John W. Enz

Air‐ground temperature coupling and subsurface propagation of annual temperature signals

[1] Borehole-based reconstructions of ground surface temperature (GST) have been widely used as indicators of paleoclimate. These reconstructions assume that heat transport within the subsurface is conductive. Climatic interpretations of GST reconstructions also assume that GST is strongly coupled to surface air temperature (SAT) on timescales of decades and longer. We examine these two assumptions using records of SAT and subsurface temperature time series from Fargo, North Dakota; Prague, Czech Republic; Cape Henlopen State Park, Delaware; and Cape Hatteras National Seashore, North Carolina. The characteristics of downward propagating annual temperature signals at each site clearly indicate that heat transport can be described as one-dimensional conduction in a homogeneous medium. Extrapolations of subsurface observations to the ground surface yield estimates of annual GST signals and allow comparisons to annual SAT signals. All annual GST signals are modestly attenuated and negligibly phase shifted relative to SAT. The four sites collectively demonstrate that differences between annual GST and SAT signals arise in both summer and winter seasons, in amounts dependent on the climatic setting of each site. INDEX TERMS: 1645 Global Change: Solid Earth; 1875 Hydrology: Unsaturated zone; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 3367 Meteorology and Atmospheric Dynamics: Theoretical modeling; KEYWORDS: heat transport, air-ground termperature coupling, paleoclimate

Daily, seasonal, and annual relationships between air and subsurface temperatures

[1] Inversions of borehole temperature profiles that reconstruct past ground surface temperature (GST) changes have been used to estimate historical changes in surface air temperature (SAT). Paleoclimatic interpretations of GST reconstructions are based on the assumption that GST and SAT changes are closely coupled over decades, centuries, and longer. This assumption has been the subject of some debate because of known differences between GST and SAT at timescales of hours, days, seasons, and years. We investigate GST and SAT relationships on daily, seasonal, and annual timescales to identify and characterize the principal meteorological changes that lead to short-term differences between GST and SAT and consider the effects of those differences on coupling between the two temperatures over much longer time periods. We use observational SAT and subsurface data from Fargo, North Dakota; Prague, Czech Republic; Cape Henlopen State Park, Delaware; and Cape Hatteras National Seashore, North Carolina. These records comprise intradaily observations that span parts of one or two decades. We compare subsurface temperature observations to calculations from a conductive subsurface model driven with daily SAT as the surface boundary condition and show that daily differences exist between observed and modeled subsurface temperatures. We also analyze year-to-year spectral decompositions of daily SAT and subsurface temperature time series and show that dissimilarities between mean annual GST and SAT are attributable to differences in annual amplitudes of the two temperature signals. The seasonal partitioning of these amplitude differences varies from year to year and from site to site, responding to variable evapotranspiration and cryogenic effects. Variable year-to-year differences between mean annual GST and SAT are closely estimated using results from a multivariate regression model that associates the partial influences of seasonal meteorological conditions with the attenuation of annual GST amplitudes.

Conduction‐dominated heat transport of the annual temperature signal in soil

[1] Conductive heat transport of temperature signals into the subsurface is a central assumption of ground surface temperature (GST) reconstructions derived from presentday temperatures in deep boreholes. Here we test this assumption and its implications for annual relationships between GST and surface air temperature (SAT) by analyzing two decades of shallow soil temperature (0.01–11.7 m) and SAT time series measured at Fargo, North Dakota. We spectrally decompose each of these temperature time series to determine the amplitude and phase of the annual signal at each depth. Conductive heat transport of a harmonic temperature signal in a homogeneous medium is characterized theoretically by exponential amplitude attenuation and linear phase shift with depth. We show that transport of the annual signal in the soil at Fargo follows these theoretical characterizations of conduction closely: the depth dependence of both the natural logarithm of the amplitude and the phase shift are highly linear. Interval wave velocities and thermal diffusivities calculated as functions of depth suggest a diffusivity gradient in the upper meter of the soil. We estimate the annual signal at the ground surface by extrapolating amplitude and phase shift regression lines upward to the surface. We compare this estimate of the annual signal at the ground surface to the annual signal contained in the SAT and show the ground surface signal to be attenuated � 20% and negligibly phase shifted relative to the SAT. INDEX TERMS: 1645 Global Change: Solid Earth; 1875 Hydrology: Unsaturated zone; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 3367 Meteorology and Atmospheric Dynamics: Theoretical modeling; KEYWORDS: paleoclimate, soil temperature, conductive heat transport

A decade of air–ground temperature exchange from Fargo, North Dakota

Abstract In borehole paleoclimatology, it is commonly assumed that a direct coupling exists between air and ground temperatures. This assumption is valid only if variables affecting ground-surface temperature exchange have remained constant through time. In an analysis of a 9-year record of air and ground temperature data, we found that several critical variables changed in ways that cause decoupling between air and ground temperatures. Mean-annual ground temperatures in the upper 12 m increased by 0.93±0.09°C during the study. Air temperatures used as model-forcing signals generated ground temperatures that exhibit no significant increase. The decoupling of winter air and ground temperatures is due to snow cover and latent energy effects. Maximum residual temperatures for freezing, summer and thawing modeling periods averaged ±0.18°C, ±0.30°C, and ±0.75°C, respectively. Duration of winter snow cover increased during the time of record and correlates with winter air–ground temperature differences (r2=0.71). Annual values of modeled latent energy of ground freezing show a dependence upon total precipitation 60 days prior to ground freezing.

Transpiration and evapotranspiration from maize as related to leaf area index

Abstract Evapotranspiration and transpiration from maize ( Zea mays L.) were measured with precision weighing lysimeters in 1983 and 1984. Climatic parameters were measured to calculate potential evapotranspiration, while leaf area index was measured twice weekly. Crop coefficients were determined from daily values of evapotranspiration (or transpiration)/potential evapotranspiration. Non-linear models were tested and showed that 72–86% of the variation in crop coefficient for evapotranspiration could be described knowing leaf area index. Model testing with transpiration showed that 90–95% of the variation in crop coefficient could be described knowing leaf area index.

Evaporation and energy balance for bare and stubble covered soil

Daily solar, reflected, and net radiation, air and dew-point temperature, wind speed, and soil heat flux were compared with daily evaporation for a bare and stubble covered soil surface for 45 days during spring 1984. Surface temperatures and radiation fluxes were monitored every 15 min from sunrise to sunset on five of these days. Daily net radiation was always greater for the bare soil surface, averaging 11 443 kJ m−2 compared to 10 483 kJ m−2 for the stubble covered soil. Temperature of the bare soil surface was always greater than the stubble covered surface except for overcast conditions with wet surfaces when they were about equal. Maximum differences were 14.5°C. Evaporation was always greater from the bare surface until it was dry (<8% water). Evaporation was then greater from the stubble covered surface because of its greater available water. There was no correlation between daily evaporation from either surface and the net or solar radiation. Daily evaporation was correlated only with the number of days after precipitation and with wind speed. Reduced evaporation from the stubble covered soil was related to reduced wind speed and lower surface temperatures in the stubble.

Quality and variability of long term climate data relative to agriculture

Abstract Climate data quality concerns prompted this study. The objective was to ascertain the impact of air temperature inhomogeneities on derived weather variables, such as growing-degree days, frost dates and heat stress, and to develop adjustment methodologies. Daily weather data spanning 1893–1989 from 34 Iowa stations were used for computation. Direct comparisons for each weather station and derived variable were made between all possible weather station combinations and a geographical mean data set. The latter was computed by using all available weather stations. For all derived weather variables studied, most weather stations revealed some type of notable inhomogeneity over the long period. Homogeneous weather stations were indeed rare. Individual time series exhibiting different levels of stability were common as were those exhibiting sharp discontinuities. Many, but not all, discontinuities could be traced to station histories. The use of the geographical mean data sets provided a rapid method to screen candidate weather stations, and could be used to correct discontinuities if they were not too numerous. More precise adjustments could, however, be made by using one or two nearby weather stations that were identified as being extremely stable. This process was more tedious because the stable periods were usually short. Climate resources are a valuable asset for mankind. However, data quality needs to be scrutinized before subsequent climate analyses.

Evaluation of energy balance and water use by spring wheat during a normal and a dry season

The energy balance for spring wheat (Triticum aestivum L.) was related to growing season water availability. In 1981, with favorable soil moisture, evapotranspiration (ET) was about 92% of net radiation (Rn). Under dry conditions in 1982, ET was about 60% of Rn and sensible heat (A) was 30% of Rn. Midday canopy temperatures tended to be warmer than air temperature in 1982 and cooler than air temperature in 1981. Evapotranspiration was highly correlated to Rn in 1981 (R2 = 0.80 and 0.81). Net radiation was also highly correlated to solar radiation (R2 = 0.99 and 0.98) although Rn was a smaller fraction of solar radiation under the drier conditions of 1982. Net radiation alone was a good predictor of ET under the favorable soil water conditions of 1981 in this subhumid environment.