The current 88D networks
do not measure rainfall rate. The 88D's measure the reflectivity factor and radial velocity. From the radial velocity, spectrum width can be computed as well.
From the reflectivity factor it is possible to "back out" an instantaneous rainfall rate using one of several different formulas depending on the environment. Again, this is
not observed by the 88D; it is a post processed field. From the derived rainfall rates it is possible to then to a time integration to generate an estimated rainfall product.
As for rainfall rates, the smaller the time measurement, the more useful the information. Examples:
2" per hour could be composed of:
- 0.01" per minute for 10 minutes (0.1" total), 0" per minute for 30 minutes (0" total), then 0.095" per minute for 20 minutes (1.9" total); or
- 0.033" per minute for 60 minutes (2" total); or
- 0.066" per minute for 30 minutes (2" total, then 0" per minute for 30 minutes (0" total)
All three of those are equivalent to 2" per hour, however at any given moment the rain rate was different. By decreasing the time component of the rate, you give a higher quality report containing more information about current rainfall intensity. However, without including how long that rate continued, it's impossible to know how much rain fell.
With that said, the best report would be to do one of the following:
- Rain rate of 0.1" per minute for 10 minutes; or
- Total rainfall of 1" over the last 10 minutes
Both of these reports are equivalent (1" in 10 minutes) and allows the NWS to compute the missing information (total rain in [1] and rain rate in [2]).