Sensitivity of Tropical Anvils to Ice Aggregation

In tropical convection, the microphysical properties of ice in anvils are a major factor in determining cloud-radiative forcing and cloud-climate feedbacks. One large source of uncertainty in predicting the ice number, size, shape, and sedimentation in storm anvils is establishing the efficiency of ice self-aggregation processes. Ice crystal adhesion decreases as temperature decreases, but aggregation process rates are difficult to measure and depend on complex environmental factors. The overarching goal of this study is to identify how uncertainty related to ice aggregation efficiencies affects anvil properties and cloud radiative feedbacks, and how we may constrain this uncertainty in the future.  

We exploit a high-resolution database of simulated convective systems being produced for the NASA INvestigation of Convective UpdraftS (INCUS) mission with the Regional Atmospheric Modeling System (RAMS), which features a bin-emulating microphysics scheme. The INCUS LES dataset represents an expanding collection of diverse storm morphologies in a variety of maritime and continental (sub)tropical environments, including scattered congestus, multicell convective clouds, squall lines, and tropical cyclones. To address our science goals, ice aggregation efficiencies for temperatures -20 to -50 ◦C are perturbed in the INCUS simulation ensemble. The perturbed aggregation efficiencies all fall within the spread of uncertainty of those obtained from previous laboratory and modeling studies. The simulations are then run through the Community Radiative Transfer Model (CRTM), and storm anvils are tracked and separated into their optically thick and thin components.  

Overall, small changes to ice aggregation efficiencies below -20 °C can significantly reduce or expand anvil extent, lifetime, depth, and their associated cloud radiative effects. Thick anvils are more sensitive to changes in aggregation than thin anvils, with a 25% reduction in thin anvil area and a near complete dissipation of thick anvils. As such, anvil cooling feedbacks are more sensitive to ice aggregation than cloud warming effects, with changes in outgoing longwave radiation on the order of 100 W/m^2. This analysis demonstrates how poorly constrained ice self-aggregation efficiencies are within observations and numerical models, proving a critical need for more observations and physical understanding of ice aggregation processes.