Assessing trends in lower tropospheric heat content in the central United States using equivalent temperature.

ABSTRACT Isobaric equivalent temperature ( TE) is the temperature that an air parcel would have if all associated water vapour were condensed and the resulting latent heat used to increase the temperature of the parcel. It is therefore an ideal metric for assessing changes in (1) total near-surface heat content associated with both temperature and moisture content and (2) the joint behaviour of temperature and humidity, which is relevant to both lower atmospheric stability and human heat stress during extreme temperature events. We present results from an analysis of 50 years (1961-2010) of daily TE and its temperature and moisture components at seven stations in the central United States. The annual means of daily TEmax and TEmin increased at all stations during the period of analysis with the largest changes occurring in TEmin, largely as a result of increasing minimum air temperature. At western locations significant increases in the annual mean TEmax were also observed, resulting from a combination of increases in Tmax and humidity. Despite small summer ( JJA) trends in maximum air temperature, summer TE trends were generally larger than their annual counterparts. The timing of the observed variations and the resulting spatial pattern are consistent with observed changes in meridional moisture flux associated with the Great Plains low-level jet. Heat waves in the region were found to be characterized by increasing TEmin, primarily resulting from increases in minimum air temperature. At western stations, heat waves were also characterized by increasing TEmax as a result of positive trends in humidity. In most cases, equivalent temperature provides a perspective on local environmental change that differs from what is provided by consideration of temperature alone. [ABSTRACT FROM AUTHOR]