Impact of Integrated Vertical Overlap of Cumulus and Stratus on the Global Precipitation and Radiation Processes in the Seoul National University Atmosphere Model Version 0 With a Unified Convection Scheme (SAM0‐UNICON)

Abstract

The previously proposed parameterization for the integrated vertical overlap of cumulus and stratus is implemented online into the cloud microphysics and radiation schemes of the Seoul National University Atmosphere Model version 0 with a Unified Convection Scheme (SAM0-UNICON). Instead of a single-merged cloud, the modified radiation scheme handles cumulus, stratus, and stratiform snow, separately, with each type having its own optical properties and vertical overlap structures. The integrated cloud overlap parameterization implemented into the cloud microphysics schemes do not reduce the biases of surface precipitation rate (PRECT) and cloud radiative forcing. Although it changes the overlap structures of clouds and precipitation areas, as well as the associated cloud microphysical processes either directly or indirectly, strong cancelation occurs among these terms, resulting in small changes to the global-mean PRECT and cloud radiative forcing. The integrated cloud overlap parameterization implemented into the radiation scheme has a substantial impact on the simulated climate: the global-mean cloud radiative forcing decreases substantially, mainly due to the separate treatment of radiative properties of individual cumulus, stratus, and stratiform snow, and PRECT exhibits strong regional responses. Sensitivity simulations showed that vertical cloud overlap exerts a weaker influence on the global-mean PRECT than the previous off-line simulations, implying that the indirect effect offsets the direct effect. In contrast to the off-line simulations, the enhanced randomness of cumulus overlap increases PRECT over the western Pacific warm pool region. Our study indicates that vertical cloud overlap has substantial impacts on global climate through complex interactions with other physical processes.

Impact of Interactive Vertical Overlap of Cumulus and Stratus on Global Aerosol, Precipitation, and Radiation Processes in the Seoul National University Atmosphere Model Version 0 With a Unified Convection Scheme (SAM0‐UNICON)

“…In contrast to negative (Figure d), global mean ΔPRECT is negative (Figure a), which is not negligible, even though it is much smaller than the regional changes in magnitude. This result is also different from the previous sensitivity test using noninteractive cumulus tilting (Figure 10 in Park et al, ), in which global mean PRECT increases as cumulus is more maximally overlapped vertically. Not only the global mean values but also the spatial patterns of ΔPRECT in Figure a differ substantially from that of Figure a of Park, Shin, et al (), and the magnitude of regional ΔPRECT with interactive tilting is smaller than that of noninteractive tilting.…”

Global Climate Simulated by the Seoul National University Atmosphere Model Version 0 with a Unified Convection Scheme (SAM0-UNICON)

“…As a contribution to phase 6 of the Coupled Model Intercomparison Project (CMIP6), the most recent CMIP project, we summarize various aspects of the global climate simulated by the Seoul National University (SNU) Atmosphere Model version 0 with a Unified Convection Scheme (SAM0-UNICON) driven by the CMIP6 forcing data. SAM0-UNICON, one of the international general circulation models (GCMs) participating in CMIP6, is based on the Community Atmosphere Model version 5 (CAM5; Neale et al 2010;Park et al 2014). SAM0-UNICON coupled with the identical land, ocean, and sea ice models of the Community Earth System Model version 1 (CESM1; Hurrell et al 2013) will be referred to as Seoul National University Earth System Model version 0 (SEM0).…”

“…More specifically, CAM5's shallow (Park and Bretherton 2009) and deep convection schemes (Zhang and McFarlane 1995;Neale et al 2008;Richter and Rasch 2008) are replaced by the Unified Convection Scheme (UNICON; Park 2014a,b), and the treatment of the convective detrainment process in the CAM5 cloud macrophysics scheme (Park et al 2014) is modified (Park et al 2017). UNICON is a processbased model of subgrid convective plumes and mesoscale organized flow that does not rely on any equilibrium assumptions, such as convective available potential energy (CAPE) closure used in CAM5's deep convection scheme or convective inhibition (CIN) closure used in CAM5's shallow convection scheme and simulates all dry-moist, forced-free, and shallow-deep convection within a single framework in a seamless, consistent, and unified way (Park 2014a). It has been shown that UNICON successfully simulates various transitional phenomena associated with convection (e.g., the diurnal cycle of precipitation and the Madden-Julian oscillation (MJO; Madden and Julian 1971) without degrading the mean climate (Park 2014b;Yoo et al 2015;Ahn et al 2019).…”

“…UNICON is a processbased model of subgrid convective plumes and mesoscale organized flow that does not rely on any equilibrium assumptions, such as convective available potential energy (CAPE) closure used in CAM5's deep convection scheme or convective inhibition (CIN) closure used in CAM5's shallow convection scheme and simulates all dry-moist, forced-free, and shallow-deep convection within a single framework in a seamless, consistent, and unified way (Park 2014a). It has been shown that UNICON successfully simulates various transitional phenomena associated with convection (e.g., the diurnal cycle of precipitation and the Madden-Julian oscillation (MJO; Madden and Julian 1971) without degrading the mean climate (Park 2014b;Yoo et al 2015;Ahn et al 2019). This improvement is mainly due to the explicit parameterization of complex interactions among convective updrafts, convective downdrafts, and mesoscale organized flow driven by subgrid cold pools, and associated simulation of convective plume memory across the time step, which cannot be simulated by CAM5's shallow and deep convection schemes (Park 2014a).…”

“…It has been shown that UNICON successfully simulates various transitional phenomena associated with convection (e.g., the diurnal cycle of precipitation and the Madden-Julian oscillation (MJO; Madden and Julian 1971) without degrading the mean climate (Park 2014b;Yoo et al 2015;Ahn et al 2019). This improvement is mainly due to the explicit parameterization of complex interactions among convective updrafts, convective downdrafts, and mesoscale organized flow driven by subgrid cold pools, and associated simulation of convective plume memory across the time step, which cannot be simulated by CAM5's shallow and deep convection schemes (Park 2014a). Following Tiedtke (1993) and Teixeira and Kim (2008), the revised cloud macrophysics scheme (Park et al 2017) diagnoses additional detrained cumulus that are not overlapped horizontally with cumulus and stratus clouds in each grid layer, by assuming a steady state balance between the detrainment rate of cumulus condensates and the dissipation rate of detrained condensates by entrainment mixing.…”