Heavy rain is a topic which lacks the intense drama of, say, melting ice. Nevertheless, changes in heavy rain frequency are important because they are cited as evidence of AGW and because, unlike Arctic ice, increased flooding could have global social impact.
This particular post concerns a tiny part of the subject – something I term “precipitation time-of-observation bias” (“P-TOB” for short).
P-TOB is not mentioned in climate science literature, so far as I can tell. This contrasts with P-TOB’s well-known cousin (temperature time of bias, “T-TOB”) which plays an important role in temperature reconstruction. P-TOB, unlike its cousin, goes without note.
The lack of literature reference certainly makes me pause and wonder if my reasoning is wrongheaded and P-TOB actually does not exist. Or, maybe my search has missed the explanation of how P-TOB is removed or avoided in studies, or maybe on balance any effect is too small to matter – I don’t know. But, nothing conjectured, nothing learned, and public errors can be excellent teachers. So, here goes.
Here’s the basic idea: a lot of intense rain occurs as thundershowers, especially on summer afternoons. If the 24-hr rainfall observation occurs during an afternoon thundershower then that thundershower may be split in two and recorded as two moderate rains rather than one intense rain. A record-keeping problem develops if the observation time is later changed to the unstormy early morning period. Such a change would mean that afternoon thundershowers are no longer split and are instead recorded as one intense rain rather than two moderate rains.
This shift in observation time (P-TOB) may make it appear that thundershowers have become more intense while in reality they have not changed.
Here is an illustration which may help:
If the daily rainfall total is taken at “A” ( see the red line) then the total afternoon storm is captured into one day of the record, as an intense event. However, if the observation is taken at “B” (the blue line) then the storm is split between two days and recorded as two moderate events.
If the time of observation changes from B to A and that change is not recognized when records are examined then one may think that there has been a shift towards intense precipitation events. However, that apparent shift may actually be an illusion created by the change in observation time.
That’s the basic P-TOB idea.