Showing papers in "Quarterly Journal of the Royal Meteorological Society in 1990"

A scheme for predicting layer clouds and their water content in a general circulation model

Abstract: A cloud water variable has been included in the Meteorological Office atmospheric general circulation model. the cloud amount and water content are generated by a scheme which assumes a distribution of thermodynamic and water content variables about their grid-box-mean values. A parametrization of the rate of depletion of cloud water by precipitation is included. the model's boundary layer mixing is reformulated in terms of variables conserved during changes of state of water, allowing a more realistic calculation of atmospheric stability in cloudy regions. The scheme produces realistic cloud distributions and the predicted cloud water content verifies well against microwave radiometer data in all regions except the mid-latitudes of the summer hemisphere. the parametrization of cloud optical properties in terms of the cloud water content will be an important future development of the model.

The theoretical relationship between foliage temperature and canopy resistance in sparse crops

Abstract: One-dimensional, sparse-crop interaction theory is reformulated to allow calculation of the canopy resistance from measurements of foliage temperature. A submodel is introduced to describe eddy diffusion within the canopy which provides a simple, empirical simulation of the reported behavior obtained from a second-order closure model. The sensitivity of the calculated canopy resistance to the parameters and formulas assumed in the model is investigated. The calculation is shown to exhibit a significant but acceptable sensitivity to extreme changes in canopy aerodynamics, and to changes in the surface resistance of the substrate beneath the canopy at high and intermediate values of leaf area index. In very sparse crops changes in the surface resistance of the substrate are shown to contaminate the calculated canopy resistance, tending to amplify the apparent response to changes in water availability. The theory is developed to allow the use of a measurement of substrate temperature as an option to mitigate this contamination.

Computational aspects of moisture transport in global models of the atmosphere

Abstract: Computational aspects of methods used to simulate the transport of water vapour in a global atmospheric general circulation model are examined. A set of properties useful in characterizing numerical methods for modelling atmospheric transport are identified. Spectral and semi-Lagrangian methods, which are very different in terms of these desired properties are compared. The extent to which the schemes do not satisfy certain properties of the continuous equations provides a measure of one component of the error of the solution. For the spectral scheme, negative specific humidities q indicate such an error component. Conventional semi-Lagrangian schemes are also susceptible to generating negative values. In addition, they are not inherently conservative. Shape-preserving semi-Lagrangian methods do not generate negative values, but still are non-conservative. the degree to which the advection process does not conserve mass provides a measure of another error associated with the numerical solution. the negative error is shown to be large for the spectral transport scheme, measured either locally or globally. Measured globally, the semi-Lagrangian transport schemes' conservation errors are equally large. Locally, the correction of this error can be made very much smaller, relative to physical processes in the model. The study highlights the computational problems which still exist within the better numerical methods used to simulate the transport of water vapour, and demonstrates the care with which one must apply computational constraints to the solution. the spectral and semi-Lagrangian transport schemes produce very different climatologies in model simulations. A comparison of these climatologies will appear elsewhere.

A model to estimate the source area contributing to turbulent exchange in the surface layer over patchy terrain

Abstract: The characteristics of turbulence in the surface layer are well understood over spatially homogeneous and smooth surfaces. Current theoretical knowledge is based on measurements over such ‘ideal’, flat and homogeneous surfaces. Over heterogeneous surfaces of a patchy nature (e.g. urban areas, mixed agricultural crops, forest etc.) turbulence is subject to considerable spatial variability over a wide range of scales. A ‘source area’ is defined as the portion of the upstream surface containing the effective sources and sinks contributing to the turbulent exchange processes at a given point in the surface layer. It is noted that the term ‘source area’ refers to sources and sinks on the surface only and does not consider production or dissipation within the atmosphere. the contributions of surface elements are treated as ‘plumes’ of heat or vapour or momentum-deficiency. A small-perturbation approach to diffusion theory and a plume-diffusion model are used in a numerical source-area model (SAM) to estimate this region, depending on meteorological scaling parameters and the reference location. This model identifies a ‘maximum effect source location’ and the upwind, downwind and lateral dimensions of regions containing various levels of the total contributing effect. It accounts for the different source weight of each point on the surface, depending on its position relative to the reference location. Although the source area is not directly measurable, the results of SAM compare favourably with observed effects of changing source areas. A sensitivity test of SAM results and non-linear regression leads to a set of simple equations (mini-SAM) approximating SAM. This short-cut model provides easily obtainable estimates of the source-area dimensions and the relative importance of surface elements for a given measurement of turbulent exchange.

On the cloud absorption anomaly

Abstract: This paper provides an overview of the subject of absorption of solar radiation by water clouds in the earth's atmosphere. the paper summarizes the available evidence which points to disagreements between theoretical and observed values of both cloud absorption and reflection. the importance of these discrepancies, particularly to remote sensing of clouds as well as to studies of cloud physics and earth radiation budgets, is emphasized. Existing cloud absorption and reflection measurements are reviewed and the persistent differences that exist between calculated and measured near-infrared cloud albedos are highlighted. Various explanations for these reflection and absorption discrepancies are discussed and a simple outline of the theory of cloud absorption is provided. This outline is used to examine the large-droplet hypothesis as well as the effects of absorbing aerosol and enhanced water vapour continuum absorption. A further hypothesis regarding the effects of cloud inhomogeneities is also examined. While the theory of cloud absorption is not completely understood, especially with regard to inhomogeneous clouds, the underlying conclusion of this paper points to the need for better measurements of solar radiation in clouds, water vapour absorption and microphysics properties of clouds.

Regimes in the wintertime circulation over northern extratropics. I: Observational evidence

Abstract: Regimes of the northern extratropical circulation in winter are identified in this paper as clusters of atmospheric states in a low-dimensional phase space generated by the leading EOFs of eddy geopotential fields In order to define the clusters, our algorithm seeks points corresponding to local maxima for the density of atmospheric states; subsequently, a cluster is defined around each density maximum as that portion of the phase space in which the observed density can be locally approximated by a unimodal function Two analyses were performed, using a 5-dimensional and a 3-dimensional space respectively, and they provided consistent results Six clusters were found the largest cluster includes 40% of the fields in our sample; its centroid is close to the climatological winter state, but it possesses a positive projection on the Pacific-North American (PNA) pattern the other five clusters represent anomalous flow regimes and include 52% of the fields One of them shows a low amplitude of the planetary waves; the remaining four represent states with large wave amplitude but different phases the variability between clusters accounts for the bimodality in the amplitude of planetary waves detected in previous observational studies Our analysis reveals that this bimodality is much enhanced in the region of the phase space where the PNA index is negative, and the separation among the clusters is stronger Finally, frequencies of transitions between clusters are presented, which show an asymmetric behaviour in the transitions between regimes with low and high amplitude of planetary waves

Numerical prediction of thunderstorms—has its time come?

Abstract: An approach to developing numerical prediction of convective storms and storm environments over the next decade is outlined. the principal scientific challenges are described, along with some early progress made in meeting them.

Nieuwstadt's stable boundary layer revisited

Abstract: Currently no theory of the stably stratified atmospheric boundary layer (SBL) is generally accepted as definitive even for idealized cases. Nieuwstadt's theory, though a promising candidate, faces objections relating to upper boundary and steady-state conditions, internal wave effects and the consistency of the model outside the strong-stability limit. the aim of this paper is to examine the objections, improve the model, draw further deductions and compare with numerical models. We shall deduce from Nieuwstadt's model that Bo = RfcG2|f|/√3, where Bo is the surface buoyancy-flux, G the geostrophic wind speed, f the Coriolis parameter and Rfc the critical value of the flux Richardson number Rf. Higher-order expansion shows this value is an upper bound corresponding to the stable limit L/h 0. This is consistent with a similar bound on Bo derived from independent energy arguments. By contrast, within present idealizations there is no bound on the surface cooling rate. Comparisons with a new series of large eddy simulations (LES) and other numerical models support the interpretation of Nieuwstadt's SBL as an idealized limiting case. the theory explains from first principles the observed sensitivity to small slopes. Interaction between sharp inversions and slopes may cause turbulence in thin layers. Coupling with the surface boundary conditions is a likely cause of intermittent turbulence, and explains features of Brost and Wyngaard's second-order closure study. the formal singularity at the top of the SBL gives time-scales for approach to inertial and heat equilibrium. Both, for separate reasons, are O(|f|−1). A natural extension to moderately stable and near-neutral conditions agrees well with numerical model results, and provides a complete prediction of SBL structure from given surface heat flux and synoptic pressure gradient. the near-neutral regime is narrow at high Rossby number. Comparison with LES supports both the local scaling approach and the gross predictions of the theory. the model gives insight into the limitations of Rossby-similarity formulae. In particular, restrictions of domain size hd (or similar background stability effects) may not become negligible even when h/hd 0. Hence matching to neutral conditions is important in predicting even quite stable boundary layers. Wave effects, though non-local in some respects, do not fundamentally change the Nieuwstadt picture for the mean structure. Retention in the model of the original value for Rfc is recommended even if wave radiation perturbs local Rf. In summary, the adapted Nieuwstadt theory seems to provide a definitive framework for the idealized SBL, from which other ‘perturbation’ effects may be assessed. the value of Bo/G2|f| gives a criterion for (a) quasi-steady or (b) intermittent SBL regimes.

Some measurements of turbulence over an antarctic ice shelf

Abstract: Profiles of wind speed, temperature and turbulent fluxes of heat and momentum in the lowest 32 m have been measured at a station on an antarctic ice shelf. During the antarctic winter the surface layer often shows strong static stability, with temperature gradients as large as 1 Km−1 in the lowest few metres. the surface inversion is destroyed during periods of high wind speed but the wind profile shows significant deviation from the expected logarithmic form under such conditions. Measurements of stress at 5 m indicate that the roughness length of the snow surface is about lO−4 m. At 5 m height, the variations of the dimensionless wind shear, ϕm, and potential temperature gradient, ϕT, agree with previously determined forms of the Monin-Obukhov similarity functions. Above 5 m, the behaviour of ϕm and ϕT is only qualitatively similar and surface-layer similarity theory does not provide a good description of the profiles. Turbulence length scales have been deduced from vertical velocity power spectra. Under near-neutral conditions, the ratio of turbulence length scale to measurement height is observed to decrease with increasing height of measurement. This observation is consistent with the variation of turbulence length scale with height implied by the measurements of ϕm. It is suggested that the stability of the overlying atmosphere restricts the depth of the turbulent boundary layer and hence the length scales of turbulence within this layer. Increasing stability causes a decrease of turbulence length scales at all levels. The ratios of turbulence kinetic energy to stress and temperature variance to heat flux are examined. Measurements are somewhat scattered, but the distribution of values varies little with height or stability. the form of the distribution suggests that large-scale motions, possibly internal gravity waves, may be playing an important role in boundary-layer processes.

Analytical model for the growth of the coastal internal boundary layer during onshore flow

Abstract: A model of near-neutral and convective steady-state internal boundary layer evolution is presented. the model deals with the internal boundary layer that forms over land in coastal and lake areas during onshore winds. Near the ground, the growth of the internal boundary layer is controlled by friction velocity in accordance with surface layer theory. Farther downwind, the growth is determined by the atmospheric stability and friction velocity within the internal boundary layer, and the temperature gradient in the air above. the wind profile inside the internal boundary layer is assumed to follow Obukhov similarity theory. an expression for the strength of the inversion that caps the layer is derived and used in the model. A comparison is carried out with independent experimental observations of internal boundary layer growth in the sea-land transition. Kinematic heat flux through the top of the internal boundary layer is described by the formulation − h = 0.2s + 2.5 uT/gh + the Zilitinkevich correction. the terms 0.2s and 2.5uT/gh account for convective and mechanical turbulence, respectively, and the Zilitinkevich correction is a turbulent kinetic-energy storage term that ensures finite growth rate of the internal boundary layer near the ground. the relative importance of mechanical and convective turbulence as well as the Zilitinkevich correction is discussed. the Zilitinkevich correction dominates the growth process of the internal boundary layer when it is lower than roughly 50m. As the layer grows, the importance of the Zilitinkevich correction diminishes. Then mechanical turbulence dominates the growth process until the internal boundary layer has reached a height of approximately −1.4L. Further growth is controlled mainly by convective turbulence. Conditions of high wind speed and large values of the potential temperature gradient over water result in a deep zone where the growth of the internal boundary layer is controlled by mechanical turbulence. With near-zero potential temperature gradient over water, the zone becomes shallow and may vanish.

Buoyancy reversal and cloud-top entrainment instability

Abstract: In 1980, Randall suggested that cloud top entrainment instability, a runaway positive feedback between mixing, evaporative cooling and entrainment, may cause the transition from the subtropical stratocumulus layers, frequently observed over the cold water of the eastern oceans, to tradewind cumulus. Predicting how the mean position of this transition, and the associated large change in albedo, would vary with, and feed back on, a change in climate is important for any credible model of the climate system. We have used numerical and laboratory simulations, as well as field observations, to study entrainment in buoyancy-reversing systems. We consider a system in which an upper fluid stably overlies a second lower fluid; but some mixtures are less buoyant than either fluid. the laboratory results show that buoyancy reversal does not cause runaway ‘explosive entrainment’ unless D (the ratio of the density difference between the most cooled mixed parcel and the lower fluid to the density difference across the unmixed layers, a measure of CEI potential) exceeds 1.3, which is far greater than the range of D observed in stratocumulus. For D below this threshold, buoyancy reversal leads to weak circulations rather reminiscent of Rayleigh-Benard convection in the lower layer, but only slightly distorts the interface. For D > 1.3 an entrainment tongue of upper fluid penetrates a sizable fraction of the depth of the lower layer and entrainment is explosively rapid. Numerical experiments suggest that even when 0 < D < 1.3, entrainment can break up idealized subtropical marine stratocumulus within about an hour if radiation and surface fluxes are removed, but comparison of numerical simulations with the laboratory experiments suggests that the numerically simulated entrainment rate may be substantially too large in this regime owing to the inability to accurately produce and resolve the entraining eddies. Our observations of stratocumulus from FIRE typically had a strong stable layer above the cloud top inversions, leading to some ambiguity in defining D, but clearly show that stable stratocumulus layers often have 0 < D < 0.2. We suggest that sudden breakup of stratocumulus due to runaway entrainment is unlikely, but evaporatively enhanced entrainment may aid the transition from stratocumulus to tradewind cumulus in conjunction with some other mechanism, such as short-wave absorption by liquid water, that can help dynamically decouple the cloud layer from the surface layer.

Onset of sahelian drought viewed as a fluctuation-induced transition

Abstract: A statistical analysis of representative sets of rainfall data in the west Sahel region is performed and a jump transition in the late sixties is detected. It is suggested that the main features associated with the record can be accounted for by a fluctuation-induced transition between two stable precipitation regimes: a ‘humid’ and a ‘dry’ state. A minimal model involving the dominant nonlinearities is constructed and its parameters determined from the data. an attempt is made to arrive at a statistical prediction of the duration of the alternate regimes through the relative stability of the two stable states as deduced from mean values and higher moments of residence times. the behaviour predicted by the model is in good qualitative agreement with the record.

Observations of boundary-layer structure over complex terrain

Abstract: Turbulence data collected in an area of complex terrain using a number of instrument packages attached to the tether cable of a captive balloon are presented. Eddy correlation measurements of the turbulent momentum flux are used to determine the magnitude of the roughness length from the drag coefficient and also to show that the roughness length is a useful concept in parametrizing the surface stress in such areas. the relationship between the terrain characteristics and the roughness length is examined using a simple formula, results from a numerical model and previous experimental estimates of the roughness length for complex terrain. It is found that a formula based on numerical model results for separated flows gives reasonable predictions and that the measured roughness length is representative of an area of order 100 km2. Non-dimensionalized turbulence data are compared with similar data obtained over flat homogeneous surfaces. This comparison suggests that the structure of the turbulent boundary layer over complex terrain is remarkably similar to that over flat homogeneous surfaces. Because of the limitations of fixed point measurements in complex terrain, results from a numerical study of turbulent flow over two-dimensional sinusoidal orography are shown to indicate the magnitudes of the perturbations introduced into the boundary-layer flow by large undulations in the underlying surface. the model results demonstrate the validity of the analysis carried out with the data.

Frontal instability generated by tropospheric potential vorticity anomalies

Abstract: The development of secondary disturbances on a two-dimensional front is described using a linear stability analysis. A two-stage process is envisaged in which, first, strong frontogenesis produces condensation at the front. This leads to the generation of a lower tropospheric zone of high potential vorticity (PV) lying along the front. As frontogenesis weakens, the second stage is entered in which an essentially steady front is susceptible to the growth of linear normal modes along the front owing to the existence of the PV anomaly or, equivalently, the frontal rainband. Thus for a typical band of width 160 km a frontal wave of some 800 km is produced with a growth rate of about 1 day−1. the dynamics of these modes is examined by considering both the realistic frontal flow as well as idealized strips of PV anomalies. the idealized case is potentially applicable to a variety of meteorological situations such as upper jet streaks. Useful insight is gained from an analysis of the energetics of the modes. This shows that for narrow frontal PV anomalies the waves main energy source is the basic kinetic energy and further they have a negative vertical heat flux. Recent observations suggest that frontal waves are indeed characterized by such a heat flux. As most frontal zones involve significant condensation, and therefore are likely to exhibit a lower tropospheric PV maximum, it is stressed that studies of frontal waves need to include this aspect of the frontal structure.

A numerical study of tropical cyclone motion using a barotropic model. I: The role of vortex asymmetries

Abstract: The motion of an initially symmetric vortex on a beta plane and the motion of initially asymmetric vortices on a beta planes are studied using a nondivergent barotropic model It is assumed that there is no basic flow, the effects of a basic flow will be addressed in part II Parameter values such as vortex size and strength and computational domain size are chosen for their relevance to tropical cyclones the simultaneous evolution of the asymmetric vorticity and streamfunction fields is investigated in terms of a partitioning of the flow introduced by Kasahara and Platzman in which all asymmetries are regarded as part of the vortex environment It is shown that in this partitioning the asymmetric streamflow provides a 'steering current' for the vortex to a very close approximation For early times, typically 24 h for the parameters chosen, the development of the asymmetries in the vortices studied can be largely understood in terms of a simple analytical theory in which the vortex centre is fixed and the initial absolute vorticity distribution is rearranged by advection by the (initial) symmetric circulation The shearing effect of this mechanism alone would appear to preclude the establishment of a true steady state in the flows considered the numerical calculation for the initially symmetric vortex on a beta plane is used to assess averaging procedures for computing the environmental wind field of a tropical cyclone from observed wind data the results show that the averaging regions that have been commonly employed in observational studies are too large to properly characterize the environmental flow at the cyclone centre and they suggest caution in making inferences from these studies in relation to physical mechanisms such as beta drift The calculations for initially asymmetric vortices on an f plane show that the effects of the asymmetry on motion depend strongly on the scale of the asymmetry compared with that of the vortex When the scale of the asymmetry is relatively small, the asymmetry is rapidly degraded by tangential shear and its effect on motion is not sustained In contrast, larger asymmetries have a more persistent effect on motion When the motion takes place on a beta plane, asymmetries induced by the advection of planetary vorticity ultimately dominate those present initially and, irrespective of the scale of the initial asymmetry, the vortex track finally turns to the north-west as in the case of the initially symmetric vortex Finally, as a case of extreme asymmetry we study the mutual interaction of a strong vortex and a weak one the behaviour is similar to that just described, the weak vortex merging with the stronger one the analytical theory also proves useful in understanding the evolution of initial vortex asymmetries The interpretations of the calculations add insight into the dynamics of tropical cyclone motion, complementing the findings of recent studies by Chan and Williams and Fiorino and Elsberry

Large‐eddy simulation of the up‐slope boundary layer

Abstract: Large-eddy simulation (LES) is used to determine the turbulent structure of the steady-state up-slope boundary layer (UBL) which forms at a uniformly heated inclined or vertical plane surface below a stratified atmosphere at rest At proper scales, the problem depends solely on the slope angle a and the surface roughness height z 0 a relative to a length scale H which is a function of stratification and surface heating We cover 2° ≤ α ≤ 90° and 3 × 10 -5 ≤ z 0 /H ≤ 3 × 10 -2 by a series of LESs Simulations using different grids show that the results are only weakly sensitive to truncation errors the UBL approaches a steady-state mean-profile by slowly decaying oscillations; its frequency equals the Brunt-Vaisala frequency times sin α In the LES, convergence towards steady state has been enforced by adjusting the fields according to integral steady-state conditions For α ≤ 10°, a well-mixed layer is formed which causes a strong temperature inversion and strong down-slope flow at the outer edge of the UBL Large-scale coherence is weak with some indications of cross-slope rolls for small inclination angles and longitudinal rolls for steep slopes Several mean quantities have been tabulated as a function of slope angle and surface roughness and approximating power laws are given the dependence of mean quantities on the slope angle is difficult to explain with simple models, but the influence of surface roughness closely follows earlier results

Extended range predictions with ECMWF models : influence of horizontal resolution on systematic error and forecast skill

Abstract: The influence of horizontal resolution on model systematic error and skill scores are examined for both winter and summer seasons from a set of extended-range integrations of the European Centre for Medium Range Weather Forecasts (ECMWF) numerical weather prediction model. the forecast data are composed of 24 30-day integrations, two per month from successive 12 GMT operational analyses, from April 1985 to March 1986 at four (triangular truncation) horizontal spectral resolutions of T21, T42, T63 and T106. Systematic errors in wind, temperature, heat and momentum fluxes are discussed, both in the tropics and extratropics. Dynamical reasons for differences between various model resolutions and between simulations and observations are proposed. Diagnosis of model systematic error is aided by comparing zonal-mean diagnostics of the observed flow in the northern hemisphere winter and southern hemisphere winter. the impact of resolution on regional simulations of rainfall during the Indian and African monsoons is also discussed. In the extratropical troposphere, the behaviour of the T21 model is quite distinct from the higher-resolution models. the southern hemisphere flow is weak, and in both hemispheres horizontal momentum fluxes are severely underestimated. the systematic errors of the T42, T63 and T106 models are quite similar to each other in the extratropical troposphere. Errors at these higher resolutions resemble the observed difference (in zonal-mean wind, temperature and eddy fluxes) between diagnostics in the northern hemisphere winter and southern hemisphere winter, suggesting that orographic forcing in this version of the ECMWF model is inadequate, despite the inclusion of envelope orography. In the extratropical stratosphere, the behaviours of the T21 and T42 models resemble each other, both having a cold polar bias, apparently associated with inadequate dynamical heating and therefore relaxation to radiative equilibrium. This in turn is related to insufficient Rossby wave focusing into the polar vortex at T21 and T42. the T63 and T106 resolution integrations have much smaller stratospheric cold-pole biases. In the tropics, there are serious systematic errors at all resolutions; however, some of these mean errors increase with increasing resolution. These include the global divergent and nondivergent wind errors, and more regional simulations such as the monsoon flow, where simulation of diabatic heating associated with convective activity is crucial. Since the local values of moisture flux convergence at T106 are effectively much noisier than the larger-scale values of such fluxes used to drive the convection scheme at T21, it is speculated that diabatic heating fields are more likely to project onto the relevant tropical meteorological modes at low model resolution. On the other hand, it is shown that the ability to resolve local orographic features is important in accurately simulating local tropical precipitation maxima over land. Despite having smaller systematic errors in the northern hemisphere, 10-day mean T21 model anomaly correlation coefficient scores were worse than those of higher-resolution models, at least up to day 20. However, there was no indication that the higher-resolution models had significantly different extended-range skill characteristics. On the other hand, the increase of tropical systematic error with increasing horizontal resolution may indicate that the potential of higher-resolution models in extended-range prediction may be underestimated in this study. the asymptotic spread between forecasts initialized 24 hours apart indicated that all models substantially underestimated low-frequency variability (by up to 50% in the extended summer period). Bearing in mind the computational burden of longer timescale integrations with complex numerical models of the atmosphere, the evidence presented in this paper suggests that the T106 resolution may be unnecessarily high for both extended-range forecasting, and for climate simulation studies. However, at present, the cost of integrating at T63 resolution may also be excessive for many climate studies. It would appear from our analysis, that in the troposphere the behaviour of the T42 model is comparable with the higher-resolution models, and is a satisfactory compromise for many purposes, particularly bearing in mind the impact of resolution on tropical systematic errors. However, it would appear that the extreme sensitivity of the 1985/86 ECMWF model climate drift to resolution in the range between T21 and T42, and the apparent inability of the ECMWF T21 model to simulate the correct internal nonlinear dynamics of the extratropics, make questionable its use for climate studies at resolution lower than T42.

The influence of soil wetness distribution on short‐range rainfall forecasting in the West African Sahel

Abstract: Surface moisture supply is known to be a factor which can be important in triggering convection. This paper aims to investigate its impact on short-range numerical forecasts of tropical rainfall. The ECMWF (European Centre for Medium Range Weather Forecasts) operational forecasting model was integrated for 5 days with realistic positive and negative anomalies of the initial surface moisture distribution in the West African Sahel. the large-scale flow is found to be relatively unaffected, but significant differences occur in the rainfall forecasts for about the first 4 days. Rainfall is greatest with a moister surface, except on days 3 and 4 in the south Sahel/Savanna region, for which a decrease of rainfall occurs. the opposite result is seen when an initially drier surface is introduced. the mechanisms involved are investigated by carrying out a detailed hydrological budget, and by studying the evolution of the atmospheric profiles of moisture and temperature. We conclude that an improved surface moisture analysis is likely to result in some improvement of shortrange rainfall forecasts in the Sahel, and presumably also in other tropical regions. In the coming years it should be possible to achieve this using satellite-derived soil wetness maps.

The information content of data from satellite sounding systems: A simulation study

Abstract: The potential of the TIROS Operational Vertical Sounder (TOVS) and the Advanced TOYS (ATOVS) for providing information on the vertical profiles of atmospheric temperature and humidity is investigated. the theoretical performances of these systems are examined primarily in terms of their ability to contribute information to a numerical weather prediction (NWP) system. the covariance of retrieval error expected from an optimal nonlinear inversion scheme, which retrieves simultaneously all the atmospheric and surface variables which significantly affect the measured radiances, is calculated theoretically and compared with the covariance of error assumed in the prior information which constrains the inversion. Performance is assessed for different combinations of radiometers, corresponding to present and future operational systems, and for different sets of assumed conditions: in the atmospheric profile, the cloud conditions, the surface characteristics and the assumed prior knowledge of these variables. The potential performance of ATOVS is shown to be a considerable improvement on the present TOVS in many respects. the value of retaining a combined infrared and microwave system is confirmed. It is demonstrated that, although the ability of these sounding systems to improve on an NWP model's analyses of temperature and humidity is rather limited, the potential for improving on the analysis of layer quantities is much greater. Some new ideas are presented for extending current retrieval methods to accommodate the additional problems presented by ATOVS data. Methods are proposed for treating the effects of microwave surface emissivity and cloud liquid water in a simultaneous inversion.

Dynamical aspects of the October storm, 1987: a study of a successful fine-mesh simulation

Abstract: A diagnostic study of a successful numerical simulation of the October storm 1987 is described. This good, non-operational forecast was obtained using the Meteorological Office's fine-mesh model run from a revised analysis at 00 GMT on 15 October 1987. the Meteorological Office's mesoscale model was also run using initial conditions at 12 GMT 15 October obtained by interpolating the above fine-mesh forecast fields to examine the benefits of very high horizontal resolution (15km grid length). Diagnostic maps and cross-sections derived are used to address two scientific issues: (i) the role of weak moist slantwise stability (or, alternatively, positive slantwise convective potential energy) (ii) the dynamical significance of the striking cloud structure (the cloud head) seen in the satellite imagery early in the storm's life. It appears that the baroclinic zone, in which the storm develops, is highly receptive to forcing by an upper trough and the warm boundary-layer air just ahead of the main surface front is characterized by substantial slantwise convective available energy. the cloud head is identified with a region of intense slantwise ascent in the warm air just above the main frontal zone. Its great meridional extent is associated with the upper tropospheric outflow and horizontal spreading of air which has recently undergone rapid slantwise ascent. the low-level temperature field exhibits, throughout the development period, a remarkable shape-preserving thermal ridge of considerable intensity. the sensitivity of the forecast to the removal of surface energy fluxes and latent heat of condensation are investigated separately. It is found that on the timescale of the simulation, surface energy fluxes have negligible impact on the explosive development. On the other hand, the contribution of latent-heat release to the development appears to dominate the dry baroclinic instability process.

The temporal distribution of amazonian rainfall and its implications for forest interception

Abstract: A model for the temporal distribution of rainfall is derived from measurements made in central Amazonian rainforest. A contrast is made with a rainfall distribution model which has been used in a stand-alone comparison of land-surface sub-models of two General Circulation Models (GCMs). In terms of the rainfall interception loss, it is shown that the central Amazonian model produces an overestimate of 35%, but is within the error bars of the measured interception loss, whereas the latter model overestimates the interception loss by more than 100%. Two other time distributions of rainfall from contrasting regions of the Amazon are derived from published data in order to investigate the possible interception variation due to rainfall pattern variability. the results illustrate the need for GCM modellers to monitor their synthesized rainfall regime against real data and to be aware of the sensitivity of the forest interception loss to rainfall distribution.

The development of narrow baroclinic zones and other small‐scale structure in the stratosphere during simulated major warmings

Abstract: Numerical simulations of stratospheric major warmings with a multi-level, primitive equation model of the stratosphere and mesosphere reveal several examples of processes giving rise to small-scale structure in the simulated flow. For a simulation of the warming of 1984/85, the following examples are cited: (1) the maintenance of the Aleutian high by injections of air with low values of potential vorticity; (2) the erosion of the main cyclonic vortex as it is stripped of air with high values of potential vorticity; (3) the merger of a pair of anticyclones at the height of the warming; (4) differential advection of potential vorticity in a vertically sheared flow; and (5) the generation in the stratosphere of narrow baroclinic zones with strong temperature gradients in the horizontal and in the vertical. For a simulation of the warming of 1979, the example given is of the in situ generation in the stratosphere of inertio-gravity waves near a developing baroclinic zone. The dynamics involved in the formation of narrow baroclinic zones in the simulated stratosphere is studied in some detail. It is shown that the ageostrophic flow (mainly the vertical motion) provides a positive feedback on the frontogenetic effect of the geostrophic (large-scale) flow. Similarities and differences with frontogenesis at the ground and in the upper troposphere are enumerated.

Variational inversion of simulated TOVS radiances using the adjoint technique

Abstract: The adjoint technique allows the variational approach for assimilating various types of observations in meteorology, including raw radiances measured by satellite, to be treated exactly and at a ‘reasonable’ cost. We have developed the tangent linear and adjoint operators of the International TOVS Processing Package radiative transfer equation. These two tools are used for treating two aspects of variational inversion of satellite sounding radiances. First of all, we apply this technique to the computation of the error covariance matrix of the retrieval/analysis of simulated TOVS sounding radiances. Using this matrix, an original method to interpret satellite sounding radiances in terms of temperature and humidity information is then presented. We can thus evaluate the number of independent parameters which are significantly estimated from radiances, given a direct radiative transfer model, observation and background error statistics. It is found that 19 HIRS channels provide six independent pieces of information while four MSU channels provide three independent pieces of information. When grouping HIRS and MSU channels, only seven independent pieces of information are available. It is shown that in cloudy conditions MSU channels bring additional significant information to HIRS channels especially at the surface. We also study the impact of cloudiness on the quality of the retrieved profiles. Numerical feasibility of 1-D variational inversion using the adjoint of the International TOVS Processing Package radiative transfer model is demonstrated. Impact of the choice of the inner product on the efficiency of the method is studied. We then discuss the quality of the inversion, which turns out to be strongly related to the quality of the atmospheric information present in the sounding radiances and to the background and observation error statistics. the gain brought by MSU channels at the surface in cloudy conditions is discussed.

Assessment of the practical utility of extended range ensemble forecasts

Abstract: Lagged-average forecast experiments, produced with the Meteorological Office 11-level general circulation model, are used to assess the benefits arising from the use of ensemble forecasts in practical monthly prediction. As measured by anomaly correlation, forecast skill is consistently positive at days 6-15, but very small on average beyond 20 days. For the monthly mean forecast, removal of an independent estimate of the model systematic error improves the average score significantly. Compared to the mean score amongst individual members, a modest increase in skill is achieved by ensemble averaging. Theoretically, however, there remains scope for a much greater impact in the future, through the use of models capable of more accurate forecasts. Local forecast skill is correlated with a measure of the agreement between ensemble members, although the overall relationship is weak beyond the medium range. Nevertheless, for areas of high agreement, the probability of obtaining useful skill is considerably higher than average at days 6-15 and 11-20. The experiments are also assessed as probability forecasts, using the Ranked Probability Score to measure skill. the ensemble probability forecast (EPF) is created by averaging the probability forecasts of the individual members. Relative to the average score for the latter, the EPF gives improved skill. For the medium-range period of the forecast (days 1-10), the use of an optimized EPF, calculated by weighting each ensemble member according to its age, leads to a small increase in skill compared to the individual forecast from the most recent analysis. This effect is not observed in the corresponding anomaly correlation results. the above results for local skill prediction are confirmed, and somewhat enhanced, in the probabilistic formulation, using the ensemble variance of forecast categories as the predictor. A simple method of issuing confidence-based adjustments to the forecast probabilities is discussed.

On the generation and propagation of the southern African coastal low

Abstract: The time-height variability of the lower marine atmosphere during the eastwards propagation of coastal lows around the coast of southern Africa is presented. These coastal lows are shallow, mesoscale disturbances that are trapped vertically by a strong subsidence inversion and horizontally, through Coriolis effects, against the steep escarpment that fringes the subcontinent. Prior to the event, warm offshore flow descending off this escarpment at about the inversion level provides an input of buoyancy which, together with the cyclonic vorticity acquired during the descent, acts to generate a coastal low rather than a high. Analysis of the data shows that coastal lows are characterized by a marked lowering of the inversion level, a switch in the direction of the winds below the inversion and a drop in the near-surface temperature, which are all consistent with the hypothesis that the coastal low is an internal Kelvin wave propagating in the marine layer. Good agreement between the observed propagation speed of the coastal low and the theoretical phase speed of such a wave reinforces this hypothesis. A model of the vertical buoyancy advection into the marine atmosphere that results from the warm offshore flow is presented. It is illustrated that an internal Kelvin wave, in the form of a coastal low, results from this buoyancy advection. Comparisons are made between coastal-low dynamics and those characteristics of other mesoscale, coastally-trapped disturbances which propagate in the marine layers of California and south-eastern Australia.

A test of a semi-implicit integration technique for a fully compressible non-hydrostatic model

Abstract: A semi-implicit scheme, in which the time-step is limited only by the advective criterion, is analysed and successfully tested in a fully compressible non-hydrostatic mesoscale model.

Extended-range predictions with ecmwf models: Interannual variability in operational model integrations

Abstract: A set of 30-day integrations made with the operational ECMWF model over a period of three and a half years (from 1985 to 1988) is studied. the impact of model reformulations during this period on the climate drift of the model is assessed, and the level of extended-range forecast skill achieved by the operational model is studied. Diagnostics of zonal average 30-day mean wind and temperature error show a systematic reduction in the extratropics over three years. It is argued that these improvements are consistent with the changes in model formulation. Eddy fluxes of heat and momentum, and levels of eddy kinetic energy are similarly improved. We also show that the ability to simulate blocking activity in the extended range has improved substantially over the three winters. From maps showing the ratio of systematic to total error it is concluded that, by the final winter, the middle-latitude extended-range error is almost entirely associated with random errors. This is supported by the growth of the dispersion, or spread, between forecasts initialized 24 hours apart. In particular, for the extended winter 1987/88, it is shown that the asymptotic level of (internal) spread is comparable with the asymptotic level of (external) skill. By contrast, in the tropics, the total error is dominated by the systematic error for all years. Despite improvements to the model, extended-range forecast skill is modest. Skill has been measured in a spatially-filtered three-dimensional phase space, spanned by rotated EOF (empirical orthogonal function) coefficients of 500 mb height, which contain the principal weather regimes in the atmosphere. On average the skill scores are consistent with those of other major centres studying dynamical extended-range prediction. In particular there is evidence of skill to day 15 and possibly to day 20 in middle latitudes. Beyond day 20 skill is marginal. We demonstrate that in the winter sample of operational-model forecasts, skill is correlated with the observed value of the first rotated EOF, which can be described as a global version of the Pacific/North American mode. By contrast, the ability to predict monthly mean fields in the tropics depends primarily on a knowledge of the boundary forcing, including sea surface temperature. It is shown that some aspects of the interannual variations in the summer monsoon circulation can be predicted, in particular the tropical easterly jet anomalies, African rainfall, and, to a much lesser extent, Indian monsoon rainfall.

Some characteristics of thunderstorms over Bathurst and Melville Islands near Darwin, Australia

Abstract: A study defining the occurrence, structure and evolution of “maritime continent” island thunderstorms over Bathurst and Melville Islands near Darwin, Australia is presented. Satellite data for the November-December pre-monsoon transition season show unambiguous records of island thunderstorms on at least 65% of days over the period 1978-1986. Storms develop most commonly between 1230LST and 1530LST and radar data indicate a mean lifetime of 2.5 h. Non-occurrence was found on only 7% of the days examined and was associated with a return to dry flow typical of the early transition season with middle level subsidence or suppressed conditions in the wake of previous convective activity. Environmental sounding data indicated that these “maritime continent” thunderstorms develop in a regime with moderate CAPE, low to moderate shear and high moisture. the dynamic regime is characteristic of multicellular storms. Examination of the evolution of the storms indicates the existence of a typical lifecycle but with considerable variability in detail. Initially, the lifecycle is consistent with the multicellular storm concept, but the developing storms tend to aggregate to form one or more mesoscale systems which in turn may develop into propagating squall lines in the mature stage. the developing storms have preferred geographic regions of genesis and intensification over the islands; and while motion of the storms is variable, it is generally consistent with the mean surface-to-700mb pressure-weighted flow.

Field measurements of wet deposition in an extended region of complex topography

Abstract: A model designed to predict the enhancement of rainfall and the wet deposition of aerosol in complex terrain has been extended and compared with field data. the results of five separate field studies performed over an extensive region of complex terrain in northern England are presented and compared with the model predictions. It is found that the main features of the data are predicted by the model in widely varying conditions but the full details of a particular case-study cannot be reproduced quantitatively owing to the complexities of the meteorology over the extended distances and collection periods. The main physical processes controlling the deposition pattern have been established and it is concluded that a simple model of the seeder-feeder process is able to give reasonable descriptions of deposition patterns in complex terrain both for individual episodes and for longer-period averages.