We were able to set up our two systems on the roof of the Minneapolis Convention Center November 21-28, 2011. We were fortunate to be able to see the roof between the snow melt from the snowfall on the 19th and before we have the snow cover through winter. We plan to go back in spring once the snow is gone. For now, here is a photo of our two systems. Both are measuring albedo (the reflectivity of the surface), absorbed sunlight, the temperature of the surface and air at four heights up to 2 m, and wind speed and direction. One system is set up over a black membrane covered with river rock (in the foreground), and the other is set up over white PVC (in the background).
Below are the first results from this experiment. One day (November 23, 2011) is shown as an example. This figure shows that the river rock surface is darker (has a lower albedo) and therefore absorbs more radiation. While this seems obvious, our ability to quantify these differences under a variety of environmental conditions (different seasons, cloudy vs. sunny days) will help us develop models that can simulate the effects of using these different surfaces to mitigate urban warming. It’s not enough to know that darker surfaces are usually warmer, we need numbers.
Net radiation is greater over the darker river rock surface than over the white PVC during daytime. Values are similar during nighttime. Noon albedo (reflectivity) is lower over the river rock than over the white PVC by 0.4.
Will Hertel has created an image that shows the features of the Twin Cities urban heat island.
The data are from September 15, 2011 at 4:00 am CDT. Urban heat islands are generally stronger in winter and during the nighttime. While we analyze data taken over the entire 24-hour day, the data at this early hour highlight the largest difference between the downtown and outlying areas on this particular day. Will interpolated among sites that include our volunteer network along with other standard temperature recording locations (such as airports).
This is one example of the data that we will be able to analyze with this project.
Thanks to those of you who have volunteered to help us with this project. We have received addresses from hundreds of interested people and are in the process of sorting through them. We’re grateful that so many of you are willing to help! With more sites being added, these preliminary data will be improved–more sites means less interpolation between existing sites. Stay tuned…
Our project was mentioned in Paul Huttner’s Updraft blog on MPR on Friday, November 11, 2011.