Showing papers in "Quarterly Journal of the Royal Meteorological Society in 2008"

Advances in simulating atmospheric variability with the ECMWF model: From synoptic to decadal time-scales

Abstract: Advances in simulating atmospheric variability with the ECMWF model are presented that stem from revisions of the convection and diffusion parametrizations. The revisions concern in particular the introduction of a variable convective adjustment time-scale, a convective entrainment rate proportional to the environmental relative humidity, as well as free tropospheric diffusion coefficients for heat and momentum based on Monin–Obukhov functional dependencies. The forecasting system is evaluated against analyses and observations using high-resolution medium-range deterministic and ensemble forecasts, monthly and seasonal integrations, and decadal integrations with coupled atmosphere-ocean models. The results show a significantly higher and more realistic level of model activity in terms of the amplitude of tropical and extratropical mesoscale, synoptic and planetary perturbations. Importantly, with the higher variability and reduced bias not only the probabilistic scores are improved, but also the midlatitude deterministic scores in the short and medium ranges. Furthermore, for the first time the model is able to represent a realistic spectrum of convectively coupled equatorial Kelvin and Rossby waves, and maintains a realistic amplitude of the Madden–Julian oscillation (MJO) during monthly forecasts. However, the propagation speed of the MJO is slower than observed. The higher tropical tropospheric wave activity also results in better stratospheric temperatures and winds through the deposition of momentum. The partitioning between convective and resolved precipitation is unaffected by the model changes with roughly 62% of the total global precipitation being of the convective type. Finally, the changes in convection and diffusion parametrizations resulted in a larger spread of the ensemble forecasts, which allowed the amplitude of the initial perturbations in the ensemble prediction system to decrease by 30%. Copyright © 2008 Royal Meteorological Society

The MOGREPS short-range ensemble prediction system

Abstract: The Met Office has recently introduced a short-range ensemble prediction system known as MOGREPS. This system consists of global and regional ensembles, with the global ensemble providing the boundary conditions and initial-condition perturbations for the regional ensemble. Perturbations to the initial conditions are calculated using the ensemble transform Kalman filter, which is a computationally-efficient version of the ensemble Kalman filter. Model uncertainties are represented in the system through a series of schemes designed to tackle the structural and subgrid-scale sources of model error. This paper describes the set-up of the system, and provides justification for the initial-condition and model perturbation schemes chosen. An outline of the structure of the perturbations generated by the system is presented, along with performance results, including verification from case studies and routine running. MOGREPS has been on trial within the operational suite at the Met Office since August 2005. On 20 October 2006 it was decided that this system should be made fully operational, with implementation expected in summer 2008. Results show a good performance. The regional ensemble is more skilful than the global ensemble, and compares favourably to the ECMWF ensemble for the forecast variables examined in this study. © Crown Copyright 2008. Reproduced with the permission of the Controller of HMSO and the Queen's Printer for Scotland. Published by John Wiley & Sons,Ltd

Can multi-model combination really enhance the prediction skill of probabilistic ensemble forecasts?

Abstract: The success of multi-model ensemble combination has been demonstrated in many studies. Given that a multi-model contains information from all participating models, including the less skilful ones, the question remains as to why, and under what conditions, a multi-model can outperform the best participating single model. It is the aim of this paper to resolve this apparent paradox. The study is based on a synthetic forecast generator, allowing the generation of perfectly-calibrated single-model ensembles of any size and skill. Additionally, the degree of ensemble under-dispersion (or overconfidence) can be prescribed. Multi-model ensembles are then constructed from both weighted and unweighted averages of these single-model ensembles. Applying this toy model, we carry out systematic model-combination experiments. We evaluate how multi-model performance depends on the skill and overconfidence of the participating single models. It turns out that multi-model ensembles can indeed locally outperform a ‘best-model’ approach, but only if the single-model ensembles are overconfident. The reason is that multi-model combination reduces overconfidence, i.e. ensemble spread is widened while average ensemble-mean error is reduced. This implies a net gain in prediction skill, because probabilistic skill scores penalize overconfidence. Under these conditions, even the addition of an objectively-poor model can improve multi-model skill. It seems that simple ensemble inflation methods cannot yield the same skill improvement. Using seasonal near-surface temperature forecasts from the DEMETER dataset, we show that the conclusions drawn from the toy-model experiments hold equally in a real multi-model ensemble prediction system. Copyright © 2008 Royal Meteorological Society

PC2: A prognostic cloud fraction and condensation scheme. I: Scheme description

Abstract: A prognostic cloud fraction and prognostic condensate scheme has been developed for the Met Office Unified Model. This is designed to replace the scheme currently used in weather forecast and climate simulations, in which cloud fraction and liquid water content are calculated diagnostically. Such a scheme overprescribes links between cloud fraction, condensate and water vapour contents. By contrast, our new prognostic cloud fraction and prognostic condensate scheme (PC2) calculates increments to prognostic variables of liquid, ice and total cloud fractions, water vapour and liquid condensate as a result of each physical process represented in the model. (Ice condensate is already represented prognostically.) This paper provides a summary of the PC2 scheme, describes how it is implemented, and discusses its relationship with other existing cloud schemes. Key aspects of the PC2 formulation are: the consistent derivation of prognostic terms, the reversible nature of the scheme under idealised forcing scenarios, the well-behaved performance in the limit of very low and very high cloud fraction, the inclusion of ice microphysical processes, and the improved representation of cloud erosion processes. A companion paper presents the results from the scheme. © Crown Copyright 2008. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

Tropical-cyclone intensification and predictability in three dimensions

Abstract: We present numerical-model experiments to investigate the dynamics of tropical-cyclone amplification and its predictability in three dimensions. For the prototype amplification problem beginning with a weak-tropical-storm-strength vortex, the emergent flow becomes highly asymmetric and dominated by deep convective vortex structures, even though the problem as posed is essentially axisymmetric. The asymmetries that develop are highly sensitive to the boundary-layer moisture distribution. When a small random moisture perturbation is added in the boundary layer at the initial time, the pattern of evolution of the flow asymmetries is changed dramatically, and a non-negligible spread in the local and azimuthally-averaged intensity results. We conclude, first, that the flow on the convective scales exhibits a degree of randomness, and only those asymmetric features that survive in an ensemble average of many realizations can be regarded as robust; and secondly, that there is an intrinsic uncertainty in the prediction of maximum intensity using either maximum-wind or minimum-surface-pressure metrics. There are clear implications for the possibility of deterministic forecasts of the mesoscale structure of tropical cyclones, which may have a major impact on the intensity and on rapid intensity changes. Some other aspects of vortex structure are addressed also, including vortex-size parameters, and sensitivity to the inclusion of different physical processes or higher spatial resolution. We investigate also the analogous problem on a β-plane, a prototype problem for tropical-cyclone motion. A new perspective on the putative role of the wind--evaporation feedback process for tropical-cyclone intensification is offered also. The results provide new insight into the fluid dynamics of the intensification process in three dimensions, and at the same time suggest limitations of deterministic prediction for the mesoscale structure. Larger-scale characteristics, such as the radius of gale-force winds and β-gyres, are found to be less variable than their mesoscale counterparts. Copyright © 2008 Royal Meteorological Society

A review of forecast error covariance statistics in atmospheric variational data assimilation. II: Modelling the forecast error covariance statistics

Abstract: This article reviews a range of leading methods to model the background error covariance matrix (the B-matrix) in modern variational data assimilation systems Owing partly to its very large rank, the B-matrix is impossible to use in an explicit fashion in an operational setting and so methods have been sought to model its important properties in a practical way Because the B-matrix is such an important component of a data assimilation system, a large effort has been made in recent years to improve its formulation Operational variational assimilation systems use a form of control variable transform to model B This transform relates variables that exist in the assimilation's ‘control space’ to variables in the forecast model's physical space The mathematical basis on which the control variable transform allows the B-matrix to be modelled is reviewed from first principles, and examples of existing transforms are brought together from the literature The method allows a large rank matrix to be represented by a relatively small number of parameters, and it is shown how information that is not provided explicitly is filled in Methods use dynamical properties of the atmosphere (eg balance relationships) and make assumptions about the way that background errors are spatially correlated (eg homogeneity and isotropy in the horizontal) It is also common to assume that the B-matrix is static The way that these, and other, assumptions are built into systems is shown The article gives an example of how a current method performs An important part of this article is a discussion of some new ideas that have been proposed to improve the method Examples include how a more appropriate use of balance relations can be made, how errors in the moist variables can be treated and how assumptions of homogeneity/isotropy and the otherwise static property of the B-matrix can be relaxed Key developments in the application of dynamics, wavelets, recursive filters and flow-dependent methods are reviewed The article ends with a round up of the methods and a discussion of future challenges that the field will need to address Copyright © 2008 Royal Meteorological Society

Primary and successive events in the Madden–Julian Oscillation

Abstract: Conventional analyses of the MJO tend to produce a repeating cycle, such that any particular feature cannot be unambiguously attributed to the current or previous event. We take advantage of the sporadic nature of the MJO and classify each observed Madden–Julian (MJ) event as either primary, with no immediately preceding MJ event, or successive, which does immediately follow a preceding event. 40% of MJ events are primary events. Precursor features of the primary events can be unambiguously attributed to that event. A suppressed convective anomaly grows and decays in situ over the Indian Ocean, prior to the start of most primary MJ events. An associated mid-tropospheric temperature anomaly destabilises the atmosphere, leading to the generation of the active MJ event. Hence, primary MJ events appear to be thermodynamically triggered by a previous dry period, although stochastic forcing may also be important. Other theories predict that boundary-layer convergence, humidity, propagation of dynamical structures around the Equator, sea surface temperatures, and lateral forcing by extratropical transients may all be important in triggering an event. Although precursor signals from these mechanisms are diagnosed from reanalysis and satellite observational data in the successive MJ events, they are all absent in the primary MJ events. Hence, it appears that these apparent precursor signals are part of the MJO once it is established, but do not play a role in the spontaneous generation of the MJO. The most frequent starting location of the primary events is the Indian Ocean, but over half of them start elsewhere, from the maritime continent to the western Pacific. Copyright © 2008 Royal Meteorological Society

TIGGE: Preliminary results on comparing and combining ensembles

Abstract: TIGGE, the THORPEX Interactive Grand Global Ensemble, is a World Weather Research Programme project to accelerate the improvements in the accuracy of 1-day to 2-week high-impact weather forecasts. This report discusses some preliminary results from predictability studies based on the ensemble data exchanged within TIGGE, and available at the TIGGE archive centres. In the first part of this work, the key characteristics of the eight ensemble systems available in the TIGGE database at the time of writing (December 2007) are compared, and the strengths and weaknesses of each system are highlighted. Then, issues related to the generation of multi-model/multi-analysis ensemble products are discussed, and some preliminary results on the potential value of combining different ensembles to generate medium-range products with a grand multi-model/multi-analysis global ensemble are presented. One of the key results documented in this work is the large difference between the performance of the single ensembles: for Z500 over the Northern Hemisphere, in the medium-range (say around forecast day 5), the difference between the worst and the best control or ensemble-mean forecasts is about 2 days of predictability, while the difference between the worst and the best probabilistic predictions can be larger, about 3 days of predictability. Another key result has been the quantification of the difference between the skill of the best ensemble system and a combined ensemble generated considering up to four different ensemble systems. Results have indicated that the difference is very small in areas where the European Centre (EC) ensemble system has a well tuned ensemble spread, equivalent to less than 6 hours of predictability in the medium range, while it is larger and more detectable in areas where the EC system has a too low ensemble spread (e.g. in the Tropics). Copyright © 2008 Royal Meteorological Society

A review of forecast error covariance statistics in atmospheric variational data assimilation. I: Characteristics and measurements of forecast error covariances

Abstract: This article reviews the characteristics of forecast error statistics in meteorological data assimilation from the substantial literature on this subject. It is shown how forecast error statistics appear in the data assimilation problem through the background error covariance matrix, B. The mathematical and physical properties of the covariances are surveyed in relation to a number of leading systems that are in use for operational weather forecasting. Different studies emphasize different aspects of B, and the known ways that B can impact the assimilation are brought together. Treating B practically in data assimilation is problematic. One such problem is in the numerical measurement of B, and five calibration methods are reviewed, including analysis of innovations, analysis of forecast differences and ensemble methods. Another problem is the prohibitive size of B. This needs special treatment in data assimilation, and is covered in a companion article (Part II). Examples are drawn from the literature that show the univariate and multivariate structure of the B-matrix, in terms of variances and correlations, which are interpreted in terms of the properties of the atmosphere. The need for an accurate quantification of forecast error statistics is emphasized. Copyright © 2008 Royal Meteorological Society

A numerical study of three catastrophic precipitating events over southern France. I: Numerical framework and synoptic ingredients

Abstract: This study examines the simulation of three torrential rain events observed on 13-14 October 1995 (the Cevennes case), 12-13 November 1999 (the Aude case) and 8-9 September 2002 (the Gard case) over the southeastern part of France using the Meso-NH non-hydrostatic mesoscale numerical model. These cases were associated with extreme Heavy Precipitation Events (HPEs) with significant precipitation amounts exceeding 500 mm in less than 24 hours. Several sets of numerical experiments were performed with 10 km and 2.5 km horizontal resolutions. In part I of this study, special attention is paid to the experimental design for obtaining realistic simulations of HPEs with the Meso-NH model, as well as the evolution of the synoptic patterns in which the rainfall events are embedded. The best 2.5 km numerical simulations show the ability of the Meso-NH model to reproduce significant quasi-stationary rainfall events. Moreover, the model fairly reproduces the low-level mesoscale environments associated with the three HPEs. The HPEs formed in a slow-evolving synoptic environment favourable for the development of convective systems (diffluent upper-level southerly flow, PV anomalies, etc.). At lower levels, a southerly to easterly moderate to intense flow provided conditionally unstable and moist air as it moved over the relatively warm Mediterranean Sea, typical for this time of the year (late summer and autumn). The two extreme cases (Gard and Aude) differ from the more classical event (Cevennes) in terms of larger low-level moisture fluxes. Weaker values of conditional convective instability, as in the Aude case, is counterbalanced by a stronger warm and moist low-level jet. The mesoscale triggering and/or sustaining ingredients for deep convection and the physical mechanisms leading to the stationarity of these rainfall events are presented and discussed in a companion paper.

A numerical study of three catastrophic precipitating events over southern France. II: Mesoscale triggering and stationarity factors

Abstract: In the western Mediterranean basin, large amounts of precipitation can accumulate in less than a day when a Mesoscale Convective System (MCS) stays over the same area for several hours. Heavy Precipitating Events (HPEs) in this region (especially southern France) are not only characterized by significant precipitation rates (typically more than 200 mm in less than 24 or 48 hours) but also by quasi-stationary behaviour. The aim of this present study is to use realistic simulations of past events to analyze and better understand the physical mechanisms which lead to the stationarity of HPEs over southern France using a high-resolution (2.5 km) non-hydrostatic mesoscale atmospheric model. We focused on three HPEs: 13-14 October 1995 (the Cevennes case), 12-13 November 1999 (the Aude case) and 8-9 September 2002 (the Gard case). The experimental design for simulating realistic structures and evolutions of the HPEs with the Meso-NH model has been presented in a companion paper. In that study, the evolution of the synoptic patterns in which the HPEs occurred were analysed. In this paper, the importance of mesoscale ingredients, such as the moist and conditionally unstable low-level jet (LLJ), is examined in terms of their ability to focus the deep convection over the same area for several hours. The Cevennes case is typical of a so-called Cevenole event in France for which orographic forcing is the primary mechanism which acts to continuously generate convective cells upwind of the Massif Central. The unusual location of the Gard event is explained by a cold pool induced by the evaporation of precipitation which acts as a relief-like feature to force the conditionally unstable and moist LLJ to rise. This constitutes a quasi-stationary forcing as the cold pool is blocked in the Rhone Valley. For the third case, diabatic cooling and the Massif Central act to enhance the precipitation but have almost no impact on the location of the anchoring point of the precipitating system.

Dust emissions over the Sahel associated with the West African monsoon intertropical discontinuity region: A representative case-study

Abstract: Near-dawn airborne lidar and dropsonde observations acquired on 7 July 2006, during the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1, were used to investigate dust mobilization, lifting and transport in the intertropical discontinuity (ITD) region over western Niger. Atmospheric reflectivity data from the LEANDRE 2 lidar system enabled us to analyse the structure of dust plumes in the context of wind and thermodynamic information provided by the WIND lidar system and dropsondes. Dust mobilization was mainly observed in two locations: (a) within the monsoon flow as the result of the passage of a density current originating from a mesoscale convective system over southwest Niger, and (b) at the leading edge of the monsoon flow where the near-surface winds and turbulence were strong, because the monsoon flow was behaving as an intrusive density current. The circulation in the head of the monsoon density current lifted the mobilized dust towards the wake, along an isentropic surface. Behind and away from the leading edge, some of the mobilized dust was observed to mix across the monsoon-harmattan interface, due to the existence of mechanical shear above the monsoon layer. The dust thus becomes available for long-range transport by the harmattan. Because dust sources are widespread over the Sahel and presumably active on many days when the ITD is located in this region during summer, dust emissions associated with the described mechanism may influence the radiation budget over West Africa.

Assimilation of radar-derived rain rates into the convective-scale model COSMO-DE at DWD

Abstract: To improve very-short-range forecasts particularly in convective situations, a version of the COSMO-Model (formerly known as LM) which simulates deep convection explicitly (horizontal grid length: 2.8 km) has been developed and is now run operationally at DWD. This model uses a prognostic type of precipitation scheme accounting for the horizontal drift of falling hydrometeors. To initialise convective-scale events, the latent heat nudging (LHN) approach has been adopted for the assimilation of surface precipitation rates derived from radar reflectivity data. It is found that a conventional LHN scheme designed for larger-scale models with diagnostic treatment of precipitation does not perform well and leads to strong overestimation of precipitation when applied to the convective-scale model with a prognostic treatment of precipitation. As illustrated here, surface precipitation and vertically integrated latent heating are far less correlated horizontally and temporally in such a model than with diagnostic precipitation, and this implies a violation of the basic assumption of LHN. Several revisions to the LHN scheme have therefore been developed in view of the characteristic model behaviour so as to re-enhance the validity of the basic assumption and to reduce greatly the overestimation of precipitation during assimilation. With the revised scheme, the model is able to simulate the precipitation patterns in good agreement with radar observations during the assimilation and the first hours of the forecast. The scheme also has a positive impact on screen-level parameters and on the longer-term climatology of the model. Extending the temporal impact of the radar observations further into the free forecast will be the focus of future research. Copyright © 2008 Royal Meteorological Society

The propagation and diurnal cycles of deep convection in northern tropical Africa

Abstract: The propagation and diurnal cycle of organized convection in northern tropical Africa are examined using five years (1999–2003) of digital infrared imagery for May–August. Reduced-dimension techniques are used to document the properties of cold clouds - proxies for deep convection and precipitation. Large-scale environments are diagnosed from global analyses. Organized convection in Africa consists of coherent sequences or episodes which span an average distance of about 1000 km and last about 25 h. A substantial fraction of events exhibits systematic propagation at regional to continental scales while undergoing decay and regeneration. Episodes with 36 h duration and 1472 km span recur at a one-per-day interval. Most episodes have phase speed of 10–20 m s−1, which is faster than most African easterly waves. Convective episodes tend to initiate in the lee of high terrain, consistent with thermal forcing from elevated heat sources. Average diurnal frequency maxima result from the superposition of local diurnal maximum with the delayed-phase arrival of systems propagating from the east. Propagation occurs with moderate low- to mid-tropospheric shear, which varies with the African easterly jet migration and West African monsoon phases. Frequent deep convection occurs with local shear maxima near high terrain. For the peak monsoon period and for 10°W–10°E, where easterly waves and convective systems are frequent, 35% of cold cloud episodes occur east of the wave trough compared with about 24% to the west. Based on the coherent behaviour of organized, propagating convection, inferences may be made regarding the prediction of precipitation beyond one or two days. Copyright © 2008 Royal Meteorological Society

Potential use of an ensemble of analyses in the ECMWF Ensemble Prediction System

Abstract: One of the crucial aspects of the design of an ensemble prediction system is the definition of the ensemble of initial states. This work investigates the use of singular vectors, an ensemble of analyses, and a combination of the two types of perturbations in the ECMWF operational ensemble prediction system. First, the similarity between perturbations generated using initial-time singular vectors (SVs) and analyses from the ensemble data assimilation (EDA) system is assessed. Results show that the EDA perturbations are less localized geographically and have a better coverage of the Tropics. EDA perturbations have also smaller scales than SV-based perturbations, and have a less evident upshear vertical tilt, which explains why they grow less with forecast time. Then, the use of EDA-based perturbations in the ECMWF ensemble prediction system is studied. Results indicate that if used alone, EDA-based perturbations lead to an under-dispersive and less skilful ensemble then the one based on initial-time SVs only. Combining the EDA and the initial-time SVs gives a system with a better agreement between ensemble spread and the error of the ensemble mean, a smaller ensemble-mean error and more skilful probabilistic forecasts than the current operational system based on initial-time and evolved SVs. Copyright © 2008 Royal Meteorological Society

Turbulence energetics in stably stratified geophysical flows: Strong and weak mixing regimes

Abstract: Traditionally, turbulence energetics is characterised by turbulent kinetic energy (TKE) and modelled using solely the TKE budget equation. In stable stratification, TKE is generated by the velocity shear and expended through viscous dissipation and work against buoyancy forces. The effect of stratification is characterised by the ratio of the buoyancy gradient to squared shear, called the Richardson number, Ri. It is widely believed that at Ri exceeding a critical value, Ric, local shear cannot maintain turbulence, and the flow becomes laminar. We revise this concept by extending the energy analysis to turbulent potential and total energies (TPE, and TTE = TKE + TPE), consider their budget equations, and conclude that TTE is a conservative parameter maintained by shear in any stratification. Hence there is no ‘energetics Ric’, in contrast to the hydrodynamic-instability threshold, Ric−instability, whose typical values vary from 0.25 to 1. We demonstrate that this interval, 0.25 < Ri < 1, separates two different turbulent regimes: strong mixing and weak mixing rather than the turbulent and the laminar regimes, as the classical concept states. This explains persistent occurrence of turbulence in the free atmosphere and deep ocean at Ri ≫ 1, clarifies the principal difference between turbulent boundary layers and free flows, and provides the basis for improving operational turbulence closure models. Copyright © 2008 Royal Meteorological Society

Analysis of the 26 July 2005 heavy rain event over Mumbai, India using the Weather Research and Forecasting (WRF) model

Abstract: A case-study of the 26 July 2005 Mumbai heavy-rain event that recorded 944 mm rainfall in 24 h with significant spatial variability was carried out using the Weather Research and Forecasting (WRF) model. The event was poorly forecasted by operational models and resulted in large human and economic losses. The present results indicate that the WRF system was able to reproduce the heavy rainfall event and the associated dynamical and thermo- dynamical features. A number of experiments were conducted with the WRF model that suggest the highly localized, heavy rain was the result of an interaction of synoptic-scale weather systems with the mesoscale, coastal land-surface features. These experiments indicate that the large-scale rising motion over the Mumbai region was synoptically forced. Analysis of the model-simulated intense, but short-lived, convective rain cells forming in the large-scale rising motion over Mumbai traces their moisture source to the north and northwesterly flow from the Arabian Sea. Synthetic sensitivity simulations without topography and without a land surface (land replaced with water) show that the large-scale synoptic flow positioned the low-pressure system over the Arabian Sea, while the mesoscale land-surface (including topography and latent heating) feedback modulated the location and intensity of the rain by changes in the winds and regional moisture convergences. Another important feature captured in the high-resolution model analysis is the formation of a mesoscale vortex over Mumbai that appears to have enhanced the conditions for localized, heavy rainfall over Mumbai. Copyright c � 2008 Royal Meteorological Society

The new VarEPS-monthly forecasting system: A first step towards seamless prediction

Abstract: A combined medium-range and monthly-forecasting forecasting system is now operational at the European Centre for Medium-Range Weather Forecasts Previously, these two systems were run separately The new combined system provides skillful predictions of small-scale, severe-weather events in the early forecast range, accurate large-scale forecast guidance up to day 15 twice a day, and large-scale guidance up to day 32 once a week In addition, the daily medium-range forecasts starting at 0000 utc are now coupled to an ocean general-circulation model after day 10 and persisted sea-surface temperature (SST) anomalies instead of persisted SSTs are applied when the atmospheric model is run in uncoupled mode Average results indicate that the monthly forecasting scores are slightly higher in the Extratropics with this new combined system In particular, the new system seems to produce better monthly forecasts for extreme events such as the 2003 heatwave over Europe or the 2007 wet summer over England However, the monthly forecasting scores in the Tropics are slightly lower than with the previous system, most likely because of the lack of ocean-atmosphere coupling during the first ten days of the forecasts This demonstrates that ocean-atmosphere coupling may be needed in medium-range forecasting and future plans include coupling the atmosphere to the ocean model from day 0 Average results based on 30 cases indicate that the medium-range forecasts benefit from the introduction of the ocean-atmosphere coupling after day 10, most especially the prediction of low-level temperature in the Tropics Copyright c � 2008 Royal Meteorological Society

A Critique of Emanuel's Hurricane Model and Potential Intensity Theory

Abstract: We present a critique of Emanuel's steady-state hurricane model, which is a precursor to his theory for hurricane potential intensity (PI) We show that a major deficiency of the theory is the tacit assumption of gradient wind balance in the boundary layer, a layer that owes its existence to gradient wind imbalance in the radial momentum equation If a more complete boundary-layer formulation is included using the gradient wind profiles obtained from Emanuel's theory, the tangential wind speed in the boundary layer becomes supergradient, invalidating the assumption of gradient wind balance We show that the degree to which the tangential wind is supergradient depends on the assumed boundary-layer depth The full boundary-layer solutions require a knowledge of the tangential wind profile above the boundary layer in the outer region where there is subsidence into the layer and they depend on the breadth of this profile This effect is not considered in Emanuel's theory We argue that a more complete theory for the steady-state hurricane would require the radial pressure gradient above the boundary layer to be prescribed or determined independently of the boundary layer The issues raised herein highlight a fundamental problem with Emanuel's theory for PI, since that theory makes the same assumptions as in the steady-state hurricane model Our current findings together with recent studies examining intense hurricanes suggest a way forward towards a more consistent theory for hurricane PI Copyright © 2008 Royal Meteorological Society

The assimilation of cloud‐affected infrared satellite radiances for numerical weather prediction

Abstract: A practical technique for the assimilation of cloud-affected infrared radiances is presented. The technique is best suited to advanced infrared sounders such as AIRS and IASI. Radiances are first pre-processed by a one-dimensional variational analysis (1D-Var) scheme, where cloud parameters (cloud-top pressure and effective cloud fraction) are retrieved simultaneously with atmospheric profile variables. The retrieved cloud parameters are then passed to a variational data assimilation system, where they are used to constrain the radiative transfer calculation in the assimilation of a reduced set of channels. The channel selection is chosen to reduce the sensitivity to errors in the forward modelling of radiation originating below the cloud top. The performance of this technique is explored by means of a 1D-Var study using simulated measurements. It is demonstrated that the technique has the potential to allow the assimilation of a significant proportion of cloud-affected infrared sounding measurements, possibly bringing valuable benefits to an operational NWP system. © Crown Copyright 2008. Reproduced with the permission of the Controller of HMSO and the Queen's Printer for Scotland. Published by John Wiley & Sons, Ltd.

The impact of extratropical transition on the downstream flow: An idealized modelling study with a straight jet

Abstract: The interaction of a tropical cyclone undergoing extratropical transition (ET) with the midlatitude synoptic-scale flow is investigated using full-physics numerical experiments with idealized initial conditions. The emphasis is on the impact on the midlatitude flow downstream of the ET event. The midlatitude flow is represented by a balanced straight jet stream. As the tropical cyclone approaches the jet, a ridge–trough couplet and a distinct jet streak form in the upper-level flow. A midlatitude cyclone develops rapidly downstream of the ET system and the further evolution is characterized by downstream baroclinic development. Based on Hovmoller diagrams, the upper-level development is interpreted as the excitation and subsequent dispersion of a Rossby wave train on the potential vorticity gradient associated with the jet. The characteristics of this wave train are sensitive to the structure of the jet and to moist processes in the midlatitudes. The tropical cyclone undergoing ET acts as a sustained forcing for the wave train and the structure of the ET system impacts the development most significantly one to two wavelengths downstream of ET. Piecewise inversion of potential vorticity, complemented by the partitioning of the flow into its rotational and divergent parts, is applied to assess the impact of the ET system quantitatively. Both the cyclonic circulation and the outflow of the tropical cyclone are important contributors to the formation and amplification of the ridge–trough couplet. The outflow anomaly reduces the eastward motion of the ridge–trough couplet significantly and thus promotes phase-locking between the tropical cyclone and the upper-level pattern. Copyright © 2008 Royal Meteorological Society

Understanding the local and global impacts of model physics changes: an aerosol example

Abstract: This study demonstrates the use of a package of diagnostic techniques to understand the local and global responses to a given physics change within a general circulation model. Here, the package is applied to the case of a change in the aerosol climatology in the forecast model of the European Centre for Medium-range Weather Forecasts. The largest difference between old and new climatologies is over the Sahara where, in particular, soil-dust aerosol is reduced. Conventional diagnostics show that the change lead to improvements in local medium-range forecast skill and reductions in seasonal-mean errors throughout the globe. To study the local physics response, short-range tendencies in weather forecasts are diagnosed. These tendencies are decomposed into the contributions from each physical process within the model. The resulting ‘initial tendency’ budget reveals how the local atmosphere responds to the aerosol change. The net tendencies also provide strong evidence to confirm that the new aerosol climatology is superior. Seasonal integrations demonstrate that the tropic-wide response can be understood in terms of equatorial waves and their enhancement by diabatic processes. The so-called ‘Rossby-wave source’ is made applicable to general circulation models and used to understand how the tropical anomalies subsequently impact on the global circulation. The mean response in the extratropics is found to be a stationary wave field. Precipitation anomalies that are co-located with extratropical divergent vorticity sources suggest the possibility for diabatic modification of the tropically forced Rossby-wave response. Copyright © 2008 Royal Meteorological Society

PC2: A prognostic cloud fraction and condensation scheme. II: Climate model simulations

Abstract: A prognostic cloud fraction and prognostic condensate scheme (PC2) has been developed for the Met Office Unified Model. A companion paper discussed the motivation for a new scheme and described its formulation in detail. In this paper we describe the results of climate model simulations, concentrating on the mechanisms by which the cloud and condensate predicted by the model change between the Control and new scheme. We demonstrate that the detrainment of condensate from the convection scheme directly into the large scale, as parametrized in the PC2 scheme, produces improved simulations of deep tropical cloud. We also show that the unphysical strong link between cloud fraction and condensed water content that is present in the Control scheme has been broken by using PC2, but that it is still challenging to produce optically thin cloud in a large-scale model. Shallow convection proves to be a difficult cloud type to parametrize using a prognostic scheme, although the PC2 scheme performs well. The use of increased vertical resolution, in both the Control and PC2, improved the simulation of cloud when compared to observations. © Crown Copyright 2008. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

On the onset of bora and the formation of rotors and jumps near a mountain gap

Abstract: This study investigates the onset phase of a strong Adriatic bora windstorm that occurred on 4 April 2002. The target area is a gap about 20 km wide embedded in the coastal mountain barrier of the Dinaric Alps that favours strong jet-like winds. Airborne-aerosol back-scatter lidar measurements on board the DLR Falcon research aircraft, together with surface and upper-air observations, are used to verify high-resolution numerical experiments conducted with the mesoscale atmospheric model RAMS and a single-layer shallow-water model (SWM). Especially during the breakthrough phase of the bora, the flow at the gap exit exhibits a complex spatial structure and temporal evolution. On a transect through the centre of the gap, a hydraulic jump forms; this is located close to the coast throughout the night, and starts to propagate downstream in the early morning. On a transect through the edge of the gap, a lee-wave-induced rotor becomes established, due to boundary-layer separation. It starts to propagate downstream about two hours after the jump. This flow evolution implies that the onset of strong winds at the coast occurs several hours earlier downstream of the centre of the gap than downwind of the edge of the gap. Consequently, the wind field in the vicinity of Rijeka airport, located downwind of the gap, is strongly inhomogeneous and transient, and represents a potential hazard to aviation. Measured bora winds at the surface exceed 20 ms−1, and the simulated wind speed in the gap wind layer exceeds 30 ms−1. The simulated turbulent kinetic energy exceeds 10 m2 s−2. RAMS indicates that wave-breaking near a critical level is the dominant mechanism for the generation of the windstorm. Gap jets can be identified downstream of several mountain passes. The simulated wave pattern above the Dinaric Alps, the wave decay with height due to directional wind shear and the strong flow descent on the leeward side of the barrier are supported by measured back-scatter intensities. Basic bora flow features, including gap jets and jumps, are remarkably well reproduced by SWM simulations. The RAMS reference run captures observed flow phenomena and the temporal flow evolution qualitatively well. A cold low-level bias, an overestimated bora inversion strength, and a slightly too-early bora onset are probably related to insufficient turbulent mixing in the boundary layer. The amplitude of trapped gravity waves, the time of the bora breakthrough and the inversion strength are found to be quite sensitive to the turbulence parametrization. Copyright © 2008 Royal Meteorological Society

The climatology of small‐scale orographic precipitation over the Olympic Mountains: Patterns and processes

Abstract: The climatology of small-scale patterns of mountain precipitation is poorly constrained, yet important for applications ranging from natural hazard assessment to understanding the geologic evolution of mountain ranges. Synthesizing four rainy seasons of high-resolution precipitation observations and mesoscale model output (from the Penn State/NCAR MM5), reveals a persistent small-scale pattern of precipitation over the ∼10 km wide, ∼800 m high ridges and valleys of the western Olympic Mountains, Washington State, USA. This pattern is characterized by a 50–70% excess accumulation over the ridge crests relative to the adjacent valleys in the annual mean. While the model shows excellent skill in simulating these patterns at seasonal time-scales, major errors exist for individual storms. Investigation of a range of storm events has revealed the following mechanism for the climatological pattern. Regions of enhanced condensation of cloud water are produced by ascent in stable flow over the windward slopes of major ridges. Synoptically generated precipitation grows by collection within these clouds, leading to enhanced precipitation which is advected by the prevailing winds. Instances of atypical patterns of precipitation suggest that under certain conditions (during periods with a low freezing level, or convective cells) fundamental changes in small-scale patterns may occur. However, case-studies and composite analysis suggest that departures from the pattern of ridge-top enhancement are rare; the basic patterns and processes appear robust to changes in temperature, winds, and background rainfall rates. Copyright © 2008 Royal Meteorological Society

Estimating observation impact without adjoint model in an ensemble Kalman filter

Abstract: We propose an ensemble sensitivity method to calculate observation impacts similar to Langland and Baker (2004) but without the need for an adjoint model, which is not always available for numerical weather prediction models. The formulation is tested on the Lorenz 40-variable model, and the results show that the observation impact estimated from the ensemble sensitivity method is similar to that from the adjoint method. Like the adjoint method, the ensemble sensitivity method is able to detect observations that have large random errors or biases. This sensitivity could be routinely calculated in an ensemble Kalman filter, thus providing a powerful tool to monitor the quality of observations and give quantitative estimations of observation impact on the forecasts. Copyright © 2008 Royal Meteorological Society

The Monte Carlo Independent Column Approximation: An assessment using several global atmospheric models

Abstract: The Monte Carlo Independent Column Approximation (McICA) computes domain-average, broadband radiative flux profiles within conventional global climate models (GCMs). While McICA is unbiased with respect to the full ICA, it generates, as a by-product, random noise. If this by-product leads to statistically significant impacts on GCM simulations, it could limit the usefulness of McICA. This paper assesses the impact of McICA's random noise on six GCMs. To this end, the GCMs performed ensembles of 14-day long simulations for various renditions of McICA, each with differing amounts of random noise. As seen in the past, low-cloud fraction and surface temperature were affected most by noise. However, all GCM simulations using operationally viable renditions of McICA showed no statistically significant impacts, even for precipitation - a highly intermittent variable that one might expect to be sensitive to random fluctuations. Two GCMs showed statistically significant responses using an academic version of McICA that generates overly large sampling noise. Time series analyses of high-resolution (i.e. typically 2-hourly) data revealed that fluctuations associated with most variables and GCMs are immune to McICA noise. Moreover, the nature of these fluctuations can vary substantially among GCMs and most often they overwhelm any noise impacts. Overall, the results presented here corroborate a range of previous studies done on one GCM at a time: random noise produced by recommended versions of McICA has statistically insignificant effects on GCM simulations. Copyright c � 2008 Royal Meteorological Society and Her Majesty in Right of Canada.

Far‐upstream precursors of heavy precipitation events on the Alpine south‐side

Abstract: This study examines the upper-level flow pattern that foreshadows the comparatively frequent co-occurrence of heavy precipitation events on the Alpine south-side together with potential vorticity (PV) streamers aloft over western Europe. It is hypothesized that each streamer is itself preceded by a dynamically distinctive and a thermodynamically active flow associated with the space–time development, and the ultimate breaking over Europe, of synoptic-scale Rossby waves. The ECMWF (European Centre for Medium-range Weather Forecasts) ERA-40 dataset is used to examine the nature of a streamer's coherent precursor pattern by deriving seasonal composites of refined Hovmoller diagrams, the upper-level pattern of the PV and its gradient, and mid-tropospheric diabatic heating. For the autumn and winter seasons, the composites reveal a significant and coherent wave-packet precursor signal in the PV field that can be traced back ∼1 week to the central and eastern Pacific. Contemporaneously, the coherent signatures in the composite PV gradient field and diabatic heating fields are conducive to triggering and/or supporting the wave signal, and also contribute to subsequent downstream wave-breaking. In spring the composite precursor PV signal appears over, and propagates eastward from, the Atlantic basin, and its limited zonal extent is attributed to the lack of a coherent PV waveguide. In summer, the signal appears essentially in situ in the Hovmoller analysis but extends across the Atlantic basin in the PV composite results. An indication is also provided of the extent to which the composite characteristics are evident in two individual events, and note is made of the study's implications for medium-range forecasting. Copyright © 2008 Royal Meteorological Society

A simple model of the hurricane boundary layer revisited

Abstract: A simple slab model for the boundary layer of a hurricane is re-examined and a small error in the original calculation is corrected. With this correction, the development of supergradient winds is a ubiquitous feature of the solutions. The boundary layer shows two types of behaviour in the inner core of the vortex depending on the depth of the layer and the maximum tangential wind speed above the layer. For small depths and/or large tangential wind speeds, large supergradient winds develop and lead to a rapid deceleration of the inflow such that the inflow becomes zero at some radius inside the radius of maximum tangential wind speed above the boundary layer. For large depths and/or small tangential wind speeds, the solutions do not become singular until within a few kilometres of the rotation axis. The transition between the two regimes is very abrupt. Interpretations are given for the foregoing behaviour. Other aspects of the boundary-layer dynamics and thermodynamics are investigated including: the dependence on mixing by shallow convection; the effects of a radially varying boundary-layer depth; the effects of downward momentum transport; the dependence of thermodynamical quantities on the boundary-layer depth; and the radial variation of equivalent potential temperature. Predicted values of the last quantity are in acceptable agreement with observations made in category-five hurricane Isabel (2003). The version with radially varying depth gives more realistic vertical velocities in the inner-core region of the vortex. The limitations and strengths of the slab model are discussed. Copyright © 2008 Royal Meteorological Society

Ensemble simulations of the cold European winter of 2005-2006

Abstract: There is only limited understanding of the processes driving year-to-year variability in European winter climate and the skill of seasonal forecasts for Europe in winter is generally low. The winter of 2005-2006 is a useful case-study because it was the coldest winter in large parts of western Europe for over a decade, and the coldest in central England since 1995-1996. Here, we present results of experiments with a range of general circulation models to investigate the importance of both the Atlantic Ocean and stratospheric circulation in producing the unusually cold winter of 2005-2006. We use models with different combinations of horizontal and stratospheric vertical resolution, allowing the sensitivity of the response to model formulation to be tested. The response to Atlantic sea-surface temperature (SST) anomalies is improved in a more recent model with higher horizontal resolution. The results show that both Atlantic SSTs and the January 2006 sudden stratospheric warming are likely to have contributed to the cold 2005-2006 European winter. © Crown Copyright 2008. Reproduced with the permission of HMSO. Published by John Wiley & Sons Ltd.