Intermittency in Precipitation: Duration, Frequency, Intensity, and Amounts Using Hourly Data

Abstract

Intermittency is a core characteristic of precipitation, not well described by data and very poorly modeled. Detailed analyses are made of near-global gridded (about 1°) hourly or 3-hourly precipitation rates from two updated observational datasets [3-hourly TRMM 3B42, version 7, and hourly CMORPH, version 1.0, bias corrected (CRT)] and from special runs of CESM from January 1998 to December 2013 to obtain hourly values. The analyses explore the intermittency of precipitation: the frequency, intensity, duration, and amounts. A comparison is made for all products using several metrics with a focus on the duration of events, and a new metric is proposed based on the ratio of the frequency of precipitation at certain rates (0.2–2 mm h−1) for hourly versus 3-hourly versus daily data. For all seasons and rain rates, TRMM values are similar in pattern to CMORPH, but durations are about 80%–85%. It is mainly over land in the monsoons that CMORPH exceeds TRMM rain durations. Observed duration of precipitation events in CMORPH over oceans are 12–15 h in the tropics and subtropics, much less than the ~20 h for CESM. Hence, the observational results differ somewhat but both are considerably different from the model, which has too much precipitation overall, and it precipitates far too often at low rates and not enough for intense rates, with the divide about 1–2 mm h−1. There is a need to properly represent precipitation phenomena and processes either explicitly or implicitly (parameterized).

High‐Frequency Intermittency in Observed and Model‐Simulated Precipitation

“…Previous work shows that if X dc i and X dc j are uncorrelated for i ≠ j, then σ 2 Trenberth et al, 2017). In this case the variance of daily means is reduced by an extra factor D from the average variance of hourly values.…”

“…High-frequency precipitation records include not only a mean diurnal cycle and longer time scale statistics, such as daily and monthly means, but also an "intermittency" component associated with the original high-frequency values (Trenberth et al, 2017). The overwhelming fraction of precipitation variance comes from day-to-day variations at each hour of the diurnal cycle.…”

“…Left-hand panels map output from ensemble Member #35 (the same member examined inTrenberth et al, 2017). Left-hand panels map bias-corrected CMORPH hourly data (0.25°latitude/longitude resolution) for composite Julys of the years 1998-2013.…”

Assessment of Water Cycle Intensification Over Land using a Multisource Global Gridded Precipitation DataSet

“…Since then, the increase in heavy precipitation has been confirmed by numerous empirical studies (e.g., Fischer & Knutti, ; Huntington, , and references therein). On the other hand, there is still some questioning on the climate models performance in simulating precipitation (Hanel et al, ; Trenberth et al, ). Most importantly, a well‐known issue is that light or moderate precipitation around the convective area is inadequately modeled even for present‐day conditions (Sherwood et al, ).…”

Projected changes in extreme precipitation over Scotland and Northern England using a high-resolution regional climate model

“…6 (summer) and 7 (winter). In both seasons, present-day intensities between 0.1 and 1.0 mm/h account for ≥ 50% of all wet hours, and intensities larger than 5 mm/h are rare ( < 2% for the 1.5 km simulation), which is a commonly found issue with PCMs (Trenberth et al 2017). The 12 km fraction of wet hours with intensity < 1 mm/h is larger than radar observations, compensated for by smaller fractions in higher intensity bins and lower skewness.…”