Generally, the daily rainfalls are recorded at fixed time intervals (e.g., from 9 am to 9 am). We can find such records of historical rainfalls for centuries. However, in such records, even the 24-hr rainfall might be split into two depending on the actual time of its start (say around 3 pm) and cease (say 3 pm of the following day). Recording of continuous rainfall has been started recently and data are available only for short periods. If I want to include the old records of daily rainfalls for my frequency analysis, what is the procedure acceptable for extracting (and adjusting if allowed) annual (or seasonal) maximum rains from such historical records?
It is customary to multiply fixed-duration daily maxima by a correction factor (Hershfield factor, Fixed-duration correction factor) of approximately 1.13 to adjust to 24-hour maxima.
Please take a look at:
1. Papalexiou, S. M., Dialynas, Y. G., & Grimaldi, S. (2016). Hershfield factor revisited: Correcting annual maximum precipitation. Journal of Hydrology, 542, 884-895.
2. Hershfield, D.M., 1961a. Technical Paper No. 40: Rainfall frequency atlas of the United States, Department of Commerce. Weather Bureau, Washington, DC.
3. WMO (2009). Guide to Hydrological Practices, Volume II: Management of Water Resources and Application of Hydrological Practices, WMO-No. 168, 6th Edition, World Meteorological Organization. (page II.5-16)
How about using the recent data which was obtained on sub-daily time scales (suppose hourly) and starting from 1am of one day to 1am of next day, determine the annual maximum 1am to 1am 24-hr rainfall for each year. Repeat this procedure for 2am to 2am rainfall; 3am to 3am rainfall, etc until reach midnite to midnite 24-hr rainfall maximum for each year. For each starting time interval, compute the mean value of the maximum for all of the years at this stating time interval. Then just find the maximum value from among all of the mean maximums for each starting interval. Take the ratio of this maximum value among all starting intervals to the mean maximum value of the 9am to 9am interval; call this ratio R.
Then moving to the earlier years that only had once daily data (e.g. 9am to 9am), multiply R by the mean maximum value of the 9am to 9am interval for each of these earlier years--this time series on average should approximate the annual maximum 24hr rainfall from among all starting time intervals.
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