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A severe bora episode which lasted from 22 to 25 July 2012 has been selected for the 3D numerical model analysis of the start of the bora near the Oštarijska Pass in the Northern Adriatic of Croatia. The 10-m wind analysis shows that the gap flow turns and rotates in the central part of the Vir Sea at the start of bora. The MM5 model validation shows that the modelled wind matches well the automatic stations measurements and the validation of the vertical profiles shows that the model adequately captured the basic thermodynamic and flow structure during the bora episode. The vertical cross sections show the turning of the horizontal wind in the first 500–800 m over a relatively small area over the Vir Sea. The analysis implies that the wind turning can be attributed to the impact of the Etesian windflow, the frontal passage and the positions of the outer islands and coastal area. The analysis of the horizontal vorticity of the surface wind shows vorticity banners downwind of the gap with high vorticity values, up to 4 × 10−5 s−1, in the central part of the Vir Sea. A secondary northeastern vorticity maximum is developed due to the orography of the coastal area and the rotational windflow.

This paper examines the synoptic control of precipitation contrasts across the Cascade Mountains of Washington State during the cool season. The average hourly precipitation for three stations on the western and eastern sides of the barrier is calculated, and composite synoptic conditions are determined for situations when precipitation is greater over the eastern slopes, similar precipitation falls on both sides, moderately greater precipitation falls on the western side, and substantially greater precipitation falls on the western side. Conditions prior to occluded/warm frontal passage are favorable for relatively equal precipitation amounts on both sides of the barrier, with extensive warm advection precipitation spreading over the region. Low-level winds are generally southerly during this period, resulting in less orographic modulation of precipitation on the north–south-oriented Cascades. In contrast, postfrontal conditions are associated with westerly flow, enhanced windward precipitation, and greater drying on the leeward (eastern) slopes. Postfrontal conditions also bring drying aloft and reduced stability at low levels, both of which contribute to the enhancement of precipitation on the western slopes. The variation of the cross-mountain precipitation ratio between El Niño and La Niña years can be explained by the associated synoptic configurations. El Niño years have weaker zonal winds and thus less orographic modulation. Furthermore, the lesser vertical stability of La Niña years enhances precipitation on the windward side of the barrier where shallow convection is released.

Surface measurements and a regional dust model were used to analyze the source, transport, and contribution of a dust event transporting with aerosol pollutant over Taiwan from 16 to 19 March, 2006. During the event, the hourly aerosol concentrations reached close to 400 μg m(-3) in northern Taiwan, and approximately 300 μg m(-3) in other areas of the island. Trajectory and regional dust models show that the dust event originated in eastern Mongolia and northern China, and the dust layer can descend from 2 to 3 km in the source area to below 1.5 km over Taiwan. On the other hand, model results show that pollution was transported near the surface from coastal China to Taiwan. During this dust event, polluted aerosol was first observed over northern Taiwan right after a frontal passage, and the concentration was strongly enhanced following the passage of the light rainfall 12h later. The descent of dusty air from the free troposphere lagged the arrival of polluted air by 7h, and was partially mixed with polluted aerosol when the transport decelerated over Taiwan. During the event, dust particles accounted for up to 60% of observed particulate matter less than 10 μm (PM10) over Taiwan, but decreased to less than 35% for particulate matter less than 2.5 μm (PM2.5) over most areas of the island. On the other hand, the long-range transport of non-dust aerosols, mainly anthropogenic pollutants, accounted for close to 30% of observed PM10 concentration in northern and western Taiwan prior to dust arrival, and the contribution of PM2.5 increased to close to 40% over the same areas. Local emission of aerosols accounted for less than 25% of PM10 concentrations in northern Taiwan, but was about 60% for PM2.5 in central and southern Taiwan because these areas are less influenced by long-range transport.

Rainfall patterns in the San Francisco Bay Area (SFBA) are highly influenced by local topography. It has been a forecasting challenge for the main US forecast models. This study investigates the ability of the Weather Research and Forecasting (WRF) model to improve upon forecasts, with particular emphasis on the rain shadow common to the southern end of the SFBA. Three rain events were evaluated: a mid-season atmospheric river (AR) event with copious rains, a typical non-AR frontal passage rain event, and an area-wide rain event in which zero rain was recorded in the southern SFBA. The results show that, with suitable choices of parameterizations, the WRF model with a resolution around 1 km can forecast the observed rainfall patterns with good accuracy, and would be suitable for operational use, especially to water and emergency managers. Additionally, the three synoptic situations were investigated for further insight into the common ingredients for either flooding rains or strong rain shadow events.

A three-dimensional photochemistry model is applied to analyze three unusual high O 3 episodes that occurred continuously in Taiwan from May 1 to 11, 2007. During this period, the first high ozone episode was observed in northern and central western Taiwan on May 4. Following this episode, the second episode was observed throughout Taiwan on May 7, and continued for several days until May 10, when the third episode was observed in central western and southwestern Taiwan. This was the worst O 3 period in Taiwan over the past two decades. During this period, 70% of the local air quality station reported an O 3 concentration of over 120 ppb, and a maximum O 3 concentration of 175 ppb was observed in background Taiwan. Based on model analyses, the sources of the three high O 3 episodes differ. The high O 3 concentration observed during the first episode is mainly attributed to the northeastward transport of O 3 precursors and concentrations from northern and central western Taiwan under southwesterlies prior to frontal passage, chemically producing O 3 over 30 ppb h −1 in northern Taiwan. During the second episode, horizontal advection of Asian outflow during the passage of an anticyclone and front contributes a maximum of 25 ppb h −1 in both northern and southern tip of Taiwan, respectively, increasing to more than 75 ppb h −1 in southwestern Taiwan because of the combination with the local source. During the third episode when the prevailing easterlies associated with a departing anticyclone to the east of Taiwan is blocked by the high central mountain, local O 3 chemical production and horizontal transport contribute a maximum of 25 ppb h −1 and 35 ppb h −1 in central and southwestern Taiwan, respectively, under a calm and high background O 3 condition. Source from Taiwan contributes more than half of the high O 3 concentration over northwestern Taiwan prior to frontal passage during the first episode, but decreases to mostly below 40% over western Taiwan during the second episode when Asian outflow occurs, and increases to more than 60% over central and southwestern Taiwan due to pollution accumulation under an anticyclonic departure.

This paper examines the springtime cloud properties in the Taiwan Strait (TS), with emphases on their dependence on synoptic controls and local processes, using a suite of in situ and remote sensing observations. Cloud properties in the TS are inferred from a combination of MODIS and in situ observations and further classified into two synoptic conditions: continental cold air surge and frontal system. The study reveals a predominance of synoptic-scale controls in regulating the cloud properties in the TS. The sensitivity of clouds to the local thermodynamic mechanisms as well as the underlying surface conditions is fundamentally dependent on synoptic-scale flow patterns. The springtime clouds over the TS are commonly a mixture of stratocumulus and alto clouds. More precisely, there is a preponderance of stratocumulus over the strait. A preferential occupancy of stratiform alto clouds is recognized during cold air surge, whereas vertical development of cloud layers (mostly the stratocumulus) is commonly observed with frontal passage. The most distinct difference between the local clouds formation associated with the two synoptic conditions is the suppression of very low cloud and fog along with cold air surge. Stratus clouds and fog are present within the northward prefrontal airflow from warmer to colder water sites, along with an increase in stability relating to lower altitudes of boundary layer clouds. Although the rainfall occurrences are about the same for both synoptic conditions, the frontal rain amounts are larger on average.

The evolution of an undular bore and its associated wind shift, spawned by the passage of a shallow surface cold front over the Southern Great Plains of the United States, is examined using surface and remote sensing observations along with output from a high-resolution numerical model simulation. Observations show that a separation between the wind shift and thermodynamic properties of the front was induced by the formation of a bore over south-central Kansas around 0200 UTC 29 November 2006. By the time the front–bore complex passed through Lamont, Oklahoma, approximately 4 h later, the bore had reached its maximum intensity and its associated wind shift preceded the trailing baroclinic zone by 20 min. Within several hours the bore decayed and a cold frontal passage, characterized by a wind shift coincident with thermodynamic properties was observed at Okmulgee, Oklahoma. Thus, a substantial transformation in both the structural and dynamical characteristics of the bore as well as its relationship to the parent surface front occurred during a short period of time. The details of this evolution are examined using output from a finescale numerical simulation, performed using the Weather Research and Forecasting (WRF) model. Analysis of the output reveals that as the bore advanced southeastward it moved into a region with a weaker surface stable layer. Consequently, the wave duct that had supported its maintenance steadily weakened resulting in dissipation of the bore. This circumstance led to a merger of the surface temperature and moisture boundaries with the orphaned wind shift, resulting in the cold frontal passage observed at Okmulgee.

This paper begins with a review of basic surface frontogenesis concepts with an emphasis on fronts located over sloping terrain adjacent to mountain barriers and fronts located in large-scale baroclinic zones close to coastlines. The impact of cold-air damming and differential diabatic heating and cooling on frontogenesis is considered through two detailed case studies of intense surface fronts. The first case, from 17 to 18 April 2002, featured the westward passage of a cold (side-door) front across coastal eastern New England in which 15°–20°C temperature decreases were observed in less than one hour. The second case, from 28 February to 4 March 1972, featured a long-lived front that affected most of the United States from the Rockies to the Atlantic coast and was noteworthy for a 50°C temperature contrast between Kansas and southern Manitoba, Canada. In the April 2002 case most of New England was initially covered by an unusually warm, dry air mass. Dynamical anticyclogenesis over eastern Canada set the stage for a favorable pressure gradient to allow chilly marine air to approach coastal New England from the east. Diabatic cooling over the chilly (5°–8°C) waters of the Gulf of Maine allowed surface pressures to remain relatively high offshore while diabatic heating over the land (31°–33°C temperatures) enabled surface pressures to fall relative to over the ocean. The resulting higher pressures offshore resulted in an onshore cold push. Frontal intensity was likely enhanced prior to leaf out and grass green-up as virtually all of the available insolation went into sensible heating. The large-scale environment in the February–March 1972 case favored the accumulation of bitterly cold arctic air in Canada. Frontal formation occurred over northern Montana and North Dakota as the arctic air moved slowly southward in conjunction with surface pressure rises east of the Canadian Rockies. The arctic air accelerated southward subsequent to lee cyclogenesis–induced pressure falls ahead of an upstream trough that crossed the Rockies. The southward acceleration of the arctic air was also facilitated by dynamic anticyclogenesis in southern Canada beneath a poleward jet-entrance region. Frontal intensity varied diurnally in response to differential diabatic heating. Three types of cyclogenesis events were observed over the lifetime of the event: 1) low-amplitude frontal waves with no upper-level support, 2) low-amplitude frontal waves that formed in a jet-entrance region, and 3) cyclones that formed ahead of advancing upper-level troughs. All cyclones were either nondeveloping or weak developments despite extreme baroclinicity, likely the result of large atmospheric static stability in the arctic frontal zone and unfavorable alongfront stretching deformation. Significant frontal–mountain interactions were observed over the Rockies and the Appalachians.

Migratory birds were surveyed at stopover sites in New Orleans, Louisiana, during seven spring and fall seasons in relation to weather conditions. Weather was classified into four synoptic scenarios distinguished primarily by positions of fronts, wind directions, and pressure characteristics. Overall numbers of migrants in spring were more numerous during the synoptic scenarios in which a cold front was about to arrive (Frontal Gulf Return) or had already passed (Post Frontal), than when airflow was off the Gulf or from the east (Coastal Return, which often preceded frontal passage). Counts of White-eyed (Vireo griseus) and Red-eyed vireos (V. olivaceus), Indigo Buntings (Passerina cyanea), and thrushes (Hylocichla and Catharus combined) showed differences among weather scenarios, mostly with high counts under Post Frontal conditions and/or low counts under Coastal Return. Post Frontal values were higher in fall than Coastal Return for overall migrants, Common Yellowthroats (Geothlypis trichas), ...

During 8–14 June 2000, a 500-hPa blocking event occurred over Mongolia and northern China (near 45°N, 108°E), which was the only case over this region in June since 1981. As the block developed, the initially weak low-level mei-yu front over southern China evolved into a system with strong baroclinity and subsequently moved south. The frontal passage over Taiwan caused temperatures to drop by 10°C, the largest in June over two decades. Using gridded analyses, manually analyzed weather maps, and satellite and surface data, the present study investigates the evolution of this mei-yu front under the influence of the block. The 925-hPa frontogenetical function is computed and effects of different processes are discussed. As the blocking event developed, concurrent ridge–trough amplification in the lower–midtroposphere produced a reversed thermal pattern. The lower-tropospheric high moved southward, and large-scale confluence and deformation were enhanced between the northerly flow and the prefrontal southwesterly flow. The location of the block, to the west-southwest of the Okhotsk Sea area, allowed it to affect the front over southern China and caused it to penetrate inside 20°N, unusual for the month of June. The distribution of the frontogenetical function indicated that the mei-yu frontogenesis and the maintenance of the front were attributed to both deformation and convergence. These two processes together counteracted the strong frontolysis along the frontal zone from diabatic effects, caused by evaporative cooling of frontal precipitation on the warm side and stronger sensible heat transfer (and daytime heating over less cloudy areas) on the cold side of the front. When deformation, convergence, and diabatic effects were all combined, the net total frontogenesis peaked slightly ahead of the frontal zone, thus contributing to the southward propagation of the front in addition to the advection by postfrontal cold air in the present case. When the front moved into the South China Sea, the cross-frontal thermal gradient diminished rapidly, mainly due to the frontolytic effect from sensible heat flux over warm waters.

A case of snow fall in the plains of the Northern Italian region Veneto is presented from a forecasters' perspective. Contrasting forecast guidance came from the ECMWF global model and the limited area model LAMI. The former showed a marked warm-moist Sirocco flow coming from the Adriatic Sea onto the coast at all levels, the latter discerned a distinct cold air flow from the north-east along the foothills of the Alps. The integrated observing network of the Centro Meteorologico di Teolo ARPA Veneto revealed this cold-air structure and helped the forecaster in the choice of the forecast and underpin the snowfall alert to the road authorities. It is argued that this feature is a crucial element for the occurrence of snowfall over the Veneto plains, and that the high-resolution numerical weather prediction model was essential in describing this mesoscale feature. The nature of the north-easterly flow is thought to be a combination of a Bora like flow and a barrier jet induced by flow blocking by the Alps.

Quantitative measurements of the impact of an urban environment on the thermal state of the atmospheric boundary layer are presented. Temperature profiles up to the height of 600 m were obtained in a continuous series of measurements by three microwave profilers MTP-5 located in different areas of Moscow. The influence of this large city on urban heat island (UHI) parameters was estimated on occasions with stationary atmospheric processes and during cases with frontal passage. Two types of UHI were identified: one with a dome of urban warmth at all levels, and another with a low warm dome in combination with a lens of cold air above.

This study describes a simple objective method to identify cases of coastal frontogenesis offshore of the Carolinas and to characterize the sensible weather associated with frontal passage at measurement sites near the coast. The identification method, based on surface hourly data from offshore and adjacent land stations, was applied to an 11-yr dataset (1984–94). A total of 379 coastal fronts was found, 70 of which eventually made landfall along the North Carolina coast; 112 that remained offshore, and 197 were termed diurnal since they remained offshore but disappeared during daylight hours. Results show that most coastal and offshore sites experience a wind shift of about 40°–70° and a warming of about 2°–3°C during the hour of frontal passage. Exceptions include sites near colder waters where the rates are markedly reduced and frontal passage is often less discernible. Excluding diurnal fronts, just over half the cases were associated with cold-air damming (CAD) during the cold season of 16 October–15 April. Most of these winter cases linked with CAD were onshore fronts. During the warm season, most fronts were diurnal, but the association with CAD was still significant. To explore the synoptic-scale environment, composite maps for the cold season were generated for all three frontal subtypes from NCEP–NCAR reanalysis data. Results show a strong surface anticyclone centered north of the region of frontogenesis for all three composites. However, several features in the synoptic-scale regimes appear to differentiate the three frontal types. For example, cyclogenesis in the Gulf of Mexico and onshore southeasterly low-level flow along the southeast Atlantic coast accompanied by warm advection distinguish onshore fronts from the other two types. The offshore fronts are accompanied by more nearly zonal flow aloft and a surface anticyclone that stalls near the New England coastline. Finally, the diurnal type is associated with much weaker pressure and height fields and an east–west elongated surface anticyclone centered much farther south than in the other cases.

This paper examines the flow conditions associated with frontal widespread and moderate precipitation over the Lago Maggiore Target Area (LMTA) of the Mesoscale Alpine Programme, and observed during the intensive observation periods (IOPs) 4 (30 September 1999), 8 (21 October), 9 (23 October) and 15 (6 November). The flow organization that led precipitation maxima to occur away from the relief instead of being concentrated on the windward southern slopes of the Alps is investigated from three ground-based Doppler radars.

The local front observed over the Kanto plain on December 5, 1996 has been analyzed as an example of one of those that ofter appear when a synoptic-scale low advances in the Japan Sea in early winter. Winds in the lower layer, blowing from the northern and western mountains since the previous night, supplied cold air to the middle of the plain. Before dawn, synoptic-scale southerly winds intensified from the south of the plain, and the local front formed along the southern margin of the cold air mass. The southerly winds over the lifted frontal surface entrained the cold air from the lower layer and transported it northward in a transition layer characterized by relatively cold southerly winds. The depths of the transition and lower layers tended to be maintained above the inland areas north of the front. The front, advanced northward, corresponding to the surplus of the northward flow in the transition layer over the cold air supply in the lower layer. As a result of the persistent cold air flow, even during the daytime, the front became stationary over the middle of the plain in the afternoon. Additionally, a very shallow (∼ 50 m) structure of the frontal head existed prior to the frontal passage through the central area of Tokyo.

ABsTRAcr.--Cold-front passage has long been associated with south-bound raptor migration in northeastern North America. We used Hawk Mountain Sanctuary's 55-year database to calculate abundance indices of 14 raptor species at the site. These indices, together with data taken from coincidental U.S. Department of Commerce daily weather maps, were used to investigate the extent to which raptor migration at Hawk Mountain is associated with the passage of cold fronts. Daily abundance indices for 12 of 14 raptor species exhibited significant increases during one or more of the three days following frontal passage. Three basic patterns emerged, which differed in the number of days after frontal passage on which peak migration occurred. The patterns fit the flight behavior of the species involved. We failed to detect a long-term linear trend in numbers of fall-season cold fronts at the site. Stepwise multipleregression analyses revealed that annual-abundance indices were not influenced by annual variation in the numbers of cold fronts passing the Sanctuary at the time of fall migration. Overall, our analyses confirm the results of earlier, short-term studies demonstrating withinyear effects of frontal passage on raptor migration. Our failure to detect among-year effects of frontal passage rates on annual counts of raptors migrating past the site supports the notion that cold fronts enhance fall migration rather than simply making the birds more visible to observers at such times by forcing them closer to the watch site. Received 6 December 1994, accepted 25 April 1995.

The detailed characteristics of a CASPII warm frontal passage are presented in this article. This storm, Intensive Operating Period (IOP) 13 (February 26–27, 1992), was observed in detail with an array of diverse instruments. It has the advantage over earlier freezing precipitation studies of having simultaneous, in situ and remote sensing measurements by aircraft and ground‐based Doppler radar. The associated precipitation was in the form of banded structures parallel to the front. Within these bands were embedded precipitation cores, some parallel to the band, some perpendicular. The warm front itself was characterized by major perturbations in its kinematic and thermodynamic features. The cores oriented parallel to the front were the result of embedded convection generated, at least in part, by the irregularities in the frontal surface. The cores oriented perpendicular to the front were closely associated with the 0°C isotherm on the underside of the frontal inversion. Precipitation phase chan...

Principal component (PC) analysis was used to study the wind and sea level pressure patterns over the island of Taiwan during the early summer rainy season. An eight-point, running-mean, low-pass filter was applied to the 3-h surface winds and sea level pressure deviations from its areal average to remove signals with a time scale of 1 day or less. The first and the second PC modes of the low-pass-filtered wind data account for 66% and 13% of the total variance, respectively. The first, the second, and the third PC modes of the low-pass-filtered pressure deviations account for 63%, 22%, and 10% of the total variance, respectively. Composite analysis of low-pass-filtered surface variables was made during a 3-day period for each TAMEX (Taiwan Area Mesoscale Experiment) frontal passage episode, using the timing of the maximum PC scores of the dominant PC modes to arrive at a general picture of the evolution of surface airflow and pressure patterns. The prefrontal southwesterly flow reached the maxim...

In this study, 500 frontal and supraorbital transcranial passages were studied in 50 cadavers and 200 crania of 500 samples. One hundred six specimens had a frontal foramen (notch), and all the samples had a supraorbital foramen (notch). The frontal passage was a foramen in 7 samples and a notch in 99 samples. As for the supraorbital passage, it was found as a foramen in 133 of specimens and as a notch in 358 of specimens. In 9 of the specimens, there were double notches or foramina. The distances from the foramina (notches) to angulus oculi medialis were measured in the cadavers. The average distance from the angulus oculi medialis to the frontal foramen (notch) was 4.50 mm, and the average distance to the supraorbital foramen (notch) was 9.87 mm. The distances from the foramina (notches) to the midline were measured in the crania. The average distances from the midline to the frontal foramen (notch) and the supraorbital foramen (notch) were 20.24 mm and 25.23 mm, respectively. The average distance between the frontal foramen (notch) and supraorbital foramen (notch) was 5.37 mm in cadavers and 4.99 mm in crania. In 200 crania, the distances of the frontal and supraorbital transcranial passages to the midline were measured. Types of these passages were also evaluated, and frequencies were calculated. Measurements were made using a digital compass, and the student t test was used in the statistical evaluation of results.

The relative influences of accretional ice crystal growth (riming) and frontal passages on winter precipitation chemistry were evaluated in level terrain in east central Illinois during the winter of 1992. Six precipitation events were analyzed during the period January to March, 1992. Five events were characterized by the passage of cold Arctic fronts and no riming was observed. Precipitation chemistry was observed to change substantially during frontal passage, reflecting the different pollutant loadings in pre- and post-frontal air masses. Precipitation that fell on March 11, 1992, associated with a warm or occluded frontal system, was moderately rimed. Application of a linear multiple regression model revealed that most of the temporal variations in precipitation chemistry during this event could be explained by changes in the degree of riming and changes in the identity of the air mass producing precipitation above the site. Accreted ice crystal mass was dominated by cloud drops with diameters between 20 and 40 μm, indicating that the composition of drops in this size range exert the most influence on precipitation chemistry.

This study focuses on cold-season (October-April) precipitation events at Iqaluit, Nunavut, that exceed the 97th percentile of daily corrected precipitation accumulation. This corresponds to a threshold of 9.5 mm. The characteristics of 194 major precipitation events are described for the 1955 - 96 period. On the basis of NCEP-NCAR and NARR reanalysis data, these events were classified into four storm tracks: south, west, Atlantic, and other. South- and Atlantic-originating systems tended to be associated with the most severe events. The duration and precipitation rate of the events, rather than the speed of motion, were critical factors influencing precipitation accumulation. Snow was the dominant precipitation type. Surface warm frontal passage was a common tropospheric feature, and the topography was important in terms of altering surface wind direction during the events. No significant trend in the occurrence of major precipitation is evident.

The orographic impact on cold fronts is investigated in the Loisach River Valley area by means of two events obtained during the German Front Experiment 1987. The discussion is focussed on the frontal passages in the Garmisch area traced from continuous recordings of three meteorological/ air-chemical stations at different heights, viz: valley floor (735m), wank peak station (1780m) and Zugspitze station (2964m); the analysis also includes some aerological data (pibal ascents at Farchant near Garmisch) and sensitive pressure data from temporary stations in the Bavarian “foreland”, respectively. In the case of the west-east (or parallel to the mountains) moving front A (October 8, 1987), with prefrontal foehn, the following features were observed: i) formation of large (up to 7hPa/100 km up-valley directed pressure gradients after the frontal passage in the foreland, ii) intrusion of cold air from the foreland into the valley (lasting for several hours, filling the whole valley and even traceable at crest height), iii) destruction of upper level foehn current during the “intrusion-phase”, iv) retardation of the front (progressing rapidly in the foreland but being retarded by the mountains located to the west). In the case of the high-reaching, southward (or perpendicular to the mountains) moving D-front (December 18/19, 1987), such striking orographic modifications were not observed. In that case the mountains located to the west obviously did not act as noticeable barrier and, therefore, the flow around and over the mountains prevented the for ation of large pressure gradients between the foreland and the valley (and an intrusion into the valley). In both events it was confirmed that trace gas measurements provide a very promising tool to localize atmospheric flows indirectly.

International audience; High tropospheric NO2 amounts are occasionally detected by space-borne spectrometers above cloudy scenes. For monitoring of near-ground air pollution such data are not directly applicable because clouds shield the highly polluted planetary boundary layer (PBL). We present a method based on trajectories which implicitly estimates the additional sub-cloud NO2 distribution in order to model concentrations at ground stations. The method is applied to a transboundary pollution transport episode which led to high NO2 vertical tropospheric column densities (VTCs) over middle Europe observed by the Global Ozone Monitoring Experiment (GOME) instrument above clouds on 17 February 2001. The case study shows that pollution originally residing near the ground in central Germany, the Ruhr area and adjacent parts of the Netherlands and Belgium has been advected to higher tropospheric levels by a passing weather front. Combining the above-cloud NO2 VTCs with trajectory information covering the GOME columns and including their sub-cloud part yields an estimate of the total NO2 distribution within the tropospheric columns. The highly polluted air masses are then traced by forward trajectories starting from the GOME columns to move further to the Alpine region and their impact there is assessed. Considering ground-based in-situ measurements in the Alpine region, we conclude that for this episode, at least 50% of the NO2 concentration recorded at the sites can be attributed to transboundary transport during the frontal passage. This study demonstrates the potential of using NO2 VTCs from GOME detected above clouds when combined with transport modelling.

An intercomparison study has been performed with four photo-oxidant dispersion models (EMEP, EURAD, LOTOS and REM3) which are currently capable of performing photo-oxidant formation calculations over larger path of Europe. The models, in principle, were run in the mode in which they are normally used, with their own input data and process descriptions. No attempt has been made to harmonize the input data. The study focused on the simulation of a six day episode (31 July-5 August, 1990) with relatively high observed ozone levels over northwest and central Europe (up to 140 ppbv) and which ended by a frontal passage. The study was mainly restricted to an analysis of the behaviour at four receptor sites across the centre of each modelling domain. Differences in yearly averaged anthropogenic emissions are in the order of 10% only. However, episode specific hourly emissions, both anthropogenic and biogenic, show much larger differences. Here up to 40% differences for anthropogenic emissions are found for central Europe while biogenic emissions differ even up to a factor 2-3. The meteorological data base for all models is created from the synoptic measurement network by different techniques. This can result in quite different model inputs as e.g. the value of the mixing height or trajectories. Other model inputs, e.g. dry deposition velocities or photolysis frequencies, show much closer agreements than expected from their derivation through different parameterizations. Model performance for the regional scale could only be observed for ozone. In central Europe, the large-scale ozone patterns are quite similar for all models. Here the temporal evolution at some receptor sites shows also a reasonable agreement with observations. However, in case a multi-component evaluation is possible it can be expected that differences between models can be revealed to a greater detail than was possible here. The model responses to emission reductions show substantial differences although the direction of the responses generally agree. The most complex model (EURAD) has the most pronounced response to anthropogenic emission reductions. The treatment of the biogenic emissions can have a large impact on scenario simulations.

The surface pressure, temperature, dew point and wind data over the South China Sea and vicinity during Winter MONEX are examined to determine the timing of the passage of cold surges at various reporting stations. It is found that for more than half of the surge cases during Winter MONEX, a surge occurs in two stages separated by a time interval of several hours to approximately one day. The first stage is often characterized by a significant rise in pressure, while the second stage by a sharp decrease in dew point temperature. Freshening of surface winds may accompany either or both stages. The time separation between the two stages is relatively short at the upstream stations but generally long at the downstream stations. The second stage is associated with a frontal passage. On the other hand, the first stage is not clearly associated with any significant synoptic events. From its very fast propagation speed which increases with an inferred depth scale, and an increase in the local cross-isob...

Air pollution and meteorological data were collected in Chicago during August and early September for 54 and 39 consecutive days in 1976 and 1977, respectively. Regular measurements were also made during these periods of the ozone-forming potential of ambient air. Rise in ozone levels was related to passage of a weather front. Minimum and maximum values of maximum hourly ozone concentrations 1-2 days and 3-5 days, respectively, after frontal passage. All O\d3 concentrations ≥\N0.08 ppm occurred when the center of a high pressure system was over or east of the Chicago region and only on the second or subsequent day after frontal passage. A rough association appeared to exist between the average ozone-forming potential (the average of values just before, during, and just after a particular time period) and the maximum hourly ozone concentrations 3 days and 4 days later, when these variables were compared for the same frontal system.

Observations, on the island of Oahu, Hawaii, of unusual rains and clouds in marine air during fresh easterly winds show that rain can form within and fall continuously from a shallow layer of warm stratocumulus in about 5 to 13 minutes. This time estimate is shown to be dependent upon where, in the rapidly moving marine cloud system, an island effect initiating raindrop growth is assumed to begin. The clouds are associated with frontal passage nearby, and they form light rains of about 1 to 8 mm/h, averaging 3.5. On the windward shore the clouds produce sparsely distributed showers, suggesting a cell-like structure of the rain development processes over the sea, whereas a few kilometers inland over the Koolau Mountain Range they produce continuous rain and then quickly dissipate. The raindrop-generating processes seem to occur largely over the island rather than over the windward sea, as has been previously suggested concerning the more common orographic showers from the northeasterly tradewind c...

The turbulent fluxes of momentum, sensible heat and latent heat associated with a cold frontal passage of October 8, 1972 over Lake Ontario are examined. There are marked increases in the momentum and moisture fluxes after the frontal passage but the effects on the sensible heat flux are more complicated. The effects on the lake surface temperature are discussed.

Two mesoscale case studies in the semi-arid climate of southeastern Montana were carried out on 1 May and 3 June 1980. I May was an unstable, rainy day with two rain periods over the mesonet area, and 3 June was a potentially unstable day, with a cold frontal passage in the afternoon producing a very intense convective event. Data from an instrumented mesoscale network (supporting the HIPLEX Montana experiment located between Miles City and Baker), a 5 cm radar, soundings, satellite (GOES), and synoptic maps were considered. The mesonet wind, temperature and moisture data were processed, computed every 15 min, and compared with radar rain patterns. The study confirmed that convergence cell development within the surface kinematic fields precedes radar echoes and is directly related to the convective event. The areas involved in the vertical motions generating storms are much larger compared to those reported in humid climates. The “areal convergence” is a better storm predictor than the maximum c...

Hourly temperature readings over a 69-day period in winter 1973–1974 are used to characterize both diurnal and longer period temperature variations in a shallow seagrass flat along the Texas coast. The average diurnal temperature variation has a range of about 2.4°C and includes 8 h of warming, starting at about 0800, followed by 16 h of relatively slow cooling. Longer term, quasi-periodic temperature variations occur at intervals of 11–13 days in response to frontal passages. Temperatures fall between 8° and 14°C in the first 2 days following the frontal passage, before rapid warming begins.

Maps of mean monthly frequency and duration of frontal incursions into the Gulf of Mexico and Caribbean Sea are presented for the 1965–72 period. The transition from the low-frequency regime of summer to the high-frequency regime of winter is quite sharp in the fall, occurring between September and October. A more gradual decrease in activity occurs in spring. During the cooler months, relative maxima in frequency exist over the western Gulf of Mexico and east of Florida, while an arch-shaped region of maximum duration extends northeastward from the Yucatan Peninsula into the central Gulf and then southeastward along the north coast of the Greater Antilles with a second maximum in the central Caribbean. The frequency and degree of penetration of cold fronts are directly related to topographic features and the position, strength and amplitude of the mid-latitude circulation. Time-sections centered around the time of frontal passage are used to present mean data for three regions. Tropical stations...

The characteristics of the lower turbulent zone (LTZ) which is associated with mountain lee waves have been investigated through the analysis of aircraft observations made near Boulder, Colo. Numerical filters and statistical analysis techniques have been applied to the data from six cases to yield vertical sections of potential temperature, horizontal wind, and turbulence intensity along the aircraft paths. One case study is presented to illustrate the structure of the LTZ and its changes during a frontal passage. In addition, the main features of all the analyses are summarized in the form of a schematic vertical section. The horizontal dimension of the LTZ varied between 25 and more than 65 km downstream of the first lee wave trough. The vertical dimensions ranged from a few hundred meters AGL at the lee wave troughs to 3 km AGL at the wave crests. Turbulence levels were light, moderate or severe over more than 90% of the total distance flown in the LTZ (nearly 1100 km). Severe turbulence was ...

Surface and upper air data for November 1974–February 1975 were examined in order to determine a method to locate and predict the movement of the wintertime arctic front over Alaska. Ten cases each of the front passing north and south over Fairbanks were studied, Frontal effects an Fairbanks were noted and methods to forecast the timing of the frontal passage at Fairbanks were determined. Movement of the arctic front over interior Alaska was found to be directly related to the location and movement of large low-pressure systems in the Gulf of Alaska. Only very light snowfall accompanied the front as it passed northward over Fairbanks. However, southward passage of the front accounted for 77% of the Fairbanks snowfall during the four mouth period. A marked temperature and moisture difference was evident between the two air masses separated by the arctic front. Periods of extremely low temperatures with clear skies and ice fog were observed when the front remained south of Fairbanks for several day...

The physical characterization of winter-time aerosol in the Detroit area studied over a 7-week period (January–March, 1983) is presented. Total suspended particle levels (

Atmospheric ozone was monitored in Michigan during the late 1880's using Schoenbein's test paper. A conversion chart was constructed to relate the Schoenbein ozone scale at various relative humidifies to ozone levels indicated by a Dasibi ozone monitor. Average monthly ozone values were highest during the spring months and lowest during the winter season. Highest recorded ozone levels accompanied southwest wind flow into Michigan. Ozone level increased with time as an air mass passed over Michigan and dropped immediately after frontal passage. These patterns are similar to patterns observed in today's data. These results strongly suggest that anthropogenic nitrogen emitted by actively growing green plants and soils is the major source of photochemical ozone precursors in earth's boundary layer.

Results of a study on back-door cold fronts for the months April through October of 1964–71 are presented. Results include information on frequency, associated air-mass duration, precipitation, temperature and dew-point temperature changes, sky cover, and rate of frontal movement. Composite 850- and 500-mb height contours are also constructed. Our findings include: (1) for the sample period, southward penetration of back-door cold fronts is a maximum in June as is frontal frequency, (2) orography plays a prominent role in funneling shallow cold air pools southward east of the Appalachians, (3) more than half of all frontal passages are associated with trace amounts or less of precipitation, (4) heaviest precipitation tends to occur in conjunction with the advancing cold fronts at the more southern and western locations and with the returning warm fronts at northern and eastern locations, (5) temperature changes following frontal passage decrease from north to south and from the coast inland (dew-...

Correlation analysis between various surface weather parameters and samples of atmospherically borne radioactive material was performed. The material collected was primarily of natural origin (i.e., Rn and Th). The analysis suggests that the natural radioactive material is transported into the Austin area within the turbulence associated with a cold frontal passage and concentrated in the lower air layers by the ensuing elevated-temperature inversion. (auth)

Artificial radioactivity in rainwater was measured during the passage of a cold front over The Netherlands. The specific radioactivity of the frontal rain increased considerably after the frontal veering of the wind. Concurrently the “age” of the fission products decreased from 45 to 17 days, indicating a supply of atomic debris from recent nuclear explosions in arctic regions. The specific radioactivity of the air increased considerably on the day of the frontal passage. Trajectories of pre-frontal and post-frontal air indicate that the pre-frontal air comes from a source region in the Mediterranean and the post-frontal air from the arctic regions. The diagram of the frontal passage is analysed.DOI: 10.1111/j.2153-3490.1960.tb01298.x

Minimum virtual height and critical frequency of the ionospheric F2-layer are analyzed by means of the superposed epoch method. Results of this analysis, on the basis of their statistical significance, suggest a relationship between frontal passage in the troposphere and F2-layer characteristics. The observed effects are interpreted qualitatively in the light of a hypothesis suggested by Martyn, and seem to be consistent with the concept of dynamic coupling between lower and upper atmosphere. The results of this analysis are also found to agree with previously reported troposphere-ionosphere correlations.

Cases are selected which are thought to present the summertime problem of whether a cold front will pass Washington, D. C., from the north “tomorrow”. Variables that measure the effective steering direction at 700 mb., the strength of zonal current at 700 mb., and the availability of cold air in Canada are found to be helpful in separating cases of effective frontal passage from other cases. The variables are combined in scatter diagrams to provide a systematic aid to forecasting the passage of the selected cold fronts.

During May 1967, R. V. Chain systematically examined a mid-ocean thermal front in the region between 29°N and 31°N and 68°W and 71°W. A prediction was made aboard ship, based on the theory of Rossby waves, of the change in current direction and of the time this frontal zone would pass through 30°N, 70°W, where a current meter at 10 meters below the sea surface was moored. The current-meter record revealed a jet-like structure on June 6, 1967, as predicted, with a central strong current to the southeast corresponding to the frontal passage. Also, the jet-like current variations that occurred 25–40 days earlier on the record are likely to be associated with another portion of the same meandering westward-moving Rossby wave. This Rossby-wave interpretation of the thermal front and the success of the prediction of the current changes at a buoy 10 meters below the sea surface are complicated by the effects of the wind. Nevertheless, the mean flow directions exhibited in the two-month-long current record do not appear related to the winds.

The relationship between frontal activity and the micropopulation of the atmosphere at altitude is described. It is shown that certain of the meteorological events associated with frontal activity quantitatively modify the micropopulation of the atmosphere. Precipitation associated with frontal passage reduces the micropopulation at altitude, whereas frontal activity with high levels of associated surface and atmospheric turbulence results in great increases in micropopulations of the upper atmosphere—particularly in those situations where surface conditions are conductive to the development of dust.