I research the mid-latitude atmospheric waves, whose variabilities drive high-impact extreme events such as heat waves, cold spells, and heavy precipitation episodes. See a short article that I contributed to AGU in 2014.
Broadly speaking, my approaches are three-fold: observations – mechanistic idealized models – state-of-the-art GCMs. I use a hierarchy of climate models, aided with a two-layer quasi-geostrophic model (the E. Coli of climate models, Held 2005), to investigate key physical mechanisms.
As the main focus of my postdoc research at Harvard, I study the dynamics of atmospheric blocking, and have provided an answer to a frequently asked question:
- what is atmospheric blocking?
- what is the causal relation between weather systems and blocking (as illustrated in the below figure)?
- what determines the frequency of blocks?
The below toolsets enabled me to make progress:
In my Ph.D. at the University of Chicago, I study the theoretical aspects of eddy-mean flow interactions and proposed a novel framework for characterizing the storm tracks. A flow field can be decomposed as mean flow and eddy:
A major question for climate studies is to quantify the nonlinear atmospheric eddies in a rigorous framework . These transient perturbations transport momentum and heat, maintain the observed atmospheric basic state, and drive the variations in jet stream. However, traditional linear theory faces great challenge on quantifying these transient perturbations.
To understand the mechanisms, I adopt a novel theoretical wave activity framework to diagnose features in reanalysis products and satellite observations. Using theory of finite-amplitude wave activity (FAWA), I have developed a new zonal momentum-wave activity cycle, as a counterpart of global energy cycle (Lorenz 1955), but with several advantages for climate diagnostics.
In contrast to most geophysical processes that are characterized by red-noise spectra, I have found that the finite-amplitude wave activity (FAWA) has a marked periodicity around 20-30 days in the Southern Hemisphere summer, consistent with the recently discovered Baroclinic Annular Mode by Dr. David Thompson and collaborators. The robust periodicity in FAWA and EKE promise a novel climate driver of extremes, and valuable for improving climate models on intra-seasonal timescales.
Specifically, my dissertation concentrate on mid-latitude, quasi-geostrophic dynamics of the atmospheric flows. Here is a schematic diagram of the atmospheric zonal momentum-wave activity cycle (Wang and Nakamura 2015, 2016):
To fully understand these contributions, I systematically investigate the wave activity in the context of geostrophic turbulence:
- The influence from the mean flow on the wave activity’s variability (Nakamura and Wang 2013; Wang and Nakamura 2015, 2016).
- The synoptic wave activity’s long-term influence on the mean flow via the diffusive flux of potential vorticity. (Wang and Lee 2016)
- The strength of inverse cascade on the eddy’s phase speeds, controlled by external parameters such as the frictional strength term. (Wang et al 2016)