Abstract
Abstract. Convective processes profoundly affect the global water and energy balance of our planet but remain a challenge for global climate modeling. Here we develop and investigate the suitability of a unified convection scheme, capable of handling both shallow and deep convection, to simulate cases of tropical oceanic convection, mid-latitude continental convection, and maritime shallow convection. To that aim, we employ large-eddy simulations (LES) as a benchmark to test and refine a unified convection scheme implemented in the Single-column Community Atmosphere Model (SCAM). Our approach is motivated by previous cloud-resolving modeling studies, which have documented the gradual transition between shallow and deep convection and its possible importance for the simulated precipitation diurnal cycle. Analysis of the LES reveals that differences between shallow and deep convection, regarding cloud-base properties as well as entrainment/detrainment rates, can be related to the evaporation of precipitation. Parameterizing such effects and accordingly modifying the University of Washington shallow convection scheme, it is found that the new unified scheme can represent both shallow and deep convection as well as tropical and mid-latitude continental convection. Compared to the default SCAM version, the new scheme especially improves relative humidity, cloud cover and mass flux profiles. The new unified scheme also removes the well-known too early onset and peak of convective precipitation over mid-latitude continental areas.
Highlights
Accurate representation of deep convection with global climate models of coarse resolution remains a nagging problem for the simulation of present-day and future climates
The three cases are simulated with SAM and with different versions of Single-column Community Atmosphere Model (SCAM), using prescribed time-dependent profiles of large-scale vertical motion and horizontal advective heating and moistening as well as surface fluxes and sea surface temperature
The first experiment employs the default version of the CAM3.5 model, in which planetary boundary layer (PBL) processes are parameterized after Holtslag and Boville (1993), shallow convection after Hack (1994) and deep convection after Zhang and McFarlane (1995)
Summary
Accurate representation of deep convection with global climate models of coarse resolution remains a nagging problem for the simulation of present-day and future climates. Cloud-resolving modeling studies have documented the gradual transition occurring from shallow to deep convection and highlighted its importance for the simulated convective diurnal cycle (e.g., Guichard et al, 2004) This may be best achieved with a unified scheme. Evaporation of precipitation (hereafter called rain evaporation) modifies the atmospheric environment and especially the structure of the planetary boundary layer (PBL), which feeds back on the convective development Including such effects in a shallow convection scheme should allow the representation of deep convection within the same scheme. Our approach bears similarities with the one of Mapes and Neale (2011) as it uses rain evaporation and its effects on cloud-base properties and mixing rates to control the transition from shallow to deep convection.
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