Convergence Line Research Articles

Triggering Mechanism of an Extreme Rainstorm Process near the Tianshan Mountains in Xinjiang, an Arid Region in China, Based on a Numerical Simulation

The current study investigated the triggering mechanism of a record-breaking heavy rain process in the area near the Tianshan Mountains in Xinjiang, an arid region in China, from July 31 to August 1, 2016, based on the simulation using the Weather Research and Forecasting (WRF) model. The results illustrated that the rainstorm system was generated in the middle atmosphere of the western Aksu region near the Tianshan Mountains and gradually evolved into a multicell linear echo during system evolution. The cold air transported from the Tianshan Mountains partly reached the low altitudes during the downhill process, and the warm southwest air from Aksu was lifted, forming oblique updraft airflow. The other part of the cold air converged with the southeastern warm air in the middle atmosphere, and the transportation and convergence of the water vapor related to the southwestern, southeastern, and oblique updraft airflows provided good water vapor conditions for the storm system. Meanwhile, the inclined upward air transported cloud water and ice-phase particles to high altitudes, mixing the two and generating a large amount of supercooled cloud water, which was very beneficial for the development and maintenance of the storm system. These conditions were favorable for power, heat, water vapor, and water condensate particles, which enabled the development and maintenance of the rainstorm system on the convergence line, thus triggering this rare rainstorm process during the movement to the northeast.

Joint influence of anomalous medium‐ and small‐scale circulations on short‐term heavy rainfall events over Beijing

AbstractDense auto‐weather station data, variational doppler radar analysis system (fast updating radar four‐dimensional variation analysis system) data, radiosonde data and ERA‐Interim reanalysis data are used to investigate the joint influence of anomalous medium and small‐scale circulations associated with short‐term heavy rainfall events, which are influenced by the interaction between Pacific subtropical high and westerly trough (Type i) over Beijing between June and August 2007–2014. This Type i primarily comprises short‐term heavy rainfall events over Beijing. The results show that short‐term heavy rainfall events appear mainly over Northeastern and Southwestern Beijing, especially Northeastern Beijing, and generally reach a peak at dusk (19:00–21:00). Large‐scale circulations (an anomalous high in the east and an anomalous low in the west over Eastern Asia) favour the southeastern and southwestern winds to transport water vapour northward to Beijing. An anomalous mesoscale cyclone moves from west to northeast via the north to Beijing at approximately 20:00. An abnormal air temperature structure (warm and wet air at whole levels), an abnormal mesoscale circulation structure (with lower‐level convergence and mid‐upper‐level divergence) and the resulting abnormal vertical upward movement are well developed over Beijing. The upward branch over the southern outskirt of the anomalous mesoscale cyclone partly overlaps with that of the anomalous small‐scale cyclonic circulations or convergence lines (anomalous thunderstorm highs), and a striking vertical upward motion is produced. One airstream of the upward branches moves into the upper divergence region (it “pumps” mid‐lower level air into the mid‐upper levels) and makes the upward motion more vigorous. Another airstream turns around and descends fiercely to the surface (downward branches of anomalous thunderstorm highs); that is, the anomalous thunderstorm highs are reinforced, and the frequency peak in the short‐term heavy rainfall events occurs at dusk. The short‐term heavy rainfall events weaken at night. The analyses of vertical velocity anomalies for anomalous medium‐scale cyclone as well as turbulent vertical velocities and diagnostic radar echoes for anomalous small‐scale circulations in the heavy rainfall event on July 21, 2012 further verify the statistical results.

Occurrence and Development of an Extreme Precipitation Event in the Ili Valley, Xinjiang, China and Analysis of Gravity Waves

We used observational data and the results from a high-resolution numerical simulation model to analyze the occurrence and development of an extreme precipitation event in the Ili Valley, Xinjiang, China on 26 June 2015. We analyzed the horizontal wavelength, period, speed, ducting, energy propagation and feedback mechanism of inertial gravity waves. A low-level convergence line was formed in the valley by the northerly and westerly winds as a result of Central Asian vortices and the trumpet-shaped topography of the Ili Valley. There was sufficient water vapor in the valley for the precipitation event to develop. A mesoscale vortex formed and developed on the low-level convergence line and the rainfall was distributed either near the convergence line or the mesoscale vortex. The low-level convergence line and the uplift caused by the terrain triggered convection, and then the convection triggered waves at lower levels. The combination of ascending motion induced by the lower level waves and the mesoscale vortex led to the development of convection, causing the precipitation to intensify. When the convection moved eastward to Gongliu County, it was coupled with the ascending phase of upper level waves, causing both the convection and precipitation to intensify again. We applied spectral analysis methods to verify that the waves were inertial gravity waves. The upper level inertial gravity waves propagated westward at a mean speed of −12 m s−1 with periods of 73–179 min and horizontal wavelengths of 50–55 km. The lower level inertial gravity waves propagated eastward at a mean speed of 8 m s−1 with periods of 73–200 min and a horizontal wavelength of 85 km. The more (less) favorable waveguide conditions determined whether the gravity waves persisted for a long (short) time and propagated for a longer (shorter) distance. Based on the mesoscale Eliassen–Palm flux theory, the wave energy of inertial gravity waves had an important effect on the maintenance and development of convection and precipitation by affecting wind strength and wind divergence. Feedback was mainly through the meridional and vertical transport of zonal momentum and the meridional transport of heat.

Are convergence lines associated with high asthma presentation days? A case-control study in Melbourne, Australia

BackgroundIn epidemic thunderstorm asthma (ETSA) events a large number of people develop asthma symptoms over a short period of time. This is thought to occur because of a unique combination of high amounts of pollen and certain meteorological conditions. However, the exact cause and mechanism of epidemic thunderstorm asthma remains unclear. ObjectivesThe objective of this study was to test the hypothesis that convergence lines may be a causative factor in ETSA events, by investigating whether convergence line weather events are associated with the occurrence of high asthma presentations days during the Victorian grass pollen season (October–December). MethodsA case control method was used. All public hospitals within 75 km of the Melbourne weather radar were included, and data were taken from 2009 to 2017 during the Victorian grass pollen season. Cases hospital days were hospitals with a high number of asthma presentations within a 24-h period, and controls were hospitals with an expected number of asthma presentations. Exposure was defined as geographical proximity of a convergence line to the hospital case or control. ResultsEighty-one case hospital days and 157 hospital day controls were included in the study. The odds of exposure to a convergence line were significantly higher for cases than for controls at all exposure distances. At 4 km, 80 of the 81 cases had been exposed to a convergence line. ConclusionConvergence lines appear to be a necessary, but not sufficient, element in the cause of epidemic thunderstorm asthma. This is the first study to show a clear link between epidemic thunderstorm asthma and convergence lines.

Using Megha‐Tropiques satellite data to constrain humidity in regional convective simulations: A northern Australian test case

AbstractConvective initiation and growth are sensitive to atmospheric conditions, especially humidity, in ways that need to be understood and quantified. It is not clear, however, how well current observations and modelling systems can serve to test this understanding. We simulate multiple cases of convergent cloud lines observed during January 2016 over northeastern Australia using the Weather Research and Forecasting (WRF) model (version 3.7.1), initialized by and nudged to reanalysis data. Overall, WRF appears to skilfully simulate the convergence lines. However, the associated convective clouds as seen by satellite were not properly captured by the model. The model humidity was tested against the current version (V3‐00) of the Megha‐Tropiques satellite, which although up to 30% too moist in the lowest retrieved layer over the dry continental interior compared to radiosondes, is usually accurate to within ±10% near the cloud lines. Unperturbed WRF simulations were found to be 10–20% too dry in this region. Compensating for this humidity bias via the model initialization produced more realistic simulations of clouds and convection, and this had a comparable or larger impact than changing model physics. This result highlights the importance of observations of water vapour that are accurate and have good spatial coverage, especially in the lower troposphere, for properly constraining simulations of convective environments and for model evaluation of moist processes.

Comparison of mechanical and thermal effects of lake urmia: a case study

Simulation of the lakes’ impact on precipitation and the quantification of the effects resulting from the contribution of different processes have proved challenging in numerical weather prediction. Lakes have differences with their surrounding lands in roughness, surface temperature and humidity content, which are the root causes of the mechanical and thermal mechanisms that lead to changes in the amount of precipitation in the downwind of a lake. In this study, the effect of Lake Urmia on precipitation in the form of snowfall is simulated for a case of heavy precipitation associated with severe weather across a significant part of the Southwest Asia on 11 Dec. 2013 using the Weather Research and Forecasting (WRF) model coupled with the Community Land Model (CLM) in simulations with and without the lake. The results illustrate that omitting the lake reduces the humidity content and precipitable water in the vertical column above the lake. In this case, however, the role of mechanical effect due to differences in roughness between the lake and its surrounding lands is noticeably larger than the thermal effect due to differences in the moisture and heat supplied by the lower surface to the air masses above. Associated with the roughness differences, a distinct change in the location of the convergence line of the antithetical currents is identified over the lake. The case highlights the importance of the proper representation of surface characteristics in mesoscale models to gain improvements on the fine-scale details of precipitation.

Characterization of the air–sea exchange mechanisms during a Mediterranean heavy precipitation event using realistic sea state modelling

Abstract. This study investigates the mechanisms acting at the air–sea interface during a heavy precipitation event that occurred between 12 and 14 October 2016 over the north-western Mediterranean area and led to large amounts of rainfall (up to 300 mm in 24 h) over the Hérault region (southern France). The study case was characterized by a very strong (>20 m s−1) easterly to south-easterly wind at low level that generated very rough seas (significant wave height of up to 6 m) along the French Riviera and the Gulf of Lion. In order to investigate the role of the waves on air–sea exchanges during such extreme events, a set of numerical experiments was designed using the Météo-France kilometre-scale AROME-France numerical weather prediction model – including the WASP (Wave-Age-dependant Stress Parametrization) sea surface turbulent flux parametrization – and the WaveWatch III wave model. Results from these sensitivity experiments in the forced or coupled modes showed that taking the waves generated by the model into account increases the surface roughness. Thus, the increase in the momentum flux induces a slowdown of the easterly low-level atmospheric flow and a displacement of the convergence line at sea. Despite strong winds and a young sea below the easterly flow, the turbulent heat fluxes upstream of the precipitating system are not significantly modified. The forecast of the heaviest precipitation is finally modified when the sea state is taken into account; notably, in terms of location, this modification is slightly larger in the forced mode than in the coupled mode, as the coupling interactively balances the wind sea, the stress and the wind.

Connecting flash flood events with radar-derived convective storm characteristics on the northwestern Mediterranean coast: knowing the present for better future scenarios adaptation

The northwestern Mediterranean coast is one of the areas most affected by flash floods and adverse weather, where also global climate scenarios are predicting an increase in precipitation extremes. This, along with the sea-level rise and the associated potential infrastructure and economical losses, represents a major risk for coastal settlements, as the one studied in this work: a major touristic area on the southern coast of Catalonia. The objective of the present paper is, thus, to understand in-depth the behavior of convective activity in the area and its link with flash flood events, to improve adverse and severe weather early warning systems in a rapidly changing weather as an adaptation measure. With this purpose, the study focuses on the period 2014–2018, analyzing the convective cells and storm motion in the region of study, through the new identification and tracking algorithm developed by the authors and applied in the Meteorological Service of Catalonia. The results show that the area is mostly affected by shallow but efficient convection, probably embedded in major weather systems with high precipitation rain rates, which tends to saturate the catchments and produce flooding. The work also shows the role of the topography as a triggering factor for storms outside the convective season, as a key feature for the convergence lines organization, and as a capping for stationary cells upstream. Improvements in the current warning system, such as different precipitation thresholds considering the time of exposure to that rain, as well as a further study are also presented as main results. These results can be extrapolated and applied to other regions affected by the same kind of heavy precipitation and flash floods.

Self‐Aggregation of Convection in Spatially Varying Sea Surface Temperatures

AbstractThe phenomenon of self‐aggregation of convection was first identified in convection‐permitting simulations of radiative convective equilibrium, characterized by homogeneous boundary conditions and in the absence of planetary rotation. In this study, we expose self‐aggregation of convection to more complex, nonhomogeneous boundary conditions and investigate its interaction with convective aggregation, as forced by large‐scale variations in sea surface temperatures (SSTs). We do this by conducting radiative convective equilibrium simulations on a spherical domain, with SST patterns that are zonally homogeneous but meridionally varying. Due to the meridional contrast in SST, a convergence line first forms, mimicking the Intertropical Convergence Zone. We nevertheless find that the convergence line breaks up and contracts zonally as a result of the self‐aggregation of convection. The contraction is significant, being here more than 50% of the original extent. The stability of the convergence line is controlled by the strength of the meridional circulation, which depends upon the imposed SST contrast. However, the process of self‐aggregation, once it is initiated, is insensitive to the strength of the SST contrast. The zonal contraction is accompanied by a slight meridional expansion and a moistening of the high latitudes, where SSTs are low. The moistening of the high latitudes can be understood from the fact that the convective cluster intensifies and expands its moist meridional low‐level outflow when it self‐aggregates zonally. Overall, our results suggest that the Intertropical Convergence Zone may be unstable to the self‐aggregation of convection, that self‐aggregation may serve as a precursor to the formation of atmospheric rivers, and that longer convergence lines are more likely to exist in regimes with strong SST gradients.

DSJRA-55 データによる東海〜関東南岸域に発生する収束線の出現特性

DSJRA-55 データによる東海〜関東南岸域に発生する収束線の出現特性

The Causes of “Vulnerable Regions” to Air Pollution in Winter in the Beijing-Tianjin-Hebei Region: A Topographic–Meteorological Impact Model Based on Adaptive Emission Constraint Technique

The Beijing-Tianjin-Hebei (BTH) region, with its complex terrain, has serious issues with air pollution. The northern and western parts of the BTH region are surrounded by the Yan Mountains and Loess Plateau (LP), and the south-central part of that region is a large open plain. Such special geographic configuration is prone to result in a concentrated pollution belt along the north-to-south direction on the eastern margin of the plateau, in addition to the influence of pollutant-emission sources and population distribution. In this study, based on an original adaptive nudging constraint method, we quantitatively analyzed the differences in the influence of emission sources under different dynamic and thermal conditions in the BTH region, which is impacted by a special large-scale leeward slope terrain. The mechanism of air pollution vulnerability and the comprehensive effects of terrain–meteorological conditions on air pollution in the BTH region were also discussed. The results indicated that the atmospheric diffusion conditions on the eastern side of the plateau were poor, and a sub-synoptic-scale “vortex sequence”, which was composed of a series of linked vortices, was identified. The corresponding atmospheric pollution convergence line stretched from Beijing to Hebei to Northern Henan in the lower atmosphere. On the eastern edge of the plateau, a “warm cover” formed by a temperature anomaly and a downdraft impeded the vertical diffusion of pollutants. Therefore, pollutants tended to converge at the eastern edge of the plateau, and the pollution belts would move longitudinally north and south along the topography of the eastern slope when south-westerly and north-easterly winds alternated. The movement generated a “train” of pollutants that were transported on the eastern edge of the plateau, which then caused air pollution to persist there. Such terrain–meteorological conditions amplified the effects of emissions by an average of 50% to 150% in the region, leading the eastern side of the LP to become a “naturally vulnerable region” to haze pollution.

Dual-Polarization Radar Observations of Deep Convection over Lake Victoria Basin in East Africa

Lake Victoria in East Africa supports the livelihood of thousands of fishermen and it is estimated that 3000–5000 human deaths occur per year over the lake. It is hypothesized that most of these fatalities are due to localized, severe winds produced by intense thunderstorms over the lake during the rainy season and larger scale, intense winds over the lake during the dry season. The intense winds produce a rough state of the lake (big wave heights) that cause fishing boats to capsize. In this region, weather radars have never been a primary tool for monitoring and nowcasting high impact weather. The Tanzania Meteorological Agency operates an S-band polarimetric radar in Mwanza, Tanzania, along the south shore of Lake Victoria. This radar collects high temporal and spatial resolution data that is now being used to detect and monitor the formation of deep convection over the lake and improve scientific understanding of storm dynamics and intensification. Nocturnal thunderstorms and convection initiation over the lake are well observed by the Mwanza radar and are strongly forced by lake and land breezes and gust fronts. Unexpected is the detection of clear air echo to ranges ≥100 km over the lake that makes it possible to observe low-level winds, gust fronts, and other convergence lines near the surface of the lake. The frequent observation of extensive clear air and low-level convergence lines opens up the opportunity to nowcast strong winds, convection initiation, and subsequent thunderstorm development and incorporate this information into a regional early warning system proposed for Lake Victoria Basin (LVB). Two weather events are presented illustrating distinctly different nocturnal convection initiation over the lake that evolve into intense morning thunderstorms. The evolution of these severe weather events was possible because of the Mwanza radar observations; satellite imagery alone was insufficient to provide prediction of storm initiation, growth, movement, and decay.

Drylines in Southern Africa: Rediscovering the Congo Air Boundary

AbstractProjected rainfall decline in southern Africa is likely to be highly sensitive to subtleties in the local atmospheric circulation. In an effort to understand the regional circulation complexities, a novel algorithm is developed to identify the Congo air boundary (CAB) in ERA-5, a high-resolution reanalysis dataset. The CAB, a forgotten feature of the circulation, is defined in the austral spring and early summer, using surface humidity gradients and near-surface wind convergence lines, and it is found to be an indicator of the location of the southern edge of the African rain belt. A related convergence-line and dryline feature, described in this paper as the Kalahari discontinuity (KD), is also identified. It is established that either a dryline CAB or KD is present in southern Africa for over 95% of days between August and December, with arc lengths typically exceeding 10°. The seasonal and diurnal cycles of the CAB and the KD are presented, and their prevalence in station observational data is confirmed. The interannual variability of the CAB latitude and detection frequency is found to explain at least 55% of interannual spring rainfall variability in southern Africa between 15° to 25°S. Links are established with the Angola and Kalahari heat lows and tropical temperate trough events.

Further Analysis of the Development of Vadose Water Profiles Over Deep Aquifers With Minimal Recharge

AbstractUnderstanding transport water from deep aquifers to the atmosphere in hyperarid environments is needed to evaluate potential waste storage facilities and reveals fundamental features of evolving scales of hydrologic processes on Earth. We seek a physical framework for the process and scales at which water and salt movement take place where there is no net recharge over long periods of time. We consider this extreme case as an end‐member of the driest possible situation, with homogeneous media. The analysis of Ross (1984, https://doi.org/10.1029/WR020i011p01627) and Selker (2017, https://doi.org/10.1002/2017WR021014) is expanded upon to provide a simple algebraic expression, derived from the governing physical processes, predicting the fluxes and the vertical extent of such systems. In much of such an unsaturated zone a counterflux region of downward liquid flow counter to upward vapor flow results from the geothermal gradient. Net upward transport is about 0.01 mm/year from the water table, which is augmented by the depletion of the residual water in the drained soil, resulting in surface fluxes of 1.0, 0.1, and 0.02 mm/year after 100 year, 10 kyr, and 150 kyr. Ten to thirty million years without recharge would be required for soil to dry to depths of 200 m. It is found that the influence of capillary flow to the upward transport of water is only influential within less than 5 m of the water table and so is insignificant to the overall flux but does imply a convergence line where vapor loss is predicted to leave behind salts delivered from above and below to the top of the capillary zone.

General Features of Radar‐Observed Boundary Layer Convergence Lines and Their Associated Convection Over a Sharp Vegetation‐Contrast Area

AbstractBoundary layer convergence lines (boundaries), an important trigger of convective storms, can be produced by land surface contrasts. This study explored a five‐year summertime radar climatology of boundaries and their associated convection in response to vegetation contrast around the bend of the Yellow River in North China. A total of 323 boundaries were identified with 44% being convection‐associated. The boundaries especially the convective boundaries were more frequent over the arid area than those over the vegetated area and tended to have an orientation parallel to the vegetation contrast line. The boundary activities collocated well with the diurnal variation in surface temperature difference across the vegetation contrast. Compared with the nonconvective boundaries, the convective boundaries formed earlier and moved faster into the inner arid area, obtained maximum length around midday, and then initiated convection. Vegetation contrast might also affect the high‐frequency location and magnitude of boundary‐associated convective precipitation.

Convection Initiation Resulting From the Interaction Between a Quasi‐Stationary Dryline and Intersecting Gust Fronts: A Case Study

AbstractThis study explores the convection initiation (CI) of a high‐impact squall line that occurred in central eastern China on 3 June 2009 based on observations and numerical modeling. The CI occurred in a scenario in which a set of intersecting gust fronts, organized in a distinct scalloped pattern, propagated toward an area of enhanced moisture produced by a near‐surface convergence line. This convergence line developed in a quasi‐stationary dryline zone. The dryline primed the preconvective environment by deepening the moist layer prior to the arrival of the intersecting gust fronts. The onset of CI occurred approximately 30 min after these intersecting gust fronts passed through the CI location, which was on the dry side of the dryline. Although these gust fronts acted as a strong signal for CI potential, CI did not occur along the entire length of the scalloped pattern of the intersecting gust fronts. The exact locations of the initiated convective cells were at the vertices of the scalloped pattern. An idealized simulation using a cloud model was conducted, demonstrating that the vertex regions were characterized by more favorable dynamical conditions for CI compared to the nonvertex regions along the scalloped outflow boundary. The greater CI probability over the vertex region was attributed to the greater magnitudes and larger vertical and horizontal extents of updrafts.

Understanding the Dynamic Contribution to Future Changes in Tropical Precipitation From Low‐Level Convergence Lines

AbstractFuture precipitation changes include contributions from both thermodynamic and dynamic processes. Given that precipitation in the tropics is commonly associated with convergence lines, we construct a simple linear regression model relating the convergence line frequency and strength to precipitation at subdaily time scales, and use it to show that changes in the convergence lines are related to the dynamic change in the precipitation. Given GCM‐predicted convergence line changes, we predict precipitation changes using the regression model. The so‐predicted precipitation change is equivalent to the dynamic component of the precipitation change identified in earlier studies that used very different methods. The difference between the precipitation change in GCMs and that predicted from changes in convergence lines accounts for thermodynamic and other potentially important dynamic contributions. More accurate predictions of future precipitation therefore require the accurate simulations of the relatively short‐lived weather features responsible for convergence lines in the tropics in GCMs.

Influence of Mountain Valley Breeze and Sea Land Breeze in Winter on Distribution of Air Pollutants in Beijing-Tianjin-Hebei Region

To reveal the effect of Mountain Valley Breeze (MVB) and Sea Land Breeze (SLB) in winter on the spatial-temporal distribution of air pollutants in the Beijing-Tianjin-Hebei region, hourly data from Automatic Weather Stations (AWS) and hourly air pollutant concentration data in December 2016 from the China National Environmental Monitoring Center were used to calculate the average wind vector fields and PM2.5 concentration fields. The change rule of MVB and SLB and its influence on the distribution of PM2.5 concentration were analyzed. The prevailing factor for the MVB days was the southerly wind (valley wind) in the Beijing-Tianjin-Hebei region from noon to afternoon, this valley wind transports air pollutants from the eastern areas of the Taihang Mountains and southwestern areas of Beijing northward. In the evening, "herringbone" convergence lines formed between the emerging mountain breeze in the western and northern parts of Beijing, as well as in the piedmont areas of the Taihang Mountains, and the southerly wind. The PM2.5 concentration increased in Beijing, Langfang, Baoding, Shijiazhuang, and Xingtai according to the concentration of the convergence lines. For the SLB days, the PM2.5 concentration increased in the piedmont areas of the Taihang Mountains due to the influence of valley wind from noon to afternoon. For the MVB days, from noon until midnight, the sea breeze appeared in the eastern coastal areas and reached the southeastern part of Tianjin, the PM2.5 concentration increased towards the front of the sea breeze. The influence of MVB and SLB on the distribution of air pollutants in the heavy pollutions process were surveyed by analyzing the temporal variation relationship between the vertical distribution of wind over 0-325 m tower (at the Institute of Atmospheric Physics) and PM2.5 concentration of urban area, and by using the Cressman method to interpolate the 10 m wind data and PM2.5 concentration data to 2D grid field. From noon to afternoon, the air pollutants were blown to Beijing by valley wind. In the evening, the air pollutants converged near the convergence lines, which were formed by the mountain breeze and southerly wind. The severe pollution zone formed in the plains of Beijing and south of Beijing. From midnight to early morning, the air pollutants in Beijing were gradually blown away by the mountain breeze and stayed south of Beijing and northwest of Tianjin. In the winter, the effect of MVB on the recycling and accumulation of air pollutants plays an important role in severe atmospheric pollution incidents in Beijing, south of Beijing, and the eastern areas of the Taihang Mountains.

Currents and Voltages Induced by Electric Field in Two Converging Single-wire Overhead Transmission Lines

The calculations of induced by electric field voltage and current for i line grounded and ungrounded ends depending on p - line convergence part length, а0 minimal distance between convergence lines, and Θ - the angle of lines convergence. Limit values of р, а0 and Θ are determined, one of which disturbance allows to neglect the electric effect of converging transmission lines.

Role of Typhoon Prapiroon (Typhoon No. 7) on the Formation Process of the Baiu Front Inducing Heavy Rain in July 2018 in Western Japan

Heavy rain in western Japan was broadly induced by the stagnation of the Baiu front during 5-7 July 2018. This study hypothesizes that cold air advection over the Sea of Japan intensified by Typhoon Prapiroon (Typhoon No. 7) was one of the triggering factors for the formation process of the Baiu front over western Japan. Typhoon Prapiroon passed over the Sea of Japan on 4 July and became extratropical at approximately 40°N on 5 July. During its passage, the strong southward pressure gradient force to the north of Typhoon Prapiroon broke down the convergence line of the Baiu front that remained at approximately 45°N before 4 July and thick cold air from the Okhotsk High flowed over the Sea of Japan. The Okhotsk High expanded toward the Sea of Japan and enhanced cold air advection to the north of western Japan. As a result, the Baiu front was stationary at approximately 35°N after 5 July. In addition, the westerly jet in the east of an upper-level trough deepened along the typhoon track was associated with the adiabatic component of the ascending motion over the isentropic upslope and was suggested to contribute to the maintenance of Baiu frontal convection.