Showing papers in "Quarterly Journal of the Royal Meteorological Society in 2006"

Radar precipitation measurement in a mountainous region

Abstract: Real-time precipitation measurement by radar in a mountainous region requires appropriate system design and innovative solutions. Of particular difficulty are the elimination of ground echoes and the correction of errors caused by shielding of the radar beam by mountain ranges. This paper presents the Swiss solution with a focus on recent modifications. A large-sample comparison between radar precipitation estimates and ground observations from a high-resolution gauge network reveals distinct improvements achieved by the modifications of the past 8 years. The verification takes into account all days of the period 1997–2004 and gauge locations spread over all of Switzerland. Particular attention was paid to the definition of a set of meaningful and robust quality parameters. This rigorous and systematic verification approach may become a standard for objective judgement of radar performance. Copyright © 2006 Royal Meteorological Society

New version of the TOMCAT/SLIMCAT off‐line chemical transport model: Intercomparison of stratospheric tracer experiments

Abstract: We describe the development of a new three-dimensional off-line chemical transport model (CTM). The CTM has been produced by combining the existing, and closely related, TOMCAT and SLIMCAT models. The new CTM (TOMCAT/SLIMCAT) has a flexible vertical coordinate which can use both σ-p and σ-θ levels. A novel approach is used for the σ-θ coordinate to ensure a smooth transition between levels. The CTM has different options for calculating vertical transport in the stratosphere, depending on the coordinate chosen. The CTM also has different options for other processes such as advection scheme, radiation scheme and meteorological forcing. We have used different configurations of the new CTM to perform tests on stratospheric tracer transport. Using ECMWF ERA-40 analyses, the σ-p coordinate model gives a stratospheric age of air which is much too low, and a tropical tape-recorder signal which propagates vertically too rapidly. Changing the coordinates to σ-θ levels, and still using the analyses to calculate the vertical motion, removes spurious vertical mixing and improves the modelled age of air significantly, although it still tends to underestimate the observations. If we use a radiation scheme to calculate the stratospheric diabatic transport in the σ-θ model we get a greater age of air, and the best overall agreement with the observations of age of air from in situ data and estimates of the tape-recorder signal from Halogen Occultation Experiment CH4 and H2O data. Based on the model results, interannual variability can cause age of air changes of up to ∼1 year in the mid/high-latitude lower stratosphere. Similar differences can be caused by changing to winds from the UK Met Office. For the year 2001, where two sets of ECMWF analyses are available, the operational analyses produce a greater stratospheric age of air than the ERA-40 re-analyses. Copyright © 2006 Royal Meteorological Society.

Structure and dynamics of the summertime Pacific–Japan teleconnection pattern

Abstract: Convective activity over the tropical western Pacific is known to influence the extratropical circulation over East Asia in the boreal summer in the form of teleconnection, called the ‘Pacific–Japan (PJ) pattern’, but its structure and dynamics have not yet been studied in depth. In this study, a composite analysis is performed for 32 monthly events of enhanced convection observed to the east of the Philippines. The composited monthly mean vorticity anomalies associated with the PJ pattern are elongated zonally with a distinct poleward tilt with height. This structure differs fundamentally from a combination of the first baroclinic mode in the tropics and the barotropic structure in midlatitudes, as has widely been accepted as a conceptual model of the PJ pattern. A wave-activity flux points polewards only in the lower troposphere, indicating that Rossby wave teleconnection occurs primarily through a low-level south-westerly jet. Those tilted anomalies over the western Pacific can effectively gain kinetic energy in the exits of the mean jet streams in the upper and lower troposphere and available potential energy (APE) in the presence of the vertically sheared jets. The enhanced convection can generate APE effectively, and the associated low-level anomalous circulation acts to increase moisture supply into the convective region while enhancing evaporation from the pre-warmed ocean surface. It is thus hypothesized that the PJ pattern may be regarded as a dynamical mode that can be effectively excited in the zonally asymmetric baroclinic mean flow associated with the Asian summer monsoon with an efficient self-sustaining mechanism through moist processes. Copyright © 2006 Royal Meteorological Society

STEPS: A probabilistic precipitation forecasting scheme which merges an extrapolation nowcast with downscaled NWP

Abstract: An ensemble-based probabilistic precipitation forecasting scheme has been developed that blends an extrapolation nowcast with a downscaled NWP forecast, known as STEPS: Short-Term Ensemble Prediction System. The uncertainties in the motion and evolution of radar-inferred precipitation fields are quantified, and the uncertainty in the evolution of the precipitation pattern is shown to be the more important. The use of ensembles allows the scheme to be used for applications that require forecasts of the probability density function of areal and temporal averages of precipitation, such as fluvial flood forecasting—a capability that has not been provided by previous probabilistic precipitation nowcast schemes. The output from a NWP forecast model is downscaled so that the small scales not represented accurately by the model are injected into the forecast using stochastic noise. This allows the scheme to better represent the distribution of precipitation rate at spatial scales finer than those adequately resolved by operational NWP. The performance of the scheme has been assessed over the month of March 2003. Performance evaluation statistics show that the scheme possesses predictive skill at lead times in excess of six hours. © Crown copyright, 2006.

Accounting for an imperfect model in 4D‐Var

Abstract: In most operational implementations of four-dimensional variational data assimilation (4D-Var), it is assumed that the model used in the data assimilation process is perfect or, at least, that errors in the model can be neglected when compared to other errors in the system. In this paper, we study how model error could be accounted for in 4D-Var. We present three approaches for the formulation of weak-constraint 4D-Var: estimating explicitly a model-error forcing term, estimating a representation of model bias or, estimating a four-dimensional model state as the control variable. The consequences of these approaches with respect to the implementation and the properties of 4D-Var are discussed. We show that 4D-Var with an additional model-error representation as part of the control variable is essentially an initial-value problem and that its characteristics are very similar to that of strong constraint 4D-Var. Taking the four-dimensional state as the control variable, however, leads to very different properties. In that case, weak-constraint 4D-Var can be interpreted as a coupling between successive strong-constraint assimilation cycles. A possible extension towards long-window 4D-Var and possibilities for evolutions of the data assimilation system are presented. Copyright © 2006 Royal Meteorological Society

The assimilation of AIRS radiance data at ECMWF

Abstract: The development of an assimilation system for radiance data from the Atmospheric InfraRed Sounder (AIRS) is described, in particular the identification of cloud contamination, bias correction and the characterization of errors in the measured radiances and radiative-transfer model. The results of assimilation experiments are presented. These show that a conservative use of AIRS radiance data (in a system already extensively observed with other satellite data) results in a small, but consistent, improvement in the quality of analyses and forecasts. Larger impacts of AIRS are found in hypothetical experiments that test the use of radiances from only a single sounding instrument. In these, the use of AIRS is found to outperform the use of data either from a single Advanced Microwave Sounding Unit-A (AMSU-A) or from a single High-resolution InfraRed Sounder (HIRS). In this hypothetical context the relative forecast performance of each sensor is found to correlate with the size and vertical scale of increments caused by the assimilation of the radiances. Copyright © 2006 Royal Meteorological Society.

Measuring forecast skill: is it real skill or is it the varying climatology?

Abstract: It is common practice to summarize the skill of weather forecasts from an accumulation of samples spanning many locations and dates. In calculating many of these scores, there is an implicit assumption that the climatological frequency of event occurrence is approximately invariant over all samples. If the event frequency actually varies among the samples, the metrics may report a skill that is different from that expected. Many common deterministic verification metrics, such as threat scores, are prone to mis-reporting skill, and probabilistic forecast metrics such as the Brier skill score and relative operating characteristic skill score can also be affected. Three examples are provided that demonstrate unexpected skill, two from synthetic data and one with actual forecast data. In the first example, positive skill was reported in a situation where metrics were calculated from a composite of forecasts that were comprised of random draws from the climatology of two distinct locations. As the difference in climatological event frequency between the two locations was increased, the reported skill also increased. A second example demonstrates that when the climatological event frequency varies among samples, the metrics may excessively weight samples with the greatest observational uncertainty. A final example demonstrates unexpectedly large skill in the equitable threat score of deterministic precipitation forecasts. Guidelines are suggested for how to adjust skill computations to minimize these effects. Copyright © 2006 Royal Meteorological Society

Assimilation experiments with CHAMP GPS radio occultation measurements

Abstract: A one-dimensional bending-angle observation operator for assimilating GPS radio occultation (RO) measurements has been integrated into the ECMWF four-dimensional variational assimilation (4D-Var) system. We have performed forecast impact experiments with 60 days of CHAMP RO measurements, in addition to the latest set of conventional and satellite data that are assimilated operationally, including radiances from the Atmospheric Infrared Sounder. It is demonstrated that the CHAMP measurements provide extremely good temperature information in the upper troposphere and lower stratosphere. In the southern hemisphere (SH), they produce a clear, statistically significant improvement in the r.m.s. forecast fit to radiosonde measurements over the day 1 to day 5 forecast range at 300, 200, 100 and 50 hPa. An improved r.m.s. fit to radiosondes is also evident at 100 hPa in the tropics. However, the observations degrade the 500 hPa geopotential height (500Z) field in the SH. This appears to be mainly caused by erroneous surface pressure increments in Antarctica. As a result, we have modified the GPS tangent-linear and adjoint routines, prior to the evaluation of the model-level pressures and geopotential heights, in order to remove the sensitivity of the model geopotential height values to the surface pressure. This improves the SH 500Z forecast scores, although a small degradation is still evident at the day 1 and day 2 forecast range. A simple method for estimating the degrees of freedom for signal (DFS) of a large variational assimilation system is noted and applied to estimate the DFS of the CHAMP measurements assimilated during a 12-hour assimilation window. The CHAMP measurements increase the total DFS of the 4D-Var system by ∼4%, and the DFS per CHAMP bending-angle profile is ∼34. Copyright © 2006 Royal Meteorological Society

A baroclinic instability test case for atmospheric model dynamical cores

Abstract: A deterministic initial-value test case for dry dynamical cores of atmospheric general-circulation models is presented that assesses the evolution of an idealized baroclinic wave in the northern hemisphere The initial zonal state is quasi-realistic and completely defined by analytic expressions which are a steady-state solution of the adiabatic inviscid primitive equations with pressure-based vertical coordinates A two-component test strategy first evaluates the ability of the discrete approximations to maintain the steady-state solution Then an overlaid perturbation is introduced which triggers the growth of a baroclinic disturbance over the course of several days The test is applied to four very different dynamical cores at varying horizontal and vertical resolutions In particular, the NASA/NCAR Finite Volume dynamics package, the National Center for Atmospheric Research spectral transform Eulerian and the semi-Lagrangian dynamical cores of the Community Atmosphere Model CAM3 are evaluated In addition, the icosahedral finite-difference model GME of the German Weather Service is tested These hydrostatic dynamical cores represent a broad range of numerical approaches and, at very high resolutions, provide independent reference solutions The paper discusses the convergence-with-resolution characteristics of the schemes and evaluates the uncertainty of the high-resolution reference solutions Copyright © 2006 Royal Meteorological Society

Fibonacci grids: A novel approach to global modelling

Abstract: Recent years have seen a resurgence of interest in a variety of non-standard computational grids for global numerical prediction. The motivation has been to reduce problems associated with the converging meridians and the polar singularities of conventional regular latitude–longitude grids. A further impetus has come from the adoption of massively parallel computers, for which it is necessary to distribute work equitably across the processors; this is more practicable for some non-standard grids. Desirable attributes of a grid for high-order spatial finite differencing are: (i) geometrical regularity; (ii) a homogeneous and approximately isotropic spatial resolution; (iii) a low proportion of the grid points where the numerical procedures require special customization (such as near coordinate singularities or grid edges); (iv) ease of parallelization. One family of grid arrangements which, to our knowledge, has never before been applied to numerical weather prediction, but which appears to offer several technical advantages, are what we shall refer to as ‘Fibonacci grids’. These grids possess virtually uniform and isotropic resolution, with an equal area for each grid point. There are only two compact singular regions on a sphere that require customized numerics. We demonstrate the practicality of this type of grid in shallow-water simulations, and discuss the prospects for efficiently using these frameworks in three-dimensional weather prediction or climate models. © Crown copyright, 2006. Royal Meteorological Society

Daytime convective development over land: A model intercomparison based on LBA observations

Abstract: This paper investigates daytime convective development over land and its representation in single-column models (SCMs) and cloud-resolving models (CRMs). A model intercomparison case is developed based on observations of the diurnal cycle and convection during the rainy season in Amazonia. The focus is on the 6 h period between sunrise and early afternoon which was identified in previous studies as critical for the diurnal cycle over summertime continents in numerical weather prediction and climate models. This period is characterized by the formation and growth of a well-mixed convective boundary layer from the early morning temperature and moisture profiles as the surface sensible- and latent-heat fluxes increase after sunrise. It proceeds with the formation of shallow convective clouds as the convective boundary layer deepens, and leads to the eventual transition from shallow to deep precipitating convection around local noon. To provide a benchmark for other models, a custom-designed set of simulations, applying increasing in time computational domain and decreasing spatial resolution, was executed. The SCMs reproduced the previously identified problem with premature development of deep convection, less than two hours after sunrise. The benchmark simulations suggest a possible route to improve SCMs by considering a time-evolving cumulus entrainment rate as convection evolves from shallow to deep and the cloud width increases up to an order of magnitude. The CRMs featuring horizontal grid length around 500 m are capable of capturing the qualitative aspects of the benchmark simulations, but there are significant differences among the models. Two-dimensional CRMs tend to simulate too rapid a transition from shallow to deep convection and too high a cloud cover. Copyright © 2006 Royal Meteorological Society

Tropical forecasting at ECMWF: The influence of physical parametrization on the mean structure of forecasts and analyses

Abstract: The period 1984–1985 has seen a dramatic improvement in the quality of the ECMWF operational forecasts for tropical regions. This improvement is discussed in terms of the model changes during this period. Revisions include the introduction of a parametrization of shallow cumulus convection, modifications to the parametrization of deep cumulus convection, a new cloud scheme, and an increase in horizontal resolution with a change in spectral truncation from T63 to T106. Impact of the various model changes is assessed through a set of forecast experiments and through parallel 10-day forecasts over a 20-day period. The results show that the improvements in tropical forecasts are mainly through the reduction of systematic errors in response to a more realistic tropical diabatic forcing. The impact of these changes on the analyses is also assessed. This is shown to be substantial, particularly for the thermal state.

The impact of urban areas on weather

Abstract: The industrial revolution led to a rapid development of urban areas. This has continued unremittingly over the last 200 years or so. In most urban areas the surface properties are heterogeneous, which has significant implications for energy budgets, water budgets and weather phenomena within the part of the earth's atmosphere that humans live. In this paper I discuss the structure of the planetary boundary layer, confining our analysis to the region above the rooftops (canopy layer) up to around the level where clouds form. It is in this part of the atmosphere that most of the weather impacting our lives occurs, and where the buildings of our cities impact the weather. In this review, observations of the structure of the urban atmospheric boundary layer are discussed. In particular the use of Doppler lidar provides measurements above the canopy layer. The impact of high-rise buildings is considered. Urban morphology impacts energy fluxes and airflow leading to phenomena such as the urban heat island and convective rainfall initiation. I discuss in situ surface-based remote sensing and satellite measurements of these effects. Measurements have been used with simple and complex numerical models to understand the complexity and balance of the interactions involved. Cities have been found to be sometimes up to 10 degC warmer than the surrounding rural areas, and to cause large increases in rainfall amounts. However, there are situations in which urban aerosol may suppress precipitation. Although much progress has been made in understanding these impacts, our knowledge remains incomplete. These limitations are identified. As city living becomes even more the norm for large numbers of people, it is imperative that we ensure that urban effects on the weather are included in development plans for the built environment of the future. Copyright © 2006 Royal Meteorological Society

The continuous ranked probability score for circular variables and its application to mesoscale forecast ensemble verification

Abstract: An analogue of the linear continuous ranked probability score is introduced that applies to probabilistic forecasts of circular quantities. This scoring rule is proper and thereby discourages hedging. The circular continuous ranked probability score reduces to angular distance when the forecast is deterministic, just as the linear continuous ranked probability score generalizes the absolute error. Furthermore, the continuous ranked probability score provides a direct way of comparing deterministic forecasts, discrete forecast ensembles, and post-processed forecast ensembles that can take the form of probability density functions. The circular continuous ranked probability score is used in this study to assess predictions of 10 m wind direction for 361 cases of mesoscale, short-range ensemble forecasts over the North American Pacic Northwest. Reference probability forecasts based on the ensemble mean and its forecast error history over the period outperform probability forecasts constructed directly from the ensemble sample statistics. These results suggest that short-term forecast uncertainty is not yet well predicted at mesoscale resolutions near the surface, despite the inclusion of multi-scheme physics diversity and surface boundary parameter perturbations in the mesoscale ensemble design. 1

Implementation of 1D+4D‐Var assimilation of precipitation‐affected microwave radiances at ECMWF. I: 1D‐Var

Abstract: This paper presents the operational implementation of a 1D+4D-Var assimilation system of rain-affected satellite observations at the European Centre for Medium-Range Weather Forecasts. The first part describes the methodology and performance analysis of the 1D-Var retrieval scheme in clouds and precipitation that uses Special Sensor Microwave/Imager microwave radiance observations for the estimation of total-column water vapour. The second part shows the global and long-term impact of these observations on both model 4D-Var analyses and medium-range forecasts. The 1D-Var scheme employs a complex observation operator that consists of linearized moist physics parametrization schemes and a multiple-scattering radiative-transfer model. The observation operator shows rather linear behaviour in most situations except in the presence of very intense precipitation suggesting a possible use even for a direct assimilation of radiances in 4D-Var. A bias correction and observation-error estimation method were implemented and indicate stable error behaviour. The 1D-Var algorithm quality control shows the largest failure number in areas with mostly frozen precipitation where the Special Sensor Microwave/Imager channels have little sensitivity to changes in hydrometeor contents. From test analyses on a global scale, a small moisture increase was computed that was greatest in dry subtropical areas. Large-scale and convective precipitation were increased similarly but showed a significantly different geographical distribution. The large-scale precipitation scheme has a stronger sensitivity to moisture changes and therefore moisture increments mainly affect stratiform precipitation distributions. While the global mean moisture fields are only weakly affected by the assimilation of rain-affected observations, the impact on local systems may be quite large. The forecast of synoptic system development through the 4D-Var analysis can be significant. Copyright © 2006 Royal Meteorological Society

Laboratory studies of the effect of cloud conditions on graupel/crystal charge transfer in thunderstorm electrification

Abstract: Collisions between vapour-grown ice crystals and a riming target, representing a graupel pellet falling in a thunderstorm, were shown by Reynolds, Brook and Gourley to transfer substantial charge, which they showed to be adequate to account for the development of charge centres leading to lightning in thunderstorms. Related experiments by Takahashi and Jayaratne et al. determined that the sign of charge transferred is dependent on the cloud liquid water content and on cloud temperature. There are marked differences between the results of Takahashi and Jayaratne in the details of the dependence they noted of the sign of graupel charging on cloud water and temperature. More recently, Pereyra et al. have shown that results somewhat similar in form to those of Takahashi are obtained by modifying the experimental technique used to prepare the clouds of ice crystals and supercooled water droplets used in the experiments. In order to help resolve the reason for the differences in charge transfer results in various studies, work has continued in the Manchester laboratory with a modified cloud chamber in which the cloud conditions of the crystals and droplets may be controlled independently. Results indicate a profound effect on the charge sign of the particle growth conditions in the two clouds involved. For example, by suitable adjustments to the water contents of the two clouds, graupel is charged negatively by rebounding ice crystal collisions at higher cloud water contents than have been noted previously. It is suggested that the most important influence on charge sign is the relative diffusional growth rate of the two ice surfaces at the moment of impact and that this is affected by an increase in cloud supersaturation experienced by the ice crystals during the cloud mixing process just prior to collision. A range of cloud conditions is used in the present work in order to help determine the reasons for the various results reported previously. Examination of some thunderstorm observations in the context of the present results points to the importance of mixing on the sign of the charge transferred during particle collisions when two cloud regions of different histories mix together. Copyright © 2006 Royal Meteorological Society

Influence of the Madden-Julian Oscillation on East African rainfall. I: Intraseasonal variability and regional dependency

Abstract: The influence of the Madden–Julian Oscillation (MJO) on rainfall amounts over Equatorial East Africa (Kenya and northern Tanzania) is analysed for the period 1979–95 at the intraseasonal (pentad) time-scale. The two rainy seasons (March to May and October to December) are considered. Intraseasonal wet events in East Africa are embedded in large-scale zonal circulation anomaly patterns along the equator, showing distinct eastward propagation. It is further found that these ‘wet’ events display a clear phasing with respect to the MJO cycle. This phasing is expressed as out-of-phase variations between the Highland and the coastal areas. Such a pattern is suggested to reflect different rain-causing mechanisms. MJO phases leading to wet spells in the western (Highland) region are those associated with the development of large-scale convection in the Africa/Indian Ocean region. These events are unambiguously related to deep convection, fuelled by low-level westerly moisture advection. MJO phases leading to wet spells in the eastern (coastal) region are often those associated with overall suppressed deep convection in the Africa/Indian Ocean region. However, these phases induce moisture advection from Indian Ocean. The possible role of stratiform rainfall or relatively shallow convection in the coastal wet spells observed in this phase is discussed. The contrasting rainfall conditions found in the two regions for the two opposite MJO phases are strongly correlated with the pressure gradient between the Indian and Atlantic Oceans. Copyright © 2006 Royal Meteorological Society

Some observations of the optical properties of clouds. I: Stratocumulus

Abstract: Aircraft measurements of broad-band solar irradiances together with radiance measurements in the 1-2 to 2-1 μm region are presented for a stratocumulus sheet. Measurements of the solar albedo and cloud physics parameters indicate that the cloud was very uniform and optically thick. Broad-band observations of albedo agreed well with calculations but observed absorption was about double the model values. Narrow-band observations of vertical reflectance at three wavelengths suggested a marked increase in effective cloud droplet radius along the cloud, which was not observed. Some evidence that the discrepancy might be due to cloud top unevenness is presented.

Sensitivity of extratropical cyclone characteristics to horizontal resolution in the ECMWF model

Abstract: The sensitivity to horizontal resolution of northern hemisphere extratropical cyclone characteristics during the wintertime (December–March) is investigated using a set of seasonal forecasts (1982–2001) with the European Centre for Medium-Range Weather Forecasts (ECMWF) model. Three different horizontal resolutions (TL95, TL159 and TL255) are employed. In order to test the realism of the simulations, the model results are compared with those obtained from ERA-40 re-analysis data. The cyclone tracking is accomplished by applying an automatic tracking scheme to six-hourly mean-sea-level pressure data. It is shown that many of the key characteristics of extratropical cyclones in the ECMWF model are highly sensitive to horizontal resolution, with the low-resolution version (TL95), for example, simulating only about 60% of the re-analysed total number of extratropical cyclones. Regions found to be particularly sensitive include the northern Pacific, the Arctic, Baffin Bay and the Labrador Sea, as well as the Mediterranean Sea. For the latter region it is shown that even the relatively high-resolution version of the model (TL255) significantly underestimates the number of cases of Genoa cyclogenesis. Furthermore, it is shown that in some regions, such as the entrance regions of the major northern hemisphere storm tracks, model deficits are insensitive to increases in horizontal resolution. The same analysis has been repeated for the high-resolution operational ECMWF analysis (2000–2004), truncated at different total wave numbers (TL95, TL159, TL255 and TL511) in order to separate dynamical effects of differences in resolution from those due to pure spectral truncation. It is found that the dynamical effect of changing horizontal resolution dominates over the truncation effect for intense cyclones, whereas the truncation effect dominates for shallow cyclones. Copyright © 2006 Royal Meteorological Society

Stratiform precipitation production over sub‐Saharan Africa and the tropical East Atlantic as observed by TRMM

Abstract: Convective systems over sub-Saharan Africa and the tropical East Atlantic have distinct geographical and seasonal variations in convective intensity and stratiform precipitation production as observed by the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar. Over the East Atlantic, convective rain rates are lower and the percentage of total rain that is stratiform is higher compared to over West Africa. In addition, the East Atlantic has more shallow precipitating convection and less non-precipitating anvil than sub-Saharan Africa. During the monsoon season, convective rain rates and the percentage of area covered by anvil decrease while the stratiform rain fraction and number of shallow convective cells increase in both regions compared to the pre-monsoon season. These observations suggest that convective sustainability, i.e. the ability of a region to continually support convection, helps determine whether a robust stratiform rain area or non-precipitating anvil forms. In addition to convective sustainability, wind shear and the Saharan Air Layer appear to play important roles in the formation and extent of the stratiform components of convective cloud systems. Strong winds associated with the African Easterly Jet and dry intrusions from the Sahara Desert may increase sublimation and evaporation at mid levels, resulting in less stratiform rain. Strong upper-level shear associated with the African Easterly Jet may further hinder stratiform rain production by displacing hydrometeors that form in the convective cells beyond the area of mesoscale rain formation. When the upper-level Tropical Easterly Jet strengthens during the monsoon season, upper-level shear is reduced dramatically and stratiform rain areas form in preference to non-raining anvil. Copyright © 2006 Royal Meteorological Society

Diagnosis and tuning of observational error in a quasi-operational data assimilation setting

Abstract: Desroziers and Ivanov proposed a method to tune error variances used for data assimilation. The implementation of this algorithm implies the computation of the trace of certain matrices which are not explicitly known. A method proposed by Girard, allowing an approximate estimation of the traces without explicit knowledge of the matrices, was then used. This paper proposes a new implementation of the Desroziers and Ivanov algorithm, including a new computation scheme for the required traces. This method is compared to Girard's in two aspects: its use in the implementation of the tuning algorithm, and the computation of a quantification of the observation impacts on the analysis known as Degrees of Freedom for Signal. Those results are illustrated by studies utilizing the French data assimilation/numerical weather-prediction system ARPEGE. The impact of a first quasi-operational tuning of variances on forecasts is shown and discussed. Copyright © 2006 Royal Meteorological Society

The Indian drought of 2002—a sub‐seasonal phenomenon?

Abstract: A major drought occurred over India in the year 2002 with a seasonal rainfall deficit of 21.5%, a result of 56% below normal rainfall in the month of July. The largest anomalies occurred in the western parts of India, when an Indian monsoon field experiment was in progress there. The present study is based primarily on data collected from a research ship that was deployed 100–250 km off the west coast of India for the experiment. Surface and upper air observations made over the eastern Arabian Sea during July 2002 are presented. Sea-surface conditions were favourable for supporting deep convection over the study area. Strong atmospheric inversions around 800 hPa prevented the growth of cumulonimbus during the first half and towards the end of July. A second inversion above the melting level was also prominently present. The strength and persistence of the inversions are unusual. Backtrajectory analysis reveals a major change in the low-level circulation during July 2002 with frequent advection of dry air from over the deserts around the eastern Arabian Sea instead of marine air from across the equator.

Investigation into the wavelength dependence of the aerosol optical depth in the Athens area

Abstract: Ground-based solar spectral irradiance measurements obtained by a spectroradiometer have been used to retrieve the aerosol optical depth, τα, and the Angstrom wavelength exponent, α. These measurements were obtained in the Athens centre in May 1995. Exponent α is frequently computed from spectral measurements both from ground-based photometers and satellite remote sensing retrievals. Although the retrieval of α has been the subject of many studies worldwide, its spectral variation is typically not considered. In this study we analyse the spectral measurements of ln τα versus ln λ, where λ is the wavelength, at six selected wavelengths which cover the spectrum from 340 to 870 nm. It is obvious that there is significant curvature in the ln ta versus ln λ relationship, and the reasons for this are examined. It was found that this curvature depends on both atmospheric turbidity and aerosol optical properties, and a second order polynomial fit provides excellent agreement. The curvature of the polynomial equation becomes greater as the aerosol loading alters from high to low. The curvature is characterized by the second derivative of α, which can be utilized in conjunction with α to characterize the spectral dependence of aerosol optical depth, and can also be used as an indicator of the relative influence of fine-versus coarse-mode particles. Copyright © 2006 Royal Meteorological Society

Medium and extended range predictability and stability of the Pacific/North American mode

Abstract: It is shown from an assessment of a small set of extended range forecasts from two centres, and from a much larger set of medium range forecasts from one centre, that variability in predictive skill is strongly related to fluctuations in the Pacific/North American (PNA) mode of low frequency variability. A hypothesis is put forward that this is associated with the dependence of large-scale instability of the forecast flow on the amplitude of the PNA mode. The hypothesis is tested in a barotropic model using as basic states, composite skilful and unskilful cases from the set of medium range forecasts and individual monthly mean fields. Results from the barotropic stability analysis suggests possible reasons for the asymmetric nature of the response of general circulation models to sea surface temperature anomalies of opposite sign, relevant to forecasting on monthly to seasonal timescales. Observational evidence for the stability hypothesis is also discussed.

Mesoscale simulations of organized convection: Importance of convective equilibrium

Abstract: The validity of convective parametrization breaks down at the resolution of mesoscale models, and the success of parametrized versus explicit treatments of convection is likely to depend on the large-scale environment. In this paper we examine the hypothesis that a key feature determining the sensitivity to the environment is whether the forcing of convection is sufficiently homogeneous and slowly varying that the convection can be considered to be in equilibrium. Two case studies of mesoscale convective systems over the UK, one where equilibrium conditions are expected and one where equilibrium is unlikely, are simulated using a mesoscale forecasting model. The time evolution of area-average convective available potential energy and the time evolution and magnitude of the timescale of convective adjustment are consistent with the hypothesis of equilibrium for case 1 and non-equilibrium for case 2. For each case, three experiments are performed with different partitionings between parametrized and explicit convection: fully parametrized convection, fully explicit convection and a simulation with significant amounts of both. In the equilibrium case, bulk properties of the convection such as area-integrated rain rates are insensitive to the treatment of convection. However, the detailed structure of the precipitation field changes; the simulation with parametrized convection behaves well and produces a smooth field that follows the forcing region, and the simulation with explicit convection has a small number of localized intense regions of precipitation that track with the mid-levelflow. For the non-equilibrium case, bulk properties of the convection such as area-integrated rain rates are sensitive to the treatment of convection. The simulation with explicit convection behaves similarly to the equilibrium case with a few localized precipitation regions. In contrast, the cumulus parametrization fails dramatically and develops intense propagating bows of precipitation that were not observed. The simulations with both parametrized and explicit convection follow the pattern seen in the other experiments, with a transition over the duration of the run from parametrized to explicit precipitation. The impact of convection on the large-scaleflow, as measured by upper-level wind and potential-vorticity perturbations, is very sensitive to the partitioning of convection for both cases. © Royal Meteorological Society, 2006. Contributions by P. A. Clark and M. E. B. Gray are Crown Copyright.

Intense convective systems in West Africa and their relationship to the African easterly jet

Abstract: For May–September 1998, convective systems in West Africa were identified from observations by the Tropical Rainfall Measuring Mission satellite Microwave Imager at 85 GHz. Using re-analysis data, the 10-day average position of the African easterly jet (AEJ) was diagnosed from the 700 hPa zonal winds. The distance from each convective system's centroid to the axis of the AEJ was calculated. Each convective system's minimum brightness temperatures were ranked so that intense convective systems were defined as those in the 10th percentile or lower. The weak (>10th percentile) and intense convective systems were represented statistically as two separate populations, the weak by the skewed Gumbel distribution and the intense by the normal distribution. From May to August, the peak in activity of weak convective systems remained south of 10°N but shifted east of 10°E. The peak in activity of the intense convective systems followed the seasonal migration of the AEJ northwards and became increasingly separate from the peak in activity of weak convective systems. The majority of both weak and intense convective systems occurred within 0.50° of high terrain. The high convective available potential energy, high-shear AEJ environment in the vicinity of high terrain appeared to have the greatest probability of generating intense convective systems in the study area. Copyright © 2006 Royal Meteorological Society

Multiple-scattering microwave radiative transfer for data assimilation applications

Abstract: A multiple-scattering radiative transfer model for microwave radiance data assimilation in global numerical weather-prediction models is presented. The model is part of the RTTOV software package and includes forward, tangent-linear, adjoint and Jacobian models. The model is based on the Eddington approximation to radiative transfer which produces mean errors of less than 0.5 K at the targeted microwave frequencies between 10 and 200 GHz. The simplified treatment of subgrid-scale cloud cover may produce biased model calculations that show a maximum at 0.5 cloud cover and may reach several degrees K. These errors may be corrected with a simple bias correction. Linearity tests indicate that, given a screening procedure that excludes situations in which the model responds nonlinearly to input perturbations, channels near 50.3, 19.35, 22.235 and 183.31 GHz may be used in global radiance data assimilation. Copyright © 2006 Royal Meteorological Society.

The interaction of the Madden-Julian Oscillation with the Maritime Continent in a GCM

Abstract: Previous studies using the Hadley Centre coupled model (HadCM3) have shown that the islands of the Maritime Continent act as an unrealistic block to the eastward propagation of the Madden-Julian Oscillation (MJO). This blocking effect is investigated using a simplified, aqua-planet version of this GCM, with various idealized configurations of the Maritime Continent islands placed on the equator, and an MJO-like convective signal forced by a propagating sea-surface temperature anomaly dipole. Results suggest that it is the orography of the islands, rather than the presence of the islands themselves, which results in the blocking of the MJO. Although the peak elevation of the orography in the GCM is very much lower than in reality, it appears to act as effective block to the eastward propagation of the low-level Kelvin wave signal which accompanies the MJO. In particular, the representation of Sumatra in the GCM, as a north-south oriented ridge straddling the equator, seems to be particularly effective at blocking the Kelvin wave signal, which in a full GCM would result in the weakening or complete extinction of the MJO signal to the east of the Maritime Continent.

The surface energy balance and boundary layer over urban street canyons

Abstract: SUMMARY A model is developed for the energy balance of an urban area, represented as a sequence of two-dimensional street canyons. The model incorporates a novel formulation for the sensible-heat flux, that has previously been validated against wind tunnel models, and a formulation for radiation that includes multiple reflections and shadowing. This energy balance model is coupled to a model for the atmospheric boundary layer. Results are analysed to establish how the physical processes combine to produce the observed features of urban climate, and to establish the roles of building form and fabric on the urban modification to climate. Over a diurnal cycle there are morning and evening transition periods when the net flux of radiation is largely balanced by the flux of heat into the surface. The urban surface has a large surface area in contact with the air, and hence a large active heat capacity, and so the urban area needs to absorb a larger amount of heat than a rural area to change the surface temperature. The morning and evening transitions are therefore prolonged over urban areas, delaying the onset of convective or stable boundary layers after sunrise and sunset. The model shows that the energy balance of the roof behaves very differently from the combined energy balance of the street canyon system of walls and street. The sensible-heat flux from the street canyon into the boundary layer is increased by the increased surface area, but is decreased by the buildings reducing the local flow speeds. The net result is that, for the two-dimensional geometry investigated here, the sensible-heat flux from the canyon is not strongly sensitive to canyon geometry. The sensible-heat flux from the roof is larger than from the street, and so the total sensible-heat flux into the boundary layer, and hence also the air temperature, is strongly dependent on the fraction of plan area occupied by roofs. The radiation budget of the street canyon, which largely drives the temperatures of the canyon surfaces, is significantly changed by the limited sky view and multiple reflections caused by the local building form. The canyon surface temperatures thus depend strongly on local building morphology. Finally, two mechanisms are suggested for how urban areas might maintain a positive sensible-heat flux at night. Firstly, if the roof material has much lower heat capacity than the street canyon surfaces, then the roof can cool the boundary-layer air faster than the street canyon surfaces cool, leading to a positive heat flux out of the street canyon. Secondly, advection decouples the boundary layer from the local surface energy balance. In this way, cool air, perhaps from a rural area, advected on to an urban surface can lead to a positive sensible-heat flux which then tends to neutralize any stable stratification in the boundary layer.

ADM‐Aeolus Doppler wind lidar Observing System Simulation Experiment

Abstract: Within the Atmospheric Dynamics Mission Aeolus (ADM-Aeolus), the European Space Agency (ESA) has approved a Doppler wind lidar (DWL) to fly on a dedicated platform orbiting dawn to dusk at 400 km altitude, planned for launch in 2008. Rigorous design trade-offs have resulted in a lidar concept capable of delivering high-quality wind component profiles, but with a limited coverage. A companion paper describes the realistic simulation of this DWL, whereas this paper sets out to assess the impact of such a lidar in meteorological analyses and forecasts. To this end, an Observing System Simulation Experiment (OSSE) is run. The superior conventional observation coverage of 1993 is used to simulate all conventional observations, although a limited set of satellite observations is simulated. As a consequence, only the northern hemisphere DWL impact in the OSSE is assumed realistic. Here, over a 15-day period with variable weather, out of 15 daily forecasts, 14 show beneficial impact of the DWL. Although the experiment is limited, it corroborates other practical and theoretical evidence that the ADM DWL will demonstrate a beneficial impact in meteorological analyses and forecasts. Copyright © 2006 Royal Meteorological Society