The Importance of Soil Moisture to Influence and Predict Summer Extreme Heat Events in North America

The role of Soil Moisture is different in affecting daily maximum (Tmax) and minimum temperature (Tmin) and is critical in predicting summer time extreme heat events. This work focuses on the land surface state in explaining how the soil moisture anomaly influences summer extreme heat, from a 13-year high resolution retrospective WRF regional climate simulation in North America. There are more daytime extreme events associated with dry soil moisture anomaly while more nighttime heatwaves favor wet anomaly. Both day and nighttime heatwave duration decrease with increasing soil moisture wetness. Two offline Noah-MP LSM (land surface model) tests are performed using the same atmospheric forcing but changing the initial soil state, to study the its impacts on 2006 daytime and 2011 nighttime heatwave. The evaporative cooling effect of increasing soil wetness to both Tmax and Tmin decays exponentially over time, with e-folding time around 20 and 15 days respectively. Tmax responses to the soil moisture anomaly more sensitively, while warmer Tmin also appears during and after heatwaves. Quantile regression between initial soil moisture anomaly and total daytime and nighttime number of hot days (NHD) for three different forecast periods agree well in the location of maxima and minima while the amplitude decreases with forecast time with high confidence. The same regression method is applied to the central North America and several strong negative soil moisture-temperature coupling regions are found in South and North Great Plain, Ohio valley in daytime and South Great Plain in nighttime with high significant level. Also, potentially more threats to human comfort and health, some positive coupling regions are located in West Ontario, around the Great Lake and Southwest arid region during nighttime. This suggests that initial soil state is crucial to influence and predict summer extreme heat over these regions.