Rotational Raman scattering in the Earth's atmosphere ... - SRON

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114 Chapter 5 for the combination of the VIS and NIR windows. However, the retrieval of cloud optical thickness is more sensitive to the noise floor than for the NUV + NIR combination. The third column of Fig. 5.7 presents retrieval simulations of the combined NUV and NIR windows when in addition to the cloud parameters also the power law scaling factor is fitted to the spectra in each window. In this case when the spectral continuum of the NUV window is described by the wavelength dependent scaling factor, the retrieval of the NIR window is not improved by the extra measurements in the NUV. This can be explained by our previous finding that the extra information of the NUV with respect to the NIR retrieval originates from the continuum signature of the NUV window. However, the NIR retrieval can be improved by the synergistic use of both spectral windows when only a wavelength-independent scaling factor is fitted to the spectrum. We can then retrieve the cloud fraction f c with a maximum sensitivity C f ≈ 1 for nearly all cloud fractions. Furthermore, we found that for the NUV + NIR spectral combination the Ring effect does not significantly contribute to the retrieval of cloud parameters. To demonstrate this we replaced the NUV reflectivity spectrum with a NUV reflectivity spectrum where we did not take inelastic Raman scattering into account. This modification of the measurement simulation hardly affected the retrieval sensitivity. The loss of sensitivity of the NUV measurement with respect to cloud top pressure is completely compensated by the sensitivity of the O 2 A absorption band in the NIR measurement. For the design of a future instrument this means that the spectral resolution requirement on the NUV measurements can be relaxed, because it is not needed to resolve the spectrally fine Ring effect features in the spectrum. Furthermore, our results implicate that it allows one to simulate the Ring effect on the spectrum with a pre-calculated Ring spectrum and certain fudge parameters, such as an amplitude of the Ring spectrum, to account for the different strength of the Ring effect depending on the atmospheric state. This commonly used approach (e.g. Chance [1998]) eases the time-consuming radiative transfer calculations that include inelastic Raman scattering considerably. Overall, the combination of the NUV and NIR window is superior to the combination of the VIS and NIR spectral range. It improves the NIR and NUV cloud retrieval significantly so that all three cloud parameters, i.e. cloud top pressure, cloud fraction and cloud optical thickness can be retrieved from the measurement with a high robustness to random-like measurement biases. This added value compared to a single NIR window approach originates from the spectral shape of the continuum in the NUV window. The relative Ring structures do not provide significant extra information in this combination. This means that the maximum benefit of the synergistic use of both spectral windows can only be achieved when the broadband spectral features are exploited in the spectrum. Therefore, the broadband radiometric calibration of the NUV requires special attention for the retrieval of cloud properties. 5.7 Summary and conclusion The majority of satellite-based reflectivity measurements are affected by clouds. To enable trace gas retrievals for cloudy scenes it is essential to retrieve auxiliary parameters that adequately describe the clouds. Our purpose was to find out which spectral window or which combination of windows is most
Retrieval of cloud properties from NUV, VIS and NIR 115 Figure 5.7: The first two columns show the retrieval sensitivities (C) to the cloud parameters p c , f c , τ c as function of cloud fraction for the spectral window combination VIS + NIR and NUV + NIR for a fixed noise floor η = 0.1%. Results are shown for the four cloud scenarios of Fig. 5.5. The third column shows the retrieval sensitivities for the combination NUV + NIR when in addition to the cloud parameters a power law (solid line) or a factor (dotted line) is fitted to the reflectivity measurement in each window, using the same cloud scenario as in Fig. 5.6.
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VRIJE UNIVERSITEIT Rotational Raman
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Contents 1 Introduction 1 1.1 Obser
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1 Introduction 1.1 Observing skylig
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Introduction 3 Extracting the wealt
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Introduction 5 and Spurr, 1997, Vou
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Introduction 7 energy: E rot (v,J)
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Introduction 9 Figure 1.4: Probabil
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Introduction 11 scattered radiance
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Introduction 13 approximation in wh
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Introduction 15 scattering have bee
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2 A doubling-adding method to inclu
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A doubling-adding method to include
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40 Chapter 3 nm with a spectral res
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42 Chapter 3 contributions of lower
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44 Chapter 3 if we use the effectiv
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46 Chapter 3 This in turn results i
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48 Chapter 3 This may be shown in a
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50 Chapter 3 The forward and adjoin
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52 Chapter 3 Here, ∆ = diag [1, 1
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54 Chapter 3 Figure 3.3: Relative R
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56 Chapter 3 biases the simulation
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58 Chapter 3 3.4.2 Approximation me
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60 Chapter 3 where I ram,app and I
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62 Chapter 3 3.5 Simulation of pola
Page 70 and 71: 64 Chapter 3 Figure 3.10: Relative
Page 72 and 73: 66 Chapter 3 dependence on the vert
Page 74 and 75: 68 Chapter 3 Figure 3.13: Same as F
Page 76 and 77: 70 Chapter 3 3.6 Summary A vector r
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Page 80 and 81: 74 Chapter 3 where S l is the expan
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Page 85 and 86: 4 Accurate modeling of spectral fin
Page 87 and 88: Accurate modeling of spectral fine-
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Page 105 and 106: Retrieval of cloud properties from
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Page 125 and 126: References Aben, I., F. Helderman,
Page 127 and 128: References 121 Ellery, A., D. Wynn-
Page 129 and 130: References 123 Lacis, A. A., J. Cho
Page 131 and 132: References 125 Schulz, F., K. Stamn
Page 133 and 134: Summary A spectrum of sunlight that
Page 135 and 136: Summary 129 Earth radiance spectrum
Page 137 and 138: Samenvatting Het door de aardatmosf
Page 139 and 140: Samenvatting 133 met een vector-mod
Page 141: List of publications Peer-reviewed
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Rotational Raman scattering in the Earth's atmosphere ... - SRON

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