Summary:
In this dissertation the spectral characteristics of the marine atmospheric
boundary layer are investigated, using data from fast sensor measurements. The
instruments (sonic anemometers and fast hygrometers) were mounted to a 20 m
mast and were taken during the CBLAST-Low Experiment in 2003 from the
experimental site of Nantucket Island (MA). The data from continuous 28 days of
measurements with a sampling frequency of 20Hz provide time series of the three
wind velocity components, temperature and humidity.
The raw data were quality controlled and necessary corrections were made to
avoid systematic errors coming from the instrumentation and then were divided
into 10 min time intervals. A subset was created with those intervals that are
stationary and the mean wind direction is from 190-2500 (South-Southwest), so
that they correspond to a pure marine boundary layer flow. The spectra examined
are those of the three wind velocity components, of temperature and humidity.
Cospectra were also derived from the same parameters.
The analysis includes scatter diagrams between the slopes within the inertial
subrange of each spectrum and cospectrum in the 10 min time interval, and
parameters such as stability, friction velocity, drag coefficient and mean
averaged wind speed. There is a clear trend in all diagrams that shows that the
slopes become steeper in more unstable conditions, even though the vast
majority of the 10 min intervals are characterised as neutral stable to very
stable. Temperature and humidity spectra have the largest dispersion of slopes
as a function of stability. All cospectra have from slightly less steep to
significantly less steep slopes within the inertial subrange compared to those
of the spectra.. There is a clear variation in the spectral characteristics of
the marine atmospheric boundary layer compared to the spectral characteristics
of the atmospheric boundary layer over land, the spectral and cospectral slopes
in the MABL within the inertial subrange are generally slightly steeper.
Keywords:
Marine atmospheric boundary layer, Spectrum, Cospectrum, Spectral analysis, Turbulence
