Sunday, February 24, 2008

February 2008 Rainfall II

The raingauge at Chez BadMom read 1.5" this morning even though it was slightly tilted over. Had it been perfectly upright, it might have held even more water.

The California-Nevada River Forecast Center shows that nearby spots received just under 1" in the last 24 hours. Breathing Treatment also wonders why my gauge readings are a little bit higher than at his house, only a few miles away.

My house is on a relatively flat area about 2 miles from the ocean. Two bands of hills (really sand dunes) stand between us and the ocean so there is no hope of ocean views unless we build an 80 foot tall house. If we were on the seaward side of a hill, we would expect higher rainfall due to orographic forcing. (That's fancy talk for the air moves uphill and the water condenses out of the air as it rises and cools.)

Rob of Are you cereus? explained that the area behind a hill also receives elevated rainfall. The water doesn't condense and start falling right away. In that time lag, the wind will carry the condensing water droplets downwind.

Additionally, the air that comes up and over the dunes can come slamming back downward in a bouyancy wave. So, there is an upside to living in Felony Flats, leeward of 140-160 foot sand dunes (Hermosa View, RB's Golden Hills, Manhattan Hill District and Manhattan Heights. I get more free water falling out of the sky and require less irrigation water.

[Incidentally, developers have been razing the small beach shacks in those areas and erecting massive houses on those sand dunes. Not surprisingly, those homes develop foundation problems very quickly.]

Past posts in the LA rainfall series:
February 2008 Rainfall
When is a moderate drought good news? Lots of rainfall statistics links there.
The whole meteorology thread.

4 comments:

  1. I measured 1.52" this last storm.

    My gauge is near the ground in an open space in the garden and though it has a clear view to the sky, I've often wondered if nearby obstacles could cause very localized wind eddys that enhance or diminish measured rainfall on a meter by meter basis. It sounds preposterous. I suppose I could test with some coffee cans scattered through the yard.

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  2. That is entirely possible. Ideally, you would want to situate your raingauge away from buildings which do cause eddies.

    Unfortunately, my neighborhood is too dense for that. In the summer, my raingauge reads low because it is partly shaded by a deciduous tree.

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  3. Indeed, I have some really neat simulations now showing very nicely that frequently the enhanced rainfall does appear on the lee side of low hills rather than the windward side.

    At my house near Thos. Oaks, we've had 2.29" so far in Feb and 16.74" since Sept.

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  4. Have you heard of CoCoRaHS? http://www.cocorahs.org/States.aspx
    It's a group of people who (using the same rain guage) report their rainfall totals every day. The idea is to have more than one source of information for rainfall totals - most of us know that it can be raining on one block and the then there's no rain on the next block.

    I'm looking into doing it soon. I just need to get through my computer slowdown.

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