An engineering approach to land-surface controlled convective precipitation

In the summer of 2022, several records of hot and dry conditions were broken in Europe, resulting in problems of water availability that are projected to increase further, especially in the Mediterranean basin. The reason for this drying trend is twofold: There is a reduction in precipitation, and an increase in evaporative demand due to climate warming. For climate mitigation and adaptation, solutions are needed to counteract this drying trend. A technological innovation that can be considered is enhancing surface evaporation by evaporating sea water using solar energy. The aim of this research is to examine whether this technology can potentially be used to address the reduction in precipitation. Therefore, we study under which conditions enhanced surface evaporation leads to more convective precipitation and how much water is required to achieve this.

For convective precipitation to occur, several conditions must be met. These include the atmospheric boundary layer (ABL) crossing the lifting condensation level (LCL), moist air parcels reaching their level of free convection (LFC), and the convective available potential energy (CAPE) surpassing a certain threshold (e.g. 400 J/kg). These conditions can be affected by turbulent fluxes of heat and moisture from the surface. Here we use a zero-dimensional mixed layer "slab" model which describes the evolution of the convective ABL height up to the LCL-crossing and the potential temperature and specific humidity of the mixed layer. From this model we obtain an implicit analytical relationship between the integrals of surface fluxes of heat and moisture that cause the LCL and ABL to cross. The relationship between these integrated surface fluxes varies depending on the initial and free atmospheric conditions.

As a case study, we examine the Ebro basin in northeastern Spain. We use the analytical expression of the LCL-crossing with the observational data from the LIAISE campaign to estimate:

• how many days during the 2021 summer months could enhanced surface evaporation theoretically have led to an LCL-crossing,

• the amount of water required in such cases, and

• the changes to the LFC and CAPE that this evaporation enhancement could cause.

Preliminary results indicate that the LCL-crossing relationship between the integrated surface fluxes strongly depends on the initial and free atmospheric temperatures. This has implications for the areas where the technology could potentially benefit the water availability. Since convective precipitation is only controlled by the surface under specific atmospheric conditions, climate warming can cause areas to go from surface controlled to being too hot for the technology to be able to trigger convective precipitation.

Our research provides a preliminary assessment of the potential of this technology to counteract the drying trend in the Mediterranean basin. Further research is needed to evaluate the amount of precipitation that can be expected from the technology, as well as the effects of the technology on local evaporative demand, evapotranspiration, and heat stress.