Publications (22)18.93 Total impact
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Article: The GlobMODEL Demonstrator: Assimilation of New Satellite Products in an Operational Meteorological Center
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ABSTRACT: As part of its Data User Element programme, the European Space Agency funded the GlobMODEL project which aimed at investigating the scientific, technical, and organizational issues associated with the use and exploitation of remotely-sensed observations, particularly from new sounders. A pilot study was performed as a ldquodemonstratorrdquo of the GlobMODEL idea, based on the use of new data, with a strong European heritage, not yet assimilated operationally. Two parallel assimilation experiments were performed, using either total column ozone or ozone profiles retrieved at the Royal Netherlands Meteorological Institute (KNMI) from the Ozone Monitoring Instrument (OMI). In both cases, the impact of assimilating OMI data in addition to the total ozone columns from the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) on the European Centre for Medium Range Weather Forecasts (ECMWF) ozone analyses was assessed by means of independent measurements. We found that the impact of OMI total columns is mainly limited to the region between 20 and 80 hPa, and is particularly important at high latitudes in the Southern hemisphere where the stratospheric ozone transport and chemical depletion are generally difficult to model with accuracy. Furthermore, the assimilation experiments carried out in this work suggest that OMI DOAS (Differential Optical Absorption Spectroscopy) total ozone columns are on average larger than SCIAMACHY total columns by up to 3 DU, while OMI total columns derived from OMI ozone profiles are on average about 8 DU larger than SCIAMACHY total columns. At the same time, the demonstrator brought to light a number of issues related to the assimilation of atmospheric composition profiles, such as the shortcomings arising when the vertical resolution of the instrument is not properly accounted for in the assimilation. The GlobMODEL demonstrator accelerated scientific and operational utilization of new observations and its results - prompted ECMWF to start the operational assimilation of OMI total column ozone data.IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 10/2009; · 1.49 Impact Factor -
Article: Can the dynamical impact of the stratosphere on the troposphere be described by large‐scale adjustment to the stratospheric PV distribution?
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ABSTRACT: Recent numerical experiments have demonstrated that the state of the stratosphere has a dynamical impact on the state of the troposphere. To account for such an effect, a number of mechanisms have been proposed in the literature, all of which amount to a large-scale adjustment of the troposphere to potential vorticity (PV) anomalies in the stratosphere. This paper analyses whether a simple PV adjustment suffices to explain the actual dynamical response of the troposphere to the state of the stratosphere, the actual response being determined by ensembles of numerical experiments run with an atmospheric general-circulation model. For this purpose, a new PV inverter is developed. It is shown that the simple PV adjustment hypothesis is inadequate. PV anomalies in the stratosphere induce, by inversion, flow anomalies in the troposphere that do not coincide spatially with the tropospheric changes determined by the numerical experiments. Moreover, the tropospheric anomalies induced by PV inversion are on a larger scale than the changes found in the numerical experiments, which are linked to the Atlantic and Pacific storm-tracks. These findings imply that the impact of the stratospheric state on the troposphere is manifested through the impact on individual synoptic-scale systems and their self-organization in the storm-tracks. Changes in these weather systems in the troposphere are not merely synoptic-scale ‘noise’ on a larger scale tropospheric response, but an integral part of the mechanism by which the state of the stratosphere impacts that of the troposphere. Copyright © 2005 Royal Meteorological Society.Quarterly Journal of the Royal Meteorological Society 12/2006; 131(606):525 - 543. · 2.91 Impact Factor -
Article: Middle atmosphere variability in the UK Meteorological Office Unified Model
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ABSTRACT: This paper describes the seasonal evolution of the stratospheric circulation simulated by a stratospheretroposphere configuration of the UK Meteorological Office (UKMO) Unified Model. Results are shown from a five-year integration of the model. The model's simulation of the stratosphere and its seasonal evolution compares well with global analyses produced by the UKMO troposphere-stratosphere data-assimilation system. The contrast between the winter circulation in the two hemispheres is well simulated. The zonal-mean winds show strong interannual variability in northern winter, while the southern hemisphere winter jet is much less variable. In northern wither the model spontaneously produces two major warmings and a number of minor warmings. In southern winter and spring the model reproduces well the break-up of the polar vortex and elements of the flow regime that often precedes this break-up. The model does, however, exhibit a number of shortcomings. Lack of conservation of potential vorticity prevents the model from capturing some of the ingredients of the flow regimes associated with stratospheric warmings and with the merger of anticyclones. There is a cold bias in the stratosphere throughout the year, with a maximum cold bias over the winter pole near the stratopause. This temperature bias appears to be due to a cooling bias in the long-wave part of the radiation scheme. The model also has unrealistically strong planetary waves in the upper stratosphere, although amplitudes in the upper troposphere and lower stratosphere are in good agreement with those derived from the UKMO analyses. This paper discusses possible future model improvements that should alleviate these problems.Quarterly Journal of the Royal Meteorological Society 12/2006; 124(549):1485 - 1525. · 2.91 Impact Factor -
Article: Dynamical evolution of the 2003 southern hemisphere stratospheric winter using Envisat trace‐gas observations
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ABSTRACT: Data from the MIPAS instrument on Envisat, supplemented by meteorological analyses from ECMWF and the Met Office, are used to study the meteorological and trace-gas evolution of the stratosphere in the southern hemisphere during winter and spring 2003. A pole-centred approach is used to interpret the data in the physically meaningful context of the evolving stratospheric polar vortex. The following salient dynamical and transport features are documented and analysed: the merger of anticyclones in the stratosphere; the development of an intense, quasi-stationary anticyclone in spring; the associated top-down breakdown of the polar vortex; the systematic descent of air into the polar vortex; and the formation of a three-dimensional structure of a tracer filament on a planetary scale. The paper confirms and extends existing paradigms of the southern hemisphere vortex evolution. The quality of the MIPAS observations is seen to be generally good, though the water vapour retrievals are unrealistic above 10 hPa in the high-latitude winter. Copyright © 2006 Royal Meteorological SocietyQuarterly Journal of the Royal Meteorological Society 06/2006; 132(619):1985 - 2008. · 2.91 Impact Factor -
Article: Sensitivity of tropospheric forecasts to stratospheric initial conditions
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ABSTRACT: A number of recent papers in the atmospheric science literature have suggested that a dynamical link exists between the stratosphere and troposphere. Numerical modelling studies have shown that the troposphere has a time-mean response to changes to the stratospheric climatological state. In this study the response of the troposphere to an imposed transient stratospheric change is examined.The study uses a high horizontal and vertical resolution numerical weather-prediction model. Experiments compare the tropospheric forecasts of two medium-range forecast ensembles which have identical tropospheric initial conditions and different stratospheric initial conditions. In three case studies described here, stratospheric initial conditions have a statistically significant impact on the tropospheric flow. The mechanism for this change involves, in its most basic step, a change to tropospheric synoptic-scale systems. A consistent change to the tropospheric synoptic-scale systems occurs in response to the stratospheric initial conditions. The aggregated impact of changes to individual synoptic systems maps strongly onto the structure of the Arctic Oscillation, particularly over the North Atlantic storm track. The relationship between the stratosphere and troposphere, while apparent in Arctic Oscillation diagnostics, does not occur on coherent, hemispheric scales. Copyright © 2004 Royal Meteorological Society.Quarterly Journal of the Royal Meteorological Society 06/2004; 130(600):1771 - 1792. · 2.91 Impact Factor -
Article: Can knowledge of the state of the stratosphere be used to improve statistical forecasts of the troposphere?
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ABSTRACT: Recent analysis of the Arctic Oscillation (AO) in the stratosphere and troposphere has suggested that predictability of the state of the tropospheric AO may be obtained from the state of the stratospheric AO. However, much of this research has been of a purely qualitative nature. We present a more thorough statistical analysis of a long AO amplitude dataset which seeks to establish the magnitude of such a link.A relationship between the AO in the lower stratosphere and on the 1000 hPa surface on a 10–45 day time-scale is revealed. The relationship accounts for ∼5% of the variance of the 1000 hPa time series at its peak value and is significant at the 5% level. Over a similar time-scale the 1000 hPa time series accounts for ≤1% of itself and is not significant at the 5% level. Further investigation of the relationship reveals that it is only present during the winter season and in particular during February and March. It is also demonstrated that using stratospheric AO amplitude data as a predictor in a simple statistical model results in a gain of skill of ∼5% over a troposphere-only statistical model. This gain in skill is not repeated if an unrelated time series is included as a predictor in the model. Copyright © 2003 Royal Meteorological SocietyQuarterly Journal of the Royal Meteorological Society 09/2003; 129(595):3205 - 3224. · 2.91 Impact Factor -
Article: Vortex dynamics and the evolution of water vapour in the stratosphere of the southern hemisphere
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ABSTRACT: The seasonal evolution of water vapour in the stratosphere of the southern hemisphere is studied by using water vapour measurements made by the Microwave Limb Sounder on the Upper Atmosphere Research Satellite. This evolution is interpreted with the aid of meteorological fields produced at the UK Meteorological Office by data assimilation. The processes governing the distribution of water vapour are clarified by focusing on the physical and dynamical conditions in and around the stratospheric polar vortex. Sustained diabatic descent in the vortex causes isopleths of water vapour mixing ratio to dip down markedly in the polar vortex, strengthening radial gradients of water vapour in the westerly jet, while stretching and folding of material lines in anticyclones adjacent to the polar vortex leads to awidening zone of weak horizontal gradients of water vapour. The circulation is discussed in terms of the dynamics of interacting vortices. Two different flow regimes are identified: (a) mid and late southern winter, with a strong polar vortex and one or more eastward-travelling anticyclone; and (b) spring, with a relatively weak polar vortex and a quasi-stationary anticyclone. The phenomenon of merger of anticyclones, already observed in the stratosphere of the northern hemisphere, is documented for the southern hemisphere. A detailed study of the tracer transport during merger is made by computing the isentropic advection of many thousands of particles. A comparison is made of the seasonal evolution of water vapour in the stratosphere of the southern and northern hemispheres.Quarterly Journal of the Royal Meteorological Society 12/1995; 122(530):423 - 450. · 2.91 Impact Factor -
Article: Lagrangian Transport Calculations Using UARS Data
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ABSTRACT: The transport of passive tracers observed by the Upper Atmosphere Research Satellite is simulated using computed three-dimensional trajectories of approx. equal 100 000 air parcels initialized on a stratospheric grid, with horizontal winds provided by the United Kingdom Meteorological Office data assimilation system, and vertical (cross isentropic) velocities computed using a fast radiation code. The conservative evolution of trace constituent fields is estimated over 20-30-day periods by assigning to each parcel the observed mixing ratio of the long-lived trace gases N2O and CH4 observed by the Cryogenic Limb Array Etalon Spectrometer (CLAES) and H2O observed by the Microwave Limb Sounder (MLS) on the initialization date. Agreement between calculated and observed fields is best inside the polar vortex and is better in the Arctic than in the Antarctic. Although there is not always detailed agreement outside the vortex, the trajectory calculations still reproduce the average large-scale characteristics of passive tracer evolution in midlatitudes. In late winter, synoptic maps from trajectory calculations reproduce all major features of the observations, including large tongues or blobs of material drawn from low latitudes into the region of the anticyclone during February-March 1993. Comparison of lower-stratospheric observations of the CLAES tracers with the calculations suggests that discontinuities seen in CLAES data in the Antarctic late winter lower stratosphere are inconsistent with passive tracer behavior. In the Arctic, and in the Antarctic late winter, MLS H2O observations show behavior that is inconsistent with calculations and with that expected for passive tracers inside the polar vortex in the middle-to-upper stratosphere. Diabatic descent rates in the Arctic lower stratosphere deduced from data are consistent with those from the calculations. In the Antarctic lower stratosphere, the calculations appear to underestimate the diabatic descent. The agreement between large-scale features of calculated and observed tracer fields supports the utility of these calculations in diagnosing trace species transport in the winter polar vortex.10/1995; -
Article: Tropical stratospheric water vapor measured by the microwave limb sounder (MLS)
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ABSTRACT: The lower stratospheric variability of equatorial water vapor, measured by the Microwave Limb Sounder (MLS), follows an annual cycle modulated by the quasi-biennial oscillation. At levels higher in the stratosphere, water vapor measurements exhibit a semi-annual oscillatory signal with the largest amplitudes at 2.2 and 1hPa. Zonal-mean cross sections of MLS water vapor are consistent with previous satellite measurements from the limb infrared monitor of the stratosphere (LIMS) and the stratospheric Aerosol and Gas Experiment 2 (SAGE 2) instruments in that they show water vapor increasing upwards and the polewards from a well defined minimum in the tropics. The minimum values vary in height between the retrieved 46 and 22hPa pressure levels.04/1995; -
Article: Three-dimensional evolution of water vapor distributions in the Northern Hemisphere stratosphere as observed by the MLS
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ABSTRACT: The three-dimensional evolution of stratospheric water vapor distributions observed by the Microwave Limb Sounder (MLS) during the period October 1991 - July 1992 is documented. The transport features inferred from the MLS water vapor distributions are corroborated using other dynamical fields, namely, nitrous oxide from the Cryogenic Limb Array Etalon Spectrometer instrument, analyzed winds from the U.K. Meteorological Office (UKMO), UKMO-derived potential vorticity, and the diabatic heating field. By taking a vortex-centered view and an along-track view, the authors observe in great detail the vertical and horizontal structure of the northern winter stratosphere. It is demonstrated that the water vapor distributions show clear signatures of the effects of diabatic descent through isentropic surfaces and quasi-horizontal transport along isentropic surfaces, and that the large-scale winter flow is organized by the interaction between the westerly polar vortex and the Aleutian high.11/1994; -
Article: On the motion of air through the stratospheric polar vortex
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ABSTRACT: Trajectory calculations using horizontal winds from the U.K. Meteorological Office data assimilation system and vertical velocities from a radiation calculation are used to simulate the three-dimensional motion of air through the stratospheric polar vortex for Northern Hemisphere (NH) and Southern Hemisphere (SH) winters since the launch of the Upper Atmosphere Research Satellite (UARS). Throughout the winter, air from the upper stratosphere moves poleward and descends into the middle stratosphere. In the SH lower to middle stratosphere, strongest descent occurs near the edge of the polar vortex, with that edge defined by mixing characteristics. The NH shows a similar pattern in late winter, but in early winter strongest descent is near the center of the vortex, except when wave activity is particularly strong. Strong barriers to latitudinal mixing exist above about 420 K throughout the winter. Below this, the polar night jet is weak in early winter, so air descending below that level mixes between polar and middle latitudes. In late winter, parcels descend less and the polar night jet moves downward, so there is less latitudinal mixing. The degree of mixing in the lower stratosphere thus depends strongly on the position and evolution of the polar night jet and on the amount of descent experienced by the air parcels; these characteristics show considerable interannual variability in both hemispheres. The computed trajectories provide a three-dimensional picture of air motion during the final warming. Large tongues of air are drawn off the vortex and stretched into increasingly long and narrow tongues extending into low latitudes. This vortex erosion process proceeds more rapidly in the NH than in he SH. In the lower stratosphere, the majority of air parcels remain confined within a lingering region of strong potential vorticity gradients into December in the SH and April in the NH, well after the vortex breaks up in the midstratosphere.11/1994; -
Article: Stratospheric Warmings During February and March 1993
[show abstract] [hide abstract]
ABSTRACT: Two stratospheric warnings during February and March 1993 are described using UKMO analyses, calculated PV and diabatic heating, and N2O observed by the CLAES instrument on the UARS. The first warming affected temperatures over a larger region. while the second produced a larger region of reversed zonal winds. Tilted baroclinic zones formed in the temperature field, and the polar vortex tilted westward with height. Narrow tongues of high PV and low N2O were drawn off the polar vortex, and irreversibly mixed. Tongues of material were drawn from low latitudes into the region between the polar vortex and the anticyclone; diabatic descent was also strongest in this region. Increased N2O over a broad region near the edge of the polar vortex indicates the importance of horizontal transport. N2O decreased in the vortex, consistent with enhanced diabatic descent during the warmings.06/1994; -
Article: Impact of research satellite observations in a data assimilation system for the troposphere and stratosphere.
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Modelling and Measurement Studies of Northern Hemisphere Ozone Loss within the SESAM Campaign. A Study of Hemispheric Scale Processes Affecting Ozone
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Article: Dynamical evolution of the 2003 southern hemisphere stratospheric winter using Envisat trace-gas observations
[show abstract] [hide abstract]
ABSTRACT: Data from the MIPAS instrument on Envisat, supplemented by meteorological analyses from ECMWF and the Met Office, are used to study the meteorological and trace-gas evolution of the stratosphere in the southern hemisphere during winter and spring 2003. A pole-centred approach is used to interpret the data in the physically meaningful context of the evolving stratospheric polar vortex. The following salient dynamical and transport features are documented and analysed: the merger of anticyclones in the stratosphere; the development of an intense, quasi-stationary anticyclone in spring; the associated top-down breakdown of the polar vortex; the systematic descent of air into the polar vortex; and the formation of a three-dimensional structure of a tracer filament on a planetary scale. The paper confirms and extends existing paradigms of the southern hemisphere vortex evolution. The quality of the MIPAS observations is seen to be generally good. though the water vapour retrievals are unrealistic above 10 hPa in the high-latitude winter. -
Article: Evaluation of ozone total column measurements by the Ozone Monitoring Instrument using a data assimilation system
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Article: The role of the south-east Asian monsoon and other seasonal features in creating the 'tape-recorder' signal in the Unified Model
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Article: Can stratospheric temperature trends be attributed to ozone depletion?
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Article: Evaluation of Envisat data using a NWP system: a vortex-centred view.
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Article: Can the dynamical impact of the stratosphere on the troposphere be described by large-scale adjustment to the stratospheric PV distribution?
[show abstract] [hide abstract]
ABSTRACT: Recent numerical experiments have demonstrated that the state of the stratosphere has a dynamical impact on the state of the troposphere. To account for such an effect, a number of mechanisms have been proposed in the literature, all of which amount to a large-scale adjustment of the troposphere to potential vorticity (PV) anomalies in the stratosphere. This paper analyses whether a simple PV adjustment suffices to explain the actual dynamical response of the troposphere to the state of the stratosphere, the actual response being determined by ensembles of numerical experiments run with an atmospheric general-circulation model. For this purpose, a new PV inverter is developed. It is shown that the simple PV adjustment hypothesis is inadequate. PV anomalies in the stratosphere induce, by inversion, flow anomalies in the troposphere that do not coincide spatially with the tropospheric changes determined by the numerical experiments. Moreover, the tropospheric anomalies induced by PV inversion are on a larger scale than the changes found in the numerical experiments, which are linked to the Atlantic and Pacific storm-tracks. These findings imply that the impact of the stratospheric state on the troposphere is manifested through the impact on individual synoptic-scale systems and their self-organization in the storm-tracks. Changes in these weather systems in the troposphere are not merely synoptic-scale noise on a larger scale tropospheric response, but an integral part of the mechanism by which the state of the stratosphere impacts that of the troposphere.
Top co-authors
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Institutions
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1995–2009
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University of Reading
- Department of Meteorology
Reading, ENG, United Kingdom
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