At the Ninth Annual GLOBE Conference, held 31 July . 05 August 2005 in Prague, the Czech Republic, GLOBE science PI Dr. David Brooks presented results from some student/teacher/scientist partnerships during the past year [Brooks, 2005]. One of these partnerships involved monitoring solar radiation (insolation) at two sites in Iowa. One site, under the direction of GLOBE teacher Kelen Panec, is at Waterloo Central Middle School and the other, under the direction of Meteorology Professor Alan Czarnetzki, is about 10 km WNW of Waterloo CMS at the University of Northern Iowa in Cedar Falls.
Solar radiation at Earth.s surface is important for any project related to agriculture and atmospheric modeling. So, even though there is no GLOBE protocol for measuring insolation, two pyranometers were installed. In keeping with GLOBE requirements for using inexpensive instruments, this project uses Model PYR pyranometers from Apogee Instruments, which cost about US each. They use silicon solar cell detectors instead of the thermopile detectors found in much more expensive pyranometers. Solar-cell-based pyranometers are widely used in agricultural applications and their performance has been studied extensively. It is possible to build solar-cell pyranometers identical in principle to commercial instruments for less than US [Brooks, 2004].
The data at Waterloo CMS and at UNI are recorded at one-minute intervals. Waterloo CMS has reported insolation data continuously since September 2004. UNI started reporting data in January 2005. Figure 1(a) shows insolation data collected at Waterloo Central Middle School for 17-24 March, 2005, at Waterloo Central Middle School. The values in boxes show the maximum and daily average insolation. .Noise. in the data indicates the presence of clouds. During this time span, 21 March was the clearest day. The data on 23 March, and other days, show the effects of solar radiation reflecting off the sides of cumulus clouds. Figure 1(b) compares the Waterloo data on 20 March with data collected at the same time at the University of Northern Iowa.
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| Figure 1(a). Insolation at Waterloo Central Middle School. |
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| Figure 1(b). Comparison of insolation at Waterloo CMS and the University of Northern Iowa, Cedar Falls, IA. |
Although these inexpensive pyranometers have limited absolute radiometric accuracy, they are entirely adequate for looking at variability in insolation. Direct measurements of spatial and temporal variability on the ground are essential for interpreting space-based measurements that are used to infer radiation at the ground. A typical sun-synchronous orbit such as followed by NASA.s Terra, Aqua, and Aura spacecraft, for example, passes views a site for just a few minutes a few times a day, and the field-of-view of their instruments at Earth.s surface may be as large as 10.s of kilometers on a side. With these viewing limitations, it is difficult to understand and separate spatial and temporal variability of the scene observed within an instrument.s ground .footprint..
Daily insolation data collected at one-minute intervals provide detailed information about the variability in insolation and, indirectly, about cloud cover and even cloud type. (See, for example, Duchon and O.Malley [1999].) The availability of two pyranometers separated by a few kilometers provides an unusual opportunity to look at both spatial and temporal variability of solar radiation. The motion of cloud fields between these two sites is clearly visible in Figure 1(b). Although clouds can often form and dissipate on a time scale of a few minutes, it is difficult to interpret the insolation data starting at about 930 minutes (15:30 CST) as anything other than the effects of the same cloud pattern passing over both stations. According to these data, the clouds have moved from 10 km from East to West in about 380 seconds, which gives a ground speed of about 26 m/s (about 58 m/h). On 20 March, the surface winds at the University of Northern Iowa were light and variable throughout the day, so this calculation is highly suspect. The most likely cause for concern is the internal data logger clocks at the two locations; if they were not set carefully to the same reference time (using a GPS receiver or www.time.gov, for example), then the time differences (and derived direction and speed) are not necessarily accurate. In any case, this work demonstrates the potential for collecting data that are extremely important for understanding spatial and temporal variability in solar irradiance over relatively small areas.
As is often the case with science data, the presentation of selected results here masks the amount of effort required to obtain a result that can be summarized in a graph or two. First of all, collaborators must establish a working relationship. Goals and procedures must be specified clearly. It takes longer to do this when the backgrounds, experiences, and goals of collaborators vary greatly. Inevitably, points that seem perfectly clear to one collaborator are not at all obvious to another collaborator. Developing a comfortable relationship is especially important and time-consuming when collaborations are conducted at a distance, with most communications conducted through the impersonal medium of e-mail.
Both these sites are still reporting pyranometer data. As is so often the case, the value of data such as these increases dramatically with the length of the data record. All the insolation data can be seen at http://www.pages.drexel.edu/~brooksdr/DRB_web_page/globeone/pyrodata.htm.
Brooks, David R., Alan Czarnetzki, Wade Geery, Kelen Panec, and Richard Roettger. Current Scientist/Teacher/Student Collaborations in Atmospheric Science, Ninth Annual GLOBE Conference, 31 July . 05 August 2005, Prague, Czech Republic.
Brooks, David R. (online documents, 2004-2005) http://www.pages.drexel.edu/~brooksdr/DRB_web_page/Solar/Solar_home_page.htm http://www.pages.drexel.edu/~brooksdr/DRB_web_page/construction/pyranometer/pyranometer.htm
Duchon, Claude E., and Mark S. O.Malley. Estimating Cloud Type from Pyranometer Observations. J. Appl. Meteor., 38, 132-141, 1999.