Showing papers on "Thunderstorm published in 1992"

A comprehensive meteorological modeling system?RAMS

Abstract: This paper presents a range of applications of the Regional Atmospheric Modeling System (RAMS), a comprehensive mesoscale meterological modeling system. Applications discussed in this paper include large eddy simulations (LES) and simulations of thunderstorms, cumulus fields, mesoscale convective systems, mid-latitude cirrus clouds, winter storms, mechanically- and thermally-forced mesoscale systems, and mesoscale atmospheric disperision. A summary of current RAMS options is also presented. Improvements to RAMS currently underway include refinements to the cloud radiation, cloud microphysics, cumulus, and surface soil/vegetative parameterization schemes, the parallelization of the code, development of a more versatile visualization capability, and research into meso-α-scale cumulus parameterization.

A simple lightning parameterization for calculating global lightning distributions

Abstract: A simple parameterization has been developed to simulate global lightning distributions. Convective cloud top height is used as the variable in the parameterization, with different formulations for continental and marine thunderstorms. The parameterization has been validated using two lightning data sets: one global and one regional. In both cases the simulated lightning distributions and frequencies are in very good agreement with the observed lightning data. This parameterization could be used for global studies of lightning climatology; the earth's electric circuit; in general circulation models for modeling global lightning activity, atmospheric NO(x) concentrations, and perhaps forest fire distributions for both the present and future climate; and, possibly, even as a short-term forecasting aid.

The Role of Boundary-Layer Convergence Zones and Horizontal Rolls in the Initiation of Thunderstorms: A Case Study

Abstract: The initiation of thunderstorms is examined through a combined observational and modeling case study. The study is based on Doppler radar, aircraft, mesonet, balloon sounding, and profiler and photographic data from the Convection Initiation and Downburst Experiment (CINDE) conducted near Denver, Colorado. The study examines the initiation of a line of thunderstorms that developed along a preexisting, quasi-stationary boundary-layer convergence line on 17 July 1987. The storms were triggered at the intersection of the convergence line with horizontal rolls where enhanced updrafts were present. The primary effect of the convergence line was to deepen the moist layer locally and provide a region potentially favorable to deep convection. The critical factor governing the time of storm development was apparently related to the attainment of a balance between horizontal vorticity in the opposing flows on either side of the convergence line. The effect was to cause the updrafts in the convergence line ...

A Preliminary Morphology of Precipitation Systems In Tropical Northern Australia

Abstract: A preliminary morphology of convective systems observed in the vicinity of Darwin, Australia is presented. Several types of tropical convection during monsoonal and break-season flow are identified using specific examples and compared with a range of systems observed worldwide in the general context of the bulk Richardson number. A significant spectrum of convective activity ranging from low shear and low Convective Available Potential Energy (CAPE), to high shear and moderate to high CAPE, typical of mid-latitude severe storms, is identified. Significant differences between monsoonal flow and break-season systems are apparent. the formation of convective lines perpendicular to the low-level shear and the apparent balance between advancing shallow cold pools and the shear are ubiquitous features. Convective-scale downdraughts and mesoscale descent appear to be responsible for redistribution of 700 mb momentum to the planetary boundary layer (PEL). The motion of the break-season squall lines appears to result from an equilibrium between the cold pool and the undisturbed environmental PEL flow. Deviations from this balance are hypothesized, to result in the observed ‘propagating’ and ‘non-propagating’ or slow-moving modes. Examples of these modes are given, showing the classical continuous development of new cells in a balanced state along the leading edge of long-lived squalls and, in the propagating case, the discontinuous development of new cells on the cold pool ahead of the squall.

Lightning induced brightening in the airglow layer

Abstract: This report describes a transient luminosity observed at the altitude of the airglow layer (about 95 km) in coincidence with a lightning flash in a tropical oceanic thunderstorm directly beneath it. This event provides new evidence of direct coupling between lightning and ionospheric events. This luminous event in the ionosphere was the only one of its kind observed during an examination of several thousand images of lightning recorded under suitable viewing conditions with Space Shuttle cameras. Several possible mechanisms and interpretations are discussed briefly.

Phenomena in Atmospheric and Environmental Electricity

Abstract: 1. History and Fundamentals of Atmospheric Electricity. History. Fundamentals. Stations for research in atmospheric electricity. 2. Ions - Aerosols - Air Conductivity. Sources, sinks, and properties of atmospheric ions. Small-ion number densities and conductivities, values and applicability. 3. Fair and Pseudo-Fair-Weather - Global Atmospheric Electricity. The problem of how to define fair-weather periods. Examples for real and pseudo-fair-weather. Conclusion concerning an applicable definition of real fair weather for use in atmospheric electricity. Examples of fair-weather recordings of global validity. Some reflections on the global atmospheric-electric circuit. 4. Phenomena Caused by Orography, Altitude, and Environmental Conditions Without Precipitation. Effects of orographical structures on atmospheric-electric parameters. Altitudinal effects on the atmospheric-electric parameters. Haze, mist and fog. Vigorous air flow across major mountains the Foehn and similar winds. Charge separation processes. Atmospheric-electric effects during solar eclipse. 5. Atmospheric-Electrical Phenomena Preceding and During Precipitation - Charge Separation Processes. Dynamic processes preceding precipitation. Steady precipitation from non-turbulent cloud layers. The melting-zone effect. Shower precipitation from unstable air masses and convective clouds. Quantitative interrelations between field, precipitation current density, and point-discharge current during different kinds of precipitation. The "Mirror-Image Effect". Chemical trace constituents in precipitations and their importance in atmospheric electricity. Thunderstorms, lightning, and atmospherics. 6. Solar-Terrestrial Relationships. Introduction. Phenomena of solar activity and of geophysical reactions applicable in research into solar-terrestrial relations. Solar-terrestrial relationships in atmospheric electricity and weather, a survey of historic and recent results and suggestion. Related sun-weather reactions. 7. Biological Effects of Electric and Magnetic Magnitudes in Man's Environment. Introduction. Categories of the electric and magnetic environment. List of the parameters to be considered. Biological effects of environmental electric and magnetic magnitudes. References. Subject Index.

Lightning Imaging Sensor (LIS) for the Earth Observing System

Abstract: Not only are scientific objectives and instrument characteristics given of a calibrated optical LIS for the EOS but also for the Tropical Rainfall Measuring Mission (TRMM) which was designed to acquire and study the distribution and variability of total lightning on a global basis. The LIS can be traced to a lightning mapper sensor planned for flight on the GOES meteorological satellites. The LIS consists of a staring imager optimized to detect and locate lightning. The LIS will detect and locate lightning with storm scale resolution (i.e., 5 to 10 km) over a large region of the Earth's surface along the orbital track of the satellite, mark the time of occurrence of the lightning, and measure the radiant energy. The LIS will have a nearly uniform 90 pct. detection efficiency within the area viewed by the sensor, and will detect intracloud and cloud-to-ground discharges during day and night conditions. Also, the LIS will monitor individual storms and storm systems long enough to obtain a measure of the lightning flashing rate when they are within the field of view of the LIS. The LIS attributes include low cost, low weight and power, low data rate, and important science. The LIS will study the hydrological cycle, general circulation and sea surface temperature variations, along with examinations of the electrical coupling of thunderstorms with the ionosphere and magnetosphere, and observations and modeling of the global electric circuit.

The Initiation and Organization of Convective Cells atop a Cold-Air Outflow Boundary

Abstract: On 13 July 1986 a cold-air outflow from thunderstorms over Illinois and Missouri propagated through the MIST (Microburst and Severe Thunderstorm) network over northern Alabama. The study of this outflow is important since the gust front was solely responsible for the initiation of numerous convective cells. Previous studies have documented the initiation of convection due to colliding gust fronts. In addition, there was a pronounced mesoscale organization of the cells atop the outflow boundary. This was most likely due to a combination of Kelvin–Helmholtz (K–H) and internal gravity (IG) wave activity. In contrast to previous cases, the K–H wave crests were oriented nearly perpendicular to the gust front within the analysis area. The resulting intersections between the circulations associated with the K–H waves and the gust front produced favorable locations for the initiation of convection. Subsequently, the convective cells remained along the updraft side of the K–H wave circulations as they pro...

Positive ground flashes produced by low-precipitation thunderstorms in Oklahoma on 26 April 1984

Abstract: A group of thunderstorms developed in western Oklahoma during the afternoon of 26 April 1984. Two of these storms initially exhibited characteristics of low-precipitation (LP) thunderstorms. Lightning ground flashes produced by these storms were mostly positive. These storms split, with one right-moving component evolving into a tornadic supercell. Ground flashes produced by the supercell, however, were predominantly negative. The highest rate of positive ground flashes (1.5 min−1) occurred during LP storm splitting and merging, when about 84% of ground flashes were positive. The maximum total ground-strike rate was 3.4 min−1and occurred during the tornadic supercell phase and when all but one of 136 ground flashes were negative. Analysis of lightning ground-strike and radar reflectivity data reveals a concentration of positive ground flashes in areas of maximum reflectivity within the LP storms; furthermore, the concentration of positive flashes appeared during storm split. After storm splitting...

Tropospheric Chemistry Over the Lower Great Plains of the United States 2. Trace Gas Profiles and Distributions

Abstract: Convective clouds and thunderstorms redistribute air pollutants vertically, and by altering the chemistry and radiative balance of the upper troposphere, these local actions can have global consequences. To study these effects, measurements of trace gases ozone, O3, carbon monoxide, CO, and odd nitrogen were made aboard the NCAR Sabreliner on 18 flights over the southern Great Plains during June 1985. To demonstrate chemical changes induced by vertical motions in the atmosphere and to facilitate comparison with computer model calculations, these data were categorized according to synoptic flow patterns. Part 1 of this two-part paper details the alternating pulses of polar and maritime air masses that dominate the vertical mixing in this region. In this paper, trace gas measurements are presented as altitude profiles (0–12 km) with statistical distributions of mixing ratios for each species in each flow pattern. The polar flow regime is characterized by northwesterly winds, subsiding air, and convective stability. Concentrations of CO and total odd nitrogen (NOy) are relatively high in the shallow planetary boundary layer (PBL) but decrease rapidly with altitude. Ozone, on the other hand, is uniformly distributed, suggesting limited photochemical production; in fact, nitric oxide, NO, mixing ratios fell below 10 ppt (parts per 1012 by volume) in the midtroposphere. The maritime regime is characterized by southerly surface winds, convective instability, and a deep PBL; uniformly high concentrations of trace gases were found up to 4 km on one flight. Severe storms occur in maritime flow, especially when capped by a dry layer, and they transport large amounts of CO, O3, and NOy into the upper troposphere. Median NO levels at high altitude exceeded 300 ppt. Lightning produces spikes of NO (but not CO) with mixing ratios sometimes exceeding 1000 ppt. This flow pattern tends to leave the midtroposphere relatively clean with concentrations of trace gases similar to those observed in the polar category. During frontal passage both stratiform and convective clouds mix pollutants more uniformly into the middle and upper levels; high mixing ratios of CO are found at all altitudes, and O3 levels are highest of any category, implicating photochemical production. These results illustrate the importance of convection in tropospheric chemistry. Use of average trace gas profiles or eddy diffusion parameterized vertical mixing can lead to errors of 30 to 50% in O3 and CO concentrations and an order of magnitude for odd nitrogen.

A modeling study of the time-averaged electric currents in the vicinity of isolated thunderstorms

Abstract: A thorough examination of the results of a time-dependent computer model of a dipole thunderstorm revealed that there are numerous similarities between the time-averaged electrical properties and the steady-state properties of an active thunderstorm. Thus, the electrical behavior of the atmosphere in the vicinity of a thunderstorm can be determined with a formulation similar to what was first described by Holzer and Saxon (1952). From the Maxwell continuity equation of electric current, a simple analytical equation was derived that expresses a thunderstorm's average current contribution to the global electric circuit in terms of the generator current within the thundercloud, the intracloud lightning current, the cloud-to-ground lightning current, the altitudes of the charge centers, and the conductivity profile of the atmosphere. This equation was found to be nearly as accurate as the more computationally expensive numerical model, even when it is applied to a thunderstorm with a reduced conductivity thundercloud, a time-varying generator current, a varying flash rate, and a changing lightning mix.

A three-dimensional numerical sensitivity study of convection over the Florida peninsula

Abstract: The Florida peninsula has the highest annual number of days with thunderstorms in the United States, partly due to sea breeze convergence. A three-dimensional mesoscale planetary boundary layer (PBL) model with the E-e turbulence closure is used to investigate the relationship between sea breeze convergence and convection over the peninsula for two ambient wind cases during typical summer days.

Temporal and Spatial Relations between Hail and Lightning

Abstract: A study of very detailed spatial and temporal data on damaging hail, associated cloud-to-ground (CG) lightning, and storm echoes in the Midwest was pursued to define their relationships and gain insight on their formation processes. Lightning activity was always closely associated with hailstreaks: the surface embodiment (4.5 km long by 1.3 km wide) of a single volume of hail generated aloft. Lightning seldom occurred where the hail fell and generally formed and moved forward in a 10–15-km2 area on either the left or right flank of the hailstreaks. These lightning centers, areas with CG flashes closely associated with hail, typically developed 9 min before hail at a point 5 km upstorm from first hail, suggesting that CG flashes began as the hailstones were developing aloft. The centers then grew in areal coverage and flash frequency until hail began and diminished shortly after hail ended, with a duration of 26 min. The hailstorm's severity was found to be well correlated to the rate of flashing ...

Three-dimensional simulation of the Denver 11 July 1988 microburst-producing storm

Abstract: Near Denver on July 11 1988, a moderate-reflectivity thunderstorm produced a microburst of unusual intensity during the test operation of the Terminal Doppler Weather Radar (TDWR) system. This microburst-producing storm, unlike those previously investigated, contained a complex updraft structure and an intense microburst downstream from the main precipitation shaft. Of special significance is that several successive airliners inadvertently encountered the microburst during final approach to Stapleton Airport. This paper focuses on the numerical investigation of this storm-via simulation with the Terminal Area Simulation System (TASS). Evolution and structure of the storm, including hazard indices based onF-factor, are presented and compared with “observed” data, including that measured by Doppler radar and aircraft flight data recorders.

Electricity in dying thunderstorms

Abstract: We made one balloon sounding of the electric field in each of two dying thunderstorms. Both balloons were launched 20 min after the storm's last lightning flash. Each sounding revealed substantial residual electrification. The peak magnitudes of electric field in the two dying storms were 35 and 71 kV/m. The charge structure in both storms appeared to consist of one internal negative charge region with positive screening layers at the upper and lower cloud boundaries. One or two negative charge regions were found below cloud base. The main charge regions had inferred charge densities of about 1 nC/m3 and were hundreds of meters thick.

Early Severe Thunderstorm Forecasting and Research by the United States weather Bureau

Abstract: In the early nineteenth century, fragmentary weather observing networks were established in portions of the United States, primarily to constitute a climatological history The invention of the telegraph in 1833 and its availability to the public in 1845 eventually led to a nationwide observation network with the formation of a national weather service in 1870 within the Signal Service of the US Army While Lt J P Finley of the Signal Service was developing techniques for the prediction of tornadoes, others, as Prof Henry A Hazen (also of the Signal Service) and the New England Meteorological Society, were studying the thunderstorm in hope of developing predictors for the forecasting of these events The thunderstorm studies were carried over to the civilian US Weather Bureau in 1891, but interest in thunderstorm and tornado research wanted as a ban on the use of the word “tornado” in forecasts, inherited from the Signal Corps, was carried into the twentieth century The growth of the a

The North Dakota Thunderstorm Project: A Cooperative Study of High Plains Thunderstorms

Abstract: The North Dakota Thunderstorm Project was conducted in the Bismarck, North Dakota, area from 12 June through 22 July 1989. The project deployed Doppler radars, cloud physics aircraft, and supporting instrumentation to study a variety of aspects of convective clouds. These included transport and dispersion; entrainment; cloud-ice initiation and evolution; storm structure, dynamics, and kinematics; atmospheric chemistry; and electrification. Of primary interest were tracer experiment that identified and tracked specific regions within evolving clouds as a means of investigating the transport, dispersion, and activation of ice-nucleating agents as well as studying basic transport and entrainment processes. Tracers included sulfur hexafluoride (SF6), carbon monoxide, ozone, radar chaff, and silver iodide. Doppler radars were used to perform studies of all scales of convection, from first-echo cases to a mesoscale convective system. An especially interesting dual-Doppler study of two splitting thunder...

The relation of radar to cloud area-time integrals and implications for rain measurements from space

Abstract: The relationships between satellite-based and radar-measured area-time integrals (ATI) for convective storms are determined, and both are shown to depend on the climatological conditional mean rain rate and the ratio of the measured cloud area to the actual rain area of the storms. The GOES precipitation index of Arkin (1986) for convective storms, an area-time integral for satellite cloud areas, is shown to be related to the ATI for radar-observed rain areas. The quality of GPI-based rainfall estimates depends on how well the cloud area is related to the rain area and the size of the sampling domain. It is also noted that the use of a GOES cloud ATI in conjunction with the radar area-time integral will improve the accuracy of rainfall estimates and allow such estimates to be made in much smaller space-time domains than the 1-month and 5-deg boxes anticipated for the Tropical Rainfall Measuring Mission.

Some Aspects of Forecasting Severe Thunderstorms during Cool-Season Return-Flow Episodes

Abstract: Historically, the Gulf of Mexico has been considered a primary source of water vapor that influences the weather for much of the United States east of the Rocky Mountains. Although severe thunderstorms and tornadoes occur most frequently during the spring and summer months, the periodic transport of Gulf moisture inland ahead of traveling baroclinic waves can result in significant severe-weather episodes during the cool season. To gain insight into the short-range skill in forecasting surface synoptic patterns associated with moisture return from the Gulf, operational numerical weather prediction models from the National Meteorological Center were examined. Sea level pressure fields from the Limited-Area Fine-Mesh Model (LFM), Nested Grid Model (NGM), and the aviation (AVN) run of the Global Spectral Model, valid 48 h after initial data time, were evaluated for three cool-season cases that preceded severe local storm outbreaks. The NGM and AVN provided useful guidance in forecasting the onset of ...

The cloud flash‐to‐ground flash ratio and other lightning occurrence characteristics in Kathmandu thunderstorms

Abstract: Lightning and thunderstorm observations have been made in Kathmandu, Nepal (274°N, 852°E) over the 21-month period March 1987 to November 1988 inclusive, using direct observations of thunder and lightning, and the registrations of a CGR3 lightning flash counter that provided records of the approximate numbers of cloud flashes, positive ground flashes, and negative ground flashes with an effective range of about 14 km The observation period effectively covered two thunderstorm seasons as almost no lightning occurred during the winter months During 1987 and 1988 the mean thunderdays per year was 42 The principal use of the CGR3 registrations was to estimate the long-term ratio of cloud flashes to ground flashes, denoted z; it was found that z was 21 in 1987 and 30 in 1988, with a two-season mean of 27 The uncertainty in ratio values is about 30% The registrations also enabled lightning flash densities to be estimated with an uncertainty of about 40% The two-season mean ground flash, cloud flash, and total flash densities were 24, 66, and 90 km−2 yr−1, respectively There were well-marked seasonal variations in occurrence, with about 82% of all lightning occurring between mid-February and the end of May During this premonsoon period there were steady rises to maximum values in May of the thunderdays per month (to 10), the monthly total flash density (to 29 km−2 month−1), and the monthly value of z (to 36) The range of values of z calculated from the records for individual thunderstorm days was 017 to infinity, but very little lightning was associated with the extreme values A distribution of values of the ratio with respect to cumulative lightning occurrence shows that 80% of all lightning was associated with values of z (calculated on a thunderstorm day basis) between 13 and 67 There is reasonably good agreement between the value of z for Kathmandu and values of z for other places at similar latitudes However, major differences have been found for low latitudes between recently measured values of z and earlier predictions of the latitudinal variation of z

The Evolution and Structure of a ``Bow-Echo Microburst'' Event. Part I: The Microburst

Abstract: On 14 July 1982, a comprehensive multi-Doppler radar dataset was collected during the life cycle of an intense microburst-producing thunderstorm during the Joint Airport Weather Studies (JAWS) Project. This is believed to be one of the first attempts to study the temporal and spatial evolution of an entire microburst-producing thunderstorm. In addition, the radar echo of the parent storm evolved into a bow-shaped echo, thus providing the first detailed dataset on this phenomenon. This microburst was numerically simulated using Srivastava's one-dimensional downdraft model in an attempt to quantify microphysical processes and downdraft propagation in the subcloud layer. Analysis of the stormwide structure using a standard multi-Doppler kinematic analysis technique and the Gal-Chen thermodynamic retrieval technique is stressed in this paper, which is Part 1 of a two-part study. Part II examines the evolution of the stormwide vorticity during the formation of the bow echo. The microburst downdraft wa...

Analysis and numerical modelling of a frontal passage associated with thunderstorm development over the Po valley and the Adriatic sea

Abstract: A case study is presented of the meteorological situation of 26 July 1987, when a moderate lee cyclone formed south of the Alps associated with a cold front moving from the northwest. A convective line developed over the Po valley and the northern Adriatic sea, slightly in advance of the position of the cold front which, over that area, was associated with a north-easterly low-level flow. The mesoscale situation over the Po valley before and after the frontal passage was complex, with different flow configurations and weather phenomena affecting the eastern and western portions of the region. The analysis of observations has been complemented by the results of numerical experiments run with a limited area, high resolution model, initialized using interpolated ECMWF analyses. Sensitivity experiments show the important roles played by the orography and by the condensation-evaporation processes in determining the mesoscale field environment favourable for the development of the organized convection.

Impacts of a Destructive and Well-Observed Cross-Country Winter Storm

Abstract: A winter storm that crossed the continental United States in mid-February 1990 produced hazardous weather across a vast area of the nation. A wide range of severe weather was reported, including heavy snowfall; freezing rain and drizzle; thunderstorms with destructive winds, lightning, large hail, and tornadoes; prolonged heavy rain with subsequent flooding; frost damage to citrus orchards; and sustained destructive winds not associated with thunderstorms. Low-end preliminary estimates of impacts included 9 deaths, 27 injuries, and $120 million of property damage. At least 35 states and southeastern Canada were adversely affected. The storm occurred during the field operations of four independent atmospheric research projects that obtained special, detailed observations of it from the Rocky Mountains to the eastern great Lakes.

Numerical Simulation of a Low-Precipitation Supercell Thunderstorm

Abstract: Numerical simulations of thunderstorms using initial conditions from an Oklahoma storm event on 26 April 1984 have been carried out using the Klemp-Wilhelmson cloud model Two thermodynamic and two horizontal wind profiles are mixed to create four initial storm environments The two initial thermodynamic profiles are designated as moderately and highly unstable Both hodographs have considerable vertical wind shear, with the extremely unstable profile having substantially stronger storm-relative curvature shear in the lowest few kilometers, as measured by the helicity of the environment Storms are initialized with two different temperature-perturbation warm bubbles A variety of storms, qualitatively covering the range observed on this day, are simulated With a strongly sheared, moderate instability environment, the two different temperature-perturbation simulations differ qualitatively, unlike the other pairs of simulations Of particular interest is the simulation carried out with the weaker temperature-perturbation It displays many of the characteristics of observed low-precipitation (LP) supercell storms Without artificially suppressing rainfall, this storm produces very little precipitation over the first 6000 s of its lifetime During this time, there is no strong downdraft at the lowest model level and vorticity at that level is much less than the values aloft We hypothesize that LP storms can be generated by smaller initial impulses than “classic” supercells in environments typically associated with a supercell storm, as speculated by Bluestein and Parks (1983), since they are more susceptible to turbulent entrainment in highly sheared environments

Initiation and Evolution of Updraft Rotation within an Incipient Supercell Thunderstorm

Abstract: One of the distinguishing characteristics of a supercell thunderstorm is the presence of a rotating updraft. During the past 30 years, various hypotheses have been proposed to explain the initiation and maintenance of rotation. However, attempts to verify the initiation process have been frustrated by the lack of multiple-Doppler radar measurements at the time that the first rotating updraft appears. Discussed in this paper are dual-Doppler radar measurements that successfully captured the initiation and evolution of rotation in the Agawam, Oklahoma, storm of 6 June 1979, which occurred during the storm-scale phase of the Severe Environmental Storms and Mesoscale Experiment (SESAME). The process leading to updraft rotation appears to follow that proposed in 1968 by Fujita and Grandoso, whereby a middle-altitude vorticity couplet formed on the downwind flanks of a strong nonrotating updraft, with cyclonic vertical vorticity on the right-forward flank and anticyclonic vertical vorticity on the left...

High-resolution numerical modelling of convective precipitation over Northern Italy

Abstract: A number of forecast experiments were performed in order to assess the capability of reproducing, by means of a limited-area numerical model, the highly structured mesoscale circulations occurring in the Po Valley of Northern Italy during a north-westerly cold front passage across the Alpine chain, with particular attention to the modelling of the effects of organized convection. The case-study occurred during summer 1987 and the model used throughout was the 1989 version of the UB/NMC Limited Area Model (University of Belgrade, National Meteorological Centre of Washington). The model was integrated both with eta, η, and sigma, ϑ, as vertical coordinates and ECMWF initialized analyses were always used as initial conditions. ECMWF initialized analysis or operational forecast fields were also used for updating in time the lateral boundary conditions. Experiments show qualitative and quantitative agreement with observations, both in upper-air geopotential height fields, in MSLP and in cumulated precipitation. Several modelling issues were also investigated, e.g. sensitivity of the results to horizontal and vertical model resolution and to the influence of the lateral boundaries poitioning, finding large effects of the latter on quantitative precipitation fields. Difficulties in modelling very localized mesoscale phenomena, e.g. organized convective thunderstorms in the Po Valley and Alpine North Foehn in the Milan area, were generally encounted.