Date of Award
Doctor of Philosophy (PhD)
W.D. Gosnold Jr.
Most borehole paleoclimate studies have been based on the assumption that the exchange of temperature between the air and ground surface remains constant. However, secular changes in boundary-layer factors can generate anomalous ground temperatures. This dissertation is an investigation of factors that cause separation of air and ground temperatures on a seasonal to inter-annual time scale, including snow cover duration, latent energy of ground freezing, daily sunlight, precipitation, and vegetation.
Continual records of surface air and near-surface ground temperatures from five sites in North Dakota show similar trends in seasonal air-ground temperature separation. Statistical regressions of mean annual ground temperatures from Fargo and Bottineau sites indicate respective warming trends of 0.93±.09°C / 9 years and 1.17±.15°C / 6 years. Mean annual air temperatures and ground temperatures calculated with a conduction model assuming direct air-ground coupling do not display any significant trends.
The effect of snow cover duration on air-ground temperature exchange was tested by comparing average winter air-ground temperature differences (T5cm - Tair) with annual duration of snow cover. The correlation coefficients between these variables are: Bottineau: r2 = .84 (6 years), Fargo: r2 = .71 (9 years), Langdon: r2 = .56 (6 years), Minot: r2 = .79 (6 years), and Streeter: r2 = .87 (6 years). Best-fit latent energy of ground freezing values were determined with a conduction model and compared with total fall precipitation. The correlation coefficients between these variables are: Bottineau: r2= .38, Fargo r2= .66, Langdon: r2= .69, Minot: r2= .95, and Streeter: r2= .01.
A least-squares linear regression of mean annual air temperatures recorded in northwestern North Dakota from 1895 to 1995 indicates a warming magnitude of 1.57±.23°C per century. This temperature time series was forced into the ground with direct coupling to generate a synthetic temperature-depth profile. Inversion of this profile yielded a ground-surface warming magnitude of 1.7°C per century. Total fall precipitation is used as a proxy for latent energy of ground freezing. Latent energy effects were modeled with direct coupling of the regional air temperature record and a temperature dependent constraint for snow cover insulation. This generated a 0.4°C per century signal.
Schmidt, William L., "Air-ground temperature exchange" (2000). Theses and Dissertations. 264.