drag has a strong influence on the zonal mean state of the mesosphere, in particular the magnitude and variability of the summer mesopause temperature and the shear on the top of the mesospheric jets. This change in the zonal mean state strongly affects the susceptibility of the mesosphere to baroclinic and/or barotropic instability and hence the formation of the 2-day wave. The 2-day wave, in turn, interacts nonlinearly with the diurnal tide, producing secondary waves and a reduction in amplitude of the diurnal tide. Previous studies with quasi-linear mechanistic tidal models have captured some semiannual variation in tidal amplitude through direct interactions between tides and gravity waves. Our fully nonlinear global circulation model results support an alternative explanation in terms of interactions between planetary waves and tides.
Geophysical Research Letters, 1996
... Received 10 January 1996; accepted 12 June 1996; . Citation: Norton, WA, and J. Thuburn (1996... more ... Received 10 January 1996; accepted 12 June 1996; . Citation: Norton, WA, and J. Thuburn (1996), The two‐day wave in a middle atmosphere GCM, Geophys. Res. Lett., 23(16), 2113–2116, doi:10.1029/96GL01956. Cited By. Please wait one moment ... ...
Geophysical Research Letters, 2002
The summer stratospheric flow is analysed in three years of ECMWF analyses. By diagnosing the ang... more The summer stratospheric flow is analysed in three years of ECMWF analyses. By diagnosing the angular rotation, it is shown that the summer stratospheric flow is close to solid-body rotation for latitudes north of 30° with the rate of rotation increasing approximately linearly with height. This flow configuration is not in the radiatively determined state. The winds in thermal wind
Geophysical Research Letters, 2000
Quarterly Journal of the Royal Meteorological Society, 1995
This paper describes a simple, but effective, procedure for producing three-dimensional tracer fi... more This paper describes a simple, but effective, procedure for producing three-dimensional tracer fields and general diagnostics based on an equivalent PV latitude-potential-temperature coordinate system. The tracer fields generated by this method are shown to agree quantitatively with independently measured profiles and total columns. Using this initialization makes short model simulations of around a week or so much more realistic. It can be used to assimilate non-global measurements, such as sonde or solar-occultation satellite data, or to transform two-dimensional model data into three-dimensional fields.
Geophys Res Lett, 1996
ABSTRACT
Several different kinds of accurate potential vorticity (PV) inversion operators, and the associa... more Several different kinds of accurate potential vorticity (PV) inversion operators, and the associated balanced models, are tested for the shallow water equations on a hemisphere in an attempt to approach the ultimate limitations of the balance, inversion, and slow-manifold concepts. The accuracies achieved are far higher than for standard balanced models accurate to one or two orders in Rossby number R or Froude number F (where F ϭ |u|/c; |u| ϭ flow speed; and c ϭ gravity wave speed). Numerical inversions, and corresponding balancedmodel integrations testing cumulative accuracy, are carried out for cases that include substantial PV anomalies in the Tropics. The balanced models in question are constructed so as to be exactly PV conserving and to have unique velocity fields (implying, incidentally, that they cannot be Hamiltonian). Mean layer depths of 1 and 2 km are tested.
1. Lighthill’s idea Lighthill’s most important idea is in our opinion the following, when express... more 1. Lighthill’s idea Lighthill’s most important idea is in our opinion the following, when expressed in a form appropriate to geophysical fluid dynamics. The idea is that the spontaneous-adjustment1 emission of inertia–gravity waves by unsteady vortical motion is sufficiently weak, in parameter regimes of interest, that the emission may be neglected when solving for the vortical motion. The sufficient weakness, in this sense, of the wave emission or ‘‘Lighthill radiation,’ ’ as we also called it, is the fundamental reason—transcending all details of mathe-1 As in Ford et al. (2000), we (a) distinguish spontaneous adjustment from Rossby or initial-condition adjustment, and (b) avoid the term ‘‘geostrophic adjustment,’ ’ since an adjustment toward balance is often an adjustment away from geostrophy. The example of a circular vortex adjusting toward ageostrophic, gradient-wind balance is enough to illustrate the point. The self-contradictory notion of ‘‘ageostrophic geostrophic adjustme...
Quarterly Journal of the Royal Meteorological Society, 2006
Analyses of observational data have suggested a link between late spring/summer conditions in the... more Analyses of observational data have suggested a link between late spring/summer conditions in the North Atlantic Ocean and atmospheric conditions over the same region in the subsequent winter. This link, and its potential value for seasonal forecasting, is investigated using a coupled climate model, HadCM3. It is found that there is memory of ocean conditions from May through to the following winter, but that the model's sea-surfacetemperature (SST) anomalies are too weak in the tropics in winter, partly because they are damped too strongly by comparison with observations and partly because of errors in the model SST anomalies in May, particularly in the east Atlantic where the mixed layer is found to be too deep. It is hypothesized that poor agreement between the winter atmospheric signals in the model and in observations is a consequence of the insufficient tropical SST anomalies. Nevertheless, it is demonstrated that, for suitable initial conditions, seasonal forecasts from 1 May could provide useful information about North Atlantic climate the following winter.
Science, 2003
CREDIT: WENDY LIMAUGE Calm above the storm. This photo shows the dark blue, clear stratosphere ab... more CREDIT: WENDY LIMAUGE Calm above the storm. This photo shows the dark blue, clear stratosphere above the cloudy, stormy troposphere. The boundary between troposphere and stratosphere, the tropopause, is near 10 km at mid-latitudes, approximately the height at which commercial aircraft fly.
Quarterly Journal of the Royal Meteorological Society, 1998
ABSTRACT
Climate models are the only viable tool for the prediction of future climates. Similarly, prospec... more Climate models are the only viable tool for the prediction of future climates. Similarly, prospects for the development of seasonal forecasts are thought to reside in the development of such models. Substantial biases
Journal of the Atmospheric Sciences, 2006
A quantitative examination of the annual cycle in the tropical tropopause temperatures, tropical ... more A quantitative examination of the annual cycle in the tropical tropopause temperatures, tropical ascent, momentum balance, and wave driving is performed using ECMWF analyses to determine how the annual cycle in tropical tropopause temperatures arises. Results show that the annual cycle in tropical tropopause temperatures is driven by the annual variation in ascent and consequent dynamical (adiabatic) cooling at the tropical tropopause. Mass divergence local to the tropical tropopause has the dominant contribution to ascent near the tropical tropopause. The annual cycle in mass divergence, and the associated meridional flow, near the tropical tropopause is driven by Eliassen–Palm (EP) flux divergence, that is, wave dissipation. The EP flux divergence near the tropical tropopause is dominated by stationary waves with both the horizontal and vertical components of the EP flux contributing. However, the largest annual cycle is in the divergence of the vertical EP flux and in particular ...
Deterministic parametrizations in global climate models rest on the assumption (or hope) that a s... more Deterministic parametrizations in global climate models rest on the assumption (or hope) that a statistical-dynamical equilibrium exists between the resolved and subgrid-scale unresolved flows. A particular choice of parameter values then takes into account only the mean of the unresolved quantities while intrinsically neglecting all higher order moments of the distribution. In the past some studies have suggested, that representing higher order moments in parameterizations of unresolved flows may improve GCMs. Convection is thought to be the most important source of gravity waves in the tropics. Unfortunately convectively triggered gravity waves are episodic in nature and highly variable in strength. Furthermore circulation of the tropical stratosphere is strongly affected by wave-mean flow interactions. It therefore seems reasonable to study the effects on the tropical stratospheric climate of a stochastic gravity wave scheme. A version of the Hines gravity wave scheme with a stochastic source strength is developed. The scheme is implemented in a 55 level version the Unified Model. The effects of different stochastic source strengths are examined. In some cases the source is assumed to represent the intensity distribution of equatorial convection. The effects of including higher order moments of the distribution are also examined.
Gravity Wave Processes, 1997
Journal of the Atmospheric Sciences, 2005
The stratospheric sudden warming in the Southern Hemisphere (SH) in September 2002 was unexpected... more The stratospheric sudden warming in the Southern Hemisphere (SH) in September 2002 was unexpected for two reasons. First, planetary wave activity in the Southern Hemisphere is very weak, and midwinter warmings have never been observed, at least not since observations of the upper stratosphere became regularly available. Second, the warming occurred in a west phase of the quasi-biennial oscillation (QBO)
Journal of Geophysical Research, 2004
1] The sensitivity of the equatorial stratospheric climate to variations in the gravity wave para... more 1] The sensitivity of the equatorial stratospheric climate to variations in the gravity wave parameterization is examined in the Unified Model. Particular emphasis is placed on the response of the annual cycle and quasi-biennial oscillation of zonal winds. Two classes of variations are considered. The first consists of variations in the source strength of the classic deterministic formulation of a Doppler-spread Doppler-shifted gravity wave scheme. The second consists of variations in the probability density function of the source strength parameter in a stochastic formulation of the same scheme. The effects on stratospheric climate of variations in the source strength of a deterministic scheme can be predicted from theory. In such a scheme, when choosing the best possible value for the source strength, a trade-off between a proper representation of the quasi-biennial oscillation and the annual cycle is inevitable. Implementation of a stochastic source strength in the resident gravity wave scheme is justified by the need to better represent the intermittent nature of the convective generation of gravity waves. The most striking effect of the stochastic parameterization is the stabilization of the quasi-biennial oscillation in multidecadal simulations. We suggest that this is due to the ability of the stochastic gravity wave parameterization to deposit a larger portion of gravity wave drag in the middle stratosphere.
Journal of Geophysical Research, 1996
Evidence is presented pointing to the existence of rich and coherent subsynoptic and mesoscale fl... more Evidence is presented pointing to the existence of rich and coherent subsynoptic and mesoscale flow features at tropopause levels. These features are related to, and evolve from, the classical V-shaped intrusions of stratospheric air down to tropospheric elevations. It is shown that ...
Journal of Geophysical Research, 1999
A global circulation model which extends from the surface to 125 km is used to study how the stre... more A global circulation model which extends from the surface to 125 km is used to study how the strength of gravity wave drag affects the dynamics of the mesosphere. The strength of gravity wave drag has a strong influence on the zonal mean state of the mesosphere, in particular the magnitude and variability of the summer mesopause temperature and the shear on the top of the mesospheric jets. This change in the zonal mean state strongly affects the susceptibility of the mesosphere to baroclinic and/or barotropic instability and hence the formation of the 2-day wave. The 2-day wave, in turn, interacts nonlinearly with the diurnal tide, producing secondary waves and a reduction in amplitude of the diurnal tide. Previous studies with quasi-linear mechanistic tidal models have captured some semiannual variation in tidal amplitude through direct interactions between tides and gravity waves. Our fully nonlinear global circulation model results support an alternative explanation in terms of interactions between planetary waves and tides.
Geophysical Research Letters, 1996
... Received 10 January 1996; accepted 12 June 1996; . Citation: Norton, WA, and J. Thuburn (1996... more ... Received 10 January 1996; accepted 12 June 1996; . Citation: Norton, WA, and J. Thuburn (1996), The two‐day wave in a middle atmosphere GCM, Geophys. Res. Lett., 23(16), 2113–2116, doi:10.1029/96GL01956. Cited By. Please wait one moment ... ...
Geophysical Research Letters, 2002
The summer stratospheric flow is analysed in three years of ECMWF analyses. By diagnosing the ang... more The summer stratospheric flow is analysed in three years of ECMWF analyses. By diagnosing the angular rotation, it is shown that the summer stratospheric flow is close to solid-body rotation for latitudes north of 30° with the rate of rotation increasing approximately linearly with height. This flow configuration is not in the radiatively determined state. The winds in thermal wind
Geophysical Research Letters, 2000
Quarterly Journal of the Royal Meteorological Society, 1995
This paper describes a simple, but effective, procedure for producing three-dimensional tracer fi... more This paper describes a simple, but effective, procedure for producing three-dimensional tracer fields and general diagnostics based on an equivalent PV latitude-potential-temperature coordinate system. The tracer fields generated by this method are shown to agree quantitatively with independently measured profiles and total columns. Using this initialization makes short model simulations of around a week or so much more realistic. It can be used to assimilate non-global measurements, such as sonde or solar-occultation satellite data, or to transform two-dimensional model data into three-dimensional fields.
Geophys Res Lett, 1996
ABSTRACT
Several different kinds of accurate potential vorticity (PV) inversion operators, and the associa... more Several different kinds of accurate potential vorticity (PV) inversion operators, and the associated balanced models, are tested for the shallow water equations on a hemisphere in an attempt to approach the ultimate limitations of the balance, inversion, and slow-manifold concepts. The accuracies achieved are far higher than for standard balanced models accurate to one or two orders in Rossby number R or Froude number F (where F ϭ |u|/c; |u| ϭ flow speed; and c ϭ gravity wave speed). Numerical inversions, and corresponding balancedmodel integrations testing cumulative accuracy, are carried out for cases that include substantial PV anomalies in the Tropics. The balanced models in question are constructed so as to be exactly PV conserving and to have unique velocity fields (implying, incidentally, that they cannot be Hamiltonian). Mean layer depths of 1 and 2 km are tested.
1. Lighthill’s idea Lighthill’s most important idea is in our opinion the following, when express... more 1. Lighthill’s idea Lighthill’s most important idea is in our opinion the following, when expressed in a form appropriate to geophysical fluid dynamics. The idea is that the spontaneous-adjustment1 emission of inertia–gravity waves by unsteady vortical motion is sufficiently weak, in parameter regimes of interest, that the emission may be neglected when solving for the vortical motion. The sufficient weakness, in this sense, of the wave emission or ‘‘Lighthill radiation,’ ’ as we also called it, is the fundamental reason—transcending all details of mathe-1 As in Ford et al. (2000), we (a) distinguish spontaneous adjustment from Rossby or initial-condition adjustment, and (b) avoid the term ‘‘geostrophic adjustment,’ ’ since an adjustment toward balance is often an adjustment away from geostrophy. The example of a circular vortex adjusting toward ageostrophic, gradient-wind balance is enough to illustrate the point. The self-contradictory notion of ‘‘ageostrophic geostrophic adjustme...
Quarterly Journal of the Royal Meteorological Society, 2006
Analyses of observational data have suggested a link between late spring/summer conditions in the... more Analyses of observational data have suggested a link between late spring/summer conditions in the North Atlantic Ocean and atmospheric conditions over the same region in the subsequent winter. This link, and its potential value for seasonal forecasting, is investigated using a coupled climate model, HadCM3. It is found that there is memory of ocean conditions from May through to the following winter, but that the model's sea-surfacetemperature (SST) anomalies are too weak in the tropics in winter, partly because they are damped too strongly by comparison with observations and partly because of errors in the model SST anomalies in May, particularly in the east Atlantic where the mixed layer is found to be too deep. It is hypothesized that poor agreement between the winter atmospheric signals in the model and in observations is a consequence of the insufficient tropical SST anomalies. Nevertheless, it is demonstrated that, for suitable initial conditions, seasonal forecasts from 1 May could provide useful information about North Atlantic climate the following winter.
Science, 2003
CREDIT: WENDY LIMAUGE Calm above the storm. This photo shows the dark blue, clear stratosphere ab... more CREDIT: WENDY LIMAUGE Calm above the storm. This photo shows the dark blue, clear stratosphere above the cloudy, stormy troposphere. The boundary between troposphere and stratosphere, the tropopause, is near 10 km at mid-latitudes, approximately the height at which commercial aircraft fly.
Quarterly Journal of the Royal Meteorological Society, 1998
ABSTRACT