Changes to the 1979–2013 summer Convective Available Potential Energy (CAPE) and extreme precipitation over North America

Abstract

Trends of monthly Convective Available Potential Energy (CAPE) and of single or multiple event extreme precipitation indices, namely RX1day, RX5day and R20mm, were analyzed for the summer of 1979–2013 over the United States (US) and southern Canada. A homogeneity test in CAPE data using the Wilcoxon-rank-sum test showed that statistically significant change points mainly occurred between 2000 and 2003 probably due to changes in humidity sensors during this period. Therefore, the trend analysis was split into two categories: 1979–2013 for regions where no statistically significant change points in CAPE were detected and 1979–2001 and 2002–2013 for regions where statistically significant change points in CAPE were detected. The results show that increasing trends in CAPE and extreme precipitation indices dominate the eastern Atlantic coast of the US and some parts of the Great Lakes. Mixed increasing and decreasing trends were detected in the US mid-west while the western US and the Canadian Pacific coast had little trend in CAPE and extreme precipitation indices. Although both increasing and decreasing trends were detected, the average trends in RX1day and RX5day were about 0.2 mm/decade and 0.5 mm/decade over 1979–2013. However, the average trend for the 1979–2001 period was 0.2 mm/decade (RX1day) and 1.2 mm/decade (RX5day) as compared 1.2 mm/decade (RX1day) and 1.7 mm/decade (RX5day) for the 2002–2013 period. Temperatures also increased more during the 2002–2013 period (0.3 °C/decade) as compared to the 1979–2001 period (0.1 °C/decade). The higher increasing trend of the recent decades suggests that increasing temperatures could potentially lead to an overall increase in CAPE and extreme precipitation over the US and southern Canada.