Abstract
AbstractAn 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. © Royal Meteorological Society, 2004. J. C. Petch's contribution is Crown copyright
Highlights
Modifications of the diurnal cycle of rainfall have even been reported (Dai 1999). These findings suggest that a proper simulation of the actual diurnal cycle of convection is required in order to capture its possible fluctuations in the context of climate sensitivities
The aim of this study is to analyse the response of cloud-resolving models (CRMs) and single-column models (SCMs) to the diurnal cycle of boundary-layer heating over land in summer for a simple, but not unrealistic, situation
This is the case over land in the tropics with the LMD general-circulation models (GCMs) (Grandpeix, personal communication). Such a predominant regime of convective organization can efficiently prevent the generation of convection by other mechanisms. Such a distinct behaviour found for the SCMs and CRMs allows us to address the following question: why does rainfall occur earlier in the SCMs than in the CRMs? We show below that this relatively well-shared behaviour of the SCMs occurs despite the fact that the SCM-simulated diurnal cycles of convection exhibit a variety of patterns
Summary
‘Convective organization’ refers to the various space and time scales of convective phenomena, and frequently to their degree of mesoscale organization. Several recent studies have shown that it is difficult for general-circulation models (GCMs) to capture the diurnal cycle of deep convection, in terms of both magnitude and phase, over the land as well as over the ocean (Dai et al 1999; Lin et al 2000; Royer et al 2000; Yang and Slingo 2001; Betts and Jakob 2002a). All these studies detected a time lag in the diurnal cycle of deep convection simulated by large-scale models, with convective rainfall occurring too early during daytime compared with observations.
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