Entraining <scp>CAPE</scp> and a simplified instability index for better assessment of environmental conditions of quasi‐stationary linear precipitation systems

Abstract In Japan, a majority of heavy rainfall events are caused by quasi‐stationary linear precipitation systems (QSLPSs), which persist for more than five hours nearly at the same location. This study examines the performance of entraining convective available potential energy (E‐CAPE), which considers effects of entraining environmental air for an ascending parcel, in evaluating the environmental potential conducive to QSLPS occurrences. A composite analysis in which meteorological variables are superposed after rotating the coordinate system to match the direction of the QSLPS, shows that a confined region of a maximum of E‐CAPE exists at about 100 km upstream of the location of three‐hourly accumulated maximum precipitation (P3max), while a wider region of large conventional CAPE is located further upstream. A comparison of E‐CAPE between QSLPSs and linear precipitation systems with shorter persistence (NQSLPSs) demonstrates that significant differences in E‐CAPE exist on both the left and right sides of the upstream region, while those in CAPE exist only on the left side of the upstream region. The differences in E‐CAPE between QSLPSs and NQSLPSs result from the differences of lapse rate and low‐ to mid‐level moisture. A kernel density plot showing the relationship between P3max and the upstream maxima of CAPE and E‐CAPE also demonstrates that both values tend to be higher for QSLPSs than for NQSLPSs. Those results suggest that E‐CAPE provides better information on the degree of conditional instability for QSLPS occurrences compared to CAPE. A new parameter, the KT‐index, which has a simpler analytical formulation than E‐CAPE and yet provides a nearly effective assessment of the environmental potential for QSLPS occurrences, is proposed.