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Russ S. Schumacher

Abstract

In this study, idealized numerical simulations are used to identify the processes responsible for initiating, organizing, and maintaining quasi-stationary convective systems that produce locally extreme rainfall amounts. Of particular interest are those convective systems that have been observed to occur near mesoscale convective vortices (MCVs) and other midlevel circulations. To simulate the lifting associated with such circulations, a low-level momentum forcing is applied to an initial state that is representative of observed extreme rain events. The initial vertical wind profile includes a sharp reversal of the vertical wind shear with height, indicative of observed low-level jets.

Deep moist convection initiates within the region of mesoscale lifting, and the resulting convective system replicates many of the features of observed systems. The low-level thermodynamic environment is nearly saturated, which is not conducive to the production of a strong surface cold pool; yet the convection quickly organizes into a back-building line. It is shown that a nearly stationary convectively generated low-level gravity wave is responsible for the linear organization, which continues for several hours. New convective cells repeatedly form on the southwest end of the line and move to the northeast, resulting in large local rainfall amounts. In the later stages of the simulated convective system, a cold pool does develop, but its interaction with the strong reverse shear at low levels is not optimized for the maintenance of deep convection along its edge. A series of sensitivity experiments shows some of the effects of hydrometeor evaporation and melting, planetary rotation, and the imposed mesoscale forcing.

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Russ S. Schumacher

Abstract

This study makes use of operational global ensemble forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) to examine the factors contributing to, or inhibiting, the development of a long-lived continental vortex and its associated rainfall. From 25 to 30 June 2007, a vortex developed and grew upscale over the southern plains of the United States. It was associated with persistent heavy rainfall, with over 100 mm of rain falling in much of Texas, Oklahoma, Kansas, and Missouri, and amounts exceeding 300 mm in southeastern Kansas. Previous research has shown that, in comparison with other rainfall events of similar temporal and spatial scales, this event was particularly difficult for numerical models to predict.

Considering the ensemble members as different possible realizations of the evolution of the event, several methods are used to examine the processes that led to the development and maintenance of the long-lived vortex and its associated rainfall, and to its apparently limited predictability. Linear statistics are calculated to identify synoptic-scale flow features that were correlated to area-averaged precipitation, and differences between composites of “dry” and “wet” ensemble members are used to pinpoint the processes that were favorable or detrimental to the system’s development. The maintenance of the vortex, and its slow movement in the southern plains, are found to be closely related to the strength of a closed midlevel anticyclone in the southwestern United States and the strength of a midlevel ridge in the northern plains. In particular, with a weaker upstream anticyclone, the shear and flow over the incipient vortex are relatively weak, which allows for slow movement and persistent heavy rains. On the other hand, when the upstream anticyclone is stronger, there is stronger northerly shear and flow, which causes the incipient vortex to move southwestward into the high terrain of Mexico and dissipate. These relatively small differences in the wind and mass fields early in the ensemble forecast, in conjunction with modifications of the synoptic and mesoscale flow by deep convection, lead to very large spread in the resulting precipitation forecasts.

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Russ S. Schumacher

Abstract

Using a method for initiating a quasi-stationary, heavy-rain-producing elevated mesoscale convective system in an idealized numerical modeling framework, a series of experiments is conducted in which a shallow layer of drier air is introduced within the near-surface stable layer. The environment is still very moist in the experiments, with changes to the column-integrated water vapor of only 0.3%–1%. The timing and general evolution of the simulated convective systems are very similar, but rainfall accumulation at the surface is changed by a much larger fraction than the reduction in moisture, with point precipitation maxima reduced by up to 29% and domain-averaged precipitation accumulations reduced by up to 15%. The differences in precipitation are partially attributed to increases in the evaporation rate in the shallow subcloud layer, though this is found to be a secondary effect. More importantly, even though the near-surface layer has strong convective inhibition in all simulations and the convective available potential energy of the most unstable parcels is unchanged, convection is less intense in the experiments with drier subcloud layers because less air originating in that layer rises in convective updrafts. An additional experiment with a cooler near-surface layer corroborates these findings. The results from these experiments suggest that convective systems assumed to be elevated are, in fact, drawing air from near the surface unless the low levels are very stable. Considering that the moisture differences imposed here are comparable to observational uncertainties in low-level temperature and moisture, the strong sensitivity of accumulated precipitation to these quantities has implications for the predictability of extreme rainfall.

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Russ S. Schumacher

Abstract

On 31 May 2013, a supercell thunderstorm initiated in west-central Oklahoma and produced a deadly tornado. This convection then grew upscale, with a nearly stationary line developing early on 1 June that produced very heavy rainfall and caused deadly flash flooding in the Oklahoma City area. Real-time convection-allowing (Δx = 4 km) model forecasts used during the Mesoscale Predictability Experiment (MPEX) provided accurate guidance regarding the timing, location, and evolution of convection in this case. However, attempts to simulate this event at higher resolution degraded the forecast, with the primary supercell failing to initiate and the evolution of the overnight MCS not resembling the observed system. Experiments to test the dependence of forecasts of this event on model resolution show that with grid spacing smaller than 4 km, mixing along the dryline in northwest Texas was more vigorous, causing low-level dry air to move more quickly eastward into Oklahoma. This drying prevented the supercell from initiating near the triple point in the higher-resolution simulations. Then, the lack of supercellular convection and its associated cold pool altered the evolution of subsequent convection. Whereas in observations and the 4-km forecast, a nearly stationary MCS developed parallel to, but displaced from, the supercell’s cold pool, the higher-resolution simulations instead had a faster-moving squall line that produced less rainfall. Although the degradation of convective forecasts at higher resolution is probably unusual and appears sensitive to the choice of boundary layer parameterization, these findings demonstrate that how numerical models treat boundary layer processes at different grid spacings can, in some cases, have profound influences on predictions of high-impact weather.

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Russ S. Schumacher

Abstract

Floods and flash floods are, by their nature, a multidisciplinary problem: they result from a convergence of atmospheric conditions, the underlying topography, hydrological processes, and the built environment. Research aimed at addressing various aspects of floods, on the other hand, often follows paths that do not directly address all of these fundamental connections. With this in mind, the NSF-sponsored Studies of Precipitation, Flooding, and Rainfall Extremes Across Disciplines (SPREAD) workshop was organized and held in Colorado during the summers of 2013 and 2014. SPREAD brought together a group of 27 graduate students from a wide variety of academic disciplines, but with the unifying theme being research interests in extreme precipitation or flooding. During the first meeting of the workshop, groups of graduate student participants designed interdisciplinary research projects that they then began work on over the intervening year, with the second meeting providing a venue to present their results. This article will outline the preliminary findings of these research efforts. Furthermore, the workshop participants had the unique and meaningful experience of visiting several locations in Colorado that had flooded in the past, and then visiting them again in the aftermath of the devastating 2013 floods. In total, the workshop resulted in several fruitful research activities that will advance understanding of precipitation and flooding. Even more importantly, the workshop fostered the development of a network of early-career researchers and practitioners who will be “multilingual” in terms of scientific disciplines, and who are poised to lead within their respective careers and across the scientific community.

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John M. Peters and Russ S. Schumacher

Abstract

This study details the development and use of an idealized modeling framework to simulate a quasi-stationary heavy-rain-producing mesoscale convective system (MCS). A 36-h composite progression of atmospheric fields computed from 26 observed warm-season heavy-rain-producing training line/adjoining stratiform (TL/AS) MCSs was used as initial and lateral boundary conditions for a numerical simulation of this MCS archetype.

A realistic TL/AS MCS initiated and evolved within a simulated mesoscale environment that featured a low-level jet terminus, maximized low-level warm-air advection, and an elevated maximum in convective available potential energy. The first stage of MCS evolution featured an eastward-moving trailing-stratiform-type MCS that generated a surface cold pool. The initial system was followed by rearward off-boundary development, where a new line of convective cells simultaneously redeveloped north of the surface cold pool boundary. Backbuilding persisted on the western end of the new line, with individual convective cells training over a fixed geographic region. The final stage was characterized by a deepening and southward surge of the cold pool, accompanied by the weakening and slow southward movement of the training line. The low-level vertical wind shear profile favored kinematic lifting along the southeastern cold pool flank over the southwestern flank, potentially explaining why convection propagated with (did not propagate with) the former (latter) outflow boundaries.

The morphological features of the simulated MCS are common among observed cases and may, therefore, be generalizable. These results suggest that they are emergent from fundamental features of the large-scale environment, such as persistent regional low-level lifting, and with the vertical environmental wind profile characteristic to TL/AS systems.

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Samantha L. Lynch and Russ S. Schumacher

Abstract

From 1 to 3 May 2010, persistent heavy rainfall occurred in the Ohio and Mississippi River valleys due to two successive quasi-stationary mesoscale convective systems (MCSs), with locations in central Tennessee accumulating more than 483 mm of rain, and the city of Nashville experiencing a historic flash flood. This study uses operational global ensemble forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) to diagnose atmospheric processes and assess forecast uncertainty in this event. Several ensemble analysis methods are used to examine the processes that led to the development and maintenance of this precipitation system. Differences between ensemble members that correctly predicted heavy precipitation and those that did not were determined, in order to pinpoint the processes that were favorable or detrimental to the system's development. Statistical analysis was used to determine how synoptic-scale flows were correlated to 5-day area-averaged precipitation. The precipitation throughout Nashville and the surrounding areas occurred ahead of an upper-level trough located over the central United States. The distribution of precipitation was found to be closely related to the strength of this trough and an associated surface cyclone. In particular, when the upper-level trough was elongated, the surface cyclone remained weaker with a narrower low-level jet from the south. This caused the plume of moisture from the Caribbean Sea to be concentrated over Tennessee and Kentucky, where, in conjunction with focused ascent, heavy rain fell. Relatively small differences in the wind and pressure fields led to important differences in the precipitation forecasts and highlighted some of the uncertainties associated with predicting this extreme rainfall event.

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Samuel J. Childs and Russ S. Schumacher

Abstract

A localized tornado and severe hail climatology is updated and enhanced for eastern Colorado. This region is one of the most active severe weather areas in the United States because of its location immediately east of the Rocky Mountains, intrusions of Gulf of Mexico moisture into a dry climate, and various small-scale topographically forced features such as the “Denver Cyclone.” Since the 1950s, both annual tornado and severe (≥1.0 in.; 1 in. = 25.4 mm) hail reports and days have been increasing across the area, but several nonmeteorological factors distort the record. Of note is a large population bias in the severe hail data, with reports aligned along major roadways and in cities, and several field projects contributing to an absence of (E)F0 tornado reports [on the (enhanced) Fujita scale] in the 1980s. In the more consistently observed period since 1997, tornado reports and days show a slight decreasing trend while severe hail reports and days show an increasing trend, although large variability exists on the county level. Eastern Colorado tornadoes are predominantly weak, rarely above (E)F1 intensity, and with a maximum just east of the northern urban corridor. Severe hail has a maximum along the foothills and shows a trend toward a larger ratio of significant (≥2.0 in.; ≥50.8 mm) hail to severe hail reports over time. Both tornadoes and severe hail have trended toward shorter seasons since 1997, mostly attributable to an earlier end to the season. By assessing current and historical trends from a more localized perspective, small-scale climatological features and local societal impacts are exposed—features that national climatologies can miss.

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John M. Peters and Russ S. Schumacher

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In this research, a numerical simulation of an observed training line/adjoining stratiform (TL/AS)-type mesoscale convective system (MCS) was used to investigate processes leading to upwind propagation of convection and quasi-stationary behavior. The studied event produced damaging flash flooding near Dubuque, Iowa, on the morning of 28 July 2011.

The simulated convective system well emulated characteristics of the observed system and produced comparable rainfall totals. In the simulation, there were two cold pool–driven convective surges that exited the region where heavy rainfall was produced. Low-level unstable flow, which was initially convectively inhibited, overrode the surface cold pool subsequent to these convective surges, was gradually lifted to the point of saturation, and reignited deep convection. Mechanisms for upstream lifting included persistent large-scale warm air advection, displacement of parcels over the surface cold pool, and an upstream mesolow that formed between 0500 and 1000 UTC.

Convection tended to propagate with the movement of the southeast portion of the outflow boundary, but did not propagate with the southwest outflow boundary. This was explained by the vertical wind shear profile over the depth of the cold pool being favorable (unfavorable) for initiation of new convection along the southeast (southwest) cold pool flank.

A combination of a southward-oriented pressure gradient force in the cold pool and upward transport of opposing southerly flow away from the boundary layer moved the outflow boundary southward. Upward transport of southerly momentum by convection along the southward-moving outflow boundary, along with convectively induced southward pressure gradient forces cut off southerly inflow to the MCS, which temporarily disrupted backbuilding.

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Russ S. Schumacher and Thomas J. Galarneau Jr.

Abstract

Global ensemble forecasts from The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) are used to quantify the magnitude of moisture transport into North America ahead of recurving tropical cyclones (TCs). Two cases in which a predecessor rain event (PRE) occurred ahead of the recurving TC—Erin (2007) and Ike (2008)—are analyzed, with ensemble members correctly predicting TC recurvature contrasted from those predicting the TC to weaken or turn southward. This analysis demonstrates that TC-related moisture transport can increase the total water vapor in the atmosphere over North America by 20 mm or more, and that the moisture transport takes place both in the boundary layer and aloft. The increased moisture does not always correspond to increased rainfall in the ensemble forecasts, however, as the location and strength of baroclinic zones and their attendant secondary circulations that can lift this moist air are also crucial to the development of heavy rains.

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