Computing Entrainment Rates in Cloud Simulations Based on a Simple and Intuitive Definition of a Cloud

Direct estimation of entrainment rates in cumulus clouds has proven to be a great challenge, even with the availability of high-resolution simulations. One of the main difficulties lies in setting reasonable boundaries to a simulated cloud. Although several methodologies have been proposed for this, these tend to use complex algorithms and the resulting cloud does not always correspond to the intuitive idea of a coherent cloudy air parcel. Here we present a new methodology for setting such boundaries in cloud resolving simulations, which is based on the simple concept of a coherent rising air parcel. Using this method we not only obtain entrainment rates consistent with previous studies, but we can also perform a detailed momentum budget on the rising thermals that allows us to assess the dynamical effects of mixing. Although such an individual-thermal approach may have limitations for determining bulk properties of convection, it is useful in terms of improving our understanding of the physical mechanisms behind the onset and development of deep convection.