scholarly journals Scale dependence of cirrus horizontal heterogeneity effects on TOA measurements – Part I: MODIS brightness temperatures in the thermal infrared

2017 ◽  
Vol 17 (13) ◽  
pp. 8489-8508 ◽  
Author(s):  
Thomas Fauchez ◽  
Steven Platnick ◽  
Kerry Meyer ◽  
Céline Cornet ◽  
Frédéric Szczap ◽  
...  
Keyword(s):  
Spatial Resolution ◽  
Optical Thickness ◽  
Approximation Error ◽  
Thermal Infrared ◽  
Sensitivity Study ◽  
Brightness Temperatures ◽  
Model Average ◽  
Horizontal Heterogeneity ◽  
Heterogeneity Effects ◽  
The Impact

Abstract. This paper presents a study on the impact of cirrus cloud heterogeneities on MODIS simulated thermal infrared (TIR) brightness temperatures (BTs) at the top of the atmosphere (TOA) as a function of spatial resolution from 50 m to 10 km. A realistic 3-D cirrus field is generated by the 3DCLOUD model (average optical thickness of 1.4, cloud-top and base altitudes at 10 and 12 km, respectively, consisting of aggregate column crystals of Deff = 20 µm), and 3-D thermal infrared radiative transfer (RT) is simulated with the 3DMCPOL code. According to previous studies, differences between 3-D BT computed from a heterogenous pixel and 1-D RT computed from a homogeneous pixel are considered dependent at nadir on two effects: (i) the optical thickness horizontal heterogeneity leading to the plane-parallel homogeneous bias (PPHB) and the (ii) horizontal radiative transport (HRT) leading to the independent pixel approximation error (IPAE). A single but realistic cirrus case is simulated and, as expected, the PPHB mainly impacts the low-spatial-resolution results (above ∼ 250 m) with averaged values of up to 5–7 K, while the IPAE mainly impacts the high-spatial-resolution results (below ∼ 250 m) with average values of up to 1–2 K. A sensitivity study has been performed in order to extend these results to various cirrus optical thicknesses and heterogeneities by sampling the cirrus in several ranges of parameters. For four optical thickness classes and four optical heterogeneity classes, we have found that, for nadir observations, the spatial resolution at which the combination of PPHB and HRT effects is the smallest, falls between 100 and 250 m. These spatial resolutions thus appear to be the best choice to retrieve cirrus optical properties with the smallest cloud heterogeneity-related total bias in the thermal infrared. For off-nadir observations, the average total effect is increased and the minimum is shifted to coarser spatial resolutions.

scholarly journals Scale dependence of cirrus heterogeneity effects. Part I: MODIS thermal infrared channels

2017 ◽  
Author(s):  
Thomas Fauchez ◽  
Steven Platnick ◽  
Kerry Meyer ◽  
Céline Cornet ◽  
Frédéric Szczap ◽  
...  
Keyword(s):  
Spatial Resolution ◽  
Optical Thickness ◽  
Approximation Error ◽  
Thermal Infrared ◽  
Sensitivity Study ◽  
Brightness Temperatures ◽  
Homogeneous Plane ◽  
Horizontal Heterogeneity ◽  
Heterogeneity Effects ◽  
The Impact

Abstract. This paper presents a study on the impact of cirrus cloud heterogeneities on MODIS simulated thermal infrared (TIR) brightness temperatures (BT) at the top of the atmosphere (TOA) as a function of spatial resolution from 50 m to 10 km. A realistic 3-D cirrus field is generated by the 3DCLOUD model, and 3-D thermal infrared radiative transfer (RT) is simulated with the 3DMCPOL code. According to previous studies, differences between 3-D BT computed from a heterogeneous pixel and 1-D RT computed from a homogeneous pixel are considered dependent, at nadir, on two effects: (i) the optical thickness horizontal heterogeneity leading to the homogeneous plane parallel bias (PPHB) and the (ii) horizontal radiative transport (HRT) leading to the independent pixel approximation error (IPAE). A unique but realistic cirrus case is simulated and, as expected, the PPHB impacts mainly the low spatial resolution results (above 250 m) with averaged values up to 5–7 K while the IPAE impacts mainly the high spatial resolution results (below 250 m) with average values up to 1–2 K. A sensitivity study has been performed in order to extend these results to various cirrus optical thicknesses and heterogeneities by sampling the cirrus in several ranges of parameters. For four optical thickness classes and four optical heterogeneity classes, we have found that, for nadir observations, the spatial resolution where the combination of PPHB and HRT effects is the smallest, falls between 100 m and 250 m. These spatial resolutions appear thus to be the best choice to retrieve cirrus optical properties with the smallest cloud heterogeneity related total bias in the thermal infrared. For off-nadir observations, the average total effect is increased and the minimum is shifted to coarser spatial resolutions.

scholarly journals Impacts of cloud heterogeneities on cirrus optical properties retrieved from space-based thermal infrared radiometry

2015 ◽  
Vol 8 (2) ◽  
pp. 633-647 ◽  
Author(s):  
T. Fauchez ◽  
P. Dubuisson ◽  
C. Cornet ◽  
F. Szczap ◽  
A. Garnier ◽  
...  
Keyword(s):  
Standard Deviation ◽  
Optical Thickness ◽  
Cirrus Clouds ◽  
Effective Diameter ◽  
Ice Crystal ◽  
Cloud Parameters ◽  
Vertical Heterogeneity ◽  
Horizontal Heterogeneity ◽  
Heterogeneity Effects ◽  
The Impact

Abstract. This paper presents a study, based on simulations, of the impact of cirrus cloud heterogeneities on the retrieval of cloud parameters (optical thickness and effective diameter) for the Imaging Infrared Radiometer (IIR) on board CALIPSO. Cirrus clouds are generated by the stochastic model 3DCLOUD for two different cloud fields and for several averaged cloud parameters. One cloud field is obtained from a cirrus observed on 25 May 2007 during the airborne campaign CIRCLE-2 and the other is a cirrus uncinus. The radiative transfer is simulated with the 3DMCPOL code. To assess the errors due to cloud heterogeneities, two related retrieval algorithms are used: (i) the split-window technique to retrieve the ice crystal effective diameter and (ii) an algorithm similar to the IIR operational algorithm to retrieve the effective emissivity and the effective optical thickness. Differences between input parameters and retrieved parameters are compared as a function of different cloud properties such as the mean optical thickness, the heterogeneity parameter and the effective diameter. The optical thickness heterogeneity for each 1 km × 1 km observation pixel is represented by the optical thickness standard deviation computed using 100 m × 100 m subpixels. We show that optical thickness heterogeneity may have a strong impact on the retrieved parameters, mainly due to the plane-parallel approximation (PPA assumption). In particular, for cirrus clouds with ice crystal diameter of approximately 10 μm, the averaged error on the retrieved effective diameter and optical thickness is about 2.5 μm (~ 25%) and −0.20 (~ 12%), respectively. Then, these biases decrease with increasing effective size due to a decrease of the cloud absorption and, thus, the PPA bias. Cloud horizontal heterogeneity effects are greater than other possible sources of retrieval errors such as those due to cloud vertical heterogeneity impact, surface temperature or atmospheric temperature profile uncertainty and IIR retrieval uncertainty. Cloud horizontal heterogeneity effects are larger than the IIR retrieval uncertainty if the standard deviation of the optical thickness, inside the observation pixel, is greater than 1.

scholarly journals Scale dependence of cirrus heterogeneity effects. Part II: MODIS NIR and SWIR channels

2018 ◽  
Vol 18 (16) ◽  
pp. 12105-12121 ◽  
Author(s):  
Thomas Fauchez ◽  
Steven Platnick ◽  
Tamás Várnai ◽  
Kerry Meyer ◽  
Céline Cornet ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Spatial Resolution ◽  
Thermal Infrared ◽  
Radiative Effects ◽  
Ice Clouds ◽  
Plane Parallel ◽  
Wide Range ◽  
The Impact ◽  
Side Illumination

Abstract. In a context of global climate change, the understanding of the radiative role of clouds is crucial. On average, ice clouds such as cirrus have a significant positive radiative effect, but under some conditions the effect may be negative. However, many uncertainties remain regarding the role of ice clouds on Earth's radiative budget and in a changing climate. Global satellite observations are particularly well suited to monitoring clouds, retrieving their characteristics and inferring their radiative impact. To retrieve ice cloud properties (optical thickness and ice crystal effective size), current operational algorithms assume that each pixel of the observed scene is plane-parallel and homogeneous, and that there is no radiative connection between neighboring pixels. Yet these retrieval assumptions are far from accurate, as real radiative transfer is 3-D. This leads to the plane-parallel and homogeneous bias (PPHB) plus the independent pixel approximation bias (IPAB), which impacts both the estimation of top-of-the-atmosphere (TOA) radiation and the retrievals. An important factor that determines the impact of these assumptions is the sensor spatial resolution. High-spatial-resolution pixels can better represent cloud variability (low PPHB), but the radiative path through the cloud can involve many pixels (high IPAB). In contrast, low-spatial-resolution pixels poorly represent the cloud variability (high PPHB), but the radiation is better contained within the pixel field of view (low IPAB). In addition, the solar and viewing geometry (as well as cloud optical properties) can modulate the magnitude of the PPHB and IPAB. In this, Part II of our study, we simulate TOA 0.86 and 2.13 µm solar reflectances over a cirrus uncinus scene produced by the 3DCLOUD model. Then, 3-D radiative transfer simulations are performed with the 3DMCPOL code at spatial resolutions ranging from 50 m to 10 km, for 12 viewing geometries and nine solar geometries. It is found that, for simulated nadir observations taken at resolution higher than 2.5 km, horizontal radiation transport (HRT) dominates biases between 3-D and 1-D reflectance calculations, but these biases are mitigated by the side illumination and shadowing effects for off-zenith solar geometries. At resolutions coarser than 2.5 km, PPHB dominates. For off-nadir observations at resolutions higher than 2.5 km, the effect that we call THEAB (tilted and homogeneous extinction approximation bias) due to the oblique line of sight passing through many cloud columns contributes to a large increase of the reflectances, but 3-D radiative effects such as shadowing and side illumination for oblique Sun are also important. At resolutions coarser than 2.5 km, the PPHB is again the dominant effect. The magnitude and resolution dependence of PPHB and IPAB is very different for visible, near-infrared and shortwave infrared channels compared with the thermal infrared channels discussed in Part I of this study. The contrast of 3-D radiative effects between solar and thermal infrared channels may be a significant issue for retrieval techniques that simultaneously use radiative measurements across a wide range of solar reflectance and infrared wavelengths.

scholarly journals Impact of cirrus clouds heterogeneities on top-of-atmosphere thermal infrared radiation

2014 ◽  
Vol 14 (11) ◽  
pp. 5599-5615 ◽  
Author(s):  
T. Fauchez ◽  
C. Cornet ◽  
F Szczap ◽  
P. Dubuisson ◽  
T. Rosambert
Keyword(s):  
Radiative Transfer ◽  
Monte Carlo Model ◽  
Thermal Infrared ◽  
Brightness Temperatures ◽  
Spectral Bands ◽  
Thickness Standard ◽  
The Difference ◽  
Scale Properties ◽  
Parallel Approximation ◽  
The Impact

Abstract. This paper presents a study of the impact of cirrus cloud heterogeneities on the thermal infrared brightness temperatures at the top of the atmosphere (TOA). Realistic 3-D cirri are generated by a cloud generator based on simplified thermodynamic and dynamic equations and on the control of invariant scale properties. The 3-D thermal infrared radiative transfer is simulated with a Monte Carlo model for three typical spectral bands in the infrared atmospheric window. Comparisons of TOA brightness temperatures resulting from 1-D and 3-D radiative transfer show significant differences for optically thick cirrus (τ > 0.3 at 532 nm) and are mainly due to the plane-parallel approximation (PPA). At the spatial resolution of 1 km × 1 km, two principal parameters control the heterogeneity effects on brightness temperatures: i) the optical thickness standard deviation inside the observation pixel, ii) the brightness temperature contrast between the top of the cirrus~and the clear-sky atmosphere. Furthermore, we show that the difference between 1-D and 3-D brightness temperatures increases with the zenith view angle from two to ten times between 0° and 60° due to the tilted independent pixel approximation (TIPA).

scholarly journals Retrieving aerosol height from the oxygen A band: a fast forward operator and sensitivity study concerning spectral resolution, instrumental noise, and surface inhomogeneity

2013 ◽  
Vol 6 (6) ◽  
pp. 10511-10550 ◽  
Author(s):  
A. Hollstein ◽  
J. Fischer
Keyword(s):  
Spectral Resolution ◽  
Optical Thickness ◽  
Principal Component ◽  
Linear Interpolation ◽  
Sensitivity Study ◽  
Aerosol Optical Thickness ◽  
Lookup Table ◽  
The Impact ◽  
Forward Operator ◽  
Aerosol Type

Abstract. Hyperspectral radiance measurements in the oxygen A band are sensitive to the vertical distribution of atmospheric scatterers, which in principle allows to retrieve aerosol height from future instruments like TROPOMI, OCO2, FLEX, and CarbonSat. Discussed in this paper is a fast and flexible forward operator for the simulation of hyperspectral radiances in the oxygen A band and, based on this scheme, a sensitivity study about the inversion quality of aerosol optical thickness, aerosol mean height, and aerosol type. The forward operator is based on a lookup table with efficient data compression based on principal component analysis. Linear interpolation and computation of partial derivatives is performed in the much smaller space of expansion coefficients rather then wavelength. Thus, this approach is computationally fast and at the same time memory efficient. The sensitivity study explores the impact of instrument design on the retrieval of aerosol optical thickness and aerosol height. Considered are signal to noise ratio, spectral resolution, and spectral sampling. Also taken into account are surface inhomogeneities and variations of the aerosol type.

scholarly journals Retrieving aerosol height from the oxygen A band: a fast forward operator and sensitivity study concerning spectral resolution, instrumental noise, and surface inhomogeneity

2014 ◽  
Vol 7 (5) ◽  
pp. 1429-1441 ◽  
Author(s):  
A. Hollstein ◽  
J. Fischer
Keyword(s):  
Spectral Resolution ◽  
Optical Thickness ◽  
Principal Component ◽  
Linear Interpolation ◽  
Sensitivity Study ◽  
Aerosol Optical Thickness ◽  
Lookup Table ◽  
The Impact ◽  
Forward Operator ◽  
Aerosol Type

Abstract. Hyperspectral radiance measurements in the oxygen A band are sensitive to the vertical distribution of atmospheric scatterers, which in principle allows the retrieval of aerosol height from future instruments like TROPOMI, OCO2, FLEX, and CarbonSat. Discussed in this paper is a fast and flexible forward operator for the simulation of hyperspectral radiances in the oxygen A band and, based on this scheme, a sensitivity study about the inversion quality of aerosol optical thickness, aerosol mean height, and aerosol type. The forward operator is based on a lookup table with efficient data compression based on principal component analysis. Linear interpolation and computation of partial derivatives is performed in the much smaller space of expansion coefficients rather than wavelength. Thus, this approach is computationally fast and, at the same time, memory efficient. The sensitivity study explores the impact of instrument design on the retrieval of aerosol optical thickness and aerosol height. Considered are signal to noise ratio, spectral resolution, and spectral sampling. Also taken into account are surface inhomogeneities and variations of the aerosol type.

scholarly journals Impact of the Revisit of Thermal Infrared Remote Sensing Observations on Evapotranspiration Uncertainty—A Sensitivity Study Using AmeriFlux Data

2019 ◽  
Vol 11 (5) ◽  
pp. 573 ◽  
Author(s):  
Pierre Guillevic ◽  
Albert Olioso ◽  
Simon Hook ◽  
Joshua Fisher ◽  
Jean-Pierre Lagouarde ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Solar Radiation ◽  
Thermal Infrared ◽  
Sensitivity Study ◽  
Time Of Day ◽  
Surface Solar Radiation ◽  
Thermal Infrared Remote Sensing ◽  
Temporal Sampling ◽  
Infrared Remote Sensing ◽  
The Impact

Thermal infrared remote sensing observations have been widely used to provide useful information on surface energy and water stress for estimating evapotranspiration (ET). However, the revisit time of current high spatial resolution (<100 m) thermal infrared remote sensing systems, sixteen days for Landsat for example, can be insufficient to reliably derive ET information for water resources management. We used in situ ET measurements from multiple Ameriflux sites to (1) evaluate different scaling methods that are commonly used to derive daytime ET estimates from time-of-day observations; and (2) quantify the impact of different revisit times on ET estimates at monthly and seasonal time scales. The scaling method based on a constant evaporative ratio between ET and the top-of-atmosphere solar radiation provided slightly better results than methods using the available energy, the surface solar radiation or the potential ET as scaling reference fluxes. On average, revisit time periods of 2, 4, 8 and 16 days resulted in ET uncertainties of 0.37, 0.55, 0.73 and 0.90 mm per day in summer, which represented 13%, 19%, 23% and 31% of the monthly average ET calculated using the one-day revisit dataset. The capability of a system to capture rapid changes in ET was significantly reduced for return periods higher than eight days. The impact of the revisit on ET depended mainly on the land cover type and seasonal climate, and was higher over areas with high ET. We did not observe significant and systematic differences between the impacts of the revisit on monthly ET estimates that are based on morning or afternoon observations. We found that four-day revisit scenarios provided a significant improvement in temporal sampling to monitor surface ET reducing by around 40% the uncertainty of ET products derived from a 16-day revisit system, such as Landsat for instance.

scholarly journals Sensitivity of aerosol optical thickness and aerosol direct radiative effect to relative humidity

2008 ◽  
Vol 8 (4) ◽  
pp. 13233-13263 ◽  
Author(s):  
H. Bian ◽  
M. Chin ◽  
J. Rodriguez ◽  
H. Yu ◽  
J. E. Penner ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Spatial Resolution ◽  
Optical Thickness ◽  
Model Comparison ◽  
Sensitivity Study ◽  
Aerosol Optical Thickness ◽  
Anthropogenic Aerosols ◽  
The Andes ◽  
Aerosol Mass ◽  
The Difference

Abstract. We present a sensitivity study on the effects of spatial and temporal resolution of atmospheric relative humidity (RH) on calculated aerosol optical thickness (AOT) and the aerosol direct radiative effects (DRE) in a global model. Using the same aerosol fields simulated in the Global Modeling Initiative (GMI) model, we find that, on a global average, the calculated AOT from RH in 1° latitude by 1.25° longitude spatial resolution is 11% higher than that in 2° by 2.5° resolution, and the corresponding DRE at the top of the atmosphere is 8–9% higher for total aerosols and 15% higher for only anthropogenic aerosols in the finer spatial resolution case. The difference is largest over surface escarpment regions (e.g. >200% over the Andes Mountains) where RH varies substantially with surface terrain. The largest zonal mean AOT difference occurs at 50–60°N (16–21%), where AOT is also relatively larger. A similar increase is also found when the time resolution of RH is increased. This increase of AOT and DRE with the increase of model resolution is due to the highly non-linear relationship between RH and the aerosol mass extinction efficiency (MEE) at high RH (>80%). Our study suggests that caution should be taken in a multi-model comparison (e.g. AeroCom) since the comparison usually deals with results coming from different spatial/temporal resolutions.

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