Showing papers in "Quarterly Journal of the Royal Meteorological Society in 1993"

Two paradigms of baroclinic‐wave life‐cycle behaviour

Abstract: Two idealized baroclinic wave-6 life cycles examined here suggest a framework of opposite extremes (a) in which to view the behaviour of real synoptic-scale disturbances in middle latitudes, and (b) in which to interrelate the synoptic and wave-theoretic viewpoints, using the ‘saturation-propagation-saturation’ (SPS) picture of wave-6 life-cycle behaviour. The two life cycles. dented by LC1 and LC2, are higher-resolution versions of the Simmons-Hoskins ‘basic’ and ‘anomalous’ cases (showing strong and weak late decay of eddy kinetic energy, EKE). They illustrate, in varying degrees. Two extreme types of behaviour here disignated ‘anticyclonic’ and ‘cyclonic’, and epitomized by strongly contrasting upper-air trough behaviour. ‘Anticyclonic’ behaviour dominates the late stages of LC1 and is characterized by backward-tilted, thinning troughs being advected anticyclonically and equatorward, as in the commoner cases of planetary-scale mid-stratospheric ‘Rossby-wave breaking’. ‘Cyclonic’ behaviour dominates LC2 and is characterized by forward-tilted, broadening troughs wrapping themselves up cyclonically and poleward. producing major cut-off cyclones in high latitudes. These morphologies are visualized by upper-air maps of potential temperature on the nominal tropopause, defined as a constant-potential-vorticity surface. Some atmospheric mid-latitude disturbances examined here, using the same visualization applied to operational analyses, show the same two extreme types of trough behaviour together with intermediate cases. The SPS picture is re-examined, using Eliassen-palm and refractive-index cross-sections. It is shown, in particular, by reference to a wave-activity theorem of Haynes, that the late stages of LC2 can be looked upon as a remarkably clear, and morphologically novel, large-amplitude counterpart of the nonlinear reflection scenario of Rossby-wave critical-layer theory. The late stages of LC1, by constrast, look more akin to an nonlinear critical-layer absorption scenario. LC2 exhibits region of largely undular PV contours adjacent to a nonlinear of irreversiby deformen PV contous. In the latter region PV rearrangement, and hence absorption of Rossby-wave activity, has largely ceased. This accounts for the more persistent EKEE in the LC2 case.

Predictability and finite-time instability of the northern winter circulation

Abstract: The finite-time instability and associated predictability of atmospheric of atmospheric circulations are defined in terms of the largest singular values, and associated singular vectors, of the linear evolution operator determined form given equations of motion. These quantities are calculated in both a barotropic and a three-level quasi-geostrophic model, using as basic states realistic large-scale northern wintertime flows that represent the climatological state, regime composites, and specific realizations of these regimes. for time-invariant basic states, the singular vectors are compared with the corresponding normal-mode solutions; it is shown that the perturbations defined (at the initial time) by the singular vectors have much larger growth rates than the normal modes, and possess a more localized spatial structure. The regimes studied have opposite values of the Pacific/North American (PNA) index, and growth rates for the barotropic basis states appear to confirm earlier studies that the barotropic instability of the negative PNA states may be larger than the corresponding positive PNA states. The evolution of the singular-vector perturbations, with emphasis on the vertical structure, is compared for time-evolving and time-invariant baroclinic basis states; the effects of nonlinearity are also discusses. It is shown that, in the baroclinic model, interactions between synoptic-scale eddies in the time-evolving basic state and in the perturbation field are fundamental for studying the predictability of transitions in the large-scale circulation. Consequently results obtained from linear calculations using very smooth basic states cannot properly account for such predictability. These results form the basis of a technique used to initialize ensembles of forecasts made with a primitive-equation model, and are described in the companion paper (Mureau et al. 1993).

Assimilation of TOVS radiance information through one-dimensional variational analysis

Abstract: In recent years difficulties have been experienced in exploiting satellite sounding data in numerical weather prediction (NWP) in the form of independently retrieved temperature and humidity profiles. Attention has now focused on methods through which the information in the radiance measurements may be assimilated more directly into the NWP system%. A scheme known as ‘one-dimensional variational analysis’ (1DVAR) has been developed at the European Centre for Medium-range Weather Forecasts as a method for extracting information from TIROS Operational Vertical Sounder radiances for use in the operational data-assimilation system. The 1DVAR scheme is based on variational principles applied to the analysis of the atmospheric profile at a single location, using a forecast profile and its error covariance as constraints. The details of the scheme are presented. Errors in 1DVAR products are correlated with those of the short-range forecast which serves as a background for the subsequent three-dimensional analysis. Methods for addressing this aspect of the assimilation problem are discussed. The characteristics of 1DVAR products and their impact on the analysis are described. A series of forecast impact experiments has been conducted and has demonstrated consistent positive impacts on forecast skill in the northern hemisphere.

Marine aerosol concentrations and estimated fluxes over the sea

Abstract: Aerosol particulate fluxes, generated at the sea surface by the action of the wind, have been derived from a comprehensive series of measurements taken in the Outer Hebrides, off the west coast of Scotland. These observations cover wind speeds from essentially zero to values in excess of 30 m s−1, and show that the particle production, over the radius range from 1 to 25 μm, is very strongly dependent upon the prevailing wind speed, especially for the larger particles. These fluxes, estimated for a height of about 10 m above the ocean surface, may be approximated by an expression comprising the sum of two log-normal distributions (in particle radius R) of the form: where the values of fI and R0i remain essentially unchanged with wind speed, but with the values of Ai being strongly dependent upon this parameter. Alternative formulations of the aerosol flux, based upon combining field observations of the dependence of whitecap coverage upon prevailing wind speed with estimates of aerosol production from laboratory-simulated whitecaps, provide poorer approximations to the aerosol observations, especially at the higher wind speeds when a substantial spume-drop contribution may be expected.

The Pressure force induced by neutral, turbulent flow over hills

Abstract: The pressure force induced by neutral, turbulent flow over small-scale hills is considered. The influence that the level of turbulence closure has on liner, analytical expressions for the force is evaluated. The two-dimensional linear expressions are then extended to three dimensions and a heuristic extension to moderate and steep slopes, for which nonlinear effects are important, is proposed. The predictions of these various expressions are compared with results from a three-dimensional numerical model employing a 1 1/2-order turbulence-closure model, and very good agreement is found. The numerical-model results supports the idea that, sufficiently far above the hills, the areally averaged velocity profile varies approximately logarithmically with height but with an enhanced, or effective, roughness length and a friction velocity equal to the square root of the total surface force per unit area and unit density. Using the theoretical results for the pressure force, discussed above, and approximate expression for the effective roughness length is obtained that agrees well with values from the model, and also matches with existing expressions that are valid for two-dimensional hills with either very low or very large slopes. Qualitative agreement is also found with the limited number of observed values of roughness length in hilly terrain. These results suggest that the effective roughness length is an accurate and convenient method of parametrizing the drag due to subgrid-scale orography in large-scale numerical-weather-prediction and climate models.

A GCM simulation of the impact of Amazonian deforestation on climate using an improved canopy representation

Abstract: To obtain a estimate of the impcat of Amazonian deforestation on local climate it is critical that the representation of the forest canopy within general circulation models (GCMs) is as realistic as possible. Recent measurements from the Amazonian forest have highlighted major weaknesses in the Meteorological Office GCM simulation of the interception of rainfall from the forest canopy. Here, we present results for Amazonia from a new 3-year control experiment which incorporates an improved representation of micrometeorological processes within the forest. A detailed assessment of the control simulation reveals that the adjusted GCM provides a realistic description of the climate of Amazonia. In determining the impact of Amazonian deforestation on climate we present a comprehensive analysis of the simulated climate following the replacement of Amazonian forest by pasture%. A comparison of the new results with those from the earlier deforestation experiment carried out by Lean and Warrilow (1989) suggests that the reductions in local rainfall (14%) and evaporation (24%) are smaller than those obtained with the previous formulation of interception. It was concluded by Lean and Warrilow that with a wet canopy, decreases in roughness in the deforested case reduce evaporation. With the introduction of the new interception formulation the canopy is less often wet, and so the effect of deforestation is reduced.

The exchange of atmospheric ammonia with vegetated surfaces. II: Fertilized vegetation

Abstract: Measurements of the exchange of gaseous ammonia (NH3) have been made over fertilized plant canopies using micrometeorological techniques. The NH3 exchange over ungrazed grassland and arable crops was examined in a range of environmental conditions: a bidirectional exchange of ammonia was observed, with warm dry conditions favouring emission (NH3 fluxes up to 24 ng m−2s−1) and cool wet conditions favouring deposition (fluxes in the range 1–35 ng m−2s−1). Limited measurements made when the ground was frozen suggest an increased surface resistance may occur in these conditions (Measured Rc up to 80 s m−1), resulting in reduced rates of deposition. The fluxes are contrasted with measurements over unfertilized vegetation, and it is suggested that the net canopy exchange is the combined result of leaf surface, stomatal and soil exchange processes; the increased tendency of fertilized vegetation to emit ammonia is attributed to the larger fixed nitrogen concentrations in these ecosystems. The bidirectional fluxes may be related to the existence of ammonia compensation points. Estimates of the stomatal compensation point for fertilized croplands were in the range 1-7 μg NH3m−3, though because of deposition to leaf surfaces, this frequently does not control net fluxes. The Mean surface concentration of ammonia X(z'o), or canopy compensation point, was therefore also estimated and found to vary, with environmental conditions, in the range 0-3 μg m−3. A simplified procedure is used to estimate the net annual exchange of ammonia over an example fertilized crop in the British Isles. This predicts an emission of fixed nitrogen of less than 1 kg per hectare, which is the net result of emission and deposition throughout the year.

The potential for stratosphere‐troposphere exchange in cut‐off‐low systems

Abstract: Three mechanisms are discussed associated with cut-off-low (COL) systems which can transfer stratospheric air into the troposphere. These mechanisms are: (i) convective erosion of the tropopause, (ii) erosion of the tropopause by turbulence associated with a jet stream, and (iii) tropopause folding around the flank of a COL. Three case studies are presented which examine the first two of these processes and provide evidence that stratosphere-troposphere exchange (STE) had occurred. Furthermore, results suggest that periodic reintensification of some COLs cause all these processes to recur or be prolonged. Their relative importance to STE is discussed and an estimate for that occurring by mechanism (i) is given for one of the case studies. Statistical data on annual COL numbers are used to calculate and annual mean transfer of ozone by this mechanism, which is found to be no more than 4% of the annual transfer for the northern hemisphere. This result suggests that convective STE in COLs is a minor contributor to the northern hemisphere STE.

Ensemble prediction using dynamically conditioned perturbations

Abstract: We apply the technique developed in the companion paper by Molteni and Palmer (1993) as a means of providing dynamically conditioned perturbations for ensemble forecasting with a primitive-equation model. Four wintertime initial states are chosen—three at random and one because of substantial development in the large-scale flow within four days, which the control forecast completely missed. A set of singular vectors are created using a quasi-geostrophic model linearized about basic states taken from data close to the chosen initial dates. These are interpolated onto the primitive-equation-model grid, and used as perturbations to the initial state. An ensemble forecast is made form the perturbed initial states. The dispersion of this ensemble is compared, for each date, with that from a second ensemble with initial perturbations constructed from 6-hour-forecast errors. Throughout the forecast period, it is found that the amplitude of the perturbations is noticeably larger using the singular vectors. The dispersion of the ensembles using the forecast-error perturbations did not indicate that the control forecast form the case with substantial development was likely to be poor. By contrast, the (envelope) dispersion of the ensemble using the singular vectors was notably larger for this case than the other three. A number of members of this ensemble were particularly skilful in predicting weather-related elements of the flow, such as low-level temperature change. It is found that the evolution of perturbations which are initially localized over the western Pacific, or western Atlantic, can develop blocking-like structures several days later over the eastern oceans. With the initial amplitudes used in this paper, the development of these structures is in part nonlinear. The growth of the singular-vector perturbations was not as large in the primitive-model as in the quasi-geostrophic model, probably due to interpolation problems exacerbated by inconsistent orographic representation. Further work to overcome these, and other, problems is indicated.

An Integrated View of the 1987 Australian Monsoon and Its Mesoscale Convective Systems. I: Horizontal Structure

Abstract: The vertical structure of monsoon thermal forcing by precipitating convection is diagnosed in terms of horizontal divergence. Airborne Doppler-radar divergence profiles from nine diverse mesoscale convective systems (MCSs) are presented. The MCSs consisted of multicellular convective elements which in time gave rise to areas of stratiform precipitation. Each of the three basic building blocks of the MCSs - convective, intermediary, and stratiform precipitation areas - has a consistent, characteristic divergence profile. Convective areas have low-level convergence, with its peak at 2-4 km altitude, and divergence above 6 km. Intermediary areas have convergence aloft, peaked near 10 km, feeding into mean ascent high in the upper troposphere. Stratiform areas have mid-level convergence, indicating a mesoscale downdraught below the melting level, and a mesoscale updraught aloft. Rawinsonde composite divergence profiles agree with the Doppler data in at least one important respect: the lower-tropospheric convergence into the MCSs peaks 2-4-km above the surface. Rawinsonde vorticity profiles show that monsoonal tropical cyclones spin-up at these elevated levels first, then later descend to the surface. Rawinsonde observations on a larger, continental scale demonstrate that at large horizontal scales only the 'gravest vertical mode' of MCS heating is felt, while the effects of shallower components of the heating (or divergence) profiles are trapped near the heating, as predicted by geostrophic adjustment theory.

On the interaction of tropical‐cyclone‐scale vortices. I: Observations

Abstract: A detailed analysis is made of the observed behaviour in interaction, tropical-cyclone-scale vortices in the western North Pacific region. It is found that all multiple-vortex interactions can be broken down into a common model of binary interaction. The classical Fujiwhara model of converging cyclonic rotation about a centroid followed by merger is rarely followed. Capture tends to occur rapidly, within several hours, and is followed by a period of relatively stable cyclonic orbit. Cyclone merger occurs by one vortex experiencing a loss of convective organization, followed by horizontal shearing and incorporation into the outer circulation of the other vortex. However, a substantial proportion of interacting cyclones escape from the interaction, and the changeover from a stable orbiting configuration occurs rapidly. A model of binary interaction is presented. Cases where swarms of mesoscale vortices are formed in convectively active regions without tropical cyclones are also documented. These vortices have a much narrower range of influence (a few hundred kilometres) then that observed for tropical cyclones. When groups of vortices come within this range they are observed to conform to the same interaction model as observed for tropical cyclones.

Comparison between the land surface response of the ECMWF model and the FIFE‐1987 data

Abstract: An averaged time series for the surface data for the 15 x 15 km FIFE site was prepared for the summer of 1987. Comparisons with 48-hr forecasts from the ECMWF model for extended periods in July, August, and October 1987 identified model errors in the incoming SW radiation in clear skies, the ground heat flux, the formulation of surface evaporation, the soil-moisture model, and the entrainment at boundary-layer top. The model clear-sky SW flux is too high at the surface by 5-10 percent. The ground heat flux is too large by a factor of 2 to 3 because of the large thermal capacity of the first soil layer (which is 7 cm thick), and a time truncation error. The surface evaporation was near zero in October 1987, rather than of order 70 W/sq m at noon. The surface evaporation falls too rapidly after rainfall, with a time-scale of a few days rather than the 7-10 d (or more) of the observations. On time-scales of more than a few days the specified 'climate layer' soil moisture, rather than the storage of precipitation, has a large control on the evapotranspiration. The boundary-layer-top entrainment is too low. This results in a moist bias in the boundary-layer mixing ratio of order 2 g/Kg in forecasts from an experimental analysis with nearly realistic surface fluxes; this because there is insufficient downward mixing of dry air.

Bayesian quality control using multivariate normal distributions

Abstract: An expression for the probability density of any distribution of observed values (given background values of known accuracy) is derived from the properties of multivariate normal distributions. This is used in the quality control of observations—‘good’ and ‘bad’ observations are assumed to have errors from a normal distribution and from a distribution giving no useful information respectively. Three Methods of quality control are presented and compared; two of these are based on the probability density derived above, and the third is based on a related maximum probability analysis. They differ in the optimality principal used: Individual Quality Control finds the most likely quality (i.e. good or bad) for each observation, given information from all the others; Simultaneous quality Control finds the most likely combination of qualities; while Variational Quality Control is based on a variational analysis which finds the most likely true values. The quality control should be considered as part of the ‘analysis’ process of using the observations; these approaches to quality control are considered as approximations to a system giving the ‘best’ analysis, based on minimizing a Bayesian loss function. Approximation are also necessary in their practical implementations; the effect of these on various operational schemes is discussed. The multi-observation framework used includes the ‘background’ check as a special case, and it is extended to deal with observations with common sources of gross error. Applications to multi-level checks, bias checks and checks for known error patterns are sketched. As a by-product the standard statistical interpolation formulae are derived from the properties of normal distributions, thus demonstrating the implicit dependence of statistical interpolation on the normal distribution.

Turbulent transfer relationships over an urban surface. I. Spectral characteristics

Abstract: SUMMARY A comprehensive study of atmospheric turbulence over an urban surface has been carried out. In this Part I of the study the spectral characteristics are analysed. Results involving all the important atmospheric variables, measured at two heights (z’/zg = 21 and 37) under unstable conditions, are presented, normalized by the respective variances/covariances, and within the Monin-Obukhov similarity framework. Although the shapes and location of the peaks of the urban spectra/cospectra are in good agreement with homogeneous surface layer data, the analysis of the normalized dissipation rates and spectral correlation coefficients reveals differences which can be attributed to the rough surface. For example, the dissipation of turbulent kinetic energy is relatively small, perhaps owing to an increase in the transport and export of locally produced turbulent energy; there is a very efficient transfer of momentum which it is suggested may be a result of wake production associated with bluff bodies, and it is found that the transfer of heat and moisture are dissimilar.

On the interaction of tropical-cyclone-scale vortices. II: Discrete vortex patches

Abstract: The interaction of vortex patches in relation to the observed scales and features reported in Part I of this paper is investigated. It is found that the initial approach of compound vortices, such as tropical cyclones, arises from distortion of their, weaker, outer vorticity fields. Care needs to be taken in applying simple concepts, such as the propagation of each vortex on the gradient of its neighbour. For similar vortices, the initial interaction consists of mutual orbit and approach with only minor fluctuations in shape. Once the vortices approach to within a critical separation distance, rapid merger occurs. A simplified expression for this critical distance is derived and it indicates that binary tropical cyclones must approach to within 150-300 km before merger of the cores can occur. When vortices have substantial differences in size or intensity, the smaller/weaker system shears into the outer circulation of the other and no core merger occurs. It is suggested that this is typical of the interaction between tropical cyclones and both the monsoon trough and embedded mesoscale convective complexes. The motion of interacting vortices consists largely of a mutual orbit, which scales according to their relative sizes and intensities. An important finding, however, is that higher-frequency meanders may develop. This indicates that a tropical cyclone interacting with a monsoonal shear zone may develop internal asymmetries and a trochoidal oscillations as a direct result of such interaction. Further, weak mesoscale vortices within the cyclone circulation may continue to cause a track meander long after they have been sheared beyond the resolving capacity of atmospheric observations.

Effective resistance to sensible‐ and latent‐heat flux in heterogeneous terrain

Abstract: Numerical-weather-prediction models resolve mean variable at the scale of their grid length only. Effective surface parameters must, therefore, be used to relate the mean properties of humidity, temperature and wind speed to the mean surface fluxes. The effective parameters can be calculated using a model with a finer grid. In this paper effective aerodynamic and surface resistances to sensible-and latent-heat flux for a surface which varies over a horizontal scale of one kilometre are calculated from a two-dimensional model with a grid size of 30m, and compared with simpler averaging techniques. The averaging techniques considered include taking the mean of the aerodynamic and surface resistance in parallel or in series, the mean of the solutions given by these two, and a heuristic model that treats the different surface covers independently but assumes that they are exposed to the same mean meteorological variables at a blending height. The heuristic model consistently estimates the effective resistances to within 25% but, in comparison to the other methods, requires more information. For heterogeneous surfaces, the effective parameters for momentum and sensible- and latent-heat transfer do not bear the same relationship to each other as they do for homogenous surfaces. Effective parameters for each type of flux, therefore, have to be calculated separately. In situations where negative sensible-heat fluxes occur over rough ground, the effective resistance to sensible-heat flux can be very large, becoming underfined in the extreme case where the area-average sensible-heat flux is zero.

Cyclone behaviour response to changes in winter southern hemisphere sea‐ice concentration

Abstract: The sensitivity of climatological cyclone behaviour in the southern hemisphere to reductions in winter antarctic sea-ice concentration is studies in general-circulation-model (GMC) experiments. An increase in the number of cyclones over most of the Weddell an Ross Seas is a response common to all cases of reduction of sea-ice concentration from 100%. There is a tendency, particularly at lower ice concentrations, for more cyclones to be found in the latitude band of about 10 to 15° of latitude width centered on the antarctic coast, and a belt of fewer cyclones to the north. The structure of the changes in cyclogenesis assumes a much lesser zonally oriented form, and all experiments induce enhanced cyclogenesis in the western Weddell Sea. For the most part thee is an increase in cyclogenesis south of about 65°S, although some longitudinal sectors show up as regions of decreased cyclogenesis even through the sea-ice concentration has been reduced there. The intensity of the extratropical cyclones has been quantified as the difference between the Mean depth of the systems and the climatological average pressure for each particular sea-ice configuration. Perhaps contrary to what might have been expected, this ‘relative’ central pressure of lows exhibits an increase over most of the sea-ice area (i.e. the cyclones become weaker in a relative sense). In an earlier study it was found that the climatological response to imposed anomalies of sea-ice concentration was that the lowering of the concentration of antarctic sea ice resulted in a lowering of sea-level pressure, particularly close to the continent. The results presented here strongly suggest that the climatological decreases in pressure result essentially from increases in cyclone numbers as opposed to the possibility that the cyclones are more intense. We have attempted to relate the regions of cyclogenesis with a measure of baroclinicity in the control simulation. We have also examined the relationship between changes in these parameters in response to anomalous sea-ice concentration. Despite some similarity between theses, our study emphasizes the important role being played by other factors, including surface heat fluxes and topography, in these experiments. This study demonstrates the utility of GCMs in the study of high-latitude cyclone sensitivity and suggests it is a valuable tool to be used in tandem with observational-based analyses.

Development of ice and precipitation in New Mexican summertime cumulus clouds

Abstract: An experiment, involving the National Center for Atmospheric Research's Kin Air aeroplane, was conducted in order to measure the microphysical properties of New Mexican summertime cumulus clouds. Since the clouds formed and developed essentially in place, over the mountains, it was possible to make multiple penetrations through a single cloud, therby observing a significant fraction of the cloud's life cycle. In this paper, the questions of primary- and secondary-ice production, and the development of precipitation particles, are addressed. Primary-ice nucleation was found to occur when the temperature within the cloud reached a value of between −10 and −12°C irrespective of whether this was in the updraught or downdraught. Drops with diameters of about 0.5 mm were often observed in concentrations of about 10L−1 before the formation of ice. which suggests a nucleation mechanism involving large drops. The maximum concentrations of ice particles observed in these clouds (up to about 1300L−1) are much greater than typical concentrations of ice particles that can be attributed to primary-ice nucleation. Evidence suggests that the most likely explanation is the Hallett-Mossop process of secondary-ice-crystal production. Ice particles generally were first observed in the downdraughts. The development of precipitation is often thought to occur via downdraught transport. followed by sedimentation or mixing of ice particles into fresh. liquid-laden turrets. The multi-thermal nature of the cloud is considered to be central to this process.

Turbulent transfer relationships over an urban surface. II: Integral statistics

Abstract: A comprehensive study of atmospheric turbulence over an urban surface has been carried out. Part II presents the normalized standard deviations of all important atmospheric variables from unstable conditions Measured at z'/z0=37 and analysed within the Monin-Obukhov similarity framework. They differ from the rural reference data in the following ways. (1)The normalized standard deviation of vertical velocity is systematically smaller. (2)The efficiency of urban transfer of heat and especially momentum are larger. (3)The humidity statistics cannot be described within the similarity framework. It is suggested that the roughness sublayer effects and large-scale space and time inhomogeneities of the motions in the suburban atmosphere are responsible for the observed dissimilarities.

Atmospheric water-vapour transport to Antarctica inferred from radiosonde data

Abstract: Data from 16 radiosonde stations are used to study the transport of water vapour in the antarctic atmosphere. Total column moisture (TCM), winds and moisture flux measurements are presented, together with an analysis of their reliability. Annual TCM values are similar—about 4 kg m−2—at all stations around the coast of East Antarctica but are much smaller on the East Antarctic Plateau. Over a period of 6 years the interannual variation (standard deviation) of the TCM is about 10% of the mean value. At the coastal stations moisture fluxes reflect the predominantly zonal easterly flow in the lower troposphere, and their meridional components are generally small. As a result of interannual variations in the strength of the atmospheric circulation, and to a lesser extent in the TCM, interannual variability of the fluxes is high, suggesting that there may be large interannual variability in the precipitation over Antarctica. From the data a water-vapour budged for East Antarctica is computed. The annual accumulation rate obtained agrees surprisingly well with glaciological estimates. However, the uncertainties are considerable as a result of measurement errors and the representativity of the stations available. The conclusion is that the data are more suitable for evaluating the regional performance of circulation models from which systematic budget estimates may be derived.

Entropy and climate. I - ERBE observations of the entropy production of the earth

Abstract: An approximate method for estimating the global distributions of the entropy fluxes flowing through the upper boundary of the climate system is introduced, and an estimate of the entropy exchange between the earth and space and the entropy production of the planet is provided. Entropy fluxes calculated from the Earth Radiation Budget Experiment measurements show how the long-wave entropy flux densities dominate the total entropy fluxes at all latitudes compared with the entropy flux densities associated with reflected sunlight, although the short-wave flux densities are important in the context of clear sky-cloudy sky net entropy flux differences. It is suggested that the entropy production of the planet is both constant for the 36 months of data considered and very near its maximum possible value. The mean value of this production is 0.68 x 10 exp 15 W/K, and the amplitude of the annual cycle is approximately 1 to 2 percent of this value.

Structure and evolution of an isolated semi‐geostrophic cyclone

Abstract: The evolution and structure of an indealized low-pressure system is studied within the framework of the semi-geostrophic dynamics and in the limit of uniform potential vorticiy. An isolated cyclone is grown from suitably chosen initial conditions, rather than studying the evolution of a longitudinally periodic train of baroclinic systems. It is shown that the resulting development is able to produce a range of flow features that compare favourably with observationally based conceptual models of cyclogenesis. These features include in particular: (i) the simultaneous occurrence of both a cold and a warm front whose alignment shows some of the characteristics of occluded frontal systems and is skin to the notion of a frontal fracture, (ii) a dry descending air-stream to the rear of the cyclone, (iii) a narrow region of maximum ascent within the warm front an its bent-back portion, (iv) a poleward traveling air-stream ahead of the cold front, and (v) a γ-shaped pattern of vertical lifting comparable with the cloud patterns as commonly observed in satellite pictures. The evolving cyclone is analysed both form Lagrangian and Eulerian viewpoints. It is demonstrated that Lagrangian criteria exist that allow for the objective definition of air-streams and flow patterns within developing cyclones. The structures of cold and warm fronts at low levels are significantly affected by the different nature of the Lagrangian trajectories within each of these regions. In particular, the air parcels in the warm-frontal region are transported rapidly towards the centre of the low, resulting in a low-level warm front with an intrinsically three-dimensional structure and an associated vorticity gradient in the along-front direction.

The sea breeze and gravity‐current frontogenesis

Abstract: The development of a sea-breeze circulation into a sea-breeze front in often observed late in the afternoon. Measurements of horizontal temperature and humidity profiles in a number of sea-breeze circulation, made using an instrumented light aircraft, are presented. On each of the four days for which data are presented an initial weak horizontal temperature gradient, extending over a distance of 5 to 10 kilometres, developed later in the into a sharp front only a few hundred metres across. The data are discussed in terms of simple gravity-current frontogenesis models applied to the development of sharp sea-breeze front from an initial diffuse transition between the air over the land sea. Examination of the equations for frontogenesis, and the result of some previous laboratory experiments on gravity-current frontogenesis, lead to the proposal that, given sufficient contrast between land and sea temperatures, the formation of a sea-breeze front depends on the balance between convergent horizontal winds which act to generate a front, and turbulent convective mixing over the land which tends to prevent its formation. the differences in driving force, wind convergence and turbulent intensity are discussed for each day on which data are available and are shown to be consistent with the theory proposed.

On the response of the tropical atmosphere to large-scale deforestation

Abstract: Recent studies on the Amazon deforestation problem predict that removal of the forest will result in a higher surface temperature, a significant reduction in evaporation and precipitation, and possibly significant changes in the tropical circulation. Here, we discuss the basic mechanisms contributing to the response of the tropical atmosphere to deforestation. A simple linear model of the tropical atmosphere is used in studying the effects of deforestation on climate. It is suggested that the impact of large-scale deforestation on the circulation of the tropical atmosphere consists of two components: the response of the tropical circulation to the negative change in precipitation (heating), and the response of the same circulation to the positive change in surface temperature. Owing to their different signs, the changes in predicted temperature and precipitation excite competing responses working in opposite directions. The predicted change in tropical circulation determines the change, if any, in atmospheric moisture convergence, which is equivalent to the change in run-off. The dependence of run-off predictions on the relative magnitudes of the predicted changes in precipitation and surface temperature implies that the predictions about run-off are highly sensitive, which explains, at least partly, the disagreement between the different models concerning the sign of the predicted change in Amazonian run-off.

Cumulus entrainment, fine‐scale mixing, and buoyancy reversal

Abstract: The formation of buoyancy reversal due to entrainment of dry environmental air, and its implication for cumulus dynamics, are discusses. Concepts originating from laboratory experiments with reacting turbulent flows, and from numerical simulations of homogeneous and isotropic turbulence, are applied to distinguish between large-scale entraining eddies developing at the interface, subsequent development of smaller-scale motions, and final homogenization by microscale processes. The physics of the microscale homogenization by molecular diffusion and sedimentation of cloud droplets is discussed. It is shown that, as a result of droplet sedimentation, much smaller negative buoyancy (buoyancy undershoots) may be generated on cloud microscale as compared with the value predicted by a classical nonlinear mixing diagram of cloudy and cloud-free air. Highly idealized numerical experiments aimed at simulating the temporal evolution of a small convective cloud were performed with and without the effects of molecular mixing that results in buoyancy reversal. It is argued that these experiments provide two limiting cases, and that the dynamics of a real cumulus cloud is located somewhere between them. The major effect of buoyancy reversal as suggested by these experiments is to increase the intermittency associated with cloud evolution. The temporal variation of cloud-top height, maximum undraughts, and minimum downdraughts increases significantly when buoyancy reversal is allowed. It is argued that results of numerical experiments, together with other laboratory and theoretical studies, cast serious doubts on the concept of cumulus entrainment being driven by the cloud-top entrainment instability.

Interannual variability of the stratospheric circulation in the southern hemisphere

Abstract: The variability from year-to-year of the seasonal evolution in the southern hemisphere stratosphere over a period of 10 years, from 1980 to 1989, is investigated using the global geopotential height derived from the data from the Stratospheric Sounding Unit on board the TIROS-NOAA satellites. As a measures of interannual variability, the variance of the zonal mean geostrophic wind over the 10 years was calculated for each day. Although the major warming hardly occurs in the southern hemisphere stratosphere, large variances can be observed during the winter and spring. The maximum variances appear at low latitudes in late autumn, move towards mid latitudes in midwinter and remain near 60°S in spring. The seasonal movement of the zonal mean westerly jet in the southern hemisphere stratosphere can be classified into two categories in terms of the location of the maximum westerlies at the 1 mb level in midwinter, namely HLJ (high-latitude-jet) years with maximum westerlies around 50°S, LLJ (low-latitude-jet) years with maximum westerlies around 40°S. During the late winter the core of the westerly jet moves polewards earlier in HLJ years in LLJ years. In association with this earlier movement, the growth of the amplitudes of planetary waves 1 and 2 during the period from winter to spring occurs earlier in HLJ years than in LLJ years. In autumn, however, the wave-1 amplitude, only for LLJ years, develops vigorously in the stratosphere and even the troposphere, before the seasonal movement of the zonal mean westerly jet branches off into the two categories. The appearances of HLJ and LLJ years occur in groups of a few years; this is quite different from the behaviour of the equatorial quasi-biennial oscillation.

An analogue‐dynamical long‐range numerical weather prediction system incorporating historical evolution

Abstract: It is convenient to regard a predicted dynamical field as a small disturbance superimposed on a historical analogue field. A Statistical technique can then be used in combination with a dynamical forecast. Using this approach, a coupled atmosphere/earth-surface analogue-dynamical model has been formulated and applied to seasonal prediction. This approach facilitates the use of information contained both in the historical data-set and in the initial field in improving the dynamical model (based solely on the latter) and shows some predictive skill.

Cloud processing of the cloud condensation nucleus spectrum and its climatological consequences

Abstract: A modelling study of the effects of aqueous phase cloud chemistry in changing the activated cloud condensation nucleus (CCN) spectrum is presented. The chemistry model is contained within a model of the development of a hill cap cloud. The aqueous phase oxidation of sulphur (IV) to sulphur (VI) by the oxidants ozone and hydrogen peroxide is considered for each of the explicit cloud-droplet categories. All gases enter cloud droplets at a finite rate which is calculated in the model. Significant modification of the CCN spectrum emerging downwind of the processing cloud is observed. This leads to the formation of strongly bimodal aerosol size distributions. The degree of modification is strongly correlated to the concentration of sulphur dioxide, and in oxidant limited situations to the concentration of hydrogen peroxide. The cloud-droplet chemistry is seen to have the largest effect on the smallest activated CCNs. This enables such particles to activate much more readily after cloud processing at critical supersaturations up to 20 times lower than originally required. A simple cloud model is used to investigate the sensitivity of this secondary activation. With updraughts typical of a stratocumulus-type cloud, droplet effective radii are seen to be lowered by up to 3 mum, 500 m above cloud base. This result is insensitive to the concentration of sulphur dioxide present in the processing cloud down to very low concentrations. If reproduced on a global scale this result could have important climatological consequences, as recently indicated by Slingo (1990). However, the reduction in effective radius is expected to be important only on local scales close to the sources of new aerosol, where the processing of the CCN spectrum by clouds may be occurring for the first time. Additional cloud processing will lead to changes in the direct radiative properties of the aerosols but will have little effect upon the radiative properties of clouds subsequently forming on them.

On the interaction of tropical‐cyclone‐scale vortices. III: Continuous barotropic vortices

Abstract: The interaction of cyclonic vortices in spherical geometry is investigated using a shallow-water model on an adaptive grid. It is found that the mutual approach and merger of binary cyclones support the compound vortex-patch findings of Part II. As interaction commences, distortion of the weak outer vorticity fields leads to a change in the advecting flow over each vortex core, which leads to mutual approach or retreat depending on the shape of the vorticity fields. Raped merger occurs when the cyclone cores approach within the critical separation distance defined in Part II. The core merger of cyclones seems to the largely independent of environment and the rotation of the earth. The initial approach can be quite sensitive, however. Non-merging binary cyclones sweep equatorial parcels of air over long trajectories towards the pole to create exceptionally strong anticyclonic gyres. These can be of sufficient strength to capture the nearest cyclone and induce the escape mode described in Part I. Some sensitivity to latitude (and hence strength of the earth-vorticity gradient), therefore, is found. A large anticyclone in a subtropical ridge location is found to be sheared and torn apart by the interacting cyclones, with little effect on merger. The observations in Part I that interactions of three cyclones could be broken down into separate binary interaction sequences is supported by experiments with three vortices. When two of the vortices merge, the interaction evolves to a small/large cyclone situation. When no merger occurs, each pair of cyclones proceeds through a distinctive orbit/escape cycle.

On the link between cloud‐top radiative properties and sub‐cloud aerosol concentrations

Abstract: Microphysical observations obtained in cumulus clouds over the sea are presented and related to background pollution levels. The sub-cloud aerosol concentrations vary from 50 to 5000 cm−3, which can hardly be considered ‘maritime’. Observed droplet size distributions are used to determine radiative properties using Mie theory. Functional expressions are derived for the extinction cross-section and the single scattering albedo as functions of the sub-cloud aerosol concentrations.