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COMPARATIVE TEMPERATURE RETRIEVALS BASED ON VIRTIS/VEX AND PMV/VENERA-15 COMPARATIVE TEMPERATURE RETRIEVALS BASED ON VIRTIS/VEX AND PMV/VENERA-15 RADIATION MEASUREMENTS OVER THE NORTHERN HEMISPHERE OF VENUSRADIATION MEASUREMENTS OVER THE NORTHERN HEMISPHERE OF VENUS
R. Haus (1), G. Arnold (1,2), and D. Kappel (1) (1) University Münster, Institute for Planetology, Münster, Germany; (2) DLR, Institute of Planetary Research, Berlin,
Germany [email protected]; [email protected] / Phone: +49-251-8339081)
MOTIVATION AND INTRODUCTION
The observed high variability of Venus’ nightside radiances in the 4-30 μm spectral range is due to the combined influence of spatial and temporal temperature and cloud profile changes. Time-averaged retrieved temperature profiles in the middle atmosphere at similar locations should agree within a few K between different data sources. Deviations may indicate that cloud composition and altitude distribution models are not optimal. Microphysical optical parameters of H2SO4 aerosols strongly differ at 4.3 versus 15 μm. A change of aerosol composition would modify the spectral features of optical parameters. This may result in different retrieved temperature profiles and cloud opacities and could eventually lead to different surface emissivity results.
A first step of work has focused on comparative temperature retrievals using both VIRTIS-M-IR/VEX and PMV/VENERA-15 nightside spectral radiance data in the vicinity of strong CO2 absorption bands located at 4.3 μm (VIRTIS) and 15 μm (PMV). Well-known prominent temperature structures like ‘cold collar’ and ‘hot dipole’ have been re-examined.
SCOPE METHOD SIMULATIONS
NIGHTSIDE TEMPERATURE RETRIEVAL
SUMMARY AND CONCLUSIONS
CLOUD FEATURE RETRIEVALRadiative transfer
and retrieval calculations using both thermal and
near-infraredradiation
measurements
1 3 5
4
6
DATA SELECTION2
Investigationof Venus’ cloud
features to improvesurface emissivity
retrievals
VIRA
Kliore, A.J., Moroz, V.I., Keating, G.M. (Eds.), 1985, The Venus International Reference Atmosphere. Adv. Space Res. 5(11), 1-305.
VeRa
Tellman, S., et al., (2009), Structure of the Venus neutral atmosphere as observed by the Radio Science experiment VeRa on Venus Express, J. Geophys. Res. 114, E00B36, 354-372.
Zasova, L.V., et al., (1999),Structure of the Venus middle atmosphere:Venera 15 Fourierspectrometer data revisited, Adv. Space Res.,23(9), 1559-1568.
Piccioni, G., et al., (2007),The Visible and Infrared Thermal ImagingSpectrometer, ESA SP,1295.
Selection of 32 PMV orbits(1150 spectra),northern hemisphere
Selection of 18 VIRTIS orbits (1370 spectra),northern hemisphere
VIRTIS/VEX and PMV/VENERA-15 measurements were used to
retrieve nightside temperature profiles in the atmosphere of
Venus in the northern hemisphere at altitudes 65 (55) - 90 km.
Both temperature sets do not differ by more than 7K (<80 km).
The profiles at different latitudes are consistent with VIRA and
VeRa data. Temperature differences to VIRA never exceed 10 K
and are typically below 7 K.
Temperatures between 55 and 75 km are sensitive to the location
of the cloud top. CT altitude in terms of unity cloud optical depth
at 1 μm was determined from spectrum fits in the near wings of
the corresponding CO2 bands. Lower CT altitudes imply the use of
higher total cloud column factors to keep the total cloud opacity
unchanged. (The cloud bottom altitude at 48 km was not varied).
The cloud top for latitudes below 55° is located nearly constant
at 72-74 km, but drops down to 67 km in polar regions.
Individual spectra show CT altitudes as low as 65 km.
The distant wings of the corresponding CO2 bands were used to
derive spectral changes of cloud optical depth that are required
to produce optimum fits of measured brightness spectra.
These preliminary results have to be interpreted with much care.
- The retrieved “optimum“ depths vary with latitude and there is
no systematic trend anywhere in the spectrum. The variations
may indicate spatial and temporal changes of cloud composition.
- Some of the suggested spectral changes may be due to errors
in the CO2 spectral line shape profiles (sub-Lorentz, line mixing)
and resulting temperature retrieval errors.
More work is underway that eventually could lead to improvements
of Venus‘ surface emissivity retrieval.
400 600 800 1000 2000 2200 2400 2600Wavenumber [cm-1]
30
40
50
60
70
80
90
100
110
Clo
ud O
ptic
al D
epth
10
15
20
25
30
35
Alti
tude
[km
]
Altitude of Weighting Function MaximumAltitudes of Weighting Function Half MaximumAltitude of Unity Cloud Optical DepthTotal Cloud Column Optical Depth
PMV15 µm
VIRTIS4.3 µm
u (1.0 µm) = 40.4
zunity (1.0 µm) = 73.5 km
CF 1.5
Altitudes of Unity Cloud Optical Depth and EffectiveSounding Level / Total Cloud Column Optical Depth
160 180 200 220 240 260 280 300 320 340Temperature [K]
50
60
70
80
90
100
110
Alti
tude
[km
]
Comparison of Retrieved Temperature Profiles
Individual Spectra Latitude 65.5°N
PMV Orbit 053
VIRTIS Orbit 147
InitialRetrieval Basic Run PMV Retrieval Final Run PMVRetrieval Basic Run VIRTIS Retrieval Final Run VIRTIS
20 30 40 50 60 70 80Latitude [°N]
50
60
70
80
90
Alt
itud
e [k
m]
230
350
225
PMV - Nightside temperature in the northern hemisphere
20 30 40 50 60 70 80Latitude [°N]
50
60
70
80
90
Alt
itu
de [
km]
Nightside Temperature Difference PMV - VIRTIS
1
For more detailed information on the thermal strucutre of Venus‘ nighttime mesosphere see for example
Zasova, L.V., et al. (2007), Structure of the Venus atmosphere, Planet. Space Sci., 55, 1712-1728.
Grassi, D., et al. (2010), Thermal structure of Venusian mesosphere as observed by VIRTIS-Venus Express, J. Geophys. Res., 115, E09007, 11 pp.
0 5 10 15 20 25 30
kc [cm-1 amagat-2 x 10-11]
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
Em
issi
vity
Retrieved Surface Emissivity E at 1.18 µmas a Function of Assumed Continuum Absorption Coefficient kc
retrieved kc values depend on
atmospheric temperature, but mainlyon the cloud model
retrieved surface emissivity from VIRTIS-M-IRmeasurements in the nightside emission windowsat 1.02, 1.10, and 1.18 µm strongly dependson assumed CO2 continuum absorption
Retrieved surface emissivity explicitlyand implicitly depends on cloud model
most probable kc is determined
from multi-spectrum analysesin the spectral range1.0 - 2.5 µm
400 600 800 1000 1200 1400 1600Wavenumber [cm-1]
170
180
190
200
210
220
230
240
250
260
Bri
ghtn
ess
Tem
pera
ture
[K
]
PMV - Profile Measuring Instrument for Venus
Orbit 53
Venera-15 Fourier Spectrometer
6.25 - 35.71 µm
15 µm CO2
2000 2100 2200 2300 2400 2500 2600 2700Wavenumber [cm-1]
170
180
190
200
210
220
230
240
250
260
Bri
ghtn
ess
Tem
pera
ture
[K
]
VIRTIS-M-IRVisible and Infrared Thermal Imaging Spectrometer
Orbit 147
Venus Express Spectro-Imager
4.3 µm CO2
3.70 - 5.11 µm
without radiancecorrection VIRTIS
Atmospheric temperature profiles are determined from comparisons between measured and iteratively recalculated top of atmosphere radiances and corresponding brightness temperatures (Smith, 1970)
(2) Final run corrections addi- tionally include retrieval of- cloud top altitude- spectrally constant cloud column factor- broad band variation of cloud depth spectral signatures- auxiliary downshift of VIRTIS band center radiance
400 600 800 1000 2000 2200 2400 2600Wavenumber [cm-1]
170
190
210
230
250
270
290
Bri
ghtn
ess
Tem
pera
ture
[K
]
Lat 30°Lat 60°Lat 85°Lat 30°, obs 70°Lat 30°, dayLat 30°, neglect MC's
Nightside, VIRA-T profiles, CF 1.5, obs 0°
Lat: latitude
obs: observation angle
day: dayside
MC: minor constituent
CF: cloud column factor
H2O
SO2
CO
Variation of Latitude,Observation Angle,Solar Illumination,Minor Constituents
PMV VIRTISSO215 µm 4.3 µm
160 180 200 220 240 260 280 300 320 340
Temperature [K]
50
60
70
80
90
Alt
itud
e [k
m]
Comparison of Temperature Profiles
Averages over
65 PMV spectra,
72 VIRTIS spectra,
5 VERA profiles
Averaged Profiles
Latitude 65°
VIRAVeRaPMVVIRTIS
Temperature Difference compared to VIRA
Difference [K]
0 5-5-10
VeRaPMVVIRTIS
400 600 800 1000 2000 2200 2400 2600Wavenumber [cm-1]
170
190
210
230
250
270
290
Bri
ghtn
ess
Tem
pera
ture
[K
] Variation of Total Cloud Column Optical Depth
PMV VIRTIS
CF: Cloud Column
Factor
OD: Cloud Optical
Depth at 1 µm
1.0 27.21.5 40.72.0 54.32.5 67.9
CF OD
Nightside, VIRA-T profile 30°, obs 0°
(1) Basic run only includes temperature retrieval
400 600 800 1000 2000 2200 2400 2600Wavenumber [cm-1]
160
170
180
190
200
210
220
230
240
250
260
Bri
ghtn
ess
Tem
pera
ture
[K]
MeasurementSimulation for T initialRetrieval Basic Run (1)Retrieval Final Run (2)VIRTIS uncorrected Meas.
Comparison of Measured and Retrieved Brightness Spectra
Individual Spectra Latitude 65.5°N
PMVOrbit 053
VIRTISOrbit 147
400 600 800 1000 2000 2200 2400 2600Wavenumber [cm-1]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Nor
mal
ized
Clo
ud C
olum
n F
acto
r
Retrieved spectral changes of cloud column factor CF and total optical depth OD
Averages over 773 PMV spectra, 1084 VIRTIS spectra and all latitues 20 - 80 °N
individual spectrum plots
look different !
normalized to CF=1.5
0
5
10
15
20
25
30
35
40
Nor
mal
ized
Clo
ud O
ptic
al D
epth
CF Average (CF-AV)OD Initial (CF=1.5)OD Change by CF-AV
regions of CO2 bands excluded to avoid
interference with temperature retrieval
PMV
CO2 band
15 µm
VIRTIS
CO2 band
4.3 µm
N
20 30 40 50 60 70 80
Latitude [°N]
64
66
68
70
72
74
76
Alt
itud
e [k
m]
Retrieved Location of Cloud Top Altitude as a Function of LatitudePMV retrieved at 8.3 µm ( 1200 cm-1)PMV referred to 1.0 µm (10000 cm-1)VIRTIS retrieved at 3.8 µm ( 2600 cm-1)VIRTIS referred to 1.0 µm (10000 cm-1)
Remark: The cloud top altitude
is defined as the level where
the total cloud optical depth
reaches unity values. Thus,
its definition depends on wave-
length. It is usually referred to
1.0 µm or 0.63 µm.
Use of 773 PMV spectra,
1084 VIRTIS spectra
This general trend well agrees with results
obtained by Ignatiev et al. (JGR 114, E00B43, 2009)
from VIRTIS-M-IR dayside observations at 1.6 µm.
