Over the last ten-plus years, our group has published papers on a wide range of high-impact meso- to synoptic-scale meteorological phenomena. This list, presented in reverse chronological order, provides short, (hopefully) accessible summaries of these papers. All papers are hosted by the American Meteorological Society, who holds the copyright to each published work, and papers that were published more than one year ago are freely accessible to all.
Prince and Evans (2022, J. Atmos. Sci.) – Convectively generated negative potential vorticity enhancing the jet stream through an inverse energy cascade during the extratropical transition of hurricane Irma
This study quantifies the contributions of negative potential vorticity generated by thunderstorms several hundred miles north of North Atlantic tropical cyclone Irma (2017) to the evolution of the synoptic-scale midlatitude pattern as Irma began to transform into an extratropical cyclone. Ascending motion within thunderstorms facilitates the horizontal and vertical redistribution of potential vorticity, with negative potential vorticity preferentially occurred to the left of the vertical wind shear along the midlatitude jet stream. Synoptic-scale deformation acting on these convectively generated anomalies results in a transfer of energy from thunderstorm to synoptic scales, potentially influencing the synoptic-scale midlatitude pattern’s evolution.
Sarro and Evans (2022, Mon. Wea. Rev.) – An updated investigation of post-transformation intensity, structural, and duration extremes for extratropically transitioning North Atlantic tropical cyclones
This study diagnoses synoptic-scale atmospheric variability distinguishing between tropical cyclones that rapidly vs. slowly transform into extratropical cyclones, intensify vs. weaken following transformation, and maintain a cold-core vs. reacquire a warm-core thermal structure following transformation. Our results emphasize the importance of phasing between the transforming cyclone and upstream trough for post-transformation intensification and synoptic-scale confluence in the downstream environment for post-transformation warm-seclusion development. Our study also presents the first composite analysis of cyclones that acquire an extratropical warm-seclusion structure immediately following transformation, termed instant warm-seclusion cyclones.
Prince and Evans (2020, Mon. Wea. Rev.) – A climatology of indirect tropical cyclone interactions in the North Atlantic and western North Pacific basins | local PDF copy
This study considers indirect tropical cyclone interactions, wherein the interaction of a lead tropical cyclone with a midlatitude trough reconfigures the immediate downstream midlatitude flow in proximity to a trailing tropical cyclone. A prime example from the western North Atlantic in 2017 is the impact to Hurricane Jose’s track and intensity associated with Hurricane Irma’s interaction with an upstream trough. An indirect interaction event occurs approximately once per year (western North Pacific) to once every other year (North Atlantic). The indirect interaction’s impact to the trailing tropical cyclone’s intensity is governed by the trailing tropical cyclone’s location with respect to the crest between the downstream ridge and subsequent downstream trough.
Schaffer et al. (2020, Mon. Wea. Rev.) – Development and evaluation of an evolution programming-based tropical cyclone intensity model | local PDF copy
This study used the evolutionary programming machine-learning technique to develop a new statistical-dynamical model (i.e., a set of statistical relationships that works with environmental predictors from a dynamical model – in this case, the GFS model) to predict tropical cyclone intensity in the North Atlantic and eastern and central North Pacific basins. Forecast skill is comparable to that of the best-performing SHIPS and LGEM statistical-dynamical models in the North Atlantic, and probabilistic rapid-intensification forecasts have forecast skill comparable to the best-performing operational rapid-intensification aids in both basins.
Nevius and Evans (2018, Wea. Forecasting) – The influence of vertical advection discretization in the WRF-ARW model on capping inversion representation in warm-season, thunderstorm-supporting environments | local PDF copy
Capping inversions forecast by the WRF-ARW model are well-known to be too smooth as compared to their observed counterparts. This study tested the hypothesis that damping (or smoothing) tied to the finite-difference approximation used to compute vertical advection is the primary cause of overly smooth forecast capping inversions. Over the sample considered, however, the data do not support this hypothesis. Single-column model experiments for strong capping inversions suggest that parameterized turbulent vertical mixing has a greater control on inversion structure, though all are too smooth compared to observations.
Evans et al. (2018, Wea. Forecasting) – An evaluation of paired regional/convection-allowing forecast vertical thermodynamic profiles in warm-season, thunderstorm-supporting environments | local PDF copy
This study evaluated the performance of paired regional and convection-allowing models at forecasting vertical thermodynamic profiles at short lead times (0-24 h) in warm-season, thunderstorm-supporting environments. Despite grid spacings in the terra incognita of numerical modeling, where boundary layer turbulence begins to be resolved by the numerical model, we find that convection-allowing models are equally skillful to their regional counterparts over the sample considered. The research has important implications for SPC operations, namely that convection-allowing models can satisfy all SPC short-lead use cases.
Prince and Evans (2018, J. Appl. Meteor. Climatol.) – A climatology of extreme South American Andean cold surges | local PDF copy
This study created a climatology of intense South American cold surges that traveled toward the Equator in the lee of the Andes Mountains. Intense cold surges were defined relative to the local climatology using standardized anomalies. These events, which occur approximately twice per year, significantly modulate the wind and temperature fields across the Amazon basin for two to eight days after forming. The composite cold surge reaches to the Equator and, in a unique finding, weakens first at and near the surface due to strong sensible heating from the underlying Amazon basin.
Keclik et al. (2017, Mon. Wea. Rev.) – The influence of assimilated upstream, pre-convective dropsonde observations on ensemble forecasts of convection initiation during the Mesoscale Predictability Experiment
This study tested the hypothesis that assimilating upstream, pre-convective, meso-alpha- to synoptic-scale dropsonde observations is sufficient to improve the predictability of convection initiation over the set of fifteen cases sampled by 2013’s Mesoscale Predictability Experiment. Both deterministic and probabilistic verification indicate negligible impact to forecast skill from assimilating these observations, likely because they were targeted to improve accumulated precipitation forecasts (which are only partially related to convection initiation) and did not effectively sample boundary-layer variability important to convection initiation.
Evans et al. (2017, Mon. Wea. Rev.) – The extratropical transition of tropical cyclones. Part I: cyclone evolution and direct impacts
This review paper documents advances in our understanding of extratropical transition since the early 2000s. Particular focus is given to extratropical transition climatologies and diagnostic identifiers, direct observations of structural change during the transformation stage of extratropical transition, improved understanding of the physics and dynamics for strong winds, large waves, and heavy precipitation accompanying transition events, and assessments of transition-related predictive skill. A companion paper describes advances in understanding of the downstream impacts of extratropical transition.
Burlingame et al. (2017, Wea. Forecasting) – The influence of PBL parameterization on the practical predictability of convection initiation during the Mesoscale Predictability Experiment (MPEX)
This study examined the influence of varying the planetary boundary layer parameterization on the practical predictability of convection initiation in WRF-ARW short-range ensemble forecasts for three MPEX events. Varying the planetary boundary layer parameterization exerts a significant control on the number of initiation events, with cooler, moister parameterizations resulting in more initiation events, but does not change predictive skill due to the duality of error between hits and false alarms. Convection initiation is only predictable to within ~100 km and ~1 h over the set of events considered.
Grunzke and Evans (2017, Mon. Wea. Rev.) – Predictability and dynamics of warm-core mesoscale vortex formation with the 8 May 2009 “super derecho” event
During the early morning of 8 May 2009, an intense warm-core mesoscale vortex formed on the northern end of a derecho-producing convective system, resulting in significant damange from southeast Kansas through central Kentucky. In this study, the predictability of the parent convective system was high, but that of the warm-core vortex was very low. Successful vortex forecasts required an initial elevated MCS to form in northwest Kansas ~6-9 h prior to vortex formation; unsuccessful forecasts did predict MCS initiation, but not until later over south-central Kansas due to an alternative forcing mechanism.
Karloski and Evans (2016, J. Climate) – Seasonal influences upon and long-term trends in the length of the Atlantic hurricane season
Motivated by several long Atlantic hurricane seasons in the mid-2000s, with named storms in April, May, and December, this study used a statistical technique known as quantile regression to evaluate long-term trends in the length of the Atlantic hurricane season. In contrast with a prior study that examined seasons through 2007, there is no statistically significant trend in season length through 2014. Looking at the seasonal-scale factors associated with early-starting and late-ending seasons, the strongest relationship is between La Nina and late-ending seasons.
Manion et al. (2015, Wea. Forecasting) – An evaluation of Advanced Dvorak Technique-derived tropical cyclone intensity estimates during extratropical transition using synthetic satellite imagery
The Dvorak technique, relating cloud patterns to near-surface maximum wind speed, is the primary method by which tropical cyclones’ intensities are determined. However, when tropical cyclones enter the midlatitudes and become extratropical, the underlying relationships between cloud organization and intensity become less reliable. This study used a new technique called synthetic satellite imagery, wherein a numerical model forecast is used to ‘predict’ satellite appearance, to attempt to quantify the skill of the objective Advanced Dvorak Technique at assessing tropical cyclone intensity during extratropical transition.
Weisman et al. (2015, Bull. Amer. Meteor. Soc.) – The Mesoscale Predictability Experiment (MPEX)
The 2013 Mesoscale Predictability Experiment aimed to test the hypothesis that assimilating targeted subsynoptic-scale (e.g., on scales of ~100 km and ~1 h) observations collected in pre-thunderstorm environments into high-resolution numerical model forecast initial atmospheric representations is sufficient to improve short-range (e.g., at lead times of 6-24 h) forecasts of downstream severe weather events. This paper outlines the motivation behind the field program, introduces the strategy through which targeted observation locations were determined, and documents the fifteen cases for which observations were collected.
Burghardt et al. (2014, Wea. Forecasting) – Assessing the predictability of convection initiation across the High Plains using an object-based approach
This study sought to determine how well convection initiation can be predicted at short lead times (6-18 h) in very-high-resolution numerical model forecasts. To do so, a novel convection initiation identification technique, in which observed and modeled thunderstorms were defined as objects, and flow-dependent verification technique, in which time and space errors were combined into a single metric, was developed. We found that convection initiation is only reliably predictable to within about 100 km and 1 h of the location and time at which it occurred.
Evans et al. (2014, J. Atmos. Sci.) – Development of an intense, warm-core mesoscale vortex assocaited with the 8 May 2009 ‘super derecho’ convective event
The dynamical processes contributing to the formation of the intense warm-core mesoscale vortex, the predictability of which was studied by Grunzke and Evans (2017), associated with the 8 May 2009 ‘super derecho’ event were the focus of this study. Using output from a high-resolution numerical simulation of the event, it was determined that the derecho’s primary system-scale air streams helped to concentrate cyclonic rotation in a deeply moist environment, the latter of which is somewhat of an atypical derecho environment.
Evans et al. (2014, Wea. Forecasting) – How do forecasters utilize output from a convection-permitting ensemble forecast system? Case study of a high-impact precipitation event
At the dawn of the operational high-resolution ensemble forecast system era, this study aimed to determine how forecasters actually use output from such a system. While no two forecasters are alike, several common themes emerged. Forecast confidence increased if the ensemble provided forecasts similar to a forecaster’s initial thoughts, aiding in the identification of a ‘most likely’ forecast scenario. If a forecaster perceives the ensemble to be biased, however, it may not be used at all. Products that help forecasters to quickly understand the range of solutions provided by the ensemble are particularly valuable in the forecast process.
Weisman et al. (2013, Wea. Forecasting) – The 8 May 2009 ‘super derecho’: analysis of a realtime explicit convective forecast
Although high-resolution numerical models had been known for some time to be capable of simulating realistic-looking mesoscale phenomena, the NCAR real-time high-resolution WRF-ARW model forecast from the 1200 UTC 7 May 2009 forecast cycle was remarkable in its ability to simulate the formation and evolution of the 8 May 2009 ‘super derecho’ event with a high degree of fidelity. This study documented the environment and structural characteristics of this system, as diagnosed from the NCAR WRF-ARW forecast output, motivating future studies (Evans et al. 2014, Grunzke and Evans 2017) into the dynamics and predictability of this event.
Evans et al. (2012, Bull. Amer. Meteor. Soc.) – The Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT) field campaign: perspectives of early career scientists
Atmospheric field campaigns typically benefit from extensive student, post-doctoral fellow, and early-career scientist participation in field operations. In turn, participating in field operations provides students, post-doctoral fellows, and early-career scientists with unique opportunities to collect observations to help evaluate theory and numerical model simulation output, gain leadership and project management experience, and network with peers. This paper, focusing on the 2010 PREDICT field campaign, details these joint benefits from the perspective of early-career scientists at an unprecedented level of detail.
Evans et al. (2011, Mon. Wea. Rev.) – Sensitivity in the overland reintensification of Tropical Cyclone Erin (2007) to near-surface soil moisture characteristics
Motivated by the unforeseen reintensification of Erin (2007) over central Oklahoma three days after landfall along the central Texas coastline, this study utilized high-resolution numerical simulations to quantify the influence of soil moisture – and the associated transfer of moisture from land to air – on Erin’s reintensification. Erin’s reintensification was primarily supported by heavy rainfall over Texas two days prior associated with Erin itself. The resulting well-above-normal soil moisture allowed inflowing air from the Gulf of Mexico to retain low-level moisture, permitting tropical-like intensification over Oklahoma in an otherwise favorable environment.
Evans and Hart (2008, Mon. Wea. Rev.) – Analysis of the wind field evolution associated with the extratropical transition of Bonnie (1998)
As tropical cyclones turn poleward and interact with the large-scale midlatitude flow during extratropical transition, their near-surface wind field typically expands in size. In this study, we presented a formal definition for this wind field expansion: the radius at which the maximum near-surface winds are found moves outward, while the expanse of strong winds increases in size. Numerical model simulations imply that cooling near the initially warm cyclone center and the increasingly asymmetric nature of the cyclone during extratropical transition are responsible for these evolutions.
Hart et al. (2006, Mon. Wea. Rev.) – Synoptic composites of the extratropical transition lifecycle of North Atlantic tropical cyclones: factors determining post-transition evolution
During extratropical transition, an initially tropical cyclone, with strongest winds near the center at/near the surface, quasi-symmetric wind and precipitation fields, and no frontal boundaries, transforms into an extratropical cyclone, with an expansive wind field and intensity that increases upward from the surface, asymmetric wind and precipitation fields, and well-defined cold and warm fronts. In this study, we identified unique atmospheric patterns associated with three during- and post-transition extremes: time to complete transition, intensity change after transition, and cyclone structure after transition.