Short-wave Trough Research Articles

The large-scale meteorological condition during the critical stage of Mesoscale Convective Complexes (MCCs) over Indian Ocean

This study analyzed the large-scale meteorological condition during Mesoscale Convective Complexes (MCCs) over the Indian Ocean. The MCCs were identified by infrared satellite imagery using an algorithm that combined information about cloud coverage, eccentricity, and the cloud lifetimes of MCCs. The data used were a combination of satellite data and reanalysis data, while the Brightness temperature (TBB) was obtained from the MTSAT IR1 satellite data. The results of this study showed that, in general, the initial stage of the MCC was characterized by strong low-level convergence and vertical convection, and largely driven by the convergence of the moisture flux in the lower troposphere. The mature stage of the MCC was characterized by weak surface convergence, strong upper-level divergence, and a shortwave ridge in the mid- and upper levels. Where there was strong surface divergence, the decay and dissipation stages were very similar, and the surface convergence left the system. The movement of most MCCs resulted from the combined contributions of advection and the propagation of surface convergence. Results from this research showed that these large convective systems tended to be formed in the vicinity of the terminus of a low-level jet that transported moist and warm air to the originating regions of the MCCs. Shortwave troughs and baroclinic zones were associated with MCC development.

Contrasting Pre-Mei-Yu and Mei-Yu Extreme Precipitation in the Yangtze River Valley: Influencing Systems and Precipitation Mechanisms

Abstract The mei-yu season over the Yangtze–Huai Rivers basin, typically occurring from mid-June to mid-July, is one of three heavy-rainfall periods over China and can contribute 50% of the annual precipitation. In this study, the first and second heaviest daily precipitation events at the Wuhan station have been selected to represent typical mei-yu and pre-mei-yu precipitation events where the differences in the atmospheric thermodynamic characteristics, precipitation nature, influencing systems, and mechanisms are investigated. During the mei-yu case, moist air mainly came from the South China Sea. Precipitation occurred south of the mei-yu front where abundant moisture and favorable thermodynamic conditions were present. The main influencing systems include a stable blocking pattern and strong and stable western Pacific subtropical high in the midtroposphere, and a small yet intense mesoscale cyclonic vortex in the low troposphere. Rainfall in Wuhan was continuous, caused by a well-organized convective line. A heavy rainband was located along the narrow band between the elongated upper-level jet (ULJ) and the low-level jet (LLJ) where the symmetric instability was found in the midtroposphere near Wuhan. Quite differently, for the pre-mei-yu precipitation case, moist air primarily came from the Beibu Gulf and the Bay of Bengal. Precipitation happened in the low-level convective instability region, where a short-wave trough in the midtroposphere and a mesoscale cyclonic vortex in the low-troposphere were found. Precipitation in Wuhan showed multiple peaks associated with independent meso-β-scale convective systems. A rainstorm occurred at the exit of the LLJ and the right entrance of the ULJ, where convective instability exited in the mid- to low troposphere.

An Investigation of Cold-Season Short-Wave Troughs in the Great Lakes Region and Their Concurrence with Lake-Effect Clouds

AbstractThe downwind shores of the Laurentian Great Lakes region often receive prolific amounts of lake-effect snowfall during the cold season (October–March). The location and intensity of this snowfall can be influenced by upper-tropospheric features such as short-wave troughs. A 7-yr cold-season climatology of 500-hPa short-wave troughs was developed for the Great Lakes region. A total of 607 short-wave troughs were identified, with an average of approximately 87 short waves per cold season. Five classes of short-wave troughs were identified on the basis of their movement through the Great Lakes region. This short-wave trough dataset was subsequently compared with the lake-effect cloud-band climatology created by N. F. Laird et al. in 2017 to determine how frequently short-wave troughs occurred concurrently with lake-effect cloud bands. Of the 607 short-wave troughs identified, 380 were concurrent with lake-effect clouds. Over 65% of these 380 short-wave troughs occurred with a lake-effect cloud band on at least four of the five Great Lakes. Short-wave troughs that rotated around the base of a long-wave trough were found to have the highest frequency of concurrence. In general, concurrence was most likely during the middle cold-season months. Further, Lake Michigan featured the highest number of concurrent events, and Lake Erie featured the fewest. It is evident that short-wave troughs are a ubiquitous feature near the Great Lakes during the cold season and have the potential to impart substantial impacts on lake-effect snowbands.

Observations of a tornado at an Automatic Weather Station in northern Tasmania

On 15 April 2009, the Automatic Weather Station (AWS) at Scottsdale, in north-eastern Tasmania, recorded a wind gust of 54 ms-1 (194 kmh-1) as an active squall line crossed the state. Investigation of the environment, instrumentation, and damage resulted in the conclusion that this was a genuine wind gust caused by a tornado passing very close to the anemometer. This was the first direct AWS observation of a tornado in Australia, and one of very few such observations in the world. In this report, we document the weather event which produced the tornado, briefly outline the synoptic situation leading to its occurrence, document additional background observations that provide context for the event, and discuss the nature of the observations made by the AWS and the method by which the wind gust observations were verified. The squall line was part of a cold front that crossed northern Tasmania on the morning of 15 April 2009. At 300 hPa, the orientation of the short-wave trough associated with the cold front changed from positively to strongly negatively tilted as it moved over central Victoria and Tasmania. The orientation of the trough and position of a jet streak within it suggested strong upper divergence and strong vertical motion. These contributed to thunderstorm development and resulted in very substantial vertical wind shear through the lower half of the troposphere, which in turn contributed to the organisation of the severe convection. Near the surface, low cloud base and strong vertical windshear in the lowest kilometre provided conditions conducive to tornado development.

Heavy Rainfall Associated with Double Low-Level Jets over Southern China. Part I: Ensemble-Based Analysis

AbstractHeavy rainfall occurred at both the inland frontal zone and coastal warm sector in southern China during 10–11 May 2014, which is a typical pattern in the early-summer rainy season. To clarify the key factors controlling the rainfall, we conduct an ensemble-based analysis using the operational global ensemble forecasts from ECMWF. The forecasts of frontal (warm sector) rainfall have a relatively small (large) spread and a small (large) bias of ensemble-mean amount, suggesting an obvious difference in the predictability. It is shown that double low-level jets (LLJs) in the southwesterly moist flow play a significant role in the heavy rainfall over southern China. The inland frontal rainband is closely related to the synoptic-system-related low-level jet (SLLJ) with maximum wind speed at 850–700 hPa, especially for its meridional wind component. The more intense cold front is accompanied by the stronger southwesterly SLLJ on the adjacent south side, favoring more precipitation near the front. The warm-sector heavy rainfall, a few hundred kilometers away from the front, is associated with the boundary layer jet (BLJ) at 925 hPa. The southerly BLJ occurs over the northern region of the South China Sea and reaches its maximum wind speed in the early morning. The variations of the BLJ are mainly induced by the surface low and related upper-level short-wave trough upstream. The large pressure gradient to the southeast of the surface low can accelerate the BLJ by increasing the geostrophic winds. The diurnal cycle of the low-level winds, seen in the climatology, also contributes in part to the development of the BLJ at night.

Observations of a tornado at an Automatic Weather Station in northern Tasmania

On 15 April 2009, the Automatic Weather Station (AWS) at Scottsdale, in north-eastern Tasmania, recorded a wind gust of 54 ms-1 (194 kmh-1) as an active squall line crossed the state. Investigation of the environment, instrumentation, and damage resulted in the conclusion that this was a genuine wind gust caused by a tornado passing very close to the anemometer. This was the first direct AWS observation of a tornado in Australia, and one of very few such observations in the world. In this report, we document the weather event which produced the tornado, briefly outline the synoptic situation leading to its occurrence, document additional background observations that provide context for the event, and discuss the nature of the observations made by the AWS and the method by which the wind gust observations were verified.The squall line was part of a cold front that crossed northern Tasmania on the morning of 15 April 2009. At 300 hPa, the orientation of the short-wave trough associated with the cold front changed from positively to strongly negatively tilted as it moved over central Victoria and Tasmania. The orientation of the trough and position of a jet streak within it suggested strong upper divergence and strong vertical motion. These contributed to thunderstorm development and resulted in very substantial vertical wind shear through the lower half of the troposphere, which in turn contributed to the organisation of the severe convection. Near the surface, low cloud base and strong vertical windshear in the lowest kilometre provided conditions conducive to tornado development.

Formation of Long-Lived Summertime Mesoscale Vortices over Central East China:Semi-Idealized Simulations Based on a 14-Year Vortex Statistic

Abstract Regions around Dabie Mountain (DBM) in the Yangtze River basin (YRB) are the source of a mesoscale vortex: the Dabie vortex (DBV). Based on a 14-yr statistical study, 11 long-lived heavy-rain-producing DBVs were composited for convection-permitting semi-idealized simulations. A control simulation, initialized 12 h before the composite vortex formation, successfully reproduced a DBV, with all the salient characteristics of the 11 events. Sensitivity experiments were designed to understand the impacts of large-scale environmental conditions, regional topography, and latent heating on DBV formation. The main results were as follows: (i) Supposition of a 500-hPa shortwave trough with an east–west-oriented lower-level transversal trough around the DBM is crucial for the formation of vortices. A nocturnal lower-level jet on the southern side of the transversal trough accelerates DBV formation by enhancing convergence, triggering/sustaining convection, and producing cyclonic vorticity. (ii) During the simulation period, the topography east of the second-step mountain ranges, including the DBM, significantly affects nearby precipitation and convective activity, whereas this is not crucial for DBV formation. (iii) Latent heating is not required for generating DBVs, but it enhances the intensity, longevity, and eastward progression of these vortices along the shear line associated with the transversal trough. (iv) The vorticity budget suggests the convergence-related (horizontal) shrinking and vertical transport dominate the cyclonic-vorticity increase associated with DBVs, whereas tilting and horizontal transport mainly act in the opposite manner.

Objective Identification of Trough Lines Using Gridded Wind Field Data

An objective method was developed to analyze longwave and shortwave trough lines in wind field data using different methods, given that these trough lines were researched in different ways. Longwave trough lines were analyzed by locating the cyclonic center and filtering candidate trough points simultaneously; the candidate longwave trough points were then traced based on distance and angle conditions. Next, candidate shortwave trough points were determined based on angular deflection and vorticity data, which were clustered and fitted to a curve for extraction. This method was applied to wind field data from the National Center for Environmental Prediction (NCEP) to analyze trough lines in East Asia and South Asia. The experimental results show that our method can effectively identify trough lines by comparing them with manual analysis results. The statistical results indicate that the method more precisely identifies longwave trough lines than shortwave trough lines, and that trough lines during the fall and winter are more accurately and effectively identified than those during the spring and summer.

Extreme Rainstorms that Caused Devastating Flooding across the East Coast of Peninsular Malaysia during November and December 2014

Abstract During the early boreal winter (northeast) monsoon (November–December), cold air frequently bursts out from intense Siberian highs toward the Chinese coast in response to the development and movement of a 500-hPa trough. The resultant strong low-level northwesterlies turn into northeasterlies across the South China Sea as “cold surges.” On interacting with the near-equatorial trough, mesoscale convective systems form north of the trough, normally giving rise to heavy downpours and severe flooding, mainly along the coastal stretch in the east coast states of Peninsular Malaysia. In November 2014, a 1-week-long episode of heavy downpours, producing more than 800 mm of rain, occurred along the coastal stretch of northeastern Peninsular Malaysia. However, during December 2014, two episodes of extreme rainfall occurred mostly over inland and mountainous areas of the east coast of Peninsular Malaysia, in particular across its northern sector. These two unusual events, which lasted a total of 11 days and delivered more than 1100 mm of precipitation, resulted in extreme and widespread flooding, as well as extensive damage, in many inland areas. Analysis shows that the stronger wind surges from the South China Sea due to very intense cold-air outbreaks of the Siberian high developed under ENSO-neutral conditions. In addition, the mesoscale convective systems that developed across the northeastern Indian Ocean (near northern Sumatra) in response to the propagation of a 500-hPa short-wave trough across the Indian subcontinent toward China were the combined factors for these unusual extreme rainfall and flooding events along the east coast of Peninsular Malaysia.

一次高影响小雨过程预报失误的原因初探

2010年冬春之交的4月，因对8~9日发生在上海地区的一次小雨过程的预报失误，导致9日最高气温的预报与实况相差近7度，市民反响强烈。本文对这一“高影响”小雨过程的预报失误原因，进行了初步分析，结果表明：低层(850 hPa)的中尺度锋生，减缓了低层短波槽系统的过境速度，使短波槽系统在过境时出现“前倾”的异常垂直结构，导致高空(500 hPa)槽线虽已过境，但小雨过程却(因低层低值系统尚未过境)不止的令预报员始料未及的现象；预报员的惯性预报思维在前期预报已出现明显偏差的情况下未及时调整，对数值模式的中尺度预报信息分析不够深入，特别是模式对弱天气过程的要素预报能力把握不准等均是最终造成这次预报失误的主要原因。文章最后还给出了上海地区春季小雨过程预报及数值模式对弱天气过程诊断量改进的几点启示。 Due to the failure forecast on a light rain of 8-9 April 2010 in Shanghai, the highest temperature forecast is fault, the error near 7˚C. In this paper, the possible reasons of failure forecast on the high-impact light rain are discussed. The results show that: the lower (850 hPa) mesoscale fron-togenesis slows down the moving speed of low-level shortwave trough, causing the shortwave trough appearing a unusual “forward-lean” vertical structure during the trough movement, lead-ing to the trough line of high-level (500 hPa) passing through Shanghai but the lower-pressure system haven't and the light rain process sustains. Obviously, the unexpected phenomenon is difficult to forecast in operation. Furthermore, the forecast results have not been timely adjusted when the early forecasts have already shown the significant deviations, because of the inertia forecasting thinking. And the mesoscale analysis based on the numerical model products is not in-depth enough. Particularly, the limited capabilities of numerical model forecast on the weak process and its weather elements, are the main reasons for ultimate forecast failure. Finally, some inspirations on the forecast of spring light rain in Shanghai and model using skill on the weak weather process are shown in this paper.

Influences of the Long-Range Transport of Biomass-Burning Pollutants on Surface Air Quality during 7-SEAS Field Campaigns

The transport of aerosols relies primarily upon air flow for conveyance; however, the air flow pattern is dominated by large-scale circulation conditions. One mission of the 2013 7-SEAS/BASELInE (Seven SouthEast Asian Studies/Biomass-burning Aerosols & Stratocumulus Environment: Lifecycles and Interactions Experiment) was to capture/confirm the downwind effect on the surface air quality due to the long-range transport of Southeast Asia biomass-burning (SEA BB) pollutants. This phenomenon was first discovered during the 2010 Dongsha experiment and directly observed by a lidar system at Hengchun in southern Taiwan during 7-SEAS/BASELInE. Through three-dimensional structural analysis, it was found that the sinking motion behind the upper-level active short wave trough is the major mechanism that enhances subsidence along the cold surge leading edge. In turn, the enhanced subsidence could bring the long-range transport of the SEA BB pollutants down to the surface. Furthermore, the HYSPLIT backward air trajectories helped identify the SEA BB pollutants in the mid-troposphere, while the fine-resolution WRF model simulation combined with dual-polarization lidar observations demonstrated the evolution of the brought-down aerosols process. An additional significant finding of this study is that the upper-level ridge-trough short wave within 20°–35°N was very active during spring 2013, highlighting the inter-annual variability of the long-range transport of SEA BB pollutants.

Multiscale Dynamics of the February 11-12, 2010, Deep South US Snowstorm Event

This study investigates the synoptic/mesoscale dynamics responsible for an unusually heavy southern US snowstorm that occurred on February 11-12, 2010, using reanalysis, observations, and numerical simulations. This record breaking snowfall event represents an example of multiple upper level and low-level jets (LLJs) and their accompanying baroclinic zones. The analysis reveals the following synoptic scale processes as significant contributors: (1) upper level jet splitting and merging, (2) advection of cold arctic air at low levels by a large anticyclone, and (3) an incoming upper level shortwave trough. In addition to the synoptic scale processes, the following mesoscale features played a major role in this snowstorm event: coexisting potential (convective) instability and conditional symmetric instability, terrain blocking, and a double LLJ development process. Sensitivity experiments including (1) limiting the orographic effects of elevated plateau in Texas and the Sierra Madre Mountains in Mexico by reducing the terrain height to 225 meters, (2) the microphysics/latent heating effects, and (3) surface fluxes on the development and intensity of the snowstorm were also conducted by turning these options off in the numerical model. Of all three experiments, the surface flux experiment displays the least amount of influence on the developing frozen precipitation bands.

Ensemble Sensitivity Analysis for Mesoscale Forecasts of Dryline Convection Initiation

AbstractTwo cases of dryline convection initiation (CI) over north Texas have been simulated (3 April 2012 and 15 May 2013) from a 50-member WRF-DART ensemble adjustment Kalman filter (EAKF) ensemble. In this study, ensemble sensitivity analysis (ESA) is applied to a convective forecast metric, maximum composite reflectivity (referred to as the response function), as a simple proxy for CI to analyze dynamic mesoscale sensitivities at the surface and aloft. Analysis reveals positional and magnitude sensitivities related to the strength and placement of important dynamic features. Convection initiation is sensitive to the evolving temperature and dewpoint fields upstream of the forecast response region in the near-CI time frame (0–12 h), prior to initiation. The sensitivity to thermodynamics is also manifest in the magnitude of dewpoint gradients along the dryline that triggers the convection. ESA additionally highlights the importance of antecedent precipitation and cold pool generation that modifies the pre-CI environment. Aloft, sensitivity of CI to a weak short-wave trough and capping inversion-level temperature is coherent, consistent, and traceable through the entire forecast period. Notwithstanding the (often) non-Gaussian distribution of ensemble member forecasts of convection, which violate the underpinnings of ESA theory, ESA is demonstrated to sufficiently identify regions that influence dryline CI. These results indicate an application of ESA for severe storm forecasting at operational centers and forecast offices as well as other mesoscale forecasting applications.

Comparison of the multi-scale features in two persistent heavy rainfall events in the middle and lower reaches of Yangtze River

Two persistent heavy rainfall (PHR) events in the middle and lower reaches of Yangtze River (MLYR) occurring in June 1982 and 1998 are studied in this paper. Though both events happened in the Meiyu season, their large-scale background and developing processes were quite different. During the PHR event in 1982, the Lake Baikal area was occupied by a strong westerly trough and the western Pacific subtropical high (WPSH) was stronger and more westward-extending than the normal years. Under such a condition, the cold–dry air and warm–moist air were continuously transported to the MLYR and favored the PHR there. For the event in 1998, the WPSH was similar to that in 1982, while the westerly trough in the Lake Baikal area was comparatively weak and a shortwave trough situating in East China contributed to advect cold–dry air to the MLYR. It is found that the high-latitude trough was closely related to the 10–30-day low-frequency oscillation while the anomaly of WPSH was linked with the combined effect of both 30–60- and 10–30-day low-frequency oscillations in the PHR event in 1982. By contrast, the 60-day low-pass perturbation demonstrated positive impact on the westward extension of WPSH and development of the Baikal trough while the 30–60-day oscillation played a role in strengthening the shortwave trough in East China and the WPSH in the case of 1998. Though the low-latitude 30–60-day oscillations contributed to the intensification and westward extension of the WPSH in both PHR events, their evolution exhibited evident differences. In the 1982 case, the 30–60-day anomalies originated from the western Indian Ocean were much more like the Madden–Julian Oscillation, while its counterpart in the 1998 case was much more similar to the first mode of the boreal summer intraseasonal oscillation.

Lake-Effect Mode and Precipitation Enhancement over the Tug Hill Plateau during OWLeS IOP2b

Improved understanding of the influence of orography on lake-effect storms is crucial for weather forecasting in many lake-effect regions. The Tug Hill Plateau of northern New York (hereafter Tug Hill), rising 500 m above eastern Lake Ontario, experiences some of the most intense snowstorms in the world. Herein the authors investigate the enhancement of lake-effect snowfall over Tug Hill during IOP2b of the Ontario Winter Lake-effect Systems (OWLeS) field campaign. During the 24-h study period, total liquid precipitation equivalent along the axis of maximum precipitation increased from 33.5 mm at a lowland (145 m MSL) site to 62.5 mm at an upland (385 m MSL) site, the latter yielding 101.5 cm of snow. However, the ratio of upland to lowland precipitation, or orographic ratio, varied with the mode of lake-effect precipitation. Strongly organized long-lake-axis parallel bands, some of which formed in association with the approach or passage of upper-level short-wave troughs, produced the highest precipitation rates but the smallest orographic ratios. Within these bands, radar echoes were deepest and strongest over Lake Ontario and the coastal lowlands and decreased in depth and median intensity over Tug Hill. In contrast, nonbanded broad-coverage periods exhibited the smallest precipitation rates and the largest orographic ratios, the latter reflecting an increase in the coverage and frequency of radar echoes over Tug Hill. These findings should aid operational forecasts and, given the predominance of broad-coverage lake-effect periods during the cool season, help explain the climatological snowfall maximum found over the Tug Hill Plateau.

An isolated tornadic supercell of 14 July 2012 in Poland — A prediction technique within the use of coarse-grid WRF simulation

On 14 July 2012 a shortwave trough with a cold front passed through Poland. A few tornadoes were reported in the north central part of the country within an isolated cyclic supercell. The cell moved along the thermal and moisture horizontal gradients and the support of a synoptic scale lift. An analysis allowed for setting up four tornado damage tracks in a distance of 100km and with a total length of 60km. Tornadoes damaged 105 buildings with predominant intensity of F1-F2/T3-T4 (maximum F3/T6) in Fujita/TORRO scale, caused 1 fatality, 10 injures and felled 500 hectares of Bory Tucholskie forest. The main aim of this article was to analyze this event and assess the possibilities of its short-term prediction. In order to achieve this, a model forecast data derived from WRF-ARW simulation with a spatial resolution of 15km and initial conditions extracted from 0000 UTC GFS was used. An analysis yielded that the cell moved in the environment of a low lifting condensation level, rich boundary layer's moisture content and a steepening vertical lapse rates that provided the presence of a thermodynamic instability. A wind vectors tilting with height and an increased vertical wind shear occurred as well. A forecasting method that combined a Universal Tornadic Index composite parameter with a convective precipitation filter showed that convective cells at 1500 UTC in the north central Poland had a potential to become tornadic. Within the use of a proposed methodology, it was possible to issue a tornado forecast for the areas where an index pointed the risk.

지역예측모델 영역 크기에 따른 집중호우 수치모의 민감도 실험

In this study, we investigated the variabilities of wind speed of 850 hPa and precipitable water over the East Asia region using the NCEP Final Analysis data from December 2001 to November 2011. A large variance of wind speed was observed in northern and eastern China during the winter period. During summer, the regions of the East China Sea, the South Sea of Japan and the East Sea show large variances in the wind speed caused by an extended North Pacific High and typhoon activities. The large variances in the wind speed in the regions are shown to be correlated with the inter-annual variability of precipitable water over the inland region of windward side of the Korean Peninsula. Based on the investigation, sensitivity tests to the domain size were performed using the WRF model version 3.6 for heavy precipitation events over the Korean Peninsula for 26 and 27 July 2011. Numerical experiments of different domain sizes were set up with 5 km horizontal and 50 levels vertical resolutions for the control and the first experimental run, and 9 km horizontal for the second experimental run. We found that the major rainfalls correspond to shortwave troughs with baroclinic structure over Northeast China and extended North Pacific High. The correlation analysis between the observation and experiments for 1-h precipitation indicated that the second experiment with the largest domain had the best performance with the correlation coefficient of 0.79 due to the synoptic-scale systems such as short-wave troughs and North Pacific High.

Climatology, Synoptic Conditions, and Misanalyses of Mississippi River Valley Drylines

Abstract The dryline is an important focal point for convection initiation. Although drylines most commonly occur on the southern Great Plains, dryline passages and subsequent severe weather outbreaks have been documented in the Mississippi River valley. This study presents a 15-yr (1999–2013) climatology of these Mississippi River valley drylines and associated severe weather. Additionally, synoptic patterns are identified that may result in drylines moving atypically far eastward into the Mississippi River valley. In total, 39 Mississippi River valley drylines (hereafter referred to as MRV drylines) were identified from the North American Regional Reanalysis (NARR) dataset through the study period. Mean and anomaly synoptic composites were created for these drylines. MRV dryline events typically occur under synoptically active conditions with an amplified upper-air pattern, a 500-hPa shortwave trough to the west or northwest of the dryline, and a strong surface cyclone to the north. These boundaries are often misanalyzed or inconsistently analyzed as cold fronts, stationary fronts, or trough axes on surface maps; of the 33 cases of identified MRV drylines for which the Weather Prediction Center archived analyses were available, only 6 were correctly analyzed as drylines. Drylines moving into the Mississippi River valley often result in severe weather outbreaks in the Mississippi River valley, the Midwest, and the southeastern United States.

Secondary Cyclogenesis along an Occluded Front Leading to Damaging Wind Gusts: Windstorm Kyrill, January 2007

Abstract Windstorm Kyrill affected large parts of Europe in January 2007 and caused widespread havoc and loss of life. In this study the formation of a secondary cyclone, Kyrill II, along the occluded front of the mature cyclone Kyrill and the occurrence of severe wind gusts as Kyrill II passed over Germany are investigated with the help of high-resolution regional climate model simulations. Kyrill underwent an explosive cyclogenesis south of Greenland as the storm crossed poleward of an intense upper-level jet stream. Later in its life cycle secondary cyclogenesis occurred just west of the British Isles. The formation of Kyrill II along the occluded front was associated (i) with frontolytic strain and (ii) with strong diabatic heating in combination with a developing upper-level shortwave trough. Sensitivity studies with reduced latent heat release feature a similar development but a weaker secondary cyclone, revealing the importance of diabatic processes during the formation of Kyrill II. Kyrill II moved farther toward Europe and its development was favored by a split jet structure aloft, which maintained the cyclone’s exceptionally deep core pressure (below 965 hPa) for at least 36 h. The occurrence of hurricane-force winds related to the strong cold front over north and central Germany is analyzed using convection-permitting simulations. The lower troposphere exhibits conditional instability, a turbulent flow, and evaporative cooling. Simulation at high spatiotemporal resolution suggests that the downward mixing of high momentum (the wind speed at 875 hPa widely exceeded 45 m s−1) accounts for widespread severe surface wind gusts, which is in agreement with observed widespread losses.

The Role of Southwesterly Flow in MCS Formation During a Heavy Rain Event in Taiwan on 12 - 13 June 2005

This paper presents a numerical study of a heavy rain event that occurred in southern Taiwan in June 2005. From 11-13 June 2005, a weak Mei-yu front moved southeastward from China to Taiwan, while mesoscale convective systems (MCSs) were developing and moving northward over the northern South China Sea (SCS). During the first day of the event the southwesterly flow intensified when a ridge associated with the Pacific high extended northwestward from the Philippines to the southern Taiwan Strait (TS). This pressure pattern produced a large northwestward pressure gradient force that created a southeasterly wind speed increase followed by intensification of the southwesterly flow through Coriolis acceleration. An low-level jet (LLJ) formed consequently and transported moisture and unstable air toward the southwestern coast of Taiwan. MCSs were triggered in the southwesterly flow because the potentially unstable air was lifted in a low-level convergence and shearing vorticity environment. They intensified, became organized, and moved northeastward overland, resulting in heavy rainfall in southern Taiwan. On the second day, low pressure formed near the southern TS because of the combined effect of a travelling short-wave trough and a pressure reduction resulting from the latent heat release by the evolving MCSs. This pressure change produced down-gradient acceleration in the northeastward direction, resulting in southwesterly flow strengthening. The local wind acceleration was smaller than that of the first day because the dominant pressure system was local scale, while that of the first day was synoptic scale.