Evaluation of the fast visible RT model RTTOV-MFASIS and...
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1 Comparison of simulated and observed reflectances Reflectance based on ICON forward simulation (left) shows similar cloud patterns as observation (middle). Largest errors occur where model cloud fields differ (right). Less cloud struture is visible in the model equivalent because the MFASIS forward simulation lacks 3D RT effects, e.g., due to cloud-top inclination (see L. Scheck et al., 11p.07). A correction to approximately account for cloud-top inclination in MFASIS is planned for RTTOV v13. The reflectance histogram (left) shows that the overall shape of the observed reflectance curve is reproduced in model calculations. Observation minus first guess departures (middle) exhibit a Gaussian shape, with clear-sky contributions promoting a peak around the mean. Contributions from different atmospheric situations are quantified (right). Here, the model equivalents have been classified by applying thresholds on the total ice and water optical depths in the vertical profile. 2 Evaluation: RTTOV-MFASIS versus RTTOV-DOM Greenland profile: lat = 75, lon = -40 ICON elev. 3km RTTOV and libRadtran forward calculations for SEVIRI 0.6μm on METEOSAT 11, 3 July 2019, 12 UTC Research and Development, Deutscher Wetterdienst, Offenbach, Germany [email protected] Christina Stumpf, Leonhard Scheck, Christina Koepken-Watts, Olaf Stiller, Liselotte Bach, Roland Potthast Evaluation of the fast visible RT model RTTOV-MFASIS and use for model cloud validation of ICON This work studies the fast RT method MFASIS for the simulation of visible satellite images available in RTTOV v12.2 and later versions. MFASIS is a lookup-table (LUT) based method (see L. Scheck et al., 11p.07). The LUTs are trained using the more accurate, 1D RT method RTTOV-DOM and are provided for SEVIRI, ABI and AHI via www.nwpsaf.eu. Here, we evaluate RTTOV-MFASIS results for the visible SEVIRI channels based on global ICON model fields by comparing them to RTTOV-DOM results in a suitable test setup and to SEVIRI observations. These investigations pave the way for further updates to MFASIS and the validation of model cloud fields. First data assimilation experiments using MFASIS in convection-resolving models have demonstrated its value by improving the representation of cloud cover and precipitation (see L. Scheck et al., 11p.07; L. Bach et al., 9.04). no BRDF values (polar night) Differences (MFASIS-DOM) for SEVIRI 0.6μm on METEOSAT 11 on 3 January 2019, 12 UTC 2.1 Large MFASIS-DOM differences over Antarctica (winter) and Greenland (summer) RTTOV-MFASIS forward simulation under real viewing geometry and observation SEVIRI 0.6μm on METEOSAT 11, 3 January 2019, 12 UTC Contribution of different cloudy situations to MFASIS forward simulation SEVIRI 0.6μm mean -0.024 stdev +0.179 rmse +0.180 sk -0.137 Comparison of first guess (RTTOV-MFASIS) and observation SEVIRI 0.6μm on METEOSAT 11, 3 January 2019, 12 UTC use model info to select clear-sky profiles 2.4 Linear water-vapour correction Impact of water-vapour correction on differences (MFASIS-DOM) SEVIRI 0.8μm on METEOSAT 11 on 3 January 2019, 12 UTC (60N - 60S) 2.2 Dependence of MFASIS-DOM differences on cloudy situations 3 Outlook Future updates to RTTOV-MFASIS: Further improve mixed-phase cloud correction, account for some 3D RT effects (approximation for cloud-top inclination), include multiple Rayleigh scattering processes by updating LUTs using RTTOV-DOM (RTTOV v13) for LUT training Extend MFASIS to account for more RT effects and more particle species (aerosols) Use neuronal network instead of lookup table (see L. Scheck et al., 11p.07) Validation of ICON and ICON-LAM (limited-area version of ICON, in preparation) using visible channel information in conjunction with all-sky infrared simulations Assimilation of visible satellite observations in global and convection-resolving models 2.3 Empirical mixed-phase cloud correction SEVIRI 0.6μm and 0.8μm on METEOSAT 11 on 3 January 2019, 12 UTC (60N - 60S) SEVIRI 0.8μm errors induced tails removed SEVIRI 0.6μm tails removed SEVIRI 0.8μm Reflectance differences in the RTTOV-MFASIS and RTTOV-DOM simulations illustrate that this simple mixed-phase cloud correction has an overall neutral to slightly positive impact. Work is ongoing to investigate improvements for mixed-phase cloud situations. Differences between model forward simulations are due to forward-operator assumptions, error dependence on viewing geometry, characteristics of the atmospheric profiles, NWP model quality and observation errors. In order to nail down the error sources, we evaluate the RTTOV-MFASIS forward operator in a test setup: We compare forward simulations using ICON model fields for 180.000 atmospheric profiles on a generic latitude-longitude grid with fixed satellite-sun viewing geometry to RTTOV-DOM results. Differences between reflectance from RTTOV-MFASIS and RTTOV-DOM simulations depend on the viewing geometry and the properties of the atmospheric profiles. For the SEVIRI instrument, errors are typically larger for the 0.8μm channel. Antarctica and Greenland are characterised by large albedos and high terrain. The assumption that all terrain is at elevation 0m in MFASIS (elevation is set to 0m in the LUT training) in combination with large albedos leads to large differences between RTTOV-MFASIS and RTTOV-DOM reflectances due to missing multiple Rayleigh scattering processes in RTTOV-DOM (and in MFASIS LUTs). Errors are significantly reduced in the respective libRadtran-DISORT/-MFASIS calculations, which account for multiple scattering processes. RTTOV v13 will include multiple Rayleigh scattering. Until then, regions of high terrain and large albedos can be flagged. The current evaluation restricts profiles to latitudes 60N - 60S. SEVIRI 0.6μm mean 0.005 stdev 0.010 rmse 0.011 sk 0.383 mean 0.003 stdev 0.018 rmse 0.018 sk -0.611 mean -0.001 stdev 0.011 rmse 0.011 sk -1.230 Comparison of first guess (RTTOV-MFASIS) and first guess (RTTOV-DOM) SEVIRI 0.6μm on METEOSAT 11, 3 January 2019, 12 UTC (60N - 60S) Reflectance differences obtained in RTTOV-MFASIS and RTTOV-DOM forward simulations and their dependence on atmospheric situations are shown below for a set of viewing geometries. In order to categorise the model-field profiles into clear-sky, water-only, ice-only and mixed-phase cloud (pure, with ice above or water below mixed-phase cloud) situations, we introduce thresholds on the total ice and water optical depths in the vertical profiles. 20 40 60 80 100 120 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 Reflectance everything converted to water everything converted to ice ice fully above water (MFASIS) ice and water homogeneously mixed 100m water above ice 200m water above ice 500m water above ice 30 35 40 45 50 55 60 65 70 0.54 0.56 0.58 0.60 0.62 0.64 Reflectance Dependence of reflectance on scattering angle for idealised cloudy scenes: MFASIS WATER ONLY ICE ONLY HOMOG. MIX REALISTIC? Idealised scenes: The SEVIRI 0.8μm channel is sensitive to water vapour in the atmosphere. RTTOV v12.3 applies a linear correction that accounts for cloud-top heights and water-vapour profiles not considered during the LUT generation. The correction successfully reduces errors in the SEVIRI 0.8μm channel to the same magnitude as other channels. MFASIS assumes separate, homogeneous ice and water clouds at fixed height. However, ground- based and in-situ observations have shown that realistic mixed-phase clouds typically have a water-only layer on top of the ice-water composite. Based on the study of several idealised scenes, RTTOV-MFASIS applies a simple correction for mixed-phase clouds by using the effective ice and water optical depths for the forward simulation.
Transcript of Evaluation of the fast visible RT model RTTOV-MFASIS and...
1 Comparison of simulated and observed reflectancesReflectance based on ICON forward simulation (left) shows similar cloud patterns as observation (middle). Largest errors occur where model cloud fields differ (right). Less cloud struture is visiblein the model equivalent because the MFASIS forward simulation lacks 3D RT effects, e.g., due tocloud-top inclination (see L. Scheck et al., 11p.07). A correction to approximately account forcloud-top inclination in MFASIS is planned for RTTOV v13.
The reflectance histogram (left) shows that the overall shape of the observed reflectance curve is reproduced in model calculations. Observation minus first guess departures (middle) exhibit aGaussian shape, with clear-sky contributions promoting a peak around the mean. Contributionsfrom different atmospheric situations are quantified (right). Here, the model equivalents have beenclassified by applying thresholds on the total ice and water optical depths in the vertical profile.
2 Evaluation: RTTOV-MFASIS versus RTTOV-DOM
Greenland profile:lat = 75, lon = -40
ICON elev. 3km
RTTOV and libRadtran forward calculationsfor SEVIRI 0.6μm on METEOSAT 11,
3 July 2019, 12 UTC
Research and Development, Deutscher Wetterdienst, Offenbach, [email protected]
Christina Stumpf, Leonhard Scheck, Christina Koepken-Watts, Olaf Stiller, Liselotte Bach, Roland Potthast
Evaluation of the fast visible RT model RTTOV-MFASISand use for model cloud validation of ICON
MFASIS WATER ONLY ICE ONLY HOMOG. MIX
REALISTIC?
This work studies the fast RT method MFASIS for the simulation of visible satellite images available in RTTOV v12.2 and later versions. MFASIS is alookup-table (LUT) based method (see L. Scheck et al., 11p.07). The LUTs are trained using the more accurate, 1D RT method RTTOV-DOM and areprovided for SEVIRI, ABI and AHI via www.nwpsaf.eu. Here, we evaluate RTTOV-MFASIS results for the visible SEVIRI channels based on global ICONmodel fields by comparing them to RTTOV-DOM results in a suitable test setup and to SEVIRI observations. These investigations pave the way for furtherupdates to MFASIS and the validation of model cloud fields. First data assimilation experiments using MFASIS in convection-resolving models havedemonstrated its value by improving the representation of cloud cover and precipitation (see L. Scheck et al., 11p.07; L. Bach et al., 9.04).
no BRDF values(polar night)
Differences (MFASIS-DOM) for SEVIRI 0.6μm on METEOSAT 11 on 3 January 2019, 12 UTC
2.1 Large MFASIS-DOM differences over Antarctica (winter) and Greenland (summer)
RTTOV-MFASIS forward simulation under real viewing geometry and observationSEVIRI 0.6μm on METEOSAT 11, 3 January 2019, 12 UTC
Contribution of different cloudysituations to MFASIS forward simulation
SEVIRI 0.6μm mean -0.024stdev +0.179rmse +0.180sk -0.137
Comparison of first guess (RTTOV-MFASIS) and observationSEVIRI 0.6μm on METEOSAT 11, 3 January 2019, 12 UTC
use model info toselect clear-sky profiles
2.4 Linear water-vapour correction
Impact of water-vapour correction on differences (MFASIS-DOM)SEVIRI 0.8μm on METEOSAT 11 on 3 January 2019, 12 UTC (60N - 60S)
2.2 Dependence of MFASIS-DOM differences on cloudy situations
3 OutlookFuture updates to RTTOV-MFASIS: Further improve mixed-phase cloud correction, account for some 3D RT effects (approximation for cloud-top inclination), include multiple Rayleighscattering processes by updating LUTs using RTTOV-DOM (RTTOV v13) for LUT training Extend MFASIS to account for more RT effects and more particle species (aerosols) Use neuronal network instead of lookup table (see L. Scheck et al., 11p.07) Validation of ICON and ICON-LAM (limited-area version of ICON, in preparation) using visiblechannel information in conjunction with all-sky infrared simulationsAssimilation of visible satellite observations in global and convection-resolving models
2.3 Empirical mixed-phase cloud correction
SEVIRI 0.6μm and 0.8μm on METEOSAT 11 on 3 January 2019, 12 UTC (60N - 60S)
SEVIRI 0.8μm
errors induced tails removed
SEVIRI 0.6μm
tails removed
SEVIRI 0.8μm
Reflectance differences in the RTTOV-MFASIS and RTTOV-DOM simulations illustrate that thissimple mixed-phase cloud correction has an overall neutral to slightly positive impact. Work isongoing to investigate improvements for mixed-phase cloud situations. Differences between model forward simulations are due to forward-operator assumptions, error
dependence on viewing geometry, characteristics of the atmospheric profiles, NWP model qualityand observation errors. In order to nail down the error sources, we evaluate the RTTOV-MFASISforward operator in a test setup: We compare forward simulations using ICON model fields for180.000 atmospheric profiles on a generic latitude-longitude grid with fixed satellite-sun viewing geometry to RTTOV-DOM results.
Differences between reflectance from RTTOV-MFASIS and RTTOV-DOM simulations depend onthe viewing geometry and the properties of the atmospheric profiles. For the SEVIRI instrument,errors are typically larger for the 0.8μm channel.
Antarctica and Greenland are characterised by large albedosand high terrain. The assumption that all terrain is at elevation0m in MFASIS (elevation is set to 0m in the LUT training) incombination with large albedos leads to large differencesbetween RTTOV-MFASIS and RTTOV-DOM reflectances dueto missing multiple Rayleigh scattering processes inRTTOV-DOM (and in MFASIS LUTs). Errors are significantlyreduced in the respective libRadtran-DISORT/-MFASIScalculations, which account for multiple scattering processes. RTTOV v13 will include multiple Rayleigh scattering. Untilthen, regions of high terrain and large albedos can be flagged.The current evaluation restricts profiles to latitudes 60N - 60S.
SEVIRI 0.6μmSEVIRI 0.6μm
SEVIRI 0.8μm
Comparison of first guess (RTTOV-MFASIS) and first guess (RTTOV-DOM)SEVIRI 0.6μm and 0.8μm on METEOSAT 11, 3 January 2019, 12 UTC (60N-60S)
mean 0.005stdev 0.010rmse 0.011sk 0.383
mean 0.003stdev 0.018rmse 0.018sk -0.611
mean -0.001stdev 0.011rmse 0.011sk -1.230
Comparison of first guess (RTTOV-MFASIS) and first guess (RTTOV-DOM)SEVIRI 0.6μm on METEOSAT 11, 3 January 2019, 12 UTC (60N - 60S)
Reflectance differences obtained in RTTOV-MFASIS and RTTOV-DOM forward simulations andtheir dependence on atmospheric situations are shown below for a set of viewing geometries.In order to categorise the model-field profiles into clear-sky, water-only, ice-only and mixed-phasecloud (pure, with ice above or water below mixed-phase cloud) situations, we introduce thresholdson the total ice and water optical depths in the vertical profiles.
20 40 60 80 100 120
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Refl
ecta
nce
everything converted to watereverything converted to ice
ice fully above water (MFASIS)
ice and water homogeneously mixed100m water above ice200m water above ice500m water above ice
30 35 40 45 50 55 60 65 70
0.54
0.56
0.58
0.60
0.62
0.64
Refl
ecta
nce
Dependence of reflectance on scattering angle for idealised cloudy scenes:
MFASIS WATER ONLY ICE ONLY HOMOG. MIX
REALISTIC?
Idealised scenes:
The SEVIRI 0.8μm channel is sensitive to water vapour in the atmosphere. RTTOV v12.3 appliesa linear correction that accounts for cloud-top heights and water-vapour profiles not consideredduring the LUT generation. The correction successfully reduces errors in the SEVIRI 0.8μmchannel to the same magnitude as other channels.
MFASIS assumes separate, homogeneous ice and water clouds at fixed height. However, ground-based and in-situ observations have shown that realistic mixed-phase clouds typically have awater-only layer on top of the ice-water composite. Based on the study of several idealisedscenes, RTTOV-MFASIS applies a simple correction for mixed-phase clouds by using the effectiveice and water optical depths for the forward simulation.
