Detecting a greenhouse-aerosol signal in the diurnal temperature cycle.

摘要

According to the Intergovernmental Panel on Climate Change's most recent publication (IPCC, 1996), increases in direct radiational forcing on the Earth attributable to greenhouse gases emissions since pre-industrial times (in the last {dollar}{lcub}sim{rcub}250{dollar} years) have been {dollar}rm 2.45 Wmsp{lcub}-2{rcub}.{dollar} Aerosol loading due in part to sulfur dioxide emissions and biomass burning also currently contributes an alteration of solar flux of approximately {dollar}rm {lcub}-{rcub}6 Wmsp{lcub}-2{rcub}{dollar} averaged over the coterminous United States and Canada. This recent and sizable alteration in net forcing and the difficulty in detecting any expected impact has fueled a scientific controversy of great magnitude.; Numerous studies have attempted to detect an anthropogenic signal in the temperature record. These investigations have focused on the average global or hemispheric temperature (Folland et al., 1984, Ellsaesser et al., 1986; Jones et al., 1986; Jones and Wigley, 1990; Oort et al., 1990), temperature change on a regional scale (Karl et al., 1984; Diaz et al., 1986), and diurnal temperature range at a regional level (Karl et al., 1993; Plummer et al., 1994), but none have focused on the cycle of daily temperature.; Using hourly data from 67 climatological stations in North America over the past 40 years, a daily temperature and dewpoint climatology was generated. This climatology fixed all data points relative to daily sunrise and sunset times which allows for examination of daytime warming and nighttime cooling regimes. Using this climatology, the following three central hypotheses directed by the current paradigm of an enhanced greenhouse-aerosol atmosphere were tested: (1) Nighttime cooling rates are decreasing under clear-sky conditions. (2) Daytime warming rates are increasing under clear-sky conditions and this increased rate is attenuated in high sulfate regions. (3) Nights are warming relative to days.; Following a careful examination, hypothesis (1) was rejected, hypothesis (2) had mixed results and hypothesis (3) was accepted. Thus, it was found that enhanced greenhouse-aerosol considerations are insufficient to explain observed changes in diurnal temperature regimes. Observed increases in cloudiness and sulfates also do not explain this apparent lack of support for the hypotheses.; The primary observed alteration in shape of diurnal temperature profiles was characterized by a relative reduction in temperatures in the few hours after sunrise and a relative increase in temperatures in the late afternoon. This observation is consistent with an urbanization signal. Changes in the overall magnitude of diurnal temperatures are consistent with circulation changes.