Severe Convective Events Research Articles

Analysis of the role of the planetary boundary layer schemes during a severe convective storm

Abstract. The role played by planetary boundary layer (PBL) in the development and evolution of a severe convective storm is studied by means of meso-scale modeling and surface and upper air observations. The severe convective precipitation event that occurred on 14 September 1999 in the northeast of the Iberian Peninsula was simulated by means of the mesoscale model MM5 (version 3) using three different PBL schemes. The numerical results show a large impact of the PBL schemes on the precipitation fields associated to the convective storm. The schemes are based on different physical assumptions: the nonlocal first order Medium-Range Forecast (MRF) and Blackadar (BLA) scheme and the local, one-and-a-half order ETA scheme. Surface and radar observations are used to validate the model results. The comparison focuses on three aspects: the evolution, the spatial distribution and the 24-h accumulated precipitation. The comparison with rain gauge observations shows that the MRF, BLA and ETA schemes predicted most of the precipitation during the morning, whereas the rain gauge stations recorded rainfall during the evening. The evaluation performed with the radar data shows that all three numerical simulations produced a realistic spatial accumulated precipitation distribution. According to the quantity distribution, all three numerical simulations were able to predict precipitation quantities comparable to the rain gauge measurements. The MRF scheme predicted the largest average accumulated precipitation and the largest average precipitation rate, whereas the ETA scheme predicted the smallest accumulated precipitation and average precipitation rate. However, the ETA scheme yielded the highest extreme precipitation rates. The performance of the three schemes is analyzed in terms of the vertical distribution of potential temperature, specific humidity and conserved variables, like equivalent potential temperature and total water content. The MRF scheme showed more evidence of enhanced mixing than did the other schemes. Due to this process, more moisture was more efficiently transported to the free atmosphere. Consequently, the MRF scheme predicts more widespread precipitation. Furthermore, the enhanced mixing led to a less sharp capping inversion. However, the stronger inversion resulting from suppressed mixing processes in the case of the ETA scheme yielded higher values of convective available potential energy (CAPE) than did the other two schemes. Consequently, the more extreme precipitation rates are simulated by MM5 when the ETA scheme is used.

The outbreak of severe storms along convergence lines northeast of the Alps. Case study of the 3 August 2001 mesoscale convective system with a pronounced bow echo

Thunderstorm development in Austria is highly influenced by orographical forcing especially in summer. However, the most severe thunderstorms occur in the lower areas north and southeast of the Alps where mountains are not high. The reason is the undisturbed initiation and intensification of thunderstorms due to the low-level wind field which is modified near the edge of the Alps. Typically, severe convective weather events in Austria develop when a slow-moving cold front approaches from west (e.g., Switzerland) and when unstable air is advected from the Mediterranean by southwesterly flow. During such conditions, a convergence zone is frequently formed north of the Alps by a low-level airflow as a result of diabatic heating and evaporative cooling along the cloud edge of the cold front. This convergence line moves faster to the east than the cold front itself and triggers new cells or intensifies existing cells. The case study presented in this work is based on analyses of data from the Austrocontrol Weather Radar Network and objective surface pressure analyses. Two supercells initiated along a convergence line due to enhanced low level wind shear on the lee side of the Alps. Both severe storms, linked to the northern slopes of the Alps, approached Austria from Bavaria which evolved into a Mesoscale Convective System with a pronounced bow echo within the orographic gap between the mountain range of Bohemian Forest and the Alps. The case study shows a typical storm configuration and how to improve nowcasting of mesoscale modified thunderstorms north of the Alps.

A diagnostic study of a summer convective precipitation event in the Czech Republic using a nonhydrostatic NWP model

Forecasting, locally, heavy convective precipitation events has been an important topic of investigation for a few decades. It has been recognized that nonhydrostatic models are able to simulate the dynamics of organized convective systems with spatial resolutions of 1 km and less, provided that adequate triggering is entered in the model. Apart from experimental model runs, there are also diagnostic case studies of convective events that employ such models and compare the results with radar data, for example. Nevertheless, the purely deterministic prediction of severe convective systems and the corresponding quantitative precipitation forecast (QPF) is far from being resolved. A research project, focused on case studies of severe convective events in the region of the Czech Republic (CR), has been running since 2000. Data from this study have been used to adapt the nonhydrostatic numerical weather prediction (NWP) Local Model of the German Weather Service (LM DWD) to use a 2.8-km horizontal resolution Small Local Model (SLM) for forecasting and/or warning of severe convection events. The results of several model applications are summarized and discussed. The historical convective event from July 22 to 23, 1998, which caused an extreme precipitation amount and a flash flood over the NE part of the CR, was analyzed with several model runs. The model runs differed in the design of the experiment (starting time of SLM integration, convective parameterization ON/OFF, modification of model orography, etc.). The results of the SLM integration indicate that the inclusion of the convective parameterization smoothes precipitation fields, which is not realistic. The structure of precipitation fields, obtained by the SLM with convective parameterization switched OFF, better corresponds to the spatial structure of radar-derived precipitation. The resulting precipitation depends on the start time of the SLM integration. The results show that the accumulated precipitation does not differ too much, provided the SLM starts its integration at least 6 h before the studied event. The fact that the SLM produces large precipitation in contrast to the routine NWP and that the area precipitation pattern corresponds to the radar precipitation is promising.

The Advanced Regional Prediction System (ARPS), storm-scale numerical weather prediction and data assimilation

In this paper, we first describe the current status of the Advanced Regional Prediction System of the Center for Analysis and Prediction of Storms at the University of Oklahoma. A brief outline of future plans is also given. Two rather successful cases of explicit prediction of tornadic thunderstorms are then presented. In the first case, a series of supercell storms that produced a historical number of tornadoes was successfully predicted more than 8 hours in advance, to within tens of kilometers in space with initiation timing errors of less than 2 hours. The general behavior and evolution of the predicted thunderstorms agree very well with radar observations. In the second case, reflectivity and radial velocity observations from Doppler radars were assimilated into the model at 15-minute intervals. The ensuing forecast, covering a period of several hours, accurately reproduced the intensification and evolution of a tornadic supercell that in reality spawned two tornadoes over a major metropolitan area. These results make us optimistic that a model system such as the ARPS will be able to deterministically predict future severe convective events with significant lead time. The paper also includes a brief description of a new 3DVAR system developed in the ARPS framework. The goal is to combine several steps of Doppler radar retrieval with the analysis of other data types into a single 3-D variational framework and later to incorporate the ARPS adjoint to establish a true 4DVAR data assimilation system that is suitable for directly assimilating a wide variety of observations for flows ranging from synoptic down to the small nonhydrostatic scales.

An Interactive Severe Weather Activity to Motivate Student Learning

An interactive Web-based severe weather activity has been developed at Iowa State University with broad applications to motivate student learning. The exercise uses an extensive archive of weather data emphasizing warm-season severe convective events and cold-season winter storms. Several variations of the activity have been developed based upon the meteorological background of students. The flexible design of the activity may allow for its use in K–12 settings, or as a significant training tool for weather forecasters outside the classroom.

Satellite multi-frequency observations of severe convective systems in the Mediterranean

In the present work two convective episodes in the Mediterranean are studied by means of nearly simultaneous Meteosat Visible-Infrared and SSM/I Passice Microwave data. The two datasets are carefully co-located and an estimate of the uncorrected errors is given in order to have a reliable pixel-by-pixel matching in the images, where possible. The aim of this work is to provide detailed observation of such cloud systems on a pure satellite basis.The two severe convective events are investigated by estimating absorbing and scattering cloud layers in terms of conceptual models available for these cloud systems. The use of microwave derived scattering and absorption indicators, coupled with the cloud top Meteosat observations, is able to describe at least qualitatively relevant features of convective clouds, such as the onset of precipitation and the structure of the anvil. Rain areas computed by two (one for Meteosat IR and one for SSM/I) single-threshold rainfall estimation techniques are compared and significant discrepancies are found.

Diagnosis of an Amplifying and Decaying Baroclinic Wave Using Wind Profiler Data

Abstract Kinematic and thermodynamic quantities derived from wind profiler triangles are used to help describe the structure of both an amplifying and decaying baroclinic wave as they traversed portions of the wind profiler demonstration network. The data provide excellent diagnoses of the cyclogenetic processes associated with the amplifying system and the cyclolytic processes associated with the decaying system. The importance of a baroclinic wave's vertical tilt and the associated profiler-derived advective patterns of the systems as they relate to surface evolution are shown to be consistent with conceptual models of baroclinic waves. These structural aspects of the observed baroclinic waves are also shown to vary substantially on short timescales. In addition, a sub-synoptic-scale feature associated with a severe convective event that developed ahead of the decaying wave trough axis was observed quite well by the profiler network. This feature's detection was dependent on the high temporal resolution...

The Causes of Severe Convective Outbreaks in Alberta. Part II: Conceptual Model and Statistical Analysis

Abstract An intercomparison of all 11 Limestone Mountain Experiment case days provided the basis for a conceptual model of severe convective outbreaks in Alberta. It is proposed that most severe convective events result when upper-level cooling, associated with an advancing, synoptic-scale trough, occurs in phase with strong surface heating over the Alberta foothills. The deep destabilization over the elevated topography acts to amplify the mountain-plain circulation and to generate mesoscale upslope moisture transport. Concurrently, the surface synoptic pressure gradient gives rise to east-northerly winds that advect the moisture-rich air of the eastern plains into the lower branch of the mountain-plain circulation. In this manner, the plains moisture is permitted to reach the convectively active foothills through underrunning of the capping lid. The end product is the initiation of well-organized, severe convective storms that move eastward with the westerly component of the midtropospheric winds. A sta...