Significant increases in summer precipitation have occurred in the midwestern United States over the last century for reasons that remain unclear. It is postulated that the expansion of irrigation and cropland in the central United States over the past 60 yr has been a major contributor to these observed increases in precipitation. As a first step toward attribution of these precipitation changes, a detailed analysis of observed daily summer precipitation frequency and intensity is conducted for the contiguous United States over multiple spatial scales and time periods from 1895 to 2011. Robust increases in precipitation frequency, total precipitation, and moderate to heavy precipitation intensity are identified during July and August in the midwestern United States. Analysis of changes in mean monthly precipitation from the early to late twentieth century initially points to increasing frequency as the source of increasing monthly precipitation in the midwestern United States during the summer, especially during August; however, these large frequency increases are not unique to the summer. On the other hand, changes in precipitation intensity and total precipitation are both greatest during July and August and coincide spatially in the midwestern United States. Additionally, the greatest intensity change occurs downwind of the most heavily irrigated regions, especially for the period between 1950 and 1980 when irrigation rapidly intensified. Thus, the seasonality and location of these regional signatures of increasing precipitation intensity (and total precipitation) are found to be broadly consistent with spatiotemporal trends in irrigation and cropland in the central United States and may be indicative of a causal link.

Infrasound—that is, acoustic waves with frequencies below the threshold of human hearing—has historically been used to detect and locate distant explosive events over global ranges (≥1,000 km). Simulations over these ranges have traditionally relied on large‐scale, synoptic meteorological information. However, infrasound propagation over shorter, local ranges (0–100 km) may be affected by smaller, mesoscale meteorological features. To identify the effects of these mesoscale meteorological features on local infrasound propagation, simulations were conducted using the Weather Research and Forecasting (WRF) meteorological model to approximate the meteorological conditions associated with a series of historical, small‐scale explosive test events that occurred at the Big Black Test Site in Bovina, Mississippi. These meteorological conditions were then incorporated into a full‐wave acoustic model to generate meteorology‐informed predictions of infrasound propagation. A series of WRF simulations was conducted with varying degrees of horizontal resolution—1, 3, and 15 km—to investigate the spatial sensitivity of these infrasound predictions. The results illustrate that convective precipitation events demonstrate potentially observable effects on local infrasound propagation due to strong, heterogeneous gradients in temperature and wind associated with the convective events themselves. Therefore, to accurately predict infrasound propagation on local scales, it may be necessary to use convection‐permitting meteorological models with a horizontal resolution ≤4 km at locations and times that support mesoscale convective activity.

Infrasound can propagate over a variety of spatiotemporal ranges and is therefore affected by spatiotemporally diverse atmospheric conditions.  However, studies of the influence of meteorology on infrasound propagation have historically utilized weather data that rely on point sources or coarser spatiotemporal resolutions, which often gloss over the effects of mesoscale meteorological phenomena.  In light of this knowledge gap, this study examines the influence of mesoscale meteorological features on infrasound propagation on local and regional scales.  To accomplish this task, output from simulations using the Weather Research and Forecasting (WRF) meteorological model is fed into an infrasound propagation model to generate infrasound predictions using realistic meteorological conditions.  The WRF simulations covered a range of horizontal resolutions – from 1 to 15 km – enabling an analysis of the sensitivity of the infrasound predictions to the horizontal resolution of the WRF output.  The main result is that convective precipitation events can appreciably alter the transmission loss patterns emanating from infrasonic sources, which is especially evident at finer horizontal resolutions.  This demonstrates that high-resolution weather data may be necessary to correctly simulate local to regional infrasound propagation, especially within warm-season, convective environments.(This work was funded by the Assistant Secretary of the Army for Acquisition, Logistics, and Technology [ASA{ALT}] under 0602784/T40/46 and 0602146/AR9/01.) Approved for public release: distribution is unlimited.

Understanding infrasound propagation is important for geophysical and military applications. Infrasound signatures can be detected from larger sources such as nuclear detonations and from smaller sources such as bridges, dams, and buildings. Infrastructure sources produce signals of lower amplitude, leading to more regional (up to 150 km) propagation. However, current methods for calculating regional infrasound propagation involve assumptions about the atmosphere, such as horizontal homogeneity, that deviate from more realistic environmental conditions and decrease the accuracy of the infrasound predictions. To remedy this issue, we have interfaced three-dimensional forecasts from the Weather Research and Forecasting (WRF) meteorological model with range-dependent parabolic equation propagation models. To test the improvement of infrasound propagation predictions with more realistic weather data, we conducted sensitivity studies with different propagation ranges and horizontal resolutions and compared them to predictions using simplified meteorological parameters. This allowed identification of the scales most ideal for resolving regional infrasound propagation given the limitations of WRF’s spatiotemporal resolutions. Additionally, we present results on quantifying uncertainty in these infrasound simulations by using multiple realizations of WRF forecasts to generate a spread of possible outcomes. Finally, we compare the simulated results to experimental data. (Approved for public release; distribution is unlimited.)