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  • Author or Editor: Gabriel T. Bromley x
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Tobias Gerken
,
Gabriel T. Bromley
, and
Paul C. Stoy

Abstract

Land management impacts atmospheric boundary layer processes, and recent trends reducing the practice of summer fallow have led to increases in precipitation and decreases in temperature in the Canadian Prairie provinces during summer. It is unclear if such trends also impact the hydrometeorology of the adjacent U.S. northern Great Plains, parts of which have seen similar changes in land management. Here, MERRA-2 reanalysis data, eddy covariance observations, and a mixed-layer (ML) atmospheric modeling framework are combined to demonstrate that the likelihood of convectively preconditioned conditions has increased by approximately 10% since the mid-1980s and is now more sensitive to further decreases in the Bowen ratio (Bo) and maximum daily net radiation in northeastern Montana. Convective season Bo in the study area has decreased from approximately 2 to 1 from the 1980s until the present, largely due to simultaneous increases in latent heat flux and decreases in sensible heat flux, consistent with observed decreases of summer fallow and increases in cropping. Daily net radiation has not changed despite a significant decrease in May and June humidity lapse rates from the 1980s to present. Future research should determine the area of the U.S. Great Plains that has seen changes in the dynamics of the atmospheric boundary layer height and lifted condensation level and their crossings as a necessary condition for convective precipitation to occur and ascertain if ongoing changes in land management will lead to future changes in convective outcomes.

Full access
Gabriel T. Bromley
,
Tobias Gerken
,
Andreas F. Prein
, and
Paul C. Stoy

Abstract

We examined climate trends in the northern North American Great Plains (NNAGP) from 1970 to 2015, a period that aligns with widespread land-use changes in this globally important agricultural region. Trends were calculated from the Climatic Research Unit (CRU) and other climate datasets using a linear regression model that accounts for temporal autocorrelation. The NNAGP warmed on an annual basis, with the largest change occurring in winter (DJF) at 0.4°C decade−1. January in particular warmed at nearly 0.9°C decade−1. The NNAGP cooled by −0.18°C decade−1 during May and June, nearly the opposite of global warming trends during the study period. The atmospheric vapor pressure deficit (VPD), which can limit crop growth, decreased in excess of −0.4 hPa decade−1 during climatological summer in the southeastern part of the study domain. Precipitation P increased in the eastern portion of the NNAGP during all seasons except fall and increased during May and June in excess of 8 mm decade−1. Climate trends in the NNAGP largely followed global trends except during the early warm season (May and June) during which 2-m air temperature T air became cooler, VPD lower, and P greater across large parts of the study region. These changes are consistent with observed agricultural intensification during the study period, namely the reduction of summer fallow and expansion of agricultural land use. Global climate model simulations indicate that observed T air trends cannot be explained by natural climate variability. However, further climate attribution experiments are necessary to understand if observed changes are caused by increased agricultural intensity or other factors.

Open access
Gabriel T. Bromley
,
Tobias Gerken
,
Andreas F. Prein
, and
Paul C. Stoy
Open access