Effects of entrainment on convective available potential energy and closure assumptions in convection parameterization

Abstract

This study investigates the effect of entrainment dilution on convective available potential energy (CAPE) and closure assumptions in convection parameterization using the sounding data from three Intensive Observation Periods (IOPs). It is shown that entrainment of the environmental air has a strong dilution effect on CAPE, and this effect depends on the degree of subsaturation of the entrained air: the drier the entrained air, the larger the effect. For CAPE‐based closure assumptions, the dilute CAPE has a moderate correlation with convective removal of CAPE. While better than for undiluted CAPE, which is virtually uncorrelated with convective CAPE removal, this correlation is not satisfactory enough for convection closure. For quasi‐equilibrium–based closures, while the free tropospheric quasi‐equilibrium assumption is a superior closure for convection when undiluted CAPE is used, both the Arakawa‐Schubert quasi‐equilibrium closure and the free tropospheric quasi‐equilibrium closure work well for dilute CAPE in all three IOPs studied. It is further shown from the CAPE definition and the large‐scale temperature budget equation that for undiluted CAPE, the free tropospheric large‐scale CAPE change and precipitation are approximately linearly related. The most important effect of entrainment dilution on CAPE and convection parameterization closure assumptions is to enhance the role of free tropospheric humidity, thereby diminishing the overwhelming role of boundary layer control on undiluted CAPE and its variation.