Modelling the diurnal cycle of deep precipitating convection over land with cloud‐resolving models and single‐column models

Abstract

SUMMARYAn idealized case-study has been designed to investigate the modelling of the diurnal cycle of deep precipitating convection over land. A simulation of this case was performed by seven single-column models (SCMs) and three cloud-resolving models (CRMs). Within this framework, a quick onset of convective rainfall is found in most SCMs, consistent with the results from general-circulation models. In contrast, CRMs do not predict rainfall before noon. A joint analysis of the results provided by both types of model indicates that convection occurs too early in most SCMs, due to crude triggering criteria and quick onsets of convective precipitation. In the CRMs, the first clouds appear before noon, but surface rainfall is delayed by a few hours to several hours. This intermediate stage, missing in all SCMs except for one, is characterized by a gradual moistening of the free troposphere and an increase of cloud-top height. Later on, convective downdraughts efficiently cool and dry the boundary layer (BL) in the CRMs. This feature is also absent in most SCMs, which tend to adjust towards more unstable states, with moister (and often more cloudy) low levels and a drier free atmosphere. This common behaviour of most SCMs with respect to deep moist convective processes occurs even though each SCM simulates a different diurnal cycle of the BL and atmospheric stability. The scatter among the SCMs results from the wide variety of representations of BL turbulence and moist convection in these models. Greater consistency is found among the CRMs, despite some differences in their representation of the daytime BL growth, which are linked to their parametrizations of BL turbulence and/or resolution.

Modelling convective processes during the suppressed phase of a Madden–Julian oscillation: Comparing single‐column models with cloud‐resolving models

“…The cloud-resolving models generally show good agreement, both with each other and with the observations. In addition, the spread among the CRMs is smaller than that among the SCMs, as in previous intercomparisons (Guichard et al, 2004). The evolution of PW in the CRMs during days 3-6 of experiment B0 shows a very different trend to the observed PW during this period.…”

“…In this study, simulations are carried out using cloud-resolving models (CRMs), which explicitly resolve cloud-scale processes, and single-column model (SCM) versions of numerical weather prediction (NWP)/global climate models (GCMs), which parametrize all cloud processes on scales smaller than that of a GCM grid cell (∼ 100 km). Previous model intercomparison studies of deep convective cloud systems using CRMs and SCMs were based upon convectively active periods of field experiments that took place over oceans (Bechtold et al, 2000;Redelsperger et al, 2000) and land (Ghan et al, 2000;Xie et al, 2002;Xu et al, 2002;Guichard et al, 2004;Grabowski et al, 2006). In order to understand the deficiencies in the representations of convective processes in GCMs and, in particular, their ability to simulate tropical variability, the present case study includes both suppressed and active periods of tropical deep convection, as well as the transitions from suppressed to active periods during the suppressed phase of the Madden-Julian oscillation (MJO), which has a period of 40-50 days (Madden and Julian, 1972).…”

Mechanisms of diurnal precipitation over the US Great Plains: a cloud resolving model perspective

“…Overall, the diurnal variation of the model precipitation from the 55-day long time series is relatively flat and less prominent compared with the observations. The above deficiencies in the simulated diurnal cycle of precipitation is consistent with what is found in many single-column GCM simulations, although the GCMs exhibit even stronger daytime precipitation (e.g., Guichard et al 2004). This deficiency is likely in part due to the fact that the model configuration does not explicitly account for the convective systems propagating into the domain-a feature that is also not present in single-column GCMs.…”

“…Those models are now being widely tested and embedded in several GCMs in place of parameterized convection and cloud processes (often referred to as superparameterization, Grabowski and Smolarkiewicz 1999;Randall et al 2003). CRMs are also used for diagnosing problems with the GCM parameterizations (e.g., Guichard et al 2004;Chaboureau et al 2004;Xie et al 2005). In this study, we investigate the important forcing mechanisms that drive the diurnal variation in convection, with a specific focus on examining the roles of the boundary heating and the large-scale dynamics in initiating deep convection.…”

Exploring metrics of extreme daily precipitation in a large ensemble of regional climate model simulations

“…Convection is not resolved in the current generation of regional climate models (RCMs), and models use parameterizations to represent convection. It is known that these parameterizations have shortcomings, for example in the representation of the diurnal cycle (Guichard et al 2004) or the sensitivity to soil conditions (Hohenegger et al 2009). …”