Aircraft observations of marine boundary layer structure in the Monterey Bay

The marine boundary layer in the coastal zone is characterized by a quite complex structure. Atmospheric turbulence data collected in the area of Monterey Bay, California, using the CIRPAS/NPS Twin Otter aircraft during the Autonomous Ocean Sampling Network (AOSN-II) project were used to study the structure of the near shore boundary layer.

Aircraft vertical soundings were used to estimate boundary layer height. Low boundary layer heights (300-600 m) were found at offshore locations with significantly lower boundary layers (below 100 m) inside Monterey Bay. Comparison of boundary layer soundings from upwind and in Monterey Bay typically shows a collapse of the boundary layer in the Bay. This could also be the effect of an expansion fan at the north part of the Bay under northwestly wind. However, in-depth analyses of various case studies suggested that the shallow boundary layers were mainly attributed to the low wind and correspondingly low turbulence in the Bay, possibly a result of lee-wave sheltering effect of the coastal mountains.

Profiles of turbulent fluxes obtained from the vertical soundings were used to estimate the vertical flux divergence of the turbulence fluxes. This analysis was intended to relate the surface fluxes with those measured at the 30-40 m AMSL flight levels. The vertical flux divergence from the soundings was found to agree with those obtained from the equations for the mean variables applied to the horizontal distribution of measured fields near sea surface.

Spectral analysis was applied to the near sea surface aircraft data in order to examine the data quality and limitations associated with the sampling procedure. The results showed that spectra follow surface layer similarity at least in the inertial subrange. However, significant differences between along and cross wind direction were found with cross wind spectra shifted to higher frequencies. Phase spectra analysis suggested the existence of kilometer scale longitudinal rolls in the boundary layer which affect the atmospheric flow even near sea surface. This result may has significant effect on flux estimation using the eddy covariance method due to possible loss of 'energy' at low frequencies for along wind sampling when the fluxes averaging length is not large enough. An alternative inertial subrange similarity method which was used to estimate surface fluxes did not suffer from this effect as expected.