bluuebunny/crossref_metadata_2025 huggingface dataset:
A statistical model of drop‐size spectra for stratocumulus clouds
Authors: Considine Geoffrey , Curry Judith A.
Published: February 2007

The purpose of this work is to present a simplified model of cloud‐droplet spectra that can be used as a tool for interpreting cloud microphysical observations in boundary‐layer clouds, and as a way to begin quantifying the couplings between cloud microphysics, dynamics and radiative properties for eventual use in cloud parametrizations. The model is steady‐state, ignores nucleation effects, and is formulated to produce horizontally averaged statistics. The major difference between this model and previous work lies in the model assumption that droplet spectra at a given level within a cloud are horizontal averages over a large number of air parcels, each of which can have a different lifting condensation level (LCL). Vertical motions are driven by buoyancy, so that liquid water and vertical velocity are simple functions of height above the LCL. This relationship, treated statistically over a large number of parcels, relates turbulent kinetic energy to horizontally averaged statistics of the droplet spectra. The broadening of droplet spectra is thus directly related to the turbulent kinetic energy within a cloud. In validating the model, results are compared with observations. It is shown that the basic droplet spectra predicted by the model are quite realistic for stratus and stratocumulus. The modelled droplet spectra broaden from cloud base to cloud top as is frequently observed, and the relationship between drop spectrum width and mean radius agrees quite well with observations.
Measurement of methane and carbon dioxide fluxes from peatland ecosystems by the conditional‐sampling technique
Authors: Beverland I. J. , Moncrieff J. B. , ÓNéill D. H. , Hargreaves K. J. , Milne R.
Published: February 2007

The conditional sampling method was used to measure methane (CH4) and carbon dioxide (CO2) fluxes from peatlands in northern Scotland. Preliminary data were obtained using a simple system. Subsequent automation made continuous flux‐measurements possible. Observed CH4 fluxes were in the range −70 to +110 μmol m−2 h−1 with a mean flux of 23 μmol m−2 h−1. Peak photosynthetic CO2 fluxes were in the range −10 to −30 mmol m−2 h−1. Nocturnal respiration ranged from 0 to +10 mmol m−2 h−1. The conditional sampling observations showed reasonable agreement with measurements of flux by eddy‐covariance, gradient and aircraft methods. Error analyses and laboratory tests were conducted to determine the precision of the flux‐measurement system. The dominant error was associated with the determination of the mixing‐ratio difference in conditionally‐sampled updraught and downdraught air. The standard error of the difference for CH4 was typically 0.15 ng g−1 (0.3 parts in 109 (p.p.b.)) using a careful high‐repetition sampling strategy with a modified gas chromatographic/flame ionization detector system. Under typical daytime atmospheric conditions this corresponded to a standard error in the flux measurement of 10 μmol m−2 h−1, which is consistent with field observations. The empirical beta factor in the conditional sampling equation was found to be insensitive to changes in turbulence intensity and atmospheric stability. Simple upscaling models were used to estimate annual carbon‐fluxes to Great Britain's peatlands of −0.5 Mt of carbon in the form of CO2 and 18 kt in the form of methane.
The dynamics of error growth and predictability in a coupled model of ENSO
Authors: Moore Andrew M. , Kleeman Richard
Published: February 2007

Using new and emerging ideas about the growth of singular vectors (‘optimal perturbations’) in dynamical systems, the dynamics of error growth and predictability in an intermediate coupled model of the El Niño Southern Oscillation (ENSO) is investigated. A mechanism is identified for error growth associated with penetrative‐convection anomalies in the atmosphere. Conditions for error growth via this mechanism are most favourable in the central Pacific where sea surface temperatures (SSTs) are relatively warm, and where changes in SST are moderately sensitive to vertical movements of the main oceanic thermocline.

The singular vectors of the coupled system were computed using the tangent‐linear coupled model and its adjoint. The singular‐value spectrum was found to be dominated by one singular vector at all times of the year. The potential for error growth in the coupled model, measured in terms of the growth of energy of the dominant singular vector, is found to vary seasonally, being greatest during the boreal spring. These seasonal variations are associated with the seasonal cycle in SST. During boreal spring and early summer, the SST in the central Pacific is at its maximum, at which time conditions are most favourable for error growth. Springtime is also the time of the ‘predictability barrier’ for ENSO. The potential for error growth is also influenced by the ENSO cycle itself. The results suggest that error growth will be enhanced during the onset of El Niño and suppressed during the onset of La Niña, which indicates that El Niño may be less predictable than La Niña.
Introduction of a local mapping factor in the spectral part of the Météo‐France global variable mesh numerical forecast model
Authors: Yessad K. , Bénard P.
Published: February 2007

For variable horizontal resolution simulations, a new formulation of spectral computations has been implemented in the ARPEGE/IFS spectral global numerical weather forecast model. The formulation makes use of a property of the conformal transformation which defines the stretched geometry: the local mapping factor is a combination of zero and first‐order Legendre polynomials only. Consequently, the mapping factor dependency in the spectral part of the computations can be introduced by simple extra multi‐diagonal operator multiplication. This idea is applied to both the semi‐implicit and the horizontal diffusion schemes to eliminate problems occurring when variable resolution is used together with either long time‐steps or steep orographies. The positive impact of these modifications is presented for semi‐Lagrangian and high‐resolution integrations. The relatively small extra computation and memory cost of the modified schemes is well suited to operational applications.
A new subgrid‐scale orographic drag parametrization: Its formulation and testing
Authors: Lott François , Miller Martin J.
Published: February 2007

A scheme for the representation of subgrid‐scale orography (SSO) in numerical weather prediction and climate models is presented. the new scheme arose in part from a desire to represent nonlinear low‐level mountain drag effects not currently parametrized. an important feature of the scheme is that it deals explicitly with a low‐level flow which is ‘blocked’, when the effective height of the subgrid‐scale orography is sufficiently high. In this new scheme, it is assumed that, for this ‘blocked’ flow, separation occurs at the mountain flanks, resulting in a form drag. This drag is parametrized on model levels which are intersected by the SSO, and provides a dynamically based replacement for envelope orography. the upper part of the low‐level flow goes over the orography and generates gravity waves. At the model resolutions considered (T106 and T213) it is assumed that the length scales characteristic of the SSO are sufficiently small for the Coriolis force to be neglected. the various parameters of the scheme are adjusted using an off‐line procedure in which the scheme is used to estimate the mountain drag and the momentum profiles above the Pyrenees; and these estimates are validated with the PYREX data. Forecasts using T106 and T213 resolutions with this new scheme, and with mean orography, show that the forecast mountain drag consistently reproduces the drag measured during PYREX whenever the flow component normal to the ridge is large. Isentropic flow diagnostics, further, show that the new scheme has a realistic impact on the flow dynamics, reinforcing the low‐level wake observed in mesoscale analyses of the flow. With this new scheme and a mean orography, the ECMWF model outperformed, in forecast skill, a version of the model which had an envelope orography and the old gravity‐wave‐drag scheme, while no longer suffering any disadvantages of envelope orography. the proposed low‐level drag parametrization should also be relevant at model horizontal resolutions much higher than T213.
A line of convection embedded in a stratocumulus‐topped boundary layer
Authors: Boers R. , Jensen J. B. , Krummel P. B.
Published: February 2007

During the Southern Ocean Cloud Experiment a line of convection embedded in an otherwise homogeneous stratocumulus layer was sampled at four levels by means of an instrumented aircraft. At the lower levels there was an abrupt change in microphysical and thermodynamic properties from the updraught region to the environment. At the upper level the change was more gradual and extended over a larger region, indicating the spreading out of the convective region underneath the strong inversion capping the boundary layer. Precipitation was observed at all levels, even in the convective region except near its centre core. Vertical‐velocity fluctuations in the convective region near the inversion were indistinguishable from the surroundings. At these high levels the horizontal wind in the convective region was from the same direction as the air close to the surface, but was at an angle of up to 60° with the horizontal wind from the surroundings. the convective region was characterized by cloud droplet concentrations that were often more than double those recorded in the surrounding cloud. This indicates that the modifications to the cloud droplet spectra associated with the onset of precipitation starts in the convective region and is enhanced in the stratiform region.
Simulation of climate change over europe using a nested regional‐climate model. II: Comparison of driving and regional model responses to a doubling of carbon dioxide
Authors: Jones R. G. , Murphy J. M. , Noguer M. , Keen A. B.
Published: February 2007

Results are assessed from a 10‐year simulation of the equilibrium response to doubled carbon dioxide (CO2) over Europe made with a nested high‐resolution regional‐climate model (RCM). the simulated changes are compared against those produced by the driving global general‐circulation model (GCM). the domain‐averaged increases in temperature and moisture content are similar in both models. Because of a stronger hydrological cycle the increases in precipitation and evaporation are larger in the RCM than in the GCM, whereas the reductions in lower and middle tropospheric relative humidity and cloud cover are smaller.

The frequency of intense precipitation events increases substantially in both models; however, the fractional changes are significantly smaller in the RCM. the proportion of precipitation associated with convection also increases in both models; however, much of the increase in intense events is explained simply by increased atmospheric moisture concentrations, especially in the RCM.

The time‐averaged precipitation changes in the RCM contain a substantial mesoscale component on scales not resolved by the GCM. Attempts are made to reconstruct this component from the changes in the large‐scale atmospheric circulation using empirical relationships calibrated from the 1 x CO2 integration. These are largely unsuccessful, indicating that simple downscaling schemes to generate high‐resolution scenarios of climate change from coarse‐grid GCM data may be of only limited applicability. Further statistical calculations suggest that longer integrations (∼ 30 years) are needed to reduce the sampling uncertainty associated with the simulated mesoscale component to an acceptable level.

The large‐scale patterns of change of surface temperature and precipitation reveal significant regional contrasts which are influenced both by changes in atmospheric circulation and regional physical feedbacks. the RCM changes are similar to those of the driving GCM except in summer. the differences in the summer changes are traced to differences between the 1 x CO2 integrations, in which the influence of the lateral boundary forcing on the RCM simulation is found to be anomalously weak. It is argued that any development of significant divergence between the RCM and GCM solutions on scales resolved by the latter may imply the need to refine or replace the one‐way nesting technique currently used in many regional modelling experiments.
Sensitivity of the asian summer monsoon to aspects of sea‐surface‐temperature anomalies in the tropical pacific ocean
Authors: Soman M. K. , Slingo Julia
Published: February 2007

The response of both the time of onset, and the strength of the Asian summer monsoon, to regional aspects of the sea surface temperature (SST) anomalies in the tropical Pacific Ocean associated with El Niño/Southern Oscillation (ENSO) has been investigated through a series of sensitivity experiments with the Universities Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM). This paper builds on the work of Ju and Slingo (1995) and on their hypothesis that the relationship between the Asian summer monsoon and ENSO involved the latitudinal position and strength of the tropical convective maximum (TCM) over Indonesia and the west Pacific in the preceding spring. the inference from their results was that the modulation of the TCM might be associated either with changes in the Walker circulation through the influence of the east Pacific SST anomalies, or with changes in the local Hadley circulation associated with the in situ SST anomalies in the west Pacific. the investigation has focused on the particular contrasting years of 1983 and 1984. the experiments described in this paper are designed to isolate the effects of the principal SST anomalies in the east and central Pacific, associated with El Niño/La Niño, from those of opposite sign which develop in the west Pacific as a complementary pattern during the mature phase of El Niño/La Niño.

The results of the experimentation suggest that, at least for the test cases of 1983 and 1984, the modulation of the Walker circulation, with implied additional subsidence over the eastern hemisphere, is the dominant mechanism whereby the Asian summer monsoon is weakened during El Niñño years. However, the late onset during El Niño years may also be associated with the complementary cold SST anomalies in the west Pacific which delay the northwards transition of the TCM. During La Niño, the modulation of the Walker circulation appears not to be the controlling factor which determines the stronger monsoons. the UGCM results suggest that the complementary warm SST anomalies in the west Pacific enhance the TCM, and it is this in situ response by the TCM that leads to an early onset and stronger monsoon. the importance of warm anomalies in the west Pacific in the development of a strong monsoon has been investigated further through a case‐study of the 1994 season. the year 1994 was an El Niño year in which the monsoon was unexpectedly active, but which was also marked by warmer than normal SSTs in the west Pacific.

The sensitivity experiments have also elucidated the role of El Niño in influencing the precursory signature of stronger subtropical westerlies over India and south‐east Asia during the winter and spring preceding weak monsoons. the results suggested that the equatorwards shift of the subtropical jet is a remote response to the warm SST anomalies in the central and east Pacific associated with El Niño.
Seasonal instability of pacific sea‐surface‐temperature anomalies
Authors: Efimov V. V. , Prusov A. V. , Shokurov M. V.
Published: February 2007

The main purpose of this study is to reveal the instability of sea‐surface‐temperature (SST) anomalies in connection with the ‘predictability barrier’ of El Niño established by Webster and Yang. the characteristics of linear instability of Pacific SST anomalies are computed using monthly COADS data and the method of Lyapunov exponents. Analysis is carried out separately for the northern and tropical part of the Pacific Ocean via approximating SST fields using a few leading empirical orthogonal functions. Changes of Lyapunov exponents throughout the year are computed and periods of the most unstable and stable seasons revealed. of most interest is the spring instability of SST anomalies both in the tropical and extratropical Pacific. This instability is crucial for the existence of a predictability barrier for an El Niño event.
A theory for vortex rossby‐waves and its application to spiral bands and intensity changes in hurricanes
Authors: Montgomery Michael T. , Kallenbach Randall J.
Published: February 2007

In this paper we examine further the physics of vortex axisymmetrization, with the goal of elucidating the dynamics of outward‐propagating spiral bands in hurricanes. the basic shysics is illustrated most simply for stable vorticity monopoles on an f‐plane. Unlike the dynamics of sheared disturbances in rectilinear shear flow, axisymmetrizing disturbances on a vortex are accompanied by outward‐propagating vortex Rossby‐waves whose restoring mechanism is associated with the radial gradient of storm vorticity. Expressions for both phase and group velocities are derived and verified; they confirm earlier speculations on the existence of vortex Rossbywaves in hurricanes. Effects of radially propagating vortex Rossby‐waves on the mean vortex are also analysed. In conjunction with sustained injection of vorticity near the radius of maximum winds, these results reveal a new mechanism of vortex intensification. the basic theory is then applied to a hurricane‐like vortex in a shallow‐water asymmetric‐balance model. the wave mechanics developed here shows promise in elucidating basic mechanisms of hurricane evolution and structure changes, such as the formation of secondary eye‐walls. Radar observations possessing adequate temporal resolution are consistent with the predictions of this work, though more refined observations are needed to quantify further the impact of mesoscale banded disturbances on the evolution of the hurricane vortex.
Stationary rossby‐wave propagation in a baroclinic atmosphere
Authors: Ambrizzi Tércio , Hoskins Brian J.
Published: February 2007

Observational studies of teleconnections in both solsticial seasons have suggested various patterns of behaviour linked mainly to westerly jets and equatorial westerlies. These have been interpreted using barotropic Rossby‐wave theory, and simulated using stationary forcing in barotropic models linearized about 300 hPa flows. In this paper the relevant propagation behaviour is investigated using a baroclinic model with three‐dimensional, climatological basic states. Time integrations are performed using localized thermal forcing as a wavemaker. the propagation results are qualitatively very similar to those obtained with the barotropic model, though there are quantitative differences. the westerly jets still act as strong waveguides. the shorter wavelengths and smaller wave‐activity speeds found with the baroclinic model are generally in better agreement with observation. Propagation into and from the upper tropospheric equatorial westerlies in the east Pacific, propagation across North America and propagation from Europe across the Arabian Gulf are all found. the extent to which upper tropospheric westerlies reach towards the equator influences the propagation into those regions. Barotropic models can only represent such behaviour if the basic state reflects the near‐equatorial upper tropospheric zonal wind structure.
A contour‐advective semi‐lagrangian numerical algorithm for simulating fine‐scale conservative dynamical fields
Authors: Dritschel David G. , Ambaum Maarten H. P.
Published: February 2007

This paper describes a novel numerical algorithm for simulating the evolution of fine‐scale conservative fields in layer‐wise two‐dimensional flows, the most important examples of which are the earth's atmosphere and oceans. the algorithm combines two radically different algorithms, one Lagrangian and the other Eulerian, to achieve an unexpected gain in computational efficiency.

The algorithm is demonstrated for multi‐layer quasi‐geostrophic flow, and results are presented for a simulation of a tilted stratospheric polar vortex and of nearly‐inviscid quasi‐geostrophic turbulence. the turbulence results contradict previous arguments and simulation results that have suggested an ultimate two‐dimensional, vertically‐coherent character of the flow. Ongoing extensions of the algorithm to the generally ageostrophic flows characteristic of planetary fluid dynamics are outlined.
Effect of relative humidity on the electrical conductivity of marine air
Authors: Kamra A. K. , Deshpande C. G. , Gopalakrishnan V.
Published: February 2007

Measurements of the atmospheric electric conductivity made in the equatorial Indian Ocean and Arabian Sea in August and September 1991 show that the value of conductivity decreases from 2.3 × 10−14 mho m−1 in the equatorial Indian Ocean, where relative humidity of the surface air is 70‐80%, to 1.1 × 10−14 mho m−1 in the region of the Somali current, where relative humidity of the surface air increases to 80‐90%. the inverse relationship observed between conductivity and relative humidity, in spite of showing a large scatter, is stronger in the case of negative than positive conductivity. the sharp increase in the sizes of ions and marine aerosol particles when the relative humidity exceeds 75‐80% is proposed as the cause of the observed decrease in conductivity in the region of the Somali current.
Relations between interannual and intraseasonal monsoon variability as diagnosed from AMIP integrations
Authors: Ferranti L. , Slingo J. M. , Palmer T. N. , Hoskins B. J.
Published: February 2007

Monsoon variability on intraseasonal and interannual time‐scales is analysed using data from five 10‐year European Centre for Medium‐Range Weather Forecasts Atmospheric Model Intercomparison Project integrations, which differ only in their initial conditions. the results show that monsoon fluctuations within a season and within different years have a common dominant mode of variability. the spatial pattern of the common dominant mode in precipitation has a pronounced zonal structure, with one band of anomalous rainfall extending from 20°N to 5°N, covering most of the land areas, with the other band, of opposite sign, lying between 5°N and 10°S, mostly over the Indian Ocean. This mode therefore describes both the active/break monsoon spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between the continental and the oceanic regime and the principal pattern of interannual variability of monsoon rainfall. In the observations the oscillations between active and break monsoon spells have similar behaviour, although the model is deficient in representing the rainfall variability over India.

On the intraseasonal time‐scale the transition between the two regimes seems to have a chaotic nature. In addition the probability density function of the principal mode is bimodal for the years in which this mode is particularly dominant. These two results indicate a possible similarity with the Lorenz 3‐component chaotic model. Northward‐propagating convective regions, simulated by the model, are not clearly associated with the phase transitions of the TCZ regime. the timing of the monsoon onset appears to be modulated by the phase of the El Niño/Southern Oscillation during the preceding season, consistent with observational studies.

The results suggest that the dominant mode may also represent some components of the observed monsoon variability. the interannual fluctuations of the dominant mode exhibit only a weak level of reproducibility compared with the relatively large predictability of a broad‐scale monsoon wind‐shear index.
Biosphere feedback on regional climate in tropical North Africa
Authors: Xue Yongkang
Published: February 2007

The impact of land‐surface degradation over the Sahel area on seasonal variations of atmospheric and hydrological components over tropical north Africa are investigated in a general‐circulation model numerical experiment, together with the mechanisms involved. The model was integrated for four years with and without vegetation change over the Sahel region with different initial atmospheric conditions. The results demonstrate that the degradation of the land surface can have a significant impact on the Sahelian regional climate. It increases the surface air temperature and reduces the precipitation, runoff and soil moisture over the Sahel region during the July‐August‐September (JAS) season. The impact is not only limited to the specified desertification area and the JAS season but is found also to the south of this area and extends into the October‐November‐December season. The changes in the annual rainfall cycle and the JAS mean surface temperature over the Sahel area are consistent with the observed climate anomalies of the past 40 years. The changes in rainfall to the south of the Sahel including eastern Africa are also in line with the observed anomalies.

The reduction in total diabatic heating rate and relative increase in subsidence motion in the upper troposphere are consistnet with the rainfall anomalies. The variations in convective heating rate, which were caused by changes in latent‐heat flux from the land surface and moisture flux convergence in the atmosphere, are the dominating factors in this process. The radiative cooling is a secondary effect. The influence of the initial conditions on the simulation of the soil water balance is also analysed.
Elliptical diagnostics of stratospheric polar vortices
Authors: Waugh Darry N. W.
Published: February 2007

Diagnostics based on the spatial moments of isopleths of a long‐lived tracer, or of potential vorticity, are presented that enable the structure and evolution of stratospheric polar vortices to be concisely summarized and quantified. The area, centre, aspect ratio and orientation of the ‘equivalent ellipse’ of the vortex, on an isentropic surface, are defined using the second‐ and lower‐order spatial moments of contours within the vortex‐edge region. By examining the variations of these ‘elliptical’ diagnostics with time and altitude, the temporal evolution and vertical structure of the polar vortices can be quantified. The usefulness of the diagnostics is assessed by examining nitrous oxide data from the Geophysical Fluid Dynamics Laboratory ‘SKYHI’ general‐circulation model. The diagnostics show, and quantify, several differences between the Arctic and Antarctic vortices in the SKYHI model. The Arctic vortex moves further off the pole, is generally more elongated, and has a more complicated vertical structure than the Antarctic vortex (with larger variations of both the vortex centre and elongation with height). The elliptical diagnostics also identify the occurrence of large‐scale Rossby‐wave breaking events, both at the vortex edge and in the subtropics, in the model.
Modelling stably stratified boundary‐layer flow over low hills
Authors: Weng W. , Chan L. , Taylor P. A. , Xu D.
Published: February 2007

The mixed spectral finite‐difference model for neutrally stratified atmospheric boundary‐layer flow over complex terrain has been extended to include the effects of stable stratification with a simple turbulent‐kinetic‐energy and mixing‐length turbulence‐closure scheme. to allow internal gravity‐wave propagation, we have constructed a steady background flow such that wind velocity and buoyancy frequency are constants above the boundary layer with the assumption that there are momentum and heat sinks to balance the flux divergences in the governing equations. Model results are compared with inviscid theory. Special emphasis is placed on the pressure or form drag on topography.
Multi‐spectral calculations of the direct radiative forcing of tropospheric sulphate and soot aerosols using a column model
Authors: Haywood J. M. , Shine K. P.
Published: February 2007

A newly developed multi‐spectral radiative‐transfer code is used to investigate the important features of the direct radiative forcing of sulphate and soot aerosol; the indirect effect of both aerosol species is not investigated in this study. the direct radiative forcing is presented for different surface albedos, solar zenith angles, relative humidities and aerosol vertical profiles together with effects upon the surface irradiance and long‐wave radiative forcing. the effect of subgrid‐scale variations in relative humidity are examined using idealized relative humidity distributions. the results show that subgrid‐scale variations in relative humidity and the spatial correlation between cloud and areas of high relative humidity should be considered in future general‐circulation model calculations of the direct forcing due to sulphate aerosol. A comparison of the direct forcing obtained by adjusting the surface albedo to that using the full multi‐spectral column calculation is performed; the results indicate that recent estimates of the climate response to the direct forcing of sulphate may be too large. the contribution to the direct forcing from cloudy‐sky regions appears to be negligible for sulphate aerosol but there is a considerable enhancement of the forcing due to soot aerosol if soot exists within or above clouds. These calculations show that a small amount of soot, relative to the sulphate mass loading, can cause a significant positive direct forcing and emphasize that the vertical profile of soot aerosol relative to cloud must be established to enable accurate assessment of the direct radiative effects of anthropogenic emissions of soot aerosol.
Convergence zones over the Greek peninsula and associated thunderstorm activity
Authors: Kotroni Vassiliki , Kallos George , Lagouvardos Konstantinos
Published: February 2007

In the frame of this study, the initiation of summer storm activity over the Greek peninsula during a prevailing weak synoptic flow is investigated using the Colorado State University‐Regional Atmospheric Modelling System. On 10 July 1994, thunderstorm activity was observed along a convergence zone which had developed following the main axis of the peninsula. the convergence zone first deepened the moist layer locally, providing a region potentially favourable to deep convection, while terrain variability and diurnal differential‐heating‐forced vertical motions. the cold air intrusion aloft, associated with the low‐level convergence, resulted in deep convection and significant storm activity. the origin of the moist air masses which fed this activity has been investigated through simulations with a Lagrangian particle dispersion model. Moist air masses, which originated from marine boundary‐layer of the Ionian Sea on the west, and north Aegean Sea on the east of the Greek peninsula, were transported over the land through the sea‐breeze mechanism and later, at the time of the storm activity, were deeply injected into the troposphere over the convergence line, at heights up to 5 km.
Vacillation cycles and blocking in a channel
Authors: Haines K. , Holland A. J.
Published: February 2007

The response to a low‐level high‐frequency wavemaker forcing in a two‐layer, β‐plane, quasi‐geostrophic channel model is examined. The wavemaker simulates regular baroclinic instability which then propagates to upper atmospheric levels to excite blocking. By altering the meridional shear in the upper layer, the large‐scale response can vary from a steady large‐amplitude split jet, very similar to observed blocks, to a weaker split with a low‐frequency vacillation cycle. The eddies will always resonantly excite the split flow, but a mixed instability process is responsible for the breakdown in cases which oscillate, and this is demonstrated using a simplified zonal stability analysis and energy‐tendency diagnostics. The re‐excitement by the eddies continues the cycle. This model provides a theory of how the meridional structure of the upper‐level winds may determine whether a large‐amplitude block can be excited or persist in the presence of similar high‐frequency eddy activity propagating up from below.
A cumulus parameterization with a prognostic closure
Authors: Pan Dzong‐Ming , Randall Davi D. A.
Published: February 2007

The paper describes the introduction of a prognostic cumulus kinetic energy (CKE) as a replacement for the quasi‐equilibrium closure hypothesis of Arakawa and Schubert (AS).

In the original version of the AS parameterization, the cloud work function, a measure of the convective available potential energy, is assumed to be maintained at ‘small’ values through a quasi‐equilibrium between the cumulus convection and the ‘large‐scale forcing’. It is argued here, however, that the distinction between the convective and large‐scale processes is ambiguous and subjective. It is demonstrated that the need for such a distinction can be avoided by relaxing the quasi‐equilibrium assumption, through the introduction of a prognostic CKE; referred to as prognostic closure. A dimensional parameter, α, is introduced to relate the CKE to the square of the cloud‐base convective mass flux. It is shown that ‘adjustment time’ defined by AS is related to α, so that when the adjustment time approaches zero the prognostic closure reduces to quasi‐equilibrium closure. A second dimensional parameter, τD, is used to determine the rate at which the CKE is dissipated. In the limit of small α and τD, the convective mass flux is formally independent of both α τD if the environmental sounding is assumed to be given, but in reality the results of a prognostic model do depend on these two parameters because they affect the time‐dependent sounding.

For simplicity, a single constant value of α is used for all cloud types in tests with a general‐circulation model, and this gives reasonably good results. Larger values of α lead to more frequent shallow cumulus convection and a cooler and more humid troposphere, in which stratiform condensation is more active and more large‐scale precipitation can reach the surface. A longer dissipation time‐scale leads to a warmer tropical troposphere. The interactions between stratiform cloudiness and convection prove to be quite important, leading to the conclusion that the convection parametrization really cannot be evaluated independently of the stratiform cloud parametrization with which it interacts.
Linear stability and single‐column analyses of several cumulus parametrization categories in a shallow‐water model
Authors: Yano Jun‐Ichi , Moncrieff Mitchell W. , McWilliams James C.
Published: February 2007

Using a mass‐flux based approach, the thermodynamic cumulus parametrization problem is reformulated in a simple atmospheric model, which is an analogue of the shallow‐water equations. The objective is to investigate basic effects of elementary representations of several parametrization categories. In particular, a linear stability analysis and a single‐column experiment are performed to infer the characteristics of each parametrization as regards its ability to simulate the large‐scale organization or coherence of tropical convection.

The moisture‐convergence closure (MC) scheme, which assumes that the ensemble of cumulus convection is controlled by the low‐level moisture convergence as in Kuo‐type scheme, predicts the largest growth at the smallest scale. Hence, although it ensures the generation of a coherent propagating structure, its scale always corresponds to the grid size. Furthermore, the MC tends to produce a catastrophic positive feedback of moist convection to the large‐scale convergence.

In contrast, the statistical equilibrium scheme, which assumes an instantaneous adjustment of the large‐scale environment to a quasi‐equilibrium state, such as Arakawa‐Schubert and moist convective adjustment schemes, asymptotes to a constant growth rate at small scales. Hence, this type of parametrization tends to generate a field like white noise with no large‐scale coherence. The lagged‐adjustment (LA) schemes, which have a short time‐lag for the cumulus growth, as in the Betts–Miller scheme, feature a finite scale selection in the linear growth rate. This ensures a smooth large‐scale coherence that is independent of the grid size, and is consistent with the scale‐separation principle.

A new type of parametrization is also tested. This convective life‐cycle (CLC) scheme represnets the life cycle of a common type of convective system made up of deep precipitating convection and a subsequent mesoscale response. It uses a buoyancy‐based closure. The growth‐rate curve is similar to the other LA schemes, but the behaviour in the zero‐dimensional (single‐column) version of the model is qualitatively different. Although the CLC scheme does not automatically saisfy the scale‐separation principle, its grid‐size dependence can be treated by a re‐normalization principle.

The result are used to interpret some reported general‐circulation‐model results regarding the impact of different parametrization schemes on the tropical atmosphere at large scales.
Vertical motion, diabatic heating, and rainfall characteristics in north Australia convective systems
Authors: Cifelli Robert , Rutledge Steven A.
Published: February 2007

Very‐high‐frequency wind‐profiler data are used to study the vertical draught structure within 13 tropical Mesoscale Convective Systems (MCSs) near Darwin, Australia during the 1989‐90 and 1990‐91 wet seasons. These studies are supported by single‐Doppler radar, soundings, and surface rainfall data to correlate radar reflectivity, thermal buoyancy, and surface rainfall patterns with vertical air‐motion structures. Because of Darwin's unique location at the southern tip of the Maritime Continent, vertical draughts in both the monsoon (maritime) and monsoon break (continental) convective regimes can be observed.

The break‐regime MCSs (six in total) were all squall lines, characterized by a leading line of convection with heavy precipitation and trailing stratiform rainfall containing a characteristic radar bright band. These MCSs exhibited a pronounced life‐cycle pattern and were all sampled by the profiler in the mature to dissipating stages. In contrast, the monsoon systems (seven in total) were composed of regions of stratiform cloud with embedded convective bands which moved on‐shore in the monsoonal flow. Results from the Darwin rain‐gauge network indicated that the majority of the total rainfall in each MCS (break and monsoon) was associated with the convective portion of the system.

The break‐regime MCSs were all characterized by a low‐level (4 km) updraught peak associated with convective cells on the leading edge of each squall line, trailed by deeper convective updraughts in the middle and upper troposphere. For the monsoon cases, the lower‐troposphere convective updraughts were typically less than those in the squall lines, yet were stronger in the upper troposphere. The low‐level differences in the convective updraughts were consistent with the smaller virtual‐temperature excess in the monsoon soundings, as well as the larger vertical radar‐reflectivity gradients observed in monsoon convection.

Consistent with the differences in vertical air‐motion patterns, diabatic heating and moistening profiles for the monsoon MCSs were characterized by a higher‐level heating and drying peak compared with the break MCSs. The results have important implications for cumulus parametrizations in numerical models since the large‐scale circulation is sensitive to the vertical distribution of diabatic heating in tropical MCSs.
Stationary gravity‐wave structure in flows with directional wind shear
Authors: Shutts G. J.
Published: February 2007

Most theoretical studies of flow over mountains assume that the wind direction is constant. For two‐dimensional orographic ridges this is not an issue, since only the component of the wind across the ridge forces internal gravity waves. A special case of rectilinear flow is when the wind component changes sign (i.e. the wind direction changes discontinuously through 180 degrees): such a point is a critical level for all stationary gravity waves. For isolated three‐dimensional mountains the gravity‐wave response comprises waves of all azimuthal orientations, and each of these may have a different critical‐level height‐depending on whether the wind backs or veers with height.

Using a combination of ray‐tracing methods and stationary‐phase solutions, the structure of the stationary, hydrostatic gravity‐wave field generated by an isolated mountain is studied for a simple wind field which backs with height. In contrast to the behaviour for unidirectional flow, wave action does not accumulate indefinitely beneath the critical level but is continuously advected downwind by the component of the wind parallel to the phase lines. Instead of the wave energy becoming unbounded following a ray as in the 2D case, the wave energy decays to zero along a trailing wake zone downwind of the mountain along phase lines. This is just an extension of the ‘asymptotic wake’ found in the linear solutions of Smith for constant flow over an isolated mountain. The work described here underlines the desirability of including the effects of selective critical‐level absorption in the gravity‐wave drag parametrization schemes of numerical weather‐prediction models.
Sensitivity to potential vorticity
Authors: Arbogast P.
Published: February 2007

A new method is proposed that computes the sensitivity to potential vorticity of any diagnosed aspect of the atmosphere. It does not require the computation of the adjoint of the full inversion procedure, but rather the tangent linear and the adjoint of the operator that relates the wind and mass fields to the distribution of potential vorticity. The formal approach employed helps in understanding the role of the balance condition. This method is applied to the study of the sensitivity of the boundary condition to the inner potential vorticity in a 2D atmosphere.
The ECMWF implementation of three‐dimensional variational assimilation (3D‐Var). I: Formulation
Authors: Courtier P. , Andersson E. , Heckley W. , Vasiljevic D. , Hamrud M. , Hollingsworth A. , Rabier F. , Fisher M. , Pailleux J.
Published: February 2007

In the first of this set of three papers, the formulation of the European Centre for Medium‐Range Weather Forecasts (ECMWF) implementation of 3D‐Var is described. In the second, the specification of the structure function is presented, and the last is devoted to the results of the extensive numerical experimentation programme which was conducted. The 3D‐Var formulation uses a spherical‐harmonic expansion, much as the ECMWF optimal interpolation (OI) scheme used an expansion of Bessel functions. This formulation is introduced using a convolution algebra over the sphere expressed directly in spectral space. It is shown that all features of the OI statistical model can be implemented within 3D‐Var. Furthermore, a non‐separable statistical model is described. In the present formulation, geostrophy is accounted for through a Hough‐modes separation of the gravity and Rossby components of the analysis increments. As in OI, the tropical analysis remains essentially non‐divergent and with a weak mass‐wind coupling. The observations used, as well as their specified statistics of errors, are presented, together with some implementation details. In the light of the results, 3D‐Var was implemented operationally at the end of January 1996.
Impact of model resolution and ensemble size on the performance of an Ensemble Prediction System
Authors: Buizza R. , Petroliagis T. , Palmer T. , Barkmeijer J. , Hamrud M. , Hollingsworth A. , Simmons A. , Wedi N.
Published: February 2007

Ensemble integrations for 14 cases are described. These integrations test the relative impact of increase in ensemble size and in the resolution of the model used to integrate the ensemble. The ensembles are evaluated using a variety of statistical tests. Some of these indicate a relative advantage of an increase in ensemble size, whilst most tests suggest a relative advantage of an increase in model resolution. However, overall, the best performance was obtained by combining enhancement in model resolution (from T63L19 to T106L31) with an increase in ensemble size (from 32 to 50 members).
Modelling the Asian summer monsoon rainfall and Eurasian winter/spring snow mass
Authors: Dong Buwen , Valdes Paul J.
Published: February 2007

The interannual variation of the south Asian summer monsoon is analysed based on sets of present day climate simulations using the UK Universities' Global Atmospheric Modelling Programme (UGAMP) generalcirculation model with different land surface parametrization schemes, different horizontal resolutions, and sea surface temperature variations. Generally, a negative relationship is found between south Eurasian winter/spring snow mass and the amount of summer monsoon rainfall over India in all simulations. However, the significance of this relationship is dependent on the land surface parametrization scheme. The simulations using the no‐flux boundary condition at the bottom of a three‐layer soil model give a strong negative correlation. This inverse relationship is the strongest over north India and the foothills of the Himalayas and is statistically significant. The model results are in good agreement with observational studies. Composite analyses suggest that less winter/spring snowfall over south Eurasia is associated with a strong Indian summer monsoon, characterized by strong south‐westerlies over the Arabian Sea in the lower troposphere in the June‐August season and heavy precipitation in early summer over north India and the foothills of the Himalayas. In contrast, heavy winter/spring snowfall delays the onset of the Indian summer monsoon through the feedback of snowmelt, soil moisture and evaporation processes, and is associated with weak summer precipitation over the two regions.

Sensitivity studies confirm that the snow mass‐Indian monsoon relationship identified in the simulation at T42 is also robust in the simulation at T31. However, a negative relationship does not exist in the simulation at T21, indicating the importance of horizontal resolution in maintaining the snow‐monsoon relationship in the UGAMP model.
Time‐scales of adjustment to radiative‐convective equilibrium in the tropical atmosphere
Authors: Tompkins Adrian M. , Craig George C.
Published: February 2007

In the tropics the assumed existence of a balanced atmospheric state of radiative‐convective equilibrium is a useful and widely utilized concept. Given an atmospheric state of radiative‐convective equilibrium which is perturbed, this paper attempts to identify the mechanisms that determine the time‐scale for the restoration of the balanced state. The perturbation could arise from large‐scale atmospheric wave motions, local‐scale convective downdraughts or sea surface temperature perturbations for example. The resulting state immediately after the perturbation is applied can be one of suppressed or convective conditions, and it is the complex response under convective conditions that is explored in this paper.

A three‐dimensional cloud resolving model is operated to a radiative‐convective equilibrium state to which sea surface temperature perturbations are then applied. It is found that the variability of the model state variables, such as temperature and total water vapour amount, can be divided by processes into an exponential adjustment to the new balanced state on a long time‐scale (15 days), superimposed by short time‐scale variability (<4 days) that is governed almost solely by the convective mass‐flux. The determination of the long time‐scale trend to equilibrium is then investigated with further numerical experiments, which demonstrate that radiation determines the adjustment time‐scale via its control of the subsidence velocities in the clear‐sky regions surrounding convection. Some implications of the results for cumulus parametrization are discussed.