Using land-based stations for air–sea interaction studies
In situ measurements representing the marine atmosphere and air–sea interaction
are taken at ships, buoys, stationary moorings and land-based towers, where
each observation platform has structural restrictions. Air–sea fluxes are often
small, and due to the limitations of the sensors, several corrections are applied.
Land-based towers are convenient for long-term observations, but one critical
aspect is the representativeness of marine conditions. Hence, a careful analysis
of the sites and the data is necessary. Based on the concept of flux footprint, we
suggest defining flux data from land-based marine micrometeorological sites in
categories depending on the type of land influence:
1) CAT1: Marine data representing open sea,
2) CAT2: Disturbed wave field resulting in physical properties different
from open sea conditions and heterogeneity of water properties in the footprint
3) CAT3: Mixed land–sea footprint, very heterogeneous conditions and
possible active carbon production/consumption.
Characterization of data would be beneficial for combined analyses using
several sites in coastal and marginal seas and evaluation/comparison of
properties and dynamics. Aerosol fluxes are a useful contribution to
characterizing a marine micrometeorological field station; for most conditions,
they change sign between land and sea sectors.
Measured fluxes from the land-based marine station Östergarnsholm are used as
an example of a land-based marine site to evaluate the categories and to present
an example of differences between open sea and coastal conditions.
At the Östergarnsholm site the surface drag is larger for CAT2 and CAT3 than
for CAT1 when wind speed is below 10 m/s. The heat and humidity fluxes show
a distinctive distinguished seasonal cycle; latent heat flux is larger for CAT2 and
CAT3 compared to CAT1. The flux of carbon dioxide is large from the coastal
and land–sea sectors, showing a large seasonal cycle and significant variability (compared to the open sea sector). Aerosol fluxes are partly dominated by sea
spray emissions comparable to those observed at other open sea conditions.