Research

2023

1. PhysGeo

   An analysis of the National Water Model for a mid-Atlantic suburban watershed

   SE. Brasher, DJ. Leathers, CL. Callahan, and 1 more author

   Physical Geography, Aug 2023

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   Discharge values from the National Water Model (NWM) were compared to USGS stream gage discharge observations for the suburban Red Clay Creek watershed (drainage area  140 km2 and mixed land-use), in Pennsylvania and Delaware, from 2016 to 2018. 18-hour retrospective simulations from the NWM were used with concurrent hourly USGS discharge observations from three locations along the Red Clay Creek. Results indicate that the mean of discharge estimates from the NWM and from USGS observations significantly differed and that the NWM generally underestimates low-flow conditions and overestimates high-flow conditions. Watershed size also impacted NWM performance (with performance degrading in smaller watersheds). A meteorological analysis determined that convective rainfall events were associated with 66% of the largest differences between NWM discharge estimates and USGS observations while mid-latitude cyclone stratiform precipitation events accounted for the other 34%. Lastly, of the largest 15 differences between the NWM and observations, 13 occurred with pre-cursor soil moisture that was below the mean (dry soil conditions), in conjunction with heavy rainfall. Given the NWM’s recent operational implementation, and its status as Prototype guidance, the results of this study present specific geographical and climatological findings that can aid in the NWM’s continued validation and improvement for similar regions.

2. AgForMet

   Influence of synoptic weather conditions on atmometers on the Delmarva Peninsula, USA

   KR. Brinson, DJ. Leathers, and SE. Brasher

   Agricultural and Forest Meteorology, Apr 2023

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   Reference evapotranspiration data from atmometers at three locations on the Delmarva Peninsula (USA) were compared to Penman-Monteith reference evapotranspiration (ETo) data across two growing seasons. Atmometer reference evapotranspiration (ETa) was found to underestimate ETo by 22.8% in 2016 and 30.4% in 2017. Stepwise linear regression was used to examine the relationship between both ET datasets and local meteorological conditions measured by Delaware Environmental Observing System (DEOS) mesonet stations that were co-located with the atmometers. Variability of ETa and ETo are well explained (R2 equal to 0.890 and 0.956, respectively) by a combination of meteorological variables, though the R2 in 2017 (R2 = 0.754) was notably lower than in 2016 (R2 = 0.890). The ET datasets were further examined by partitioning the data into days with similar synoptic conditions using a temporal synoptic index (TSI). Using the TSI results, three dominant synoptic categories were defined during the study period: High Pressure (HP), Southwest Flow (SW), and Cold Fronts (CF). Overall, the similarity between ETa and ETo was greatest on HP days, followed by CF and SW days. This relationship was primarily driven by wind speed, which had the greatest influence on ETa-ETo differences under all synoptic weather patterns. The 2016 growing season consisted of more days with synoptic conditions that are associated with smaller ETa -ETo differences than the 2017 growing season. Thus, changes in synoptic category frequency impact the nature of the ETa-ETo relationship from season to season. This study improves upon previous atmometer comparison studies by associating atmometer correction factors with synoptic weather patterns and descriptions in order to improve the utility of atmometers and remove the need for expensive meteorological equipment to correct atmometer data.

2022

1. ClimDyn

   Using Self-Organizing Maps to Detect Northern Hemisphere Cryo-Cover Transformation

   SE. Brasher, and DJ. Leathers

   Climate Dynamics, Jul 2022

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   Northern Hemisphere cryo-cover (here defined as both snow and sea ice covers) is dynamic and has displayed distinct spatial and temporal variability, as well as shifts in stability (predominantly becoming less stable) in recent decades. To better understand these transitions, this manuscript focuses on three transitional months of cryo-cover transformation, March, June and October, and implements Self-Organizing Maps (SOMs) on a MeaSuREs derived (temporal coverage of 1979–2012) cryo-cover concentration dataset. Cryo-cover transformation was analyzed on a regional and hemispheric basis with trends calculated for all areas identified by the SOMs. Results suggest cryo-cover decreases (in area and stability) during March and June as pixels shift from SOM identified stable patterns to more transient patterns. October results suggest increases in transient cryo-cover, a result of declining stability in sea ice but also increasing snow cover predominantly centered around 50 °N latitude. All months experience regions of increasing and decreasing cryo-cover trends. Decreasing trends are predominant in March and June, although some areas of increase are apparent even during these months. By contrast, October has significant area experiencing increasing trends in cryo-cover, as well as large decreasing trends in the ice domain. Given accelerated rising temperatures in the Northern Hemisphere high latitudes, transformation of cryo-cover will continue, and monitoring these changes spatially (using methodologies like SOMs) will be necessary to better understand the radiative and environmental impacts.

2021

1. IJOC

   Climatology of Northern Hemisphere Cryo-cover

   SE. Brasher, and DJ. Leathers

   International Journal of Climatology, May 2021

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   The Earth’s cryo-cover (defined here as snow cover and sea ice) is dynamic, and varies considerably on both interannual and seasonal time-scales. This study analyzes trends in Northern Hemisphere cryo-cover from 1979 to 2020 using three datasets: National Snow and Ice Data Center (NSIDC) Sea Ice Extent, Rutgers Global Snow Lab (GSL) Snow Cover Extent and MeaSuREs State of the Cryosphere values. Temporal trends of cryo-cover, both as a combined variable and as separate snow and ice components, are calculated on an annual and monthly basis. Using derived cryo-cover concentration values, pixels are categorized to be either stable (covered 75% of the time or more) or transient (covered <75% of the time), as a method to differentiate the type of cryo-cover present annually and monthly as well as associated trends and transitions from one cover type (stable or transient) to the other. Results indicate a decrease in cryo-cover since 1979 with sea ice decreasing in all months, but snow cover increasing in the autumn and early winter and decreasing during the spring. Decreasing trends in stable cryo-cover are observed during most months with several months seeing increases in transient cover. Increases in autumn snow cover (particularly in October) help to offset the large decreases in late summer/early autumn sea ice. From April to August, cryo-cover as a whole exhibits statistically significant decreasing trends. A comprehensive understanding of the climatology of cryo-cover is of great importance in understanding the radiative impacts of a changing cryosphere in a warming climate.

2020

1. IJOC

   The role of blocking circulation and emerging open water feedbacks on Greenland cold-season air temperature variability over the last century

   TJ. Ballinger, E. Hanna, RJ. Hall, and 7 more authors

   International Journal of Climatology, Oct 2020

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   Substantial marine, terrestrial, and atmospheric changes have occurred over the Greenland region during the last century. Several studies have documented record-levels of Greenland Ice Sheet (GrIS) summer melt extent during the 2000s and 2010s, but relatively little work has been carried out to assess regional climatic changes in other seasons. Here, we focus on the less studied cold-season (i.e., autumn and winter) climate, tracing the long-term (1873–2013) variability of Greenland’s air temperatures through analyses of coastal observations and model-derived outlet glacier series and their linkages with North Atlantic sea ice, sea surface temperature (SST), and atmospheric circulation indices. Through a statistical framework, large amounts of west and south Greenland temperature variance (up to r2   50%) can be explained by the seasonally-contemporaneous combination of the Greenland Blocking Index (GBI) and the North Atlantic Oscillation (NAO; hereafter the combination of GBI and NAO is termed GBI). Lagged and concomitant regional sea-ice concentration (SIC) and the Atlantic Multidecadal Oscillation (AMO) seasonal indices account for small amounts of residual air temperature variance (r2 <  10%) relative to the GBI. The correlations between GBI and cold-season temperatures are predominantly positive and statistically-significant through time, while regional SIC conditions emerge as a significant covariate from the mid-20th century through the conclusion of the study period. The inclusion of the cold-season Pacific Decadal Oscillation (PDO) in multivariate analyses bolsters the air temperature variance explained by the North Atlantic regional predictors, suggesting the remote, background climate state is important to long-term Greenland temperature variability. These findings imply that large-scale tropospheric circulation has a strong control on surface temperature over Greenland through dynamic and thermodynamic impacts and stress the importance of understanding the evolving two-way linkages between the North Atlantic marine and atmospheric environment in order to more accurately predict Greenland seasonal climate variability and change through the 21st century.

2019

1. IJOC

   A comparison of extreme precipitation event frequency and magnitude using a high-resolution rain gage network and NOAA Atlas 14 across Delaware

   DJ. Leathers, SE. Brasher, KR. Brinson, and 2 more authors

   International Journal of Climatology, Dec 2019

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   Extreme precipitation events are arguably one of the most important natural hazards in many areas of the globe, impacting nearly every societal sector. In the Northeastern United States, extreme precipitation events have been shown to be increasing with several recent events garnering national attention (i.e., Ellicott City Maryland 2018; Tropical Storm Lee 2011). The NOAA Atlas 14 product is the nation’s standard for estimating the magnitude and frequency of site-specific extreme precipitation events, containing both precipitation frequency estimates, as well as associated confidence intervals. The Atlas uses surface stations, primarily from the National Weather Service Cooperative Observer Program, and statistical methodologies to provide point-based precipitation exceedance probability estimates for several durations and potential recurrence intervals. Unfortunately, the number and quality of Cooperative Observer sites varies greatly over space and time. This research compares observed precipitation extremes from a high-resolution statewide mesonet to those estimated by the Atlas 14 product for a 10-year recurrence interval at several precipitation durations. Results of the analysis indicate that Atlas 14 underestimates the number and magnitude of extreme precipitation events across the state of Delaware at longer event durations (360- to 1,440-min). At shorter durations (5- to 240-min) the Atlas 14 estimates are more closely aligned with the observations from the high-resolution precipitation network. These results suggest that caution should be exercised when using Atlas 14 estimates for engineering standards and hydrologic studies, especially for longer duration events. Therefore, a more rapid update cycle for revision of the Atlas 14 product should be considered, as a changing climate regime may be responsible for the differences identified in this research.