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Evaluation of heat wave forecasts seamlessly across subseasonal timescales. by
We develop an extreme heat validation approach for medium-range forecast models and apply it to the NCEP coupled forecast model, for which we also attempt to diagnose sources of poor forecast skill. A weighting strategy based on the Poisson function is developed to provide a seamless transition from short-term day-by-day weather forecasts to expanding time means across subseasonal timescales. The skill of heat wave forecasts over the conterminous United States is found to be rather insensitive to the choice of skill metric; however, forecast skill does display spatial patterns that vary depending on whether daily mean, minimum, or maximum temperatures are the basis of the heat wave metric. The NCEP model fails to persist heat waves as readily as is observed. This inconsistency worsens with longer forecast lead times. Land–atmosphere feedbacks appear to be a stronger factor for heat wave maintenance at southern latitudes, but the NCEP model seems to misrepresent those feedbacks, particularly over the Southwest United States, leading to poor skill in that region. The NCEP model also has unrealistically weak coupling over agricultural areas of the northern United States, but this does not seem to degrade model skill there. Overall, we find that the Poisson weighting strategy combined with a variety of deterministic and probabilistic skill metrics provides a versatile framework for validation of dynamical model heat wave forecasts at subseasonal timescales. Climate science: evaluating forecasts of extreme heat across subseasonal timescales A validation window that broadens with time is developed to provide seamless verification of extreme heat forecasts from days to weeks. Trent Ford (Southern Illinois University), Paul Dirmeyer (COLA-George Mason University), and David Benson (George Mason University) applied several skill metrics with a Poisson function weighting strategy to verify NOAA coupled forecast system model extreme heat forecasts over the United States. The model fails to persist heat waves as readily as is observed, and this inconsistency worsens with longer forecast lead times. Land surface–atmosphere interactions appear to influence heat wave persistence, but the model misrepresents these interactions, leading to poor skill in the Southwest and Midwest regions. The Poisson weighting strategy provides a versatile framework for verifying forecasts across subseasonal timescales. Continued verification and improvement of model forecasts contributes to reducing the risk of extreme climate events. [ABSTRACT FROM AUTHOR]
Publication Date: 2018
Characteristics and Predictability of Midwestern United States Drought. by
Journal of Hydrometeorology. Nov 2021, Vol. 22 Issue 11, p3087-3105. 19p.
Characteristics and predictability of drought in the midwestern United States, spanning the from the Great Plains to the Ohio Valley, at local and regional scales are examined during 1916–2015. Given vast differences in hydroclimatic variability across the Midwest, drought is evaluated in four regions identified using a hierarchical clustering algorithm applied to an integrated drought index based on soil moisture, snow water equivalent, and 3-month runoff from land surface models forced by observed analyses. Highlighting the regions containing the Ohio Valley (OV) and Northern Great Plains (NGP), the OV demonstrates a preference for subannual droughts, the timing of which can lead to prevalent dry epochs, while the NGP demonstrates a preference for annual-to-multiannual droughts. Regional drought variations are closely related to precipitation, resulting in a higher likelihood of drought onset or demise during wet seasons: March–November in the NGP and all year in the OV, with a preference for March–May and September–November. Due to the distinct dry season in the NGP, there is a higher likelihood of longer drought persistence, as the NGP is 4 times more likely to experience drought lasting at least one year compared to the OV. While drought variability in all regions and seasons is related to atmospheric wave trains spanning the Pacific–North American sector, longer-lead predictability is limited to the OV in December–February because it is the only region/season related to slow-varying sea surface temperatures consistent with El Niño–Southern Oscillation. The wave trains in all other regions appear to be generated in the atmosphere, highlighting the importance of internal atmospheric variability in shaping Midwest drought. Significance Statement: The midwestern United States, spanning from the Great Plains to the Ohio Valley, has endured many costly and life-altering droughts. A drought in 2012 led to an estimated $34.5 billion in direct economic losses. This study aims to build a more complete understanding of drought in regions of the Midwest that could be used in drought early warning efforts. Drought is evaluated in four midwestern regions of coherent hydroclimatic variability. The regions were identified by applying a method that groups similar objects to an integrated drought index that includes soil moisture, snow water equivalent, and 3-month runoff from land surface models during 1916–2015. Highlighting the regions containing the Ohio Valley (OV) and Northern Great Plains (NGP), droughts in the NGP generally last longer than in the OV. Droughts in the NGP only begin and end during the warm and wet season while droughts in the OV can begin and end during any time of year. El Niño–Southern Oscillation (ENSO), a slow varying phenomenon of the Earth system, may be used as a source of predictability for drought onset and demise in the OV during winter. However, circulation patterns internal to the atmosphere play a key role in shaping drought in all other seasons and regions of the Midwest. [ABSTRACT FROM AUTHOR]
Publication Date: November 2021
Flash Droughts: A Review and Assessment of the Challenges Imposed by Rapid Onset Droughts in the United States by
Bulletin of the American Meteorological Society (ISSN 0003-0007) (e-ISSN 1520-0477); 99; 5; 911-919.
Given the increasing use of the term "flash drought" by the media and scientific community, it is prudent to develop a consistent definition that can be used to identify these events and to understand their salient characteristics. It is generally accepted that flash droughts occur more often during the summer due to increased evaporative demand; however, two distinct approaches have been used to identify them. The first approach focuses on their rate of intensification, whereas the second approach implicitly focuses on their duration. These conflicting notions for what constitutes a flash drought (i.e., unusually fast intensification versus short duration) introduce ambiguity that affects our ability to detect their onset, monitor their development, and understand the mechanisms that control their evolution. Here, we propose that the definition for flash drought should explicitly focus on its rate of intensification rather than its duration, with droughts that develop much more rapidly than normal identified as flash droughts. There are two primary reasons for favoring the intensification approach over the duration approach. First, longevity and impact are fundamental characteristics of drought. Thus, short-term events lasting only a few days and having minimal impacts are inconsistent with the general understanding of drought and therefore should not be considered flash droughts. Second, by focusing on their rapid rate of intensification, the proposed flash drought definition highlights the unique challenges faced by vulnerable stakeholders who have less time to prepare for its adverse effects. [ABSTRACT FROM AUTHOR]
Publication Date: 2018
A Case Study of Large Floodplain River Restoration: Two Decades of Monitoring the Merwin Preserve and Lessons Learned through Water Level Fluctuations and Uncontrolled Reconnection to a Large River by
Wetlands; Aug2022, Vol. 42 Issue 6, p1-16, 16p.
Large riverine systems are diverse and dynamic and are made up of multiple habitat types of lentic and lotic water. They are also heavily modified by humans and today nearly all habitats in many large rivers have been drastically altered. These modifications often include disconnecting lentic habitats either permanently or intermittently from the main channel. The Merwin Preserve at Spunky Bottoms (Merwin) began as a connected backwater that was leveed and drained for agriculture in the 1920s and restored in 1999, with restoration allowing it to become a disconnected backwater habitat. This status changed in 2013 when record flooding on the adjacent Illinois River overtopped and breached the levee creating an unmanaged and intermittent connection allowing the river access at moderately high river stages. During the past 20 years, the fish community at Merwin has undergone several changes that follow three drought events pre-breach, the exchange of fishes from the mainstem following the breach in 2013, and subsequent low water conditions of much of the area as river levels drop. Long term data and more intensive sampling efforts during the drought of 2012 showed relative abundance of sport fishes declined during, or immediately following, pre-breach drought events and post-breach low water conditions while relative abundance of non-sport and non-native fishes remained stable. The unique story of Merwin can provide a case study for other large river restoration projects on the effects of drought, climate change, and impacts of an unmanaged connection of a previously disconnected habitat to an adjacent large river. [ABSTRACT FROM AUTHOR]
Publication Date: August 2022
Effects of 0.5 °C less global warming on climate extremes in the contiguous United States by
Climate Dynamics. Jul2021, Vol. 57 Issue 1/2, p303-319. 17p.
The Intergovernmental Panel on Climate Change (IPCC) suggests limiting global warming to 1.5 °C compared to 2 °C would avoid dangerous impacts of anthropogenic climate change and ensure a more sustainable society. As the vulnerability to global warming is regionally dependent, this study assesses the effects of 0.5 °C less global warming on climate extremes in the United States. Eight climate extreme indices are calculated based on Coupled Model Intercomparison Project—phase 5 (CMIP5), and North American—Coordinated Regional Climate Downscaling Experiments (NA-CORDEX) with and without bias correction. We evaluate the projected changes in temperature and precipitation extremes, and examine their differences between the 1.5 and 2 °C warming targets. Under a warming climate, both CMIP5 and NA-CORDEX show intensified heat extremes and reduced cold extremes across the country, intensified and more frequent heavy precipitation in large areas of the North, prolonged dry spells in some regions of the West, South, and Midwest, and more frequent drought events in the West. Results suggest that the 0.5 °C less global warming would avoid the intensification of climate extremes by 32–46% (35–42%) for heat extremes intensity (frequency) across the country and, by 23–41% for heavy precipitation intensity in the North, South, and Southeast. The changes in annual heavy precipitation intensity are mainly contributed by winter and spring. However, impacts of the limited warming on the frequency of heavy precipitation, dry spell, and drought frequency are only evident in a few regions. Although uncertainties are found among the climate models and emission scenarios, our results highlight the benefits of limiting warming at 1.5 °C in order to reduce the risks of climate extremes associated with global warming. [ABSTRACT FROM AUTHOR]
Publication Date: July 2021
Evaluating Soil Moisture–Precipitation Interactions Using Remote Sensing: A Sensitivity Analysis by
Journal of Hydrometeorology. 19(8):1237-1253.
The complex interactions between soil moisture and precipitation are difficult to observe, and consequently there is a lack of consensus as to the sign, strength, and location of these interactions. Inconsistency between soil moisture–precipitation interaction studies can be attributed to a multitude of factors, including the difficulty of demonstrating causal relationships, dataset differences, and precipitation autocorrelation. The purpose of this study is to explore these potential confounding factors and determine which are most important for consideration when assessing statistical coupling between soil moisture and precipitation. Soil moisture is assessed via three remote sensing datasets: theAdvancedMicrowave Scanning Radiometer for EarthObserving System, the Tropical Rainfall Measuring Mission Microwave Imager, and the Essential Climate Variable Soil Moisture. Estimates of soil moisture are coupled with afternoon thunderstorm events identified by the Thunderstorm Observation by Radar (ThOR) algorithm, and dry soil or wet soil preferences for convection initiation are determined for over 16 000 thunderstorm events between 2005 and 2007. Differences in soil moisture datasets were found to have the largest impact with regard to determining wet or dry soil preferences. Precipitation autocorrelation is prevalent in the data; however, precipitation autocorrelation did not influence the results with regard to dry or wet soil preferences. Consideration of the convective environment (i.e., weakly or synoptically forced) did result in significant differences in wet/dry soil preference, but only for certain soil moisture datasets. The results suggest that observation-driven soil moisture–precipitation interaction studies should both consider the convective environment and implement multiple soil moisture datasets to assure robust results.
Publication Date: 2018
Future changes in the transitions of monthly‐to‐seasonal precipitation extremes over the Midwest in Coupled Model Intercomparison Project Phase 6 models. by
International Journal of Climatology. Jun2022, p1. 20p. 11 Illustrations, 1 Chart.
Precipitation extremes present significant risks to Midwest agriculture, water resources, and natural ecosystems. Recently, there is growing attention to the transitions of precipitation extremes, or shifts between heavy precipitation and drought, due to their profound environmental and socio‐economic impacts. Changes in Midwest precipitation extremes and transitions between extremes over the past few decades have been documented; however, their future changes are still unknown. In this study, we estimate the projected changes in transitions of precipitation extremes in the Midwest based on 17 CMIP6 models. Two Standardized Precipitation Index (SPI) based metrics, intra‐annual variability and transitions, are used to quantify the magnitude, duration, and frequency of variability and transitions between wet and dry extremes. Compared with the observation‐based precipitation datasets, the multimodel ensemble median of CMIP6 can reasonably represent the spatial patterns of SPI extremes and transitions. Climate projections show significantly intensified wet extremes across the Midwest by the end of the century, with a greater increase over the northern Midwest and the Great Lakes region. The short‐term SPI also shows intensified dry extremes over the western half of the Midwest. Consequently, there is a significant increase in the magnitude of intra‐annual variability in most areas. Projections also suggest more frequent and rapid transitions between the wet and dry extremes, especially over the Great Lakes region and northern Midwest. Seasonally, more frequent transitions from a wet spring to a dry summer (or from a dry fall to a wet winter/spring) are projected to occur; and generally, the wet and dry conditions between the transitions are projected to be more intense compared to the historical period. Furthermore, the intensified precipitation extremes and accelerated transitions are greatly alleviated under a lower emission scenario, implying that future changes in hydroclimate extremes, and impacts thereof, in the Midwest are sensitive to climate change mitigation. [ABSTRACT FROM AUTHOR]
Publication Date: June 2022
Variability and Transitions in Precipitation Extremes in the Midwest United States. by
Journal of Hydrometeorology. Mar2021, Vol. 22 Issue 3, p533-545. 13p.
Monthly to seasonal precipitation extremes, both flood and drought, are important components of regional climates worldwide, and are the subjects of numerous investigations. However, variability in and transition between precipitation extremes, and associated impacts are the subject of far fewer studies. Recent such events in the Midwest region of the United States, such as the 2011-12 flood to drought transition in the upper Mississippi River basin and the flood to drought transition experienced in parts of Kentucky, Ohio, Indiana, and Illinois in 2019, have sparked concerns of increased variability and rapid transitions between precipitation extremes and compounded economic and environmental impacts. In response to these concerns, this study focuses on characterizing variability and change in Midwest precipitation extremes and transitions between extremes over the last 70 years. Overall we find that the Midwest as a region has gotten wetter over the last seven decades, and that in general the annual maximum and median wetness, defined using the standardized precipitation index (SPI), have increased at a larger magnitude than the annual minimum. We find large areas of the southern Midwest have experienced a significant increase in the annual SPI range and associated magnitude of transition between annual maximum and minimum SPI. We additionally find wet to dry transitions between extremes have largely increased in speed (i.e., less time between extremes), while long-term changes in transition frequency are more regional within the Midwest. [ABSTRACT FROM AUTHOR]
Publication Date: March 2021
The Role of Vegetation in Flash Drought Occurrence: A Sensitivity Study Using Community Earth System Model, Version 2. by
Journal of Hydrometeorology. Apr2021, Vol. 22 Issue 4, p845-857. 13p.
Flash droughts are noted by their unusually rapid rate of onset or intensification, which makes it difficult to anticipate and prepare for them, thus resulting in severe impacts. Although the development of flash drought can be associated with certain atmospheric conditions, vegetation also plays a role in propagating flash drought. This study examines the climatology of warm season (March–September) flash drought occurrence in the United States between 1979 and 2014, and quantifies the possible impacts of vegetation on flash drought based on a set of sensitivity experiments using the Community Earth System Model, version 2 (CESM2). With atmospheric nudging, CESM2 well captures historical flash drought. Compared with NASA's Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), and National Climate Assessment–Land Data Assimilation System (NCA-LDAS), CESM2 shows agreement on the high flash drought frequency in the Great Plains and southeastern United States, but overestimates flash drought occurrence in the Midwest. The vegetation sensitivity experiments suggest that vegetation greening can significantly increase the flash drought frequency in the Great Plains and the western United States during the warm seasons through enhanced evapotranspiration. However, flash drought occurrence is not significantly affected by vegetation phenology in the eastern United States and Midwest due to weak land–atmosphere coupling. In response to vegetation greening, the extent of flash drought also increases, but the duration of flash drought is not sensitive to greening. This study highlights the importance of vegetation in flash drought development, and provides insights for improving flash drought monitoring and early warning. [ABSTRACT FROM AUTHOR]
Publication Date: April 2021
Characterizing winter season severity in the Midwest United States, Part I: Climatology and recent trends. by
International Journal of Climatology; May2022, Vol. 42 Issue 6, p3537-3552, 16p.
Severe winter weather is a staple of Midwest United States (U.S.) climate and is an important natural hazard that has significant economic and environmental impacts. Winter season severity varies both spatially and temporally in the Midwest, and past studies have documented significant changes in Midwest winter characteristics, including increases in temperature and decreases in snow depth. In this study, we use the Accumulated Winter Season Severity Index (AWSSI) to characterize winter severity across the Midwest and assess its variability and change on various spatial and temporal scales during the 1951–2020 period. AWSSI is derived from daily records of snowfall (SF), snow depth (SD), maximum surface air temperature (TMAX), and minimum surface air temperature (TMIN) during the winter season. The daily total AWSSI index value reflects the sum of points accumulated from daily values of each of these four variables surpassing pre-determined thresholds, and the total season sum reflects the overall winter season severity. We find AWSSI provides a unique perspective by which to describe climatological differences in winter season character in the Midwest. Namely, over three quarters of winter season days accumulate points in the northern Midwest, compared to just over half in the southern Midwest. Meanwhile, the relative contribution of extreme winter days to total winter severity is much larger in the southern Midwest than farther north. In addition, we find overall winter season severity has significantly decreased at only a quarter of stations and the frequency of extreme winter days has significantly decreased at only a fifth of stations, despite widespread increases in winter daily maximum and minimum temperature. The results suggest Midwest U.S. winter season severity as described by the AWSSI has not significantly changed over the past 70 years. [ABSTRACT FROM AUTHOR]
Publication Date: May 2022
Characterizing winter season severity in the Midwest United States, part II: Interannual variability. by
International Journal of Climatology; May2022, Vol. 42 Issue 6, p3499-3516, 18p.
The Accumulated Winter Season Severity Index (AWSSI) describes the “harshness” of the cold season. This work utilizes AWSSI to explore the interannual teleconnective influences that shaped winter severity between 1951 and 2020 across the Midwest U.S. Annual AWSSI total variability clusters into four distinct geographic areas, each shaped by different teleconnective influences. The regions are closely delineated by the main river basins and include the Ohio (Region 1), upper Mississippi and Missouri (Region 2), central Mississippi and southern Missouri (Region 3), and the Great Lakes (Region 4). Of outmost influence in shaping severe winters across the Midwest have been synoptic flow modifications associated with significant meandering and southern displacement of the Polar jet over the conterminous United States and/or the Midwest that depress near-surface air temperatures and increase the likelihood of snow fall and/or snow depth. These flows are most closely related to the combined influence of the negative (positive) phases of the NAO (PNA and PDO) in Region 1, PNA− in Region 2, NAO− in Region 3, and the negative (positive) phase of the PNA (TNH) across Region 4. Mild winters have been shaped by more zonal flows with the Polar jet positioned north of the Midwest and bringing milder temperatures, less snowfall, and reduced winter severity to the impacted areas. Such modifications have been related to TNH−, PNA+, and NAO+ across Regions 1, 2, and 3, respectively. Mild winters have been shaped by TNH− and to a limited extent NAO+, across Region 4. [ABSTRACT FROM AUTHOR]
Publication Date: May 2022
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