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 maximu m 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 Pa lmer's soil moisture anomaly (Z) index, the Palmer Drought Severity Index ( PDSI), and other such time series. In contrast to other water balance model s 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 Pri estley-Taylor method in lieu of Thornthwaite's method to calculate potentia l 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 geog raphical variations in soil water capacity. The Priestley-Taylor method is considered to yield more realistic estimates of evapotranspiration than Tho rnthwaite's: method since it accounts for net radiation and represents a sp ecial case of the widely used Penman-Monteith method. Total runoff is param eterized according to the Variable Infiltration Capacity model. Based on a comparison with soil moisture measurements at Peoria, Illinois, the model a ppears to simulate the variability of soil moisture anomalies (W') reasonab ly well. Analysis of the relationship between W' and daily maximum temperatures (T-m ax) shows that in the central and eastern United States during the summer, the entire frequency distribution of standardized T-max is shifted toward h igher values following a "low-W'" day (i.e., a day on which W' falls into t he bottom quartile of its frequency distribution). The shift is most pronou nced at the high end of the temperature distribution, indicating that as th e soil gets drier, hot days tend to get hotter to a greater degree than coo l days get warmer. Over the southeastern United States, where local evapotr anspiration contributes a significant portion of the moisture available for precipitation, the temperature signal is particularly prominent and persis ts for up to several weeks after the soil moisture anomaly is observed. The relationship between temperature and daily precipitation is found to be mu ch weaker and less persistent than the T-max-W' association. Thus, the freq uency of record and near-record high temperatures is shown to be sensitive to soil moisture conditions, particularly on timescales shorter than one mo nth.