Dry Tropical Nights and Wet Extreme Heat in Beijing: Atypical Configurations between High Temperature and Humidity

Generally, tropical nights [TN; minimum temperature (Tmin) >= 25 degrees C] occur under wet air conditions, while extreme heat [EH; maximum temperature (Tmax) >= 35 degrees C] occurs under dry air conditions. This can be explained by higher humidity favoring TN through reducing longwave radiation cooling, and lower humidity favoring EH through enhancing solar radiation at the surface. The present study focuses on the atypical phenomena of dry TN (30% of all TN days) and wet EH (20% of all EH days) in Beijing during July and August, 1979-2008. It was found that meteorological conditions, including large-scale circulations and specific humidity, exhibit a resemblance between typical (wet TN and dry EH) and atypical (dry TN and wet EH) cases. That is, the meteorological anomalies for dry TN are similar to those for dry EH, and the anomalies for wet EH are similar to those for wet TN. For instance, descending anomalies, which lead to lower humidity and are thus associated with dry EH, appear for more than 70% of dry TN cases. In addition, the persistence of high temperature from day to night, and from night to day, also contribute significantly to dry TN and wet EH, respectively. About 50% of dry TN days and about 70% of wet EH days are preceded by EH and TN, respectively. It can be concluded from these results that both meteorological conditions and temperature persistence contribute greatly to dry TN and wet EH.