Sigma Space MPL (Micropulse LIDAR) measures the amount and vertical distribution of volcanic ash, clouds, and other aerosols above Bariloche, Argentina airport. View Map
See below for HOW TO INTERPRET THE DATA.
HOW TO INTERPRET THE DATA
Based on the same principles as radar, the MPL transmits laser pulses into the atmosphere and receives the backscattered light.
The laser light transmitted by the MPL is polarized light. The MPL receiver measures the light scattered back from the atmosphere in two separate channels. One channel looks at the scattered light that has the same polarization as the transmitted light. This is the co-pol or co-polarization signal. The other channel looks at the scattered light whose polarization has changed, or rotated by 90 degrees, from the transmitted light. This is cross-pol or cross-polarization signal.
We are displaying two separate graphs of real-time data. One graph shows the co-pol signal. The other graph shows the ratio of the cross-pol signal to the co-pol signal, which is called the depolarization ratio. Each graph is a time series of vertical profiles with time shown along the bottom of the graph and height above the ground shown on the left side of each graph. The measured signal strength is shown by a color scale on the right side of the graphs. This signal is proportional to the amount of volcanic ash, cloud, or other aerosol particles present at a given height in the atmosphere above the airport.
Clouds and fog contain spherical liquid aerosol droplets, which scatter light without changing the polarization of the light. Such particles have a low depolarization ratio. Volcanic ash particles are non-spherical, asymmetrical in shape and when they scatter light these particles strongly change the polarization. Such particles have a high depolarization ratio.
The above figure shows a sample data set obtained on February 2nd at the Bariloche Airport. In Section A of both graphs, the MPL signal shows moderate scattering in the co-pol channel and almost no depolarization, indicating the absence of volcanic ash. In Section B of both graphs, the MPL measures strong scattering in the co-pol channel as well as a large (0.3 – 0.4) depolarization ratio. This large depolarization ratio indicates the presence of volcanic ash and gives us a vertical distribution of volcanic ash above the airport. This amount of depolarization falls in the same range as scattering seen from ice particles in cirrus clouds, which usually form above 5 km in the atmosphere. These LiDAR measurements are proportional to the amount of volcanic ash at a given height and are being calibrated for absolute accuracy.
