Chen, Yi-Hsuan陳毅軒
Assistant Research Fellow
Research Interests
My research focuses on understanding cloud and radiation processes and their roles in the Earth’s climate system, particularly on parameterizing these processes in climate models. I have extensive experience with longwave radiation, planet boundary layer, and convection schemes in climate models, including modifying existing schemes, implementing new schemes, and evaluating simulation results.
Currently, I am investigating simulated marine stratocumulus characteristics in TaiESM1 by leveraging field observation data, reanalysis, and a hierarchy of TaiESM1 simulations including single-column model, nudge experiments, AMIP and fully coupled simulations.
Representative Publications
Highlights
--- Characteristics of simulated marine stratocumulus in TaiESM1
TaiESM1 can reproduce realistic shortwave cloud radiative effects (SWCRE) over the marine stratocumulus regions, yet the underlying mechanisms remain unclear. Using observation data from the DYCOMS-II field campaign in the northeastern Pacific, along with TaiESM1 hindcast simulation for the same period, we find that TaiESM1 reproduces the observed SWCRE but underestimates the liquid water path and mixed layer height. To better understand TaiESM1’s behaviors, we conduct a detailed tendency analysis for cloud liquid. Our analysis reveals that turbulence and macrophysics are the dominant processes, though they act in opposite ways. In the cloud layer, macrophysics generates cloud liquid while turbulence acts to remove it. In the subcloud layer, the cloud liquid diffused by turbulence is subsequently removed by macrophysics. This highlights the value of combining field campaign data, hindcast simulations, and tendency analysis to gain insights into model behavior and characteristics.
--- Influence of ice cloud longwave scattering on the polar climate
Most climate models neglect cloud longwave (LW) scattering because scattering is considered negligible compared to strong LW absorption by clouds and greenhouse gases. While this rationale is valid for simulating extrapolar regions, it is questionable for the polar regions, where the atmosphere is dry and hence has weak absorption, and ice clouds that have strong scattering capability frequently occur. Using the slab-ocean Community Earth System Model, we show that ice cloud LW scattering can warm winter surface air temperature by 0.8–1.8 K in the Arctic and 1.3–1.9 K in the Antarctic, while this warming becomes much weaker in polar summer. Such scattering effect cannot be correctly assessed when sea surface temperature and sea ice are prescribed as this effect is manifested through a surface-atmosphere coupling. For further details, please check out our 2020 GRL paper (https://doi.org/10.1029/2020GL090534).
--- Exploring two coupling strategies of the boundary layer and convection schemes
Planet boundary layer (PBL) and moist convection closely couple with each other. Here we explore two coupling strategies of PBL and convection schemes in GFDL AM4, namely, (1) PBL_then_Conv, in which the convection scheme sees the state updated by the PBL scheme, and (2) PBL_and_Conv, in which both PBL and convection schemes see the same state. The AMIP results show that these coupling strategies have the strongest impact on marine shallow cumulus regime. PBL_and_Conv has weaker convection, stronger PBL activities, and more low cloud than those in the PBL_then_Conv. We hypothesize that these are because the convection scheme in PBL_and_Conv “sees” a less unstable state, leading to weaker convection.
--- Implementation and evaluation of the MYNN-EDMF scheme in GFDL AM4
GFDL AM4 underestimates marine stratocumulus amount on the west coasts of North and South America and of South Africa, leading to excessive shortwave absorption in these regions. To address this issue, we implement the Mellor-Yamada-Nakanishi-Niino Eddy-Diffusivity/Mass-Flux (MYNN-EDMF) scheme into the AM4. The major implementation challenges include (1) incompatibility of the MYNN-EDMF cloud scheme and AM4 cloud scheme, and (2) coupling the MYNN-EDMF with other schemes. The performance of the MYNN-EDMF in AM4 is evaluated using AMIP simulation. AM4 with MYNN ED shows moderate improvements in marine stratocumulus biases. However, AM4 with MYNN-EDMF worsens the already large marine stratocumulus biases, partly due to coupling with the AM4 stratiform cloud scheme.
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