a. Irradiance Maps
Digital elevation models (DEM), such as the worldwide 90m SRTM satellite
models, provide a relatively detailed topographical profile of the surface
of the earth. Extracts of these files can be used to determine the vertically
projected irradiance of each raster field on a given date and daytime,
which can then be visualized on corresponding irradiance maps.
Thermals are part of the atmospheric energy flow caused by solar irradiation.
The results have shown that irradiance maps can be used to predict where
thermals are likely to occur, particularly during the morning hours
of Alpine regions.
Irradiance maps, however, show
an instant situation, whereas heating up the surface and the air causing
the thermals takes time. This becomes evident during the afternoon hours.
There temperature maps expressing how much heat has been accumulated
at a given place and time seemd to be a better indicator for the occurrence
of thermals.
Region of Aletsch glacier on June
20, 10.00h UTC: The intensities are from green via yellow to red.
The white spots are expected thermal takeoff points
b. Temperature Maps
For practical reasons TherMap computes the heat accumulation
at the surface using a relatively simple empirical smoothing algorithm,
to approximate the evolution of the surface temperature at the usual
flight altitudes. To simulate the effect of the actual orography Thermap
considers the cooling effect of calculated forest areas and seasonal
vegetation, and makes an approximation for the effect of snow and permafrost
surfaces. With these adjustments the resulting temperature maps seemed
to be more plausible predictors of the location of thermals during the
afternoon hours.
To validate and refine the maps, IGC-flight tracks have been superimposed,
using colour codes to distinguish between climbing and descending flight
phases. This way the agreement between the hotspots and their influence
on the flights could be visualized. The map extract shown below, of
a flight across the Aletsch glacier region, illustrates that the climbing
phases coincide very well with the yellow/red hotspots.
Temperature map of Aletsch glacier region on June 28,
12.00h UTC.
Note the high level of agreement with the superimposed flight path (blue
= climbing, white = sinking)
In his publications the German glider pioneer Jochen
von Kalckreuth mentioned that thermals on slopes exceeding about
25 to 30 degrees tend to climb along the slope until they reach a smaller
slope or an edge. TherMap also considers the snow and permafrost limits,
where thermals climbing along the slope meet the cold air coming from
above, an additional reason causing thermals to take off.
c.
Thermap Pressure
Irradiance and temperature maps basically show evenly distributed irradiance
or temperatures across mountain sides with constant orientation and
slope. During hot afternoon hours the maps therefore tend to be overloaded
with hot spots and too blurred for gliding purposes. This problem also
persists if all but the hottest spots are masked out. With the thermal
pressure maps, TherMap could finally overcome these weaknesses.
The idea of the thermal pressure is based on the insight
that any air "bubble" heated above a slope develops a lift
force which can be decomposed into two components, namely one along
the line of steepest ascent of the slope, and one perpendicaular to
the slope. The first one creates a static pressure along the line
of steepest ascent. This pressure is basically distributed proportional
to the slope angle, but diminishing slowly with the distance from the
original bubble, until the residual pressure falls below a critical
value or until a takeoff point is reached. The initial lift of the
bubbles is derived from the temperature maps, from which the thermal
pressure maps can then be derived in an additional computation run based
on the principles just described.
The resulting thermal pressure maps show the local
potential for thermalsare indded more precisely. The following picture
illustrates that thermal pressure maps even provide detailed results
for southward facing slopes on hot summer afternoons.
This version has therefore until now been used for the version 1.06
of TherMap.
Thermal pressure map of June 28, 12h UTC, with the same
flight track. The thermal hotspots are shown far more precisely than
on the corresponding temperature map, as one can see by moving the cursor
on the image in order to see the original map.
d. Backdrop Filter
Despite the precision of the thermal pressure maps,
one of their limitations is that the part of the areas shaded in green,
which have a lower thermal potential, is somewhat redundant. This limits
their readability which becomes particularly obvious when generating
maps for more southern regions such as the US Sierra Nevada. Since purely
topographic maps with a sun elevation of about 22 degrees are easy to
read, it turned out to be better to replace part of the less interesting
green areas of the thermal pressure maps by the corresponding topographic
backdrop view based on the same sun azimuth as the one used for the
thermal pressure, but with a constant sun elevation of 22 degrees. Using
the same azimuth ensures consistent visual information and the relatively
low sun elevation creates good shading contrasts for the topographic
backdrops. This presentation mode has therefore been adopted in TherMap
2.0.
Filtered thermal pressure map for the same region on
July 1, 13h UTC. The filtered map is more readable and contains all
the relevant information concerning the thermal pressure distribution.
Slope Maps
Slope angles and mountain ridges have s significant
influence on the thermal activities. TherMap therefore also shows slope
maps as a complementary view. Their colouring is simply determined
by the slope angle. The resulting picture differs from the usual view
and can therefore reveal aspects which may otherways remain unnoticed.
These maps also remind us of maps drawn by ancient cartographers.
Slope map of the above region
