Evaluating the CoMorph‐A parametrization using idealized simulations of the two‐way coupling between convection and large‐scale dynamics

Abstract

AbstractWe present a new methodology to test the interactions of convection schemes with their larger scale environment. A single‐column model (SCM) using the new Met Office convection scheme, CoMorph‐A, and the new Met Office–Natural Environment Research Council cloud‐resolving model (CRM) are coupled to damped‐gravity‐wave‐derived large‐scale dynamics. The coupled models are used to investigate convective responses to stimulus forcings under the influence of interactive large‐scale dynamics. Within CoMorph‐A, the default entrainment varies with the prediction of convection size that is dependent on earlier rainfall, and the sensitivity of the SCM results to the entrainment formulation is explored. We demonstrate that the behaviour of the SCM using CoMorph‐A is now very similar to that of the CRM. For temperature or moisture stimulus applied separately, the SCM adjusts to a new equilibrium that is similar to that in the CRM, but its transient convective responses to stimuli acting to suppress convection are markedly too fast. For a combination of stimuli acting to enhance convection, the SCM responses are stronger than in the CRM. Finally, convective rainfall in the SCM is relatively insensitive to a combination of stimuli acting to enhance and suppress convection simultaneously, in agreement with the CRM. However, the SCM recovery from a non‐precipitating state is overly delayed for the default entrainment formulation but is too rapid when the entrainment rate is fixed at a low rate to represent only the deep convective state. We examined the responses to moisture stimuli of different strengths. Both models produce a monotonic increase of precipitation with column relative humidity (CRH) as well as the sharp increase of precipitation as CRH exceeds a threshold, as seen in observations. Though both models correctly capture the observed CRH threshold, differences from the observed precipitation–CRH relationship are noted. For instance, above the threshold the increase of precipitation with CRH is more abrupt in the SCM than in the CRM and observations. A similar behaviour is obtained in the SCM using low entrainment rate.