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Research Highlights September 2011:

Aerosol effects on stratocumulus water paths in a PDF-based parameterization by Guo, H, J-C Golaz and L. J. Donner.

We have incorporated multi-variate probability density functions with dynamics (MVD PDFs) into the single-column model (SCM) of GFDL AM3. In agreement with benchmark large eddy simulations (LESs), our single-column simulations produce both positive and negative liquid water path (LWP) responses to increasing aerosol concentrations. Sensitivity tests show that the dependence of sedimentation on cloud droplet size is essential to capture both the positive and negative LWP responses.

We investigated three nocturnal marine stratocumulus cases: ASTEX, RF01, and RF02.

To explore the impact of the free atmosphere relative humidity on LWP, we reduced the relative humidity in ASTEX from 70% to 25%, and refered this additional test as ‘dry ASTEX’. Our single-column simulations with MVD PDFs are capable of capturing both positive and negative LWP responses to increasing aerosol concentrations (Figure 1), and compare favorably with benchmark large eddy simulations.



Figure 1: (a) liquid water path (LWP), (b) surface precipitation rate, and (c) cloud top entrainment rate (w_e) as a function of cloud droplet number concentrations (N_d) from the MVD PDFs simulations. Shaded areas indicate ranges for different vertical resolutions (5 to 40 m) and time steps (0.5 to 3 min). `*’ and vertical bars in panel (c) indicate measurement estimates and ranges of w_e for ASTEX (Blue), RF01 (Red), and RF02 (Dark Red). Panels (d), (e) and (f) show comparisons of the MVD PDFs (solid curves) with benchmark large eddy simulations (dashed curves).

We conducted two series of sensitivity tests in which: (1) cloud water sedimentation is completely omitted; and (2) sedimentation depends on mass only. In contrast to Figure 1 in which sedimentation depends on both mass and number, LWP increases for dry ASTEX, RF01, and RF02 (Figure 2(c) and (e)). This suggests that a more realistic mass- and number-dependent sedimentation scheme is required to capture both positive and negative responses in LWP.

Figure 2: Normalized liquid water path (LWP(Nd)/LWP(Nd=30 cm-3)) and cloud top entrainment rate (w_e) with mass- and number-dependent sedimentation (a,b), with droplet sedimentation completely omitted (c,d), and with sedimentation dependent on mass only (e,f)

For more information: Guo, H., J-C. Golaz, L. J. Donner (2011), Aerosol effects on stratocumulus water paths in a PDF-based parameterization. Geophysical Research Letters, 38, L17808, doi:10.1029/2011GL048611.