Implementation and Evaluation of a Double-Plume Convective Parameterization in NCAR CAM5

Abstract

Performance of global climate models (GCMs) is strongly affected by their cumulus parameterizations (CP) used. Similar to the approach in GFDL AM4, a double-plume CP, which unifies the deep and shallow convection in one framework, is implemented and tested in NCAR Community Atmospheric Model version 5 (CAM5). Based on the University of Washington (UW) shallow convection scheme, an additional plume was added to represent the deep convection. The shallow and deep plumes share the same cloud model, but use different triggers, fractional mixing rates and closures. The scheme was tested in single column, short-term hindcast and AMIP simulations. Compared with the default combination of Zhang-McFarlane scheme and UW scheme in CAM5, the new scheme tends to produce a top-heavy mass flux profile during the active monsoon period in the single column simulations. The scheme increases the intensity of tropical precipitation, closer to TRMM observations. The new scheme increased subtropical marine boundary layer clouds and high clouds over the deep tropics, both in better agreement with observations. Sensitivity tests indicate that regime dependent fractional entrainment rates of the deep plume are desired to improve tropical precipitation distribution and upper troposphere temperature. This study suggests that a double-plume approach is a promising way to combine shallow and deep convections in a unified framework.

Description and Evaluation of a New Deep Convective Cloud Model Considering In‐Cloud Inhomogeneity

“…Another is to investigate whether the dry bias and early peak of summer rainfall over the US Great Plains can be alleviated using the new approach. In addition, it would be interesting to add it as a deep plume into the recently developed double‐plume schemes (Chu et al., 2021; M. Zhao et al., 2018). Finally, various detailed optimization can be explored.…”

Improved Climate Simulation by Using a Double‐Plume Convection Scheme in a Global Model

“…In the DP scheme, $${\varepsilon }_{0}$$ is replaced by a constant value of 3 km −1 for the shallow plume (Zhao et al., 2018). For the deep plume of DP, a mixing rate dependent on the relative humidity (RH) is applied to inhibit deep convection in a hostile environment (Chu et al., 2021). For the detrainment, UW considers the turbulent mixing ($${\delta }_{m}$$) at the lateral boundary (similar to the entrainment) and the constrained detrainment ($${\delta }_{c}$$) occurring at the top of a plume.…”

“…In particular, it exhibits some problematic tropical variabilities. In this study, we demonstrate that by replacing the original Zhang-Mcfarlane (ZM) deep (Neale et al, 2008;Zhang & McFarlane, 1995;hereafter, ZM) and University of Washington (UW) shallow ) convection scheme of CAM5 with a double-plume (DP) scheme (Chu et al, 2021;Zhao et al, 2018), the model performance has been improved in various aspects, especially with regard to its representation of precipitation and tropical variability. Therefore, a detailed comparison of these two schemes within GRIST was carried out to study the model sensitivity related to convective parameterizations.…”

“…The deep and shallow plumes share the same buoyancy-sorting method as UW for computing entrainment and detrainment, except for a smaller lateral mixing rate for the deep plume to allow it to penetrate to higher altitudes. This version of the DP scheme was recently modified and introduced into CAM5 (Chu et al, 2021), in which the deep and shallow plumes are parallel-coupled following Zhao et al (2018). The deep plume of DP has a very different cloud model and trigger assumption from the ZM scheme.…”

“…Nevertheless, the frequency of intense precipitation was still underestimated (cf. Chu et al, 2021).…”