LIDAR Determination of the Mixed Layer Height

An air pollution dispersion and transport model based on experiment is very essential in closely evaluating and assessing the fast degrading air quality in Metro Manila. An important parameter for this to be reliably done is the mixed layer height or the depth of the convective boundary layer (CBL).

The mixed layer or CBL is a thin layer of the atmosphere closest to the ground where pollutants are trapped. Pollutants are transported from the ground to the top of the mixed layer because of turbulence primarily caused by solar induced convection.

Traditional observations of the mixed layer are done with radiosonde, wiresonde, pilot balloon, tetheredsonde, aircraft, towers, among others. These in situ measurements have their own limitations. Towers, for example, have very limited height range, radiosondes have poor space and time resolutions and measurement of the mixed layer height cannot be done continuously. The latter is also true for an instrument aircraft. Active remote sensors like the LIDAR can be used for an extended period of time to study even the dynamics of the boundary layer. It can even be used to scan large volume of the atmosphere at a much shorter time than possible with in situ measurements. Also, in situ measurements only see a local mixed layer top that is not representative of the average over the desired area. The LIDAR technique allows for horizontal averaging and thus provides a more representative value of the mixed layer height than do in situ measurements. It is an effective technique in determining the height of the mixed layer as there is a dramatic decrease in aerosol loading above the mixed layer as can be seen in the LIDAR backscatter signal.

[Presented at the Samahang Pisika ng Pilipinas (SPP) Congress, 1999.]