Susan Wahl

Date of Award

8-2009

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geography & Geographic Information Science

First Advisor

Paul E. Todhunter

Abstract

The influence of solar variability on climate has received increasing attention. Much of the research has dealt with fluctuating ultraviolet radiation and its effects in the stratosphere; decadal signatures in cloud cover and their relationship to cosmic rays; and correlations of surface air temperature with various proxies (e.g., sunspot numbers; 10Be from ice cores and marine sediments). Much of the research has focused on changing solar radiation within the upper atmosphere and the propagating effects into the troposphere. However, there is a missing component involving differential surface heating as a function of soil composition and corresponding spectral radiation absorption properties that result in changing barometric pressure and atmospheric circulation patterns. Size and bonding of molecules composing crystalline structures within soils combined with the bending, stretching, rotation, vibration, and translation frequencies determine the type of photons absorbed. The availability of photons fluctuates as the Sun’s output varies. This study attempts to extract the influence of changing spectral radiation on near-surface soil temperatures for six meteorological stations operated by the Natural Resources Conservation Service (NRCS). The six stations are part of the Soil Climate Analysis Network (SCAN) and are located within arid/semiarid regions of the United States (New Mexico, Nevada, Arizona, Idaho, Oregon, and Washington). 2-inch (5-cm) soil temperature measurements were extracted for June, July, and August of 2004 and 2005 and correlated with top-of-atmosphere solar spectral irradiance data from the Spectral Irradiance Monitor (SIM) on board the Solar Radiation and Climate Experiment (SORCE) spacecraft.

Initial analyses yielded low correlation coefficients, with the exception of the site in Oregon. Soil moisture had a large influence on soil temperature for the other five stations. However, soil temperature for the station in Oregon did not have a strong connection to soil moisture and produced R = 0.47 (T = 4.8; α = 0.001) between soil temperature and ultraviolet irradiance. In addition, the sites in Arizona and Washington contained measurements with no soil moisture content. When the data was separated into categories of 0% soil moisture and positive soil moisture, soil temperatures responded to changing solar irradiance. Both sites produced moderate positive correlations between soil temperature and longer wavelength radiation (visible and middle-infrared), with the maximum correlations occurring at 1500-1600 nm irradiance. For the Arizona site, R = 0.61 (T = 4.4; α = 0.001) for soil temperature measurements when no soil moisture was present. The site in Washington also produced moderate correlations between soil temperature and longer wavelength radiation (R = 5.8; T = 3.7; α = 0.01), however, this occurred for measurements containing positive soil moisture. Insignificant correlations occurred when no soil moisture content was present.

The results indicate that soil chemistry may influence the type of spectral radiation absorbed. This could potentially result in temporal fluctuations in surface heating due to changes in solar output. Surface heating as a function of changing solar spectral irradiance may influence barometric pressure and localized circulation patterns as a result of spatial distribution of geochemical composition. Based on measurements from SIM, visible and near-infrared radiation is inversely correlated with ultraviolet radiation. Therefore, the sites in Washington and Arizona may have greater surface heating during times of reduced solar activity, while the site in Oregon indicates greater surface heating during times of increasing solar activity. This shows that responses to solar variability are regional, with some areas being positively correlated with solar activity, others have inverse responses, and some have no connection to changing spectral irradiance.

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