Dependence of extreme daily maximum temperatures on antecedent soil moisture in the contiguous United States during summer

The paper presents an analysis of the dependence of summertime daily maximum temperature on antecedent soil moisture using daily surface observations from a selection of stations in the contiguous United States and daily time series of soil moisture computed with a simple local water balance model. The computed soil moisture time series are offered its an alternative to Palmer's soil moisture anomaly (Z) index, the Palmer Drought Severity Index (PDSI), and other such time series. In contrast to other water balance models that have been designed for the computation of soil moisture time series, the model herein is driven by daily rather than monthly data, uses the Priestley-Taylor method in lieu of Thornthwaite's method to calculate potential evapotranspiration, allows for runoff during dry periods as well as when soil moisture is not at field capacity, includes a crude scheme for taking into account the effects of snowmelt on the water balance, and permits geographical variations in soil water capacity. The Priestley-Taylor method is considered to yield more realistic estimates of evapotranspiration than Thornthwaite's: method since it accounts for net radiation and represents a special case of the widely used Penman-Monteith method. Total runoff is parameterized according to the Variable Infiltration Capacity model. Based on a comparison with soil moisture measurements at Peoria, Illinois, the model appears to simulate the variability of soil moisture anomalies (W') reasonably well. Analysis of the relationship between W' and daily maximum temperatures (T-max) shows that in the central and eastern United States during the summer, the entire frequency distribution of standardized T-max is shifted toward higher values following a "low-W'" day (i.e., a day on which W' falls into the bottom quartile of its frequency distribution). The shift is most pronounced at the high end of the temperature distribution, indicating that as the soil gets drier, hot days tend to get hotter to a greater degree than cool days get warmer. Over the southeastern United States, where local evapotranspiration contributes a significant portion of the moisture available for precipitation, the temperature signal is particularly prominent and persists for up to several weeks after the soil moisture anomaly is observed. The relationship between temperature and daily precipitation is found to be much weaker and less persistent than the T-max-W' association. Thus, the frequency of record and near-record high temperatures is shown to be sensitive to soil moisture conditions, particularly on timescales shorter than one month.