Showing papers in "Tellus A in 1983"

Characteristics of northern hemisphere blocking as determined from a long time series of observational data

Abstract: 30 years of observational 500 mb geopotential height data have been used to assess the characteristics of northern hemisphere blocking situations. A zonal index suitable for identification of blockings is defined and translated into a computer program. Characteristics of blocking situations have been computed and are presented as statistics. As expected, there are 2 preferred regions for blocking, the Atlantic region and the Pacific region. The results show that the number of days with blocked flow has a maximum over the eastern part of the Atlantic region, while the maximum is found over the western part of the Pacific region. The annual variation shows that there is an extended maximum from February through April in the Atlantic region, while there is a pronounced maximum in January in the Pacific region. The occurrence of simultaneous blocking in the 2 regions has also been investigated. The results show that there is no preference for a connection. Investigation of individual blocking situations reveals that the shortest ones are also formed in preferred geographical locations, and are not just random configurations in the changing pattern of waves in the Westerlies. There is a tendency for blocking episodes to seem to be concentrated in certain geographical locations, the longer the duration of the episode. The results also show that long-lasting episodes are notably more frequent in the Atlantic region than in the Pacific. DOI: 10.1111/j.1600-0870.1983.tb00210.x

Lagged average forecasting, an alternative to Monte Carlo forecasting

Abstract: In order to use the information present in past observations and simultaneously to take advantage of the benefits of stochastic dynamic prediction we formulate the lagged average forecast (LAF) method. In a LAF, just as in a Monte Carlo forecast (MCF), sample statistics are calculated from an ensemble of forecasts. Each LAF ensemble member is an ordinary dynamical forecast (ODF) started from the initial conditions observed at a time lagging the start of the forecast period by a different amount. These forecasts are averaged at their proper verification times to obtain an LAF. The LAF method is operationally feasible since the LAF ensemble members are produced during the normal operational cycle. To test the LAF method, we use a two-layer, f-plane, highly truncated spectral model, forced by asymmetric Newtonian heating of the lower layer. In the experiments, a long run is generated by the primitive equation version of the model which is taken to represent nature, while forecasts are made by the quasigeostrophic version of the model. On the basis of forecast skill, the LAF and MCF are superior to the ODF; this occurs principally because ensemble averaging hedges the LAF and MCF toward the climate mean. The LAF, MCF and ODF are all improved when tempered by a simple regression filter; this procedure yields different weights for the different members of the LAF ensemble. The tempered LAF is the most skillful of the forecast methods tested. The LAF and MCF can provide a priori estimates of forecast skill because there is a strong correlation between the dispersion of the ensemble and the loss of predictability. In this way the time at which individual forecasts lose their skill can be predicted. The application of the LAF method to more realistic models and to monthly or seasonally averaged forecasts is briefly discussed. DOI: 10.1111/j.1600-0870.1983.tb00189.x

West African water vapor variations between recent contrasting Subsaharan rainy seasons

Abstract: Investigation is made of the interannual and intraseasonal variability of the southwest monsoon layer's thickness and moisture content over West Africa, and its water vapor supply to this region from the tropical Atlantic, for the Subsaharan (10°–20° N) rainy season of July–September Results for the very deficient 1968 1971 and 1972 Subsaharan rainy seasons whose departures from the 1941–74 mean ranged between −078σ and − 139σ are compared with those for the near-average rainy seasons of 1967, 1969, and 1975 (–027σ to + 006σ) The observational base consists of monthly mean rawinsonde data from West African stations and seasonal estimates of the tropical Atlantic near-surface specific humidity field Subsaharan drought does not appear to be associated with the northward supply of unusually dry surface air to West Africa from the tropical Atlantic Over West Africa, the magnitude of the vertically integrated monsoon layer advective water vapor flux tends to be proportional to the layer thickness The extremely poor 1972 Subsaharan rainy season (–139σ) coincided with particularly shallow southwesterly flow across the Gulf of Guinea coast (–5°N); thicker monsoon layers are found characteristic of the less severe 1968 (–078°) and 1971 (–088°) droughts However, the monsoon layer depth here during the nondrought study years does not always exceed that for drought months The direction of the low-level water vapor flux above Dakar, a sensitive Subsaharan location (−15°N) on the West African coast, shows a stronger tendency to be from north of west during unproductive Subsaharan rainy seasons (− 078σ to − 139σ) than in near-average ones (–027σ to + 006σ) This suggests the southwest monsoon does not extend as far north along the West African coast during Subsaharan droughts as in more abundant rainy seasons In contrast to the foregoing Gulf of Guinea results, the monsoon layer over the interior of the Subsaharan zone at ˜ 13°N exhibits minimal variability for the drought study years The better developed nondrought southwesterly flows here tended to be thicker than those characteristic of most drought months DOI: 101111/j1600-08701983tb00197x

Aircraft measurements of atmospheric kinetic energy spectra

Abstract: Wind velocity data obtained from a jet airliner have been used to construct kinetic energy spectra over the range of wavelengths from 2.5 to 2500 km. The spectra exhibit an approximate −5/3 slope for wavelengths of less than about 150 km. steepening to about −2.2 at larger scales. These results support and extend the measurements of spatial and temporal spectra of Vinnichenko and Dutton (1969). Brown and Robinson (1979). and Balsley and Carter (1982). and are generally consistent with the recent results of Nastrom and Gage (1983). DOI: 10.1111/j.1600-0870.1983.tb00212.x

Satellite observations of the southern hemisphere monsoon during Winter MONEX

Abstract: Time-longitude strips of infrared GMS satellite imagery have been used to study the southern hemisphere summer monsoon during Winter MONEX. This representation reveals a sudden monsoon onset in late December characterized by a transition to spatially organized convection. Active and break regions of the monsoon show a strong coherence in time but are very longitudinally dependent. The latitudinal dependence, on the other hand, is quite weak. The dominant spatial scale over which changes in convection take place is of order 35 degrees longitude by 15 degrees latitude. These synoptic scale convective regions can move in either the eastward or westward direction. Typical phase speeds of movement range between 3 and 8 degrees longitude per day. Tropical cyclones in the region are revealed to be embedded in the large-scale monsoon circulation. DOI: 10.1111/j.1600-0870.1983.tb00196.x

Seasonal distributions of diabatic heating during the First GARP Global Experiment

Abstract: The seasonal and annual global distributions of diabatic heating during the First GARP Global Experiment (FGGE) are estimated using the isentropic mass continuity equation. The data used are from the FGGE Level IIIa analyses generated by the United States National Meteorological Center. Spatially and temporally coherent diabatic heating distributions are obtained from the isentropic planetary scale mass circulation that is forced by large-scale heat sources and sinks. The diabatic heating in the Northern Hemisphere is closely associated with the distribution of land and ocean. While cooling prevails over Eurasia and North America and heating occurs along the east coasts of these two continents during northern winter, a contrasting situation exists during northern summer with heating over most of the continents and cooling over the oceans where subtropical anticyclonic circulations prevail. Heating in the tropics, mostly from moist convection, has considerable longitudinal variations which change seasonally. Of particular importance is the seasonal progression of the primary centre of heating within the planetary scale Asiatic monsoon which is located over the western equatorial Pacific during northern winter and near the Philippines and Southeast China during northern summer. DOI: 10.1111/j.1600-0870.1983.tb00201.x

The yearly averaged air temperature in Rome from 1782 to 1975

Abstract: Air temperature data of Rome are analysed to point out the variations caused by natural events and human activities. In particular, the influence of the growth of the city on the minimum average temperature is evident. It is also found that the air temperature seems to be affected by the solar cycle.

A first look at wavenumber spectra from GASP data

Abstract: A preliminary analysis of Global Atmospheric Sampling Program (GASP) data has been made to explore the character of mesoscale atmospheric wavenumber spectra. The results show a k −5/3 spectral slope over spatial scales ranging from about 100 km to about 400–700 km. At larger scales there is evidence of a transition to steeper wavenumber dependence. These results are briefly discussed in terms of two-dimensional stratified turbulence and internal waves. DOI: 10.1111/j.1600-0870.1983.tb00213.x

A heat‐flux history length scale for the nocturnal boundary layer

Abstract: A height scale, H = ∫ Q H dt /(ρC ρ Δθs), based on the night-time heat-flux ( Q H ) history is proposed here as being a relevant scale for the nocturnal boundary layer temperature structure. This scale, together with the surface temperature decrease since sunset, Δθs. are shown to be sufficient to define a common similarity shape for the potential temperature profile. An equation for this shape based on Wangara field experiment data is Δθ/Δθ s = exp (–0.77 z / H ). This curve is then tested against the Koorin boundary-layer data set, and is shown to provide a good fit to the data. Such a potential-temperature profile equation suggests that the top of the boundary layer is at about 5H , and that 23% of the cooling within that depth is caused directly by radiation. The definition for H, above, is shown to be a simple means of expressing much of the same information as Nieuwstadt's rate equation. DOI: 10.1111/j.1600-0870.1983.tb00199.x

Effects of orography on Mediterranean lee cyclogenesis and its relationship to European blocking

Abstract: The role of orographically-induced cyclogenesis in the lee of the Alps in favouring the onset of Euro-Atlantic blocking is investigated by means of numerical simulations of a real case. The model used is the ECMWF global grid-point model as operational in August 1980. While the essential part that the local orography (the Alps) plays in triggering the development of cyclonic disturbances is confirmed for the first time using a global model, the relationship between Alpine cyclogenesis and European blocking remains less clear. The importance of global orography in generating and maintaining the blocking pattern is subsequently investigated and its crucial role evidenced and discussed. DOI: 10.1111/j.1600-0870.1983.tb00203.x

Patterns of India monsoon rainfall anomalies

Abstract: Pattern characteristics of annual rainfall anomalies over India during 1900–72 are studied from precipitation records of 31 regional subdivisions and series of annual cyclone frequency and number of break monsoon days. Methods include spatial correlation, pattern stratification, and principal component and spectral analyses. Pattern stratification with respect to an all-India rainfall index yields largest concordant departures in the Central and Western portions of the country. Pattern stratification with respect to the largest annual frequency of cyclonic storms in the Bay of Bengal and Arabian Sea show large positive rainfall departures over Eastern. Southeastern and Western India, and negative departures in the North and a limited area of the Southwest: for the collective of years with smallest storm frequency a partly inverse rainfall anomaly pattern is obtained. Pattern stratification with respect to the years of most numerous break monsoon days yields deficient rainfall in a large, broadly zonally oriented band across Central India, and positive departures in most other parts of ther country. Stratification with respect to the least numerous break monsoon days yields a broadly inverse pattern. The first eight principal component patterns account for 74% of the total variance. The first component resembles the pattern obtained by stratification with respect to the all-India rainfall index, the two time series being highly positively correlated. The third component is approximately inverse to the pattern constructed from stratification with respect to the most numerous break monsoon days, the two time series being negatively correlated. The seventh component is similar to the pattern derived from stratification with respect to the largest frequency of cyclonic storms: this is consistent with the positive correlation between the two time series. No physical significance is apparent for the other principal components. The combination of the various analysis techniques thus serves to identify spatial and temporal pattern characteristics of possible physical significance. DOI: 10.1111/j.1600-0870.1983.tb00206.x

Barotropic oscillations of the Mediterranean and Adriatic Seas

Abstract: The periods and structures of several of the lowest barotropic free modes of oscillation of the combined Mediterranean—Adriatic system are computed. The computations take account of the two-dimensional structure of the Mediterranean—Adriatic system, the actual bottom topography, and the earth's rotation. To separate the effects of each of these factors, calculations are carried out first for uniform depth, then for variable depth but no rotation, and then for variable depth and rotation. The results show that the effect of variable bottom topography is more important than rotation. The computed periods for the lowest four modes of the Mediterranean Sea are 38.5 h. 11.4 h, 8.4 h, and 7.4 h. In addition, the periods and structures of several of the lowest modes of the Adriatic Sea are computed without rotation on a high resolution grid. The location of the mouth of the Adriatic Sea is determined by the combined Mediterranean-Adriatic calculation. By imposing the mouth in the proper location, we found that the computed periods of the lowest modes agree with observed periods. The computed periods of the three lowest modes in the Adriatic Sea are 21.9 h, 10.7 h, and 6.7 h. DOI: 10.1111/j.1600-0870.1983.tb00216.x

A time dependent, two-layer frontal model of buoyant plume dynamics

Abstract: The dynamics of buoyant plumes, such as those observed at the mouths of the Mississippi and Connecticut Rivers is investigated by considering the one dimensional, time dependent behavior of a two-layer, frontal model. The equations governing the flow in the main body of the plume are the long wave equations. The flow at the frontal boundary is modeled using jump conditions which incorporate the important dissipative processes. These equations are solved numerically using a shock patching technique that is accurate to second order in the long wave region and to first order at the front. The solutions demonstrate the controlling influence of the inlet on the flow field and the importance of time-dependent processes for the dynamics of buoyant plumes. DOI: 10.1111/j.1600-0870.1983.tb00186.x

A numerical study on cyclic behaviour of polar ice sheets

Abstract: Possible cyclic behaviour of polar ice sheets is studied with a numerical ice-flow model. The model includes a calculation of bedrock adjustment and temperature field in the ice sheet. Basal water is traced and affects ice-mass discharge. Relaxation oscillations occur only for low ice-accumulation rates (less than 5 cm ice depth per year). For larger ice-accumulation rates, 2 steady-state regimes exist, depending on temperature: an ice sheet frozen to the bottom, and an ice sheet subject to basal melting every where. The volume of the latter is considerably smaller (about 65% of that of the “cold sheet”). In general, the mean basal ice temperature decreases with increasing ice accumulation rate. This is due to the advection term in the heat equation (which has been ignored in earlier studies of ice-sheet surging). DOI: 10.1111/j.1600-0870.1983.tb00187.x

Stability properties of cylindrically curved mean flows

Abstract: This article describes results from numerical experiments which isolate the influence of horizontal curvature upon various linear properties of growing baroclinic eddies. Cylindrically symmetric mean flows on an f-plane are studied; they have either strong or weak curvature. The radius of curvature is twice as large for weak as it is for strong curvature. Strong curvature reduces the growth rates and this is most evident only for the short waves. The baroclinic energy conversion is positive (from mean flow to eddy) and little influenced by curvature. The barotropic conversion is made more negative by stronger curvature; it is responsible for the growth rate reductions. The regions of positive and negative barotropic conversion are distributed about the mean flow jet in such a way that the eddies are tending to straighten out that jet. These results compare favorably with some previous analyses of long-wave flows by others. DOI: 10.1111/j.1600-0870.1983.tb00194.x

On the theory of the katabatic slope wind

Abstract: The known Prandtl slope wind problem is considered for the case when the turbulent field in the surface layer is specified in accordance with Monin-Obukhov similarity theory. The solution of such a problem turns out to be a particular case of the solution for another more general problem published recently. On the basis of the former solution, simple analytical and graphical relationships have been obtained between external parameters of the problem, and those characteristics of the katabatic slope wind which seem to be most available from the observations. The paper strives to encourage the carrying out of special slope wind observations. DOI: 10.1111/j.1600-0870.1983.tb00198.x

On the effect of model resolution on numerical simulation of blocking

Abstract: A numerical case study of blocking is discussed briefly. The results indicate that model resolution seems to play a major role in the success of the simulation. It is argued that the effect of the initial conditions is somewhat less important than normally considered. The model used for all the experiments is the ECMWF spectral model. DOI: 10.1111/j.1600-0870.1983.tb00182.x

Ocean response to mesoscale atmospheric forcing

Abstract: Many processes have been proposed as possible forcing mechanisms for mesoscale oceanic variability. The present study shows that atmospheric forcing can be an important source of mesoscale variability in the ocean. We show that the response is linearly proportional to the product of the time scale of the storm and its intensity. We clarify the point that for storms with scales considerably smaller than the barotropic Rossby radius of deformation, the oceanic stratification and the horizontal extent of the storm are the only factors determining the penetration depth of the response, implying that it is not the Rossby radius of deformation but rather the scale of penetration depth ( h = ( f / N ) L ) that characterizes the response. In exploring the effect of differing eddy-viscosity parametization on oceanic response, we find no significant qualitative differences, although as one might expect we find quantitative differences in the results. The role of the mixed layer is considered very important in the transfer of surface stresses down into the system. The mixed layer does not seem to be important in determining the characteristic lengths of the problem, however, at least for storms that give a penetration depth considerably larger than the mixed layer (for a mixed layer on the order of 20 m, the storm should be larger than a few kilometers). The non-linear advection terms seem to affect the adjustment process more by reducing the associated wave energy than by modifying the characteristics of the geostrophic response. Finally, making the stratification more realistic has no significant impact on the resulting oceanic response. DOI: 10.1111/j.1600-0870.1983.tb00205.x

Inter-annual and seasonal variations in the structure and energetics of the atmosphere over northeast Brazil

Abstract: Inter-annual and seasonal variations of energetics over northeast Brazil are discussed. A cylindrical boundary having 11 aerological stations has been chosen for the study. Mass and energy fluxes are evaluated for the dry and wet seasons and also for the dry and wet years. During the wet period, there is inward mass flux in the lower levels and outward mass flux in the upper levels. During the dry period there is inward mass flux in the middle levels with outward mass flux above and below. Variations in the vertical profiles of the energy parameters suggest that the rainfall variations in the NE Brazil are not of local origin but are connected to the variations of the general circulation in the tropics. Water vapor flux calculations showed that during the wet period there is a net import of water vapor and during the dry period a net export. DOI: 10.1111/j.1600-0870.1983.tb00191.x

On the response of ocean currents to atmospheric cooling

Abstract: The response of steady ocean currents to forced atmospheric cooling is examined by a simplified two-layer model. The study focuses on currents, such as the Gulf Stream, which lose their heat to the atmosphere as they flow from one region to another. The model is inviscid and includes no coupling between the ocean and the atmosphere. Approximate solutions for specified cooling processes acting steadily on currents whose undisturbed speed is uniform are obtained analytically using a uniformly valid power series expansion. It is found that upon encountering a region of cooling, the interface steepens and the whole current is displaced horizontally. The streamlines in the upper portion of the light layer are displaced to the right (looking downstream) whereas the streamlines in the lower portion of the upper layer are displaced to the left. These movements result from a combined effect of advection and “thermal wind” motion. For actual currents and heat losses in the ocean, the predicted interface steepening is of the same order as the slope upstream and the predicted horizontal displacements during strong cooling processes can be as high as ˜ 100 km. Possible application of this theory to the separated Gulf Stream which loses heat to the atmosphere as it flows from Cape Hatteras toward the northeast is discussed. The model predicts that during the late winter the position of the Gulf Stream front will be farther to the south than it is during the summer and that the slope of the interface will be larger in winter. Both processes agree qualitatively with the observed seasonal variability of the Stream. DOI: 10.1111/j.1600-0870.1983.tb00185.x

Large-scale fluctuations in a long-range integration of the ECMWF spectral model

Abstract: The ECMWF spectral model with a triangular T21 truncation has been integrated for a 6-year period. The chronology of the atmosphere simulated by the model is studied through Principal Component Analysis of the 500 mb geopotential field. The annual cycle is found to be predominant, but the interannual variability is shown to be large and the model proves able to generate long and persistent anomalies, even though sea-surface temperatures change only with a climatological annual variation. DOI: 10.1111/j.1600-0870.1983.tb00195.x

On the mechanics of the deep-water flow in the Bornholm Channel

Abstract: A model for stationary and longitudinally homogeneous 2-layer rotating canal flow subject to friction is proposed for the deep-water flow in the Bornholm Channel. Using parameter values estimated from the results of a field survey carried out in conjunction with a long term program for monitoring the inflow of high-saline water to the Baltic, diagnostic calculations for the position of the pycnocline have been performed. The results correspond to an encouraging degree to those observed in nature and indicate the existence ofa frictional coupling between the down-and cross-stream pressure gradients in deep-water. DOI: 10.1111/j.1600-0870.1983.tb00192.x

Linear non-divergent mass-wind laws on the sphere

Abstract: The properties of 3 types of linear non-divergent mass-wind laws on the sphere have been examined: linear-balance equation, spherical harmonic expansions of the linearized primitive equations, and Rossby-Hough expansions. Both the symmetric and antisymmetric non-zonal cases are examined. The results show that all 3 methods are virtually equivalent for the antisymmetric case, but differ considerably for the symmetric case. All 3 methods, whether derived from the primitive equations or from a filtering approximation, appear to be singular at the equator in the symmetric case. DOI: 10.1111/j.1600-0870.1983.tb00181.x

The use of dynamical constraints in the analysis of mesoscale rawinsonde data

Abstract: This study seeks to generate dynamically consistent analyses of mesoscale rawinsonde data by coupling analyses of winds and geopotentials on pressure surfaces. The approach used here follows in a general way the one taken by Lewis and Bloom (1978) in their analysis of surface data; the observations are first objectively analyzed onto a regular grid in space and time, and the gridded fields are then adjusted in a least-squares sense using a set of forecast equations of horizonatal momentum as dynamical constraints. However, the scheme described in this paper has a more consistent treatment of the adjustments to the fields in time. The adjustment process is tested on objectively analyzed rawinsonde soundings from two cases from the data archives of the National Severe Storms Laboratory (NSSL); 8 June 1966 and 22 May 1966. The principal results of this study are: (1) the initial winds and geopotentials produce unphysical residuals when they are put into the momentum constraints; (2) the adjusted fields have greatly reduced residuals, (3) the adjustments to the wind and geopotential fields have root-mean-square values of 1–2 m s -1 and 30–50 m 2 s -2 respectively, and (4) the vertical velocity fields computed from the adjusted winds are more consistent with the weather events that occurred in the two data cases. DOI: 10.1111/j.1600-0870.1983.tb00211.x

The development characteristics of quasi-geostrophic baroclinic disturbances

Abstract: An overall qualitative analysis is made for the development characteristics of quasi-geostrophic pure baroclinic disturbances by using both integral properties and the WKB method. The results obtained by these two methods are consistent, but the WKB method yields more conclusions. The sufficient condition for stability and the necessary condition for instability are investigated. A local individual disturbance can be unstable only if it is located in the layer where the instability criteria is satisfied locally. For an arbitrarily given initial condition, the instability might be first realized locally, and then spread into adjacent levels. More attention is paid to the problems of development. A disturbance is called developing (decaying) if its energy increases (decreases). In a vertical shear zonal flow, all characteristics of a disturbance, such as total wave length, vertical wave length, orientation of axis (trough-ridge line), amplitude and energy usually change with time during the evolutional process. The development of a disturbance depends only on the orientation of its axis and its location related to the basic current. A developing (decaying) disturbance enlarges (shortens) its total wave length and vertical wave length, and gradually turns its axis more vertically (more tilted). If the meridional gradient of zonal mean potential vorticity is every where positive during the period of growth (decay), the maximum amplitude of the disturbance moves toward (out from) the jet and is accompanied by a vertical energy flux toward (out from) the jet. In the unstable case, the energy flux is more complicated. It is also pointed out that wave action might not exist in the unstable case, but the abovementioned conclusions are all still valid. DOI: 10.1111/j.1600-0870.1983.tb00209.x

The atmospheric energy budgets over North America, the North Atlantic and Europe based on ECMWF analyses and forecasts

Abstract: The kinetic energy, sensible heat and latent heat budgets and the total energy balance of the earth-atmosphere system are studied over North America, the North Atlantic and Europe. The data consist of 1 year (December 1979—November 1980) of ECMWF initialized objective grid point s-coordinate analyses and the ensemble of forecasts for day 3. The results quantit atively confirm the thermally direct meridional circulation over North America with generation of kinetic energy and adiabatic heating in the troposphere; kinetic energy and sensible heat are exported out of the region. The opposite holds for the North Atlantic area. The residuals of the energy budgets give a realistic approximation of the unresolved sub-grid scale processes (friction, diabatic heating, moisture sources/sinks) in the analysed data. Budgets from the forecast data reproduce the main features present in the analysed data, but also indicate some defects in the parameterization of the sub-grid scale processes in the ECMWF model. The analysed data, taking 4 analyses per day separately, exhibit diurnal variations, the magnitude of which is consistent with observed small diurnal wind variations. The total energy balance obtained for the 3 areas is reasonable and points to a considerable heat convergence in the North Atlantic ocean. DOI: 10.1111/j.1600-0870.1983.tb00183.x

On low-order non-linear stochastic-dynamic systems

Abstract: A low-order system of equations derived from the advective equation with forcing and dissipation is investigated. A stochastic-dynamic treatment of these equations based on the closure assumption of the neglect of third and higher moments is carried out. For this system, the steady states, including those created by the closure assumption, are determined, and their linear stability properties are found. It turns out that 5 steady states exist in addition to the 3 related to the deterministic equations. Among the 8 steady states, 3 are stable. 2 of them are the steady states coming from the deterministic problem. The asymptotic solutions in a stochastic-dynamic sense are determined without any closure assumption. It is shown that the 3 stable steady states for the system with closure form a subset of the continuum of asymptotic states for the system without closure. Several examples showing the differences between the asymptotic solutions of the 2 systems are calculated. Numerical experiments based on backward integrations in time show that the system with closure also has a limit cycle characterized by a closed curve in the mean values and in each point a degenerate uncertainty ellipse consisting of a line segment. DOI: 10.1111/j.1600-0870.1983.tb00180.x

African wave disturbances in a general circulation model

Abstract: Evidence is presented to show that African wave disturbances are reproduced in a general circulation simulation. The model used is the general circulation model developed by the Goddard Laboratory for Atmospheric Sciences of the National Aeronautics and Space Agency. The model was integrated in order to simulate the summer of 1974. A synoptic analysis of the simulated data over Africa for the month of July was done. The results of the analysis show that wave disturbances are generated by the model; the behavior and the structure of the simulated disturbances are similar to those observed over tropical Africa during the northern summer.

Statistical predictability and spectra of air temperature over the northern hemisphere

Abstract: Parametric models and statistical predictability of mean annual air temperature, both at individual stations (MAAT) and zonally averaged (MZAT), are evaluated. The time series have lengths from 125 to 315 years for MAAT and 86 years (1891–1976) for MZAT. The optimal parametric models of MAAT are shown to be closely approximated by a first-order Markov sequence with a small characteristic time scale, so that their statistical predictability is minimal. Variations in MZAT should be described with more complicated models; the limits of statistical predictability amount to between 4 and 5 years. The temperature spectra are estimated, and the relatively high statistical predictability is shown to result from the concentration of spectral energy at low frequencies. DOI: 10.1111/j.1600-0870.1983.tb00184.x