Regional Aerosol-Cloud Analysis: Vertical Distributions and Seasonal Fluctuations from CALIOP

Aerosols are tiny suspended particles that play a multi-faceted role in the atmosphere. They act as cloud condensation nuclei (CCN) during cloud formation by providing a platform for water vapor to condense. In addition, some aerosols may reflect solar radiation back into space, while others have the ability to absorb solar radiation. Clouds have the ability to reflect, absorb, transmit, and emit both terrestrial and solar radiation depending on the type of cloud (liquid, ice, or mixed), the cloud thickness, and it's altitude in the atmosphere. Aerosols can also interact with clouds in a variety of ways that modify their properties such as the cloud drop number concentration, the cloud lifetime, and precipitation. The outcomes of these interactions are dependent upon the aerosol type (absorbing, reflecting, or mixed), aerosol amount (high or low concentration), and where the aerosol is located with respect to the clouds (above, below, or mixed within the cloud layer). Because of these processes, aerosols and their interactions with clouds play an important role in Earth's energy budget. The extent of the impact is still highly uncertain.

Until recent advancements in satellite instrumentation, aerosol type and vertical distribution were challenging to study. In 2006 to ameliorate this issue NASA launched CALIPSO, which contains the LiDAR instrument CALIOP. CALIPSO is a vertical profiling satellite that was maneuvered into orbit joining the group of Earth-observing satellites referred to as the A-train. This study uses Level 2 Lidar Vertical Feature Mask (VFM) data (CAL_LID_L2_VFM-Valstage1) for the years 2006-2015. The VFM data provides aerosol types in categories based on their optical properties, including those that are reflective, such as sulfur containing continental pollution, and those considered absorbing, such as dust and smoke. The CALIOP aerosol types under investigation in this study are clean marine, dust, polluted dust, clean continental, polluted continental, and smoke. In addition to aerosols, cloud types and their heights in the atmosphere will also be determined using the VFM data. The cloud types are low overcast (transparent), low overcast (opaque), transition stratocumulus, low, broken cumulus, altocumulus (transparent), altostratus (opaque), cirrus (transparent), and deep convective (opaque).

In order to study the intricate relationships between aerosols and clouds, a regional analysis is conducted using fourteen different regions. These regions were chosen for observation based on fire count data from Aqua's MODIS, an Earth-observing instrument also in the A-train constellation, for locations known to have occurrences of biomass burning. Aerosols from biomass burning can have diverse effects on cloud properties making their interactions an important topic of study. Simply stated, biomass-burning aerosols may increase cloud formation by providing additional CCN to the atmosphere creating longer lasting non-precipitating clouds, or conversely, may provide larger CCN creating a higher chance of precipitating clouds. Additionally, by absorbing solar radiation, these aerosols may cause cloud evaporation due to the direct heating of the atmosphere by the aerosol when present in the environment surrounding the clouds.

The fourteen regions of focus are: North America-West (NAW), North America-East (NAE), Central America (CEN), South America-Central (SAC), Eastern Europe (EEU), Mediterranean (MED), Central Africa (CAF), South Africa (SAF), Siberia (SIB), India (IND), China (CHN), Southeast Asia (SEA), Oceania (OCE), and Australia (AUS). Gridded profiles were created based on the latitude and longitude of these specific regions. Each satellite pass through these regions was extracted and averaged over a nine year time period to create a climatological average of the dominant aerosol and cloud type, their maximum and average heights, and their seasonal fluctuations in the atmosphere.

There are still many unknowns in regards to aerosols and clouds and their overall effect on the climate, which makes continued research in this field a high priority. By knowing the location and vertical distribution of aerosols in the atmosphere in relation to nearby clouds along with their seasonality, it will allow for a better understanding of aerosol-cloud-climate interactions.