Showing papers in "Quarterly Journal of the Royal Meteorological Society in 2005"

The ERA‐40 re‐analysis

Abstract: ERA-40 is a re-analysis of meteorological observations from September 1957 to August 2002 produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) in collaboration with many institutions. The observing system changed considerably over this re-analysis period, with assimilable data provided by a succession of satellite-borne instruments from the 1970s onwards, supplemented by increasing numbers of observations from aircraft, ocean-buoys and other surface platforms, but with a declining number of radiosonde ascents since the late 1980s. The observations used in ERA-40 were accumulated from many sources. The first part of this paper describes the data acquisition and the principal changes in data type and coverage over the period. It also describes the data assimilation system used for ERA-40. This benefited from many of the changes introduced into operational forecasting since the mid-1990s, when the systems used for the 15-year ECMWF re-analysis (ERA-15) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis were implemented. Several of the improvements are discussed. General aspects of the production of the analyses are also summarized. A number of results indicative of the overall performance of the data assimilation system, and implicitly of the observing system, are presented and discussed. The comparison of background (short-range) forecasts and analyses with observations, the consistency of the global mass budget, the magnitude of differences between analysis and background fields and the accuracy of medium-range forecasts run from the ERA-40 analyses are illustrated. Several results demonstrate the marked improvement that was made to the observing system for the southern hemisphere in the 1970s, particularly towards the end of the decade. In contrast, the synoptic quality of the analysis for the northern hemisphere is sufficient to provide forecasts that remain skilful well into the medium range for all years. Two particular problems are also examined: excessive precipitation over tropical oceans and a too strong Brewer-Dobson circulation, both of which are pronounced in later years. Several other aspects of the quality of the re-analyses revealed by monitoring and validation studies are summarized. Expectations that the ‘second-generation’ ERA-40 re-analysis would provide products that are better than those from the firstgeneration ERA-15 and NCEP/NCAR re-analyses are found to have been met in most cases. © Royal Meteorological Society, 2005. The contributions of N. A. Rayner and R. W. Saunders are Crown copyright.

Review of the vapour pressures of ice and supercooled water for atmospheric applications

Abstract: The vapour pressures of ice and supercooled water are reviewed with an emphasis on atmospheric applications. Parametrizations are given for the vapour pressure, molar heat capacity, and latent heat of vaporization of both ice and liquid water. For ice, the experimental vapour pressure data are in agreement with a derivation from the Clapeyron equation. Below 200 K cubic ice may affect the vapour pressure of ice both in the atmosphere and in the laboratory. All of the commonly used parametrizations for the vapour pressure of supercooled water are extrapolations that were not originally intended for use below the freezing point. In addition, the World Meteorological Organization definition of the vapour pressure of supercooled water contains an easily overlooked typographical error. Recent data on the molar heat capacity of supercooled water are used to derive its vapour pressure. Nevertheless, the uncertainty is such that measurements of the deliquescence and freezing behaviour of aerosol particles are beginning to be limited by uncertainties in the thermodynamics of supercooled water. Copyright © 2005 Royal Meteorological Society

A new dynamical core for the Met Office's global and regional modelling of the atmosphere

Abstract: A computational scheme suitable for numerical weather prediction and climate modelling over a wide range of length scales is described. Its formulation is non-hydrostatic and fully compressible, and shallow atmosphere approximations are not made. Semi-implicit, semi-Lagrangian time-integration methods are used. The scheme forms the dynamical core of the unified model used at the Met Office for all its operational numerical weather prediction and in its climate studies. © Crown copyright, 2005. Royal Meteorological Society

Diagnosis of observation, background and analysis-error statistics in observation space

Abstract: Most operational assimilation schemes rely on linear estimation theory. Under this assumption, it is shown how simple consistency diagnostics can be obtained for the covariances of observation, background and estimation errors in observation space. Those diagnostics are shown to be nearly cost-free since they only combine quantities available after the analysis, i.e. observed values and their background and analysis counterparts in observation space. A first application of such diagnostics is presented on analyses provided by the French 4D-Var assimilation. A procedure to refine background and observation-error variances is also proposed and tested in a simple toy analysis problem. The possibility to diagnose cross-correlations between observation errors is also investigated in this same simple framework. A spectral interpretation of the diagnosed covariances is finally presented, which allows us to highlight the role of the scale separation between background and observation errors.

Operational multivariate ocean data assimilation

Abstract: A fully three-dimensional, multivariate, optimum-interpolation ocean data assimilation system has been developed that produces simultaneous analyses of temperature, salinity, geopotential and vector velocity. The system is run in real-time, and can be executed as a stand-alone analysis or cycled with an ocean forecast model in a sequential incremental update cycle. Additional capabilities have been built into the system, including flow-dependent background-error correlations and background-error variances that vary in space and evolve from one analysis cycle to the next. The ocean data types assimilated include: remotely sensed sea surface temperature, sea surface height, and sea-ice concentration; plus in situ surface and sub-surface observations of temperature, salinity, and currents from a variety of sources, such as ships, buoys, expendable bathythermographs, conductivity-temperature-depth sensors, and profiling floats. An ocean data quality-control system is fully integrated with the multivariate analysis, and includes feedback of forecast fields and prediction errors in the quality control of new observations. The system is operational at the US Navy oceanographic production centres both in global and in regional applications. It is being implemented as the data assimilation component of the Hybrid Coordinate Ocean Model as part of the US contribution to the Global Ocean Data Assimilation Experiment, and in a limited-area ensemble-based forecasting system that will be used in an adaptive sampling, targeted observation application.

Aerosol impact on the dynamics and microphysics of deep convective clouds

Abstract: Mechanisms through which atmospheric aerosols affect cloud microphysics, dynamics and precipitation are investigated using a spectral microphysics two-dimensional cloud model. A significant effect of aerosols on cloud microphysics and dynamics has been found. Maritime aerosols lead to a rapid formation of raindrops that fall down through cloud updraughts increasing the loading in the lower part of a cloud. This is, supposedly, one of the reasons for comparatively low updraughts in maritime convective clouds. An increase in the concentration of small cloud condensation nuclei (CCN) leads to the formation of a large number of small droplets with a low collision rate, resulting in a time delay of raindrop formation. Such a delay prevents a decrease in the vertical velocity caused by the falling raindrops and thus increases the duration of the diffusion droplet growth stage, increasing latent heat release by condensation. The additional water that rises to the freezing level increases latent heat release by freezing. As a result, clouds developing in continental-type aerosol tend to have larger vertical velocities and to attain higher levels. The results show that a decrease in precipitation efficiency of single cumulus clouds arising in micro-physically continental air is attributable to a greater loss of the precipitating mass due to a greater sublimation of ice and evaporation of drops while they are falling from higher levels through a deep layer of dry air outside cloud updraughts. By affecting precipitation, atmospheric aerosols influence the net heating of the atmosphere. Simulations show that aerosols also change the vertical distribution of latent heat release, increasing the level of the heating peak. Clouds arising under continental aerosol conditions produce as a rule stronger downdraughts and stronger convergence in the boundary layer. Being triggered by larger dynamical forcing, secondary clouds arising in microphysically continental air are stronger and can, according to the results of simulations, form a squall line. The squall line formation was simulated both under maritime (GATE-74) and continental (PRE-STORM) thermodynamic conditions. In the maritime aerosol cases, clouds developing under similar thermodynamic conditions do not produce strong downdraughts and do not lead to squall line formation. Thus, the ‘aerosol effect’ on precipitation can be understood only in combination with the ‘dynamical effect’ of aerosols. Simulations allow us to suggest that aerosols, which decrease the precipitation efficiency of most single clouds, can contribute to the formation of very intensive convective clouds and thunderstorms (e.g. squall lines, etc.) accompanied by very high precipitation rates. Affecting precipitation, net atmospheric heating and its vertical distribution, as well as cloud depth and cloud coverage, atmospheric aerosols (including anthropogenic ones) influence atmospheric motions and radiation balance at different scales, from convective to, possibly, global ones. Copyright © 2005 Royal Meteorological Society.

Bias and data assimilation

Abstract: All data assimilation systems are affected by biases, caused by problems with the data, by approximations in the observation operators used to simulate the data, by limitations of the assimilating model, or by the assimilation methodology itself. A clear symptom of bias in the assimilation is the presence of systematic features in the analysis increments, such as large persistent mean values or regularly recurring spatial structures. Bias can also be detected by monitoring statistics of observed-minus-background residuals for different instruments. Bias-aware assimilation methods are designed to estimate and correct systematic errors jointly with the model state variables. Such methods require attribution of a bias to a particular source, and its characterization in terms of some well-defined set of parameters. They can be formulated either in a variational or sequential estimation framework by augmenting the system state with the bias parameters.

A kinetic energy backscatter algorithm for use in ensemble prediction systems

Abstract: Physical justification is provided for the use of kinetic energy backscatter in forecast models, particularly in respect of ensemble prediction systems. The rate of energy backscatter to scales near the truncation limit is controlled by a total energy dissipation function involving contributions from numerical diffusion, mountain drag and deep convection. A cellular automaton is used to generate evolving patterns that, together with the dissipation function, define a stream-function forcing field. Each member of the European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble forecast system is perturbed by a different realization of this backscatter forcing, and the resulting increase in ensemble spread, if not excessive, has a beneficial impact on probabilistic measures of forecast skill. The input of small-scale kinetic energy by the backscatter algorithm also helps to correct a known problem with the energy spectrum in the ECMWF model—the absence of the observed −5/3 spectral slope in the mesoscales. Copyright © 2005 Royal Meteorological Society.

Ensemble Kalman filtering

Abstract: An ensemble Kalman filter (EnKF) has been implemented at the Canadian Meteorological Centre to provide an ensemble of initial conditions for the medium-range ensemble prediction system. This demonstrates that the EnKF can be used for operational atmospheric data assimilation. We show how the EnKF relates to the Kalman filter. In particular, to make the ensemble approximation feasible, we have to use a fairly small ensemble with many less members than either the number of model coordinates, or the number of independent observations, or the (unknown) dimension of the dynamical system. To nevertheless obtain good results, we must (i) counter the tendency of the ensemble spread to underestimate the true error, and (ii) localize the ensemble covariances. The localization is severe and leads to imbalance in the initial conditions. The operational EnKF is used to investigate to what extent our system respects the underlying hypotheses of both the Kalman filter and its ensemble approximation. In particular, we quantify the imbalance in the initial conditions and the magnitude of the model-error component. The occurrence of imbalance constrains the ways in which time interpolation can be performed and in which parametrized model error can be added. With this study we hope to obtain and provide guidance for further improvements to the EnKF. Copyright © 2005 Royal Meteorological Society

Ensemble-derived stationary and flow-dependent background-error covariances: Evaluation in a quasi-operational NWP setting

Abstract: In this study several approaches for obtaining more accurate background-error covariances for atmospheric data assimilation are evaluated. Experiments are conducted by replacing the covariances in the operational three-dimensional variational analysis system at the Canadian Meteorological Centre. In the current system, these covariances are computed using the so-called NMC method that is known to suffer from several deficiencies. The approaches evaluated in this study attempt to more realistically sample the probability distribution of background error by simulating (using a Monte Carlo approach) the error generated at each stage of the forecast-analysis process. The ensemble Kalman filter and a simpler approach applied to an existing forecast-analysis system are both used to generate these error samples. In addition, error samples are generated directly from the covariances of the operational system to allow the effects of sampling error to be quantified. Several strategies for estimating the full covariance matrix from a relatively small number of error samples are then employed. Approaches include the use of a spatially localized ensemble representation of the correlations that allows the usual assumptions of homogeneity and isotropy to be relaxed. In addition, the use of a weighted average between such a covariance matrix and a covariance matrix with homogeneous and isotropic correlations is evaluated. Several diagnostic results from the estimated background-error covariances are presented in addition to verification statistics computed from two-week forecast-analysis experiments. Modest forecast improvements are obtained by using the new background-error covariance estimates, mostly in the southern hemisphere. However, additional results suggest that further improvements may be gained by increasing the number of error samples and a preliminary quantitative estimate of the expected gain is computed. © Crown copyright, 2005. Royal Meteorological Society

The diurnal cycle of the West African monsoon circulation

Abstract: SUMMARY Using numerical model analyses, it is shown that there is a coherent diurnal cycle of the West African monsoon winds. As has been observed in previous studies of arid and semi-arid areas, the winds are at their weakest in the afternoon when the convective boundary layer (CBL) is deep, and intensify overnight when the boundary-layer turbulence is much weaker. This diurnal cycle is maximized in the northern part of the monsoon layer, where the meridional pressure gradient and the diurnal cycle of the CBL are both strong. The diurnal cycle can also be resolved in surface and upper-air data, which show how the nocturnal meridional circulation acts to stratify the lower part of the monsoon layer. In contrast, mixing in the daytime CBL acts to maintain the baroclinicity, as has been observed in laboratory flows. This pattern has implications for the efficiency of the monsoon circulation in the continental water budget, as well as in mixing of trace gases and aerosols between the surface layer and the free troposphere. Vertical mixing occurs by day, while meridional advection, with isentropic upgliding and downgliding, is most efficient at night. Finally, high-resolution observations from the JET2000 experiment are used to show that there is mesoscale structure in the diurnally varying monsoon circulation. In the nocturnal flows, local circulations have been observed and appear to represent a response to recent deep convective events. In contrast, the daytime CBL properties at these scales have been shown in a previous study to map closely onto patterns of soil moisture, with horizontal advection playing a weaker role.

Effects of stochastic parametrizations in the Lorenz '96 system

Abstract: Stochastic parametrization of the effects of unresolved variables is studied in the context of the Lorenz '96 system These parametrizations are found to produce clear improvements in correspondence between the model and ‘true’ climatologies; they similarly provide clear improvements in all ensemble forecast verification measures investigated, including accuracy of ensemble means and ensemble probability estimation, and including measures operating on both scalar (each resolved forecast variable evaluated individually) and vector (all forecast variables evaluated simultaneously) predictands Scalar accuracy measures for non-ensemble (ie single integration) forecasts are, however, degraded The results depend very strongly on both the amplitude (standard deviation) and time-scale of the stochastic forcing, but only weakly on its spatial scale In general there seems not to be a single clear optimum combination of time-scale and amplitude, but rather there exists a range of combinations producing similar results Copyright © 2005 Royal Meteorological Society

Evaluation of probabilistic prediction systems for a scalar variable

Abstract: SUMMARY A systematic study is performed of a number of scores that can be used for objective validation of probabilistic prediction of scalar variables: Rank Histog rams, Discrete and Continuous Ranked Probability Scores (DRPS and CRPS respectively). The reliability-resolution-uncertainty decomposition, defined by Murphy (1972a and 1972b) for the DRPS, and extented here to the CRPS, is studied in detail. The decomposition is applied to the results of the Ensemble Prediction Systems of ECMWF and NCEP. Comparison is made with the decomposition of the CRPS defined by Hersbach (2000). The pos sibility of determining an accurate reliabilityresolution decomposition of the RPS’s is severely limited b y the unavoidably (relatively) small number of available realizations of the prediction system. The Hersbach decomposition may be an appriopriate compromise between the competing needs for accuracy and practical computability.

Parametrization of ice‐particle size distributions for mid‐latitude stratiform cloud

Abstract: SUMMARY Particle size distributions measured by the UK C-130 aircraft in ice stratiform cloud around the British Isles are analysed. Probability distribution functions over large scales show that the zeroth, second and fourth moments (equivalent to concentration, ice water content and radar reflectivity) as well as mean particle size have monomodal distributions. Rescaling of the size distributions requiring knowledge of two moments reveals a ‘universal’ distribution that has been fitted with analytically integrable functions. The existence of the ‘universal’ distribution implies that two-moment microphysics schemes are adequate to represent particle size distributions (PSDs). In large-scale models it may be difficult to predict two moments, and so power laws between moments have been found as functions of in-cloud temperature. This means that a model capable of predicting ice water content and temperature can predict ice PSDs to use for calculations requiring knowledge of the size distribution (e.g. precipitation rate, radar reflectivity) or to make direct use of the power laws relating moments of the size distribution.

Overview of global data assimilation developments in numerical weather‐prediction centres

Abstract: Recent data assimilation developments which have taken place at numerical weather-prediction centres are briefly discussed, from the perspectives of both the importance of data and algorithmic developments. The increase in quality and quantity of satellite data is seen to play a major role in the improvement of forecast performance, particularly in the southern hemisphere. Further optimization of the use of observations is possible through the proper evaluation of data impact and the optimization of the amount of data to be assimilated. The generalized advent of four-dimensional variational assimilation is presented, and trends in the specification of error statistics are described. Finally, a more interactive forecasting system including an adaptive component is a new challenge to bring additional improvement to the forecasting of high-impact weather. Copyright © 2005 Royal Meteorological Society

Ensemble data assimilation for an eddy-resolving ocean model of the Australian region

Abstract: The Bluelink Ocean Data Assimilation System (BODAS) is an ensemble optimal interpolation system applied to a global ocean general-circulation model with 10 km resolution around Australia. BODAS derives estimates of forecast-error covariances (FECs) from a stationary 72-member ensemble of intraseasonal model anomalies. The FECs are localized around each observation to reduce the negative effects of sampling error and to increase the rank of the ensemble. The FECs have characteristics that reflect the length-scales and the anisotropy of the ocean circulation in different regions. BODAS assimilates in situ and satellite-derived observations of temperature, salinity and sea-level anomaly. Results from a 13-year ocean re-analysis demonstrate that the reanalysed fields are often in very good agreement with withheld observations, and provide a good synoptic representation of the eddy field around Australia. Copyright © 2005 Royal Meteorological Society

Mechanisms of ENSO-forcing of hemispherically symmetric precipitation variability

Abstract: Summary The patterns of precipitation anomalies forced by the El Nino-Southern Oscillation during Northern Hemisphere winter and spring are remarkably hemispherically symmetric and, in the mid-latitudes, have a prominent zonally symmetric component. Observations of global precipitation variability and the moisture budget within atmospheric reanalyses are examined to argue that the zonally symmetric component is caused by interactions between transient eddies and tropically-forced changes in the subtropical jets. During El Nino events the jets strengthen in each hemisphere and shift equatorward. Changes in the subtropical jet influence the transient eddy momentum fluxes and the eddy-driven mean meridional circulation. During El Nino events eddy-driven ascent in the mid-latitudes of each hemisphere is accompanied by low level convergence and brings increased precipitation. This changes in the transient eddy and stationary eddy moisture fluxes almost exactly cancel each other and, in sum, do not contribute to the zonal mean precipitation anomalies. Propagation of anomalous stationary waves disrupts the zonal symmetry. Flow around the deeper Aleutian Low and the eastward extension of the Pacific jet stream supply the moisture for increased precipitation over the eastern North Pacific and the western seaboard of the United States while transient eddy moisture convergence supplies the moisture for increased precipitation over the southern United States. In each case increased precipitation is fundamentally caused by anomalous ascent forced by anomalous heat and vorticity fluxes.

Broadening of droplet size distributions from entrainment and mixing in a cumulus cloud

Abstract: Quantitative predictions of the relationship between the droplet size-distribution width and entrainment in warm cumulus have been elusive, largely because of the difficulty in representing the extent of the scales involved. A new modelling framework is presented as a first step toward quantitative predictions of droplet size distributions resulting from entrainment, consisting of a three-dimensional cloud model coupled with a Lagrangian microphysical parcel model. The cloud model represents turbulent cloud dynamics but parametrizes microphysical processes such as condensation, and the parcel model complements this approach by performing explicit microphysical calculations within the kinematic and thermodynamic constraints established by the cloud model. The parcel model is run along trajectories all ending at the same point in the cloud, and the individual droplet size distributions are averaged together at this point to represent the turbulent mixing together of the droplets produced by these different parcel trajectories. The results replicate some important features of observed cloud droplet size distributions, including large widths, the continued presence of small droplets high in the clouds, and the bimodal structure. The origin of these features in these calculations is the variability introduced by entrainment, which leads to possibilities for droplets to encounter varying supersaturation histories during their transit through the cloud to the point of observation. Droplet sizes larger than those calculated for adiabatic ascent are also produced, with possible implications for coalescence initiation. Copyright © 2005 Royal Meteorological Society

A multivariate balance operator for variational ocean data assimilation

Abstract: It is common in meteorological applications of variational assimilation to specify the error covariances of the model background state implicitly via a transformation from model space where variables are highly correlated to a control space where variables can be considered to be approximately uncorrelated. An important part of this transformation is a balance operator which effectively establishes the multivariate component of the error covariances. The use of this technique in ocean data assimilation is less common. This paper describes a balance operator that can be used in a variable transformation for oceanographic applications of three- and four-dimensional variational assimilation. The proposed balance operator has been implemented in an incremental variational data assimilation system for a global ocean general-circulation model. Evidence that the balance operator can explain a significant percentage of background-error variance is presented. The multivariate analysis structures implied by the balance operator are illustrated using single-observation experiments. Copyright © 2005 Royal Meteorological Society

An overview of the variational assimilation in the ALADIN/France numerical weather‐prediction system

Abstract: We present an overview of the 3D-Var data assimilation in the framework of the ALADIN/France model. The purpose of this system is to provide improved precipitation forecasts at mesoscale and in the short range, up to 18 hours. The goal of the paper is threefold. Firstly, we present initial considerations for the design of the 3D-Var system. Secondly, we discuss in more detail the specification of the background-error covariance matrix, by comparing three different error simulation techniques, namely two variants of the NMC method and an ensemble-based approach. The formal, diagnostic and impact studies have led to the selection of the ensemblebased covariances for the ALADIN/France assimilation. Thirdly, scores of quantitative precipitation forecasts are shown in order to illustrate the robustness and the preliminary meteorological performance of the ALADIN/France assimilation suite. The results indicate that the tested configuration improves some aspects of the precipitation forecast, while being neutral for others, when compared with the spin-up model. We conclude the paper by providing a more explicit insight into the future evolution of limited-area variational analysis towards convective-scale data assimilation. Copyright © 2005 Royal Meteorological Society

Analysis of the African easterly jet, using aircraft observations from the JET2000 experiment

Abstract: Analyses of the African easterly jet (AEJ) are presented which are based on meridional transects of high-resolution dropsonde observations made during JET2000, an aircraft campaign conducted in the last week of August 2000. The observations have confirmed that the AEJ is closely defined by geostrophic balance. The baroclinicity between the extreme northern and southern profiles accurately determines the altitude of the jet core, while the location and morphology of the jet core correspond to a locally-defined geostrophic wind measure. The potential-vorticity (PV) structure has also been found to accord with theoretical expectations, with distinctive positive- and negative-PV anomalies equatorward and poleward of the jet core respectively. The thermodynamic structure of the AEJ environment can be categorized into coherent layers. The monsoon layer is a humid zone connected to the land surface, extending northwards into the Sahel and increasing in depth towards the south. This layer is affected by the land surface on diurnal time-scales, through the growing convective mixed layer and through shallow cumulus clouds. Above the monsoon layer is the Saharan air layer (SAL), which can be identified as a layer of low static-stability and low PV. The SAL is deep where it merges with the Saharan boundary-layer in the north, and becomes thinner toward the south. It has been shown that the boundaries of the SAL can be approximated to good accuracy as adiabatic surfaces, meaning that the SAL comprises air which is adiabatically connected to the land surface via the Saharan boundary-layer. The upper region of the SAL is identified as a layer of high relative-humidity where altocumulus and stratocumulus layers are observed. Finally, the troposphere above the SAL is again almost pseudoadiabatic, with small baroclinicity which determines the closure of the AEJ core aloft. Through inspection of thermodynamic tracers, evidence of convective and lateral transport and exchange between these layers is also presented. Copyright © 2005 Royal Meteorological Society

Consistent approximate models of the global atmosphere: shallow, deep, hydrostatic, quasi-hydrostatic and non-hydrostatic

Abstract: We study global atmosphere models that are at least as accurate as the hydrostatic primitive equations (HPEs), reviewing known results and reporting some new ones. The HPEs make spherical geopotential and shallow atmosphere approximations in addition to the hydrostatic approximation. As is well known, a consistent application of the shallow atmosphere approximation requires omission of those Coriolis terms that vary as the cosine of latitude and of certain other terms in the components of the momentum equation. An approximate model is here regarded as consistent if it formally preserves conservation principles for axial angular momentum, energy and potential vorticity, and (following R. Muller) if its momentum component equations have Lagrange's form. Within these criteria, four consistent approximate global models, including the HPEs themselves, are identified in a height-coordinate framework. The four models, each of which includes the spherical geopotential approximation, correspond to whether the shallow atmosphere and hydrostatic (or quasi-hydrostatic) approximations are individually made or not made. Restrictions on representing the spatial variation of apparent gravity occur. Solution methods and the situation in a pressure-coordinate framework are discussed. © Crown copyright 2005.

Probabilistic physically based cloud screening of satellite infrared imagery for operational sea surface temperature retrieval

Abstract: SUMMARY We propose and demonstrate a fully probabilistic (Bayesian) approach to the detection of cloudy pixels in thermal infrared (TIR) imagery observed from satellite over oceans. Using this approach, we show how to exploit the prior information and the fast forward modelling capability that are typically available in the operational context to obtain improved cloud detection. The probability of clear sky for each pixel is estimated by applying Bayes’ theorem, and we describe how to apply Bayes’ theorem to this problem in general terms. Joint probability density functions (PDFs) of the observations in the TIR channels are needed; the PDFs for clear conditions are calculable from forward modelling and those for cloudy conditions have been obtained empirically. Using analysis fields from numerical weather prediction as prior information, we apply the approach to imagery representative of imagers on polar-orbiting platforms. In comparison with the established cloud-screening scheme, the new technique decreases both the rate of failure to detect cloud contamination and the false-alarm rate by one quarter. The rate of occurrence of cloud-screening-related errors of >1 K in area-averaged SSTs is reduced by 83%.

A pre-operational variational data assimilation system for a non-hydrostatic model at the Japan Meteorological Agency: formulation and preliminary results

Abstract: A new variational data assimilation system for a non-hydrostatic model is being developed at the Japan Meteorological Agency (JMA) for operational use. Known as the JMA non-hydrostatic model variational data assimilation (JNoVA) system, it mainly functions as a four-dimensional variational data assimilation system, although it has an option to be used as a three-dimensional system. The set of control variables consists of initial conditions of unbalanced horizontal winds, large-scale components of potential temperature and surface pressure, unbalanced temperature and pseudo relative humidity. In the control variable transformation, hydrostatic balance and geostrophic balance are considered explicitly and the effect of the surface friction is also considered implicitly. When calculating the background-error covariances by the NMC method, a low-pass filter is introduced to remove noise in potential temperature and surface pressure that degrades the quality of the balanced winds. The cut-off wavelength of the low-pass filter is set to 300 km, which is the scale at which the model's kinetic energy spectrum transits to the shallower slope characterizing mesoscale motions. An adjoint model of the JMA non-hydrostatic model has been developed from scratch by hand for this system. Although some of the physics are simplified, all physical processes except the radiation are considered. A preliminary data assimilation experiment with the JNoVA has been done for a heavy rainfall event. The results show that the quantitative precipitation forecast (in terms of the intensity, timing and position of the event) from the analysis by the JNoVA is improved over the forecast from the analysis by a four-dimensional variational system that employs the JMA hydrostatic spectral model. Copyright © 2005 Royal Meteorological Society

Modelling tropical atmospheric convection in the context of the weak temperature gradient approximation

Abstract: A cumulus ensemble model is used to simulate the interaction between tropical atmospheric convection and the large-scale tropical environment in the context of Sobel and Bretherton's (2000) weak temperature gradient approximation. In this approximation, gravity waves are assumed to redistribute buoyancy anomalies over a broad area of the tropics, thus maintaining the local virtual-temperature profile close to the large-scale mean. This result is implemented in the model by imposing the advective effects of a hypothetical mean vertical velocity which is just sufficient to counteract the local heating induced by convection and radiation. The implied vertical advection in the moisture equation and entrainment of air from the surrounding environment have major effects on the evolution of convection in the model. The precipitation produced by the model mimics the results of a very simple model of tropical precipitation introduced by Raymond (2000), in that the mean rainfall rate predicted by the cumulus ensemble model is, to a good approximation, a function only of the mean column precipitable water. The evolution of the precipitable water, and hence the precipitation rate, is a result of the imbalance between the surface flux of moist entropy into the domain and the radiative loss of entropy out of the top of the domain. This evolution leads to a statistically steady solution in which the resulting precipitation rate is a unique function of the entropy flux imbalance. These results support the hypothesis that tropical precipitation averaged over distance scales of a few hundred kilometres and time scales of a day is a consequence only of local thermodynamic factors. Copyright © 2005 Royal Meteorological Society

Convective-scale assimilation of radar data : Progress and challenges

Abstract: Active research has been carried out in recent years to assimilate high-resolution observations into numerical models to improve precipitation forecasting. Considerable progress has been made although great scientific and technological challenges still exist. This paper reviews techniques used in convective-scale data assimilation research. Experiences in the assimilation of radar observations into high-resolution numerical models are presented. A number of future challenges in convective-scale data assimilation are discussed. Copyright © 2005 Royal Meteorological Society

The role of the basic state in the ENSO–monsoon relationship and implications for predictability

Abstract: The impact of systematic model errors on a coupled simulation of the Asian Summer monsoon and its interannual variability is studied. Although the mean monsoon climate is reasonably well captured, systematic errors in the equatorial Pacific mean that the monsoon-ENSO teleconnection is rather poorly represented in the GCM. A system of ocean-surface heat flux adjustments is implemented in the tropical Pacific and Indian Oceans in order to reduce the systematic biases. In this version of the GCM, the monsoon-ENSO teleconnection is better simulated, particularly the lag-lead relationships in which weak monsoons precede the peak of El Nino. In part this is related to changes in the characteristics of El Nino, which has a more realistic evolution in its developing phase. A stronger ENSO amplitude in the new model version also feeds back to further strengthen the teleconnection. These results have important implications for the use of coupled models for seasonal prediction of systems such as the monsoon, and suggest that some form of flux correction may have significant benefits where model systematic error compromises important teleconnections and modes of interannual variability.

Resistance and heat-transfer laws for stable and neutral planetary boundary layers: Old theory advanced and re-evaluated

Abstract: The planetary boundary layer (PBL) resistance and heat-transfer laws express the surface fluxes of momentum and heat through the PBL governing parameters. Since the late sixties, the dimensionless coefficients (A, B and C) in these laws were considered as single-valued functions of internal stability parameters: µ=u*/|f|Ls in the steady state PBLs, or h/Ls in the evolving PBLs (u* is the friction velocity, f is the Coriolis parameter, Ls is the surface Monin–Obukhov length, and h is the PBL depth). Numerous studies revealed very wide spread of data in empirical plots of A, B and C versus µ or h/Ls. It is not surprising that the above laws, although included in all modern textbooks on boundary-layer meteorology, are not practically used. In the present paper the resistance and heat-transfer laws are revised, accounting for the free-flow stability, baroclinicity and the rise of a capping inversion. The coefficients A, B and C become functions not only of µ or h/Ls, but also of the external stability parameter µN=N/|f| (where N is the Brunt–Vaisala frequency in the free atmosphere above the PBL), the parameter of baroclinicity µΓ=Γ/N (or the free-flow Richardson number Ri=(N/Γ)2=µΓ−2, where Γ is the geostrophic wind shear), and the ratio of the actual and equilibrium PBL depths h/hE. Moreover, the coefficient C is redefined to account for the effect of a capping inversion. It follows that A, B and C can be considered as single-valued functions of µ only in the steady-state, barotropic, nocturnal (that is short-lived) PBL. On the other hand, the advanced laws cover a wide range of the PBL regimes. They are validated through large-eddy simulations of different types of PBLs: truly neutral, conventionally neutral, nocturnal and long-lived. This new development explains why prior formulations performed so poorly, and promotes advanced resistance and heat-transfer laws as practical tools for use in environmental modelling applications. Copyright © 2005 Royal Meteorological Society.

Issues in targeted observing

Abstract: This paper summarizes successes and limitations of targeted observing field programmes starting from the Fronts and Atlantic Storm-Track Experiment in 1997 through recent programmes targeting winter storms and tropical cyclones. These field programmes have produced average reductions in short-range forecast errors of about 10 per cent over regional verification areas, and maximum forecast error reductions as large as 50 per cent in certain cases. The majority of targeting cases investigated so far involve sets of dropsondes and other observation data that provide partial coverage of target areas. The primary scientific challenges for targeting include the refinement of objective methods that can identify optimal times and locations for targeted observations, as well as identify the specific types of satellite and in situ measurements that are required for the improvement of numerical weather forecasts. The most advanced targeting procedures, at present, include: the ensemble transform Kalman Filter, Hessian singular vectors, and observation-space targeting using the adjoint of a variational data assimilation procedure. Targeted observing remains an active research topic in numerical weather prediction, with plans for continued refinement of objective targeting procedures, and field tests of new satellite and in situ observing systems. Copyright © 2005 Royal Meteorological Society

Tropical plumes and extreme precipitation in subtropical and tropical West Africa

Abstract: Three cases of extreme precipitation in October, January and March/April over subtropical north-western and tropical West Africa have been selected in order to study the moisture transports, precipitation generation mechanisms and large-scale dynamics involved. All cases show strong tropical–extratropical interactions and elongated cloud bands extending from the Tropics into the subtropics, usually referred to as tropical plumes (TPs). Investigations are based on observational data and high-resolution output from simulations using the University of Wisconsin–Nonhydrostatic Modeling System. Trajectory analyses show that moisture is transported from the tropical easterly mid-level flow, the monsoonal southerlies and the north-easterly trade winds to provide precipitable water and potential instability at mid-tropospheric levels. The instability is released through ascent related both to weak quasi-geostrophic forcing to the east of an upper-level trough and to the strong, inertially unstable and highly divergent upper-level subtropical jet (STJ) accompanying the TP. In the January and March cases the passage of a precursor upper-level trough over the same location precedes the development and initiates the moisture transport from the deep Tropics. The associated north-westerly cold advection supports the generation of precipitation by triggering convection over the tropical Atlantic Ocean, whose outflow feeds into the STJ over Africa, and by enhancing the vertical mass transport within the intertropical discontinuity over and near West Africa. In October, when the still active African monsoon facilitates the extraction of tropical moisture, a wave in the tropical easterlies and strong trade winds from the southern hemisphere are dominant factors. Low-level cyclogenesis and frontogenesis only play roles in the late stages of the developments. The elongated, mostly positively tilted potential-vorticity (PV) streamers associated with the observed troughs form as a result of an equatorward transport of high-PV air downstream of a large ridge over the North Atlantic. The rapid amplification of the ridge is achieved through a combination of negative horizontal PV advection and diabatic reduction of upper-level PV through latent heating within a cloud band that forms in connection with an explosive baroclinic development near the east coast of North America. Copyright © 2005 Royal Meteorological Society