Institut für Physik und Meteorologie

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The impact of irrigated biomass plantations on mesoscale climate in coastal arid regions
(2015) Branch, Oliver; Wulfmeyer, Volker
Large-scale agroforestry in coastal arid and semi-arid regions could provide a geoengineering solution to anthropogenic climate change. Since agroforestry may impact on mesoscale climate in unknown ways, urgent research into potential impacts of large-plantations is needed to fully assess the viability and optimal placement for such schemes. Validated mesoscale simulations provide insights into feedbacks between land surface and atmosphere, particularly with respect to convective processes. Simulations of irrigated Simmondsia chinensis (jojoba) plantations were carried out with the WRF-NOAH atmosphere-land surface model using prescribed land surface and plant parameters. A sub-surface irrigation algorithm was developed based on critical soil moisture stress levels and implemented into the model code. The simulation of desert and plantation land surfaces was validated with field data from two sites in the Negev Desert - an arid desert site and a 400 ha jojoba plantation. For desert and vegetated surfaces, the model output of diurnal meteorological quantities and energy fluxes generally match well with the respective observations. Diurnal 2m-temperatures over the desert and plantation are matched by the model to within ± 0.2 °C and ± 1.5 °C, respectively. Wind speeds for both surfaces match to within 0.5 ms−1 and plantation latent heat is reproduced to within ± 20 Wm−2. Subsequent to validation, larger plantations of 100 km × 100 km were then simulated in two coastal arid regions, Israel and Oman over a period of one month and compared with control runs, without plantations. In Oman, convection and precipitation were triggered or enhanced by the plantation over multiple days whereas in Israel almost no impacts were observed. Two mechanisms were responsible for observed convection initiation: turbulent vertical transport of scalars due to increased surface heating and roughness as well as a low pressure-induced convergence at the canopy leeside. The main contributors to the surface heating effect were reduced albedo and the high water-use efficiency exhibited by specialist desert species. The combination of increased net surface radiation and high stomatal resistances significantly limited transpiration and led to a surplus in sensible heat flux compared with the surrounding soils (> 100 Wm−2). In Oman, convection initiation triggered by the plantation tended to occur on days when a high mid-tropospheric temperature lapse rate and significant surface air humidity were present. Israel exhibits more stable lapse rates during summer and drier conditions aloft, both of which suppressed convection significantly, even with a similar land surface perturbation. The initiation of moist convection at the mesoscale is therefore strongly controlled by prevailing synoptic conditions. A regional climatological analysis of temperature and humidity ECMWF reanalysis data and station precipitation data indicate that the south-west of North America has particularly suitable conditions for impacts. Coastal locations in Baja California and the Sonoran Desert exhibit a seasonal concurrence of monsoonal instability, high surface humidity and integrated column water vapor, but at the same time low precipitation. Therefore plantation impacts on convection there are likely and could be beneficial in terms of higher amounts of precipitation. These findings indicate that mesoscale convective events can be triggered by large plantations within arid and semi-arid regions and that these effects may be controllable via judicious placement of such schemes. Thus arid agroforestry has the potential not only to increase precipitation and reverse desertification within arid and semi-arid regions, but also to mitigate climate change if implemented on very large scales.

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