Quantifying the role of individual surface properties in atmospheric feedbacks and land-atmosphere interactions
Lague, Marysa Monique
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The land plays a critical role in the coupled Earth System. While it is intuitive to think of the impact of climate on the vegetated land surface, it is also true that changes in land surface properties can modify climate, on both local and global scales. Land surface properties such as albedo, evaporative resistance, and aerodynamic roughness modulate fluxes of energy to the atmosphere. Albedo controls how much incoming shortwave radiation is absorbed by the land surface, and thus how much energy must be either stored by the land surface, or returned to the atmosphere in the form of longwave radiation, sensible heat, or latent heat flux (evaporation). Evaporative resistance modifies the partitioning of turbulent energy fluxes between sensible and latent heat, thus modifying the amount of moisture fluxed from the land to the atmosphere. Aerodynamic resistance effects the efficiency of turbulent mixing with the atmosphere. While the general role of each of these surface properties in the surface energy budget is understood, it is not known which of these surface properties has the largest impact on the climate experienced by the land surface, or where each of these surface properties plays the largest role in influencing surface climate. Moreover, changes in any one of these surface properties can modify the climate experienced by the land surface both directly - that is, simply by changing the magnitude of individual surface energy fluxes - and indirectly, by driving atmospheric feedbacks. Atmospheric feedbacks are responses of the atmosphere to initial changes in surface fluxes, which can then feedback on the surface energy budget, both locally and remotely - that is, a change in the land surface in one location can modify the surface energy budget in remote regions, via ecoclimate teleconnections. In this dissertation, I separate and quantify the role of each of three individual surface properties associated with vegetation change - albedo, evaporative resistance, and aerodynamic resistance - using an idealized land surface model (the Simple Land Interface Model, SLIM) coupled to a complex Earth System Model. Additionally, I separate and quantify the magnitude of change in surface climate coming directly from the land surface, and the magnitude of change coming from atmospheric responses to those initial changes in the land surface. Albedo: I show that albedo has the largest direct impact on land surface temperatures and energy fluxes in regions that are sunny and dry, such as the sub-tropics. Albedo plays a less important direct role in high latitudes because there is less insolation (thus, the same change in albedo leads to a smaller change in absorbed energy than it would at a lower latitude). Albedo leads to increased energy absorption in the tropics, but does not directly lead to a large amount of warming, as the moist tropics can shed excess absorbed energy through evaporation (latent heat flux), rather than surface warming. Decreasing land albedo leads to more total energy absorbed by the land system, and thus released to the bottom of the atmosphere. As such, darkening the land surface leads to a net divergence of energy transport by the atmosphere away from the continents towards the ocean. In some regions, such as off the west coast of South America, this energy convergence over the oceans leads to increased low cloud cover. Historical changes in albedo resulting from vegetation change lead to both warming and cooling regional temperature signals, primarily resulting from afforestation of abandoned cropland in the mid-latitudes, and deforestation for agriculture in the tropics. Evaporative Resistance: Evaporative resistance does not directly control the total amount of energy absorbed by the land surface; it controls the partitioning between sensible and latent heat fluxes. I show that the direct effect of changes in evaporative resistance has the largest impact on surface temperatures and fluxes in regions with larege latent heat fluxes - that is, areas with substantial water available on the land surface and large amounts of energy absorbed by the land surface, such as the tropics. However, I show that the effect of evaporative resistance on land surface climate is greatly amplified by atmospheric interactions - in particular, by changes in cloud cover. I show that changes in evaporative resistance have the largest impact on terrestrial temperatures over the northern mid-latitudes, where reduced land evaporation leads to reductions in low cloud cover, which in turn lead to increased sunlight reaching the land surface in these regions. The increased solar radiation reaching the land surface is the largest driver of warming in response to evaporative resistance. I demonstrate that changes in evaporative resistance can lead to large-scale changes in atmospheric energy transport. However, atmospheric energy transport only responds to changes in evaporative resistance over regions where there are strong cloud feedbacks to modified evaporation from the land surface. It is the cloud feedback that allows for evaporation to modify the total amount of energy absorbed by the land surface - and thus released back to the base of the atmosphere. Historical land use change has resulted in substantial changes in evapotranspiration in Earth System Models. Using SLIM, I show that the modeled changes in evapotranspiration between 1850 and 2000 are responsible for more surface temperature change than the changes in albedo driven by vegetation change over the same period. Surface Roughness: Changes in surface roughness change how efficiently the land can exchange energy with the atmosphere through turbulent mixing. I show how changes in the aerodynamic roughness of the surface (which varies with vegetation height and patchiness) strongly control the radiative skin temperature of the land surface, but have a much weaker influence on the 2m air temperature. Unlike albedo and evaporative resistance, atmospheric feedbacks to changes in surface roughness do not play a large role in controlling the pattern and magnitude of the response of surface temperatures and fluxes to changes in surface roughness. Because surface roughness does not modify the total amount of energy absorbed by the land surface, changes in surface roughness have very little impact on large-scale atmospheric circulation. However, I show that changes in surface roughness do have strong impacts on near-surface wind speeds. This work clearly demonstrates the importance of atmospheric feedbacks to change in the land surface, and quantifies the effects of individual land surface properties on the larger climate system. Outline: Chapter 1 provides relevant background knowledge relating to land-atmosphere inter- actions. Chapter 2 is a detailed description of the Simple Land Interface Model, which was developed in order to address the issue of separating the individual effects of different land surface properties associated with vegetation. Chapter 3 explores the sensitivity of the climate system to incremental idealized, global-scale changes in individual land surface properties, with a particular focus on the surface energy budget. Chapter 4 considers the effects of idealized land surface property changes on large-scale atmospheric circulation using both a complex and an idealized Earth System Model. Chapter 5 focusses on the pattern of land surface property change associated with historical vegetation change from 1850 to the present day.
- Atmospheric sciences