COMPARATIVE TEMPERATURE RETRIEVALS BASED ON VIRTIS/VEX AND PMV/VENERA-15 RADIATION MEASUREMENTS OVER...

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 rainer.haus@uni-muenster.de; garnold@uni-muenster.de / 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.