10-05-2014, 11:25 AM
Determining the refractive index structure constant using high-resolution radiosonde data
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ABSTRACT
Within the framework of the Atmosphere and Climate Explorer (ACE+) radio-
occultation mission work has been carried out to determine the effects of scintillation
on its radio links. To that end a method to derive estimates of the refractive index
structure constant (Cn2) from high-resolution radiosonde data has been developed.
Data from four locations, from high to low latitudes, has been used, covering from
one up to four years of radiosonde measurements. From north to south the locations
are: Lerwick, Camborne, Gibraltar and St. Helena. A rigorous statistical analysis has
been performed, which seems to confirm the usefulness of these data to determine Cn2
with no assumptions regarding the statistics of turbulent layers.
INTRODUCTION
This work has been carried out within the context of the preparatory work of the
Atmosphere and Climate Explorer (ACE+), ESA [2004]. ACE+ proposed to use 3
radio links in occultation to determine atmospheric temperature and water vapour.
The frequencies proposed are 10, 17 and 23 GHz. The transmitter and receiver are
located on two different Low Earth Orbit (LEO) satellites.
From the attenuations measured at the receiving satellite, the water vapour and
temperature can be retrieved due to the different absorption at the three frequencies.
Since this technique uses the amplitude (or intensity) of the radio frequency
signal, measured in a finite period of time, scintillation may have an impact in the
accuracy of the estimation of the atmospheric attenuation. The time available for the
attenuation measurement is limited due to the velocity of the two satellites and the
required resolution of the temperature and water vapour profiles.
Turbulence
Richardson [1922] first proposed a qualitative description of turbulence by imagining
it as a process of decay as it proceeds through an energy cascade, in which eddies
subdivide into ever smaller eddies until they disappear by means of heat dissipation
through molecular viscosity. This cascade begins at the outer scale wavenumber, with
an eddy size equal to the outer scale length L0, and continues on until the eddies are
equal the inner scale length l0.The main energy losses occur in the energy dissipation
region, which is separated from the energy input region by the inertial range. All the
energy is thus transmitted without any significant losses through the inertial range to
the viscous dissipation region. The energy transfer through the spectrum from small to
large wavenumbers, or from large scale eddies to small-scale ones.
Radiosondes
Standard meteorological radiosondes have been used to derive Cn2 , Warnock &
VanZandt [1985], VanZandt et al [1978] and Vasseur [1999]. Radiosonde launches
are generally carried out at synoptic times (0, 6, 12 and 18 UTC) across the globe. In
more than 700 sites launches are carried out twice a day and in more than 300, four
times a day. These measurements are carried out as part of the global meteorological
network coordinated by the World Meteorological Organization.
HIGH RESOLUTION RADIOSONDE DATA
High quality, high-resolution radiosonde data is increasingly becoming available for
scientific applications. Some research organisations have started to store and archive
the full resolution data (instead of only the standard and significant levels) from the
operational radiosonde launches. The acquisition of these data is justified when the
quality and response time of the equipment and sensors is adequate.
The British Atmospheric Data Centre has been archiving the high-resolution data
of the Vaisala RS80L radiosondes performed by the UK Met Office for around twenty
sites. This data is in the Vaisala PC-CORA binary format.
These data yield values for pressure, temperature, humidity, wind speed and
direction. Wind speed and direction are not directly measured by the radiosonde.
These are calculated from the position of the sonde at successive time intervals. The
equipment used to obtain the data is the Vaisala RS80L radiosonde.
Potential Refractive Index Gradient
The potential refractive index 'vertical' gradient, M, is needed to compute the
refractive index structure constant. This is not a 'full' gradient, meaning it does not
comprise all derivatives to variables the refractive index is dependent of. This is
because the only relevant gradient, or variation of the refractive index, is the one due
to turbulence alone. To that end, the refractive index variation must be inspected in
terms of conservative additives.