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Met Office (meet + office)
Selected AbstractsThe development of a new dust uplift scheme in the Met Office Unified ModelÔMETEOROLOGICAL APPLICATIONS, Issue 4 2009D. Ackerley Abstract Aeolian mineral dust aerosol is an important consideration in the Earth's radiation budget as well as a source of nutrients to oceanic and land biota. The modelling of aeolian mineral dust has been improving consistently despite the relatively sparse observations to constrain them. This study documents the development of a new dust emissions scheme in the Met Office Unified ModelÔ (MetUM) based on the Dust Entrainment and Deposition (DEAD) module. Four separate case studies are used to test and constrain the model output. Initial testing was undertaken on a large dust event over North Africa in March 2006 with the model constrained using AERONET data. The second case study involved testing the capability of the model to represent dust events in the Middle East without being re-tuned from the March 2006 case in the Sahara. While the model is unable to capture some of the daytime variation in AERONET AOD there is good agreement between the model and observed dust events. In the final two case studies new observations from in situ aircraft data during the Dust Outflow and Deposition to the Ocean (DODO) campaigns in February and August 2006 were used. These recent observations provided further data on dust size distributions and vertical profiles to constrain the model. The modelled DODO cases were also compared to AERONET data to make sure the radiative properties of the dust were comparable to observations. Copyright © 2009 Royal Meteorological Society and Crown Copyright [source] Soil state and surface hydrology diagnosis based on MOSES in the Met Office Nimrod nowcasting systemMETEOROLOGICAL APPLICATIONS, Issue 2 2006R. N. B. Smith Abstract A system has been developed and made operational at the Met Office for the real-time diagnosis of soil state and surface hydrology. It is based on the Met Office Surface Exchanges Scheme (MOSES) modified to take account of unresolved soil and topographic heterogeneity when calculating surface runoff by incorporating a Probability Distributed Moisture (PDM) scheme developed by the Centre for Ecology and Hydrology. The implementation of MOSES-PDM in the Met Office's Nimrod nowcasting system is described. High resolution soil characteristics and land cover data, together with Nimrod's analyses of precipitation amount and type, cloud cover and near-surface atmospheric variables are used to drive MOSES-PDM. Hourly values of snowmelt, runoff, net surface radiation, evaporation, potential evaporation, soil temperature, soil moisture and soil moisture deficit are calculated on a 5 km grid. Copyright © 2006 Royal Meteorological Society. [source] MONIM: the new Met Office Night Illumination ModelMETEOROLOGICAL APPLICATIONS, Issue 3 2004S. J. Revell This paper describes a new model developed by the Met Office to predict night-time light levels. The Met Office Night Illumination Model (MONIM) predicts light levels both in the visible (photopic) range and in the waveband to which night vision goggles (NVGs) are sensitive. The model will be used operationally for support of night-time flying operations. The model is described in detail and its light-level forecasts are compared with observations. Copyright © 2004 Royal Meteorological Society [source] The application of AVHRR data for the detection of volcanic ash in a Volcanic Ash Advisory CentreMETEOROLOGICAL APPLICATIONS, Issue 4 2003S. C. Watkin A volcanic ash detection product using AVHRR data has been developed for use in the London Volcanic Ash Advisory Centre, operated by forecasters at the Met Office. The product is an image that shows the brightness temperature difference between two infrared channels, which can be used to discriminate between clouds containing volcanic ash particles and those containing water droplets or ice crystals. Factors such as water vapour, water droplet and ice crystal content, opacity, cloud top temperature, ash particle size, surface emissivity properties and instrumental effects all have an effect on the spectral signal represented in the image. AVHRR imagery has been used to study the volcanic clouds ejected from eruptions of Mount Etna (Italy) in July 2001, Hekla (Iceland) in February 2000 and Mount Spurr (USA) in September 1992 to demonstrate the application of this product during volcanic events. The volcanic ash detection product provides essential information that can help forecasters locate volcanic ash and give appropriate guidance through advisory statements to the aviation industry and thus help avoid dangerous and expensive encounters between aircraft and volcanic ash. Copyright © 2003 Royal Meteorological Society. [source] Wind speed-up in the Dover Straits with the Met Office New Dynamics ModelMETEOROLOGICAL APPLICATIONS, Issue 3 2003Rachel Anne Capon It is part of British sailing and forecasting folklore that the wind speed increases in the Dover Straits when there is an established wind-flow ,westerly/south-westerly or easterly/north-easterly ,along the English Channel. However the underlying mechanism of the phenomenon is unclear. We have used the Met Office ,New Dynamics' mesoscale model to perform a case study on an occasion when this phenomenon was observed in the Channel but not forecast well by the operational model, UM 4.5. Results are presented showing the sensitivity of forecasts to horizontal resolution (down to 2 km) and to vertical resolution. We probe the physical mechanism of the Channel jet by altering the surrounding orography and the land or sea surface roughness. Both the orography and the surface roughness are shown to influence the jet formation. Copyright © 2003 Royal Meteorological Society [source] Use of medium-range ensembles at the Met Office 2: Applications for medium-range forecastingMETEOROLOGICAL APPLICATIONS, Issue 3 2002M V Young The term ,medium range' is taken to refer to forecasts for lead times ranging from about 2 or 3 days ahead up to about 10 days ahead. A wide variety of numerical model products are available to the forecaster nowadays, and one of the most important of these is the ECMWF Ensemble Prediction System (EPS). This paper shows how forecasters at the Met Office use these products, in particular the EPS, in an operational environment in the production of medium-range forecasts for a variety of customers, and illustrates some of the techniques involved. Particular reference is made to the PREVIN post-processing system for the EPS which is described in the companion paper by Legg et al. (2002). Forecast products illustrated take the form of synoptic charts (produced primarily via Field Modification software), text guidance and other graphical formats. The probabilistic approach to forecasting is discussed with reference to various examples, in particular the application of the EPS in providing early warnings of severe weather for which risk assessment is increasingly important. A central theme of this paper is the vital role played by forecasters in interpreting the output from the models in terms of the likely weather elements, and using the EPS to help assess confidence levels for a particular forecast as well as possible alternative synoptic evolutions. Verification statistics are presented which demonstrate how the EPS helps the forecaster to add value to the wide range of individual deterministic model products and that furthermore, the forecaster can improve upon many probabilistic products derived directly from the ensemble. Copyright © 2002 Royal Meteorological Society. [source] Can 4D-Var use dynamical information from targeted observations of a baroclinic structure?THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 651 2010E. A. Irvine Abstract Targeted observations are generally taken in regions of high baroclinicity, but often show little impact. One plausible explanation is that important dynamical information, such as upshear tilt, is not extracted from the targeted observations by the data assimilation scheme and used to correct initial condition error. This is investigated by generating pseudo targeted observations which contain a singular vector (SV) structure that is not present in the background field or routine observations, i.e. assuming that the background has an initial condition error with tilted growing structure. Experiments were performed for a single case-study with varying numbers of pseudo targeted observations. These were assimilated by the Met Office four-dimensional variational (4D-Var) data assimilation scheme, which uses a 6 h window for observations and background-error covariances calculated using the National Meteorological Centre (NMC) method. The forecasts were run using the operational Met Office Unified Model on a 24 km grid. The results presented clearly demonstrate that a 6 h window 4D-Var system is capable of extracting baroclinic information from a limited set of observations and using it to correct initial condition error. To capture the SV structure well (projection of 0.72 in total energy), 50 sondes over an area of 1×106 km2 were required. When the SV was represented by only eight sondes along an example targeting flight track covering a smaller area, the projection onto the SV structure was lower; the resulting forecast perturbations showed an SV structure with increased tilt and reduced initial energy. The total energy contained in the perturbations decreased as the SV structure was less well described by the set of observations (i.e. as fewer pseudo observations were assimilated). The assimilated perturbation had lower energy than the SV unless the pseudo observations were assimilated with the dropsonde observation errors halved from operational values. Copyright © 2010 Royal Meteorological Society [source] Medium-range multimodel ensemble combination and calibrationTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 640 2009Christine Johnson Abstract As part of its contribution to The Observing System Research and Predictability Experiment (THORPEX), the Met Office has developed a global, 15 day multimodel ensemble. The multimodel ensemble combines ensembles from the European Centre for Medium-Range Weather Forecasts (ECMWF), Met Office and National Centers for Environmental Prediction (NCEP) and is calibrated to give further improvements. The ensemble post-processing includes bias correction, model-dependent weights and variance adjustment, all of which are based on linear-filter estimates using past forecast-verification pairs, calculated separately for each grid point and forecast lead time. Verification shows that the multimodel ensemble gives an improvement in comparison with a calibrated single-model ensemble, particularly for surface temperature. However, the benefits are smaller for mean-sea-level pressure (mslp) and 500 hPa height. This is attributed to the higher degree of forecast-error similarity between the component ensembles for mslp and 500 hPa height than for temperature. The results also show only small improvements from the use of the model-dependent weights and the variance adjustment. This is because the component ensembles have similar levels of skill, and the multimodel ensemble variance is already generally well calibrated. In conclusion, we demonstrate that the multimodel ensemble does give benefit over a single-model ensemble. However, as expected, the benefits are small if the ensembles are similar to each other and further post-processing gives only relatively small improvements. © Crown Copyright 2009. Reproduced with the permission of HMSO. Published by John Wiley & Sons Ltd. [source] Assimilation of IASI at the Met Office and assessment of its impact through observing system experimentsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 639 2009F. Hilton Abstract Observations from the Infrared Atmospheric Sounding Interferometer (IASI), onboard EUMETSAT's MetOp satellite, have been assimilated at the Met Office in global and regional numerical weather-prediction systems since 27 November 2007. Pre-operational trials of IASI assimilation in the global model during the summer of 2007 delivered a positive impact on forecasts approximately twice as large as that shown by the Atmospheric InfraRed Sounder (AIRS) on the EOS-Aqua satellite. A series of observing system experiments confirmed the relative performance of IASI and AIRS, and showed that impact from IASI is equivalent to a single Advanced Microwave Sounding Unit-A (AMSU-A) combined with a single Microwave Humidity Sounder (MHS). The results of an IASI assimilation trial for the winter of 2007 were consistent with those of the summer trial, although the impact was slightly lower overall. The assessment of impact is strongly dependent on the variables and methods chosen for verification: assimilation trials with the regional model showed similar improvements to the large-scale fields (e.g. mean-sea-level pressure and geopotential height) as seen in the global model, but no forecast impact was seen for variables such as visibility and rain-rate. © Crown Copyright 2009. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd. [source] Boundary layer mechanisms in extratropical cyclonesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 623 2007Robert J. Beare Abstract This paper revisits the mechanism for the interaction of the boundary layer with extratropical cyclones. Two diagnostic approaches are compared: Ekman pumping and potential vorticity. Ekman pumping derives from the boundary layer stress which induces convergence and ascent. boundary layer potential vorticity contains in a single quantity both the vorticity and stratification. These quantities are compared for an idealized extratropical cyclone life cycle simulated with the Met Office Unified Model. A significant component of the boundary layer stress and thus Ekman pumping at occlusion is forced by the cold conveyor-belt jet in the unstable boundary layer. In contrast, much of the boundary layer depth-averaged potential vorticity is contained within the stable warm-sector region. Inversion of the warm-sector PV indicates a small local deepening of about 2.5 hPa. Moreover, switching off the boundary layer mixing in the unstable cold sector has much more impact than in the stable warm sector. The sensitivity of the cyclone and its boundary layer to basic-state jet strength is then investigated. The maximum friction velocity scales closely with the initial maximum jet strength. This demonstrates the important role of the large-scale flow in organizing the boundary layer structure. Changes in the minimum pressure produced by altering the boundary layer parametrization correspond closely to changes in the surface stress averaged over the cyclone. Different operational changes to the boundary layer scheme produce small and compensating changes to the cyclone minimum pressure over three days. © Crown Copyright 2007. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd [source] Northern Hemisphere stratospheric summer from MIPAS observationsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 622 2007W. A. Lahoz 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 Northern Hemisphere during spring and summer 2003. A Pole-centred approach, together with sequences of vertical profiles along the viewing tracks of the MIPAS instrument, is used to interpret the data in the physically meaningful context of the evolving summertime high. During April the vortex break-up and build-up of the summertime high gives rise, in the mid-stratosphere, to a ,frozen-in' anticyclone (FrIAC), over the Pole, encircled by vortex fragments at ,50°N. As the summer moves on, the FrIACs and vortex fragments are gradually smoothed out but they persist in the mid- and upper stratosphere until July,August as roughly zonally symmetric W-shaped tracer isopleths. The persistence of the W shows the slowness of isentropic mixing processes at these levels during the summer. As the summertime high becomes dominant during June,August, net photochemical ozone loss produces a low ozone pool in the lower and mid-stratosphere. Finally, as the summertime high decays and the wintertime polar vortex builds up from September onward, the low ozone pool extends vertically throughout the stratosphere, and the tracer isopleths at high latitudes start to dip, showing the effects of wintertime diabatic descent. Of these features, to our knowledge, the W-shaped tracer isopleths have not been observed previously. Copyright © 2007 Royal Meteorological Society [source] Orographic flow-blocking scheme characteristicsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 603 2004A. R. Brown Abstract The characteristics of the orographic flow blocking parametrization used in the Met Office Unified Model are examined in a one-dimensional framework. This approach allows feedback of the parametrized stresses on the winds resulting, in some cases, in stresses very different from those that would be obtained without this feedback. The key controlling non-dimensional parameters are identified, and the implications of the results for the use of the scheme in climate and numerical weather-prediction models are discussed. Copyright © 2004 Royal Meteorological Society [source] The extratropical transition of hurricane Irene (1999): A potential-vorticity perspectiveTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 598 2004A. Agusti-panareda Abstract Extratropical transition (ET) of tropical cyclones is common in all ocean basins where tropical cyclones recurve polewards. After the tropical cyclone experiences ET, a rapid deepening can take place resulting in the development of a very large and deep extratropical cyclone. The ET of hurricane Irene (1999) was an example of such an ,explosive' ET. Irene formed in the Caribbean and experienced ET as it moved poleward, resulting in a low-pressure system which deepened 39 hPa in 24 hours (according to the Met Office analyses). The extent to which the hurricane was responsible for the explosive extratropical development has been determined by performing Met Office Unified Model forecasts from initial states with and without the hurricane. The circulation and temperature anomalies associated with the hurricane were removed from the initial state using potential-vorticity inversion. The moisture anomaly co-located with the hurricane core was also removed. The results show that an extratropical cyclogenesis event takes place regardless of the presence of the hurricane in the initial conditions. However, the hurricane makes a significant difference to the track and central mean-sea-level pressure evolution of the resulting extratropical cyclone. When Irene was present the track of the extratropical cyclone was more zonal and the cyclone deepening rate was twice as fast as when Irene was not present. These effects appear to be particularly associated with a negative potential-vorticity anomaly and enhanced divergent flow in the region of the upper-level outflow of the transforming hurricane rather than with the hurricane vortex. Results also show that the presence of the hurricane resulted in a significant downstream surface-low development in the eastern Atlantic. Copyright © 2004 Royal Meteorological Society. [source] A high-resolution modelling case study of a severe weather event over New ZealandATMOSPHERIC SCIENCE LETTERS, Issue 3 2008Stuart Webster Abstract In this article, the ability of the Met Office Unified Model to simulate the severe weather over the South Island of New Zealand, on 8 January 2004 is investigated. Simulations were run at horizontal resolutions of between 60 and 1 km. The modelled broad-scale rainfall and wind features, most notably the area-averaged accumulated rainfall, were found to converge with resolution. At the highest resolutions, all the observed rainfall and wind features of this event were captured well by the model. Even the 12-km-resolution model is able to resolve the broad elongated ridge-like structure of the Southern Alps and qualitatively capture the main features of the rainfall and wind fields. Copyright © 2008 Royal Meteorological Society and Crown Copyright 2008, published by John Wiley & Sons, Ltd. [source] Use of a stochastic precipitation nowcast scheme for fluvial flood forecasting and warningATMOSPHERIC SCIENCE LETTERS, Issue 1 2005Clive Pierce Abstract In collaboration with the Bureau of Meteorology (Melbourne, Australia), the Met Office (Joint Centre for Hydro,Meteorological Research, UK) has developed a stochastic precipitation nowcast scheme, designed to model and predict the PDF of surface rain rate and rain accumulation in space and time. Here we demonstrate the range of probabilistic products generated by the scheme, and their potential applications for fluvial flood forecasting and warning. With the aid of a hydrological model (the PDM), we consider the use of ensembles of predicted catchment rain accumulation in evaluating the range of possible river flow responses from a given catchment. When employed in conjunction with a catchment specific, cost-based decision-making model, we highlight the value of PDFs of forecast catchment rainfall accumulation and river flow as an aid to objective decision making within the flood warning process. Crown Copyright 2005. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd. [source] Singular vectors and excess growths in semi-implicit non-hydrostatic modelsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 619 2006T. J. PAYNE Abstract In this note we show how the use of a semi-implicit time discretization in a linearized non-hydrostatic model can permit very large non-modal perturbation energy growth. This occurs in a single normal-length time step when a hydrostatically unbalanced increment at initial time is mapped to an approximately balanced increment. The energy growth is typically far larger than that of the leading meteorological singular vector (SV) over the course of a 12-hour optimization time interval so, unless action is taken, the leading SVs obtained with this discretization are meteorologically spurious. We show that the excess growths can be prevented by imposing a condition on the relation between the predictor (explicit) step and corrector (implicit) step of the semi-implicit method. We apply this condition to the Met Office's ,perturbation forecast' model to obtain SVs with admissible growth rates. Copyright © 2006 Royal Meteorological Society [source] | ||||||||||