Predicting vertical concentration profiles in the marine atmospheric
boundary layer with a Markov chain random walk model
Abstract
In an effort to better represent aerosol transport in meso- and
global-scale models, large eddy simulations (LES) from the NCAR
Turbulence with Particles (NTLP) code are used to develop a Markov chain
random walk model that predicts aerosol particle vertical profiles in a
cloud-free marine atmospheric boundary layer (MABL). The evolution of
vertical concentration profiles are simulated for a range of aerosol
particle sizes and in a neutral and an unstable boundary layer. For the
neutral boundary layer we find, based on the LES statistics, that there
exist temporal correlation structures for particle positions, meaning
that over short time intervals (T= 500 s, or T/Tneut= 0.25), particles
near the bottom of the boundary are more likely to remain near the
bottom of the boundary layer than being abruptly transported to the top,
and vice versa. For the unstable boundary layer, a similar time interval
of T= 500 s (T/Teddy= 0.39) exhibits weaker temporal correlation
compared to the neutral case due to the strong non-local convective
motions. In the limit of a large time interval, T= 2000 s (T/Teddy=
1.56), particles have been mixed throughout the MABL and virtually no
correlation exists. We leverage this information to parameterize a
Markov chain random walk model that accurately predicts the evolution of
vertical concentration profiles for the range of particle size and
stability tested in LES, even over short time intervals which exhibit
substantial correlation. The new methodology has significant potential
to be applied at the subgrid level for coarser-scale weather and climate
models.