2. Ground-based remote sensing

2.1. Background, motivation and state of the science

Ground-based lidars have been used since the early 1970s to quantitatively monitor the stratospheric aerosol layer. In some cases they have measured plumes from volcanic eruptions in remote locations on earth before the eruptions were identified. They provide a relatively inexpensive method to regularly measure the stratospheric layer from a given site with high altitude resolution (tens or hundreds of meters) and time resolution (tens of minutes). The fundamental quantity measured is backscattered light. Multiple laser wavelengths can be used to infer particle size and polarization effects can used to infer non-spherical particle shape.

2.2. Role within SSiRC

Long-term monitoring of the stratospheric layer by lidars has been very valuable in tracking the dispersion and decay of eruption plumes. The same is true for smoke injected into the stratosphere by large forest fires. Each event is a potential natural experiment to compare the measured aerosol with model calculations of transport and evolution of the aerosol. The long-term records have also been used to characterize transport effects such as the quasibiennial oscillation (QBO) and tropospheric-stratospheric exchange.

Besides long-term monitoring, ground-based lidar measurements can complement the SSiRC objectives by providing aerosol profiles before, during, and after campaigns. Balloon and aircraft measurements allow far more detailed measurements of aerosols than the lidar, but coverage in time provided by the lidar can often be very useful in interpreting the cloud and aerosol conditions present during the flights.

2.3. Activities

2.3.1. Long-term, regular monitoring and campaign support

There are many lidars in operation around the world with most in the northern hemisphere. Many participate in networks, some of which are listed below:

Of these, the Network for the Detection of Atmospheric Composition Change (NDACC), formerly the Network for the Detection of Stratospheric Change, has historically made the most contribution to stratospheric aerosol research. The lidar programs (aerosol, ozone, and temperature) have participated in calibration and validation activities specifically to promote reliable and accurate measurements. The data is readily available in Ames format files from primary and complementary sites around the world. The instruments tend to be funded for regular monitoring, but might need additional funds to support an intensive campaign.

The other networks tend to be aimed more towards tropospheric aerosols, but many would easily retrieve eruption plumes in the stratosphere. The funding and schedule of observations vary widely for the various lidar groups.

2.3.2.Comparison of lidar aerosol profiles

The NASA CALIOP space-based lidar is currently being used to compare NDACC lidars at locations around the world. This will lead to more consistant datasets that can be used to quantitatively compare aerosol loading in the stratosphere.

2.4. Gaps in current research plan