5 Dynamik der Atmosphäre - IAP > Microwave Physics

5 Dynamik der Atmosphäre
Die Verteilung der meisten Spezies in der Atmosphäre wird bestimmt durch chemische
Prozesse und durch Transportphänomene. Das Gebiet der Dynamik der Atmosphäre
behandelt ganz allgemein Bewegungsvorgänge in der Atmosphäre. So wird sie nicht nur
für die Bestimmung der Verteilung von Gasen, sondern besonders auch in der dynamischen Meteorologie eingesetzt, insbesondere in Modellen für die Wettervorhersage. Der
Charakter von atmosphärischen Bewegungen hängt stark von deren horizontalen Dimensionen ab. Dabei wird eine extrem mannigfaltige Skala abgedeckt, die von ca. 10−7 m
(mittlere freie Weglänge) bis zu 107 m (planetare Wellen) reichen kann. Durch die immer schnelleren Computer sind auf dem Gebiet der angewandten Atmosphärendynamik
in den letzten Jahren enorme Fortschritte erzielt worden. Nicht nur werden Wetterprognosen mittels Modellen über mehrere Tage bestimmt, sondern auch die grossräumige
Verteilung reaktiver Gase in der mittleren Atmosphäre zu bestimmen, ist möglich sowie ihre Bewegungen über Tage zu extrapolieren, oder die Herkunft von Luftpaketen
rückwärts zu berechnen. Für die Beschreibung der atmosphärischen Bewegungen wird
die Atmosphäre als Flüssigkeit betrachtet und man verwendet die klassischen Gesetze
der Hydrodynamik und der Thermodynamik. Die Zirkulation der Atmosphäre kann im
Wesentlichen mit Hilfe von drei Erhaltungssätzen beschrieben werden:
• Erhaltung des Impulses (Newton)
• Erhaltung der Masse (Kontinuitätsgleichung)
• Erhaltung der Energie (1.Satz der Thermodynamik)
Für atmosphärische Bewegungen sind primär die folgenden Kräfte von Bedeutung:
• Druckgradienten-Kraft
• Gravitationskraft
• Reibungskraft
In einem rotierenden Koordinaten-System, wie dies für die Erde der Fall ist, kommen
zusätzlich Scheinkräfte dazu:
• Zentrifugalkraft
• Corioliskraft
Da es sich bei der Atmosphärendynamik um ein komplexes Gebiet handelt, wollen wir
nur einige Grundlagen erarbeiten. Für eine ausführliche Behandlung muss auf die Literatur oder auf eine allfällige spätere Vorlesung verwiesen werden.
69
5 Dynamik der Atmosphäre
5.1 Koordinatensysteme
Es ist üblich zur Beschreibung dynamischer Prozesse in der Atmosphäre sphärische Koordinaten zu verwenden. Dabei rotiert das Koordinatensystem mit der Erde mit. Bezeichnen wir mit φ die geographische Breite, mit λ die geographische Länge und mit r
den Abstand vom Erdmittelpunkt, wobei r ≈ RE = 6370km und RE der Erdradius ist,
so erhalten wir für die Geschwindigkeitskomponenten:
u = dx/dt = RE cos ϕ
v = dy/dt = RE
dϕ
dt
w = dz/dt.
dλ
dt
(5.1)
(5.2)
(5.3)
Dabei ist u die zonale, v die meridionale Komponente und w diejenige in der Höhe.
Häufig wird als vertikale Komponente auch die Druckhöhe, die potentielle Temperatur
oder die geopotentielle Höhe genommen.
5.2 Wichtige Kräfte
5.2.1 Druck-Gradientenkraft
Unter der Wirkung eines Druckgradienten erfährt Luft eine Kraft, die sog. DruckGradientenkraft, F~p :
1~
(5.4)
F~p = − ∇p.
ρ
Diese Kraft steht senkrecht zu den Isobarenflächen und zeigt vom höheren zum tieferen
Druck. Der Abstand der Isobaren gibt ein Mass für die Grösse des Luftdruckgradienten.
Die Wirkung der vertikalen Komponente wird sozusagen dadurch eliminiert, dass der
Luftdruck mit der Höhe abnimmt, so dass hydrostatisches Gleichgewicht herrscht. Typische Werte für den Druckgradienten sind: in vertikaler Richtung ca. 1mb auf 8m und
in horizontaler Richtung ca. 1mb auf 10km!
5.2.2 Corioliskraft
Jede Masse, die sich in einem mit der Winkelgeschwindigkeit ω
~ rotierenden Bezugssystem
mit der Geschwindigkeit ~v relativ zu diesem System bewegt, erfährt eine Scheinkraft,
genannt Corioliskraft, F~c , pro Masseneinheit:
F~c = 2~v × ω
~.
(5.5)
Bezogen auf die Erde ist ω = Ω = 2π/Sterntag = 7.292·10−5 sec−1 . Für die Corioliskraft
pro Einheitsmasse erhalten wir bei zonaler Geschwindigkeit, u, und bei der geografischen
Breite ϕ
dv
= 2Ωu sin ϕ
(5.6)
dt
70
5.2 Wichtige Kräfte
und analog bei meridionaler Bewegung
du
= −2Ωv sin ϕ.
dt
Dabei bezeichnet man mit f den Coriolisparameter
f = 2Ω sin ϕ.
(5.7)
(5.8)
f ist am Aequator gleich null und an den Polen f = ±1.46 · 10−4 sec−1 .
Die Corioliskraft wirkt senkrecht auf die Bewegungsrichtung und zwar nach rechts auf
der nördlichen Halbkugel und nach links auf der südlichen Halbkugel. Da die Corioliskraft immer senkrecht auf die Bewegungsrichtung wirkt, kann sie keine Arbeit verrichten
und kann daher die Bewegungsenergie eines Luftpakets nicht verändern. Die Corioliskraft
ist am grössten an den Polen, am Aequator verschwindet sie. Die horizontale Komponente lässt sich auch schreiben mit
F~c = −f ~k × ~v .
(5.9)
H
Dabei ist ~k ist der Einheitsvektor in z-Richtung.
5.2.3 Reibungskraft
Der für die Dynamik der Atmosphäre wichtigste Fall von Reibung ist die Bodenreibung,
die sich in der sog. planetaren Grenzschicht (ungefähr bis 1km über den Boden) auswirkt.
In diesem Bereich ist die Reibungskraft von ähnlicher Grösse wie die anderen Komponenten in horizontaler Richtung. Die Reibungskraft ist proportional der Geschwindigkeit
und es gilt
F~R = −a~v .
(5.10)
5.2.4 Allgemeine Bewegungsgleichung
Aus der Summe der betrachteten Kräfte erhalten wir die allgemeine Bewegungsgleichung
für ein Luftvolumen
d~v
= F~p + F~c + F~R + F~G
(5.11)
dt
resp.
1~
d~v
~ × ~v − a~v + ~g .
= − ∇p
− 2Ω
(5.12)
dt
ρ
v
Der Beschleunigungsterm d~
kann aufgespalten werden in eine rein zeitliche Geschwindt
digkeitsänderung bei festgehaltenem Ort, d.h. ∂~v /∂t, und in eine Feldbeschleunigung.
Dieser zweite Term kommt dadurch zustande, dass ein Luftpaket bei seiner Bewegung
in einem gegebenen Geschwindigkeitsfeld an einen Ort gelangt, wo die Strömung eine
andere Geschwindigkeit hat. Es gilt allgemein
d
∂
~
=
+ ~v · ∇.
(5.13)
dt
∂t
Durch diese Feldbeschleunigung wird die Bewegungsgleichung quadratisch in der Geschwindigkeit.
71
5 Dynamik der Atmosphäre
5.2.5 Geostrophischer Wind
Die Bewegungsgleichung für eine horizontale Bewegung lautet
d~v
= F~p + F~cH + F~R
dt
1~
= − ∇p
− f ~k × ~v − a~v .
ρ
(5.14)
(5.15)
Oberhalb einer Höhe von etwa 1km kann die Reibung vernachlässigt werden, so dass nur
Druckgradienten- und Corioliskraft wirken. Ein Luftpaket sei anfänglich in Ruhe. Infolge
eines Druckgradienten wird es beschleunigt, was aber sofort eine Corioliskraft bewirkt
und zu einer Ablenkung nach rechts (auf der Nordhalbkugel) führt. Die Geschwindigkeit
erhält also eine Komponente senkrecht zum Druckgefälle bis Gleichgewicht herrscht. Es
gilt dann
1~
(5.16)
f ~k × ~v = − ∇p.
ρ
Durch Multiplikation mit ~k von links erhalten wir
1~ ~
~
~
k × ∇p.
−k × k × ~v = ~v =
fρ
(5.17)
Die so erhaltene Geschwindigkeit, v~g , nennen wir geostrophischer Wind
v~g =
1 ~ ~
k × ∇p.
ρ·f
(5.18)
Der geostrophische Wind weht parallel zu den Isobaren. Auf der Nordhalbkugel liegt
dabei der tiefere Luftdruck links von der Bewegungsrichtung. Ein Tiefdruckgebiet wird
in derselben Richtung umweht, wie die Erde rotiert (zyklonaler Wind). Geostrophischer
Wind um ein Hochdruckgebiet ist ergo antizyklonaler Wind. Je enger die Isobaren, desto stärker der Wind. Da der geostrophische Wind senkrecht zum Gefälle des Luftdrucks
weht, kann er Druckunterschiede nicht ausgleichen. Reibung in der planetaren Grenzschicht bewirkt, dass die Windgeschwindigkeit subgeostrophisch wird. Auch im stationären Fall gibt es nun eine Komponente in Richtung des Druckgefälles und es können
somit Druckunterschiede ausgeglichen werden.
5.2.6 Primitive Gleichungen
Unter den primitiven Gleichungen versteht man die grundlegenden Gleichungen, die
für die Beschreibung der zeitlichen Entwicklung atmosphärischer Bewegung benötigt
werden. Diese Gleichungen sind eine Synthese aus der allgemeinen Bewegungsgleichung
nach Newton
1~
d~v
~ × ~v − a~v + ~g ,
− 2Ω
(5.19)
= − ∇p
dt
ρ
der Gasgleichung
p = ρRT,
(5.20)
72
5.2 Wichtige Kräfte
dem ersten Hauptsatz der Thermodynamik
dT
R T dp Q̇
=
+
dt
cp ρ dt cp
(5.21)
und der Kontinuitätsgleichung
∂p
~ · ~v = 0.
+ ρ∇
∂t
Das ganze Set der Gleichungen in sphärischen Koordinaten lautet
(5.22)
du
uv
1 ∂φ
=
tan ϕ + 2Ω sin ϕv −
+ Fλ
dt
R
R cos ϕ ∂λ
dv
u2
1 ∂φ
= − tan ϕ − 2Ω sin ϕu −
+ Fϕ
dt
R
R ∂ϕ
∂φ
RT
=
∂z
H
(5.24)
dT
RT
Q̇
+w
=
dt
cp H
cp
(5.26)
1 ∂u
1 ∂(v cos ϕ) 1 ∂ρ · w
+
+
=0
R cos ϕ ∂λ R cos ϕ
∂ϕ
ρ ∂z
(5.27)
d
∂
u
∂
v ∂
∂
=
+
+
+w .
dt
∂t R cos ϕ ∂λ R ∂ϕ
∂z
(5.28)
(5.23)
(5.25)
Mit F sind die Komponenten der Reibungskraft gemeint.
73
Constants and Conversions for Atmospheric Science
Universal constants
Universal gravitational constant
Universal gas constant in SI units
Gas constant in chemical units
Speed of light
Planck’s constant
Stefan-Boltzmann constant
Constant in Wien’s displacement law
Boltzmann’s constant
Avogadro’s number
Loschmidt number
G
R∗
(Rc )∗
c
h
σ
λmax T
k
NA
L
=
=
=
=
=
=
=
=
=
=
6.67 × 10−11 N m2 kg−2
8.3143 J K−1 mol−1
0.0821 L atm K−1 mol−1
2.998 × 108 m s−1
6.626 × 10−34 J s
5.67 × 10−8 J s−1 m−2 K−4
2.897 × 10−3 m K
1.38 × 10−23 J K−1 molecule−1
6.022 × 1023 molecules mol−1
2.69 × 1025 molecules m−3
ρ0
Rd
Md
cp
cv
g/cp
K
=
=
=
=
=
=
=
1.25 kg m−3
287 J K−1 kg−1
28.97 kg kmol−1
1004 J K−1 kg−1
717 J K−1 kg−1
9.8 × 10−3 K m−1
2.40 × 10−2 J m−1 s−1 K−1
ρwater
ρice
Rv
Mw
ε
cpw
cvw
cw
ci
Lv
=
=
=
=
=
=
=
=
=
103 kg m−3
0.917 × 103 kg m−3
461 J K−1 kg−1
18.016 kg kmol−1
Mw /Md = 0.622
1952 J deg−1 kg−1
1463 J deg−1 kg−1
4218 J K−1 kg−1
2106 J K−1 kg−1
2.50 × 106 J kg−1
2.25 × 106 J kg−1
2.85 × 106 J kg−1
3.34 × 105 J kg−1
Air
Typical density of air at sea level
Gas constant for dry air
Effective molecular mass for dry air
Specific heat of dry air, constant pressure
Specific heat of dry air, constant volume
Dry adiabatic lapse rate
Thermal conductivity at 0◦ C
(independent of pressure)
Water substance
Density of liquid water at 0◦ C
Density of ice at 0◦ C
Gas constant for water vapor
Molecular mass for H2 O
Molecular weight ratio of H2 O to dry air
Specific heat of water vapor at constant pressure
Specific heat of water vapor at constant volume
Specific heat of liquid water at 0◦ C
Specific heat of ice at 0◦ C
Latent heat of vaporization at 0◦ C
Latent heat of vaporization at 100◦C
Latent heat of sublimation (H2 O)
Latent heat of fusion (H2 O)
Ls
Lf
Constants and Conversions for Atmospheric Science (continued)
Earth and Sun
Acceleration due to gravity at sea level
Mass of the Earth
Mass of the Earth’s atmosphere
Radius of the Earth
Area of the surface of the Earth
Mass of an atmospheric column
Atmosphere to Pascals
Rotation rate of Earth
Mass of the sun
Radius of the sun
Mean earth-sun distance
Solar flux
Average intensity of solar radiation
ma
1 atm
Ω
m
r
d
Es
Is
=
=
=
=
=
=
=
=
=
=
=
=
=
9.81 N kg−1
5.97 × 1024 kg
5.3 × 1018 kg
6.37 × 106 m
5.10 × 101 4 m2
1.017 × 104 kg m−2
1.01325 × 105 Pa
7.292 × 10−5 s−1
1.99 × 1030 kg
6.96 × 108 m
1.50 × 1011 m = 1.00 AU
3.85 × 1026 W
2.00 × 107 W m−2 sr−1
TC
TK
1 hPa
1 m3
1d
1 cal
1◦ lat
1◦ lon
1 knot
1 m s−1
1 Sv
1 DU
=
=
=
=
=
=
=
=
=
=
=
=
5
9
g0
m⊕
me
RE
Units and Conversions
Fahrenheit-Celsius conversion
Kelvin-Celsius conversion
Hectopascal conversions
Cubic meters to liters
Days to seconds
Calories to Joules
Latitude conversions
Longitude conversions
Knots to miles per hour
Meters per second to knots
Sverdrups to m3 s−1
Dobson unit
(TF − 32)
TC + 273.15
1 mb = 103 dynes cm−2
1000 L
86,400 s
4.1855 J
60 nautical mi = 111 km = 69 statute mi
111 km × cos(latitude)
1 nautical mi/h = 1.15 statute mi/h
1.9426 kt
106 m3 s−1
2.6 × 1016 molecules O3 cm−2
http://www.grss-ieee.org/menu.taf?menu=Publications&detail=newsletter
Cumulative Issue #140
September 2006
ISSN 0274-6338
Editor: Adriano Camps
UNIVERSITY PROFILE
INSTITUTE OF APPLIED PHYSICS, UNIVERSITY OF BERN,
SWITZERLAND MICROWAVE REMOTE SENSING OF THE ATMOSPHERE
Niklaus Kämpfer, Christian Mätzler, Emmanuel Brocard, Dietrich Feist, Alexander Haefele, Klemens
Hocke, Lorenz Martin, June Morland, Stefan Müller, Marc Schneebeli
Sidlerstr. 5, CH-3012 Bern, Switzerland
http://www.iapmw.unibe.ch
E-mail: [email protected]
1. Introduction
The Institute of Applied Physics (IAP) is a member of the
Faculty of Natural Sciences at the University of Berne,
Switzerland. It participates in the teaching activities of the
Faculty and conducts research in the field of electromagnetic radiation from microwave to X-rays and the investigation of interaction mechanisms of radiation with matter.
The methodologies range from the generation of intensive
monochromatic light to very sensitive methods for the
detection of submillimeter waves and to photon counting.
Scientists working in this field of research are supported
by technical and administrative staff leading to a total of
approx. 70 persons working at the IAP. The research projects are manifold and range from environmental studies to
applications of laser light in medicine or information technology. Research is mostly carried out in cooperation with
other national and international institutes as well as with
partners from industry.
Since many years the department of microwave physics
at the Institute of Applied Physics, University of Bern, has
been active in the field of remote sensing of the environment. Radiometers at microwave frequencies and up to the
sub-mm region have been developed and successfully
operated from the ground, from aircraft and from space in
order to determine the altitude distribution of constituents
like water vapour, ozone, chlorine monoxide and others.
Research in the field of water vapour and ozone in the middle atmosphere has become a key research topic at the IAP
in recent years. During the last years we were successful in
designing, optimizing and operating instruments for the
measurement of water vapour. An overview of our expertise in this field is given below.
2. Water vapour
2.1. The role of water vapour
Water vapour, H2O, in the middle atmosphere plays a multiple role. It is active chemically, radiatively, and acts as a
tracer for dynamical processes. As a consequence any
change in its abundance may have different impacts. H2O
is the primary source of the OH radical
and thus is involved in a large number of
chemical processes, including ozone
depletion. The formation of Polar
Stratospheric Clouds, PSC, is affected by
the amount of water vapour. Finally, H2O
determines the temperature and hence the
circulation of the middle atmosphere, the
stratosphere and mesosphere, by contributing to their long wave cooling.
Results of the investigations of several
authors suggest that increasing H2O has
made a significant contribution to the
recent downward trend in stratospheric
temperatures and that further cooling
from water vapour might play an important role in future climate change and
ozone evolution. Since CH4 oxidation
presently contributes approximately half
Figure 1. Block diagram of the middle atmospheric water vapour radiometer at 22 GHz of the peak H2O abundance in the middle
IEEE Geoscience and Remote Sensing Society Newsletter • June 2006
13
atmosphere, it can be concluded that any significant
change in tropospheric CH4 amount will, in turn cause an
increase of H2O. The effect of such an increase in tropospheric methane on the middle atmosphere would be a cooling that would be accompanied by a strengthened general
circulation, intensified dynamic heating, and a reduction in
the mean age of middle-atmospheric air. An increase of
water vapour, together with a decrease in temperature, will
also affect ozone chemistry that again affects temperature.
Ozone depletion and global warming might interact to produce rapid climate change.
Despite the fact of the importance of water vapour for
atmospheric chemistry and physics the available data sets
are relatively sparse. Measurements from the ground in the
middle atmosphere have so far only been possible by
microwave remote sensing techniques. In situ measure-
ments by balloons are difficult and limited to altitudes
below approx. 25 km. However water vapour has been
measured since several years by satellite sensors, mainly
HALOE and SAGE and more recently MLS on AURA.
2.2. Microwave remote sensing
of water vapour
Water vapour in the middle atmosphere
Middle atmospheric water vapour is measured with
microwave radiometers that detect the pressure-broadened
emission lines of the species under investigation. By
means of a retrieval technique it is possible to estimate the
altitude distribution from the measured spectra. We dispose of two instruments for the measurement of H2O-spectra. One at 183 GHz called AMSOS (Airborne Microwave
Stratospheric Observing System) and another one at 22
GHz, called MIAWARA (MIddle Atmospheric WAter
vapour RAdiometer). The rotational transition at 183 GHz
has much higher line intensity than the one at 22 GHz that
makes it particularly suitable for observations at high altitudes or from aircraft. However from low altitudes such as
from Bern a transition with low opacity has to be chosen in
order to be able to ”see“ through the troposphere.
MIAWARA: Middle Atmospheric Water
Vapour Radiometer
Figure 2. MIAWARA operated at Bern University. Clearly visible is
the big corrugated horn antenna (6° HPBW), the flat mirror for
pointing in different directions, big Dewar for total power calibration with liquid nitrogen.
Figure 3. Profiles of the volume-mixing ratio of water vapour in ppm
measured at Bern by the MIAWARA instrument
14
MIAWARA has been optimized for high sensitivity, reliability and ease of use also at remote places. A block diagram of MIAWARA is given in Figure 1 and an impression of the instrument gives Figure 2. The system temperature of this uncooled radiometer is 135K. The instrument
is equipped with two spectrometers with different bandwidth and resolution for the retrieval from the lower
stratosphere up the upper mesosphere. The calibration of
MIAWARA is done with so-called tipping-curve and balanced calibration schemes. With the use of tipping-curve
calibrations the instrument operates as a stand-alone
instrument. Due to its large altitude coverage of 20-70 km
MIAWARA is also a valuable ground-based instrument for
satellite validations. An example of profiles obtained in
the last two years is shown in Figure 3. Since autumn
2005 MIAWARA has been part of the NDACC (former
NDSC), the Network for the Detection of Stratospheric
Composition Change.
An excellent chance to test the campaign suitability of
MIAWARA was offered in January / February 2004 when
we were invited to participate in the field campaign called
LAUTLOS (Lapland upper tropospheric lower stratospheric water vapour validation project) in Sodankylä
(northern Finland) where water vapour was measured with
IEEE Geoscience and Remote Sensing Society Newsletter • June 2006
different balloon sensors up to altitudes of approx. 25 km.
Data from MIAWARA could thus complement the balloon
data for altitudes up to the mesopause. Probably for the
first time ever it was possible to measure water vapour
profiles from the ground to 80 km at the same place.
AMSOS: Airborne Millimeter and Submillimeter
Observing System
For several years, our institute performed measurements
of water vapour in the middle atmosphere with an airborne
microwave radiometer. In collaboration with the Swiss Air
Force we have the opportunity to use a Lear jet for flight
campaigns (Cover figure, top). Pressure broadened spectra
as measured at different latitudes are shown in Figure 4.
As the tropopause in the tropics is located at much higher
altitudes than in polar regions the contribution in the
wings is higher in tropical regions in contrast to polar
spectra. Since 1998, the instrument has taken part in several campaigns on an almost yearly basis but during different seasons. An impression of the latitudinal water
vapour distribution as measured in the campaign in 2002
is given in Figure 5.
The most mobile and delicate sector of the global water
cycle is tropospheric water, some 99% of which consisting
of water vapour. Called humidity in meteorological terms,
water vapour is a significant greenhouse gas and has a short
residence time of about 1 week in the troposphere. Therefore
water vapour is not well mixed, but shows the temporal and
spatial structure of atmospheric turbulence. The vertically
integrated mass of water vapour (IWV) is about 20 kg/m2.
Clouds, consisting of small water drops or ice crystals,
appear like the foam in a glass of beer; they are subject to
constant changes between droplet evaporation, formation,
growth through various processes, and precipitation. Still,
Water vapour in the troposphere
In contrast to research of water vapour in the middle
atmosphere, research in the field of water vapour in the troposphere addresses different questions, the link, however,
being related to climate issues.
Figure 5. Latitudinal survey of water vapour in autumn 2002 with
superimposed values of the potential vorticity indicating the location
of the polar vortex. The upper part of the troposphere in tropical
regions is clearly seen.
Figure 4. Typical spectra measured with AMSOS at different latitudes with different contributions of water vapour at upper tropospheric levels.
Figure 6. Absorption as a function of frequency for cloud liquid,
water vapour and oxygen in the microwave part of the spectrum.
Indicated are channels used by ASMUWARA for the retrieval of
water vapour column density and profiles in the troposphere, of column amount of liquid water as well a of temperature profiles in the
troposphere.
IEEE Geoscience and Remote Sensing Society Newsletter • June 2006
15
Figure 7. Example of zenith mapped integrated water vapour measurements in kg m-2 of the whole sky. Black points symbolise measurement points.
Figure 8. Maps of integrated liquid water (left) and corresponding photographs of the sky. The values are zenith mapped and the colouring
corresponds to the range from 0 kg m-2 (blue) to 0.5 kg m-2 (white)
16
clouds produce strong radiative feedbacks and therefore are
important for atmospheric processes and climate.
Several instruments are operated at IAP for the detection
of tropospheric water vapour and liquid water in the frame
of the project STARTWAVE (STudies in Atmospheric
Radiative Transfer and Water Vapour Effects). This project
aims to investigate the role which water vapour plays in the
climate system, and in particular its interaction with radiation. Within this framework, an ongoing water vapour database project was set up which comprises integrated water
vapour (IWV) measurements made over the last ten years
by ground-based microwave radiometers, Global
Positioning System (GPS) receivers and sun photometers
located throughout Switzerland at altitudes between 330
and 3584 m. At Bern (46.95? N, 7.44? E) tropospheric and
stratospheric water vapour profiles are obtained on a regular basis and integrated liquid water, which is important for
cloud characterisation, is also measured. A key instrument
in this project is an all sky scanning and multiwavelength
radiometer called ASMUWAR (Cover figure, bottom). It
detects sky radiation at frequencies from 20 to 150 GHz
plus in addition is equipped with sensors at two infrared
wavelength bands for cloud temperature. Frequencies from
18 to 31 GHz are used for the detection of water vapour and
cloud liquid whereas the cluster of frequencies around the
oxygen line at 60 GHz is used to retrieve the temperature
profile (Figure 6).
Hemispheric images of ASMUWARA allow the detection of atmospheric structures in water vapour and clouds.
An example is shown in Figures 7 and 8.
A further objective of ASMUWARA is to make use of
information about the dynamics of the atmosphere as seen
in the different spectral bands of thermal radiation. The
simplest dynamics is a vertical profile of the horizontal
velocity field, assuming Taylor’s assumption of ‘frozen’
air parcels. First, the idea was to use time series of full sky
images. Unfortunately, due to the long scan duration of 15
minutes, the correlation between successive views was
often poor. Therefore we decided to limit the scan to the
main wind direction (zenith angle range from -45° to
+45°) with a repetition rate of 2 min, including calibration. An example of a time series of such scans for narrowband infrared data is given in Figure 3 showing clouds
at different altitudes as expressed by the different temperatures. Straight lines represent constant horizontal motion.
The steeper the line the larger is the angular velocity. The
tilts of selected structures are calculated by cross correlation over given time periods, and by the application of a
known temperature profile the angular velocities are
transformed to a wind profile. A retrieval using the data of
Figure 9 is shown in Figure 10. The wind speed at
IEEE Geoscience and Remote Sensing Society Newsletter • June 2006
Bantiger indicates that the wind field was stronger at Bern
than at Payerne, which would explain our higher values.
Although this application is very new and thus preliminary, the potential is quite apparent. At present the
microwave radiometer data are too noisy to add significant
information, but future investigations will try to make use
of other channels as well.
3. Ozone
Since more than a decade a microwave radiometer has been
operated at the University of Bern in the frame of the
NDACC (Network for the detection of atmospheric composition change) for the detection of middle atmospheric
ozone. The instrument is measuring the emission line of
ozone at 142.17504 GHz in a total power mode. Spectra
are obtained every 10 seconds and are then integrated providing ozone profiles in the altitude range from approx. 20
– 70 km with a time resolution of 30 minutes. Based on the
measurements performed on a regular basis since 1994 at
Bern a long time series of ozone profiles is now available
that allows investigating trends at different altitudes. An
impression of the available data is given in Figure 11. Our
data are provided to the NDACC database where they are
accessible to researchers worldwide
(http://www.ndacc.org/).
Figure 9. Time series of narrowband IR radiometer (9.6 to 11.5 mm)
data of ASMUWARA.
5.Summary
Microwave remote sensing is a powerful tool for providing information of the state of the atmosphere, valuable
data that is needed in atmospheric and climate research.
The altitude distribution of trace gases such as ozone or
water vapour can be inferred from pressure-broadened
emission lines with a time resolution of the order of minutes to hours and almost independent on weather conditions. Microwave radiometers also provide information
about the column density of water
vapour and liquid water with high time
resolution. By special scanning techniques it is possible to retrieve sky maps
representing these parameters of interest. The Institute of Applied Physics at
the University of Bern, Switzerland, is
devoted to microwave remote sensing of
the atmosphere from the ground and aircraft since many years. Details about the
institute and its activities can be found
on the web at http://www.iap.unibe.ch/.
A downloading area of relevant publications from researcher at the IAP can be Figure 11. Volume
Bern, Switzerland.
found on this webpage as well.
Figure 10. Wind profile above Bern (blue points with error bars,
height steps corresponding to temperature steps of 11 K) retrieved
from the data of Figure 3, in comparison with the radiosonde wind
profile from Payerne (green) and wind speed from the nearby TV
tower Bantiger (magenta).
mixing ratio (VMR) profiles in parts per million of ozone over
IEEE Geoscience and Remote Sensing Society Newsletter • June 2006
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