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Abstract The magnitude and frequency of heavy precipitation are expected to increase under warming temperatures caused by climate change. These trends have emerged in observational records but with much larger evidence on a daily rather than a subdaily scale. Here, we quantify long‐term changes in heavy precipitation frequency in the United States using hourly observations in 1949–2020 from 332 gauges. We demonstrate that, when analyzed collectively, the frequencies of heavy precipitation at multiple durations from hourly to daily exhibit an increase that cannot be explained by natural climate variability. Upward trends are significant at ∼20%–40% of the gauges throughout the country except for the coastal western and southeastern regions, with higher percentages for longer durations. We also show that the frequency of hourly heavy precipitation has mainly grown after ∼2000, thus explaining the limited evidence of trends at the subdaily scale reported in past studies. 

Two-sample tests are widely used in hydrologic and climate studies to investigate whether two samples of a variable of interest could be considered drawn from different populations. Despite this, the information on the power (i.e., the probability of correctly rejecting the null hypothesis) of these tests applied to hydroclimatic variables is limited. Here, this need is addressed considering four popular two-sample tests applied to daily and extreme precipitation, and annual peak flow series. The chosen tests assess differences in location (t-Student and Wilcoxon) and distribution (Kolmogorov–Smirnov and likelihood-ratio). The power was quantified through Monte Carlo simulations relying on pairs of realistic samples of the three variables with equal size, generated with a procedure based on suitable parametric distributions and copulas. After showing that differences in sample skewness are monotonically related to differences in spread, power surfaces were built as a function of the relative changes in location and spread of the samples and utilized to interpret three case studies comparing samples of observed precipitation and discharge series in the U.S. It was found that (1) the t-Student applied to the log-transformed samples has the same power as the Wilcoxon test; (2) location (distribution) tests perform better than distribution (location) tests for small (moderate-to-large) differences in spread and skewness; (3) the power is relatively lower (higher) if the differences in location and spread or skewness have concordant (discordant) sign; and (4) the power increases with the sample size but could be quite low for tests applied to extreme precipitation and discharge records that are commonly short. This work provides useful recommendations for selecting and interpreting two-sample tests in a broad range of hydroclimatic applications. 

Increasing empirical evidence has been showing that, over the last decades, the frequency of daily heavy precipitation has risen in some regions of the United States (U.S.); less evidence has instead been presented at subdaily resolutions. In this study, we describe the challenges and opportunities associated with the detection of trends in subdaily heavy P in the U.S. using Version 2 of the Hourly Precipitation Data (HPD) from the National Climatic Data Center (NCDC). This dataset comprises records from 1897 gages which we found to be affected by several issues preventing their use in trend studies, including long periods with missing observations, changes of instruments, and different signal resolutions (largely, 0.254 and 2.54 mm). Despite this, after proper checks, we were able to identify 370 gages with ≥40 years of statistically homogenous data in 1950-2010 that cover the U.S. with a good density. To improve the ability to detect trends, we designed a framework that quantifies the degree to which the observed over-threshold series above a given empirical q-quantile are consistent with stationary count time series with the same marginal distribution and serial correlation structure as the observations. We also applied the false discovery rate test to account for spatial dependence and multiplicity of the local tests. Analyses were performed for the signals aggregated at Δt = 1, 2, 3, 6, 12, and 24 h and for q = 0.95, 0.97, and 0.99, finding that most gages exhibit increasing trends across all Δt’s and that their statistical significance increases with Δt and decreases with q, but only for Δt ≥ 2 h. This might indicate that the physical generating mechanisms of precipitation have changed in a way that leads to larger accumulations over durations >1 h but similar intensities within 1 h. An alternative possible explanation for these outcomes is instead that the coarse signal resolution (2.54 mm) reduces the power of the test for trend detection as Δt decreases. Investigating these issues will be the subject of our immediate future work. 

Theoretical arguments and modeling experiments suggest that heavy precipitation is expected to intensify in a warmer climate. These projections have been supported by observational evidence at the daily scale, whereas the lack of long-term subdaily records has limited the ability to conduct analyses at shorter durations. In this study, we address this research gap using the Hourly Precipitation Data (HPD) from the National Climatic Data Center (NCDC). Due to the presence of relatively large periods with missing observations, we first implement a procedure to reconstruct probable missing zeros using the Analysis of Record Calibration (AORC) hourly gridded product. After the reconstruction, we identify 1404 gages with more than 75% (median of 94%) of complete records in the period 1979-2019 that cover the continental U.S. with good density. We then perform trend test analyses on the hourly observations where, at each gage, (1) independent events are identified, (2) peak-over- threshold series above the 90th, 95th, and 99th quantiles are extracted, and (3) trend tests are performed on the annual frequency and mean intensity of the POT series. After accounting for field significance, we find that hourly heavy precipitation exhibits statistically significant trends that are increasing for the frequency (+1% - +2% every year) but decreasing for the intensity (- 0.4 mm/h - -1.8 mm/h every 10 years). This is true in most of the country, except for some areas in the Southwest and South regions. Analyses repeated with the signals aggregated at 2, 3, 6, 12, and 24 hours lead to similar patterns, although a lower number of statistically significant trends is found as the duration increases. Overall, the statistical evidence of the trends is higher when focusing on the frequency rather than the intensity of heavy precipitation, and it is reduced when considering higher quantiles likely because of the lower test power. The results of this study are useful for the validation of climate and atmospheric models and the incorporation of nonstationarities due to global warming in intensity-duration- frequency curves of extreme precipitation used for infrastructure design.