Initial Aerosol Concentration Is Key Contributor to Low-Level Cloud Reflectivity

Penner, J. E., University of Michigan

Aerosol Properties

Aerosol

Penner, J., Dong, X., Chen. Y., Observational evidence of a change in radiative forcing due to the indirect aerosol effect, Nature, Vol. 427, 15 January 2004.


Cloud optical depth, as determined from the parcel model, is indicated by the dots. Red lines show best fit data of cloud liquid water path and optical depth determined from ARM solar and microwave radiometer measurements.


Cloud optical depth, as determined from the parcel model, is indicated by the dots. Red lines show best fit data of cloud liquid water path and optical depth determined from ARM solar and microwave radiometer measurements.

A key variable in simulating climate is the effect of aerosols on cloud reflectivity. Most climate scientists agree that, as clouds form, the greater the initial aerosol concentration, the more cloud droplets are produced of smaller average size, all other variables being equal. Smaller droplets produce a greater optical depth, making the clouds more reflective and less transmissive of solar radiation. Proving this for any individual cloud case, however, is very difficult, because cloud formation involves a highly complex combination of variables. These variables include aerosol number and size, vertical updraft speed (which drives the amount of condensed water), and temperature and humidity profiles.

Research reported in Nature (15 Jan 2004) provides corroborating evidence that for low-level clouds, aerosols do affect cloud properties relative to the initial aerosol concentration. Researchers used data from low-level clouds at two sites operated by the Department of Energy's Atmospheric Radiation Measurement (ARM) Program to compare direct observations of high and low aerosol environments with simulations from a simple parcel model. Atmospheric conditions at the Southern Great Plains site in Oklahoma are representative of typical high aerosol concentrations (polluted), while the North Slope of Alaska site is representative of typical Arctic low aerosol concentrations (clean).

They first compared cloud optical depth to the observed liquid water path (column amount of condensed water). As expected, results showed that in the cleaner environment, lower optical depth was produced for the same amount of condensed water. Their next step was to try and simulate a similar relationship using a simple model. Their parcel model simulates a parcel of air—imagine a balloon filled with air particles—as it is lifted to the point of condensation and beyond. Water begins to condense on some of the particles at the condensation level and continues to condense as the particle is lifted further.

As shown in the figure, the model results agree very well with the data curve, indicating that the difference in cloud characteristics is predominately due to the differences in initial aerosol concentrations at the two sites. For low-level clouds, this corroboration confirms that aerosol concentrations do affect cloud properties. Arctic clouds were also shown to be less reflective for the same liquid water path and environmental conditions, because of bigger, but fewer, droplets.

By sampling a wide variety of atmospheric conditions, the ARM sites allow researchers to investigate these important aerosol effects. These results show that aerosols do impact cloud properties in significant and understandable ways. Such effects must be included in climate models in order to understand climate change during the current and previous centuries, because aerosol concentrations are a constantly changing part of the environment.