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John M. Peters and Paul J. Roebber

Abstract

This study examines the degree to which the downscale cascade of information from synoptic-scale motions constrains error growth in simulations of a particular type of heavy-rain-producing mesoscale convective system known as training lines. A total of 21 cases of training convection over a 7-yr period from 2000 to 2006 that produced extreme rainfall were dynamically downscaled from reanalysis data using a high-resolution convection-permitting configuration of the Weather Research and Forecasting Model. The NCEP/Department of Energy (DOE)-II and Interim ECMWF Re-Analysis (ERA-Interim), representing lower- and higher-resolution datasets, respectively, were used for this purpose. In most cases the model simulations were able to reproduce qualitative aspects of observed storm structure, including subjectively classified mesoscale convective system archetype and training characteristics, despite the absence of mesoscale features in the reanalysis datasets used to provide initial conditions and lateral boundary conditions to the simulations. Furthermore, models were capable of predicting that a heavy-precipitation event would occur in nearly every case. Increasing the horizontal resolution of the reanalysis dataset used for initial conditions and lateral boundary conditions did not result in measurable improvement in simulated precipitation placement skill relative to observations. A quantitative relationship between a measure of synoptic-scale uncertainty in the atmospheric state and error in the model forecast accumulated precipitation was established, with larger synoptic-scale uncertainty tending to be associated with larger model error. This result suggests that synoptic-scale uncertainty in numerical weather prediction model simulations partially controls error in the placement of heavy convective precipitation.

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Russ S. Schumacher and John M. Peters

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This study investigates the influences of low-level atmospheric water vapor on the precipitation produced by simulated warm-season midlatitude mesoscale convective systems (MCSs). In a series of semi-idealized numerical model experiments using initial conditions gleaned from composite environments from observed cases, small increases in moisture were applied to the model initial conditions over a layer either 600 m or 1 km deep. The precipitation produced by the MCS increased with larger moisture perturbations as expected, but the rainfall changes were disproportionate to the magnitude of the moisture perturbations. The experiment with the largest perturbation had a water vapor mixing ratio increase of approximately 2 g kg−1 over the lowest 1 km, corresponding to a 3.4% increase in vertically integrated water vapor, and the area-integrated MCS precipitation in this experiment increased by nearly 60% over the control. The locations of the heaviest rainfall also changed in response to differences in the strength and depth of the convectively generated cold pool. The MCSs in environments with larger initial moisture perturbations developed stronger cold pools, and the convection remained close to the outflow boundary, whereas the convective line was displaced farther behind the outflow boundary in the control and the simulations with smaller moisture perturbations. The high sensitivity of both the amount and location of MCS rainfall to small changes in low-level moisture demonstrates how small moisture errors in numerical weather prediction models may lead to large errors in their forecasts of MCS placement and behavior.

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John M. Peters and Russ S. Schumacher

Abstract

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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John M. Peters and Russ S. Schumacher

Abstract

In this research, rotated principal component analysis was applied to the atmospheric fields associated with a large sample of heavy-rain-producing mesoscale convective systems (MCSs). Cluster analysis in the subspace defined by the leading two resulting principal components revealed two subtypes with distinct synoptic and mesoscale characteristics, which are referred to as warm-season-type and synoptic-type events, respectively. Subsequent composite analysis showed that both subtypes typically occurred on the cool side of a quasi-stationary, low-level frontal boundary, within a region of locally maximized low-level convergence and warm advection. Synoptic-type events, which tended to exhibit greater horizontal extent than warm-season-type events, typically occurred downstream of a progressive upper-level trough, along a low-level potential temperature gradient with the warmest air to the south and southeast. Warm-season-type events, on the other hand, occurred within the right-entrance region of a minimally to anticyclonically curved upper-level jet streak, along a low-level potential temperature gradient with the warmest low-level air to the southwest. Synoptic-scale forcing for ascent was stronger in synoptic-type events, while low-level moisture was greater in warm-season-type events. Warm-season-type events were frequently preceded by the passage of a trailing-stratiform- (TS) type MCS, whereas synoptic-type events often occurred prior to the passage of a TS-type system. Analysis of the composite vertical wind profiles at the event location suggests that quasi-stationary behavior in warm-season events predominantly resulted from upstream propagation that nearly canceled advection by the mean steering flow, whereas in the case of synoptic-type events training predominantly resulted from system motion that paralleled a front.

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John M. Peters, Erik R. Nielsen, Matthew D. Parker, Stacey M. Hitchcock, and Russ S. Schumacher

Abstract

This article investigates errors in forecasts of the environment near an elevated mesoscale convective system (MCS) in Iowa on 24–25 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. The eastern flank of this MCS produced an outflow boundary (OFB) and moved southeastward along this OFB as a squall line. The western flank of the MCS remained quasi stationary approximately 100 km north of the system’s OFB and produced localized flooding. A total of 16 radiosondes were launched near the MCS’s eastern flank and 4 were launched near the MCS’s western flank.

Convective available potential energy (CAPE) increased and convective inhibition (CIN) decreased substantially in observations during the 4 h prior to the arrival of the squall line. In contrast, the model analyses and forecasts substantially underpredicted CAPE and overpredicted CIN owing to their underrepresentation of moisture. Numerical simulations that placed the MCS at varying distances too far to the northeast were analyzed. MCS displacement error was strongly correlated with models’ underrepresentation of low-level moisture and their associated overrepresentation of the vertical distance between a parcel’s initial height and its level of free convection (, which is correlated with CIN). The overpredicted in models resulted in air parcels requiring unrealistically far northeastward travel in a region of gradual meso-α-scale lift before these parcels initiated convection. These results suggest that erroneous MCS predictions by NWP models may sometimes result from poorly analyzed low-level moisture fields.

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