bluuebunny/crossref_metadata_2025 huggingface dataset:
The parametrization of entrainment in cloudy boundary layers
Authors: Lock A. P.
Published: February 2007

A parametrization for the entrainment rate in shear‐free convective cloud‐capped boundary layers is derived. The only information it requires is the external mixed‐layer turbulence forcings (the surface buoyancy flux and the net radiative‐divergence profile), the inversion jumps of temperature and humidity, the cloud‐top liquid water mixing ratio and the cloud and mixed‐layer depths. Despite this simplicity it is found to compare well against both a wide range of large‐eddy simulations and observations of stratocumulus.

The parametrization is an extension of those derived previously for the idealised cases of smoke clouds, where turbulence is driven by combinations of surface heating and radiative cooling, and liquid water clouds driven solely by buoyancy reversal. The radiative forcing is specified as an indirect forcing, through the buoyant production of turbulence within the boundary layer and a direct forcing, which promotes deepening of the boundary layer when undulations in the cloud top cause part of the cooling to occur within the horizontally averaged inversion. Condensation of water in saturated air reduces the strength of both these terms but otherwise, for radiatively driven liquid water clouds (where evaporative cooling of entrained air does not generate buoyancy reversal), the parametrization is unchanged from that for smoke clouds.

Where evaporative cooling of entrained air is strong enough to generate buoyancy reversal, and therefore drive convective motions, not only does it provide an additional turbulence source, but it is also found to compensate for the reduction in strength of the radiative‐forcing terms by the presence of saturated air. Allowing for this enhancement, the entrainment rate is predicted with remarkable accuracy by the sum of previously derived parametrizations for the rates that would have been generated by radiative cooling and buoyancy reversal acting in isolation.
A novel approach to atmospheric dispersion modelling: The Puff‐Particle Model
Authors: De Haan Peter , Rotach Mathias W.
Published: February 2007

The Puff–Particle Model (PPM) uses a new approach to problems of three‐dimensional atmospheric dispersion from micro‐ to mesoscale. The pollutant particles are grouped in clusters treated as Gaussian puffs, which are dispersed making use of the concept of relative diffusion. The centre of mass of each puff is moved along a stochastic trajectory. This trajectory is derived from particle trajectories given by a Lagrangian stochastic‐dispersion model. In this way, the PPM retains the advantages of traditional puff models and those of particle models, and is able to take into account the correct probability density function of the stochastic velocity components. The effect of meandering (caused by turbulent eddies larger than the puff but not resolved by the flow field) is simulated by the puff centre trajectories, yielding a complete description of dispersion. The PPM is validated using measurements from three tracer experiments in Copenhagen, Lillestrøm and Kincaid.
Estimation of area‐averaged moisture flux
Authors: Mahrt L. , Kotwica K. , Sun Jielun , Macpherson J. I. , Desjardins R. L.
Published: February 2007

Toward the goal of predicting area‐averaged evapotranspiration, the evaporative fraction is modelled in terms of surface radiation temperature, air temperature, solar zenith angle, Normalized Difference of the Vegetation Index and albedo. Previous relationships break down when applied simultaneously to a variety of surfaces. The zenith angle is required to account for the influence of shaded ground on the remotely sensed surface radiation temperatures. The model of evaporative fraction is developed from Canadian Twin Otter aircraft data from the BOReal Ecosystem‐Atmosphere Study and the California Ozone Deposition Experiment. The model consists of separate submodels for active agriculture, forested surfaces, bare soil/senescent vegetation and water surfaces. These broad land‐use groups are effectively determined from remotely sensed red and near‐infrared reflectances.

The area‐averaged moisture flux predicted by the model compares favourably with that computed from independent data over forested surfaces. However, the model performs poorly over bare soil/senescent surfaces, and data for a wider variety of surface conditions are required before the generality of the model can be evaluated.
The horizontal and vertical structure of east Asian winter monsoon pressure surges
Authors: Compo Gilbert P. , Kiladis George N. , Webster Peter J.
Published: February 2007

East Asian cold‐air outbreaks, accompanied by increasing surface pressure (‘pressure surges’), are shown to be an important aspect of the subseasonal variability of the winter monsoon system. In this study the statistical linear relationship between pressure surges, tropical convection, and tropospheric circulation is assessed using a ten‐year data set (1985/86‐1994/95) of the European Centre for Medium‐Range Weather Forecasts gridded operational analyses. From spectral analysis the pressure, wind, and temperature fluctuations indicative of strong pressure surges are found to have statistically‐significant spectral peaks at submonthly periods (6 to 30 days). Linear regression analysis is used to detail the time evolution of the dominant horizontal and vertical structure of east Asian pressure surges. Surges are shown to relate significantly to circulation anomalies in both the meridional and zonal components of the lower‐tropospheric wind. Relationships between east Asian pressure surges in the submonthly band and tropical circulation anomalies are found over the Bay of Bengal, the eastern Indian Ocean, Indonesia, and the western Pacific regions. Submonthly surges over the South China Sea are related to strong surges and convective activity south of Indonesia, over the South China Sea, the eastern Indian Ocean, and the Philippine regions. Surge‐enhanced convective activity is found to precede an enhancement of the local east Asian Hadley cell. Submonthly surges over the Philippine Sea are related to periods of westerly‐wind anomalies and convective activity in the western Pacific. Upper‐level wave activity over western Asia precedes submonthly surges. the wave activity amplifies in the region of the Pacific jet stream, and can be traced dispersing equatoward through the region of upper‐level westerlies over the eastern tropical Pacific.
An analysis of a relatively rare case of continental blocking
Authors: Lupo Anthony R. , Bosart Lance F.
Published: February 2007

Planetary‐ and synoptic‐scale analyses of a relatively rare continental blocking event that occurred over North America during the spring of 1980 are undertaken to determine whether or not this event was different from its counterparts which occur over oceanic regions. the planetary‐scale analysis demonstrates that during the spring season a ridge was located further inland over the North American continent and amplified with respect to climatology. the position of this ridge may have been linked to a broad region of colder‐than‐normal sea surface temperatures found over the north central Pacific during the spring season and much of the previous winter. Simple ‘Sutcliffe‐type’ and thermodynamic analyses of the accompanying lower‐tropospheric warm anomaly associated with the ridging show that lower‐tropospheric temperature advection and subsidence associated with anticyclonic‐vorticity advection by the time‐mean thermal wind produced much of the anomalous warmth.

A simple synoptic‐scale analysis was performed using both the Zwack‐Okossi (ZO) equation and potential vorticity (PV) thinking approaches. These complementary analyses demonstrated that synoptic‐scale cyclones were instrumental in the formation and maintenance (and/or intensification) of this blocking event. the PV analysis demonstrated that low‐PV air was swept polewards and then was advected over the blocking region sustaining the broad region of low potential vorticity associated with the block over North America. the ZO analysis showed that the advection of anticyclonic vorticity was the most important mechanism forcing geopotential‐height rises at 500 hPa over the block centre. the region of low PV and ZO height rises could be associated with the anticyclonic‐shear side of an upstream jet maximum typically found in association with developing and/or intensifying blocking events. Thus, negative PV advection correlated significantly with calculated ZO height rises. Finally, it is suggested that a favourable phase relationship between the upstream cyclones and the large‐scale ridge is necessary for block development or intensification.
Accounting for subgrid‐scale cloud variability in a multi‐layer 1d solar radiative transfer algorithm
Authors: Oreopoulos Lazaros , Barker Howard W.
Published: February 2007

A multi‐layer, 1D solar radiative transfer algorithm that accounts for subgrid‐scale cloud variability is presented. This algorithm is efficient and suitable for use in large‐scale models such as global climate and weather prediction models. While it is built on the same principles as standard multi‐layer 1D codes, there are two major differences. First, it is assumed that for all cloudy layers all the time, frequency distributions of optical depth τ are described by gamma probability density functionspr(τ) and characterized by mean optical depth τ and a variance‐related parameterv.Albedos and transmittances for individual layers are estimated by integrals over all τ of the plane‐parallel, homogeneous two‐stream approximation equations weighted bypr(τ). Thus, the model is referred to as the gamma‐weighted two‐stream approximation. Second, in an attempt to counteract the use of horizontally homogeneous fluxes, a method was devised that often reduces layer values of τ.

The gamma‐weighted two‐stream approximation was implemented in a well known broadband column model and the parametrizations upon which it is built were tested using 2D and 3D inhomogeneous cloud fields generated by a bounded cascade model and cloud‐resolving models. All fields resolved the lowest 20 km of the atmosphere into at least 30 layers. Reference calculations were obtained by: (i) applying the 1D‐plane‐parallel, homogeneous model to each column and averaging (the independent column approximation); and (ii) a 3D Monte Carlo algorithm. the gamma‐weighted two‐stream approximation, the regular plane‐parallel, homogeneous, and two other 1D models operated on horizontally‐averaged versions of the fields (i.e. 1D vectors of cloud fraction, τ, andv). For several demanding cases, the gamma‐weighted two‐stream approximation reduced plane‐parallel, homogeneous‐biases for TOA albedo and surface irradiance by typically more than 85%. Moreover, its estimates of atmospheric heating rates usually differed from the independent column approximation and Monte Carlo values by less than 10%. This translates into heating rate errors that are four to eight times smaller than those associated with conventional 1D plane‐parallel, homogeneous algorithms. In a large‐scale model, a multi‐layer solar code with the gamma‐weighted two‐stream approximation should require about twice as much CPU time as its plane‐parallel, homogeneous counterpart.
Variational quality control
Authors: Anderson Erik , Järvinen Heikki
Published: February 2007

We describe the implementation of variational quality control in the operational four‐dimensional variational data assimilation scheme at the European Centre for Medium‐Range Weather Forecasts (ECMWF). the quality control takes place during the iterative solution of the variational analysis problem itself, rather than in a separate step prior to the main analysis. This ensures that the quality control is consistent with the analysis in terms of error statistics, background and model constraints, in particular. All observational data are used and quality‐controlled simultaneously. the probability of gross error is computed for each observation, given the preliminary analysis at each iteration. the weight of each observation is smoothly decreased with increased likelihood for gross error. the formulation, the implementation details and results from data assimilation and forecast experiments are presented.
A numerical study of tropical‐cyclone structure: Quasi‐stationary spiral bands
Authors: Weber Harry C.
Published: February 2007

The evolution of quasi‐stationary spiral bands of vorticity, on the periphery of an initially symmetrical tropical‐cyclone‐like vortex moving on a β‐plane, is investigated using a semi‐spectral nondivergent nonlinear barotropic numerical model, formulated in a cylindrical coordinate system moving with the vortex. the results obtained with the numerical model are supported by analytic solutions of the linear barotropic vorticity equation in a moving frame of reference.

The analysis of the model data shows the slow evolution and vortex‐relative translation of a spectrum of Rossby waves, forming cyclonically curved spiral bands of positive and negative vorticity anomalies in the rear‐right quadrant of the moving vortex. the analytic solutions of the linear barotropic vorticity equation and examination of the vorticity tendency in the numerical model indicate that the generation of the spiral bands is mainly a result of the vortex drift and the advection of planetary vorticity by the vortex wind field. Investigation of the kinematical structure of the trailing spiral bands of vorticity and their vicinity suggests that the cyclonic spiral bands in the numerical model may be related, either directly or indirectly, to the existence of quasi‐stationary principal spiral bands of convection in real tropical cyclones.
The effect of land‐surface feedbacks on the monsoon circulation
Authors: Ferranti L. , Slingo J. M. , Palmer T. N. , Hoskins B. J.
Published: February 2007

Summary the effects of feedbacks from land‐surface forcing on intraseasonal monsoon activity are studied by performing idealized sensitivity experiments with a general circulation model.

In agreement with observations, the simulated intraseasonal monsoon activity is mainly described by irregular alternations of active spells and break spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between a continental and an oceanic regime. In the model, the spatial characteristic of the intraseasonal monsoon variability is a robust feature which is primarily related to an internal mode of variability of the system, rather than to a response to land‐surface feedbacks. Experimentation indicates that the simulation of northward propagating events, related to transitions in the regime, does not require the inclusion of interactive surface hydrological processes. This suggests that the transitions are also mainly related to internal atmospheric dynamics.

The temporal characteristics of the fluctuations between the two TCZ regimes, however, are influenced by an interactive surface. the low‐frequency intraseasonal monsoon variability is enhanced by hydrological surface feedbacks. When the surface interacts with the atmosphere, the active and break regimes of the monsoon are equally likely. In the absence of surface feedbacks, the probability distribution is modified and the changes depend on the land‐surface conditions imposed. the results show that the probability of a monsoon break exceeds that of an active phase when the imposed land‐surface conditions are based on climatological values for July. This asymmetry in the probability distribution affects intraseasonal monsoon variability. In turn, the time‐mean monsoon circulation, depending as it does on the statistics of the intraseasonal oscillations (such as frequency of occurrence and mean amplitude), is also modified by the surface feedbacks. It follows that the surface conditions play a role in the interannual predictability of the time‐mean monsoon.
Primary ice nucleation in orographic cirrus clouds: A numerical simulation of the microphysics
Authors: Spice A. , Johnson D. W. , Brown P. R. A. , Darlison A. G. , Saunders C. P. R.
Published: February 2007

Ice particle production by the processes of homogeneous freezing and heterogeneous nucleation in orographic cirrus clouds is studied using a simple adiabatic ascending‐air‐parcel model. Homogeneous freezing rates in the model are based on a formulation by Jeffery and Austin, which matches with observed rates, modified according to the solution effect. Two alternative ice nuclei (IN) activation spectra are applied, one dependent upon temperature and the other upon ice supersaturation. Simulations are performed for air parcels with initial dew‐point temperatures, Td, in the range from −20 to −45°C and constant vertical velocities, w, of 0.1 to 5.0 m s‐1. In modelled clouds with Td ⩾ 30°C, heterogeneous nucleation initiated by IN is found to dominate ice formation, since homogeneous freezing rates are so low. In modelled clouds with Td ⩽ −35°C, pure liquidwater homogeneous freezing rates are large, but inclusion of sufficient quantities of IN may completely suppress aqueous solution droplet growth, dilution and homogeneous freezing, with heterogeneous nucleation then remaining dominant even at very low temperatures. the IN number concentrations required for homogeneous freezing suppression, for a fixed cloud condensation nucleus (CCN) activation spectrum, are found to increase with increasing w‐heterogeneous nucleation at low temperatures is therefore much more viable at the low w values found in non‐orographic compared with orographic cirrus clouds. the common conception of a deficit of IN aloft is challenged, and, because of the apparent sensitivity of cirrus cloud microphysical and radiative properties to these aerosol particles, IN measurements at cirrus levels, in addition to CCN measurements, are suggested as vital. the model results suggest that it may be possible to interpret measurements of peak liquid‐water content, or possibly maximum droplet radius, in orographic cirrus clouds in terms of the dominant ice nucleation mode.
Aircraft observations of sea‐breeze frontal structure
Authors: Wood R. , Stromberg I. M. , Jonas P. R.
Published: February 2007

Detailed aircraft observations of sea‐breeze frontal structure and dynamics are presented for two cases of well defined sea‐breeze fronts near the east coast of England. In the first case the sea‐breeze was advancing into a well mixed convective boundary layer with strong turbulence and an offshore breeze of around 3 m s‐1. In the second case the sea‐breeze was penetrating into a convective boundary layer characterized by weaker turbulence and an offshore wind speed of 2‐3 m s‐1. Indeed, during the course of the measurements a stable internal boundary layer was forming in the early evening. Cross‐sections of the frontal structure are derived from aircraft traverses at a range of heights along a fixed line normal to the coast. A typical head‐like structure is observed in both cases, with a region of strong mixing immediately seawards of the head. an approximately exponential fall‐off in dissipation rate with distance seaward of the leading edge of the front is found. Heat and momentum fluxes are used to derive turbulent kinetic energy (TKE) budgets for the mixing region and indicate that turbulence in this mixing region is maintained by TKE generated by strong shear at the top of the cold‐air inflow. In the first case the shear production of TKE is almost twice that of the second case, suggesting that the presence of ambient turbulence in the convective boundary layer has a significant effect upon the frontal dynamics. In the second case, as the turbulence decayed, a curious wave‐like structure appeared behind the leading edge of the front. the waves have wavelengths of 1‐3 km and it is unlikely that they were caused by Kelvin‐Helmholtz instability. It is suggested that the waves might be a solitary wave‐train emerging as the sea‐breeze interacts with a low‐level stable layer forming in the early evening.
Surface‐flux aggregation in heterogeneous terrain
Authors: Hasager C. B. , Jensen N. O.
Published: February 2007

Aggregation of surface fluxes in non‐homogeneous terrain is a difficult task due to the highly nonlinear processes governing especially the momentum flux. Results of the current work are high‐resolution maps of land‐surface momentum flux in a strongly heterogeneous mid‐latitude area in Europe, and aggregated values of the aerodynamic roughness lengths. This paper describes the development of a flux aggregation model based on a linearized set of the atmospheric flow equations solved by fast Fourier transforms. This is computationally efficient, therefore a large amount of data can be handled quickly. the input data to the model are high‐resolution (30 m x 30 m) roughness maps derived from classified satellite images and assigned Meteorological field values within a region (100 km x 100 km). the model deals with neutral flow only. the momentum‐flux results from this objective (physically based) method compare well with other models and experimental field data.
Retrieval of surface and atmospheric parameters over land from SSM/I: Potential and limitations
Authors: Prigent C. , Rossow W. R.
Published: February 2007

The potential to retrieve surface temperature, water vapour content and cloud liquid water path from the Special Sensor Microwave/Imager (SSM/I) observations over land is examined, using a variational inversion scheme. the method relies upon prior estimates of the monthly mean land surface microwave emissivities and exploits a priori information on the atmospheric situation (International Satellite Cloud Climatology Project (ISCCP) data and National Center for Environmental Prediction analysis). the accuracy of the surface temperature retrieval is of the order of 1 K, does not strongly depend on surface type and is not very sensitive to the presence of thin clouds. the sensitivity to the water vapour is very low except in the most arid areas. With an accuracy of ∼ 0.1 kg m−2, the SSM/I retrieval does not properly characterize thin clouds, but the major cloud structures are well delineated and qualitative agreement is observed between the SSM/I and ISCCP estimations. This inversion process also has some ability to detect rain events.
Ensemble‐based targeting experiments during FASTEX: The effect of dropsonde data from the lear jet
Authors: Szunyogh I. , Toth Z. , Emanuel K. A. , Bishop C. H. , Snyder C. , Morss R. E. , Woolen J. , Marchok T.
Published: February 2007

In this study we evaluate the performance of the Ensemble Transform (ET) technique, which is one of several targeting methods used in real time during the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). ‘Targeted’ observations were taken adaptively in those upstream areas identified in real time as most relevant for improving the initial conditions for forecasts of synoptic‐scale storms developing downstream. the upstream areas were identified as regions where the effect of extra observations at a future analysis time could produce the largest decrease in the largest likely forecast error at a preselected later verification time at a given downstream location. the ET technique selects these observational areas out of a large number of possible deployment locations of observational resources via a linear transformation of an ensemble of forecasts.

The analysis and forecast effects of special targeted observations associated with seven Intensive Observing Periods (IOPs) during FASTEX were investigated. the most important result of the present study is that the ET technique, based on the National Centers for Environmental Prediction (NCEP) operational global ensemble, was able to identify upstream areas that had significant contribution to the quality of selected future downstream forecast features. Moreover, the technique could reliably distinguish between the areas of greatest contribution.

Though the overall impact of the targeted data on forecast quality is positive, there were cases when the extra data degraded the forecasts. Our analysis indicates that large amplification of ensemble perturbations from the targeted area into the verification area is a good indicator of potential forecast improvement.
Adaptive observations during FASTEX: A systematic survey of upstream flights
Authors: Bergot Thierry
Published: February 2007

The effectiveness of the recent concept of adaptive observations is tested on 20 cases for which Fronts and Atlantic Storm‐Track EXperiment (FASTEX) upstream dropsondes were available.

In the first stage, the influence of the targeted data is tested without taking into account the real evolution of the atmosphere. On average, the inclusion of these targeted data significantly influences the forecasts (mean impact of about 9 hPa at 36 h). However, a large spread of impacts is observed and at 36 h (close to the mean verifying time) the mean impact is about 7.8 hPa for National Center for Atmospheric Research Learjet 36 (L36) flights and 10.8 hPa for National Oceanic and Atmospheric Administration Gulfstream IV (GIV) flights, with different behaviours. These tests also indicate that the targeted observations may indirectly affect the mediumrange forecast of the subsequent lows. the redundancy of targeted data with respect to conventional no‐FASTEX data (defined as the difference in impact between forecasts with and without conventional data), is about 40% for L36 flights and 14% for GIV flights, at 36 h. These differences between L36 and GIV flights (for the temporal characteristics of the impact signals and the redundancy) might be due either to the location of the data (L36 data were closer to the observationally dense North American continent), or to the difference between targeting techniques used to guide the planes. It seems also that adaptive observations are clearly constrained by the conventional observing network, and the sampling strategy used is also a crucial problem to solve.

In the second stage, the skill of forecasts with targeted dropsondes is evaluated. the inclusion of these targeted data leads to a mixture of improvement and degradation cases, the mean improvement is very weak and has the same order of magnitude as the verifying‐analysis errors. However, a study of cases suggests that the adaptive‐observation effectiveness is high when the predictability is weak. Otherwise, the assimilation system seems unable to reduce the errors in the most unstable directions, and these deficiencies blur the impact of the targeted data. This suggests that the success of adaptive observations also depends on the assimilation system used.
As assessment of the singular‐vector approach to targeted observing using the FASTEX dataset
Authors: Gelaro R. , Langland R. H. , Rohaly G. D. , Rosmond T. E.
Published: February 2007

In this study, we investigate whether the results of assimilating special targeted observations from the Fronts and Atlantic Storm‐Track EXperiment (FASTEX) in an operational forecast model support the underlying principles of the singular‐vector (SV) approach to targeted observing. A simple framework is presented that allows explicit examination of the changes made to the analysis in the subspace of the leading SVs from assimilation of the observations. the impact of this component on the forecast provides a key measure of the effectiveness of SV‐based targeting.

Results confirm that the impact of the additional observations occurs primarily as a result of changes to the analysis in the subspace of the leading SVs. These changes account for a small fraction of the total targeting increment at initial time, but explain a large fraction of the response of the forecast at the verification time. the results also confirm that analysis errors in the middle and lower troposphere are an important source of error in forecasts of extratropical cyclones.

While moist processes can play an important role in the forecast‐error evolution, SVs that exclude these processes can remain an effective targeting tool. This is because the location of maximum sensitivity will not necessarily differ from that identified by the dry SVs. It is also shown that the locations of the leading (target) SVs can be computed accurately with lead times of up to 48 hours, allowing ample time for the deployment of observational resources.
Four‐dimensional variational analyses of FASTEX situations using special observations
Authors: Desroziers Gerald , Pouponneau Béatrice , Thépaut Jean‐Noel , Janisková Marta , Veersé Fabrice
Published: February 2007

This paper focuses on Intensive Observation Period 17 of the Fronts and Atlantic Storm‐Track EXperiment (FASTEX): the FASTEX special soundings obtained during that period are added to the conventional dataset and assimilated with a 4‐dimensional variational (4D‐Var) scheme under development at Météo‐France. Results show a consistent use of the high density FASTEX ship soundings by the 4D‐Var formulation and an improvement brought by this new scheme over 3D‐Var when using only the vertical profiles synchronous with synoptic time. the application of the 4D‐Var analysis to dropsondes launched by a system‐relative flight designed by the Joint Centre for Mesoscale Meteorology also produces a fine‐scale description of sub‐structures of the mature system.
Pacific and atlantic ‘bomb‐like’ deepenings in mature phase: A comparative study
Authors: Lemaǐtre Y. , Protat A. , Bouniol D.
Published: February 2007

The mesoscale dynamics of a secondary cyclone sampled during the field phase of the Fronts and Atlantic Storm‐Track Experiment (FASTEX) are documented using airborne Doppler radar data. the high quality of the mesoscale airborne Doppler dataset collected within this secondary cyclone is highlighted. This study demonstrates, however, the need to combine in the near future airborne Doppler radar and dropsonde measurements, in order to access multiscale processes involved in the mature stage of this FASTEX cyclone both in clear‐air and in precipitating regions. the studied secondary cyclone developed on the trailing cold front of a parent low situated over Greenland, and underwent the most explosive deepening (roughly −54 mb in 24 hours) sampled in the multiscale sampling area during FASTEX. the main flows responsible for the mesoscale organization of the cyclone are: a warm airflow emerging from the base of the synoptic warm conveyor belt that generates the most active part of the cloud head; a dry intrusion approaching in the dry slot the cyclone centre; a cold conveyor belt that contributes to the upward mass transport towards the upper part of the polar‐front cloud band; and a cold northerly flow in the western part of the cloud head, which tends to wrap around the low centre and is likely to release convective instability through air‐sea interactions. This study also documents a slight shift in position between the high vertical‐vorticity core associated with the active part of the cloud head and the location of the pressure minimum.

The detailed three‐dimensional structure of this Atlantic secondary cyclone is also compared with that gathered within another ‘bomb‐like’ deepening (defined as more than a 24 mb surface pressure drop in 24 hours) observed over the Pacific ocean during the Coastal Observation and Simulation with Topography (COAST) experiment. Some similarities are found. the main differences appear to be linked to the warm seclusion that is exclusive to the studied Pacific cyclone. More pronounced ascending motions are found within the cloud head of the Pacific ‘reference’ case, and the deepening results from a more constructive effect of geostrophic and non‐geostrophic processes than for the Atlantic ‘reference’ case.
Mesoscale dynamics of a deepening secondary cyclone in FASTEX IOP16: Three‐dimensional structure retrieved from dropsonde data
Authors: Bouniol D. , Protat A. , Lemaǐtre Y.
Published: February 2007

The mesoscale dynamics of a secondary cyclone sampled during the field phase of the Fronts and Atlantic Storm‐Track EXperiment (FASTEX) Intensive Observation Period (IOP) 16 are documented using airborne dropsonde data. the method proposed to recover the mesoscale analytical three‐dimensional (3‐D) dynamic fields from the dropsonde measurements is evaluated using simulated fields, airborne in situ measurements, and airborne Doppler radar measurements. These simulations and comparisons indicate that the method is able to resolve the mesoscale structures within the studied secondary cyclone both in the regions of clear air and precipitation. It appears, however, that the most accurate description of the 3‐D dynamic fields in both these regions would be obtained by combining Doppler and dropsonde measurements.

The studied secondary cyclone is a fast moving frontal wave that developed on the trailing cold front of a parent low situated over Greenland. Using the dropsonde‐derived 3‐D fields, the mesoscale flows involved in the organization of the secondary cyclone are described. the potential‐vorticity and equivalent‐potential‐vorticity fields (deduced from the basic analytical fields) indicate that the atmosphere is roughly neutral to moist slantwise instability. This configuration is known to significantly promote frontogenesis. the vertical configuration of the jets and circulations within this secondary cyclone are compared with the coupling‐uncoupling scheme of Shapiro. A slight coupling is found between the thermally indirect circulation and the ageostrophic circulation. the obtained coupling is, however, not as developed as in the ideal case described by Shapiro, which may explain the small upward velocities in our case.

Airborne Doppler radar and dropsonde measurements are combined so as to infer the multi‐scale processes involved in the mesoscale and convective‐scale organization of the Atlantic secondary cyclones and to evaluate the new emerging theoretical interpretations of secondary cyclogenesis.
Intensification of tropical‐cyclone‐like vortices in uniform zonal background‐flows
Authors: Dengler K. , Keyser D.
Published: February 2007

The effect of uniform zonal background‐flows on the intensification of tropical‐cyclone‐like vortices is investigated using a numerical three‐layer shallow‐water model that includes parametrizations of convection, sea‐surface energy‐exchange, and boundary‐layer friction.

Calculations on an f ‐plane showed that during the developing stage, an initially weak vortex intensified more rapidly in the presence of a uniform zonal background‐flow. Compared to an environment at rest, the uniform zonal background‐flow resulted in increased boundary‐layer convergence collocated with increased boundary‐layer mixing ratio (as a result of surface moisture fluxes), producing stronger convection and more rapid intensification. After the vortex achieved hurricane strength in the presence of an easterly (westerly) background‐flow, a region of convectively stable air formed to the south (north) of the vortex centre as a result of small values of surface moisture flux and the downward transport of relatively dry middle‐layer air. This convectively stable air penetrated slowly into regions of boundary‐layer convergence, thereby weakening convection and reducing the maximum intensity of the vortex compared to the case without background flow. This effect was more pronounced for stronger background‐flows.

Calculations on a β‐plane showed that a westerly background‐flow is more favourable for intensification than an easterly background‐flow of the same strength. In the former, the vortex always moved into a region of convectively unstable air, whereas, in the latter, the vortex progressively moved into a region of convectively stable air produced by subsidence and drying of the boundary layer. Because of the relatively high vortex drift speed in the easterly background‐flow case, there was not sufficient time for surface moisture fluxes to moisten the boundary‐layer air rapidly enough to eliminate the convectively stable region ahead of the vortex, resulting in the overall weakening of convective activity and a less intense storm than in the westerly flow case.

The foregoing calculations suggest that the imposed background‐flow affects the development of the vortices in the shallow‐water model by establishing an environment in which boundary‐layer convergence and surface moisture fluxes may interact to produce stratifications that either enhance or suppress convection, the occurrence of which is necessary for the intensification of the vortices in the model.
Nonlinear equatorial Kelvin waves and CISK. I: Small‐amplitude approximation and the trailing edge of a cloud region
Authors: Cho Han‐Ru
Published: February 2007

Kelvin wave‐CISK (Conditional Instability of the Second Kind) is studied as a possible explanation for the intraseasonal oscillations of the equatorial troposphere. The small‐amplitude approximation will be made in part I of this paper. The governing equations of the small‐amplitude approximation are nonlinear, however, because of the cumulus parametrization scheme used. It was found that: (1) Kelvin waves when coupled with cumulus healing are never unstable if the cumulus heating rate is smaller than the adiabatic cooling rate; (2) the solution is not equatorially confined if the cumulus heating rate is larger than the adiabatic cooling rate; (3) since a Kelvin wave is not a dispersive wave, the method of characteristics is used to solve the governing equations when the cumulus heating rate is smaller than the adiabatic cooling rate. It was found that the cumulus parametrization scheme often used in theoretical studies of CISK is incompatible with the so called 'jump condition' of the generalized solutions often required by hyperbolic systems of equations.
The use of TOVS level‐1b radiances in the NCEP SSI analysis system
Authors: McNally A. P. , Derber J. C. , Wu W. , Katz B. B.
Published: February 2007

A methodology is described for the direct assimilation of TOVS (TIROS‐N (Television Infra‐Red Observation Satellite) Operational Vertical Sounder) radiance data that have not undergone the pre‐processing (such as limb adjustment and cloud clearing) that is usually applied to make radiances suitable for use in linear retrieval algorithms. Inherent nonlinearities in the observations can be treated successfully within a variations analysis scheme such as Spectral Statistical Interpolation (SSI) thus making radiance pre‐processing unnecessary. The benefit of using unprocessed (or level‐lb) data is demonstrated in terms of an improved ability to quality control and bias correct the observations within the analysis. The assimilation of level‐1b radiance data in the SSI has shown a significant positive impact on the quality of the analysis compared with the use of pre‐processed radiances, and a significant improvement in forecast skill for the tropics and southern hemisphere.
Use of a neural‐network‐based long‐wave radiative‐transfer scheme in the ECMWF atmospheric model
Authors: Chevallier F. , Morcrette J.‐J. , Chéruy F. , Scott N. A.
Published: February 2007

The definition of an approach for radiative‐transfer modelling that would enable computation limes suitable for climate studies and a satisfactory accuracy, has proved to be a challenge for modellers. A fast radiative‐transfer model is tested at ECMWF: NeuroFlux. It is based on an artificial neural‐network technique used in conjunction with a classical cloud approximation (the multilayer grey‐body model). The accuracy of (he method is assessed through code‐by‐code comparisons, climate simulations and ten‐day forecasts with the ECMWF model. The accuracy of NeuroFlux appears to be comparable to the accuracy of the ECMWF operational scheme, with a negligible impact on the simulations, while its computing time is seven times faster.
TIGR‐like atmospheric‐profile databases for accurate radiative‐flux computation
Authors: Chevallier F. , Chédin A. , Cheruy F. , Morcrette J.‐J.
Published: February 2007

This note summarizes the characteristics of different topological methods for sampling vertical profiles of heterogeneous variables, like atmospheric temperature and water‐vapour concentration. The methods presented follow the approach developed for the successive Thermodynamic Initial Guess Retrieval (TIGR) databases at Laboratoire de Météorologie Dynamique. The most recent one is applied to select limited numbers of profiles (some thousands) by the sampling of a much larger dataset (more than one million profiles) from the European Centre for Medium‐Range Weather Forecasts short‐range forecasts. The sampled datasets are then used for (raining a neural‐net work‐based radiative‐flux profile computation model (NeuroFlux).
Three‐dimensional modelling of ice accretion density
Authors: Szilder Krzysztof , Lozowski Edward P.
Published: February 2007

A three‐dimensional, random‐walk, ballistic model is used to simulate the density and structure of an ice accretion formed by impinging supercooled droplets. The model is calibrated using experimental observations of the ice density at the stagnation line on a fixed circular cylinder. A relationship has been established between the Macklin parameter, used commonly to relate atmospheric conditions with ice accretion density, and model parameters. The model is applicable over a wide range of growth conditions, including, in the limits, growth where the droplets freeze on impact and growth where the droplets spread over and percolate into the existing ice structure.
Evolution of disturbances and singular vectors in the shallow‐water semi‐geostrophic model
Authors: Ren Shuzhan
Published: February 2007

The first part of this work is devoted to exploring the instability mechanism of the shallow‐water semi‐geostrophic model by investigating the evolution of disturbances under constant‐shear and cosine‐type basic flows. For the constant‐shear basic‐flow case, the wave‐packet analysis method is applied to study the evolution of potential and total energy and the meridional structure of disturbances under me assumption of a spatially slowly varying basic state. The results are found to be in good agreement with numerical calculations even when the assumption is slightly relaxed. For the cosine‐type basic‐flow case, both unstable normal modes and continuous spectra exist if the stability criterion is violated. Numerical results show that the relative importance of regular normal‐mode and continuous‐spectrum disturbances in the evolution process depends on the configuration of initial disturbances and their projections onto the normal modes and continuous spectra.

In the second part of this work, in order to search for initial disturbances based on which the growth of certain norms of disturbances can be optimized, singular vectors of different norms (metrics) are computed. It is found that of all the norms, the structures of singular vectors are mainly determined by the configuration of the zonal basic flow and less affected by model parameters such as the Rossby number, rotating Froude number, and amplitude of topography. However, when the shear of the basic flow is strong, the optimal singular value can be sensitive to the model parameters. When the stability criterion is violated, the optimal singular values of all the norms have clear wave‐number preferences. The peaks of spectra of singular values of the energy‐type norms, including potential, kinetic and total‐energy norms, correspond to very weak normal‐mode instability. However, the optimal singular value of the enstrophy norm has almost the same wave‐number preference as unstable normal modes.
A parametrization of the effects of cloud and precipitation overlap for use in general‐circulation models
Authors: Jakob Christian , Klein Stephen A.
Published: February 2007

The necessity for treating the effects of vertically varying cloud fraction when parametrizing microphysical processes in general‐circulation models (GCMs) was recently highlighted by Jakob and Klein. In this study a parametrization to include such effects in a GCM is developed, and the new scheme is applied in the ECMWF global model. The basic idea of the new scheme is to separate the model's rain and snow fluxes into a cloudy and a clear‐sky part. The scheme is tested using the subgrid‐scale precipitation model of Jakob and Klein as a benchmark. The impact of the new scheme on the model climate is also investigated.

It is shown that the new parametrization leads to a better representation of the effects of cloud and precipitation overlap, and that it alleviates most of the problems connected with their treatment in the current scheme. Due to the better treatment of cloud and precipitation overlap the new parametrization leads to a reduction in precipitation evaporation and an increase in accretion rates. When tested in seasonal model integrations the new scheme produces a drier tropical mid‐troposphere with consequences for the hydrological cycle.
Seasonal and interannual variability of the stratosphere diagnosed from UKMO TOVS analyses
Authors: Scaife A. A. , Austin J. , Butchart N. , Pawson S. , Keil M. , Nash J. , James I. N.
Published: February 2007

UK Meteorological Office (UKMO) TOVS analyses are used to show that the record of satellite radiance measurements is now long enough to capture the vast majority of year‐to‐year variability in the stratosphere. Given this, a climatology of the stratosphere is constructed and used to identify robust but detailed features of the seasonal evolution in the northern and southern hemispheres. Interannual variability is also investigated. In the northern winter stratosphere, the interannual variability in geopotential height increases monotonically towards the pole. In contrast, in the southern hemisphere the largest interannual variability occurs in a collar of high values surrounding a low‐variability region over the pole. For the Tropics, we show that quasi‐biennial and semiannual oscillations in the tropical zonal wind can be derived from the geopotential‐height analyses. However, comparisons of tropical winds from three different methods of analysis show that the current meridional resolution of the analysis limits the accuracy of the stratospheric winds over the equator. Finally, the satellite radiance data are used to derive updated temperature trends for the stratosphere. These show a cooling trend throughout most of the stratosphere that increases with height above the tropopause and exceeds 2 K per decade at the stratopause.
Small‐amplitude coastally trapped disturbances and the reduced‐gravity shallow‐water approximation
Authors: Durran Dale R.
Published: February 2007

Solutions are obtained for linear hydrostatic disturbances propagating parallel to the face of an uninterrupted topographic step in an infinitely deep, stably stratified fluid on an β‐plane. These waves are vertically trapped because their frequencies are smaller than the Coriolis parameter and the height of the topographic step is finite. These waves are referred to as step‐trapped Kelvin waves, because they are dynamically similar to internal Kelvin waves throughout the layer of fluid below the top of the topographic step. These waves appear to provide an idealized, semi‐analytic model for the coastally trapped disturbances observed to propagate parallel to mountainous coastlines in several parts of the world.

Computations are performed for a basic state with uniform static stability and for a three‐layer basic state in which the two lowest layers represent the marine boundary layer and a strong capping inversion. One might suppose that the linear dynamics of hydrostatic disturbances in the three‐layer basic state could be well approximated by a reduced‐gravity shallow‐water model, but this is not the case. In particular, the reduced‐gravity shallow‐water model does not provide reliable estimates for the phase speed of linear step‐trapped Kelvin waves. This defect suggests that detailed quantitative comparisons between marine boundary‐layer flows and the reduced‐gravity shallow‐water system may not have any intrinsic physical significance. Nevertheless, these results do not preclude the possibility of constructing useful qualitative analogies between marine boundary‐layer flows and the reduced‐gravity shallow‐water model.