Factors Influencing the Cold-Season Diurnal Temperature Range in the United States

Abstract

This study examines the contributions of sunshine duration, snow cover extent, and the atmospheric circulation to variations of the cold-season diurnal temperature range (DTR) in eight regions of the contiguous United States. The goal of the research is to facilitate the interpretation of long-term changes in the DTR in light of the possible anthropogenic role in these trends. For the cold seasons (Nov?Mar) between 1958/59 and 1994/95, daily surface observations at more than 200 stations from the First Summary of the Day (FSOD) dataset as well as selected daily fields from the NCEP?NCAR 40-Year Reanalysis Project are analyzed using compositing, correlation, and regression techniques. For each region, a sea level pressure anomaly pattern is identified that is linearly related to daily variations in the DTR. It is found that the presence of positive sea level pressure anomalies over a region, clear skies, and the absence of snow on the ground all favor high values of the regionally averaged DTR. The strength of these associations varies geographically because of the effects of nonlinear relationships, the frequency of snow cover, and the complexity of local dynamics. The cold-season trends of several variables for the period 1965/66?1994/95 are also analyzed. During the 30-yr period of record, the central and southern United States experienced a decrease in the DTR, while the northeast, Pacific coast, and portions of the interior west experienced an increase. Variations in the DTR-related sea level pressure patterns and sunshine duration explain significant fractions of the DTR increase in the coastal Northwest and the DTR decrease in the south-central states. The DTR trends over the rest of the country are largely unrelated to linear trends in sunshine duration, snow cover, or the sea level pressure field. The spatial pattern of DTR trends is reproduced when homogeneity-adjusted data from the Global Historical Climatology Network are used in lieu of FSOD data. Hence, it appears that the geographical pattern of trends is not a result of inhomogeneities in the FSOD data. The findings presented here suggest that many of the observed cold-season trends in the DTR are not induced by linearly related changes in the atmospheric circulation and, therefore, are attributable either to internal nonlinear relationships in the climate system or to anthropogenic factors such as urbanization and increasing concentrations of greenhouse gases and tropospheric aerosols.