Land surface drying and its feedback to surface air temperature
Currently, I am working on understanding changes in the hydrological cycle and its couplings to temperature over land. Will we have a drier or a more moist homeland? Under what conditions will land get drier/wetter? When does drying of the land surface impact surface air temperature? Can we find consistency across different regions?
Duan, S. Q., Findell, K. L., Fueglistaler, S. A. Coupling of changes in temperature extremes and the hydrological cycle over land, PNAS, 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 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