Publication Date:
2013-04-21
Description:
[1] Numerical simulations and airborne measurements are used to evaluate the impact of physical processes on synoptically-forced, midlatitude cirrus ice concentrations. The agreement within a factor of two between ice concentrations measured with independent techniques (replicators and optical imaging probes) provides confidence in the accuracy of the in situ measurements. We use a computationally efficient modeling approach that incorporates the key cirrus physical processes, such that thousands of cloud cases can be simulated and the model results can be statistically compared with observations. One-dimensional simulations with detailed treatments of cloud microphysical processes are driven by temperatures and vertical winds extracted from meteorological analyses. Small-scale temperature and vertical wind perturbations associated with mesoscale waves are superimposed on the analysis fields. The comparisons between simulated and observed ice concentration statistics are examined for simulations that include (or exclude) physical processes such as homogeneous freezing nucleation, heterogeneous nucleation, sedimentation, and aggregation. We find that in simulations with only homogeneous freezing nucleation, ice concentration statistics are very sensitive to the specified mesoscale wave vertical wind perturbations. With the frequency distribution of vertical winds adjusted to agree with aircraft observations, we obtain good agreement between the simulated and observed ice concentration frequency distributions. Both the observations and simulations indicate that relatively high ice concentrations (≥1000 L − 1 ) occur rarely in these clouds (less than 1% of the time). Simulations including both homogeneous and heterogeneous nucleation indicate that even with moderate concentrations of ice nuclei (20 L − 1 ), heterogeneous nucleation is an important ice production process, particularly for relatively low ice concentrations and warm temperatures. With enhanced ice nuclei concentrations (100 L − 1 ), heterogeneous nucleation dominates ice production in the model and homogeneous nucleation plays a secondary role. We find that it is critically important to include the impact of sedimentation on the evolution of ice concentrations when comparing model results with observations; calculations that only indicate the ice concentrations just after nucleation events give excessive concentrations. Ice crystal collection efficiencies are poorly constrained at low temperatures, and we find that aggregation can significantly reduce ice concentrations. We evaluate the robustness of the results given various model assumptions and uncertainties (e.g., time periods simulated, geographic domains simulated, ice crystal habit, trajectory paths, and initial relative humidity profiles). We find that neither the agreement between observed and simulated ice crystal statistics nor the sensitivities indicated by the simulations are significantly affected by these model assumptions. The mesoscale wave temperature and vertical wind perturbations that control ice concentrations (to first order) are not resolved in large-scale models, and assumptions must be made about the dependence of subgrid-scale vertical winds on resolved fields. We recommend evaluation of the vertical wind speeds used in global-model cirrus parameterizations by comparison with airborne measurements of vertical winds.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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