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Theor. Appl. Climatol. 77, 1–7 (2004)
DOI 10.1007/s00704-004-0038-7
Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Davos Dorf, Switzerland
Greenhouse effect and altitude gradients over
the Alps – by surface longwave radiation
measurements and model calculated LOR
R. Philipona, B. Dürr, and C. Marty
With 3 Figures
Received March 27, 2003; revised January 3, 2004; accepted January 5, 2004
Published online February 25, 2004 # Springer-Verlag 2004
Summary
The greenhouse effect has been investigated predominantly
with satellite measurements, but more than 90% of the
greenhouse radiative flux affecting Earth’s surface temperature and humidity originates from a 1000 meter layer
above the surface. Here we show that substantial improvements on surface longwave radiation measurements and
very good agreement with radiative transfer model
calculations allow the clear-sky greenhouse effect be
determined with measured surface longwave radiation and
calculated longwave outgoing radiation at the top of the
atmosphere. The cloud radiative forcing is determined by
measured net longwave fluxes and added to the clear-sky
greenhouse effect to determine the all-sky greenhouse
effect. Longwave radiation measurements at different
altitudes were used to determine the clear-sky and all-sky
annual and seasonal greenhouse effect and altitude
gradients over the Alps. Linear altitude gradients are
measured for clear-sky situations, whereas the all-sky
greenhouse effect is strongly influenced by varying, cloud
amounts at different altitudes. Large diurnal and seasonal
variations show the importance of surface heating and
cooling effects and demonstrate the strong coupling of the
greenhouse effect to surface temperature and humidity.
1. Introduction
Temperature rises at the Earth’s surface during
the past decades are most likely related to an
enhanced greenhouse effect (e.g. Mitchell et al.,
1995; Tett et al., 1999; Harries et al., 2001;
Houghton et al., 2001). Observational determinations of the greenhouse effect are so far mostly
based on satellite measurements by determining
the clear-sky greenhouse effect (Raval and
Ramanathan, 1989; Ramanathan et al., 1989) and
the top of the atmosphere (ToA) cloud forcing
(Harrison et al., 1990; Stephens and Greenwald,
1991) from space. Attempts are being made to
establish correlations between ToA and surface
cloud forcing parameters, so that surface parameters can be derived from satellite observations
(Harshvardhan et al., 1990). Global distributions
of surface and atmospheric cloud forcing parameters have been derived using parameterized
radiation models with satellite meteorological
data from the Int. Satellite Cloud Climatology
Project (ISCCP), and directly measured ToA
radiative fluxes from the Earth Radiation Budget
Experiment (ERBE) (Gupta et al., 1993). Ship
and airborne radiometer measurements were
used to investigate the tropical super greenhouse
effect and to determine tropospheric profiles of
the clear-sky greenhouse effect over the equatorial Pacific Ocean (Lubin, 1994; Valero et al.,
1997).
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R. Philipona et al.
Atmospheric longwave downward radiation
(LDR) measured at the surface is a function of
the amount of greenhouse gases and the atmospheric state, and reflects the atmospheric greenhouse effect acting on the Earth’s surface.
Longwave upward radiation (LUR) is a function
of surface temperature and emissivity. Longwave
radiation measurements have been largely improved in recent years (Philipona et al., 1995;
Lorenz et al., 1996; Philipona, 2001). Two International Pyrgeometer and Absolute Sky-scanning
Radiometer Comparisons (IPASRC-I) (Philipona
et al., 2001) and (IPASRC-II) (Marty et al.,
2003) show that with optimized pyrgeometers,
uncertainty levels of 1 Wm2 or 0.4% for
LDR are reached. Furthermore, comparisons
between clear-sky measurements and radiative
transfer model calculations (MODTRAN and
LBLRTM) show differences within 1.5 Wm2
or 0.5% between absolute longwave measurements and calculated irradiances.
This paper reports on greenhouse effect determination by surface longwave radiation measurements and radiative transfer model calculations.
Investigations are based on six years of accurate
measurements at eleven stations of the Alpine
Surface Radiation Budget (ASRB) network
(Philipona et al., 1996) (latitude 46 N) at altitudes between 370 and 3580 m a.s.l.. Good agreement between clear-sky longwave downward
radiation measurements and radiative transfer
model calculations at the radiosonde station
Payerne and other ASRB stations (D€
urr et al.,
2004) encourage determining the ToA longwave
outgoing radiation by the MODTRAN radiative
transfer model. Linear relations are formulated
between measured longwave downward and
upward radiation and the clear-sky greenhouse
effect. These relations and measured seasonal
and annual clear-sky LDR values are used to
determine the clear-sky greenhouse effect. The
all-sky greenhouse effect is determined by adding the longwave cloud radiative forcing to the
clear-sky greenhouse effect.
2. Greenhouse effect and cloud
radiative forcing
The greenhouse effect (or greenhouse radiative
forcing) (G) is defined as the longwave upward
radiative flux (LUR) emitted by the Earth surface
at a certain location minus the longwave outgoing
radiative flux (LOR) escaping to space at the ToA
above that location (Raval and Ramanathan,
1989). The greenhouse effect is related to the
net radiative heating (H) of the entire surfaceto-atmosphere column. Clouds are known to prevent the escape of terrestrial longwave radiation
and change the net radiative heating. To describe
the effects of clouds on H from observations,
the cloud radiative forcing (CF) (Charlock and
Ramanathan, 1985) has been defined as CF ¼
Hall –Hclr, where ‘all’ stand for all-sky and ‘clr’
for clear-sky situations. The net radiative heating
at the top of the atmosphere (H(T)) is composed
of the net radiative heating in the atmosphere
(H(A)) and at the surface (H(S)) and the cloud
radiative forcing effect therefore becomes
CF(T) ¼ CF(A) þ CF(S) (Gupta et al., 1993).
The greenhouse effect can be determined from
space at the top of the atmosphere (G(T)) by using
satellite measurements or from the surface (G(S))
by using surface longwave radiation measurements and a radiative transfer model to calculate
LOR at the ToA. For clear-sky situations both
methods rely on the same parameters and therefore the same (Gclr) results from space and from
the surface. For all-sky situations however, the
greenhouse effect (Gall) is affected by thermal
effects of clouds and therefore the respective
longwave radiative cloud forcing (CFLW) is
added to the clear-sky greenhouse effect
Gall ¼ Gclr þ CFLW :
ð1Þ
Although the longwave cloud radiative forcing in
the atmosphere CFLW(A) is small (Harshvardhan
et al., 1990; Gupta et al., 1993), it is different
from zero. The all-sky greenhouse effect determined from the surface therefore is not necessarily the same as the greenhouse effect determined
from space. However, for surface related climate
change investigations, the cloud radiative forcing
and the greenhouse effect at the surface Gall(S)
are the relevant driving forces.
3. Longwave radiation
and greenhouse effect
Longwave downward radiation is strongly
related to temperature and humidity at the
Earth’s surface. Under clear skies more than
90% of LDR is from the first 1000 meter above
Greenhouse effect and altitude gradients over the Alps
3
Fig. 1. Clear-sky longwave downward
irradiance spectra from different layers
above the surface and percentage of
LDR and water vapour content (WC)
in these layers. The uppermost Gaussian shaped curve shows the longwave
upward radiation LUR emitted by the
Earth’s surface (surface emission plus
LDR reflection). Calculations are made
with the radiative transfer model
MODTRAN v4.2 and the Payerne
radiosonde profile of Aug 11, 1998 in
the wavelength range of 2.5 to 200 mm
the surface. MODTRAN v4.2 (Berk et al., 2000)
model calculations (Fig. 1) show that 39% of
LDR originates from the first 10 meters even
though this layer only contains 0.3% of the total
water vapor content in the atmosphere. Spectra
from the first 100 meter show that the water band
at 6.7 mm and above 25 mm as well as the 15 mm
CO2 band are already saturated around the center
of the bands. Spectra from 1 000 and 100 000
meter demonstrate that from very high altitudes,
Fig. 2. Relations of night- and daytime clear-sky greenhouse effect Gclr versus longwave downward LDR and upward LUR
radiation for Payerne and Versuchsfeld. The relations are based on clear-sky LDR and LUR measurements, and MODTRAN
calculated LOR that includes atmospheric profiles from Payerne and surface temperature inferred from measured LUR. Linear
regressions of Gclr versus LDR (left side) are finally used to determine Gclr with measured average LDR values
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R. Philipona et al.
only in the atmospheric window region between
8 mm and 13 mm, some 6% of LDR is added,
mainly in the water vapor continuum and the
9.6 mm ozone band.
As stated above, measurements agree very well
with radiative transfer model calculated clear-sky
LDR values (Philipona et al., 2001; Marty et al.,
2003). Comparisons of ASRB longwave downward radiation measurements with MODTRAN
calculations show an average difference of
0.3 Wm2 with Stdev of 3.5 Wm2 , for a subset
of 274 selected nighttime clear-sky cases at the
radiosonde station Payerne (D€
urr et al., 2004).
Hence, the good agreement between measurements and model calculations allows calculating
ToA longwave outgoing radiation with accurate
measurements of atmospheric parameters (radiosonde) and of surface longwave upward radiation.
Relations have been established between
clear-sky longwave radiation and the greenhouse
effect for four ASRB stations (Payerne 490 m,
Davos 1610 m, Versuchsfeld 2540 m and
Weissfluhjoch 2690 m), which all have downward and upward radiation measurements. About
60 nighttime and 25 daytime case studies, distributed over the year, are used with measured
LDR and LUR, and MODTRAN calculated
LOR that includes radiosonde profiles from
Payerne, station air temperature and skin temperature inferred from LUR. Gclr versus LDR relations for Payerne (490 m a.s.l.) and Versuchsfeld
(2540 m a.s.l.) are shown in Fig. 2 (left). Large
differences of LUR during nighttime emission
(cooling phase) and daytime shortwave absorption (warming phase) made it necessary to calculate separate LDR relations for day and night.
These relations are altitude dependent and correlation coefficients of 0.93 for night and 0.88 for
day relations were found. Day correlations are
lower because solar shortwave radiation produces thermal effects, which disturb the equilibrium between LUR and LDR. In contrast, Gclr
versus LUR relations (Fig. 2, right) that are
almost uniform for day and night, show much
higher correlation coefficients of 0.98 and 0.99
and have almost no altitude dependence.
4. Greenhouse effect over the Alps
Annual and seasonal mean values of the clearsky and all-sky greenhouse effect and of the
longwave cloud radiative forcing over the Alps
are based on six years of measurements of two
minute average samples at the eleven ASRBstations. Seasonal mean values are restricted to
June, July and August for summer and December,
January and February for winter mean values
respectively. Clear-sky situations are separated
from all-sky measurements using the Clear-Sky
Index (CSI) (Marty and Philipona, 2000). Since a
number of stations do not have LUR measurements the clear-sky greenhouse effect is determined for all ASRB stations with measured
average LDRclr values and the respective day or
night Gclr versus LDR linear regressions (Fig. 2,
left). For stations at which Gclr versus LDR could
not explicitly be determined, due to missing LUR
measurements, the relation of a station at similar
altitude was used. The longwave cloud forcing is
calculated using all-sky minus clear-sky longwave net radiation (Marty et al., 2002) and the
all-sky greenhouse effect is determined using
Eq. (1). In Table 1 a comparison of Gclr values
determined with LDR and with LUR relations is
shown for those stations which have LUR measurements. Differences are larger during winter
and at stations at higher altitude with snow covered surfaces. The longwave cloud radiative forcing CFLW and the all-sky greenhouse effect Gall
(Gclr from LDR measurements plus CFLW) is also
shown.
The greenhouse effect in the Alps is shown in
Fig. 3, with annual and seasonal clear-sky night
and day values (left graphs) and all-sky night and
day values (right graphs). The clear-sky greenhouse effect shows explicit altitude dependence
with high linear correlation and an annual mean
gradient of 1.5 Wm2 =100 m during nighttime
(Fig. 3, upper left). Considerable higher gradients
of up to 3.1 Wm2 =100 m, are observed during
daytime (Fig. 3, lower left). These larger gradients are primarily caused by higher daytime surface temperatures on mostly snow free surfaces
at low altitude stations. Summer and winter show
higher respectively lower greenhouse forcing but
gradients are similar for night respective day
situations. Lowest linear correlation of 0.91 was
found for daytime summer greenhouse effect,
which is due to less stable atmospheric situations
during summer time.
Negative altitude gradients are caused by
stronger decrease of LUR with altitude compared
Greenhouse effect and altitude gradients over the Alps
5
Table 1. Annual and seasonal night and day clear-sky Gclr and all-sky greenhouse effect Gall and longwave cloud forcing CFLW
at 4 ASRB-stations. For comparison the clear-sky Gclr is determined with measured LDR and LUR and the respective relations
ASRB-station
Altitude
a.s.l.
Gclr
(LDR rel.) [Wm2 ]
Gclr
(LUR rel.) [Wm2 ]
CFLW
[Wm2 ]
Year
Su
Year
Su
Year
Wi
Su
Year
Wi
Su
Wi
Wi
Gall
[Wm2 ]
Night
Payerne
Davos
Versuchsfeld
Weissfluhjoch
490 m
1610 m
2540 m
2690 m
90
69
63
55
69
51
47
43
104
90
82
75
94
68
57
66
73
42
35
53
109
92
83
87
25
24
29
42
40
19
19
35
16
27
40
45
115
93
92
97
109
70
66
77
120
117
122
120
Day
Payerne
Davos
Versuchsfeld
Weissfluhjoch
490 m
1610 m
2540 m
2690 m
161
119
105
87
121
90
77
64
184
157
142
142
157
107
81
90
101
65
53
65
184
155
128
151
56
41
44
56
61
30
30
43
38
51
54
61
216
160
149
143
181
120
107
107
222
208
196
203
Fig. 3. Nighttime (above) and daytime (below) annual and seasonal mean clear-sky (left) and all-sky (right) greenhouse effect
at ASRB-stations versus altitude. Altitude gradients and trend lines are also shown
to that of LOR. For night situations this effect is
reduced if clouds are present and lower altitude
gradients around 0.9 Wm2 =100 m result for
the all-sky greenhouse effect (Fig. 3, upper
right). Due to frequent convective clouds at
mountain peak stations this reduction is most
pronounced during summer. Gornergrat being
influenced by southern dryer weather conditions
and sitting on a high plateau remote from surrounding mountain peaks has lower cloud forcing
during the night. Daytime all-sky greenhouse
effect values (Fig. 3, lower right) increased due
to the cloud forcing with minor effect on the
gradients except for the summer gradient which
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R. Philipona et al.
lowered due to the same reason as for the night
situation. A rather high winter all-sky greenhouse
effect can be observed over Payerne, which is
due to frequent stratoform cloud layers over the
Swiss central plateau, North of the Alps.
Acknowledgments
5. Conclusions
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Authors’ addresses: Rolf Philipona (e-mail: r.philipona@
pmodwrc.ch) and Bruno D€
urr, Physikalisch-Meterologisches
Observatorium Davos and World Radiation Center,
CH-7260 Davos Dorf, Switzerland; Christoph Marty, Swiss
Federal Institute for Snow and Avalanche Research Davos
CH-7260 Davos Dorf, Switzerland.