Tropical cirrus in high-resolution global and regional models

Abstract

Cirrus clouds remain a large source of uncertainty in predictions of future climate. It is difficult to simulate cirrus because they have very thin optical depths yet cover large horizontal extents. Representation of cirrus in models is difficult to verify due to the limited observations that are comparable to model output. Yet as models continue to increase horizontal and vertical resolutions, cirrus cloud microphysics and parameterizations become an increasingly important driver of model uncertainty. This dissertation is part of a recent effort to reduce uncertainties from high clouds and improve understanding of TTL cirrus. We use high-resolution models on global and regional scales to understand the contribution of ice microphysics and cirrus clouds to the model spread or uncertainty. We investigate the origins of cirrus clouds in high-resolution simulations of radiative convective equilibrium. The origins of cirrus have been a source of debate for decades and this dissertation provides valuable insight into the role of convection in cirrus cloud formation. While convection is a key source of cirrus and water vapor in the upper-troposphere, there are many in-situ formed cirrus that persist far from convection. While the models generally represent large-scale tropical convection with fidelity, there are large regional differences in cloud properties and top-of-atmosphere radiative fluxes. We focus on the Tropical Western Pacific (TWP) as a case study for the local and small-scale features available with the high spatiotemporal resolution of GSRMs. Each model simulates unique cloud characteristics that are persistent across seasons. We show that these models simulate the difficult-to-observe thin tropical tropopause layer (TTL) cirrus. The models typically scatter around observations but have persistent biases, which stem from the different microphysics and dynamics used in each model. In order to separate microphysical and dynamical effects in the models, we use a single model regional domain sensitivity study to expose the role of microphysics on tropical cirrus. Microphysics can play a significant role in cirrus properties that can have a disproportionate impact on top-of-atmosphere (TOA) radiative fluxes. Sensitivity to ice microphysics seems to explain some of the model spread. Furthermore, we explore how the ice nucleation scheme and the presence of large-scale ascent impacts cirrus clouds in simulations with convective organization. We introduce passive tracers to the model in order to investigate the origin and life cycle of cirrus clouds. The structure and degree of convective organization seems to play a key role in the amount of in-situ cirrus in these simulations. While the response of anvil clouds is similar to other RCE models when the SST is increased by 4K, the response of in-situ cirrus is uncertain; they decrease in frequency of occurrence in two of three simulations. Nonetheless, all simulations show a warming of in-situ cirrus clouds when the SST is increased by 4K.