Rainfall measurement

Archive of NZ Weather & Climate
Forum rules
These topics are a read-only archive and may be subject to out-of-date information.

For today's weather discussion head to: New Zealand Weather & Climate
RWood
Posts: 3745
Joined: Sat 24/01/2004 16:56
Location: Wellington
Has thanked: 188 times
Been thanked: 123 times

Rainfall measurement

Unread post by RWood »

TonyT, I would appreciate it if you could give an indication of the comparability issues for manual and automated rainfall devices...NIWA has been publishing monthly data for both types at Milford Sd for a few years now - the AWS values are consistently lower by up to 20%, but of course in the absence of any other information this is not very enlightening. At Kelburn during the period of overlap the comparison was much closer, with the AWS reading a few % higher.
User avatar
TonyT
Moderator
Posts: 2878
Joined: Thu 08/05/2003 11:09
Location: Amberley, North Canterbury
Has thanked: 352 times
Been thanked: 1146 times

Unread post by TonyT »

I'm not aware of any issues between the two. There are not really any physical differences between the gauges insofar as the way they collect rain, what is different is the way the rain is measured once it gets inside the funnel. In a manual gauge the rain is measured repeatedly using a glass tube, whereas in the automatic gauge there is usually a tipping bucket mechanism. However, if we assume that both processes are operating properly (ie the manual observer reliably and consistently uses the glass tube, and the calibration on the tipping buckets is correct) then they should both produce the same results FOR THE SAME EXPOSURE.

This last point is the key one I suspect. Put a manual guage on the ground and an AWS gauge at 2m on a mast and they may produce slightly different results. Install the AWS 1km down the road (as may have happened in Milford Sound) and they will certainly produce different results. Add in the complex topography of a place like Milford (steep slopes all around, plenty of potential for minor sheltering and/or enhancement effects due to topography, and different wind flows over and around nearby slopes), and I guess the difference of 20% is certainly explainable.
RWood
Posts: 3745
Joined: Sat 24/01/2004 16:56
Location: Wellington
Has thanked: 188 times
Been thanked: 123 times

Unread post by RWood »

Thanks for that... :)
Locked