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Extreme rainfall over Southwestern United States/Northwestern Mexico (SWUNWM) has been mostly investigated during wet seasons, while no or little attention has been paid to extreme rainfall during dry seasons despite its vital importance for sustaining vegetation and ecosystems. Here we examine the top 1% rainfall over SWUNWM in June, the driest month on average, and assess how it is affected by the ocean with a 50 km-resolution global climate model. Comparing millennia-long simulations with and without the ocean, we find that the ocean does not change the pattern and magnitude of atmospheric circulation associated with June extreme rainfall, but significantly enhances rainfall intensity. This intensification is attributed to a larger variability of atmospheric moisture content enhanced mainly by the sea surface temperature (SST) in the tropical Pacific. The similarities in the atmospheric circulation associated with, and the temporal characteristics of, June extreme rainfall between the two simulations point to a dominant control of extreme rainfall dynamics by atmospheric intrinsic processes and atmosphere-land coupling. These modeling results imply that the predictability of occurrence of June extreme rainfall over SWUNWM is limited by atmospheric intrinsic dynamics and atmosphere-land coupling, while reliable predictions of its intensity likely require a faithful simulation of SST variability, especially in the tropical Pacific. [ABSTRACT FROM AUTHOR]

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• Rainfall momentum increases with altitude. • A new parameter, momentum content M mm , is proposed to study rain erosivity. • M mm is related to the DSD shape and the pressure (air density) at the site. • Soil particle detachment by drops is magnified at higher altitude locations. Soil erosion is a mechanical process described by indices, such as kinetic energy or momentum of rainfall (M), which alters the protective cover of the ground surface. Optical disdrometer (PWS100) data, obtained between 2014 and 2018 at four sites across Mexico, were used to study the features of power and linear relationships between M and rainfall amounts (Acc). A comparison of the measurements showed that PWS100 usually estimated larger amounts of rainwater (PBIAS from 2 to 25%) with respect to gauges. The M-Acc relationships exhibited low dispersion, and the coefficients of the fitted expressions suggest an increase in M based on site altitude. Normalization of M based on the rainwater amount (M / Acc in N m−2 mm−1, hereafter M mm) depended on the median volumetric diameter, D 0 , and speed of raindrop fall. The D 0 was used as a proxy of raindrop spectrum broadness. Effect size results exhibited large values (d > 1.1) when M mm means were compared between coastal and continental platforms. Furthermore, y -coefficients of the M mm – D 0 relationships suggested an effect of the altitude (related to the air density in the sampling site) over M mm estimates. Based on the v (D) empirical formulas, a set of expressions were proposed to fit the M mm observations of the four sampling sites using D 0 and the corresponding atmospheric pressure or air density estimates. The results of performance measures showed that all equations predicted M mm adequately thus suggesting that this erosivity parameter depended on both the amount of precipitation and the altitude (expressed as air density or atmospheric pressure) of the site at which the measurements were performed. These findings reinforce ideas regarding magnification of soil particle detachment via individual drops at higher altitude locations, as the force exerted is more significant with a decrease of air pressure and density. [ABSTRACT FROM AUTHOR]

This research evaluates the partial-area effect and its relationship with the rainfall intensity–duration–frequency (IDF) equations. In the Rational Method, if the critical rainfall duration is shorter than the time of concentration, the partial-area effect occurs. We proved that the partial area could exist for the general ID equation i = a / (b + t d) c , only when c > 1 . For these equations, in the application of the Rational Method, the maximum discharge at basin outlet occurs for rainfall duration ( t d ) equal to b/(c−1). Nevertheless, for that case, the Depth Duration Frequency (DDF) has a maximum at that rainfall duration. These situations are present in engineering practice and will be discussed in this paper. Research was done to look for IDF equations with c > 1 in hydrologic engineering practice. It found 640 inconsistent IDF equations ( c > 1 ) in four countries (Brazil, Mexico, India, and USA), which means that a fundamental principle for building consistent IDF equations (i.e., c > 1 ), published in the scientific literature since 1998, did not reach the hydrologic engineering practice fully. We provided some analysis regarding this gap between theory and engineering practice. [ABSTRACT FROM AUTHOR]