Precipitation Efficiency over the United Arab Emirates Obtained from Numerical Simulations with the CReSS Model

To conduct effective cloud seeding experiments over the United Arab Emirates (UAE), we need to investigate the possibility of precipitation enhancement using numerical models. Precipitation Efficiency (PE) is generally defined as the ratio of surface precipitation to sources associated with precipitation and is an important physical parameter in vapor, cloud, and surface precipitation budgets for various cloud systems. Cloud Microphysics Precipitation Efficiency (CMPE) is defined as the ratio of the surface rainfall rate to the sum of the condensation and deposition rates of supersaturated water vapor (Sui et al., 2005; 2007).

In this study, numerical simulations have been carried out using Cloud Resolving Storm Simulator (CReSS) model (Tsuboki and Sakakibara, 2007) to investigate the PE. Numerical simulations with 5 km (CReSS_5km) and 1 km (CReSS_1km) horizontal resolutions were conducted for the case of September 11, 2015.

The research domain area falls into three categories, D0 over UAE and Oman, D1 over desert below 300 m terrain height, D2 over mountain area over 300 m terrain height. In this event, most of accumulated surface precipitation amount is distributed in D1 and D2 over the UAE. Horizontal distribution of column maximum radar reflectivity shows similar pattern between radar observation and CReSS simulation over both desert and mountain areas. And cloud base and freezing level are located at about 3 km and 4.6 km heights. The diurnal convective clouds induced by solar radiation more intensively develop over the mountain area than over the desert area in terms of their areal occupation and cloud top height (2 km difference). Among the production terms, water vapor condensation rate to cloud water dominates sink term in water vapor budget of PE than water vapor deposition rates to ice, snow, and graupel. The domain averaged surface rainfall intensity is much higher in D2 than in D1, but PE is slightly higher in D2 than in D1. The domain-averaged rainfall intensities in both D1 and D2 simulated by CReSS_5km are roughly three times higher than those simulated by CReSS_1km, but PEs simulated by CReSS_5km and CReSS_1km do not show such a large difference. Rather small values of PEs about 0.1 in D1 and D2 and from CReSS_5km and CReSS_1km simulations are due to intensive evaporation of rain drops below cloud base. Two thirds of rainwater at cloud base evaporates before reaching the ground.

At the conference, the possibility of rain enhancement by hygroscopic and glaciogenic seeding will be discussed based on the PE, the ratio of water vapor evaporation/sublimation from cloud particles to water vapor condensation/deposition to cloud particles, and detail analysis of precipitation mechanisms.