Gary Lackmann | North Carolina State University (original) (raw)

Papers by Gary Lackmann

Weather and Forecasting, Jul 12, 2023

We used object-oriented verification and self-organizing maps (SOMs) to identify patterns in envi... more We used object-oriented verification and self-organizing maps (SOMs) to identify patterns in environmental parameters correlating with mesoscale snowband predictive skill by the High-Resolution Ensemble Forecast (HREF) system between 2017 and 2022. First, HREF snowband forecasts for 305 banding events were verified based on similarities between forecast and observed feature properties. HREF members performed comparably, demonstrating large positional errors, but the non-time-lagged High-Resolution Rapid Refresh member demonstrated the greatest overall skill. Observed banding events were clustered by 500-hPa geopotential height anomalies, mean sea level pressure, vertical velocity, frontogenesis, and saturation equivalent potential vorticity from the European Centre for Medium-Range Weather Forecasts Reanalysis version 5 using SOMs. Clusters reaffirmed the presence of midlevel frontogenesis, ascent, and reduced stability in most banding cases, and the predominant synoptic environments conducive to band development. Clusters were compared to determine whether patterns in the variables were correlated with predictive skill. Strength of upward motion was correlated with skill, with the strongest upward motion cases verifying 10% better than the weakest upward motion cases due to smaller positional error. Additionally, events with a single region of strong upward motion verified better than events with disorganized, but comparably intense, upward motion. The magnitude of frontogenesis was uncorrelated with skill, but events with more upright frontogenesis collocated with the band centroid were better predicted than events with shallower slopes and low-level frontogenesis displaced toward warmer air. The skill variance associated with different vertical motion magnitudes could assist forecasters in modulating forecast confidence, while the most common types of errors documented here may be beneficial to model developers in refining HREF member snowfall forecasts. Significance Statement High-resolution numerical weather prediction (NWP) models generally have limited predictive skill for mesoscale snowband forecasts. Even so, some snowbands are forecast by NWP models with much greater skill than others. In this work, we apply artificial intelligence to group snowband events based on atmospheric conditions and then determine whether different groups are easier or harder for models to predict. Identification of these groups could help forecasters know when to trust or be skeptical of NWP output and help developers improve snowband formation processes in NWP models.

Weather and Forecasting, Oct 1, 2019

Monthly Weather Review, Mar 1, 2010

American Meteorological Society eBooks, 2011

We present multi-seasonal simulations representative of present-day and future thermodynamic envi... more We present multi-seasonal simulations representative of present-day and future thermodynamic environments using the global Model for Prediction Across Scales-Atmosphere (MPAS) version 5.1 with high resolution (15 km) throughout the Northern Hemisphere. We select ten simulation years with varying phases of El Niño-Southern Oscillation 10 (ENSO) and integrate each for 14.5 months. We use analysed sea surface temperature (SST) patterns for present-day simulations. For the future climate simulations, we alter present-day SSTs by applying monthly-averaged temperature changes derived from a 20-member ensemble of Coupled Model Intercomparison Project Phase 5 (CMIP5) general circulation models (GCMs) following the Representative Concentration Pathway (RCP) 8.5 emissions scenario. Daily sea ice fields, obtained from the monthly-averaged CMIP5 ensemble mean sea ice, are used for present-day and future simulations. 15 The present-day simulations provide a reasonable reproduction of large-scale atmospheric features in the Northern Hemisphere such as the wintertime midlatitude storm tracks, upper-tropospheric jets, and maritime sea-level pressure features as well as annual precipitation patterns across the tropics. The simulations also adequately represent tropical cyclone (TC) characteristics such as strength, spatial distribution, and seasonal cycles for most of Northern Hemispheric basins. These results demonstrate the applicability of these model simulations for future studies examining climate change effects on 20 various Northern Hemispheric phenomena, and, more generally, the utility of MPAS for studying climate change at spatial scales generally unachievable in GCMs. Plain Language Summary. We expect that high-impact weather events will change in a warmer climate. Computational constraints limit global climate models to resolutions that are too coarse to fully capture many societally significant weather 25 events, such as tropical cyclones and flooding rains in middle-latitude low-pressure systems. While these global models often provide reliable projections of changes in mean temperatures and global circulation patterns, they cannot tell us how intense, high-impact events may be altered in response to climate change. Here, we present a novel set of atmospheric simulations designed to address changes in high-impact weather events. The model covers the globe, but has higher resolution in the Northern Hemisphere. We simulate ten years sampling a range of tropical climate conditions, as represented 30 by observed ocean surface temperatures, and we carry out simulations for current and projected late 21st-century climate

Bulletin of the American Meteorological Society, Jul 1, 2004

split cold front is a cold front in the midtroposphere-usually centered near the 700-hPa level-th... more split cold front is a cold front in the midtroposphere-usually centered near the 700-hPa level-that is located at least 200 km ahead of the surface cold front, creating a forward-tipped cold front structure. This structure has been shown to favor the development of convective precipitation along the upper-level cold front due to a thermally direct frontogenetical circulation that can provide the trigger for convective initiation. The split front is usually marked by a rapid decrease in moisture aloft during its passage, more so than temperature, so quantities such as 700-hPa equivalent-potential temperature (<9) or wetbulb potential temperature (0) are best to use for identification of a split front. Due to the drier air and quasigeostrophic descent induced by cold advection aloft behind the split front, the eventual passage of the surface cold front is often dry in these situations. Therefore, the distribution of precipitation associated with a split front system or cold front aloft may differ significantly from that of the traditional Norwegian cyclone model, where precipitation would be concentrated along the surface cold front.

Weather and Forecasting, Oct 1, 2003

Appalachian cold-air damming (CAD) is characterized by the development of a cool, stable air mass... more Appalachian cold-air damming (CAD) is characterized by the development of a cool, stable air mass that is advected southwestward along the eastern slopes of the Appalachian Mountains by low-level ageostrophic flow. Operational forecasters have identified the demise of CAD as a major forecasting challenge, in part because numerical weather prediction models have a tendency to erode the cold air too quickly. Previous studies have considered the role of clouds and precipitation in the initiation and maintenance of CAD; generally, precipitation is thought to reinforce CAD due to the cooling and stabilization resulting from evaporation. Here, the impact of precipitation on CAD during a situation where the lower-tropospheric air mass was near saturation prior to the arrival of precipitation is considered. Previous studies have indicated that the passage of a cold front can bring about CAD demise, as the synopticscale flow becomes northwesterly behind the front and low-level stable air is scoured. Additional complexity is evident in the case of split cold fronts (or cold fronts aloft). In these situations, precipitation bands are found well to the east of the surface cold front and may be accompanied by severe weather. Here, the impact of a split-front rainband on a mature CAD event from 14 February 2000 is investigated. The coastal front, marking the eastern boundary of the CAD region, made significant inland progress as the split-front rainband passed. Computations from Eta Model forecast fields revealed substantial latent heat release above the cold dome during the passage of the rainband. The CAD cold dome persisted longer in an MM5 model numerical simulation in which the effects of latent heat were withheld relative to both a full-physics control run and to observations. A third model simulation where the low levels of the cold dome were initially dried showed that once saturation occurred, the cold dome began to erode. Analysis of model output and observations suggests that, in this case, precipitation contributed to the retreat of the cold dome through lowertropospheric pressure falls, an isallobaric wind response, and a resultant inland jump of the coastal front.

Weather and Forecasting, Oct 1, 1999

Warm, moist southwesterly airflow into the northwestern United States during the cold season can ... more Warm, moist southwesterly airflow into the northwestern United States during the cold season can result in rapid snowmelt and flooding. The objectives of this research are to document characteristic synoptic flow patterns accompanying cold-season (November-March) flooding events, and isolate flow anomalies associated with the moisture transport during a representative event. The first objective is accomplished through a 46-case composite spanning the years 1962-88; the second objective is addressed through diagnosis of a flooding event that occurred on 17-18 January 1986. The 46-case composite is constructed for a 6-day period centered at 1200 UTC on the day of heavy precipitation onset (denoted 0). Composite 500-hPa geopotential height anomaly fields reveal anomalous ridging over the Bering Sea preceding the precipitation event, a negative anomaly over the Gulf of Alaska throughout the composite evolution, and a positive anomaly over the southwestern Unites States and adjacent eastern Pacific Ocean during and after the event. The gulf trough and southwestern ridge lead to enhanced southwesterly geostrophic flow into the northwestern United States at 0. A positive temperature anomaly at the 850-hPa level advances northeastward into the northwestern United States by 0 , and expands over much of the United States by ϩ48. Piecewise geostrophic moisture transport computations for 17-18 January 1986, based on quasigeostrophic potential vorticity inversion, demonstrate that the transport of moisture into the northwestern United States is largely associated with a duo of mobile cyclones that track from the subtropical Pacific Ocean toward British Columbia. There is also a smaller contribution from a stationary anticyclone over the southwestern United States. These results indicate that the role of the planetary-scale flow, as depicted in the composite analyses, is to provide a persistent storm track, while the moisture flow within this storm track is modulated by cyclone-scale dynamics.

Geophysical Research Letters, Nov 22, 2022

Extreme heat is investigated in a series of high‐resolution time‐slice global simulations compari... more Extreme heat is investigated in a series of high‐resolution time‐slice global simulations comparing the current and late‐21st century climates. An increase in climate‐relative extreme heat is found in the region surrounding the Black Sea. Similarities between the synoptic‐scale flows in current and future heat events combined with a decrease in future summer precipitation suggests that the increased future severity stems from strengthened land‐atmosphere feedbacks driven primarily by the changes in precipitation. The resulting intensification of heat events beyond the mean warming driven by climate change could generate significant future heat hazards in vulnerable regions. Given the continental cool bias in the present‐day simulations, the resulting estimates of future extreme heat are likely to be conservative.

Theory indicates that tropical cyclone intensity should respond to changes in the vertical temper... more Theory indicates that tropical cyclone intensity should respond to changes in the vertical temperature profile. While the sensitivity of tropical cyclone intensity to sea surface temperature is well understood, less is known about sensitivity to the temperature profile. In this paper, we combine historical data analysis and idealised modelling to explore the extent to which historical tropospheric warming and lower stratospheric cooling can explain observed trends in the tropical cyclone intensity distribution. Observations and modelling agree that historical global temperature profile changes coincide with higher lifetime maximum intensities. But observations suggest the response depends on the tropical cyclone intensity itself. Historical lower-and upper-tropospheric temperatures in hurricane environments have warmed significantly faster than the tropical mean. In addition, hurricane-strength storms have intensified at twice the rate of weaker storms per unit warming at the surface and at 300-hPa. Idealized simulations respond in the expected sense to various imposed changes in the temperature profile and agree with tropical cyclones operating as heat engines. Yet lower stratospheric temperature changes have little influence. Idealised modelling further shows an increasing altitude of the TC outflow but little change in outflow temperature. This enables increased efficiency for strong tropical cyclones despite the warming upper troposphere. Observed sensitivities are generally larger than modelled sensitivities, suggesting that observed tropical cyclone intensity change responds to a combination of the temperature profile change and other environmental factors. Non-Technical Summary. Understanding how tropical cyclones (TCs) are changing is key for the protection of lives and livelihoods in vulnerable regions. We know that warm oceans generally favour TC activity. Less is known about the role of air temperature above the oceans and extending into the lower stratosphere. Our analysis of historical records and computer simulations suggests that TCs strengthen in response to historical temperature change while also being influenced by other environmental factors. 1 Introduction Understanding how tropical cyclones (TCs) and their impacts respond to climate change is of critical scientific and societal importance (e.g., Knutson et al., 2020). However, TC response to environmental change is complex and multi-faceted. Here, we use observations and idealized models to examine the TC response to changes in the environmental temperature profile. Historical global surface temperature trend analyses show significant warming since the mid 1970s, attributed to anthropogenic forcing (Meehl et al., 2004; 2012). Yet trends in the vertical thermal structure and their attribution are less well understood (O'Gorman and Singh, 2013; Prein et al., 2017). Since the mid 1970s most datasets show that the troposphere has warmed while the lower stratosphere has cooled (e.g., Thompson et al., 2012; Philipona et al., 2018). However, analysing these trends is particularly challenging in the global tropics because of sparse long-term historical temperature profile records and the potential for artificial trends driven by observing system changes (Thorne et al., 2011).

Journal of Climate, Apr 14, 2021

Persistent anomalies (PAs) are associated with a variety of impactful weather extremes, prompting... more Persistent anomalies (PAs) are associated with a variety of impactful weather extremes, prompting research into how their characteristics will respond to climate change. Previous studies, however, have not provided conclusive results, owing to the complexity of the phenomenon and to difficulties in general circulation model (GCM) representations of PAs. Here, we diagnose PA activity in ten years of current and projected future output from global, high-resolution (15-km mesh) time-slice simulations performed with the Model for Prediction Across Scales-Atmosphere (MPAS-A). These time slices span a range of ENSO states. They include high-resolution representations of sea-surface temperatures and GCM-based sea ice for present and future climates. Future projections, based on the RCP8.5 scenario, exhibit strong Arctic amplification and tropical upper warming, providing a valuable experiment with which to assess the impact of climate change on PA frequency. The MPAS-A present-climate simulations reproduce the main centers of observed PA activity, but with an eastward shift in the North Pacific and reduced amplitude in the North Atlantic. The overall frequency of positive PAs in the future simulations is similar to that in the present-day simulations, while negative PAs become less frequent. Although some regional changes emerge, the small, generally negative changes in PA frequency and meridional circulation index indicate that climate change does not lead to increased persistence of midlatitude flow anomalies or increased waviness in these simulations.

23rd Conference on Weather Analysis and Forecasting/19th Conference on Numerical Weather Prediction (1-5 June 2009), Jun 3, 2009

18th Conference on Atmospheric BioGeosciences/28th Conference on Agricultural and Forest Meteorology/28th Conference on Hurricanes and Tropical Meteorology
(28 April–2 May 2008), May 1, 2008

Weather and climate dynamics, Jul 1, 2022

Theory indicates that tropical cyclone (TC) intensity should respond to environmental temperature... more Theory indicates that tropical cyclone (TC) intensity should respond to environmental temperature changes near the surface and in the TC outflow layer. While the sensitivity of TC intensity to sea surface temperature is well understood, less is known about the role of upper-level stratification. In this paper, we combine historical data analysis and idealised modelling to explore the extent to which historical low-level warming and upper-level stratification can explain observed trends in the TC intensity distribution. Observations and modelling agree that historical global environmental temperature changes coincide with higher lifetime maximum intensities. Observations suggest the response depends on the TC intensity itself. Hurricane-strength storms have intensified at twice the rate of weaker storms per unit surface and upper-tropospheric warming, and we find faster warming of low-level temperatures in hurricane environments than the tropical mean. Idealised simulations respond in the expected sense to various imposed changes in the near-surface temperature and upper-level stratification representing present-day and end-of-century thermal profiles and agree with TCs operating as heat engines. Removing upper-tropospheric warming or stratospheric cooling from end-of-century experiments results in much smaller changes in potential intensity or realised intensity than between present day and the end of the century. A larger proportional change in thermodynamic disequilibrium compared to thermodynamic efficiency in our simulations suggests that disequilibrium, not efficiency, is responsible for much of the intensity increase from present day to the end of the century. The limited change in efficiency is attributable to nearly constant outflow temperature in the simulated TCs among the experiments. Observed sensitivities are generally larger than modelled sensitivities, suggesting that observed TC intensity change responds to a combination of the temperature change and other environmental factors.

Quarterly Journal of the Royal Meteorological Society, Jul 1, 2022

The meteorology community primarily assesses tropical cyclone (TC) forecast skill using track and... more The meteorology community primarily assesses tropical cyclone (TC) forecast skill using track and intensity errors. These metrics are frequently uncorrelated and can offer conflicting information about model performance. Continued improvements in intensity forecasting require improved representation of physical processes over multiple scales, and model verification of TC spatial structure can contribute to these improvements. To date, there are limited studies into forecast model representation of wind fields. More work is needed to better understand model deficiencies in skillfully predicting TC size metrics. In this study, we demonstrate an object‐based approach that can reveal structural differences in TC wind fields. Object‐based methods have been underutilized, and these methods, along with spatial metrics, can serve as additional verification methods for assessing storm structure in both observations and model simulations. Specifically, we illustrate this approach by examining a major difference between the Tiedtke and Kain–Fritsch cumulus parametrization schemes: The Tiedtke scheme includes convective momentum transport while the Kain–Fritsch scheme does not. We create three experiments of Hurricane Isabel (2003) using the Weather Research and Forecasting model using the Kain–Fritsch and Tiedtke cumulus parametrization schemes and an altered Tiedtke scheme with convective momentum transport turned off. Within the three experiments, we generate a small ensemble of four simulations to avoid drawing erroneous conclusions due to growth of numerical noise. Then, we use object‐based methods to measure and compare spatial attributes of the low‐level wind fields to confirm the dominant influence of momentum transport in influencing the TC spatial structure. Our spatial metric approach offers an objective suite of structural attributes that could be useful in diverse applications. A future goal is to use spatial metrics in systematic verification studies of TCs in operational model forecasts and climate model simulations, which may offer great benefit to operational forecasters and numerical model developers.

Monthly Weather Review, Oct 10, 2019

Environments that accompany mesoscale snowbands in extratropical cyclones feature strong midlevel... more Environments that accompany mesoscale snowbands in extratropical cyclones feature strong midlevel frontogenesis and weak symmetric stability, conditions conducive to vigorous ascent. Prior observational and numerical studies document the occurrence of upward vertical velocities in excess of 1 m s 21 near the comma head of winter cyclones. These values roughly correspond to the terminal fall velocity of snow; snow lofting has been measured directly with vertically pointing radars. Here, we investigate the occurrence of lower-tropospheric snow lofting near mesoscale bands and its contribution to snowfall heterogeneity. We test the hypothesis that hydrometeor lofting substantially influences snowfall distributions by analyzing the vertical snow flux in case-study simulations, by computing snow trajectories, and by testing sensitivity of simulated snowbands to parameterized snow terminal fall velocity and advection. These experiments confirm the presence of upward snow flux in the lower troposphere upstream of simulated mesoscale snowbands for two events (27 January 2015 and 2 February 2016). The band of lowertropospheric lofting played a more important role in the January 2015 case relative to the February 2016 event. Lofting enhances the horizontal advection of snow by increasing hydrometeor residence time aloft, influencing the surface snowfall distribution. Experimental simulations illustrate that while lofting and advection influence the snow distribution, these processes reduce snowfall heterogeneity, contrary to our initial hypothesis. Our findings indicate that considerable horizontal displacement can occur between the locations of strongest ascent and heaviest surface snowfall. Numerical forecasts of snowbands are sensitive to formulations of terminal fall velocity of snow in microphysical parameterizations due to this lofting and transport process.

Journal of Hydrometeorology, Nov 1, 2017

This is a preliminary PDF of the author-produced manuscript that has been peer-reviewed and accep... more This is a preliminary PDF of the author-produced manuscript that has been peer-reviewed and accepted for publication. Since it is being posted so soon after acceptance, it has not yet been copyedited, formatted, or processed by AMS Publications. This preliminary version of the manuscript may be downloaded, distributed, and cited, but please be aware that there will be visual differences and possibly some content differences between this version and the final published version.