Seeding Effects on Summertime Mixed-Phase Convective Clouds over the United Arab Emirates Simulated Using CReSS with Simple Hygroscopic Seeding Scheme

In order to enhance precipitation efficiently at any areas, it is important to investigate not only the cloud and precipitation microphysical characteristics of target clouds but also the optimal seeding methods. For the investigation, some approaches using cloud resolving models are necessary as well as observations and laboratory experiments.

For numerical models that express cloud water and rainwater as single-moment variables, including the original version of Cloud Resolving Storm Simulator (CReSS) model (Tsuboki and Sakakibara, 2007), sensitivity experiments of precipitation to the increase/decrease of cloud condensation nuclei (CCN) have been performed by changing the prescribed number concentration of cloud water. Based on the results, a possible maximum seeding effect has been discussed. However, this method cannot accurately represent the actual seeding effects. Therefore, in this study, we introduced a cloud microphysics scheme (revised version of the MRI-NHM4ICE scheme based on the Farley scheme) into CReSS, which expresses all the hydrometeors as double-moment variables. Further we introduced a new simplified hygroscopic seeding scheme into CReSS and investigated its performance in simulating hygroscopic seeding of mixed-phase convective clouds over the UAE and its surroundings.

The new hygroscopic seeding scheme was built on the CCN activation scheme newly introduced in CReSS (Murakami et al., 2019). The coefficients, C and k, of the CCN activation spectrum proposed by Twomey (1959) can be changed at grid points where a given atmospheric condition meets so as to simulate more realistic and efficient hygroscopic seeding. This facilitates an investigation of optimal seeding conditions. The coefficients of background CCN spectra at initial time are given as a vertical profile, which also enable us to study aerosol effects on microphysical structures of multi-layer clouds associated with synoptic scale disturbances.

First, we conducted idealized simulations of summertime mixed-phase convective cloud formation, where typical summertime atmospheric profiles over the UAE were assumed and convection was initiated by warm bubble with different magnitudes. We investigated hygroscopic seeding effects on surface rainfall by changing the value of C from background value of 300 cm^-3 to 30 cm^-3 (LSEED case) or 3000 cm^-3 (HSEED case) at grid points with updraft velocity velocities > 0.3 m s^-1 just below cloud base.

HSEED case showed a remarkable increase in surface precipitation, but also showed a remarkable dependence of seeding effects on cloud characteristics (mainly cloud top height) and cloud microphysics schemes. In parallel to hygroscopic seeding simulations, we are validating/improving cloud microphysics schemes against aircraft observation data collected over the UAE in the summer of 2017.

At the conference, seeding effects on summertime mixed-phase convective clouds over the UAE, which are simulated by the improved numerical model using more realistic and appropriate seeding conditions, will be discussed with focuses on the surface precipitation, microphysical structures and precipitation processes in clouds.