Factors influencing the cold-season diurnal temperature range in the United States

This study examines the contributions of sunshine duration, snow cover exte nt, and the atmospheric circulation to variations of the cold-season diurna l 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 tre nds. For the cold seasons (Nov-Mar) between 1958/59 and 1994/95, daily surf ace observations at more than 200 stations from the First Summary of the Da y (FSOD) dataset as well as selected daily fields from the NCEP-NCAR 40-Yea r Reanalysis Project are analyzed using compositing, correlation, and regre ssion techniques. For each region, a sea level pressure anomaly pattern is identified that is linearly related to daily variations in the DTR. It is f ound that the presence of positive sea level pressure anomalies over a regi on, clear skies, and the absence of snow on the ground all favor high value s of the regionally averaged DTR. The strength of these associations varies geographically because of the effects of nonlinear relationships, the freq uency 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 south ern United States experienced a decrease in the DTR, while the northeast, P acific coast, and portions of the interior west experienced an increase. Va riations in the DTR-related sea level pressure patterns and sunshine durati on explain significant fractions of the DTR increase in the coastal Northwe st and the DTR decrease in the south-central states. The DTR trends over th e rest of the country are largely unrelated to linear trends in sunshine du ration, snow cover, or the sea level pressure field. The spatial pattern of DTR trends is reproduced when homogeneity-adjusted data from the Global Hi storical Climatology Network are used in lieu of FSOD data. Hence, it appea rs that the geographical pattern of trends is not a result of inhomogeneiti es in the FSOD data. The findings presented here suggest that many of the o bserved cold-season trends in the DTR are not induced by linearly related c hanges in the atmospheric circulation and, therefore, are attributable eith er to internal nonlinear relationships in the climate system or to anthropo genic factors such as urbanization and increasing concentrations of greenho use gases and tropospheric aerosols.