Abstract

Dual-Doppler radar analyses of three tornadoes associated with a multicellular line of storms are presented. The F2–F3 intensity tornadoes occurred on 15 June 1988 near Denver, Colorado, during the Terminal Doppler Weather Radar (TDWR) Project. These tornadoes developed from misocyclones of no larger than 2 km in diameter that formed along the collision of two surface outflows. The misocyclones were observed to build in height and intensify with time, coincident with rapid storm growth overhead. All three misocyclones were clearly associated with the maximum storm updrafts. Downdrafts and associated outflows did not play a role in the formation of one of the tornadoes, but may have contributed to the genesis of the other two tornadoes. It is clear that a downdraft is not a necessary condition for the formation of a nonsupercell tornado, but when present, likely plays a role in determining the timing and intensity of the tornado. This is achieved by the downdraft and outflow causing an increase in the magnitude of the low-level convergence and updraft.

Vertical vorticity production terms were examined for each tornado. Given the close proximity in time and space of the tornadoes, there was surprising variability in the magnitudes and locations of the stretching, tilting, and advection terms for each tornado. In general, however, the predominant contribution to positive vertical vorticity and tornadogenesis was from vorticity stretching in the 0.2–2.0-km layer resulting from intensification of low-level convergence and storm updrafts. Above 2.0 km, increased vertical vorticity resulted from a redistribution of low-level vorticity vertically. Small areas of positive vorticity tilting were found within the regions of large streamwise vorticity just prior to tornadogenesis but not during the formative stages of the mesocyclones, amplifying the already strong contributions to tornadogenesis from vertical stretching of the vortices.

The spatial resolution of the data presented here is as high as any documented in tornado literature. However, limitations in what features are actually resolvable became strikingly apparent and are discussed in the paper.

Abstract

Thirty-one microburst-producing storms from northeast Colorado were studied using single and multiple Doppler radar for the purpose of identifying radar signatures that indicated the development of a downdraft capable of producing a microburst. Descending reflectivity cores, increasing radial convergence within cloud, rotation and reflectivity notches were found to be microburst precursors, appearing typically 2–6 min prior to initial surface outflow. Descending maximum reflectivity cores coincident with increasing radial convergence within cloud (3–8 km AGL) or near cloud base is believed to be a good indicator of storm downdraft and microburst predictor, especially when coupled with low θ _e air above cloud base and a dry adiabatic lapse rate below cloud base. Three conceptual models have been drawn, based on the 31 events, to summarize the radar signatures of importance in low, moderate, and high-reflectivity microburst-producing storms.

Experience indicates that Doppler radar may aid in providing 0–10 min nowcasts of microbursts. This requires the rapid perusal and assimilation of a large quantity of radar data by the forecaster. To improve upon this effort, a forecaster-computer environment is proposed to allow the forecaster to readily view radar reflectivity and Doppler velocity information in both unprocessed and analyzed form. Use of multi-image radar displays and time-height profiles of quantitative radar estimates of reflectivity and radial shear are suggested to provide an environment where rapid progress can be made in developing techniques to nowcast microbursts.

Abstract

The evolution of cumulus clouds over a variety of radar-detected, boundary layer convergence features in eastern Colorado has been examined using Geostationary Operational Environmental Satellite (GOES) imagery and Weather Surveillance Radar-1988 Doppler (WSR-88D) data. While convective storms formed above horizontal rolls in the absence of any additional surface forcing, the most intense storms initiated in regions above: gust fronts, gust front interaction with horizontal rolls, and terrain-induced stationary convergence zones. The onset of vigorous cloud growth leading to storm development was characterized by cloud tops that reached subfreezing temperatures and exhibited large cooling rates at cloud top 15 min prior to the first detection of 10-dBZ radar echoes aloft and 30 min before 35 dBZ. The rate of cloud-top temperature change was found to be important for discriminating between weakly precipitating storms (<35 dBZ) and vigorous convective storms (>35 dBZ). Results from this study have been used to increase the lead time of thunderstorm initiation nowcasts with the NCAR automated, convective storm nowcasting system. This improvement is demonstrated at two operational forecast offices in Virginia and New Mexico.

Abstract

The enhanced observation period during the HIGHWAY field campaign in East Africa provided the opportunity to obtain continuous ground-based radar observations over the Lake Victoria basin. This provided insight into thunderstorm initiation processes and thunderstorm evolution. This insight is significant for it can lead to nowcasting thunderstorms over Lake Victoria, which is particularly important because of the >200 000 fishers using the lake daily and the extremely high number of drownings resulting from capsized boats caused by large waves and high winds from thunderstorms. Radar data from the south shoreline of Lake Victoria made it possible to observe thunderstorm activity over the entire lake. Unexpectedly the radar returns from high concentration of insects over the lake made it possible for the radar to observe boundary layer convergence lines. With this information a radar-trained forecaster could provide nowcasts of severe storm locations and by using extrapolation techniques provide nowcasts of their future location. In addition, rules for forecasting the timing and extent of nighttime thunderstorm activity over the lake based on radar monitoring of earlier activity along the northeast land/lake region are provided. While there are many obstacles to overcome, it is hoped that in the near future this possible life-saving information can be provided to Lake Victoria boaters.

Abstract

The data-rich International H₂O Project (IHOP_2002) experiment is used to study convective storm initiation and subsequent evolution for all days of the experiment. Initiation episodes were almost evenly divided between those triggered along surface-based convergence lines and elevated initiation episodes that showed no associated surface convergence. The elevated episodes occurred mostly at night, and the surface-based episodes occurred during the afternoon and evening. Surface-based initiations were mostly associated with synoptic fronts and gust fronts and less so with drylines and bores. Elevated initiations were frequently associated with observable convergent or confluent features in the Rapid Update Cycle (RUC) wind analysis fields between 900 and 600 hPa. The RUC10 3-h forecast of the precipitation initiation episodes were correct 44% of the time, allowing a tolerance of 250 km in space and for the forecast being early by one period. However, the accuracy was closely tied to the scale of the initiation mechanism, being highest for synoptic frontal features and lowest for gust fronts.

Gust fronts were a primary feature influencing the evolution of the initiated storms. Almost one-half of the storm complexes associated with initiation episodes did not produce surface gust fronts. Storm systems that did not produce gust fronts most often lived 2–6 h while those that did frequently lived at least 8 h. The largest and longest-lived storm complexes had well-developed intense gust fronts that influenced the propagation of the storm system. The RUC10 was generally not successful in forecasting the evolution and motion of the larger, more intense storm complexes; presumably this was because it did not produce strong gust fronts.

Implications for forecasting convective storm initiation and evolution are discussed.

Abstract

A conceptual model is presented for developing a new tool for nowcasting severe thunderstorms using existing operational data. Selected output from two operational, automated, weather detection and forecasting systems have been combined together within a fuzzy logic–based, data fusion system to test the concept and produce 15-min nowcasts of severe weather. The NCAR Auto-Nowcast System provides information and nowcasts on the evolving boundary layer and storm initiation, growth, and decay. The National Severe Storms Laboratory Warning Decision Support System (WDSS) identifies severe weather attributes within storms and provides storm-centric and specific detections of strong winds, mesocyclones, tornadoes, and probabilities of hail and severe hail. A modified version of the Auto-Nowcast System is employed as the engine for combining the Auto-Nowcast gridded output with the object-based WDSS output. Severe thunderstorm nowcasts are compared with available spotter reports for a multicellular, hail-producing squall-line event and a tornadic supercell event. Proof of concept is demonstrated and the results are encouraging as some skill is observed with the 15-min nowcasts. Many challenges still exist in producing a robust tool and these challenges are discussed.

Abstract

Flash flooding represents a significant hazard to human safety and a threat to property. Simulation and prediction of floods in complex urban settings requires high-resolution precipitation estimates and distributed hydrologic modeling. The need for reliable flash flood forecasting has increased in recent years, especially in urban communities, because of the high costs associated with flood occurrences. Several storm nowcast systems use radar to provide quantitative precipitation forecasts that can potentially afford great benefits to flood warning and short-term forecasting in urban settings. In this paper, the potential benefits of high-resolution weather radar data, physically based distributed hydrologic modeling, and quantitative precipitation nowcasting for urban hydrology and flash flood prediction were demonstrated by forcing a physically based distributed hydrologic model with precipitation forecasts made by a convective storm nowcast system to predict flash floods in a small, highly urbanized catchment in Denver, Colorado. Two rainfall events on 5 and 8 July 2001 in the Harvard Gulch watershed are presented that correspond to times during which the storm nowcast system was operated. Results clearly indicate that high-resolution radar-rainfall estimates and advanced nowcasting can potentially lead to improvements in flood warning and forecasting in urban watersheds, even for short-lived events on small catchments. At lead times of 70 min before the occurrence of peak discharge, forecast accuracies of approximately 17% in peak discharge and 10 min in peak timing were achieved for a 10 km² highly urbanized catchment.

Abstract

Doppler weather radar data from the Joint Airport Weather Studies (JAWS) Project are used to determine the horizontal and vertical structure of airflow within microbursts. Typically, the associated downdraft is about 1 km wide and begins to spread horizontally at a height below 1 km. The median time from initial divergence at the surface to maximum differential wind velocity across the microburst is 5 min. The height of maximum differential velocity is ∼75 m. The median velocity differential is 22 m s⁻¹ over an average distance of 3.1 km. The outflow is asymmetric, averaging twice as strong along the maximum shear axis compared to the minimum axis.

Doppler radar could be an effective means for identifying microbursts and warning aircraft of wind shear hazards. For microburst detection such a radar must be able to measure wind velocities in clear air as well as in heavy rain and hail. Scan update rates should be approximately every 2 min and the lowest few hundred meters of the atmosphere must be observed. Ground clutter must be considerably reduced from levels typically obtained with present Doppler radars. New antenna technology and signal processing techniques may solve this problem. Automated range and velocity unfolding is required, as well as automated identification and dissemination techniques.

Abstract

The Beijing 2008 Forecast Demonstration Project (B08FDP) included a variety of nowcasting systems from China, Australia, Canada, and the United States. A goal of the B08FDP was to demonstrate state-of-the-art nowcasting systems within a mutual operational setting. The nowcasting systems were a mix of radar echo extrapolation methods, numerical models, techniques that blended numerical model and extrapolation methods, and systems incorporating forecaster input. This paper focuses on the skill of the nowcasting systems to forecast convective storms that threatened or affected the Summer Olympic Games held in Beijing, China. The topography surrounding Beijing provided unique challenges in that it often enhanced the degree and extent of storm initiation, growth, and dissipation, which took place over short time and space scales. The skill levels of the numerical techniques were inconsistent from hour to hour and day to day and it was speculated that without assimilation of real-time radar reflectivity and Doppler velocity fields to support model initialization, particularly for weakly forced convective events, it would be very difficult for models to provide accurate forecasts on the nowcasting time and space scales. Automated blending techniques tended to be no more skillful than extrapolation since they depended heavily on the models to provide storm initiation, growth, and dissipation. However, even with the cited limitations among individual nowcasting systems, the Chinese Olympic forecasters considered the B08FDP human consensus forecasts to be useful. Key to the success of the human forecasts was the development of nowcasting rules predicated on the character of Beijing convective weather realized over the previous two summers. Based on the B08FDP experience, the status of nowcasting convective storms and future directions are presented.

Abstract

East African countries benefit economically from the largest freshwater lake in Africa: Lake Victoria (LV). Around 30 million people live along its coastline, and 5.4 million people subsist on its fishing industry. However, more than 1000 fishermen die annually by high-wave conditions often produced by severe convective wind phenomena, which marks this lake one of the deadliest places in the world for hazardous weather impacts. The World Meteorological Organization launched the 3-yr High Impact Weather Lake System (HIGHWAY) project, with the main objective to reduce loss of lives and economic goods in the lake basin and improve the resilience of the local communities. The project conducted a field campaign in 2019 aiming to provide forecasters with high-resolution observations and to study the storm life cycle over the lake basin. The research discussed here used the S-band polarimetric Tanzania radar from the field campaign to investigate the diurnal cycle of the convective mode over the lake. We classified the lake storms occurring during the two wet seasons into six different convective modes and present their diurnal evolution, organization, and main radar-based attributes, thereby extending the knowledge of convection on the lake. The result is the creation of a “convection catalog for Lake Victoria,” using the operational forecast lake sectors, and defining the exact times for the different timeslots resulting from the HIGHWAY project for the marine forecast. This will inform methods to improve the marine operational forecasts for Lake Victoria, and to provide the basis for new standard operation procedures (SOP) for severe weather surveillance and warning.