Precipitation Intensity Trend Detection using Hourly and Daily Observations in Portland, Oregon

Abstract

Abstract:The intensity of precipitation is expected to increase in response to climate change, but the regions where this may occur are unclear. The lack of certainty from climate models warrants an examination of trends in observational records. However, the temporal resolution of records may affect the success of trend detection. Daily observations are often used, but may be too coarse to detect changes. Sub-daily records may improve detection, but their value is not yet quantified. Using daily and hourly records from 24 rain gages in Portland, Oregon (OR), trends in precipitation intensity and volume are examined for the period of 1999-2015. Daily intensity is measured using the Simple Daily Intensity Index, and this method is adapted to measure hourly scale intensity. Kendall's tau, a non-parametric correlation coefficient, is used for monotonic trend detection. Field significance and tests for spatial autocorrelation using Moran's Index are used to determine the significance of group hypothesis tests. Results indicate that the hourly data is superior in trend detection when compared with daily data; more trends are detected with hourly scale data at both the 5% and 10% significance levels. Hourly records showed a significant increase in 6 of 12 months, while daily records showed a significant increase in 4 of 12 months at the 10% significance level. At both scales increasing trends were concentrated in spring and summer months, while no winter trends were detected. Volume was shown to be increasing in most months experiencing increased intensity, and is a probable driver of the intensity trends observed.

Impacts of climate change on characteristics of daily‐scale rainfall events based on nine selected GCMs under four CMIP5 RCP scenarios in Qu River basin, east China

“…For finer temporal resolution, such as hourly rainfall, the changes may be more obvious, particularly for extreme rainfall. Cooley and Chang () demonstrated that daily data is a bit weaker in trend detection when compared with hourly data, and Utsumi et al . () also indicated that daily‐scale extreme rainfall is less sensitive to climate warming than sub‐daily scale extreme rainfall.…”

Detecting Change in Rainstorm Properties from 1977-2016 and Associated Future Flood Risks in Portland, Oregon

“…At the 5% significance level daily records were significant in March while hourly records were significant in more months including January, March, April, October, and November. This is consistent with expectations that daily scale records suppress intensity trend detection compared with hourly scale records (Cooley and Chang, 2017). Compared to temperature, precipitation occurs at smaller spatial and temporal scales that are more difficult to measure (Boer et al, 2000).…”

“…Although the resolution of climate models steadily increases, the ability of these models to deliver precipitation projections that resemble realistic precipitation at the hourly scale remain novel (Xu et al, 2005;Seneviratne et al 2006;Prein et al, 2016). However, the reliance on daily data provides a potential obstacle to detection of trends (Cooley and Chang, 2017). It is possible that the use annual time scales and daily data may contribute to the incoherent precipitation trends observed.…”

How does sub‐hourly rainfall intermittency bias the climatology of hourly and daily rainfalls? Examples from arid and wet tropical Australia

“…The goal is to quantify this effect using data having high temporal resolution. As noted above, many authors note that even hourly data are used relatively little owing to the limited access to such data (Bartolini et al ., ; Beranova et al ., ; Cooley and Chang, ). Only in special investigations are high‐resolution data commonly available, as exemplified by the work of Larsen and Teves (), who analyzed data from a dense array of 21 laser disdrometers that reported tallies of drop counts and sizes every 60 s. The availability of such data remains very limited.…”