Scaling Artificial Heat Islands to Enhance Precipitation in Arid Regions

Abstract

Abstract. Potential for regional climate engineering is gaining interest as a means of solving regional environmental problems like water scarcity and high temperatures. In the hyper-arid United Arab Emirates (UAE), water scarcity is reaching crisis point due to high consumption and over-extraction, and is exacerbated further with climate change. To counteract this problem, the UAE has conducted cloud seeding operations and intensive desalination for many years, but is now considering other means of increasing water resources. Very large ‘Artificial Black Surfaces’ (ABS), made of black mesh/black painted/solar PV panels have been proposed as a means of enhancing convective precipitation, via surface heating and amplification of vertical motion. Under the influence of the daily UAE sea breeze, this can lead to convection initiation under the right conditions. Currently it is not known how strong this rainfall enhancement would be, nor what scale of black surface would need to be employed. This study simulates the impacts at different ABS scales using the WRF-NoahMP model chain and investigates impacts on precipitation quantities and underlying convective processes. Simulations of five square ABS of 10, 20, 30, 40, and 50 km sizes were made on four one-day cases over a 24-hour period. These were compared with a Control model run, with no land use change, to quantify impacts. The ABS themselves were simulated by altering land cover static data, and prescribing a unique set of land surface parameters like albedo and roughness length. On all four days, rainfall is enhanced by low-albedo surfaces of 20 km or larger, primarily through a reduction of convection inhibition and production of convergence lines and buoyant updrafts. The 10 km square ABS had very little impact. From 20 km upwards there is a strong scale-dependency, with ABS size influencing the strength of convective processes and volume of rainfall. In terms of rainfall increases, the 20 km produces a mean rainfall increase, over the Control simulation, of 571,616 m3 day-1, 30 km (~1 mil. m3 day-1), 40 km (~1.5 mil. m3 day-1), and 50 km (~ 2.3 mil. m3 day-1). If we assume that such rainfall events happen only in 10 days in a year, this would equate to respective annual water supplies for >31,000, >50,000, >79,000, and >125,000 extra people yr-1, at UAE per capita consumption rates. Thus, artificial heat islands made from black panels or solar PV offer a means of enhancing rainfall in arid regions like the UAE, and should be made a high priority for further research.