Rain Velocity Affects Rain Guages?

[Wes Carter]

I just read this article on LiveScience.com. The author states that because we have underestimating the velocity of falling rain we have been overestimating the amount of rainfall by up to 20%. I can understand how it might affect doppler radar, which is mentioned. But how could this affect the readings of rain guages? They specifically mention "specialized rain guages". What kind of rain guages would be affected by velocity?

Link to article

 

[Chris Nuttall]

I wish I could access and read the actual journal article. Since I can't, I can't really comment on their findings. Just off the top of my head, a "specialized rain gauge" that would be affected by fall velocity be tipping buckets. There are several known scenarios that would result in underestimation of rainfall, especially during convective events with high wind.

That's why I want to see the details in their findings. I'm interested to see what types of rain gauges they are talking about.

 

[Mike Smith]

I second Chris' comments regarding tipping buckets.

An anecdote: I live about one mile from the tipping bucket at AAO and my conventional rain gauge almost always reads higher after a period of intense rainfall.

 

[Greg Stumpf]

An anecdote: I live about one mile from the tipping bucket at AAO and my conventional rain gauge almost always reads higher after a period of intense rainfall.

I agree that if the gauge's buckets cannot keep tipping fast enough, there will be undercatchment. However, is the article considering the rate of rainfall or the fall velocity of the drops? If the latter, what would be the effects?

 

[Jeff Snyder]

I don't have access to AGU journals (Geophysical Research Letters is published by AGU), but the abstract can be read at http://www.agu.org/pubs/crossref/2009/2008GL037111.shtml

I agree -- I have no idea how this would affect the raingauge observations. Since rainrate calculations based on DSDs do use a drop size - fall velocity relation [typically in a power-law (e.g. Atlas and Ulbrich 1977), exponential (e.g. Atlas, 1973), or polynomial (e.g. Brandes et al. 2002)], an inaccurate fallspeed velocity relation will result in incorrect rainrate calculations based on observed or modeled DSDs. If one uses DSDs to calculate radar reflectivity factor and rainrate, then one could end up with an incorrect Z-R relation. As such, an incorrect velocity relation could end up affecting rainrate estimates from radars (through an incorrect Z-R relation if such relation is derived from observed or modeled DSDs).

The article notes that the increased fall velocity occurs for a very short time immediately after drop breakup or collision, which doesn't seem like something that appreciably affects modeled or DSD-calculated rainrates. As measured by gauges, though, this doesn't appear to affect anything. A raingauge doesn't really care about fall speed of the drops, with the exception of some known biases associated with particular raingauge designs (e.g. under-reporting by tipping bucket raingauges in very intense rainrates, etc).

 

[Wes Carter]

Thanks for the replies. I cannot find the original paper but if I did we would have to translate it. According to the article it was done at a Mexican university. But the tipping bucket guages makes sense. I was just thinking that my dual cylinder type guage would not care about velocity unless it caused a portion of the rain to splash out, which would be noticeable.

If I find the original paper, I'll post a link.

 

[Chris Nuttall]

The full article can be purchased from Jeff's link to the abstract, but it costs $9.