Monthly Solar Radiation in the Southeast Based on Solmet Data

Southeastern Geographer Vol. XX, No. 2, November 1980, pp. 134-144 MONTHLY SOLAR RADIATION IN THE SOUTHEAST BASED ON SOLMET DATA Vernon Meentemeyer and Mary Busken Kimsey The National Oceanic and Atmospheric Administration (NOAA) has recognized the emerging need for good baseline data for solar planning and engineering by providing, through the National Climatic Center (NCC), new solar radiation data for the United States. (1) The purpose of this paper is to produce solar radiation maps for the southeastern United States using this new Solar-Meteorological (SOLMET) data. All maps were produced using the Synagraphic Mapping (SYMAP) computer cartography program. (2) However, the SYMAP-derived maps display patterns and total values which in some cases are markedly different from the maps traditionally cited in the Climatic Atlas of the United States. (3) A secondary purpose of this paper is to examine these differences and to compare the mapped solar radiation values with data collected at Athens and at Atlanta, Georgia. The maps of monthly and annual solar radiation for the coterminous United States that appear in the Climatic Atlas of the United States are based upon data collected from a national network of 104 pyranometer stations. These maps have been widely reproduced in many publications , especially in documents on solar energy planning and research. Unfortunately there are substantial errors in these data, ranging from ±5 percent to ±30 percent. (4) Instrument "drift" and calibration problems, data manipulation, and data presentation all contributed to these errors. Consequently, data collection at all stations was terminated in 1972 despite the need for solar radiation data. The following year, a recommendation was made by NOAA officials that the best data collected by the pyranometer network be "rehabilitated." Under contract with the U.S. Department of Energy, NOAA assumed the task of rehabilitating the faulty data. (5) Dr. Meentemeyer is Associate Professor of Geography at the University of Georgia in Athens, GA 30602. Ms. Kimsey is Energy Resources Planner, Northeast Georgia Area Development and Planning Commission in Athens, GA 30605. Vol. XX, No. 2 135 Twenty-six stations with good data were selected. These data were checked for errors, reformatted, and corrected for instrument and other errors. The next step was the creation of a separate regression equation of the measured but rehabilitated data with records of hours of bright sunshine and sky cover conditions. Since sunshine hours and sky cover are measured at many stations and tend to correlate well with global solar radiation, it would then be possible to estimate the solar radiation received at many additional stations. Monthly and annual solar radiation totals were derived from regression equations for an additional 222 stations , making a total of 248 data stations. (6) Of the total 248 SOLMET data stations, 32 (Table 1) have been used to prepare the monthly and annual solar radiation maps for the Southeast (Figs. 1-13). Data from 13 stations of the old pyranometer network have been used to produce January, July, and annual maps (Figs. 14-16). Even 32 stations are not sufficient for detailed mapping. In areas of steep terrain such as the Appalachian mountain region, and along coastal areas which are affected by a sea breeze, the solar radiation values may vary from those indicated on the maps. Clearly a denser monitoring network is needed. Also, the possibility of truncated isolines on the periphery of each map was visually checked using data from stations outside the area mapped. Only in the eastern North Carolina area were manual adjustments of the original computer isolines found to be necessary . ANALYSIS. A visual examination of the monthly maps based on SOLMET (Figs. 1-12) reveals some striking seasonal changes in patterns. The contour pattern in January, February, and March displays primarily a horizontal (latitudinal) pattern which shifts to a vertical (meridional) pattern in June, July, and August. The range in the mean daily solar radiation across the Southeast presents a similar seasonal variation. In January there is a range of over 100 calories/square centimeter (cal · cm~2) across the map with high values in southern Florida and low values in north-central Tennessee. As the year progresses the range in solar radiation across the Southeast decreases until a...