Microphysical and dynamical controls on cirrus cloud optical depth distributions

We assess microphysical and dynamical controls on cirrus cloud optical depth distributions [P( σ )] along idealized air parcel trajectories. We find P( σ ) shape depends primarily on the ratio of the ice crystal fallout timescale to timescales of other microphysical and dynamical processes. With homogeneous freezing only, two P( σ ) regimes emerged. In the limited fallout regime, relatively slow fallout allows complete depletion of the ice supersaturation, and P( σ ) has a peak at large optical depth values ( σ > 1). In contrast, in the fallout‐dominated regime, relatively rapid fallout results in persistent high‐ice supersaturation and multiple freezing events, and P( σ ) has a monotonically decreasing shape dominated by small optical depth values. The addition of heterogeneous freezing alters the homogeneous‐freezing P( σ ) shape only in the limited fallout regime. Here glaciated ice nuclei (IN) do not inhibit homogeneous freezing but can change P( σ ) by reducing the optical depth of the P( σ ) peak and adding a monotonically decreasing tail at low optical depth values. Surprisingly, glaciated IN do not significantly change P( σ ) values or shape in the fallout‐dominated regime. Fluctuations in vertical velocity and accompanying temperature changes have relatively little impact on P( σ ) unless the fluctuation timescales are shorter than fallout timescales, but longer than ice crystal growth timescales. As temperature fluctuations increase in amplitude, new freezing events affect P( σ ) as long as fluctuation timescales approach or exceed freezing timescales. Our modeled P( σ ) qualitatively resemble observed P( σ ), indicating these results could aid in GCM cirrus P( σ ) parameterization and help diagnose the controls on cirrus P( σ ).