Thesis (Ph. D.)--University of Washington, 2000During the past several decades, much of the United States has experienced a warming, particularly at night, and a marked decrease in the diurnal temperature range (DTR). Observed variations in many variables have been cited as possible causes of the reported temperature trends. These include increasing cloudiness, rising concentrations of tropospheric aerosols, urbanization and other changes in land cover, ablation of snow cover, and changes in atmospheric circulation patterns. The purpose of this work is to sort out some of the interrelationships among these factors. Through a variety of analyses of historical daily meteorological observations, differences in the effects of cloudiness, land surface conditions, and the atmospheric circulation on daytime versus nighttime temperatures are examined.It is found that conditions at the land surface have a significant impact on daily maximum temperatures and the DTR. During summer, the tendency for daytime temperatures to be higher when the land surface is dry, which has been well-documented on monthly timescales, is manifested in daily data by a considerable increase in the incidence of near-record high temperatures on days on which the soil is depleted of moisture. In addition, enhanced evapotranspiration associated with the onset and growth of vegetation appears to play a role in the warm season dip of the climatological-mean DTR that is prominent over much of the eastern United States. During winter, snow cover suppresses the DTR under clear skies and reduces the effect of cloudiness on the DTR in the north-central United States.During the period 1966--1995, the cold season (November--March) DTR decreased over the central and southern United States, but increased over the Northeast, the Pacific Coast, and parts of the interior West. On a national scale, the impact of changes in the sea level pressure field on linear trends in the DTR is found to be small. Thus, much of the long-term trend in the DTR is not linearly related to changes in the atmospheric circulation, but may be attributable either to nonlinear relationships with other changing variables or to anthropogenic factors such as urbanization and the rise in the concentrations of tropospheric aerosols and greenhouse gases
