The Impact of Hybrid Usage of a Cumulus Parameterization Scheme on Tropical Convection and Large‐Scale Circulations in a Global Cloud‐System Resolving Model

Abstract

The impact of activating a cumulus parameterization scheme in the global nonhydrostatic icosahedral atmospheric model (NICAM) coupled with a one‐dimensional (1‐D) mixed‐layer ocean model is assessed using a cloud‐system‐resolving, 14‐km mesh. The Chikira‐Sugiyama (CS) scheme, which employs an entrainment rate sensitive to the humidity of the environment, can consider congestus clouds in the tropics when used in conventional global climate models. Congestus clouds are underresolved in the default 14‐km mesh NICAM. In the present study, boreal summer NICAM simulations are performed with and without the CS scheme, and several different scheme parameters are evaluated. The results show that the horizontal scale of convection and precipitable water increased in the tropics when using the CS scheme. Model adjustments were apparent at two different timescales: a rapid adjustment within the first week and a slower adjustment at 1 to 2 months. Both effects were magnified in the simulations that employed smaller values for the parameter that defines the fractional of loss of buoyancy‐generated energy in parameterized convection. The upward branch of the Hadley circulation shifted northward, and the Walker circulation was enhanced when the CS scheme was activated. These large‐scale adjustments suggested that increased midtropospheric moisture in the tropics tends to favor larger organized convective activities, which require an abundant supply of moisture, which, in this case, is available to the north of the equatorial West Pacific Ocean.

The Nonhydrostatic ICosahedral Atmospheric Model for CMIP6 HighResMIP simulations (NICAM16-S): experimental design, model description, and impacts of model updates

“…The land surface model, Minimal Advanced Treatments of Surface Interaction and RunOff (MATSIRO) (Takata et al, 2003) solves land states such as soil temperature, soil moisture, and land surface fluxes. Ocean surface fluxes are calculated following Louis (1979) with a modified roughness length for strong surface wind conditions (Fairall et al, 2003;Moon et al, 2007). The conventional orographic gravity wave drag scheme (McFarlane, 1987) is used to introduce the effect of vertically propagating subgrid-scale orographic gravity waves on the momentum tendency of the atmosphere.…”

Data Assimilation for Climate Research: Model Parameter Estimation of Large‐Scale Condensation Scheme

“…We generated the initial parameter ensemble by independent random numbers from a Normal distribution whose mean B 1,default and SD B 1,spread are 0.10 and 0.05, respectively (cf. Miyakawa et al (2018) showed that the parameters in a cumulus parameterization scheme were sensitive to precipitation bias in NICAM. The experimental results are verified with observed LWP, OLR, and OSR.…”

“…Estimating B 1 changed the temperature mainly for the lower and middle troposphere (1,000-600 hPa). Cumulus parameterization plays important role in GCMs to regulate clouds and OLD in TR (Miyakawa et al, 2018). As increased B 1 accelerated autoconversion, vapor and clouds remained for less time in the atmosphere.…”

“…For further improvement, it would be necessary to estimate other relevant parameters such as the ones in cumulus parameterization schemes. Miyakawa et al (2018) showed that the parameters in a cumulus parameterization scheme were sensitive to precipitation bias in NICAM.…”

“…To solve the mixed impact, we may need to explore estimating uncertain parameters beyond the LSC scheme. Cumulus parameterization plays important role in GCMs to regulate clouds and OLD in TR (Miyakawa et al, 2018). In TR, estimating parameter(s) of the cumulus parameterization may be necessary to mitigate temperature bias without detrimental impacts.…”