Description

GFS saSAS Shallow Convection Module is an updated version of the previous mass-flux shallow convection scheme with scale and aerosol awareness and parameterizes the effect of shallow convection on the environment. GFS saSAS Shallow Convection Module is similar to GFS saSAS Deep Convection Module but with a few key differences. First, no quasi-equilibrium assumption is used for any grid size and the shallow cloud base mass flux is parameterized using a mean updraft velocity. Further, there are no convective downdrafts, the entrainment rate is greater than for deep convection, and the shallow convection is limited to not extend over the level where \(p=0.7p_{sfc}\). The paramerization of scale and aerosol awareness follows that of the SAMF deep convection scheme, although it can be interpreted as only having the "static" and "feedback" control portions, since the "dynamic" control is not necessary to find the cloud base mass flux.

The previous version of the shallow convection scheme (shalcnv.f) is described in Han and Pan (2011) [47] and differences between the shallow and deep convection schemes are presented in Han and Pan (2011) [47] and Han et al. (2017) [49] . Details of scale- and aerosol-aware parameterizations are described in Han et al. (2017) [49] .

In further update for FY19 GFS implementation, interaction with turbulent kinetic energy (TKE), which is a prognostic variable used in a scale-aware TKE-based moist EDMF vertical turbulent mixing scheme, is included. Entrainment rates in updrafts are proportional to sub-cloud mean TKE. TKE is transported by cumulus convection. TKE contribution from cumulus convection is deduced from cumulus mass flux. On the other hand, tracers such as ozone and aerosol are also transported by cumulus convection.

To reduce too much convective cooling at the cloud top, the convection schemes have been modified for the rain conversion rate, entrainment and detrainment rates, overshooting layers, and maximum allowable cloudbase mass flux (as of June 2018).

GFS saSAS Scheme Updates in GFSv17

The updates to the SAMF parameterization described above, between GFSv16 and GFSv17 are carefully outlined in Bengtsson and Han (2024) [7]. The main updates include:

Implementation of a positive definition mass-flux scheme and a method for removing the negative tracers (Han et al. 2022 [51])
Introduction of a new closure based on a prognostic evolution of the convective updraft area fraction in both shallow and deep convection (Bengtsson et al. 2022 [9])
Introduction of 3D effects of cold-pool dynamics and stochastic initiation using self-organizing cellular_automata (Bengtsson et al. 2021 [8])
Introduction of environmental wind shear and TKE dependence in convection, to seek improvements in hurricane forecast prediction (Han et al. 2024 [52])
Stricter convective initiation criteria to allow for more CAPE to build up to address a low CAPE bias in GFSv16 (Han et al. 2021 [50])
Reduction of convective rain evaporation rate to address a systematic cold bias near the surface in GFSv16 (Han et al. 2021 [50])

Intraphysics Communication

Argument Table

General Algorithm

GFS samfshalcnv General Algorithm