Currently, I am splitting 50% - 50% on two projects. For one part, I run large ensembles of CESM2 experiments to examine the role of atmospheric circulation vs. soil moisture on heatwave events. For the other, I study different facets of the precipitation characteristics.
* Welcome to chat more if you run models. I hope to learn about other people's experiences :)
Hydroclimatic changes over land
We have a generally good understanding of the first principles of how the global hydrological cycle changes with warming. However, the picture gets complicated and predictions become uncertain if we want to know about short-time events over land. Unlike the ocean, land has limited soil water and therefore it dries. Drying of the land surface exerts a nonlinear impact on the hydrological cycle and surface air temperature. Furthermore, land surface properties vary vastly from region to region. Out of these complexities, model representations of the land properties and the land-atmosphere coupling are also highly uncertain. All these factors confound our understanding of whether, when, and where we will have a drier or a more moist homeland. How does drying/moistening of the land surface feedback to surface air temperature? Can we find some consistent rules of change across different regions and across different models?
In this study, we reveal to you consistent patterns of changes in multiple key hydroclimatic variables over land (hard to get!), and show you the coherent mechanistic links between these variables.
Duan, S. Q., Findell, K. L., Fueglistaler, S. A. Coherent mechanistic patterns of land hydroclimatic changes, GRL, 50, e2022GL102285. https://doi.org/10.1029/2022GL102285 Editor's Highlight NOAA Climate Highlight
Also, welcome to check out a long paper on connecting the land surface-based and the atmospheric dynamics-based perspectives on amplified warming over tropical land
Duan, S. Q., Mckinnon, K. A., Simpson, I. R. Two perspectives on amplified warming over tropical land, JCLI, under review
Temperature distribution change
Duan, S. Q., Findell, K. L., Wright, J. S. Three regimes of temperature distribution change over dry land, moist land, and oceanic surfaces, GRL, 47, e2020GL090997. https://doi.org/10.1029/2020GL090997
Previous studies report that extreme temperature events will occur more often and become more extreme in the future, yet there is no consensus on how much this increased likelihood of extreme heat events is due to a shift of temperature distribution mean or a changed temperature distribution shape. We found that summertime local temperature distribution changes can be summarized into three regimes according to surface properties: dry land exhibits a shift of the entire distribution with pronounced warming in the mean; moist land shows a smaller change in the mean compared with dry land but features an elongated upper tail relative to the mean; oceanic surface shows a smaller shift in the mean relative to land surface, with no significant elongation of the upper tail. The elongated upper tail over moist land indicates an amplified warming of extreme hot days. This amplified extreme warming over moist land is compounded on top of the land-ocean contrast in mean warming, and is related to suppressed evaporation and associated land surface feedbacks.
Land warms more than the ocean;
Dry land warms more than moist land.
Moist land in the tropics/subtropics features amplified warming in the extreme relative to the mean, indicating an elongated upper tail.
On using stable water isotope (HDO) to constrain convection
Here in our study, we simulate HDO in a bulk plume model of cumulus convection, and test the sensitivity of HDO to three convective parameters: entrainment/detrainment rate, raindrop re-evaporation fraction, and the distance of the raindrop fall/get-lofted before its re-evaporation.
However, we find that at a given relative humidity, the possible range of HDO is small: its range is comparable to both the measurement uncertainty of tropical mean profile and the structural uncertainty of a single-column model. Therefore, we conclude that the mean tropical HDO profile is unlikely to add information about free tropospheric convective processes in a bulk-plume framework that cannot already be learned from relative humidity alone. Our message is, if we want to explore physics using water isotope, free tropospheric convection is not a good place to apply--apply it for tracing water sources for the topical tropopause layer and/or the boundary layer instead.
Duan, S. Q., Wright, J. S., & Romps, D. M. (2018). On the utility (or futility) of using stable water isotopes to constrain the bulk properties of tropical convection. Journal of Advances in Modeling Earth Systems, 10. https://doi.org/10.1002/2017MS001074
Stable water isotope has a property that heavy isotopes preferably stay in the condensed phase during phase change. This phenomenon is called fractionation. Due to the property of fractionation, water isotope can track water history, and can potentially serve as proxies for physics that we cannot directly measure.
In the recent decade, people are proposing that water isotopes are promising to add constraints on convective physics which are hard to measure and simulate. Satellite measurements of water isotopes and isotope-enabled numerical models are also more developed to lend people better tools to make further explorations.
Sketch of our bulk-plume model of the tropical atmosphere. q and q' denote the mass fractions of H2O and HDO.
𝞭D (HDO) profiles at a given RH value from our model solutions with three free parameters (entrainmet rate, reevaporation fraction and evaporation distance) ranging in their plausible ranges. The colors show the cumulative distribution function of 𝞭D for the subset solutions with that RH value.
Rapid intensification of tropical cyclones and potentially related environmental factors