Justin T. Schoof

Changes in the seasonality of precipitation over the contiguous USA

[1] Consequences of possible changes in annual total precipitation are dictated, in part, by the timing of precipitation events and changes therein. Herein, we investigated historical changes in precipitation seasonality over the US using observed station precipitation records to compute a standard seasonality index (SI) and the day of year on which certain percentiles of the annual total precipitation were achieved (percentile day of year). The mean SI from the majority of stations exhibited no difference in 1971-2000 relative to 30-year periods earlier in the century. However, analysis of the day of year on which certain percentiles of annual total precipitation were achieved indicated spatially coherent patterns of change. In some regions, the mean day of the year on which the 50th percentile of annual precipitation was achieved differed by 20-30 days between 1971-2000 and both 1911-1940 and 1941-1970. Output from the 10-Atmosphere-Ocean General Circulation Models (AOGCM) simulations of 1971-2000, 2046-2065, and 2081-2100 was used to determine whether AOGCMs are capable of representing the seasonal distribution of precipitation and to examine possible future changes. Many of the AOGCMs qualitatively captured spatial patterns of seasonality during 1971-2000, but there was considerable divergence between AOGCMs in terms of future changes. In both the west and southeast, 7 of 10 AOGCMs indicated later attainment of the 50th percentile accumulation in 2047-2065, implying a possible reversal of the twentieth-century tendency toward relative increases in precipitation receipt during winter and early spring over the southeast. However, this is also a region characterized by considerable interannual variability in the percentile day of year during the historical period.

On the Proper Order of Markov Chain Model for Daily Precipitation Occurrence in the Contiguous United States

Abstract Markov chains are widely used tools for modeling daily precipitation occurrence. Given the assumption that the Markov chain model is the right model for daily precipitation occurrence, the choice of Markov model order was examined on a monthly basis for 831 stations in the contiguous United States using long-term data. The model order was first identified using the Bayesian information criteria (BIC). The maximum-likelihood estimates of the Markov transition probabilities were computed from 100 bootstrapped samples and were then used to generate 50-yr precipitation occurrence series. The distributions of dry- and wet-spell lengths in the resulting series were then compared with observations using a two-sample Kolmogorov–Smirnov (K-S) test. The results suggest that the most parsimonious model, as identified by the BIC, usually (in approximately 68% of the cases) reproduced the wet- and dry-spell length distributions. However, the K-S test often indicated a second-order model when the BIC indicated...

Importance of the SRES in projections of climate change impacts on near-surface wind regimes

Analyses presented herein are designed to determine whether downscaled projections from the ECHAM5[MPI-OM1 Atmosphere-Ocean Global Climate Model (AOGCM) exhibit a high degree of sensitivity to the variations in the emission scenario (SRES) applied, and whether the projections of possible northern European wind climates lie beyond the range of conditions experienced in the historical period. For the study domain that extends from 51° to 71° N and 3° to 33° E, in the near-term (i.e. the middle of the current century) natural variability exceed the climate change signal. By the end of the twenty-first century there is evidence for small magnitude declines in most metrics of the wind climate relative to the 1961-1990 control period and for possible increases in extreme wind speeds, but the regionally averaged changes are of small magnitude relative to variations in two simulations of the control period. The analyses thus indicate that unlike precipitation and thermal regimes, the influence of the SRES on projections of the wind climate is small compared to natural variability and other sources of uncertainty in the downscaled projections. Results presented herein indicate that the AOGCM used to downscale wind climates has greater influence on the downscaled projections than stochastic effects within individual AOCGM simulations or the emission scenario applied.

Modeling Agricultural Watersheds with the Soil and Water Assessment Tool (SWAT): Calibration and Validation with a Novel Procedure for Spatially Explicit HRUs

Applications of the Soil and Water Assessment Tool (SWAT) model typically involve delineation of a watershed into subwatersheds/subbasins that are then further subdivided into hydrologic response units (HRUs) which are homogeneous areas of aggregated soil, landuse, and slope and are the smallest modeling units used within the model. In a given standard SWAT application, multiple potential HRUs (farm fields) in a subbasin are usually aggregated into a single HRU feature. In other words, the standard version of the model combines multiple potential HRUs (farm fields) with the same landuse/landcover, soil, and slope, but located at different places of a subbasin (spatially non-unique), and considers them as one HRU. In this study, ArcGIS pre-processing procedures were developed to spatially define a one-to-one match between farm fields and HRUs (spatially unique HRUs) within a subbasin prior to SWAT simulations to facilitate input processing, input/output mapping, and further analysis at the individual farm field level. Model input data such as landuse/landcover (LULC), soil, crop rotation, and other management data were processed through these HRUs. The SWAT model was then calibrated/validated for Raccoon River watershed in Iowa for 2002–2010 and Big Creek River watershed in Illinois for 2000–2003. SWAT was able to replicate annual, monthly, and daily streamflow, as well as sediment, nitrate and mineral phosphorous within recommended accuracy in most cases. The one-to-one match between farm fields and HRUs created and used in this study is a first step in performing LULC change, climate change impact, and other analyses in a more spatially explicit manner.

High‐resolution projections of 21st century daily precipitation for the contiguous U.S.

Changes in precipitation have the potential to produce wide ranging impacts across human and natural systems. Here precipitation projections from select Atmosphere-Ocean General Circulation Models and Earth Systems Models participating in Phase 5 of the Coupled Model Intercomparison Project are downscaled to a high-resolution (0.25° × 0.25°) grid covering the contiguous U.S. to improve spatial and temporal characteristics of the model-derived projections and derive multiple descriptors of 21st century precipitation climate. Projections for the Northeast, Pacific Northwest, and the high elevations of the Rocky Mountains are characterized by increases in total annual precipitation, with the magnitude depending strongly on the level of radiative forcing. Parts of the southern U.S. are projected to experience moderate precipitation decreases under all forcing scenarios. Increases in total annual precipitation are associated primarily with changes in precipitation intensity during the cold season. Significant precipitation decreases are projected for parts of the southern U.S. in all seasons except autumn and are associated primarily with changes in precipitation occurrence. Many locations in the eastern U.S. are projected to experience longer extreme dry spells and longer extreme wet spells, reflecting an increase in the serial correlation of precipitation. Conversely, many western locations are projected to experience shorter dry spells and wet spells, reflecting a decrease in the serial correlation of precipitation. Most locations are projected to experience an increase in extreme precipitation, reflected in increases in the mean annual single-day maximum precipitation and the number of heavy (>10 mm) and very heavy (>20 mm) precipitation days.

Nitrogen deposition to and cycling in a deciduous forest.

The project described here seeks to answer questions regarding the role increased nitrogen (N) deposition is playing in enhanced carbon (C) sequestration in temperate mid-latitude forests, using detailed measurements from an AmeriFlux tower in southern Indiana (Morgan-Monroe State Forest, or MMSF). The measurements indicate an average atmosphere-surface N flux of approximately 6 mg-N m(-2) day(-1) during the 2000 growing season, with approximately 40% coming from dry deposition of ammonia (NH3), nitric acid (HNO3), and particle-bound N. Wet deposition and throughfall measurements indicate significant canopy uptake of N (particularly NH4+) at the site, leading to a net canopy exchange (NCE) of -6 kg-N ha(-1) for the growing season. These data are used in combination with data on the aboveground C:N ratio, litterfall flux, and soil net N mineralization rates to indicate the level of potential perturbation of C sequestration at this site.

Assessing the fidelity of AOGCM-simulated relationships between large-scale modes of climate variability and wind speeds

The ability of atmosphere-ocean general circulation models (AOGCMs) to reproduce associations between surface/near-surface variables over the United States (US) and large-scale modes of climate variability has implications for evaluating possible biases in future climate projections and is an important model diagnostic. Indices of three such modes (El Nino–Southern Oscillation (ENSO), the Arctic Oscillation (AO), and the Pacific-North American (PNA) pattern) and their relationships to wind speeds over the contiguous US are derived using historical reanalysis products and then used to evaluate the fidelity of AOGCM simulations of both the climate modes and the teleconnections. In the reanalysis data, the response of middle and upper troposphere wind speeds to ENSO phase is found to be essentially symmetric with anomalies of opposite sign occurring during the warm and cold phases. The AO and PNA phases are both associated with higher wind speeds relative to “neutral” conditions. AOGCMs from Phase 5 of the Coupled Model Intercomparison Project (CMIP5) produce AO- and PNA-like spatial patterns which exhibit good accord with those from NCEP-NCAR Reanalysis. The AOGCM-derived climate indices also exhibit general agreement with reanalysis-derived indices in terms of the frequencies associated with highest variance, although the agreement is better for AO and PNA than it is for ENSO. The AOGCMs are in good agreement with the NNR in terms of representation of the influence of the AO and PNA on winds over the contiguous US. However, for ENSO, AOGCMs fail to consistently capture the observed relationship between La Nina and near-surface to middle troposphere winds.

Simulation of targeted pollutant-mitigation-strategies to reduce nitrate and sediment hotspots in agricultural watershed

About 50% of U.S. water pollution problems are caused by non-point source (NPS) pollution, primarily sediment and nutrients from agricultural areas, despite the widespread implementation of agricultural Best Management Practices (BMPs). However, the effectiveness of implementation strategies and type of BMPs at watershed scale are still not well understood. In this study, the Soil and Water Assessment Tool (SWAT) ecohydrological model was used to assess the effectiveness of pollutant mitigation strategies in the Raccoon River watershed (RRW) in west-central Iowa, USA. We analyzed fourteen management scenarios based on systematic combinations of five strategies: fertilizer/manure management, changing row-crop land to perennial grass, vegetative filter strips, cover crops and shallower tile drainage systems, specifically aimed at reducing nitrate and total suspended sediment yields from hotspot areas in the RRW. Moreover, we assessed implications of climate change on management practices, and the impacts of management practices on water availability, row crop yield, and total agricultural production. Our results indicate that sufficient reduction of nitrate load may require either implementation of multiple management practices (38.5% with current setup) or conversion of extensive areas into perennial grass (up to 49.7%) to meet and maintain the drinking water standard. However, climate change may undermine the effectiveness of management practices, especially late in the 21st century, cutting the reduction by up to 65% for nitrate and more for sediment loads. Further, though our approach is targeted, it resulted in a slight decrease (~5%) in watershed average crop yield and hence an overall reduction in total crop production, mainly due to the conversion of row-crop lands to perennial grass. Such yield reductions could be quite spatially heterogeneously distributed (0 to 40%).

Oppressive Heat Events in Illinois Related to Antecedent Wet Soils

AbstractExtreme heat events have been connected with antecedent soil moisture in many global regions, such that dry soils increase sensible heat content of the near-surface atmosphere and impede precipitation through boundary layer growth. However, negative soil moisture–temperature feedbacks (dry soils = higher temperatures) are founded on investigations of maximum temperature that neglect the potentially important latent heating component provided by soil moisture. In this study, the association of spring soil moisture and subsequent summer oppressive heat events is quantified, defined by equivalent temperature. The advantage of equivalent temperature over maximum temperature is that it accounts for both the temperature and moisture components of atmospheric heat content. Quantile regression and composite analysis are used to determine the association between spring soil moisture and summer oppressive heat events using a 25-yr station observation record in Illinois. A consistent response of summer oppre...

Characterizing extreme and oppressive heat waves in Illinois

Heat waves are characteristic features of summertime climate in the Midwest United States, and can have significant agricultural, hydrological, and societal impacts. Historically, heat waves in the Midwest state of Illinois have been either extreme (high temperature, low humidity) or oppressive (high temperature, high humidity) in nature, but our knowledge of the factors determining which heat wave type occurs is limited. We use self-organizing maps to classify synoptic-scale atmospheric circulation patterns associated with oppressive and extreme heat events, and analysis of variance to evaluate the atmospheric and land surface features responsible for differences in humidity that characterize the two. We find the majority of extreme and oppressive heat events are associated with similar synoptic-scale atmospheric conditions. Additionally, both locally-evaporated moisture and advected moisture sources were important for determining which of the two heat wave types occurred. Specifically, oppressive heat waves were characterized by abundant antecedent precipitation, surplus soil moisture, and elevated evapotranspiration and related atmospheric humidity. Lower humidity levels during extreme heat wave events were driven by relative reductions in evapotranspiration due to limited soil water content. Overall, our results suggest that the onset of heat waves in Illinois is primarily driven by circulation features in the upper atmosphere; however, the distinction of extreme or oppressive heat wave is due to differences in boundary layer humidity, driven in part by land surface moisture availability for evapotranspiration.