A Bin and a Bulk Microphysics Scheme Can Be More Alike Than Two Bin Schemes

Abstract

Modeling clouds is hard. As noted in great detail in Morrison, van Lier-Walqui, Fridlind, et al. (2020), the two biggest obstacles are (a) the sheer number of cloud and precipitation particles to predict and (b) the lack of the fundamental understanding of microphysical processes involved. With quintillions of droplets in a typical cloud, it is infeasible to predict the evolution of each droplet and its environment. In addition, there does not exist a fundamental governing equation or benchmark model that can help inform the modeling of microphysics in larger scale, unlike other subgrid physical processes such as radiation, turbulence, and convection. Clouds are thus only modeled from a macroscopic perspective with its parameterizations determined empirically. Aside from the more recently developed Lagrangian superdroplet method (e.g., Shima et al., 2009), the two most well-established microphysics scheme types are the bulk microphysics scheme and the bin microphysics scheme. Bin schemes keep track of the number of droplets of within discrete size or mass ranges (i.e., size-resolved or mass-resolved), allowing the particle size distribution (PSD) to evolve freely but is therefore computationally expensive (A. P. Khain et al., 2015). On the other hand, bulk schemes predict quantities proportional to moments of the PSD (i.e., moment-resolved), typically mass and number concentration (e.g.,

Bulk Microphysics Schemes May Perform Better With a Unified Cloud‐Rain Category

“…However, the bin schemes should not be treated as “truth” due to numerical broadening (Grabowski et al., 2019; Hill et al., 2023; Morrison et al., 2018) and uncertainty in any scheme about the fundamental physical processes. Such issues result in large differences among bin schemes (Hill et al., 2023; Hu & Igel, 2023a; VanZanten et al., 2011; Xue et al., 2017). We specifically use two reference bin schemes because we recognize that neither is perfect.…”

“…The continued struggle of these traditional schemes to accurately simulate warm-phase precipitation (Jing et al, 2017;Kay et al, 2018) has led to a few studies and discussions on the feasibility of a bulk scheme with atypical liquid water treatment (Kogan & Belochitski, 2012;Liu et al, 2023;Szyrmer et al, 2005;van Lier-Walqui et al, 2020). More recently, the idea of a unified liquid water category bulk scheme has been revisited in Igel et al (2022;hereafter I22) and (Hu & Igel, 2023a;hereafter HI23). I22 found that a unified category (1category, 4-moment) bulk scheme was noticeably superior compared to a separate category (2-category, 2moment) bulk scheme when representing collision-coalescence, even though the number of total predicted moments and assumed gamma distributions are the same.…”

“…More recently, the idea of a unified liquid water category bulk scheme has been revisited in Igel et al. (2022; hereafter I22) and (Hu & Igel, 2023a; hereafter HI23). I22 found that a unified category (1‐category, 4‐moment) bulk scheme was noticeably superior compared to a separate category (2‐category, 2‐moment) bulk scheme when representing collision‐coalescence, even though the number of total predicted moments and assumed gamma distributions are the same.…”