Tropical Cirrus Are Highly Sensitive to Ice Microphysics Within a Nudged Global Storm‐Resolving Model

• Published in: Geophysical research letters Vol. 51; no. 1
• Main Authors: Atlas, R. L., Bretherton, C. S., Sokol, A. B., Blossey, P. N., Khairoutdinov, M. F.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.01.2024; Wiley
• Subjects: Aircraft; Aircraft observations; anvil; Atmospheric models; Bias; Climate and weather; Climate models; Cloud physics; Earth; global cloud‐resolving model; global storm‐resolving model; I.R. radiation; Ice clouds; Ice crystals; ice microphysics; Infrared radiation; longwave cloud radiative effect; Microphysics; Modelling; Moisture content; Physics; Representations; Satellites; Simulation; Storms; Thunderstorms; tropical cirrus; Tropical environments; Troposphere; Variability; Water content
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL105868

Cirrus dominate the longwave radiative budget of the tropics. For the first time, the variability in cirrus properties and longwave cloud radiative effects (CREs) that arises from using different microphysical schemes within nudged global storm‐resolving simulations from a single model, is quantified. Nudging allows us to compute radiative biases precisely using coincident satellite measurements and to fix the large‐scale dynamics across our set of simulations to isolate the influence of microphysics. We run 5‐day simulations with four commonly‐used microphysics schemes of varying complexity (SAM1MOM, Thompson, M2005 and P3) and find that the tropical average longwave CRE varies over 20 W m−2 between schemes. P3 best reproduces observed longwave CRE. M2005 and P3 simulate cirrus with realistic frozen water path but unrealistically high ice crystal number concentrations which commonly hit limiters and lack the variability and dependence on frozen water content seen in aircraft observations. Thompson and SAM1MOM have too little cirrus. Plain Language Summary Recently, advancements in computing have made it possible for atmospheric scientists to simulate Earth's global atmosphere with higher resolution than ever before. This new generation of models, called global‐storm resolving models, have a horizontal grid spacing of just a few kilometers, which permits the formation of thunderstorms. As a result, they simulate clouds more realistically than traditionally climate and weather models and are a great tool for diagnosing cloud biases in atmospheric models. Here, we run a single global storm‐resolving model with four different representations of cloud physics called M2005, P3, SAM1MOM and Thompson. We evaluate simulated tropical cirrus, which are stratiform ice clouds at the top of the troposphere that reduce the amount of infrared radiation emitted by the Earth, with satellite and aircraft data to see which representations have the best performance. SAM1MOM and Thompson make too little cirrus causing too much infrared radiation to be emitted, M2005 makes too much cirrus, causing too little infrared radiation to be emitted, and P3 makes about the right amount. Key Points Nudged global storm‐resolving simulations are valuable for microphysics sensitivity studies Mean tropical longwave cloud radiative effect varies over 20 W m−2 depending on microphysics scheme Two‐moment schemes outperform simpler one‐moment and partial double‐moment schemes, and P3 has the smallest longwave radiative bias