Geoscience Reference
In-Depth Information
interpolation methods are available and are commonly used. The first spatial
interpolation methods used in France were made up of simple regressions between
temperature and relief. These methods would then be improved by several different
researchers, including those researchers who worked on the
Detailed Climate Map
of France
[ER30 81]. Topographic factors, such as altitude, slope, and exposure are
variables that continue to be used in these methods. In regions where there is not
such a large contrast in relief, it is common to use interpolation methods that are less
dependent on environmental factors, in other words the method of kriging is used.
The work carried out in this field takes into consideration land use, which influences
the energy exchanges that occur within the boundary layer. In this section a
statistical method of interpolation used for maximum air temperatures in Brittany,
France, will be presented. The images that were used for this particular method were
NOAA-AVHRR images and they correspond to data that were gathered on April 10,
1997 [DUB 02]. The region of Brittany has interesting topography (even if the
altitude level never exceeds 400 m), because of the existence of a particular
peninsula. By studying this unique peninsula, the combined effects of latitude and
distance from the coast can be easily explained. The diversity of land use makes it
possible to use infrared data that are produced by NOAA-AVHRR satellites. For the
purposes of our research these satellites had a spatial resolution of 1.1 km.
For this study 45 weather stations from the Météo-France network were chosen.
For a region, which has a total surface area of 27,000 km
2
, there was, on average,
one measurement site for every 600 km
2
. The study took place April 10, 1997 and
occurred under anticyclone conditions. High pressure had established itself across
North-Western Europe on this date, and all across France there were clear skies and
a slight easterly wind, which brought masses of dry continental air to the region of
Brittany. Anticyclone conditions are the best conditions for comparing and
contrasting temperatures in relation to their environment. Slight winds mean that the
air does not move and as a result this type of weather is likely to create local thermal
contrasts.
The infrared radiation measurements carried out by the NOAA satellites are used
to provide information about surface temperatures. The AVHRR radiometer, which
can be found on board the NOAA-14, measures temperatures in wavelengths
located within the ranges of 10.3 and 11.3 μm (channel 4), and between 11.5 and
12.5 μm (channel 5). It is, therefore, necessary to take into consideration the
problems that are associated with atmospheric absorption and which disrupt the
signal recorded by the satellite. This is why infrared radiation is used from channels
4 and 5 in the satellite. The split-window technique is a linear equation used to link
the temperature of an unknown surface to a particular brightness temperature
measured by the sensor. The use of this technique partially solves the problem that
is associated with atmospheric absorption. There are many different techniques and
formulae to be found in the literature [OTT 92], but for the purpose of this study the
Vidal Madjar method has been used:
Ts = T4 + 2.78(T4-T5) - 1.35
[3.1]
