My primary research interests are:
- Infrared radiation and interactions with the surface and clouds
- Cloud parameterizations, including cloud microphysics, cloud fraction, and cumulus parameterizations
- Climate modeling and diagnostics
Research projects:
I investigate how changes of surface longwave (LW) emissivity in the Sahara and Sahel influence simulated regional climate and beyond. Surface emissivity is a function of surface types and usually varies with wavelength. However, most general circulation models (GCMs) neglect this spectral dependency and assume surface emissivity as a constant over the all LW bands (usually close or equals to unity). This assumption can lead to biases in climate simulations over the Sahara and Sahel, where the surface LW emissivity can be as low as 0.6-0.7 over the atmospheric infrared window band due to the features of sand surface. In order to assess the extent to which the emissivity treatment in the Sahara and Sahel influences the simulated regional climate, my colleagues and I have implemented a realistic surface emissivity dataset into the NCAR CESM, carried out climate simulations, and analyzed the simulation results.
Compared to the simulation that treats surface as blackbody, the simulation with realistic surface emissivity treatment shows that surface air temperature increases over the Sahara and Sahel. This is mainly because the inclusion of realistic, non-blackbody surface emissivity decreases the surface emission and keeps more LW energy at the surface, which in turn increases the surface temperature. In addition to that, the planetary boundary layer over the Sahara and Sahel becomes warmer and wetter, favoring more convection and produce more convective rainfall, especially in the Sahara. The inclusion of surface emissivity in the Sahara and Sahel also leads to changes in moisture flux convergence in the adjacent regions, resulting in precipitation changes, in particular south of the Sahel.
Our study highlights the need to treat surface emissivity realistically in the GCMs, in order to represent the surface-atmosphere LW coupling processes in a more physical way.
For further details, please refer to our paper published in the Journal of Climate.
I compared two widely-used longwave and shortwave radiation schemes in climate models, namely CLIRAD (CLImate and RADiation) developed by the NASA Goddard Space Flight Center, and RRTMG (Rapid Radiative Transfer Model for GCMs) developed by Atmospheric & Environmental Research. This comparison focused on their gaseous absorption treatment, radiative transfer solver, cloud overlap assumption, and simulation results with different standard atmospheric profiles, such as mid-latitude summer and tropics. I also implemented the CLIRAD into the NCAR CESM climate model (the default is RRTMG) as a part of the project for Taiwan Earth System Model (TaiESM).
In order to better represent cloud microphysical processes and aerosol-cloud interactions in cumulus parameterization schemes, I incorporated a two-moment (mass and number), two hydrometeor species (cloud water and rain) warm-rain scheme into the Zhang-McFarlane deep cumulus parameterization. The unique feature of this new scheme was that raindrops can move upward or downward depending on their terminal velocity and the updraft velocity. This treatment was more physical than other current schemes which assumes raindrops either leave the updraft immediately or only move upward with the updraft. Sensitivity tests of this new scheme using the NCAR CAM5 single column model showed that the portion of convective precipitation decreased while the total precipitation remains unchanged, suggesting a way to address the overestimation of convective precipitation in CAM5 and other climate models.
For further details, please refer to my Master thesis.