Strong Impacts of Vertical Velocity on Cloud Microphysics and Implications for Aerosol Indirect Effects: Despite widely recognized importance, aerosol indirect effects are still full of uncertainty, even controversy for some aspects. One reason for the uncertainty/controversy is that aerosol effects are often intertwined with changes in cloud dynamics such as vertical velocity and separation of aerosol indirect effects from dynamical effects poses a confounding challenge, esp., in observations. DOE scientists at the Brookhaven National Laboratory and collaborators have taken on the issue using data collected in cumulus clouds during the Routine AAF [Atmospheric Radiation Measurement (ARM) Aerial Facility] Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign, with a focus on the effect of vertical velocity on cloud droplet number concentration and relative dispersion, two cloud microphysical properties critical for quantifying aerosol indirect effects. This observational study confirms their early theoretical prediction that variation of vertical velocity leads to a negative correlation between cloud droplet number concentration and relative dispersion: an increase in vertical velocity leads to an increase in cloud droplet number concentration but a decrease in relative dispersion, opposite to that caused by changes in aerosol loading . The study demonstrates that one can capitalize on the opposing effects of aerosol loading and vertical velocity to help discern their relative importance. Reference: Lu, C., Y. Liu, S. Niu, and A. M. Vogelmann (2012), Observed impacts of vertical velocity on cloud microphysics and implications for aerosol indirect effects, Geophys. Res. Lett.,doi:10.1029/2012GL053599 (accepted) Contact: Dorothy Koch, SC23.1, 301-903-0105.

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Motivation● One reason for the uncertainty in aerosol effects

is that aerosol effects are often intertwined with changes in other factors, such as vertical velocity. Separating aerosol effects from dynamical effects poses a confounding challenge, and such observational studies are particularly rare.

Approach● Simultaneous measurements of vertical velocity

and cloud droplet size distributions in cumuli collected during the RACORO field campaign are analyzed to empirically quantify the effect of vertical velocity on cloud droplet number concentration, relative dispersion and their relationship.

Result● Effect of vertical velocity on relative dispersion

and its relationship with droplet concentration is opposite to that associated with aerosol loading. The opposing relationships can be used to help discern the relative importance of aerosol and dynamical effects.

Observed Impacts of Vertical Velocity on Cloud Microphysics and Implications for Aerosol Indirect Effects

Lu, C., Y. Liu, S. Niu, and A. M. Vogelmann (2012), Observed impacts of vertical velocity on cloud microphysics and implications for aerosol indirect effects, Geophys. Res. Lett.,doi:10.1029/2012GL053599 (accepted)

Joint probability density functions (PDF) of relative dispersion (ε) vs. vertical velocity (w) along horizontalaircraft legs for each cumulus flight (date given in legend).The color (contour) represents the frequency of

occurrence; Only the mesh grids (0.1 for ε and 0.5 m s-1 w)that have percentages ≥ 0.3% are shown. the total numbers of samples in the six flights are 3,641, 4,586, 2,829, 3,525, 776 and 4,115, respectively. The red lines denote weighted least squares power-law fits of the data.