Abstract
Abstract. Cloud macrophysics schemes are unique parameterizations for general circulation models. We propose an approach based on a probability density function (PDF) that utilizes cloud condensates and saturation ratios to replace the assumption of critical relative humidity (RH). We test this approach, called the Global Forecast System (GFS) – Taiwan Earth System Model (TaiESM) – Sundqvist (GTS) scheme, using the macrophysics scheme within the Community Atmosphere Model version 5.3 (CAM5.3) framework. Via single-column model results, the new approach simulates the cloud fraction (CF)–RH distributions closer to those of the observations when compared to those of the default CAM5.3 scheme. We also validate the impact of the GTS scheme on global climate simulations with satellite observations. The simulated CF is comparable to CloudSat/Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data. Comparisons of the vertical distributions of CF and cloud water content (CWC), as functions of large-scale dynamic and thermodynamic parameters, with the CloudSat/CALIPSO data suggest that the GTS scheme can closely simulate observations. This is particularly noticeable for thermodynamic parameters, such as RH, upper-tropospheric temperature, and total precipitable water, implying that our scheme can simulate variation in CF associated with RH more reliably than the default scheme. Changes in CF and CWC would affect climatic fields and large-scale circulation via cloud–radiation interaction. Both climatological means and annual cycles of many of the GTS-simulated variables are improved compared with the default scheme, particularly with respect to water vapor and RH fields. Different PDF shapes in the GTS scheme also significantly affect global simulations.
Highlights
We presented a macrophysics parameterization based on a probability density function (PDF) called the Global Forecast System (GFS)–Taiwan Earth System Model (TaiESM)–Sundqvist (GTS) cloud macrophysics scheme, which is based on Sundqvist’s cloud macrophysics concept for global models and the recent modification of the cloud macrophysics in the NCAR CESM model by Park et al (2014)
The GTS scheme especially excludes the assumption of a prescribed critical relative humidity threshold (RHc), which is included in the default cloud macrophysics schemes, by determining the width of the PDF based on grid hydrometeors and saturation ratio
In a single-column model setup, we noticed, according to the pair-wise comparisons shown and discussed in Figs. 3 and 4, the use of PDFbased treatments for parameterizing both liquid and ice cloud fraction (CF) in the GTS schemes contributed to the CF–RH distributions
Summary
Global weather and climate models commonly use cloud macrophysics parameterization to calculate the subgrid cloud fraction (CF) and/or large-scale cloud condensate, as well as cloud overlap, which is required in cloud microphysics and radiation schemes (Slingo, 1987; Sundqvist, 1988; Sundqvist et al, 1989; Smith, 1990; Tiedtke, 1993; Xu and Randall, 1996; Rasch and Kristjansson, 1998; Jakob and Klein, 2000; Tompkins, 2002; Zhang et al, 2003; Wilson et al, 2008a, b; Chabourea and Bechtold, 2002; Park et al, 2014, 2016). We re-derived this equation by describing the change in the distribution width δ with grid-mean cloud condensates and saturation ratio using the basic assumption of uniform distribution from Sundqvist et al (1989) rather than using the RHc-derived δc, thereby eliminating unnecessary use of the RHc while retaining the PDF assumption for the entire scheme This modified macrophysics scheme is named the GFS–TaiESM–Sundqvist (GTS) scheme version 1.0 (GTS v1.0). Park et al (2014) discussed a similar approach wherein a triangular PDF was used to diagnose cloud liquid water as well as the liquid cloud fraction, and suggested that the PDF width could be computed internally rather than specified, to consistently diagnose both CF and cloud liquid water as in macrophysics These authors mentioned that such stratus cloud macrophysics could be applied across any horizontal and vertical resolution of a GCM grid, they did not formally implement and test this idea using their scheme. We conduct a series of sensitivity tests related to the supersaturation ratio assumption, which is applied when calculating the ice cloud fraction in the GTS scheme
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