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Sats and the City
Jamais Cascio, 21 Jul 05

urbanSE_USA.jpgGlobal climate models are pretty good -- better than some give them credit for -- but they're not perfect. There are still elements of atmospheric systems that they don't adequately cover. One such element is the effect of cities upon the climate. Arguably, this is not a disaster, as cities cover all of about 0.2 percent of the planet's land surface. But cities are growing, both in number and in size, and will soon hold half the planet's population. Even if the overall effect of urban development on the global climate is slight, results like the "urban heat island" effect certainly alter the local climates around cities.

How, then, could climate scientists account for cities in their models? J. Marshall Shepherd, at NASA's Goddard Space Flight Center, and Menglin Jin, at the University of Maryland-College Park, have authored a paper (in the May edition of Bulletin of the American Meteorological Society) with one possible answer. Their solution uses satellites.

What is it about cities that changes the atmosphere?

According to scientific studies presented at a recent American Geophysical Union session organized by the authors, "the construction of buildings, parking lots, houses, urban areas dramatically change the smoothness of a surface, thermal conductivity (the ability of a material to transmit heat), hydraulic conductivity (measure of the ability of soil to transmit water), albedo (reflectivity off of Earth's surfaces) emissivity (the ratio of radiation emitted by a body or surface) and vegetation cover."

It turns out that satellites such as NASA’s Terra, Aqua, Landsat, and Tropical Rainfall Measuring Mission (TRMM) are particularly useful for measuring the difference in effect between urban and non-urban areas, providing useful data on hydrology, surface reflectivity, atmospheric heat, and similar geophysical results. As a result, the authors argue that scientists should be able to comb through existing datasets and determine many of the urban characteristics over the years. The remaining issues for deriving good urban effect climate models all have to do with how cities are laid out, and are not readily amenable to satellite discovery. These include:

  • "Roughness length" calculations -- systems for determining the general smoothness of the surface, including the varying heights of buildings, parks, and roads;
  • "Urban geometry database" -- a standardized dataset of building density, coverage and height, information that's possible to assemble but not available in forms readily usable by climate scientists;
  • "Updated urban cover database" -- a listing of growth rates for cities, as most are growing in size so quickly that information from just ten years ago may be effectively useless for understanding present conditions.

    Various other factors can be a part of the models, too, depending upon the degree of complexity desired: street orientation; building materials used; height of vegetation "canopy;" and many more.

    Although the BAMS issue is behind a subscriber wall, NASA has made the article available for download (PDF). It's brief -- six pages of text, plus a few more of graphs and references -- and while it has enough climate science jargon to reassure the reader that it is, in fact, from a scientific journal, it's also relatively comprehensible for lay readers. For those of you interested in getting a glimpse of how models evolve, this is a fairly good and relevant introduction.

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    Comments

    You can see this in weather radar, as well.
    Watch a storm approaching a city. See it divide
    and go around on each side. Doesn't happen every time, but it does happen frequently.

    -jsq


    Posted by: John S. Quarterman on 23 Jul 05



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