Our research group concentrates on the interactions between land and atmospheric processes. Our special strength is in the linkages between process understanding, their observational characterizations, and examinations of their contribution to the Earth System through their incorporation in complex models, in particular the latest version of the Community Atmosphere Model (CAM) and Community Land Model (CLM).
Our current and recent research emphasizes four topics: a) global surface land energy and water balance; b) role of and improvements in modeled stomatal functioning in controlling water and carbon fluxes and their climate impacts; c) dynamics of tropical land covers; d) dynamics of climatological boundary layers in controlling occurrences of precipitation. The following sections provide more details.
Global Surface Land Energy and Water Balance
Much of the incident solar radiation is absorbed at the surface, which also receives a large amount of energy from atmospheric downward long wave radiation. Variability of these fluxes due to clouds and aerosols and how the land surface responds to them is a major element of climate and its modeling.
Our recent research on modeling land surface response has emphasized the 3D structure of vegetation canopies. In particular, we have worked with scientists at Beijing Normal University (BNU) (Yuan et al., 2013) to develop a 3D 3-layer canopy model for the absorption and reflection of incident solar radiation, using as a building block earlier studies of the optical properties of the spherical bush (e.g. Dickinson et al., 2008).
We have also worked extensively with Kaicun Wang at BNU to develop observational constraints for global land-surface processes. We have in particular shown how standard meteorological variability measurements can be used to characterize the global inter-annual variability of land aerosol optical depths (Wang and Dickinson et al. 2009). We have developed a method to use standard meteorological data, available for many decades, to determine inter-annual and decadal variations of evapotranspiration (ET) ( Wang and Dickinson et al., 2010a and 2010b). We have also developed a method to use standard meteorological data, available for many decades, to determine inter-annual and decadal variations of evapotranspiration (ET), We have reviewed all the current approaches to obtaining ET (Wang and Dickinson, 2012) and provided a critical assessment of measurements of downward long wave radiation ( Wang and Dickinson, 2013). We have critically assessed current measurements of incident solar radiation and effects of aerosols and clouds on its variability. We have, in particular, assessed the dependence of day-night temperature differences (DTR) on variability of solar radiation. Although they are well correlated, these are controlled by too many other factors to be useful for a quantitative determination of incident solar. However, DTR gives a consistent estimate of the impact of aerosols on past decadal variability. In particular, DTR indicates that solar dimming partially balanced the warming of greenhouse gases from the 1950s to the 1980s likely a result of increase of global aerosols, but contributed little after that (Wang and Dickinson, 2013).
Modeling Stomatal Functioning
Little holes in leaves i.e. their “stomates” are a major control on ET, the transfer of water from its soil storage to the atmosphere. How much resistance they impose on this water movement depends on the plants requirements for carbon assimilation, how much CO2 is in the air, and if present, on drought stress. ET cools the land, while the accompanying assimilation of CO2 reduces the amounts stored in the atmosphere and causing global warming.
Several past studies have indicated that stomatal closure from increased CO2 contributes to global warming. However, because of a recent report to the contrary with extensive press coverage, we re-examined this issue (Pu and Dickinson et al. 2012 & 2013).
Our modeling studies not only confirmed and quantified the stomatal contribution to global warming, they discovered a significant new effect. That is, we found that the warming effect of stomatal closure over tropical continents leads to a monsoon-like response with a large increase of precipitation. Calculation of stomatal resistance to ET in response to carbon assimilation requires numerical iterations. Ying Sun found this procedure in CLM to have deficiencies in sometimes failing to converge and in requiring an ET biasing assumption. They provided an improved Newton-Raphson iteration (Sun et al., 2012).
For assimilating carbon, CO2 must not only pass through the stomates but also through various paths internal to the leaf, whose collective effect is referred as “mesophyll” resistance. Although of the same magnitude as leaf stomatal resistance, the mesophyll resistance has not previously been included in modeling of carbon assimilation. Sun et al. (2013) has developed a parameterization for this term and we are currently studying its impact. For current CO2 levels, earlier parameterizations ignoring their term have compensated in parameters fitting. However, such fitting does not recognize the changes of stomatal functioning with changed levels of atmospheric CO2. Thus, we are currently studying the impacts of this new parameterization on past and future levels of CO2 concentrations.
Roots provide soil water to plants. Doing so requires a hydraulic suction from the roots to the soil. This is supplied by leaves developing negative water potential and applying this potential through the plant, e.g., through the xylem for trees, to the roots. Leaf water potential is another control on leaves. Climate models currently impose this control in an adhoc fashion by making stomatal resistance depend on soil water potential. Our student Binyan Yan is developing dynamic model of roots to provide their control on stomatal resistance.
Dynamics of Tropical Vegetation
On treatments of mesophyll resistance, dynamics roots and 3D canopy radiation provide a basis for more realistic treatment of global dynamics of vegetation as needed for projecting future land storage of carbon. In focusing on tropical forests and savannas, we are beginning to address several further issues. The dynamic treatment of tropical trees needs to include a distribution of tree heights with lower canopy levels shaded by higher canopies. It also needs to recognize multiple growth strategies, differing at the forest edge and in savannas than deep in closed canopy forests. Finally, the current fire parameterization needs to be improved to be applicable to tropical conditions. In particular, grass should be recognized a much more flammable than trees. An improved model should be able to represent the transitions between forest and savanna as part of a climate model, and establish to what degree such transitions represent an Earth system “tipping point”.
Boundary layer Controls on Precipitation
We are using a time-dependent slab model of the atmospheric boundary layers to explore the details of various mechanisms conditioning the occurrence in absence of precipitation over the US Great Plains and similar regions elsewhere in the world. In our initial study with this model (Pu and Dickinson, 2013), boundary layer gradients of pressure are obtained from observations, and various patterns these gradients take (e.g., diurnal, seasonal, and inter-annual variations) are related to patterns of vertical velocity, in turn related to patterns of precipitation. Further study will determine the mechanisms producing the observed pressure patterns, which can be land process generated thermal patterns, or pressure gradient variability imposed by conditions over the Gulf, or upper level barotropic or potential vorticity anomalies.
References to Unpublished Papers
Pu, B., and R.E. Dickinson, 2013: Diurnal spatial variability of Great Plains summer precipitation related to the dynamics of the low-level jet (submitted).
Yuan, H., R. E. Dickinson, Y. Dai, M. J. Shaikh, L. Zhou, W. F. Shangguan, and D. Ji (2013), A 3-D canopy radiative transfer model for global climate modeling: description, validation and application, J Climate, (Accepted).