Showing papers in "Quarterly Journal of the Royal Meteorological Society in 2007"

On the use and significance of isentropic potential vorticity maps

Abstract: The two main principles underlying the use of isentropic maps of potential vorticity to represent dynamical processes in the atmosphere are reviewed, including the extension of those principles to take the lower boundary condition into account. the first is the familiar Lagrangian conservation principle, for potential vorticity (PV) and potential temperature, which holds approximately when advective processes dominate frictional and diabatic ones. the second is the principle of ‘invertibility’ of the PV distribution, which holds whether or not diabatic and frictional processes are important. the invertibility principle states that if the total mass under each isentropic surface is specified, then a knowledge of the global distribution of PV on each isentropic surface and of potential temperature at the lower boundary (which within certain limitations can be considered to be part of the PV distribution) is sufficient to deduce, diagnostically, all the other dynamical fields, such as winds, temperatures, geopotential heights, static stabilities, and vertical velocities, under a suitable balance condition. the statement that vertical velocities can be deduced is related to the well-known omega equation principle, and depends on having sufficient information about diabatic and frictional processes. Quasi-geostrophic, semigeostrophic, and ‘nonlinear normal mode initialization’ realizations of the balance condition are discussed. an important constraint on the mass-weighted integral of PV over a material volume and on its possible diabatic and frictional change is noted. Some basic examples are given, both from operational weather analyses and from idealized theoretical models, to illustrate the insights that can be gained from this approach and to indicate its relation to classical synoptic and air-mass concepts. Included are discussions of (a) the structure, origin and persistence of cutoff cyclones and blocking anticyclones, (b) the physical mechanisms of Rossby wave propagation, baroclinic instability, and barotropic instability, and (c) the spatially and temporally nonuniform way in which such waves and instabilities may become strongly nonlinear, as in an occluding cyclone or in the formation of an upper air shear line. Connections with principles derived from synoptic experience are indicated, such as the ‘PVA rule’ concerning positive vorticity advection on upper air charts, and the role of disturbances of upper air origin, in combination with low-level warm advection, in triggering latent heat release to produce explosive cyclonic development. In all cases it is found that time sequences of isentropic potential vorticity and surface potential temperature charts—which succinctly summarize the combined effects of vorticity advection, thermal advection, and vertical motion without requiring explicit knowledge of the vertical motion field—lead to a very clear and complete picture of the dynamics. This picture is remarkably simple in many cases of real meteorological interest. It involves, in principle, no sacrifices in quantitative accuracy beyond what is inherent in the concept of balance, as used for instance in the initialization of numerical weather forecasts.

Stochastic representation of model uncertainties in the ECMWF ensemble prediction system

Abstract: SUMMARY A stochastic representation of random error associated with parametrized physical processes (‘stochastic physics’) is described, and its impact in the European Centre for Medium-Range Weather Forecasts Ensemble Prediction System (ECMWF EPS) is discussed. Model random errors associated with physical parametrizations are simulated by multiplying the total parametrized tendencies by a random number sampled from a uniform distribution between 0.5 and 1.5. A number of diagnostics are described and a choice of parameters is made. It is shown how the scheme increases the spread of the ensemble, and improves the skill of the probabilistic prediction of weather parameters such as precipitation. A choice of stochastic parameters is made for operational implementation. The scheme was implemented successfully in the operational ECMWF EPS on 21 October 1998.

The ECMWF operational implementation of four-dimensional variational assimilation. I: Experimental results with simplified physics

Abstract: A comprehensive set of physical parametrizations has been linearized for use in the European Centre for Medium-Range Weather Forecasts (ECMWF's) incremental four-dimensional variational (4D-Var) system described in Part I. The following processes are represented: vertical diffusion, subgrid-scale orographic effects, large-scale precipitation, deep moist convection and long-wave radiation. The tangent-linear approximation is examined for finite-size perturbations. Significant improvements are illustrated for surface wind and specific humidity with respect to a simplified vertical diffusion scheme. Singular vectors computed over 6 hours (compatible with the 4D-Var assimilation window) have lower amplification rates when the improved physical package is included, due to a more realistic description of dissipative processes, even though latent-heat release contributes to amplify the potential energy of perturbations in rainy areas. A direct consequence is a larger value of the observation term of the cost-function at the end of the minimization process when improved physics is included in 4D-Var. However, the larger departure of the analysis state from observations in the lower-resolution inner-loop is in better agreement with the behaviour of the full nonlinear model at high resolution. More precisely, the improved physics produces smaller discontinuities in the value of the cost-function when going from low to high resolution. In order to reduce the computational cost of the linear physics, a new configuration of the incremental 4D-Var system using two outer-loops is defined. In a first outer-loop, a minimization is performed at low resolution with simplified physics (50 iterations), while in the second loop a second minimization is performed with improved physics (20 iterations) after an update of the model trajectory at high resolution. In this configuration the extra cost of the physics is only 25%, and results from a 2-week assimilation period show positive impacts in terms of quality of the forecasts in the Tropics (reduced spin-down of precipitation, lower root-mean-square errors in wind scores). This 4D-Var configuration with improved physics and two outer-loops was implemented operationally at ECMWF in November 1997.

The Development and Verification of A Cloud Prediction Scheme For the Ecmwf Model

Abstract: This paper describes the development of a fractional cloud cover scheme which was implemented operationally in the ECMWF medium range forecast model in May 1985. the scheme is based on a diagnostic approach in which cloudiness is related empirically to the large-scale model variables, including convective activity. an example of the performance of the scheme is given, showing that a fair degree of skill is achieved in forecasting tropical and extratropical cloudiness. an attempt to verify the results using retrieved cloudiness from Nimbus 7 is described. Considerable difficulties were experienced due to the different height classifications of clouds in the model and in the satellite data. the importance of cloud radiative properties as well as cloud amount is considered briefly, based on a comparison of model earth radiation budget diagnostics with similar data from the NOAA polar-orbiting satellites. the sensitivity of the simulated outgoing long-wave radiance to changes in the prescribed cloud liquid water content is discussed. Two examples of the effects of cloudradiation interaction on boundary layer processes are described which demonstrate the importance of an integrated approach to the treatment of clouds, radiation and turbulent fluxes.

Review of atmospheric turbulence over cities

Abstract: This paper provides a comprehensive, critical review of turbulence observations over cities. More than fifty studies are analysed with their experimental conditions summarized in an appendix. The main results are based on 14 high-quality experiments which met criteria based on stringent experimental requirements. The observations are presented as non-dimensional statistics to facilitate comparison between urban studies and work conducted over other rough, inhomogeneous surfaces. Wake production associated with bluff bodies, and the inhomogeneous distribution of sources and sinks of scalars, result in a roughness sub-layer which for the studies reviewed extends to about 2.5 to 3 times the height of the buildings. It is shown that within this region the basis of several traditional micrometeorological approaches to describe the turbulent exchange is in doubt. There are strong similarities to flow over plant canopies, and many of the turbulence characteristics can be interpreted in the framework of a plane mixing layer. Future field observations should concentrate on the turbulent exchange near the top and within the urban canopy as well as within the urban boundary layer.

The Met Office global four‐dimensional variational data assimilation scheme

Abstract: The Met Office has developed a 4D-Var data assimilation system, which was implemented in the global forecast system on 5 October 2004. This followed a development path based on the previous 3D-Var configuration, with many aspects kept in common. A 4D-Var capability was provided by the introduction of a linear perturbation forecast model based on the Unified Model, the non-hydrostatic grid-point model producing our operational forecasts. There were clear advantages in verification of forecasts compared to the equivalent 3D-Var configuration, with an improvement of 2.6% in a composite skill score verified against observations during pre-operational trials. The largest differences in model evolution occur in storm-track regions in the extratropics. Overall, improvements in verification scores as measured against observations were larger than those measured against analyses, particularly at upper levels. There is an improvement in verification of surface parameters (10 m wind, 1.5 m temperature and relative humidity) against analyses. The strongest impact on fitting observations is seen for satellite radiances with weighting functions peaking in the stratosphere and upper troposphere. The largest changes to assimilation increments occurred in the top model levels, particularly wind increments which became much larger near the model top. Similarities were found in the signal of 4D-Var versus 3D-Var for models at two different resolutions, from which we infer that low-resolution trials remain valid for exploring some aspects of 4D-Var before confirmation in full-scale tests. © Crown Copyright 2007. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd

Variational assimilation of meteorological observations with the adjoint vorticity equation. II: Numerical results

Abstract: The adjoint variational approach to data assimilation described in the first part of this paper is used, with the same vorticity equation model, to assimilate northern hemisphere radiosonde observations of wind and geopotential distributed over a 24-hour period. Except over the eastern Pacific Ocean, where no observations are available, the variational assimilation reconstructs all structures of the flow resolvable by the model to an accuracy of about 30 m for geopotential heights and 8 m s−1 for wind vectors. A particular structure, the Aleutian depression, is reconstructed even though it was not covered by the available observations. The assimilation produces unrealistic small-scale noise which can be reduced by adding an appropriate smoothing term to the distance function minimized in the variational process. Detailed study of the minimization strongly suggests that the distance function varies quadratically with respect to the model's initial conditions. This implies that the tangent linear equation of the model suffices to describe the 24-hour evolution of the forecast error.

Monsoon convection in the Himalayan region as seen by the TRMM Precipitation Radar

Abstract: Three-dimensional structure of summer monsoon convection in the Himalayan region and its overall variability are examined by analyzing data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar over the June–September seasons of 2002 and 2003. Statistics are compiled for both convective and stratiform components of the observed radar echoes. Deep intense convective echoes (40 dBZ echo reaching heights > 10 km) occur primarily just upstream (south) of and over the lower elevations of the Himalayan barrier, especially in the northwestern concave indentation of the barrier. The deep intense convective echoes are vertically erect, consistent with the relatively weak environmental shear. They sometimes extend above 17 km, indicating that exceptionally strong updraughts loft graupel to high altitudes. Occasionally, scattered isolated deep intense convective echoes occur over the Tibetan Plateau. Wide intense convective echoes (40 dBZ echo > 1000 km2 in horizontal dimension) also occur preferentially just upstream of and over the lower elevations of the Himalayas, most frequently in the northwestern indentation of the barrier. The wide intense echoes have an additional tendency to occur along the central portion of the Himalayas, and they seldom if ever occur over the Tibetan Plateau. The wide intense echoes exhibit three mesoscale structures: amorphous areas, lines parallel to the mountain barrier, and arc-shaped squall lines perpendicular to and propagating parallel to the steep Himalayan barrier. The latter are rare, generally weaker than those seen in other parts of the world, and occur when a midlevel jet is aligned with the Himalayan escarpment. Deep and wide intense convective echoes over the northwestern subcontinent tend to occur where the low-level moist layer of monsoon air from the Arabian Sea meets dry downslope flow, in a manner reminiscent of severe convection leeward of the Rocky Mountains in the central USA. As the low-level layer of moist air from the sea moves over the hot arid northwestern subcontinent, it is capped by an elevated layer of dry air advected off the Afghan or Tibetan Plateau. The capped low-level monsoonal airflow accumulates instability via surface heating until this instability is released by orographically induced lifting immediately adjacent to or directly over the foothills of the Himalayas. Broad (>50 000 km2 in area) stratiform echoes occur in the eastern and central portions of the Himalayan region in connection with Bay of Bengal depressions. Their centroids are most frequent just upstream of the Himalayas, in the region of the concave indentation of the barrier at the eastern end of the range. The steep topography apparently enhances the formation and longevity of the broad stratiform echoes. Monsoonal depressions provide a moist maritime environment for the convection, evidently allowing mesoscale systems to develop larger stratiform echoes than in the western Himalayan region. Copyright © 2007 Royal Meteorological Society

Adaptive bias correction for satellite data in a numerical weather prediction system

Abstract: Adaptive bias corrections for satellite radiances need to separate the observation bias from the systematic errors in the background in order to prevent the analysis from drifting towards its own climate. The variational bias correction scheme (VarBC) is a particular adaptive scheme that is embedded inside the assimilation system. VarBC is compared with an offline adaptive and a static bias correction scheme. In simulation, the three schemes are exposed to artificial shifts in the observations and the background. The VarBC scheme can be considered as a good compromise between the static and the offline adaptive schemes. It shows some skill in distinguishing between the background-error and the observation biases when other unbiased observations are available to anchor the system. Tests of VarBC in a real numerical weather prediction (NWP) environment show a significant reduction in the misfit with radiosonde observations (especially in the stratosphere) due to NWP model error. The scheme adapts to an instrument error with only minimal disruption to the analysis. In VarBC, the bias is constrained by the fit to observations—such as radiosondes—that are not bias-corrected to the NWP model. In parts of the atmosphere where no radiosonde observations are available, the radiosonde network still imposes an indirect constraint on the system, which can be enhanced by applying a mask to VarBC. Copyright © 2007 Royal Meteorological Society

A modelling and observational study of the relationship between sea surface temperature in the North‐West atlantic and the atmospheric general circulation

Abstract: Results from four pairs of 50-day wintertime integrations of the Meteorological Office's 5-level general circulation model, with warm and cold sea surface temperature (s.s.t.) anomalies of about 3 K in the northwest Atlantic, are described. Difference fields between the warm and cold integrations are statistically significant at the 1% level with positive geopotential height over the central north Atlantic, and weaker negative height over Europe. the storm track over the Atlantic is displaced from its normal position. Results from four further pairs of integrations with halved s.s.t. anomalies are also described. the response is approximately linear, with systematic differences in 500 mb geopotential height over the Atlantic, parts of which are just significant at the 10% level with half the full s.s.t. anomaly. Overall, however, the model's response is weaker than could be obtained with tropical s.s.t. anomalies of the same magnitude. Results from the model integrations are compared with results from an observational study of the relationship between wintertime s.s.t. in the north-west Atlantic, and mean sea level pressure and 500mb height. Two independent 30-year periods were chosen for study, thus minimizing the influence of long-term trends in s.s.t. Over the Atlantic and Europe the model results compared well with the observations. With s.s.t. data lagging the atmosphere by one month, the observational study appears to show that the s.s.t. anomalies are initially forced by perturbations in the atmospheric circulation. With s.s.t. data leading the atmosphere by one month results show that atmospheric and s.s.t. anomalies are most persistent in the period October to December. Throughout the winter these lagged relationships are much weaker and not statistically significant. Diagnostics of E-vector divergence from the GCM experiments are used to suggest that anomalous baroclinic wave activity over the Atlantic is important as a momentum forcing for the anomalous time-mean flow pattern. On the other hand, the role of thermal forcing, provided by anomalous diabatic heating and transient eddy heat flux convergence, may be important. to substantiate this statement, a simple linear steadystate two-layer model of the response to extratropical thermal forcing is described. With a suitable basic state flow, and a mid tropospheric heat source (given mainly by the transient eddy heat flux convergence), the response is shown to be equivalent barotropic with a downstream ridge and ascent over the thermal source. Conversely, results from an ocean mixed layer model are discussed which suggest that warm s.s.t. anomalies could be maintained by a positive surface pressure perturbation positioned downstream of the anomaly, through anomalous northward advection of warm ocean water by Ekman drift currents. This northward advection would balance the sensible and latent heat loss into the atmosphere over the s.s.t. anomaly. Hence it is possible that some positive ocean-atmosphere feedback may account for the persistence of such atmospheric and oceanic anomalies.

An objective definition of the Indian summer monsoon season and a new perspective on the ENSO–monsoon relationship

Abstract: The concept of an interannually varying Indian summer monsoon season is introduced here, considering that the duration of the primary driving of the Indian monsoon – the large-scale meridional gradient of the deep tropospheric heat source – may vary from one year to another. Onset (withdrawal) is defined as the day when the tropospheric heat source shifts from south to north (north to south). This physical principle leads to a new thermodynamic index of the seasonal mean monsoon. While the traditional measure of seasonal rainfall, averaged from 1 June to 30 September, indicates a breakdown of the ENSO–monsoon relationship in recent decades, it is argued that this breakdown is partly due to the inappropriate definition of a fixed monsoon season. With a new physically based definition of the seasonal mean, the ENSO–monsoon relationship has remained steady over the decades. El Nino (La Nina) events contract (expand) the season, and thus decrease (increase) the seasonal mean monsoon by setting up persistent negative (positive) tropospheric temperature (TT) anomalies over the southern Eurasian region. Thus, we propose a new pathway, whereby the Indian summer monsoon could be influenced by remote climatic phenomena via modification of TT over Eurasia. Diagnostics of the onset and withdrawal processes suggest that onset delay is due to the enhanced adiabatic zubsidence that inhibits vertical mixing of sensible heating from warm landmass during the pre-monsoon months. On the other hand, the major factor that determines whether the withdrawal is early or late is the horizontal advective cooling. Most of the late (early) onsets and early (late) withdrawals are associated with El Nino (La Nina). This link between the ENSO and the monsoon is realized through vertical and horizontal advections associated with the stationary waves in the upper troposphere set up by the tropical ENSO heating.

Lessons on orographic precipitation from the Mesoscale Alpine Programme

Abstract: Although moisture-laden airflow towards a mountain is a necessary ingredient, the results from the Mesoscale Alpine Programme (MAP) demonstrate that detailed knowledge of the orographically modified flow is crucial for predicting the intensity, location and duration of orographic precipitation. Understanding the orographically modified flow as it occurs in the Alps is difficult since it depends on the static stability of the flow at low levels, which is heavily influenced by synoptic conditions, the complex effects of latent heating, and the mountain shape, which has important and complicated variations on scales ranging from a few to hundreds of kilometres. Central themes in all of the precipitation-related MAP studies are the ways in which the complex Alpine orography influences the moist, stratified airflow to produce the observed precipitation patterns, by determining the location and rate of upward air motion and triggering fine-scale motions and microphysical processes that locally enhance the growth and fallout of precipitation. In this paper we review the major findings from the MAP observations and describe some new research directions that have been stimulated by MAP results. Copyright  2007 Royal Meteorological Society

Scandinavian pattern and its climatic impact

Abstract: a LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China b Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan Maintenance mechanisms of the Scandinavian teleconnection pattern and its possible impact on the Eurasian climate are investigated on the basis of long-term monthly data. Its upstream portion over the North Atlantic is forced and maintained by feedback forcing from transient eddies migrating along the nearby storm track, with an additional contribution from incoming Rossby wave activity from further upstream. The primary anomaly centre over the Scandinavian Peninsula and the downstream portion of the pattern manifest themselves basically as Rossby waves propagating towards central Siberia and east Asia, under the modest feedback forcing from transient eddies. The pattern shows apparent seasonality in its dynamical properties, including the wave-train orientation and wavelength, under the seasonally varying transient-eddy feedback forcing and waveguide structure for Rossby waves. In cold seasons, the positive phase of the pattern causes cold-air accumulation over a vast area extending from western Siberia to the regions around Lake Baikal and Lake Balkhash, while giving rise to decreased precipitation over northeastern Europe, western Siberia and some of the Arctic coastal regions. The pattern also changes the North Atlantic sea surface temperature differently between autumn and winter.

Flash flood forecasting: What are the limits of predictability?

Abstract: Flash floods may occur suddenly and be accompanied by other hazards such as landslides, mud flows, damage to infrastructure and even death. In the UK such events are comparatively rare occurring on average only once or twice per year. Warning systems must depend upon the accurate real-time provision of rainfall information, high-resolution numerical weather forecasts and the operation of hydrological model systems in addition to forecast delivery procedures not discussed in this paper. In this paper we review how flash floods are forecast considering the limitations and uncertainty involved in both the meteorological and hydrological aspects of forecasting systems. Data assimilation and the use of ensembles are both key elements across disciplines. Assessing the susceptibility of river catchments to extreme flooding is considered, and statistical methods of estimating the likelihood of extreme rainfall and floods within a changing climate are examined. Ways of constraining flash flood forecasts are noted as one way to improve forecast performance in the future. Copyright © 2007 Royal Meteorological Society

Using numerical weather prediction to assess climate models

Abstract: Estimates of climate change remain uncertain—hampering strategic decision making in many sectors. In large part this uncertainty arises from uncertainty in the computational representation of known physical processes. This model component of climate change uncertainty is increasingly being assessed using perturbed model experiments. Some such model perturbations have, for example, led to headline global warming estimates of as much as 12 °C. These experiments consider many differently perturbed versions of a given base model and assess the likelihood of each perturbed model's climate prediction based on how well it simulates present-day climate. In these experiments, the computational cost of the model assessment is extremely high unless one assumes that the climate anomalies associated with different model perturbations can be combined linearly. Here we demonstrate a different method, which harnesses the power of the data assimilation system to assess directly the perturbed physics of a model. Data assimilation involves the incorporation of daily observations to produce initial conditions (analyses) for numerical weather prediction (NWP). The method used here quantifies systematic initial tendencies in the first few time steps of a model forecast. After suitable temporal averaging, these initial tendencies imply systematic imbalances in the physical processes associated with model error. We show how these tendencies can be used to produce probability weightings for each model that could be used in the construction of probability distribution functions of climate change. The approach typically costs 5% of the cost of a 100-year coupled model simulation that might otherwise be used to assess the simulation of present-day climate. Importantly, since the approach is amenable to linear analysis, it could further reduce the cost of model assessment by several orders of magnitude: making the exercise computationally feasible. The initial tendency approach can only assess ‘fast physics’ perturbations, i.e. perturbations that have an impact on weather forecasts as well as climate. However, recent publications suggest that most of the present model parameter uncertainty is associated with fast physics. If such a test were adopted, assessment of the ability to simulate present-day climate would then only be required for models that ‘pass’ the fast physics test. The study highlights the advantages of a more seamless approach to forecasting that combines NWP, climate forecasting, and all scales in-between. Copyright © 2007 Royal Meteorological Society

Observation and background adjoint sensitivity in the adaptive observation-targeting problem

Abstract: Recent observation-targeting field experiments, such as the Fronts and Atlantic Storm-Track Experiment (FASTEX) and the NORth Pacific Experiment (NORPEX), have demonstrated that by using objective adjoint techniques it is possible, in advance, to identify regions of the atmosphere where forecast-error growth in numerical forecast models is maximally sensitive to the error in the initial conditions. 'Qpically, such techniques produce a field of the sensitivity of some aspect of the forecast to the analysis field. This analysis sensitivity field is then used to identify promising targets for the deployment of additional observations during the flight-planning phase of field experiments. While FASTEX and, particularly. NORPEX had a number of successful ‘hits’, where the addition of dropsondes reduced the forecast error, there were also failures. None of the objective techniques have involved any consideration of the characteristics of the data-assimilation systems used in the analysis of the targeted observations. In particular, the interaction with the background field, interactions with other observations, and the background- and observationerror characteristics have been ignored. This can lead to potential mis-sampling, conflict with other observations, and inefficient use of aircraft and expendables. In this study, the adjoint of a simplified data-assimilation system is used to determine directly the sensitivity of the forecast aspect to the observations and the background field. The procedure is illustrated by using simplified linear contexts such as the one- and two-dimensional horizontal univariate problem and the one-dimensional direct radiance assimilation problem. Adaptive targeting tools, such as a single-observation sensitivity map and a marginal observation sensitivity vector, are devised and tested. The possibility of determining when the forecast would be sensitive to the background field and/or the observations is demonstrated. Such dependencies are shown to be a function of the specified observation- and background-error variances, the characteristic scales of analysis sensitivity vector and background-error correlations, and the properties of forward (observation) operators. Although the present experiments concerned simplified assimilation systems and observation networks, extension of the technique to real situations is quite feasible. Obtaining the adjoint of a full three-dimensional variational assimilation system is straightforward; moreover, the target areas are small and contain relatively few observations so the computational requirements are modest. Finally, the data-assimilation adjoint theory can be used for a posteriori assessment of those sources of forecast error which are due to errors in the initial analysis.

Ice supersaturation in the ECMWF integrated forecast system

Abstract: A parametrization for ice supersaturation is introduced into the ECMWF Integrated Forecast System (IFS), compatible with the cloud scheme that allows partial cloud coverage. It is based on the simple, but often justifiable, diagnostic assumption that the ice nucleation and subsequent depositional growth time-scales are short compared to the model time step, thus supersaturation is only permitted in the clear-sky portion of the grid cell. Results from model integrations using the new scheme are presented, which is demonstrated to increase upper-tropospheric humidity, decrease high-level cloud cover and, to a much lesser extent, cloud ice amounts, all as expected from simple arguments. Evaluation of the relative distribution of supersaturated humidity amounts shows good agreement with the observed climatology derived from in situ aircraft observations. With the new scheme, the global distribution of frequency of occurrence of supersaturated regions compares well with remotely sensed microwave limb sounder (MLS) data, with the most marked errors of underprediction occurring in regions where the model is known to underpredict deep convection. Finally, it is also demonstrated that the new scheme leads to improved predictions of permanent contrail cloud over southern England, which indirectly implies upper-tropospheric humidity fields are better represented for this region.

The role of the ocean in the Madden-Julian Oscillation : Implications for MJO prediction

Abstract: The role of coupled processes in the Madden–Julian Oscillation (MJO) is investigated in the European Centre for Medium-Range Weather Forecasts Monthly Forecasting System. A series of forecasts initialized daily for 47 days during the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA-COARE) period are performed with sea surface temperatures (SSTs) provided by persistence of initial conditions, and coupling to either a full dynamical ocean model with vertical resolution in the upper ocean typical of coupled models (10 m), or, a 1D mixed layer ocean model with high (∼1 m) vertical resolution in the upper ocean. The experiment with the full dynamical model shows improved skill compared with the persisted SST experiment, indicating a role for coupled processes in the MJO. The experiment with the mixed layer model shows a further improvement in skill over the full dynamical ocean, particularly for the phases of the MJO where the convection is active over the Indian Ocean or West Pacific. This further improvement comes about from an enhanced sensitivity of the SST to the surface flux anomalies associated with the MJO. Additional sensitivity experiments reveal that the improved representation of the diurnal cycle which results from the increased vertical resolution is a significant factor in the improved skill of the experiments with the mixed layer model. Copyright © 2007 Royal Meteorological Society

A Study of the Diurnal Variation of Stratocumulus Using A Multiple Mixed Layer Model

Abstract: A multiple mixed layer model of the cloud-topped boundary layer is developed to investigate the diurnal variation of stratocumulus. A simple representation of the microphysical properties of the cloud layer is included which enables high resolution, interactive radiation calculations to be made. A constraint on the buoyancy flux profile is introduced which permits the previously well-mixed layer to separate into two independently driven layers, thereby avoiding many of the unrealistic aspects of single-layer models and enabling the model to reproduce features seen in observational studies. the diagnosis of this decoupling, which generally occurs during the morning, is discussed and is shown to have a seasonal and latitudinal sensitivity due to its dependence on cloud layer short-wave absorption. Comparisons of results with those obtained when separation is not permitted show significant differences. In particular, the cloud layer displays a much enhanced diurnal variation in thickness when separation is allowed, with the minimum occurring in the afternoon. These results are shown to produce much better agreement with observations. the likely consequences of separation for boundary layer evolution are discussed. the surface energy balance can be quite strongly affected, suggesting that it is important to resolve this diurnal variation, especially in areas where stratocumulus is a dominant climatological feature. The sensitivity of the results to model assumptions are investigated and the limitations of the model are also assessed.

On the boundary‐layer structure over highly complex terrain: Key findings from MAP

Abstract: Within MAP, one of the scientific projects was devoted to 'Boundary Layers in Complex Terrain'. In a number of subprojects, boundary-layer issues were addressed and detailed high-resolution multi-sensor observations were combined with simulation by models allowing for adequate parametrization of turbulence processes. In this contribution, the projects are briefly introduced and an attempt is made to summarize their key findings and to put them into a joint perspective. Spatial variability is found to be large but strictly related to topography and therefore allowing for possible parametrization. Traditional boundary-layer scaling approaches cannot simply be applied over highly complex topography, but some of the MAP findings suggest the potential for suitable extensions of those scaling relations to cover various cases of complex terrain. The mean boundary-layer structure and thermally driven flows in narrow valleys are found not to be generally in line with previous results from larger valleys elsewhere. Furthermore, local circulations are reported to contribute considerably to exchange between valley and free troposphere. In particular, the range of their effects on the lower atmosphere seems to be larger than just turbulent transport within the planetary boundary layer would suggest. Thus in larger-scale numerical models where the topography is not resolved, possible sub-grid parametrizations for local exchange seem to be in order. Copyright  2007 Royal Meteorological Society

Aerosol optical depth and Ångström exponent climatology at El Arenosillo AERONET site (Huelva, Spain)

Abstract: Routine aerosol measurements have been carried out at INTA–El Arenosillo (Huelva, Spain) since February 2000 with a Cimel sun photometer integrated in the global aerosol monitoring network AERONET. A 5-year aerosol database allows characterization and classification of aerosol properties, which define the local aerosol climatology. In this work two basic parameters for aerosol characterization, aerosol optical depth (AOD) and Angstrom exponent, α, are used for aerosol analysis. The mean AOD is 0.18 ( ± 0.14) and the mean α is 1.05 ( ± 0.43). The AOD presents two peaks during the year, at the end of the winter and during the summer, which are related to the seasonal patterns of the desert dust aerosols which arrive at the south-western Iberian Peninsula from North Africa. The Angstrom exponent presents two frequency modes, related to the two main aerosol types present at El Arenosillo: coastal marine aerosols and desert dust. A main aerosol scenario (66% of data) is defined as coastal marine aerosols, with the influence of local sources of continental and polluted aerosols. Continental aerosols are present in 11% of cases, while the desert dust has a very relevant occurrence in around 20% of the data. Copyright © 2007 Royal Meteorological Society

The new ECMWF VAREPS (Variable Resolution Ensemble Prediction System)

Abstract: The European Centre for Medium-Range Weather Forecasts (ECMWF) Variable Resolution Ensemble Prediction System (VAREPS) is a system designed to provide skilful predictions of small-scale, severe-weather events in the early forecast range, and accurate large-scale forecast guidance in the extended forecast range (say beyond forecast day 7). In this work, first the rationale behind VAREPS is presented, and then the performance of VAREPS with a truncation at forecast day 7, i.e. TL399L40(d0–7) and TL255L40(d7–15), is discussed and compared to the performance of two constant resolution systems, a TL255L40 and a TL319L40 (this latter one requires similar computing resources to VAREPS). Average results based on up to 111 cases indicate that VAREPS has a higher forecast-time-integrated skill, and it provides better forecasts in the early forecast range without losing accuracy in the long forecast range. In the early forecast range, the differences in forecast performance can be very large and responsible for substantial improvements in the prediction of weather variables such as surface wind, significant wave height and total precipitation, as was shown in two case-studies. Average results have also shown that the VAREPS extension to 15 days (the old EPS system was run operationally only up to forecast day 10) will provide users with some skilful extended-range forecasts. Copyright © 2007 Royal Meteorological Society

Tropical cyclogenesis sensitivity to environmental parameters in radiative–convective equilibrium

Abstract: In this study, the relationship between the likelihood of tropical cyclogenesis and external environmental forcings is explored in the simplest idealized modelling framework possible: radiative-convective equilibrium on a doubly periodic f-plane. In such an environment, control of the equilibrium environmental sounding is reduced to three parameters: the sea-surface temperature, the Coriolis parameter, and the imposed background surface wind speed. Cloud-resolving mesoscale model simulations are used to generate environments of radiative-convective equilibrium determined by these three factors. The favourability of these environments for tropical cyclogenesis is measured in three ways: in terms of the maximum potential intensity (MPI) of the sounding, based on the thermodynamic theory of Emanuel; in terms of the ‘genesis potential’ determined by an empirical genesis parameter; and in terms of the propensity of weak initial vortices in these environments to form into tropical cyclones. The simulated environments of radiative—convective equilibrium with no vertical wind shear are found to be very favourable for tropical cyclogenesis. Weak initial vortices always transition to a tropical cyclone, even for rather low sea-surface temperatures. However, the time required for these vortices to make the transition from a weak, mid-level vortex to a rapidly developing tropical cyclone decreases as the MPI increases, indicating the importance of MPI in enhancing the frequency of cyclogenesis. The relationship between this ‘time to genesis’ and the thermodynamic parameters is explored. The time to genesis is found to be very highly (negatively) correlated to MPI, with little or no relationship to convective instability, Coriolis parameter, mid-level humidity, or the empirical genesis parameter. In some cases, tropical cyclones are found to form spontaneously from random convection. This formation is due to a cooperative interaction between large-scale moisture, long-wave radiation, and locally enhanced sea-surface fluxes, similar to the ‘aggregation’ of convection found in previous studies. Copyright © 2007 Royal Meteorological Society

Airborne observations of the impact of a convective system on the planetary boundary layer thermodynamics and aerosol distribution in the inter-tropical discontinuity region of the West African Monsoon

Abstract: The impact of a convective system downdraught and associated gravity current on the monsoon and harmattan flow structure as well as on aerosol vertical distribution over the Sahel and the Sahara is investigated using dropsondes and an airborne lidar. Complementary ground-based and satellite observations, as well as European Centre for Medium-range Weather Forecasts analyses are also used. The mission was conducted on 5 June 2006, in the framework of the African Monsoon Multidisciplinary Analysis Special Observing Period. The targeted area was the Saharan heat low region, and Mali in particular, over which the inter-tropical discontinuity was strongly perturbed by the convective system. To the north, away from the influence of the gravity current, the atmosphere exhibited a two-layer structure, with a well mixed 3 km deep internal boundary layer capped by a stable layer 2-2.5 km deep layer referred to as the Saharan aerosol layer (SAL). The aerosol loading in the internal boundary layer (as determined by lidar) was observed to be much less than in the SAL above. Plumes of dust generated by strong low-level winds west of the Hoggar as observed in Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) images, and possibly injected in the upper SAL through orographic lifting, contributed to the dust loading of the elevated SAL over much of the domain observed by the lidar. In the northernmost part of the domain investigated, the depth of the SAL top was decreased as the result of subsidence enhanced by the presence of an elevated jet associated with the monsoon trough and/or related to the return circulation of plain-mountain wind system involving the Atlas range. Associated with the gravity current passing over dust sources, a large aerosol plume was observed by means of lidar measurements and SEVIRI imagery. The plume was seen to reach heights of approximately 3 km above ground level a few tens of kilometres behind the leading edge. Lidar measurements suggest that a fraction of the dust lifted by the gravity current is mixed into the SAL, and in some instances injected above the SAL.

Modification of turbulence characteristics in flow over hills

Abstract: A new model has been developed to investigate turbulence in flows over two-dimensional hills: the von Mises transformation was applied in mean momentum equations, and turbulence equations (second-order-closure type) were solved in a coordinate system aligned with streamlines; the hill perturbation pressure was calculated by means of potential flow theory. the model predictions were compared with some experimental results from the Askervien Hill project with good agreement. It was found that apart from rapid distortion the curvature of streamlines has an important dynamical effect on turbulence. the curvature effect is shown to be particularly pronounced in the vicinity of the inner layer height on the hill top, where it attenuates turbulence. Overall, the salient features of the hill top turbulence profile were successfully explained by analysing the turbulence conservation equations in streamline coordinates.

On the robustness of changes in extreme precipitation over Europe from two high resolution climate change simulations

Abstract: Two Regional Climate Model (RCM) projections of changes in extreme precipitation over Europe are assessed and compared. This provides insight into the importance of RCM formulation in representing changes in climate extremes at high spatial resolution. The models concerned are two recent Hadley Centre RCMs, HadRM2 and HadRM3, and are applied at a horizontal resolution of approximately 50 km over Europe, nested within the Hadley Centre coupled Atmosphere Ocean General Circulation Model (AOGCM), HadCM2. The simulation periods are thirty years with fixed concentrations of greenhouse gases representing the climate of 1961–1990 and twenty years representing transient climate change for 2080–2100. The use of common boundary conditions to drive the two RCMs allows us to determine whether their different formulations significantly alter the downscaled projections. The RCM simulations of precipitation extremes are compared with observations from a dense rain-gauge network over Great Britain, aggregated to the grid used by the RCMs. Both RCMs simulate realistically extreme precipitation occurring over timescales of one to thirty days and for return periods of two to twenty years. In particular, relative errors in the magnitude of extreme precipitation are generally no larger than those in the mean. The two regional models show different patterns of errors for daily precipitation extremes, with the main difference in the western and upland areas of Great Britain where they are underestimated in HadRM2 and overestimated in HadRM3. Change in extremes over all land areas in the domain show increases in intensity everywhere (except for the Iberian peninsula and Mediterranean coast) with most of these significant at the 5% level. Projected increases are greatest for those extremes which are the rarest and shortest duration (i.e. the most intense), both in relative and thus absolute terms. The large-scale patterns of these changes are very similar in the two RCMs implying they are generally robust to the RCM formulation changes. Given the demonstrated quality of the models this enhances our confidence in the projected changes and suggests that they are mainly conditioned by the large-scale response in the driving GCM. Copyright Crown Copyright 2007. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd

The Influence of Diffusional Growth Rates On the Charge Transfer Accompanying Rebounding Collisions Between Ice Crystals and Soft Hailstones

Abstract: SUMMARY Laboratory experiments designed to investigate the charge transfer accompanying rebounding collisions between ice crystals and soft hailstones were performed inside a cold room. They constitute an extension of those conducted with the same apparatus, reported in 1983 by Jayaratne et al. In particular, the range of temperature was extended (to cover -1.5"C to -35°C) and specific tests were performed in an effort to establish relationships between the sign and magnitude of the charging and the growth characteristics of both types of hydrometeor. Significant charging was obtained only when the interacting surfaces were growing by vapour diffusion. The sign of the current, I, flowing to the simulated soft hailstone target in these circumstances was a sensitive function of time, liquid water content, and temperature. The complete set of results is qualitatively consistent with the hypothesis that I is positive if the target surface is growing more rapidly from the vapour than the ice crystals and is negative for the opposite case. In formulating a qualitative model of this hypothesis, account is taken of the contribution made to the diffusional growth of the target by the flux of vapour from droplets freezing onto its surface. Crude calculations designed to assess the implications of these studies with respect to thunderstorm electrification, yield the conclusion that negative soft hailstone charging is likely to predominate over positive in most circumstances.