Planetary boundary layer (PBL) depth is important to a wide range of atmospheric processes including cloud formation, aerosol mixing and transport, and chemical mixing and transport. The CAPI working group has proposed development of an ARM PBL height VAP to make this important value available to the ARM and broader scientific community.
Numerous instruments and algorithms may be used for PBL height detection, each with their own strengths and weaknesses. We plan to implement several different algorithms for PBL height detection using different instruments as the comparisons between the different methods will provide useful information about the reliability of the PBL height retrievals. We will develop this VAP in a phased approach, in which with we start with relatively simple algorithms and successively add new instruments/algorithms. Additionally, we will initially focus on the convective daytime boundary layer, as it is more easily detected and algorithms are more reliable than for the stable nighttime boundary layer.
In the first phase of this VAP (covered by this ECR), we will focus on implementing methods for radiosondes, ceilometer, and MPL as these instruments exist at all ARM sites. Two radiosonde methods (Heffter and bulk Richardson) and three ceilometer/MPL algorithms (1D gradient, wavelet, combined 2D gradient and wavelet) have been identified. More details on these methods are given in the implementation plan. Code for these algorithms will be provided by ARM scientists, and will be implemented in ISDE. In the next stage of development, which will be defined in more detail at a later date, we will explore implementing retrievals for more advanced instrumentation (AERI, Raman Lidar, Doppler Lidar, Radar wind profilers) that do not exist at all sites.
Initial evaluation of the VAP will focus on several IOP periods: MC3E, Azores, CARES, and AMIE. As part of the VAP development, we will also develop a database of PI PBL height retrievals for these IOP periods. We do not plan to run a formal inter-comparison, but will use this database to 1) evaluate the implementation of the VAP methods, 2) identify promising methods to implement in the next state of development, 3) identify areas of uncertainty and methods of flagging PBL height retrievals, and 4) encourage science PIs to submit their retrievals as formal ARM PI products.
A draft PBL height implementation plan has been developed and is available from Sally upon request.