Satellite based Temperature Profile Determination using Passive Microwave and Radio Occultation Instruments
Berichte aus dem Institut für Umweltphysik, Bd. 2
Axel von Engeln
ISBN 978-3-89722-453-7
217 Seiten, Erscheinungsjahr: 2000
Preis: 40.50 EUR
Stichworte/keywords: Atmosphäre , Fernerkundung , Temperaturprofil , Mikrowellen , Radio Okkultation
Abstract
This work investigates two different satellite based methods to probe
the temperature profile of the atmosphere. On the one hand, passive
microwave emissions, on the other hand, radio occultation.
The actual characteristics of the passive microwave instrument
Millimeter-Wave Atmospheric Sounder (MAS) were used for an
investigation on the possibilities of these type of instruments. The
MAS instrument operated onboard the Space Shuttle in the years 1992,
1993, and 1994. The characteristics of the radio occultation
instrument were based on the GNSS Receiver for Atmospheric Sounding
(GRAS); this instrument is currently developed by the European Space
Agency.
The investigation of the different instrument was performed with the
optimal estimation method, which uses a priori data for the
inversion process. It offers a very elegant way to characterize the
error of the retrieved parameters. The influence of the temperature
profile and the magnetic field of the Earth on the retrieval error of
the temperature are determined.
Synthetic retrieval calculations showed that the MAS instrument allows
the determination of the temperature profile between 20 and 90 km,
with a minimum error of about 2 K in the stratosphere and errors up
to 5 K in the mesosphere. The resolution varies between 4 km in the
stratosphere and 10 km in the mesosphere.
The developed retrieval algorithm was used to derive stratospheric and
mesospheric temperature profiles from actual observations of the MAS
instrument onboard of the Space Shuttle. A validation with different
temperature measuring instruments onboard the Upper Atmosphere
Research Satellite (UARS) was performed. Overall, agreement between
the instruments was found. Additionally, a validation with ground
based Lidar data was performed.
Possible improvements for a future MAS, like the antenna size, the
instrument noise, and the frequency resolution were investigated.
Mainly the reduction in the noise of the instrument results in a direct
improvement of the retrieval error at all altitude levels.
The GRAS instrument allows the determination of the temperature
profile between about 0 and 40 km, above the signal-to-noise ratio is
too low. The retrieval error is below 1 K for altitudes up
to 30 km, when water vapor is either not present or perfectly known.
Otherwise, the water vapor profile can be retrieved, leading to an
increase of the temperature retrieval error at levels where water vapor is
present. The water vapor profile can be determined up to 5 km for a
dry atmosphere and up to 8 km for a moist atmosphere. The resolution
for the temperature profile is 0.5 km throughout the troposphere and
lower stratosphere and decreases to 1 km at the middle
stratosphere. The water vapor profile resolution is 0.5 km.
The measurements of the two instruments have been combined using the
optimal estimation method to retrieve one hybrid temperature profile,
spanning an altitude interval from 0 to 90 km. The measurement
errors entering the retrieval calculation were derived from the MAS
and the GRAS instrument specifications. The actual MAS characteristics
have been modified to represent a reasonable modern passive microwave
receiver, e.g., better antenna, different system noise temperatures.
The obtained accuracy of the temperature profile depends on the chosen
a priori constraint of the retrieval calculation. A very conservative
estimate was assumed in this study, yielding a retrieval error of
around 4 K in the mesosphere, and below 1 K for
all altitudes up to 35 km.
