Showing papers in "Tellus A in 1990"

On the operational predictability of blocking

Abstract: The entire 7-year archive of ECMWF operational analysis and forecast data is used to assess the skill of the Centre's model in short- and medium-range forecasting of atmospheric blocking. The assessment covers 7100-day periods, from 1 December to 10 March of all winters from 1980-81 to 1986-87, inclusive. A slightly modified version of the Legenas and Okland objective zonal index is used to quantify both observed and forecast occurrence of blocking. The study is performed on 500 hPa geopotential height and on Euro-Atlantic and Pacific blocking separately. It is found that blocking frequency is severely underestimated in medium-range forecasts; the model is, on average, reasonably skilful if the initial conditions are blocked, but blocking onset is poorly represented if it occurs more than a few days into the forecast. This inability in entering the blocking regime has a substantial impact on the systematic error of the model. DOI: 10.1034/j.1600-0870.1990.t01-2-00003.x

Climatological analysis of Mediterranean cyclones using ECMWF data

Abstract: A thoroughly objective method for the definition, selection and tracing of Mediterranean region cyclones is presented. The method is applied to the ECMWF 1982-1987 analyzed datasets to show monthly cyclone frequencies, cyclonic tracks and vertical variation of average relative vorticity. Day-to-night changes and vertical variation of cyclonic frequencies/vorticities indicate the importance of the sea thermal effect in the eastern Mediterranean. In the western Mediterranean and to a lesser extent in the Cyprus region, the lee cyclogenetic effect is very pronounced. Monthly cyclone tracks are presented and they clearly indicate the preferred routes of cyclonic movements. DOI: 10.1034/j.1600-0870.1990.00007.x

Variational assimilation of meteorological observations with the direct and adjoint shallow-water equations

Abstract: Experiments of variational assimilation, similar to those already performed by the authors on a vorticity equation model are performed on a shallow-water equation model. The variational algorithm requires the computation of the gradient of the distance function to be minimized with respect to the model state at the beginning of the assimilation period. As in the previous experiments, this gradient is computed by using the adjoint equations of the model. Northern Hemisphere observations of wind and geopotential, distributed at the 500 mb level over a 24-h time period, are assimilated with a spectral model truncated at degree 21. The results confirm the results previously obtained, namely that the variational process reconstructs to a satisfactory degree of accuracy the meteorological structures of the flow. In addition: (i) Gravity wave noise can be efficiently eliminated by adding an appropriate penalty term to the distance function, and by introducing in the variational process a nonlinear normal mode initialization algorithm. The latter has the effect of improving the numerical conditioning of the variational process. (ii) The quality of forecasts produced from the results of variational assimilation is similar to the quality of shallow-water equation forecasts produced from the results of operational assimilations, which use many more data and more realistic models. Assimilations performed with a model truncated at degree 42 produce similar results. They also show that the numerical efficiency of the variational process, as measured by the number of descent steps necessary to reach convergence, is almost insensitive to the dimension of the model phase space. Finally, study of the variations of the distance function suggests that, as in the case of the vorticity equation, the tangent linear approximation to the model equations is valid in the conditions of data assimilation. DOI: 10.1034/j.1600-0870.1990.t01-4-00004.x

The role of transient eddies in low‐frequency zonal variations of the Southern Hemisphere circulation

Abstract: The dominant modes of low-frequency variation of the Southern Hemisphere (SH) circulation in winter have been identified using 15 years of monthly mean analyses of the SH troposphere. The two leading modes are primarily zonally-symmetric, representing out-of-phase variations of geopotential height between middle and high latitudes in one case and between the tropics and middle latitudes in the other. There are accompanying variations of zonal wind and temperature associated with these modes, which have an equivalent barotropic vertical structure in the extratropics. The two modes represent 25% of the variance of the monthly mean height in the SH. The high-latitude mode is also the dominant pattern of variation of 5-day average height anomalies, indicating that this is an important mode on time scales from about a week to a season. Using composites of monthly mean fields and transient eddy statistics for opposite extremes of this high latitude mode, the interaction between transient eddies and the anomalous mean flow is described. During one extreme of the variation, there is an increased height gradient, a stronger zonal jet and more baroclinicity at high latitudes. Although there may be some doubt about the reliability of the transient eddy statistics, they show an enhanced storm track at high latitudes, with transient eddy fluxes which help to maintain the anomalous mean flow. DOI: 10.1034/j.1600-0870.1990.00005.x

Can chaos and intransitivity lead to interannual variability

Abstract: We suggest that the atmosphere–ocean–earth system is unlikely to be intransitive, i.e. to admit two or more possible climates, any one of which, once established, will persist forever. Our reasoning is that even if the system would be intransitive if the external heating could be held fixed, say as in summer, the new heating patterns that actually accompany the advance of the seasons will break up any established summer circulation, and an alternative circulation may develop during the following summer, particularly if chaos has prevailed during the intervening winter. We introduce a very-low-order geostrophic baroclinic “general circulation” model, which may be run with or without seasonal variations of heating. Under perpetual summer conditions the model is intransitive, admitting either weakly oscillating or strongly oscillating westerly flow, while under perpetual winter conditions it is chaotic. When seasonal variations of heating are introduced, weak oscillations prevail through some summers and strong oscillations prevail through others, thus lending support to our original suggestion. We develop some additional properties of the model as a dynamical system, and we speculate as to whether its behaviour has a counterpart in the real world. DOI: 10.1034/j.1600-0870.1990.t01-2-00005.x

The role of synoptic/planetary-scale interactions during the development of a blocking anticyclone

Abstract: The period 19–21 January 1979 marked the development of a blocking anticyclone over the North Atlantic Ocean preceded by explosive cyclogenesis about 500 km south of Nova Scotia. Using fields derived from GLA analyzes (4° lat × 5° long) of the FGGE SOP-I data set, the general behavior of this block is diagnosed using the extended height tendency equation. This equation preserves much of the simplicity of the quasi-geostrophic form, but replaces the geostrophic wind and relative vorticity by the observed value. Three-dimensionally varying static stability and strong diabatic heating are also allowed in the extended form. To further analyze the relative importance of planetary-scale, synoptic-scale, and scale-interaction forcing of this block, height tendencies were solved from a scale-partitioned form of height tendency equation. The scale partitioning is accomplished using the Barnes objective analysis scheme. Results indicate that vorticity advection was the primary forcing mechanism during the block development. Growth in this mechanism occurred during and extended beyond the period of explosive cyclogenesis and was located downstream from the cyclone event. In fact, much of the vorticity advection was attributed to the northward advection of negative relative vorticity east of a jet streak that formed between the cyclone and anticyclone. The scale interactions implied by this relationship between the cyclone and anticyclone were confirmed in the partitioned height tendencies. The scale interaction component was consistently larger than the other two and was particularly significant during the block development. This component was followed in importance by the synoptic-scale component, although the latter was significant only in the vorticity advection term. Interestingly, despite pronounced northward warm air advection, the direct forcing of the block by thermal advection was relatively small. Rather, the thermal forcing was strongest in the upstream cyclone, which in view of the subsequent role of scale interactions in the block development suggests an indirect role for thermal advection. DOI: 10.1034/j.1600-0870.1990.00015.x

The North Atlantic Oscillation in the Atlantic‐European SLP*

Abstract: An analysis of the signature of the North Atlantic Oscillation (NAO) in the Atlantic-European sea level pressure (SLP) is presented for observed (German Weather Service) and ECMWF T21 model data. The former time series consists of 1881–1984 January to December fields and the latter of 42 monthly fields from 3 permanent January simulations. The NAO is shown to be one of the dominant eigenmodes of SLP for all calendar months. A very similar NAO anomaly pattern is filtered from the T21 model data, whereby the correspondence between model and reality is far better than for the mean states. Typical extreme states of the NAO, i.e. high- and low-index situations are reconstructed from the EOF (empirical orthogonal functions) and the mean fields. The annual cycle of these states and the differences between observed and simulated January extremes of the NAO are discussed. DOI: 10.1034/j.1600-0870.1990.t01-3-00001.x

On the response of the horizontal mean vertical density distribution in a fjord to low-frequency density fluctuations in the coastal water

Abstract: Baroclinic water exchange through a fjord mouth, driven by a slowly varying density field outside the mouth, is modelled by a simple quasi-steady frictionless model It is assumed that a certain fraction of the horizontal pressure difference between the coastal water and the fjord is used to accelerate the fluid into the mouth The continuous vertical density distribution in the fjord, which changes in response to the water exchange, is modelled using a time-dependent, one-dimensional advective-diffusive ‘filling-box’ type of model The model has been tested against an almost one-year-long time series of salinity and temperature from the Orsta fjord (horizontal surface area about 15 km 2 ) on the Norwegian west coast It is found that for this particular fjord, the mean externally forced baroclinic water exchange is one order of magnitude greater than the mean water exchange driven by the estuarine circulation (600 and 60 m 3 s -1 respectively) Such a vigorous water exchange between a fjord and the external area implies that the time-averaged concentrations of many biological and chemical species above the sill level in the fjord are approximately equal to those in the coastal water outside the fjords DOI: 101034/j1600-08701990t01-1-00010x

Diurnal variation of California coastal stratocumulus from two days of boundary layer soundings

Abstract: Boundary layer soundings show the diurnal variation of California coastal stratocumulus. During the period of solar heating, cloud base rises and the cloud layer thins, while the subcloud layer becomes more stable, suggesting some daytime uncoupling of the cloud layer from the surface. DOI: 10.1034/j.1600-0870.1990.t01-1-00007.x

Constraints on the global sources of methane and an analysis of recent budgets

Abstract: Recent observations show that the concentration of methane before being affected by human activities was about 650 ppbv; now it is about 1690 ppbv and has increased at an average rate of 16.5 ppbv/yr over the last decade. The present lifetime of methane is 8–12 years. These facts produce constraints on the global budgets of methane and particularly on the anthropogenic fraction (ratio of anthropogenic to total emission rates). Taking into account the possibilities that the lifetime of methane has gotten longer over the last century because of a possible decline in the natural sinks and that emissions from natural sources have also changed, we show that the anthropogenic fraction should be between 40%–70%, and the total present emissions should be between 420–620 Tg/yr. Budgets that do not meet these conditions would be inconsistent with one or more of the observations mentioned above. When we analyzed 11 budgets of methane published over the last decade, we found that only 2 meet these conditions. DOI: 10.1034/j.1600-0889.1990.t01-1-00001.x

Modelling the Baltic Sea as thirteen sub-basins with vertical resolution

Abstract: A mathematical model of the Baltic Sea is presented and explored. The model divides the Baltic Sea into 13 sub-basins. The dynamics in each sub-basin is reduced to a one–dimensional boundary layer problem with vertical mean velocities based upon in- and outflows from surrounding basins. The turbulent exchange coefficients are calculated using a two-equation model of turbulence. The boundary conditions consider wind stress as well as the heat exchange between water and air. Sea ice is neglected in the model; instead, the water temperature is kept at the freezing point when the temperature is below freezing. The model is tested against observations from the severe sea ice winter 1986/87 and found to simulate the surface water temperature reasonably in all sub-basins during the whole winter. The thermal response due to sea ice has, however, to be considered in some sub-basins, particularly during spring warming. DOI: 10.1034/j.1600-0870.1990.00006.x

A coupled one-dimensional sea ice–ocean model applied to a semi-enclosed basin

Abstract: The objectives of the present study are to formulate and explore a coupled sea ice–ocean model for semi-enclosed basins. Both thermodynamic and dynamic processes are included. Variations in the on–off shore directions are considered, and the model is applied to the Bothnian Bay, where ridging due to southerly winds is a common feature. The importance of the different ice processes and their simulation are discussed. It is concluded that a one-dimensional model approach to the winter sea ice season can be used, giving realistic information about ice formation, growth and decay. Horizontal gradients in the sea and in the ice, however, imply that a two- or three-dimensional model is in general needed, particularly for the initial advance of the ice edge and for the ice deformation calculation. DOI: 10.1034/j.1600-0870.1990.t01-3-00007.x

A model‐based diagnostic study of the development and maintenance mechanism of two polar lows

Abstract: The life cycles of two polar lows in the Norwegian Sea have been studied. Both lows were observed, analysed and forecast by the Norwegian mesoscale model system. The system is run operationally 4 times a day and provides high spatial and temporal resolution data for diagnostics in addition to daily forecasts. Both lows start developing as upper level vorticity maxima approach low-level baroclinic zones. One of the lows decays after the initial baroclinic intensification phase, while the other develops further. This second phase, the maintenance phase, seems to be related to organized convection. The possibility for air-sea interaction instability is discussed. The role of the underlying surface in creating conditions favourable for polar low developments is addressed as is the predictability of these mesoscale cyclones. DOI: 10.1034/j.1600-0870.1990.00009.x

Semi-Lagrangian moisture transport in the NMC spectral model

Abstract: The moisture advection component of the National Meteorological Center global spectral model is replaced with a semi-Lagrangian transport scheme. Several interpolation schemes, both monotonic and nonmonotonic, which were not eliminated from contention in earlier studies with simple test systems, are compared for moisture transport in five-day forecasts. The observed conservation properties of the schemes are considered first. Many of the schemes are found to be unsatisfactory with respect to conservation although several are very good. The best nonmonotonic interpolants are Hermite cubic and a rational cubic both coupled with a cubic derivative estimate. The best monotonic interpolants are these same two with the cubic derivative estimate modified to satisfy the appropriate C° monotonicity condition. The conservation aspects of these four schemes are better than the original spectral scheme if their computational sources are compared to the negative mixing ratios introduced by the spectral method. The skill of the semi-Lagrangian forecasts as measured by the traditional global skill scores (anomaly correlation and rms) is the same as that of the original spectral model. The characteristics of the precipitation in the semi-Lagrangian forecasts are very different from those of the spectral forecast. While the major precipitation events are similar in the two in both magnitude and location, the patterns of light precipitation are very different. The spectral model produces regions of light precipitation which are attributable to spectral truncation. The semi-Lagrangian method shows no similar spurious precipitation. DOI: 10.1034/j.1600-0870.1990.t01-3-00002.x

Cyclonic vortices over South America

Abstract: It is observed from satellite cloud imagery that about 100 neutral vortices per year cross the South American continent between 15-60°S, and almost as many are generated or intensified in the same latitude belt between 30-70°W. There are more cyclogeneses in summer than in winter in contrast to the common idea that winter is the season of maximum extratropical cyclogenesis. The frequency of neutral vortices is higher in the transition seasons than in summer and winter. Neutral vortices move toward the east with a speed of about 20 m/s -1 , while the cyclones which develop over the region move south-eastward more slowly with an average speed of 12.5 m/s -1 . Vortices crossing the Andes with cyclonic vorticity in excess of 2 × 10 -5 s -1 have a 40% probability of producing intense convective systems near the east coast of the continent. DOI: 10.1034/j.1600-0870.1990.00016.x

The fractal dimension of relative lagrangian motion

Abstract: 14 clusters of drifters are studied to obtain the fractal dimension of relative trajectories between pairs of drifters. Averaging over all experiments, the relative trajectories have a fractal dimension of 1.3 over separation scales ranging from 10 m to 4000 m. The clusters were deployed in Lake Erie, the Atlantic Equatorial Undercurrent, and in coastal waters off the south shore of Long Island. The average fractal dimension was quite similar for groups of experiments in each of these 3 areas, but it fluctuates over a greater range (1.59 to 1.12) when we consider individual experiments. This and consideration of variation of the generalized fractal dimension for higher moments, raise the possibility that trajectories are multifractal. The estimates we obtain for fractal dimension of relative lagrangian motion should be regarded as first estimates, pending analysis of more substantial data sets. DOI: 10.1034/j.1600-0870.1990.t01-4-00005.x

On the influence of the Alps on a cold front

Abstract: This paper discusses the synoptic- scale environment and the scale dependent features of a cold front which crossed southern Bavaria on 8 October 1987. The analysis is based on rawinsonde, Doppler radar, aircraft and surface data. The observations reveal that the front was of the ana-type in the synoptic-scale. Its leading edge exhibited small-scale, bore-like characteristics which are typical for an atmospheric gravity current interacting with a stable layer at the ground. The leading edge was marked by several spectacular roll clouds; their generation was favored by south-foehn in the pre-frontal area and by post-frontal precipitation. The low-level front was retarded by the Alps whereas the upper level front crossed the Alps without being distorted. The analysis shows that the cross-frontal gradients in pressure and temperature were stronger close to the Alps than at a greater distance from the obstacle. The data suggest that the middle tropospheric extrusion of cold air towards the front in a downward direction and from behind, which is a characteristic synoptic-scale feature of an ana-front, is weakened in the vicinity of the orography. DOI: 10.1034/j.1600-0870.1990.00013.x

Sensitivity of limited area model data assimilation to lateral boundary condition fields

Abstract: Experiments with a limited area model forecasting system have been carried out to show the importance of accurate lateral boundary conditions, also during data assimilation forecast cycles. Advection of observed information from data-dense areas to data-sparse areas is an important process in any data assimilation, and for limited area assimilation, this process should also work properly across the lateral boundaries. A case study including a rapid cyclogenesis in the vicinity of Iceland is used to illustrate the deterioration of forecast quality as a result of too old forecast boundaries during data assimilation. DOI: 10.1034/j.1600-0870.1990.00010.x

Dynamic coupling of sea ice and water for an ice field with free boundaries

Abstract: Ice-ocean dynamics is investigated using field data and a transient ice-ocean model. The measurements were made in April 1975 in the Bothnian Bay in an ice patch (100 km across) with free boundaries. The data consist of velocity time series of wind, ice, and currents at depths of 7, 10, 20 and 30 m from the ice. The ice-ocean model is used to examine the characteristics of the observations; the model consists of an unsteady free ice drift model and a transient Ekman flow model, which uses a two-equation turbulence model to achieve closure. For the surface wind at the altitude of 10 m, the estimated wind factor was 1.9%, and the deviation angle 21°, the ice-wind correlation being 0.90. The wind factor value is slightly low, probably due to biased wind data. The optimized oceanic boundary layer parameters are: roughness length 0.05 m and displacement height (due to ice ridge keels) 5 m. The model simulations predict significant inertial oscillations in the whole ice–ocean system, but in the ice measurements, they are missing and appear as highly damped in the current measurements. The reason for this discrepancy is probably due to the internal friction of the ice. Using the ocean model forced by observed ice drift data, a better fit to the current data was achieved and the inertial oscillations in the current calculations were correctly damped. DOI: 10.1034/j.1600-0870.1990.t01-2-00007.x

Chaotic dynamics, Markov processes and climate predictability

Abstract: A systematic method for mapping deterministic chaos in general, and atmospheric and climate dynamics in particular, into a Markovian process is developed. The possibility of using the Markovian description for carrying out long-term statistical predictions is suggested, and illustrated on simple models and on data pertaining to sea surface temperature anomalies which can be reduced to one-dimensional maps. DOI: 10.1034/j.1600-0870.1990.t01-2-00001.x

The three-dimensional distribution of atmospheric heating during the GWE

Abstract: The three-dimensional global distributions of time-averaged atmospheric heating for January, April, July and October 1979 are estimated from the ECMWF GWE Level IIIb data set. Heating rates are calculated through a vertical integration of the isentropic equation of mass continuity. Estimates of the vertical variation of heating are presented in isobaric coordinates through interpolation of the vertical profiles of heating from isentropic to isobaric coordinates. The horizontal distributions of heating and vertical profiles from various climatological regimes of the planetary circulation provide insight into the four-dimensional structure of the thermal forcing of the atmosphere. The large-scale structure of the heating distributions appears spatially and temporally consistent with known features of the global circulation and the seasonal evolution. Major features of the global distributions include the heating in regions of deep moist convection over South America, equatorial Africa, the ITCZ, the Asian monsoon circulation and the oceanic cyclone tracks of the Northern Hemisphere. The primary centres of heating migrate meridionally and zonally with changing seasons. The meridional migration is linked directly with the annual variation of the latitude of maximum incoming solar radiation. The zonal migration is linked with the planetary scale distribution of continents and oceans and the land-sea surface temperature distribution as determined by the surface energy balance and energy transport within the atmosphere itself. Throughout the tropical-subtropical regions, the strongest heating occurs above 600 mb in association with deep convection. Within the primary centres of heating associated with the Asian monsoon circulation, the maximum heating in the vertical profile is near 400 mb. Over the oceanic storm tracks of the Northern Hemisphere, heating occurs through most of the troposphere. The lower tropospheric heating is maximized in January and in that month is significantly stronger than the heating in the middle troposphere. In July there is negligible heating in the lower troposphere. Profiles from the high latitude continental regions such as central Asia and Canada are in close agreement showing cooling through most of the troposphere in winter and modest heating in summer. Profiles from desert regions over the Sahara and western Australia are also in close agreement showing a transition from low level heating to middle and upper level cooling. Cooling occurs throughout the free troposphere in regions of subtropical anticyclonic circulations over the eastern North and South Pacific Oceans. DOI: 10.1034/j.1600-0870.1990.t01-1-00001.x

The time scale of land surface hydrology in response to initial soil moisture anomalies : a case study

Abstract: The sensitivity of a general circulation model to changes in the initial soil moisture distribution is illustrated through the detailed analysis of a case study. Two summer climate simulations are compared where initial soil moisture was obtained from a climatological distribution or from a previous integration of the model. In a third experiment, the climatological distribution of soil moisture is used as a fixed boundary condition throughout the integration. At low latitudes, the initial difference in the zonal mean soil moisture causes a significant difference in the soil temperature or land-surface evapotranspiration, but persists only up to 15 days. At high latitudes, the initial difference is more persistent, lasting through the end of the integration (50 days). In particular, large soil moisture differences persist over Siberia or North America during the whole integration and cause persistent temperature differences of a few degrees in the same regions. At high or middle latitudes, the soil-moisture values at the end of spring may therefore significantly influence the climate of the subsequent summer. Potential evapotranspiration determines to a large extent the characteristic time scale of the ground hydrology. DOI: 10.1034/j.1600-0870.1990.t01-1-00006.x

The role of angular momentum in the splitting of isolated eddies

Abstract: The question of which oceanic eddies can split and break up is addressed with the aid of two simplified analytical models which rely on the conservation of integrated angular momentum. First, an inviscid barotropic model with an initial round vortex is considered. The conditions necessary for the breakup of the vortex without exchanging angular momentum with its environment are examined. A solution for the final state is obtained without solving for the highly nonlinear transient splitting process. It is found that only cyclonic eddies meet the necessary condition for splitting — anticyclonic eddies can never split, no matter what their structure is . The cyclones are subject to a critical intensity above which breaking is possible and below which splitting is impossible. Specifically, cyclones with relative vorticity higher than f (where f is the (uniform) Coriolis parameter) can split into 2 eddies, whereas cyclones with a vorticity higher than f /3 can split into 3 or 4 vortices. The peculiar asymmetry between anticyclones and cyclones is a result of the conservation of integrated angular momentum. This can be demonstrated by noting that during the splitting process, the newly formed eddies are pushed away from their original center of rotation acquiring planetary torque. Therefore, in order for splitting to occur, the torque of the parent eddy must be large enough to accommodate for this addition of planetary torque. It turns out that only cyclones, which typically have more absolute angular momentum than their anticyclonic counterparts (because they rotate in the same sense as the spin of the earth), have enough torque to allow splitting. The above analysis is also applied to the splitting of a fully nonlinear zero potential vorticity lens. As in the barotropic anticyclonic cases, splitting is strictly impossible because the parent eddy does not have enough angular momentum. DOI: 10.1034/j.1600-0870.1990.t01-2-00006.x

The organization of moist convection by internal gravity waves

Abstract: A two-dimensional, Boussinesq approximated model is used to investigate the grouping of moist convection and its connection with internal gravity waves. The effect of moisture has been taken into account as a flow-dependent heating. The model sensitivity to parameters has been studied systematically. Numerical simulations are started from a state of rest with a conditionally unstable stratification. Distinct individual convective cells appear soon after small random perturbations are introduced. These cells later become organized into groups, as the equilibrium state of the model is approached asymptotically in the form of an oscillation. The oscillation of the condensational heating generates gravity waves, which are embedded in the subsidence of the convective cells and thus propagate in a stably stratified environment. The gravity waves modulate the generation of new convective cells which in turn introduce new heat sources, giving rise to additional gravity waves, and so on. If the lapse rate of the reference temperature profile is unstable, the oscillation of the condensational heating will persist for a long time and more updrafts will appear. If the dissipation is weak, the waves can propagate a long distance without being significantly damped. In these situations, strong nonlinear interactions between waves and moist updrafts are possible which leads to the grouping of convective clouds. DOI: 10.1034/j.1600-0870.1990.t01-1-00005.x

Short‐term systematic errors in global forecasts: their estimation and removal

Abstract: We have found spatial and temporal coherencies in the departures of short-range-forecasts from observed and analysed geopotential fields, which can be used diagnostically and as the basis for adjustment of forecast products. We identify characteristics of the differences between forecast and observed, and forecast and analysed values, and we demonstrate the significant impact on forecast accuracy which may be gained through the routine estimation and removal of these discrepancies. The removal of these discrepancies in the data assimilation step of the forecast cycle was found to have little impact, in contrast with the effect of direct correction of forecast products in conjunction with post processing. DOI: 10.1034/j.1600-0870.1990.00001.x

The structure and evolution of a wintertime occluded front

Abstract: A study, making use of serial radiosonde soundings, weather radar measurements and aircraft reports, has been made of the structure of a deeply occluded wintertime front. The sounding interval during the frontal passage was exceptionally short. The synoptic-scale front-relative flow could be described using the concepts of cyclone warm and cold conveyor belt flows (WCB and CCB) only in its initial stages. During the final stages of the occlusion process, the WCB became diffuse, the CCB weakened and penetrated below the occluded front, and a third relatively dry, cool, and diffluent airflow of mid-tropospheric origin was established above the occluded front. The subsynoptic structure of the occlusion was characterized by a warm frontal zone and, lying above it, a series of “warm tongues” and “cold surges” with a wavelength of about 100 km. The related circulations were also responsible for the formation of the observed banded structures in the snowfall, and agreed with the observed distributions of wind, humidity and temperature. As a whole, the warm tongues and the cold surges together with their related circulations transferred warm moist air upwards and northwards as required in the occlusion process. The observed wavelength, as well as the thermal and kinematic features of the circulations agreed well with the theory of conditional symmetric instability (CSI), though in this case, only small areas with pure CSI were found. The most significant area with CSI was found behind the occluded surface front at the top of the moist layer, while the other areas of CSI, found at higher elevations both above and on the occluded frontal surface were clearly smaller. This was considered to be related to the time history and the life-cycle of the roll circulations probably produced by the CSI. DOI: 10.1034/j.1600-0870.1990.00012.x

The dynamics of coastal troughs and coastal fronts

Abstract: Mesoscale structures in the wind and pressure fields are demonstrated by case studies. Two main types are identified. The one is characteristic for the coast of Northern Norway and is called coastal trough in this paper. The other, typical for the south-eastern coast, has similarities to the New England phenomenon named coastal front. Both occur during the winter season. The structures are quasi-permanent in certain weather conditions, which suggests a description in terms of a stationary model in two space dimensions, assuming that the variables do not change in a direction parallel to the straight coastline. The scaled equations depend on two non-dimensional numbers, which in general are small, permitting a series expansion of the perturbation variables. Solutions of zero and first order are derived, essentially by analytical methods. The size and structure of the solutions are compared to the case studies. The results seems to verify the initial hypothesis, namely that the trough is caused mainly by differential heating, while the front also depends on the discontinuity in surface friction. DOI: 10.1034/j.1600-0870.1990.t01-3-00004.x

On the effects of horizontal resolution and diffusion in a two-layer general circulation model with a zonally symmetric forcing

Abstract: The general circulation of a two-layer spectral model with a simple symmetrical forcing is determined from long-term integrations. It is found that the zonal as well as the meridional mean flow is clearly influenced by the horizontal resolution of the model. Related to that, it is also found that the meridional eddy transport of momentum is increased significantly by an increase of the horizontal resolution. As a main result of the present study, it is demonstrated, that the response of the model to an increase of the horizontal resolution can also be obtained to a considerable extent by using a suitable, strongly scale-dependent, linear diffusion in the model. Considering especially the spectral distribution of kinetic energy, it is found that an increase of the horizontal resolution as well as the use of a selective horizontal diffusion on the smaller scales results in a significant increase of the eddy kinetic energy on the large scales connected with an enhancement of the baroclinicity of the zonal mean flow. As to be expected, the effect of increasing horizontal resolution or the scale selective diffusion is also to decrease the kinetic energy on the smaller scales, with the result that the energy spectrum is approaching a – 3-power law for a wider range of wavenumbers. DOI: 10.1034/j.1600-0870.1990.t01-4-00003.x

On the asymmetry of baroclinic instability between easterly and westerly shear

Abstract: A complementary analysis of Charney-model baroclinic instability for a zonal current with easterly shear is presented. In contrast to an infinite number of unstable modes (the Charney, Green, and Burger modes) in westerly shearing currents, in easterly shearing flow, only a single unstable mode exists, corresponding to the Charney mode. For the same magnititude of vertical shear, the maximum growth rate in easterly shear is substantially smaller while the preferred wave length is considerably longer than the corresponding counterparts in westerly shear. The steering level of the most unstable mode is higher than one density scale height for easterly shear, while lower than one half density scale height for westerly shear. The asymmetry of baroclinic instability arises from the asymmetric variation of the basic state potential vorticity gradient with respect to the sign of the vertical shear due to the combined effects of the meridional variation of the Coriolis parameter and the vertical variation of density stratification. DOI: 10.1034/j.1600-0870.1990.t01-3-00005.x

Model simulations of an intense meso-β scale cyclone The role of condensation parameterization

Abstract: On 23 July 1985, an unusual mesoscale storm struck the Baltic Sea coast of Sweden. Locally, more than 50 mm of precipitation fell in 12 h and mean winds exceeding 25 m s -1 were recorded. The failure of operational models to predict this storm, as well as the damage it caused (2 people were killed), raised important questions. What physical process caused this rare event? Could it be predicted with existing data, and if so, how? Diagnostic studies reveal the following features. The development took place in a very humid air mass, which possessed weak conditional stability. The synoptic-scale environment provided large relative vorticity, which helped to spin up the mesoscale vortex. Calculations show significant adiabatic forcing initially, but then latent heat release takes over as the main driving mechanism. Numerical simulations of this storm show that it can be successfully simulated with a regional model, provided that condensation and latent heat release are treated in a consistent manner. This is demonstrated, e.g., by comparing latent heating distributions in 2 different condensation schemes. The simulations, furthermore, demonstrate the crucial role of good humidity data for this type of simulations. DOI: 10.1034/j.1600-0870.1990.00008.x