UK Meteorological Office Unified Research Articles

The usability of 250 m resolution data from the UK Meteorological Office Unified Model as input data for a hydrological model

AbstractThis article examines whether very‐high resolution (250 m) data provided by the United Kingdom Meteorological Office Unified Model (UM) could be used as an input to a hydrological model for hydrological forecasting purposes. A summer convective event and a winter stratiform event are both considered and it is found that in these two cases, despite errors in both the positioning and timing of storms the predictions were of a sufficient quality to provide beneficial inputs for hydrological modelling. There are encouraging indications that this technique may be valuable in improving flood forecasting generally. Copyright © 2008 Royal Meteorological Society

Seasonal cycles of mixing ratio and 13C in atmospheric methane at Suva, Fiji

A series of clean air samples has been collected at a coastal site near Suva, Fiji (18°08′S, 178°26′E) by researchers at the University of the South Pacific. These samples, covering the period 1994 to mid‐2002, have been analyzed for methane mixing ratio and δ13C and provide the first ever time series of these species reported for this part of the tropical South Pacific. The data show large variability when compared to similar time series of the same species measured farther south in the extratropical Pacific. In particular, summer variability at the Fiji site is high, especially through La Niña conditions. A modeling study was carried out using a modified version of the UK Meteorological Office's Unified Model (a general circulation model) and TM2 (a chemical transport model driven by stored meteorological fields). These showed that a large amount of the variability in the methane mixing ratio and its δ13C can be attributed to complex tropical meteorology in the region changing the rate of transport of methane from the Northern into the Southern Hemispheres. Enhanced interhemispheric transport occurred during the summer months, especially during La Niña conditions which lead to the suppression of expected minima in the methane mixing ratio caused by OH oxidation. Although enriched signals in δ13C were expected at the site caused by intrusions of methane emitted from tropical biomass burning in Indonesia, relatively few of these events could be identified in the time series.

Predictive skill of the UKMO Unified Model in the lower stratosphere

This paper studies the short-term (up to 10 days) predictive skill of the UK Meteorological Office Unified Model (UM) in the lower stratosphere by applying conventional verification statistics used in numerical weather prediction studies. Simulations from four versions of the UM, each of them having a different representation of the stratosphere, are used to assess the dependence of the model's skill on stratospheric vertical resolution, location of the highest model full-level and location of the highest model half-level. Results are shown for two winter periods: February 1994 (northern winter with a weak wave-number two warming) and October 1994 (southern winter with a typical top-down break-up of the stratospheric polar vortex). the results of this paper indicate that the model's skill in the lower stratosphere depends on a number of factors, including flow structure, stratospheric vertical resolution and the location of the highest model full-level. By contrast, the model forecast skill in the mid troposphere is insensitive to the stratospheric representation on the 10-day time-scale. the highest skill (10 days and above for temperature and geopotential height at 50 hPa) is associated with steady and deep stratospheric flows which span the upper troposphere. the poorest skill is associated with rapidly varying flows; flows with shallow vertical structure also tend to have relatively poor skill. In all cases the model's skill is a significant improvement over persistence. the model has higher forecast skill in the lower stratosphere than in the mid troposphere; it also has higher skill in northern winter than in southern winter: these differences in skill can be ascribed to the flow regime in the lower stratosphere being dominated by lower wave numbers than in the mid troposphere, and to larger initialization errors in the southern hemisphere. It is shown that higher stratospheric vertical resolution does not necessarily give higher skill in the lower stratosphere; this is particularly true in northern winter. the model results show evidence of links between tropospheric and stratospheric errors.

The simulation of the Asian summer monsoon, and its sensitivity to horizontal resolution, in the UK meteorological office unified model

AbstractThe quality of the simulation of the Asian summer monsoon in the climate version of the UK Meteorological Office's Unified Model, and the impact upon this of increased horizontal resolution, is investigated using two atmosphere‐only model runs forced with observed sea surface temperatures (SSTs) and sea ice extents. the runs each cover the period from 1979 to 1988, but have different horizontal resolutions, with one at climate resolution (2.5° latitude by 3.75° longitude, about 300 km at midlatitudes) and the other at global forecast model resolution (0.833° latitude by 1.25° longitude, approximately 100 km at midlatitudes).The characteristic monsoon circulation and the spatial distribution of precipitation are in good agreement with observations. However, the model has a tendency to overestimate the strength of the monsoon, and also exhibits an early monsoon onset. the large‐scale interannual variations in circulation appear to be simulated reasonably well (as far as can be determined using this short dataset), although the magnitude of the interannual variability of precipitation is overestimated. However, the regional circulation and precipitation changes between El Niño and La Niña years show some significant differences between the model and the observations. the dominant mode of intraseasonal variability seen in both model simulations is, in agreement with observations, associated with the active/break cycle of the monsoon (although this only explains about 10% of the total variance in both simulations). There is some evidence that the SST changes associated with El Niño may produce a coherent forcing of the secondary (east‐west) mode of intraseasonal variability during the onset phase of the monsoon in the model. However, comparison with observations suggests that this may not be representative of what occurs in the real atmosphere. There is no evidence that the SST variations are causing the system to prefer either the active or the break monsoon phase, as was suggested by Palmer.With increased model horizontal resolution, extra detail is provided in the precipitation distribution, but the mean monsoon simulation is scarcely altered and the systematic errors remain. the interannual variations in circulation and precipitation appear not to be greatly altered, and the overall pattern of intraseasonal variability is also unaffected. This study suggests that the systematic errors in the monsoon simulation are not a result of poor horizontal resolution, but may be due to problems with the model physics.

A microphysically based precipitation scheme for the UK meteorological office unified model

AbstractMany numerical weather prediction models are moving towards more prognostic schemes for the prediction of ice and liquid water contents within clouds. This paper describes a large‐scale cloud and precipitation scheme developed for the UK Meteorological Office's Unified Model. It uses physically based transfer equations to predict ice as a prognostic variable. We review similar schemes and then describe our new scheme, giving examples of its performance in mesoscale forecasts compared with the current operational scheme and with observations. the microphysical processes occurring in a frontal cloud are well modelled. the prediction of supercooled stratocumulus cloud is much improved.

The simulation of the Asian summer monsoon, and its sensitivity to horizontal resolution, in the UK Meteorological Office Unified Model

The simulation of the Asian summer monsoon, and its sensitivity to horizontal resolution, in the UK Meteorological Office Unified Model

Predictive skill of the UKMO unified model in the lower stratosphere

AbstractThis paper studies the short‐term (up to 10 days) predictive skill of the UK Meteorological Office Unified Model (UM) in the lower stratosphere by applying conventional verification statistics used in numerical weather prediction studies. Simulations from four versions of the UM, each of them having a different representation of the stratosphere, are used to assess the dependence of the model's skill on stratospheric vertical resolution, location of the highest model full‐level and location of the highest model half‐level. Results are shown for two winter periods: February 1994 (northern winter with a weak wave‐number two warming) and October 1994 (southern winter with a typical top‐down break‐up of the stratospheric polar vortex). the results of this paper indicate that the model's skill in the lower stratosphere depends on a number of factors, including flow structure, stratospheric vertical resolution and the location of the highest model full‐level. By contrast, the model forecast skill in the mid troposphere is insensitive to the stratospheric representation on the 10‐day time‐scale. the highest skill (10 days and above for temperature and geopotential height at 50 hPa) is associated with steady and deep stratospheric flows which span the upper troposphere. the poorest skill is associated with rapidly varying flows; flows with shallow vertical structure also tend to have relatively poor skill. In all cases the model's skill is a significant improvement over persistence. the model has higher forecast skill in the lower stratosphere than in the mid troposphere; it also has higher skill in northern winter than in southern winter: these differences in skill can be ascribed to the flow regime in the lower stratosphere being dominated by lower wave numbers than in the mid troposphere, and to larger initialization errors in the southern hemisphere. It is shown that higher stratospheric vertical resolution does not necessarily give higher skill in the lower stratosphere; this is particularly true in northern winter. the model results show evidence of links between tropospheric and stratospheric errors.

A microphysically based precipitation scheme for the UK Meteorological Office Unified Model

A microphysically based precipitation scheme for the UK Meteorological Office Unified Model

Analysis of an ex‐tropical cyclone after its reintensification as a warm‐core extratropical cyclone

AbstractEx‐hurricane Lili reintensified as an extratropical cyclone before travelling across the data‐rich region of the British Isles on 28 October 1996. The cyclone centre passed close to a Mesosphere‐Stratosphere‐Troposphere (MST) radar, providing continuous profiles of wind etc. which were used to evaluate diagnostics from the mesoscale version of the operational UK Meteorological Office Unified Model. The paper presents a mesoanalysis of the mature extratropical cyclone using model output together with radar and satellite observations. The combined analysis reveals a vertically extensive warm core three‐quarters surrounded by a low‐level jet reaching over 40 m s−1. There was an associated eye, relatively free of cloud and partly surrounded by a hook cloud producing extensive heavy rain, which was itself encircled by cooler dry‐intrusion air. The stratospheric part of the dry intrusion (and its potential‐vorticity (PV) anomaly) descended within a tropopause fold around the cloud hook generally to below 400 hPa, with small pockets penetrating significantly lower. The cyclone's reintensification as an extratropical cyclone was related to its interaction with the stratospheric PV anomaly. This interaction commenced immediately after the decay of the strong moist ascent and associated deep column of diabatically generated positive PV that had characterized the earlier tropical‐cyclone phase. Following reintensification, the dry‐intrusion air entered the eye region of the extratropical cyclone over a deep layer. The mesoscale model represented many aspects of the cyclone structure well but it underestimated the dryness of the dry‐intrusion air entering the eye. The MST radar vividly depicted the region of moist boundary‐layer air responsible for the hook cloud rising up into the region of the lowered tropopause.

Analysis of an ex-tropical cyclone after its reintensification as a warm-core extratropical cyclone

Ex-hurricane Lili reintensified as an extratropical cyclone before travelling across the data-rich region of the British Isles on 28 October 1996. The cyclone centre passed close to a Mesosphere-Stratosphere-Troposphere (MST) radar, providing continuous profiles of wind etc. which were used to evaluate diagnostics from the mesoscale version of the operational UK Meteorological Office Unified Model. The paper presents a mesoanalysis of the mature extratropical cyclone using model output together with radar and satellite observations. The combined analysis reveals a vertically extensive warm core three-quarters surrounded by a low-level jet reaching over 40 m s−1. There was an associated eye, relatively free of cloud and partly surrounded by a hook cloud producing extensive heavy rain, which was itself encircled by cooler dry-intrusion air. The stratospheric part of the dry intrusion (and its potential-vorticity (PV) anomaly) descended within a tropopause fold around the cloud hook generally to below 400 hPa, with small pockets penetrating significantly lower. The cyclone's reintensification as an extratropical cyclone was related to its interaction with the stratospheric PV anomaly. This interaction commenced immediately after the decay of the strong moist ascent and associated deep column of diabatically generated positive PV that had characterized the earlier tropical-cyclone phase. Following reintensification, the dry-intrusion air entered the eye region of the extratropical cyclone over a deep layer. The mesoscale model represented many aspects of the cyclone structure well but it underestimated the dryness of the dry-intrusion air entering the eye. The MST radar vividly depicted the region of moist boundary-layer air responsible for the hook cloud rising up into the region of the lowered tropopause.

Middle atmosphere variability in the UK Meteorological Office Unified Model

AbstractThis 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.

Middle atmosphere variability in the UK Meteorological Office Unified Model

Middle atmosphere variability in the UK Meteorological Office Unified Model

A new gravity-wave-drag scheme incorporating anisotropic orography and low-level wave breaking: Impact upon the climate of the UK Meteorological Office Unified Model

A new anisotropic gravity-wave-drag parametrization-scheme which represents high-drag states modelled on hydraulic jump, flow blocking and internal-wave-reflection theory, including trapped lee-waves, is presented. The scheme is shown to represent the breaking of waves over mountains better than previous schemes by comparison with mesoscale simulations of PYREX case studies. Extended simulations of the Meteorological Office Unified Model at climate resolution are presented, showing the impact of the new scheme and a combination of the scheme with a new orographic-roughness parametrization. Results show a changed distribution of surface-gravity-wave surface-stress, mostly due to mountain anisotropy, and a greater tendency for lower-troposphere wavc-drag. Inclusion of the orographic-roughness parametrization halves the gravity-wave stress, the combined effect of these schemes for climate integrations being a slight improvement to the tropospheric flow.

A new gravity‐wave‐drag scheme incorporating anisotropic orography and low‐level wave breaking: Impact upon the climate of the UK Meteorological Office Unified Model

AbstractA new anisotropic gravity‐wave‐drag parametrization‐scheme which represents high‐drag states modelled on hydraulic jump, flow blocking and internal‐wave‐reflection theory, including trapped lee‐waves, is presented. The scheme is shown to represent the breaking of waves over mountains better than previous schemes by comparison with mesoscale simulations of PYREX case studies. Extended simulations of the Meteorological Office Unified Model at climate resolution are presented, showing the impact of the new scheme and a combination of the scheme with a new orographic‐roughness parametrization. Results show a changed distribution of surface‐gravity‐wave surface‐stress, mostly due to mountain anisotropy, and a greater tendency for lower‐troposphere wavc‐drag. Inclusion of the orographic‐roughness parametrization halves the gravity‐wave stress, the combined effect of these schemes for climate integrations being a slight improvement to the tropospheric flow.

A mesoscale simulation of land surface heterogeneity from HAPEX-Sahel

A mesoscale version of the UK Meteorological Office Unified Model has been used to study the evolution of the planetary boundary layer over heterogeneous terrain in the HAPEX-Sahel region. The model simulates a typical diurnal cycle during the dry down period of the intensive observation period on 8 October 1992. The model has a grid length of 10 km, with a domain covering a region of approximately 1000 km 2, centred on the HAPEX experiment. Satellite images are used to generate a mesoscale land cover map over the region. To initialise the soil moisture, a simple soil moisture accounting model was developed. This generates soil moisture patterns from daily rainfall estimates over the entire mesoscale domain. Where possible, the land surface scheme has been calibrated with measurements taken during the experiment. Moisture availability is the principle component in determining the partition of energy at the surface. The mean midday Bowen ratio of the square is 0.82. However, spatial contrasts in soil moisture availability lead to a range of midday sensible heat fluxes of between 100 W m −2 and 280 W m −2. The pattern of surface fluxes is closely related to the mesoscale variation of the seasonal rainfall. Comparisons of the model output with surface meteorology, surface fluxes, sonde and aircraft measurements are encouraging. Both model and climate observations show similar gradients at the surface across the square. Features of the aircraft measurements are well represented by the model. Anomalous increases in latent heat flux with height measured by the aircraft are also found in the model. This may be related to high entrainment fluxes at the inversion separating contrasting air flows. Heat and moisture budget calculations show the importance of large scale flow in the evolution of the boundary layer. Periods of low level south-westerly flow for example produce considerably larger inputs of moisture into the planetary boundary layer than surface evaporation. In this study, the influence of surface flux variability on the boundary layer is limited. However, scaling surface fluxes up to larger scales must take some account of the considerable variation in soil moisture.

The sensitivity of climate simulations to the specification of mixed phase clouds

In the prognostic stratiform cloud scheme used in the UK Meteorological Office Unified Model, mixed phase clouds are assumed to exist between 0 and −15 °C. Recent observations of such clouds, carried out using the C-130 aircraft of the Meteorological Research Flight, suggest that a smaller range of 0 to −9°C may be more appropriate. The sensitivity of cloud and radiation fields as simulated by a 5 × 7.5° latitude-longitude version of the Unified Model to such a reduction in the temperature range of mixed phase clouds is considered. Using a smaller temperature range systematic errors in the radiation budget of the model are reduced in mid-latitudes, bringing the model into closer agreement with ERBE data. The sensitivity of model albedo to an increase in the temperature range over which mixed phase clouds are assumed to exist, suggested by previous observational studies, is also considered together with the impact of removing the mixed phase part of the precipitation parametrization altogether.

Studies with a flexible new radiation code. I: Choosing a configuration for a large‐scale model

AbstractA comprehensive new radiation code based on the two‐stream equations in both the long‐wave and short‐wave spectral regions is described. The spectral resolution of the code is variable, enabling it to be used in a wide range of applications. Because of its flexibility, the code is well‐suited to the investigation of the sensitivity of radiative calculations to changes in the way in which physical processes are parametrized. The gaseous transmission data are derived from a line‐by‐line model. Particular attention is directed towards the treatment of the water vapour continuum, the overlap between gases, and the sensitivity to changing the carbon dioxide concentrations.The performance of the code is examined both at high spectral resolution and in a lower‐resolution configuration designed for the UK Meteorological Office Unified Forecast/Climate Model (UM). Particularly for use in the UM, the code must be shown to perform satisfactorily across the whole range of atmospheric conditions. Comparisons are therefore made with reference calculations in both the long‐wave and the short‐wave, in clear and cloudy skies, and the accuracy with which various processes may be represented is studied.For the cloudy calculations in the short‐wave, a new method is presented for deriving the single‐scattering properties in broad bands, based on the analytic expression for the reflectivity of an optically thick cloud. This minimizes the errors in calculating the short‐wave radiative properties of water clouds when the spectral resolution is reduced to that designed for the UM. In contrast, for ice clouds the errors are minimized by deriving the single‐scattering properties using linear averaging, as appropriate for optically thin clouds. In the long‐wave, the vertical distribution of the radiative heating in cirrus clouds is examined at high spectral resolution. The effect of scattering of long‐wave radiation, usually ignored in large‐scale models, is examined in some detail and is explained using a simple model. Taking all these studies into account, it is concluded that the configuration designed for the UM retains the generality of the code, without significantly compromising the overall accuracy.