2011-09-27 18:49:50Ubanization increases rainfall
Paul D

chillcast@googlemail...
82.18.130.183

Interesting massive experinment:

http://environmentalresearchweb.org/cws/article/news/47295

Seems that China is one big experiment into the influence of urbanization on rainfall!

Interesting consequences are that when you concrete over large tracts of land, you are going to cause flooding in the place you don't want it.

2011-09-27 18:58:31
nealjking

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91.33.125.141

Sounds odd.

2011-09-28 00:22:39
adelady

amgnificent@gmail...
124.171.82.190

Not at all, neal.  This happens to be one of my hobby horses.    

Being a bit of an antique, I grew up in the era when very few suburban streets had concrete footpaths and, especially, gutters.   The roadside 'gutters' were really what permaculture people would call swales.  A deepish depression where water collected runoff and soaked into the soil and kept street trees alive through the dry months.     Yards were large and had fruit trees and a bit of a veg garden, and very little in the way of concrete.   Water from roofs was collected in rainwater tanks and the overflow from those did not go to a street drain, but ran along a brick or tile gutter - not sealed beneath by concrete - usually along a line of trees or hedge and any excess finished up in a patch of bamboo or some other plant that would take up as much water as it could get.

In other words, rainfall was directed and most of it soaked into soil in very much the same way it had been before suburbia  took over the farmland which had taken over the bush. 

Nowadays, a favoured word from the oldfart vocabulary, road surfaces and the attached gutters hermetically seal the soil beneath from any rainwater replenishment.   The clay soil of this area shrinks and cracks and swells unevenly because moisture levels are so low and water soakage is so erratic.  The result?  We need more and more street drainage to deal with flooding from the (very) occasional downpour.  The old pipes beneath the roads send up mighty gushers when they collapse under the stresses of mains pressure water within and moving soil around them. 

Pah!

2011-09-28 00:34:10
nealjking

nealjking@gmail...
91.33.125.141

adelady,

You're talking about flooding; but the article is about rainfall - specifically, about downpours.

2011-09-28 01:09:48
adelady

amgnificent@gmail...
124.171.82.190

And it's also about the resulting flooding.     That's OK.    My hobby horse got its exercise for the week.

2011-09-28 01:15:36
Paul D

chillcast@googlemail...
82.18.130.183

It's interesting that many have pointed to the lack of drain cleaning and drain maintence for flooding in urban areas in the UK (often climate change skeptics).
I doubt if this finding will stop urban sprawl increasing, but it is certainly something for planners to consider. Larger drains and improved infrastructure may be a default requirement.

2011-09-28 01:17:36
nealjking

nealjking@gmail...
91.33.125.141

adelady,

Your comment about flooding makes sense; but throws no light on my original question, which was, How would urbanization increase downpourss?

2011-09-28 01:26:31
Paul D

chillcast@googlemail...
82.18.130.183

The urban heat island effect??
Particles from cars and other sources of emissions?

In which case switching to low carbon energy would stop it.

2011-09-28 02:07:28
Albatross
Julian Brimelow
stomatalaperture@gmail...
199.126.232.206

Hi all,

There is some evidence in the literature, for Atlanta, that a city can increase rainfall and lightning activity under certain flow regimes-- but that is typically downwind of the city, not necessarily over the city.  

From Rose et al. (2008):

"This study verifies Atlanta’s propensity to conjointly enhance cloud-to-ground lightning and precipitation production in the absence of strong synoptic forcing. However, because of variability in aerosol characteristics and the dynamics of land use change, it may be a simplification to assume that this observed enhancement will be persistent across all scales of analysis."

Diem (2008) also has a paper.  There is also this essay from Sheperd and Mote (2009).  Just Google "urban rainfall enhancement".

In conrtrast, a colleague of mine has conuducted transects using an instrumented vehicle and has found that urban areas, and even large toens have a lower water vapour mixing ratios than the surrounding agricultural land during the summer months.  So the urban areas are both warmer and drier (the latter probably b/c of reducded evapotranspiration on account of all the buildings and infrastructure).  That would tend to reduce or inhibit thunderstorm formation and intensity.