CCN and Vertical Velocity Influences

Science

Cloud microphysics and cloud condensation nuclei (CCN) spectral measurements from two summertime aircraft cloud research projects off the central California (CA) coast—Marine Stratus/Stratocumulus Experiment (MASE) and Physics of Stratocumulus Tops (POST)—are presented and analyzed. MASE was always polluted (high CCN, N_c (N_CCN), and droplet concentrations, N_c), while POST had clean (low N_CCN and N_c), polluted, and intermediate concentrations.

Impact

Both projects show the suppression of cloud supersaturation (S) by higher CCN concentrations ( N_CCN) shown in Figure 1. POST showed a strong positive correlation between N_CCN and N_c (correlation coefficient, R = 0.86) but MASE showed this R to be -0.36. Vertical velocity (W) also influences N_c-. W was positively related to N_c in POST (R = 0.60), while the standard deviation of W (δ_W, which is thought to be more pertinent to stratus clouds) was positively related to N_c (R = 0.51) in MASE.

Six POST and 12 MASE cloud penetrations showed abrupt district divisions in terms of N_c and W (POST) or δ_W (MASE), as displayed in Figure 2. These divided penetrations demonstrated the influence of W or δ_W on N_c because all 12 of the MASE divided cloud penetrations showed significantly higher δ_W and N_c within the same divisions of these clouds than the adjacent divisions with lower N_c and lower δ_W, as shown in Figure 2.

Twomey (1959) demonstrated that the relative influences of N_CCN and W on N_c depends on the slope of the CCN spectra. When the slope (k) of the cumulative CCN spectrum is low (shallow), N_CCN variations have more influence on N_c than W variations. When k is high (steep; greater differences in N_CCN for the same S differences), W variations have more influence on N_c. CCN spectra generally show systematic variations of k over S; k decreases with S. At high S (~1%) k is low (<0.3) and at low S (< 0.1%) k tends to be high (~1.0). High S in cleaner air masses such as POST ( N_CCN < 400 cm-3 in Figure 1) result in greater influence of N_CCN on N_c. Hence, the high R for N_CCN-N_c of POST (0.86). The low cloud S (~ 0.1%) that is forced by higher N_CCN (Figure 1, > 600 cm-3), such as was often measured during MASE, makes relevant only the high k of the CCN spectra that is usually observed at or below 0.1%. This makes δ_W variations more influential to N_c than N_CCN.

Summary

Clean stratus clouds are more greatly influenced by CCN concentration differences than by vertical velocity differences, but polluted stratus clouds are more influenced by vertical velocity variations than by CCN concentration variations. This seems to put a limit on the indirect aerosol effect because very high CCN concentrations do not have as much influence as otherwise expected. This also provides plausibility for the negative R for N_CCN- N_c observed in MASE.

Twomey, S. 1959. “The nuclei of natural cloud formation, II; The supersaturation in natural clouds and the variation of cloud droplet concentration.” Pure and Applied Geophysics 43: 243-249.