Davis Application Note 28 says:

RAINFALL RATE
Parameters Used: Rain Total (actually, rain rate is a measured
variable in the sense that it is measured by the ISS and transmitted
to the display console, whereas all other calculated variables are
determined by the console from data received from the ISS.)

Formula:
Under normal conditions, rain rate data is sent with a nominal
interval of 10 to 12 seconds. Every time a rain tip or click occurs, a
new rain rate value is computed (from the timer values) and the rate
timers are reset to zero. Rain rate is calculated based on the time
between successive tips of the rain collector. The rain rate value is
the highest rate since the last transmitted rain rate data packet.
(Under most conditions, however, a rain tip will not occur every 10 to
12 seconds.) If there have been no rain tips since the last rain rate
data transmission, then the rain rate based on the time since that
last tip is indicated. This results in slowly decaying rate values as
a rain storm ends, instead of showing a rain rate which abruptly drops
to zero. This results in a more realistic representation of the actual
rain event. If this time exceeds roughly 15 minutes, than the rain
rate value is reset to zero. This period of time was chosen because 15
minutes is defined by the U.S. National Weather Service as
intervening time upon which one rain "event" is considered separate
from another rain "event". This is also the shortest period of time
that the Umbrella will be seen on the display console after the onset
of rain.

(Apologies if the line length isn't consistent above - copied from a
PDF document.)

John Dann
www.weatherstations.co.uk