Estimation of Convective Entrainment Properties from Cloud-Resolving Model and Large-Eddy Simulations During TWP-ICE

Description

The fractional entrainment rate in convective clouds is an important parameter in current convective parameterization schemes of climate models. In this work, we estimate entrainment rates and associate entrained thermodynamic properties using a 1-km-resolution cloud-resolving model (CRM) simulation of convective clouds from TWP-ICE. The cloud population is divided into different types, characterized by cloud-top heights. The entrainment rates and moist static energy that is entrained or detrained are determined by analyzing the budget of moist static energy for each cloud type. Results show that the entrained air is a mixture of approximately equal amount of cloud air and environmental air, and the detrained air is a mixture of ~ 80% of cloud air and 20% of the air with saturation moist static energy at the environmental temperature. After taking into account the difference in moist static energy between the entrained air and the mean environment, the estimated fractional entrainment rate is much larger than those used in current convective parameterization schemes. The bulk plume-based entrainment rate can be interpreted as an effective entrainment rate that would be required in order to achieve the same degree of dilution of updraft air if the entrained air were to carry the environmental mean properties. To verify our results from the relatively coarse resolution (1 km) CRM simulation, we analyzed 10 hours of Giga-LES simulation with 0.1 km resolution for the active Australian monsoon period of the TWP-ICE experiment. By coarse-graining the high resolution LES data to 1 km resolution and performing similar entrainment rate calculation on both the 0.1 and 1 km Giga-LES data, we confirmed that the entrainment rates calculated using the coarse resolution data are credible. It is shown that the characteristics of entrainment rates estimated using both the 0.1 km-resolution data and CRM-resolution coarse-grained data are similar. For each cloud category, the entrainment rate is high near cloud base and top, but low in the middle of clouds. We also examined the relationship between entrainment rate and several parameters commonly used in previous studies for parameterizing entrainment rates, including inverse of height, in-cloud vertical velocity, buoyancy and its vertical gradient. We found that entrainment rate is best related to the inverse of in-cloud vertical velocity by a second order polynomial for both shallow and deep convection.