A Microphysics Guide to Cirrus Clouds

Cirrus clouds still represent one of the largest uncertainties in the prediction of the Earth's climate (IPCC, 2013) since their microphysical and radiative properties are not yet well known. The major reason fis that it is difficult to measure the respective parameters on fast-flying, high altitude aircraft. Another problem is that aircraft measurements cannot capture the evolution of the cirrus clouds properties with time. In most cases, the measurements are shown along the flight tracks or as function of temperature. The most common parameters that are measured in cirrus clouds -besides the meteorological variables- are ice water content (IWC), number of ice crystals (N_ice), mean mass size (R_ice) and relative humidity (with respect to ice, RH_ice), and sometimes vertical velocity. However, it is difficult to conclude on the history of ice nucleation and development of microphysical properties from these observations. Our study aims to provide a guide to cirrus microphysics, which is compiled from an extensive set of model simulations covering the broad range of atmospheric conditions for cirrus formation and evolution. We then portray the model results in the same parameter space as the field measurements, i.e. in the temperature - IWC parameter space. From this representation of simulated cirrus, we can assign, to a certain degree, the formation mechanism and history to specific combinations of IWC, N_ice, R_ice and RH_ice inside and outside of cirrus as a function of temperature. Method: The information in the microphysis cirrus guide is computed by using the detailed microphysical model MAID (Gensch et al., 2008, ERL). MAID is a box model that is operated along idealized or realistic air mass trajectories. It includes homogeneous and heterogeneous freezing, where the freezing threshold and number of ice nuclei can be varied for heterogeneous ice nucleation. Diffusional growth, evaporation, and sublimation as well as Lagrangian tracking and sedimenation of ice particles are included in MAID. About 500 idealized cirrus scenarios are represented in the cirrus guide by varying the freezing mechanism (pure homomgeneous or combined heterogeneous followed by homomgeneous freezing), the number of ice nuclei and freezing threshold, the temperature and vertical velocity and the strength of sedimentation. In one set of scenarios, temperature fluctuations are superposed to the constant temperature changes of the idealized scenarios. Results: Figure 1 shows the cirrus guide for IWC under typical atmospheric conditions (see legend of the figure). The annotations show which conditions lead to a certain IWC. For example, a high IWC can mostly be assigned to homogeneous freezing in high updrafts and is connected with a high number of ice crystals. This cirrus analysis approach is validated by evaluating data sets from multiple field campaigns we have conducted in the last ten years. It can be shown that the field observations indeed show the characteristics expected from the cirrus guide. For example, high/low IWCs are indeed found together with high/low N_ice. Fig. 1. MAID model simulations of IWC versus temperature. The scenario is for heterogeneous+homogeneous freezing, freezing threshold 110% (mineral dust), number of ice nuclei is 0.01 1/ccm, sedimentation parameter f=0.9 (mediate sedimenation). The grey lines are median, minimum and maximum IWC derived from field measurements (Schiller et al, 2008, JGR and Luebke et al, 2013, ACP).