Perturbed physics ensemble simulations of cirrus on the cloud system-resolving scale

In this study, the effects of uncertainties in the parameterization of ice microphysical processes and initial conditions on the variability of cirrus microphysical and radiative properties are investigated in a series of cloud system-resolving perturbed physics ensemble (PPE) and initial condition ensemble (ICE) simulations. Three cirrus cases representative of mid-latitude, subtropical and tropical cirrus are examined.

It is found that the variability in cirrus properties induced by perturbing uncertain parameters in ice microphysics parameterizations outweighs the variability induced by perturbing the initial conditions in mid-latitude and subtropical cirrus. However, in tropical anvil cirrus the variability in the PPE and ICE simulations is about the same order of magnitude. The cirrus properties showing the largest sensitivity to the uncertainties in the parameterization of ice microphysical processes are the vertical distribution of ice water content (IWC) and cloud thickness whereas the overall cirrus cloud cover is only marginally affected. We find that changes in cirrus ice water path and outgoing longwave radiation are controlled primarily by changes in IWC and cloud thickness. The change in the vertical distribution of cloud fraction and cloud thickness is caused by changes in cirrus cloud base whereas cloud top is not sensitive to either perturbed physics or perturbed initial conditions. In all cirrus cases, the top three parameters controlling the microphysical variability and radiative impact of cirrus clouds are ice fall speeds, ice autoconversion size thresholds and heterogeneous ice nucleation parameters. Changes in the ice deposition coefficient do not affect the ice water path and outgoing longwave radiation. Similarly, changes in the number concentration of aerosols available for homogeneous freezing have virtually no effect on the microphysical and radiative properties of mid-latitude and subtropical cirrus but only little impact on tropical anvil cirrus. Overall, the sensitivity of cirrus microphysical and radiative properties to uncertainties in ice microphysics is largest for mid-latitude cirrus and smallest for tropical anvil cirrus.